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Reschke 3 Internet-Draft greenbytes 4 Intended status: Standards Track February 8, 2010 5 Expires: August 12, 2010 7 Application of RFC 2231 Encoding to 8 Hypertext Transfer Protocol (HTTP) Header Fields 9 draft-reschke-rfc2231-in-http-09 11 Abstract 13 By default, message header field parameters in Hypertext Transfer 14 Protocol (HTTP) messages can not carry characters outside the ISO- 15 8859-1 character set. RFC 2231 defines an escaping mechanism for use 16 in Multipurpose Internet Mail Extensions (MIME) headers. This 17 document specifies a profile of that encoding suitable for use in 18 HTTP header fields. 20 Editorial Note (To be removed by RFC Editor before publication) 22 There are multiple HTTP header fields that already use RFC 2231 23 encoding in practice (Content-Disposition) or might use it in the 24 future (Link). The purpose of this document is to provide a single 25 place where the generic aspects of RFC 2231 encoding in HTTP header 26 fields are defined. 28 Distribution of this document is unlimited. Although this is not a 29 work item of the HTTPbis Working Group, comments should be sent to 30 the Hypertext Transfer Protocol (HTTP) mailing list at 31 ietf-http-wg@w3.org [1], which may be joined by sending a message 32 with subject "subscribe" to ietf-http-wg-request@w3.org [2]. 34 Discussions of the HTTPbis Working Group are archived at 35 . 37 XML versions, latest edits and the issues list for this document are 38 available from 39 . A 40 collection of test cases is available at 41 . 43 Note: as of January 2010, there were at least three independent 44 implementations of the encoding defined in Section 3.2: Konqueror 45 (trunk), Mozilla Firefox, and Opera. 47 Status of this Memo 48 This Internet-Draft is submitted to IETF in full conformance with the 49 provisions of BCP 78 and BCP 79. 51 Internet-Drafts are working documents of the Internet Engineering 52 Task Force (IETF), its areas, and its working groups. Note that 53 other groups may also distribute working documents as Internet- 54 Drafts. 56 Internet-Drafts are draft documents valid for a maximum of six months 57 and may be updated, replaced, or obsoleted by other documents at any 58 time. It is inappropriate to use Internet-Drafts as reference 59 material or to cite them other than as "work in progress." 61 The list of current Internet-Drafts can be accessed at 62 http://www.ietf.org/ietf/1id-abstracts.txt. 64 The list of Internet-Draft Shadow Directories can be accessed at 65 http://www.ietf.org/shadow.html. 67 This Internet-Draft will expire on August 12, 2010. 69 Copyright Notice 71 Copyright (c) 2010 IETF Trust and the persons identified as the 72 document authors. All rights reserved. 74 This document is subject to BCP 78 and the IETF Trust's Legal 75 Provisions Relating to IETF Documents 76 (http://trustee.ietf.org/license-info) in effect on the date of 77 publication of this document. Please review these documents 78 carefully, as they describe your rights and restrictions with respect 79 to this document. Code Components extracted from this document must 80 include Simplified BSD License text as described in Section 4.e of 81 the Trust Legal Provisions and are provided without warranty as 82 described in the BSD License. 84 Table of Contents 86 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 87 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4 88 3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4 89 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 5 90 3.2. Parameter Value Character Set and Language Information . . 5 91 3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7 92 3.3. Language specification in Encoded Words . . . . . . . . . 8 93 4. Guidelines for Usage in HTTP Header Field Definitions . . . . 8 94 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 9 95 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9 96 4.3. Using Multiple Instances for Internationalization . . . . 9 97 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 98 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 99 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 100 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 101 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 102 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 103 Appendix A. Document History and Future Plans (to be removed 104 by RFC Editor before publication) . . . . . . . . . . 12 105 Appendix B. Change Log (to be removed by RFC Editor before 106 publication) . . . . . . . . . . . . . . . . . . . . 12 107 B.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 12 108 B.2. Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 12 109 B.3. Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 13 110 B.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 13 111 B.5. Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 13 112 B.6. Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 13 113 B.7. Since draft-reschke-rfc2231-in-http-06 . . . . . . . . . . 13 114 B.8. Since draft-reschke-rfc2231-in-http-07 . . . . . . . . . . 13 115 B.9. Since draft-reschke-rfc2231-in-http-08 . . . . . . . . . . 13 116 Appendix C. Resolved issues (to be removed by RFC Editor 117 before publication) . . . . . . . . . . . . . . . . . 13 118 C.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 119 C.2. tokengrammar . . . . . . . . . . . . . . . . . . . . . . . 13 120 C.3. attrcharvstoken . . . . . . . . . . . . . . . . . . . . . 14 121 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 123 1. Introduction 125 By default, message header field parameters in HTTP ([RFC2616]) 126 messages can not carry characters outside the ISO-8859-1 character 127 set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping 128 mechanism for use in MIME headers. This document specifies a profile 129 of that encoding for use in HTTP header fields. 131 Note: this profile does not apply to message payloads transmitted 132 over HTTP, such as when using the media type "multipart/form-data" 133 ([RFC2388]). 135 2. Notational Conventions 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 139 document are to be interpreted as described in [RFC2119]. 141 This specification uses the ABNF (Augmented Backus-Naur Form) 142 notation defined in [RFC5234]. The following core rules are included 143 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), 144 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP 145 (linear white space). 147 Note that this specification uses the term "character set" for 148 consistency with other IETF specifications such as RFC 2277 (see 149 [RFC2277], Section 3). A more accurate term would be "character 150 encoding" (a mapping of code points to octet sequences). 152 3. A Profile of RFC 2231 for Use in HTTP 154 RFC 2231 defines several extensions to MIME. The sections below 155 discuss if and how they apply to HTTP. 157 In short: 159 o Parameter Continuations aren't needed (Section 3.1), 161 o Character Set and Language Information are useful, therefore a 162 simple subset is specified (Section 3.2), and 164 o Language Specifications in Encoded Words aren't needed 165 (Section 3.3). 167 3.1. Parameter Continuations 169 Section 3 of [RFC2231] defines a mechanism that deals with the length 170 limitations that apply to MIME headers. These limitations do not 171 apply to HTTP ([RFC2616], Section 19.4.7). 173 Thus in HTTP, senders MUST NOT use parameter continuations, and 174 therefore recipients do not need to support them. 176 3.2. Parameter Value Character Set and Language Information 178 Section 4 of [RFC2231] specifies how to embed language information 179 into parameter values, and also how to encode non-ASCII characters, 180 dealing with restrictions both in MIME and HTTP header parameters. 182 However, RFC 2231 does not specify a mandatory-to-implement character 183 set, making it hard for senders to decide which character set to use. 184 Thus, recipients implementing this specification MUST support the 185 character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629]. 187 Furthermore, RFC 2231 allows leaving out the character set 188 information. The profile defined by this specification does not 189 allow that. 191 The syntax for parameters is defined in Section 3.6 of [RFC2616] 192 (with RFC 2616 implied LWS translated to RFC 5234 LWSP): 194 parameter = attribute LWSP "=" LWSP value 196 attribute = token 197 value = token / quoted-string 199 quoted-string = 200 token = 202 This specification extends the grammar to: 204 parameter = reg-parameter / ext-parameter 206 reg-parameter = attribute LWSP "=" LWSP value 208 ext-parameter = attribute "*" LWSP "=" LWSP ext-value 210 ext-value = charset "'" [ language ] "'" value-chars 211 ; extended-initial-value, 212 ; defined in [RFC2231], Section 7 214 charset = "UTF-8" / "ISO-8859-1" / mime-charset 216 mime-charset = 1*mime-charsetc 217 mime-charsetc = ALPHA / DIGIT 218 / "!" / "#" / "$" / "%" / "&" 219 / "+" / "-" / "^" / "_" / "`" 220 / "{" / "}" / "~" 221 ; as in Section 2.3 of [RFC2978] 222 ; except that the single quote is not included 224 language = 226 value-chars = *( pct-encoded / attr-char ) 228 pct-encoded = "%" HEXDIG HEXDIG 229 ; see [RFC3986], Section 2.1 231 attr-char = ALPHA / DIGIT 232 / "!" / "#" / "$" / "&" / "+" / "-" / "." 233 / "^" / "_" / "`" / "|" / "~" 234 ; token except ( "*" / "'" / "%" ) 236 Thus, a parameter is either regular parameter (reg-parameter), as 237 previously defined in Section 3.6 of [RFC2616], or an extended 238 parameter (ext-parameter). 240 Extended parameters are those where the left hand side of the 241 assignment ends with an asterisk character. 243 The value part of an extended parameter (ext-value) is a token that 244 consists of three parts: the REQUIRED character set name (charset), 245 the OPTIONAL language information (language), and a character 246 sequence representing the actual value (value-chars), separated by 247 single quote characters. Note that both character set names and 248 language tags are restricted to the US-ASCII character set, and are 249 matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646], 250 Section 2.1.1). 252 Inside the value part, characters not contained in attr-char are 253 encoded into an octet sequence using the specified character set. 254 That octet sequence then is percent-encoded as specified in Section 255 2.1 of [RFC3986]. 257 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629]) 258 or "ISO-8859-1" ([ISO-8859-1]). Extension character sets (ext- 259 charset) are reserved for future use. 261 Note: recipients should be prepared to handle encoding errors, 262 such as malformed or incomplete percent escape sequences, or non- 263 decodable octet sequences, in a robust manner. This specification 264 does not mandate any specific behavior, for instance the following 265 strategies are all acceptable: 267 * ignoring the parameter, 269 * stripping a non-decodable octet sequence, 271 * substituting a non-decodable octet sequence by a replacement 272 character, such as the Unicode character U+FFFD (Replacement 273 Character). 275 Note: the RFC 2616 token production ([RFC2616], Section 2.2) 276 differs from the production used in RFC 2231 (imported from 277 Section 5.1 of [RFC2045]) in that curly braces ("{" and "}") are 278 excluded. Thus, these two characters are excluded from the attr- 279 char production as well. 281 Note: the ABNF defined here differs from the one in 282 Section 2.3 of [RFC2978] in that it does not allow the single 283 quote character (see also RFC Editor Errata ID 1912 [3]). In 284 practice, no character set names using that character have been 285 registered at the time of this writing. 287 3.2.1. Examples 289 Non-extended notation, using "token": 291 foo: bar; title=Economy 293 Non-extended notation, using "quoted-string": 295 foo: bar; title="US-$ rates" 297 Extended notation, using the unicode character U+00A3 (POUND SIGN): 299 foo: bar; title*=iso-8859-1'en'%A3%20rates 301 Note: the Unicode pound sign character U+00A3 was encoded using ISO- 302 8859-1 into the single octet A3, then percent-encoded. Also note 303 that the space character was encoded as %20, as it is not contained 304 in attr-char. 306 Extended notation, using the unicode characters U+00A3 (POUND SIGN) 307 and U+20AC (EURO SIGN): 309 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates 311 Note: the unicode pound sign character U+00A3 was encoded using UTF-8 312 into the octet sequence C2 A3, then percent-encoded. Likewise, the 313 unicode euro sign character U+20AC was encoded into the octet 314 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG 315 allows both lower-case and upper-case character, so recipients must 316 understand both, and that the language information is optional, while 317 the character set is not. 319 3.3. Language specification in Encoded Words 321 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to 322 also support language specification in encoded words. Although the 323 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section 324 2.2), it's not clear to which header field exactly it applies, and 325 whether it is implemented in practice (see 326 for details). 328 Thus, the RFC 2231 profile defined by this specification does not 329 include this feature. 331 4. Guidelines for Usage in HTTP Header Field Definitions 333 Specifications of HTTP header fields that use the extensions defined 334 in Section 3.2 should clearly state that. A simple way to achieve 335 this is to normatively reference this specification, and to include 336 the ext-value production into the ABNF for that header field. 338 For instance: 340 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param 341 title-param = "title" LWSP "=" LWSP value 342 / "title*" LWSP "=" LWSP ext-value 343 ext-value = 345 [[rfcno: Note to RFC Editor: in the figure above, please replace 346 "xxxx" by the RFC number assigned to this specification.]] 348 4.1. When to Use the Extension 350 Section 4.2 of [RFC2277] requires that protocol elements containing 351 text are able to carry language information. Thus, the ext-value 352 production should always be used when the parameter value is of 353 textual nature. 355 Furthermore, the extension should also be used whenever the parameter 356 value needs to carry characters not present in the US-ASCII 357 ([USASCII]) character set (note that it would be unacceptable to 358 define a new parameter that would be restricted to a subset of the 359 Unicode character set). 361 4.2. Error Handling 363 Header specifications that include parameters should also specify 364 whether same-named parameters can occur multiple times. If 365 repetitions are not allowed (and this is believed to be the common 366 case), the specification should state whether regular or the extended 367 syntax takes precedence. In the latter case, this could be used by 368 producers to use both formats without breaking recipients that do not 369 understand the syntax. 371 Example: 373 foo: bar; title="EURO exchange rates"; 374 title*=utf-8''%e2%82%ac%20exchange%20rates 376 In this case, the sender provides an ASCII version of the title for 377 legacy recipients, but also includes an internationalized version for 378 recipients understanding this specification -- the latter obviously 379 should prefer the new syntax over the old one. 381 Note: at the time of this writing, many implementations failed to 382 ignore the form they do not understand, or prioritize the ASCII 383 form although the extended syntax was present. 385 4.3. Using Multiple Instances for Internationalization 387 It is expected that in many cases, internationalization of parameters 388 in response headers is implemented using server driven content 389 negotiation ([RFC2616], Section 12.1) using the Accept-Language 390 header ([RFC2616], Section 14.4). However, the format described in 391 this specification also allows using multiple instances providing 392 multiple languages in a single header. Specifications that want to 393 take advantage of this should clearly specify the expected processing 394 by the recipient. 396 Example: 398 foo: bar; title*=utf-8'en'Document%20Title; 399 title*=utf-8'de'Titel%20des%20Dokuments 401 5. Security Considerations 403 This document does not discuss security issues and is not believed to 404 raise any security issues not already endemic in HTTP. 406 6. IANA Considerations 408 There are no IANA Considerations related to this specification. 410 7. Acknowledgements 412 Thanks to Martin Duerst and Frank Ellermann for help figuring out 413 ABNF details, and to Benjamin Carlyle and Roar Lauritzsen for 414 implementer's feedback. 416 8. References 418 8.1. Normative References 420 [ISO-8859-1] 421 International Organization for Standardization, 422 "Information technology -- 8-bit single-byte coded graphic 423 character sets -- Part 1: Latin alphabet No. 1", ISO/ 424 IEC 8859-1:1998, 1998. 426 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 427 Requirement Levels", BCP 14, RFC 2119, March 1997. 429 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 430 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 431 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 433 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 434 10646", RFC 3629, STD 63, November 2003. 436 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 437 Specifications: ABNF", STD 68, RFC 5234, January 2008. 439 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying 440 Languages", BCP 47, RFC 5646, September 2009. 442 8.2. Informative References 444 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 445 Extensions (MIME) Part One: Format of Internet Message 446 Bodies", RFC 2045, November 1996. 448 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 449 Part Three: Message Header Extensions for Non-ASCII Text", 450 RFC 2047, November 1996. 452 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded 453 Word Extensions: 454 Character Sets, Languages, and Continuations", RFC 2231, 455 November 1997. 457 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and 458 Languages", BCP 18, RFC 2277, January 1998. 460 [RFC2388] Masinter, L., "Returning Values from Forms: multipart/ 461 form-data", RFC 2388, August 1998. 463 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration 464 Procedures", BCP 19, RFC 2978, October 2000. 466 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 467 Resource Identifier (URI): Generic Syntax", RFC 3986, 468 STD 66, January 2005. 470 [USASCII] American National Standards Institute, "Coded Character 471 Set -- 7-bit American Standard Code for Information 472 Interchange", ANSI X3.4, 1986. 474 URIs 476 [1] 478 [2] 480 [3] 482 Appendix A. Document History and Future Plans (to be removed by RFC 483 Editor before publication) 485 Problems with the internationalization of the HTTP Content- 486 Disposition header field have been known for many years (see test 487 cases at ). 489 During IETF 72 490 (), the 491 HTTPbis Working Group shortly discussed how to deal with the 492 underspecification of (1) Content-Disposition, and its (2) 493 internationalization aspects. Back then, there was rough consensus 494 in the room to move the definition into a separate draft. 496 This specification addresses problem (2), by defining a simple subset 497 of the encoding format defined in RFC 2231. A separate 498 specification, draft-reschke-rfc2183-in-http, is planned to address 499 problem (1). Note that this approach was chosen because Content- 500 Disposition is just an example for an HTTP header field using this 501 kind of encoding. Another example is the currently proposed Link 502 header field (draft-nottingham-http-link-header). 504 This document is planned to be published on the IETF Standards Track, 505 so that other standards-track level documents can depend on it, such 506 as the new specification of Content-Disposition, or potentially 507 future revisions of the HTTP Link Header specification. 509 Also note that this document specifies a proper subset of the 510 extensions defined in RFC 2231, but does not normatively refer to it. 511 Thus, RFC 2231 can be revised separately, should the email community 512 decide to. 514 Appendix B. Change Log (to be removed by RFC Editor before publication) 516 B.1. Since draft-reschke-rfc2231-in-http-00 518 Use RFC5234-style ABNF, closer to the one used in RFC 2231. 520 Make RFC 2231 dependency informative, so this specification can 521 evolve independently. 523 Explain the ABNF in prose. 525 B.2. Since draft-reschke-rfc2231-in-http-01 527 Remove unneeded RFC5137 notation (code point vs character). 529 B.3. Since draft-reschke-rfc2231-in-http-02 531 And and resolve issues "charset", "repeats" and "rfc4646". 533 B.4. Since draft-reschke-rfc2231-in-http-03 535 And and resolve issue "charsetmatch". 537 B.5. Since draft-reschke-rfc2231-in-http-04 539 Add and resolve issues "badseq" and "tokenquotcharset". 541 B.6. Since draft-reschke-rfc2231-in-http-05 543 Say "header field" instead of "header" in the context of HTTP. 545 B.7. Since draft-reschke-rfc2231-in-http-06 547 Add an appendix discussing document history and future plans, to be 548 removed before publication. 550 B.8. Since draft-reschke-rfc2231-in-http-07 552 Add and resolve issues "impl" and "rel-2388". 554 B.9. Since draft-reschke-rfc2231-in-http-08 556 Editorial improvements. Add and resolve issues "attrcharvstoken" and 557 "tokengrammar". 559 Appendix C. Resolved issues (to be removed by RFC Editor before 560 publication) 562 Issues that were either rejected or resolved in this version of this 563 document. 565 C.1. edit 567 Type: edit 569 julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for 570 editorial fixes/enhancements. 572 C.2. tokengrammar 574 In Section 3.2: 576 Type: edit 578 julian.reschke@greenbytes.de (2010-02-04): Benjamin Carlyle noticed 579 (off-list) that token in RFC 2231 / RFC 2045 allows "{" and "}", 580 while HTTP does not. Minimally, we need to point out the difference. 582 Resolution (2010-02-04): Add a note pointing out (and explaining) the 583 difference. 585 C.3. attrcharvstoken 587 In Section 3.2: 589 Type: change 591 julian.reschke@greenbytes.de (2010-02-04): For some reason, attr-char 592 fails to be token - 2231specials; it includes ":", but fails to 593 include a few other characters from token. (reported by Benjamin 594 Carlyle) 596 Resolution (2010-02-04): Revise attr-char so it really is token \ ( 597 "*" / "%" / "'" ) 599 Author's Address 601 Julian F. Reschke 602 greenbytes GmbH 603 Hafenweg 16 604 Muenster, NW 48155 605 Germany 607 Email: julian.reschke@greenbytes.de 608 URI: http://greenbytes.de/tech/webdav/