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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track January 19, 2010
5 Expires: July 23, 2010
7 Application of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Header Fields
9 draft-reschke-rfc2231-in-http-08
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 July 23, 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 . . . . . . . . . . . . . . . . 8
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 . . . . . . . . . . . . . . . . . . 10
103 Appendix A. Document History and Future Plans (to be removed
104 by RFC Editor before publication) . . . . . . . . . . 11
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 . . . . . . . . . . 12
110 B.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 12
111 B.5. Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 12
112 B.6. Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 12
113 B.7. Since draft-reschke-rfc2231-in-http-06 . . . . . . . . . . 13
114 B.8. Since draft-reschke-rfc2231-in-http-07 . . . . . . . . . . 13
115 Appendix C. Resolved issues (to be removed by RFC Editor
116 before publication) . . . . . . . . . . . . . . . . . 13
117 C.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
118 C.2. impl . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
119 C.3. rel-2388 . . . . . . . . . . . . . . . . . . . . . . . . . 13
120 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
122 1. Introduction
124 By default, message header field parameters in HTTP ([RFC2616])
125 messages can not carry characters outside the ISO-8859-1 character
126 set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping
127 mechanism for use in MIME headers. This document specifies a profile
128 of that encoding for use in HTTP header fields.
130 Note: this profile does not apply to message payloads transmitted
131 over HTTP, such as when using the media type "multipart/form-data"
132 ([RFC2388]).
134 2. Notational Conventions
136 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
137 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
138 document are to be interpreted as described in [RFC2119].
140 This specification uses the ABNF (Augmented Backus-Naur Form)
141 notation defined in [RFC5234]. The following core rules are included
142 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
143 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
144 (linear white space).
146 Note that this specification uses the term "character set" for
147 consistency with other IETF specifications such as RFC 2277 (see
148 [RFC2277], Section 3). A more accurate term would be "character
149 encoding" (a mapping of code points to octet sequences).
151 3. A Profile of RFC 2231 for Use in HTTP
153 RFC 2231 defines several extensions to MIME. The sections below
154 discuss if and how they apply to HTTP.
156 In short:
158 o Parameter Continuations aren't needed (Section 3.1),
160 o Character Set and Language Information are useful, therefore a
161 simple subset is specified (Section 3.2), and
163 o Language Specifications in Encoded Words aren't needed
164 (Section 3.3).
166 3.1. Parameter Continuations
168 Section 3 of [RFC2231] defines a mechanism that deals with the length
169 limitations that apply to MIME headers. These limitations do not
170 apply to HTTP ([RFC2616], Section 19.4.7).
172 Thus in HTTP, senders MUST NOT use parameter continuations, and
173 therefore recipients do not need to support them.
175 3.2. Parameter Value Character Set and Language Information
177 Section 4 of [RFC2231] specifies how to embed language information
178 into parameter values, and also how to encode non-ASCII characters,
179 dealing with restrictions both in MIME and HTTP header parameters.
181 However, RFC 2231 does not specify a mandatory-to-implement character
182 set, making it hard for senders to decide which character set to use.
183 Thus, recipients implementing this specification MUST support the
184 character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
186 Furthermore, RFC 2231 allows leaving out the character set
187 information. The profile defined by this specification does not
188 allow that.
190 The syntax for parameters is defined in Section 3.6 of [RFC2616]
191 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
193 parameter = attribute LWSP "=" LWSP value
195 attribute = token
196 value = token / quoted-string
198 quoted-string =
199 token =
201 This specification extends the grammar to:
203 parameter = reg-parameter / ext-parameter
205 reg-parameter = attribute LWSP "=" LWSP value
207 ext-parameter = attribute "*" LWSP "=" LWSP ext-value
209 ext-value = charset "'" [ language ] "'" value-chars
210 ; extended-initial-value,
211 ; defined in [RFC2231], Section 7
213 charset = "UTF-8" / "ISO-8859-1" / mime-charset
215 mime-charset = 1*mime-charsetc
216 mime-charsetc = ALPHA / DIGIT
217 / "!" / "#" / "$" / "%" / "&"
218 / "+" / "-" / "^" / "_" / "`"
219 / "{" / "}" / "~"
220 ; as in Section 2.3 of [RFC2978]
221 ; except that the single quote is not included
223 language =
225 value-chars = *( pct-encoded / attr-char )
227 pct-encoded = "%" HEXDIG HEXDIG
228 ; see [RFC3986], Section 2.1
230 attr-char = ALPHA / DIGIT
231 / "-" / "." / "_" / "~" / ":"
232 / "!" / "$" / "&" / "+"
234 Thus, a parameter is either regular parameter (reg-parameter), as
235 previously defined in Section 3.6 of [RFC2616], or an extended
236 parameter (ext-parameter).
238 Extended parameters are those where the left hand side of the
239 assignment ends with an asterisk character.
241 The value part of an extended parameter (ext-value) is a token that
242 consists of three parts: the REQUIRED character set name (charset),
243 the OPTIONAL language information (language), and a character
244 sequence representing the actual value (value-chars), separated by
245 single quote characters. Note that both character set names and
246 language tags are restricted to the US-ASCII character set, and are
247 matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646],
248 Section 2.1.1).
250 Inside the value part, characters not contained in attr-char are
251 encoded into an octet sequence using the specified character set.
252 That octet sequence then is percent-encoded as specified in Section
253 2.1 of [RFC3986].
255 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
256 or "ISO-8859-1" ([ISO-8859-1]). Extension character sets (ext-
257 charset) are reserved for future use.
259 Note: recipients should be prepared to handle encoding errors,
260 such as malformed or incomplete percent escape sequences, or non-
261 decodable octet sequences, in a robust manner. This specification
262 does not mandate any specific behavior, for instance the following
263 strategies are all acceptable:
265 * ignoring the parameter,
267 * stripping a non-decodable octet sequence,
269 * substituting a non-decodable octet sequence by a replacement
270 character, such as the Unicode character U+FFFD (Replacement
271 Character).
273 Note: the ABNF defined here differs from the one in
274 Section 2.3 of [RFC2978] in that it does not allow the single
275 quote character (see also RFC Editor Errata ID 1912 [3]). In
276 practice, no character set names using that character have been
277 registered at the time of this writing.
279 3.2.1. Examples
281 Non-extended notation, using "token":
283 foo: bar; title=Economy
285 Non-extended notation, using "quoted-string":
287 foo: bar; title="US-$ rates"
289 Extended notation, using the unicode character U+00A3 (POUND SIGN):
291 foo: bar; title*=iso-8859-1'en'%A3%20rates
293 Note: the Unicode pound sign character U+00A3 was encoded using ISO-
294 8859-1 into the single octet A3, then percent-encoded. Also note
295 that the space character was encoded as %20, as it is not contained
296 in attr-char.
298 Extended notation, using the unicode characters U+00A3 (POUND SIGN)
299 and U+20AC (EURO SIGN):
301 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
303 Note: the unicode pound sign character U+00A3 was encoded using UTF-8
304 into the octet sequence C2 A3, then percent-encoded. Likewise, the
305 unicode euro sign character U+20AC was encoded into the octet
306 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
307 allows both lower-case and upper-case character, so recipients must
308 understand both, and that the language information is optional, while
309 the character set is not.
311 3.3. Language specification in Encoded Words
313 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
314 also support language specification in encoded words. Although the
315 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
316 2.2), it's not clear to which header field exactly it applies, and
317 whether it is implemented in practice (see
318 for details).
320 Thus, the RFC 2231 profile defined by this specification does not
321 include this feature.
323 4. Guidelines for Usage in HTTP Header Field Definitions
325 Specifications of HTTP header fields that use the extensions defined
326 in Section 3.2 should clearly state that. A simple way to achieve
327 this is to normatively reference this specification, and to include
328 the ext-value production into the ABNF for that header field.
330 For instance:
332 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
333 title-param = "title" LWSP "=" LWSP value
334 / "title*" LWSP "=" LWSP ext-value
335 ext-value =
337 [[rfcno: Note to RFC Editor: in the figure above, please replace
338 "xxxx" by the RFC number assigned to this specification.]]
340 4.1. When to Use the Extension
342 Section 4.2 of [RFC2277] requires that protocol elements containing
343 text can carry language information. Thus, the ext-value production
344 should always be used when the parameter value is of textual nature.
346 Furthermore, the extension should also be used whenever the parameter
347 value needs to carry characters not present in the US-ASCII
348 ([USASCII]) character set (note that it would be unacceptable to
349 define a new parameter that would be restricted to a subset of the
350 Unicode character set).
352 4.2. Error Handling
354 Header specifications that include parameters should also specify
355 whether same-named parameters can occur multiple times. If
356 repetitions are not allowed (and this is believed to be the common
357 case), the specification should state whether regular or the extended
358 syntax takes precedence. In the latter case, this could be used by
359 producers to use both formats without breaking recipients that do not
360 understand the syntax.
362 Example:
364 foo: bar; title="EURO exchange rates";
365 title*=utf-8''%e2%82%ac%20exchange%20rates
367 In this case, the sender provides an ASCII version of the title for
368 legacy recipients, but also includes an internationalized version for
369 recipients understanding this specification -- the latter obviously
370 should prefer the new syntax over the old one.
372 Note: at the time of this writing, many implementations failed to
373 ignore the form they do not understand, or prioritize the ASCII
374 form although the extended syntax was present.
376 4.3. Using Multiple Instances for Internationalization
378 It is expected that in many cases, internationalization of parameters
379 in response headers is implemented using server driven content
380 negotiation ([RFC2616], Section 12.1) using the Accept-Language
381 header ([RFC2616], Section 14.4). However, the format described in
382 this specification also allows to use multiple instances providing
383 multiple languages in a single header. Specifications that want to
384 take advantage of this should clearly specify the expected processing
385 by the recipient.
387 Example:
389 foo: bar; title*=utf-8'en'Document%20Title;
390 title*=utf-8'de'Titel%20des%20Dokuments
392 5. Security Considerations
394 This document does not discuss security issues and is not believed to
395 raise any security issues not already endemic in HTTP.
397 6. IANA Considerations
399 There are no IANA Considerations related to this specification.
401 7. Acknowledgements
403 Thanks to Martin Duerst and Frank Ellermann for help figuring out
404 ABNF details, and to Roar Lauritzsen for implementer's feedback.
406 8. References
408 8.1. Normative References
410 [ISO-8859-1]
411 International Organization for Standardization,
412 "Information technology -- 8-bit single-byte coded graphic
413 character sets -- Part 1: Latin alphabet No. 1", ISO/
414 IEC 8859-1:1998, 1998.
416 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
417 Requirement Levels", BCP 14, RFC 2119, March 1997.
419 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
420 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
421 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
423 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
424 10646", RFC 3629, STD 63, November 2003.
426 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
427 Specifications: ABNF", STD 68, RFC 5234, January 2008.
429 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
430 Languages", BCP 47, RFC 5646, September 2009.
432 8.2. Informative References
434 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
435 Part Three: Message Header Extensions for Non-ASCII Text",
436 RFC 2047, November 1996.
438 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
439 Word Extensions:
440 Character Sets, Languages, and Continuations", RFC 2231,
441 November 1997.
443 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
444 Languages", BCP 18, RFC 2277, January 1998.
446 [RFC2388] Masinter, L., "Returning Values from Forms: multipart/
447 form-data", RFC 2388, August 1998.
449 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
450 Procedures", BCP 19, RFC 2978, October 2000.
452 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
453 Resource Identifier (URI): Generic Syntax", RFC 3986,
454 STD 66, January 2005.
456 [USASCII] American National Standards Institute, "Coded Character
457 Set -- 7-bit American Standard Code for Information
458 Interchange", ANSI X3.4, 1986.
460 URIs
462 [1]
464 [2]
466 [3]
468 Appendix A. Document History and Future Plans (to be removed by RFC
469 Editor before publication)
471 Problems with the internationalization of the HTTP Content-
472 Disposition header field have been known for many years (see test
473 cases at ).
475 During IETF 72
476 (), the
477 HTTPbis Working Group shortly discussed how to deal with the
478 underspecification of (1) Content-Disposition, and its (2)
479 internationalization aspects. Back then, there was rough consensus
480 in the room to move the definition into a separate draft.
482 This specification addresses problem (2), by defining a simple subset
483 of the encoding format defined in RFC 2231. A separate
484 specification, draft-reschke-rfc2183-in-http, is planned to address
485 problem (1). Note that this approach was chosen because Content-
486 Disposition is just an example for an HTTP header field using this
487 kind of encoding. Another example is the currently proposed Link
488 header field (draft-nottingham-http-link-header).
490 This document is planned to be published on the IETF Standards Track,
491 so that other standards-track level documents can depend on it, such
492 as the new specification of Content-Disposition, or potentially
493 future revisions of the HTTP Link Header specification.
495 Also note that this document specifies a proper subset of the
496 extensions defined in RFC 2231, but does not normatively refer to it.
497 Thus, RFC 2231 can be revised separately, should the email community
498 decide to.
500 Appendix B. Change Log (to be removed by RFC Editor before publication)
502 B.1. Since draft-reschke-rfc2231-in-http-00
504 Use RFC5234-style ABNF, closer to the one used in RFC 2231.
506 Make RFC 2231 dependency informative, so this specification can
507 evolve independently.
509 Explain the ABNF in prose.
511 B.2. Since draft-reschke-rfc2231-in-http-01
513 Remove unneeded RFC5137 notation (code point vs character).
515 B.3. Since draft-reschke-rfc2231-in-http-02
517 And and resolve issues "charset", "repeats" and "rfc4646".
519 B.4. Since draft-reschke-rfc2231-in-http-03
521 And and resolve issue "charsetmatch".
523 B.5. Since draft-reschke-rfc2231-in-http-04
525 Add and resolve issues "badseq" and "tokenquotcharset".
527 B.6. Since draft-reschke-rfc2231-in-http-05
529 Say "header field" instead of "header" in the context of HTTP.
531 B.7. Since draft-reschke-rfc2231-in-http-06
533 Add an appendix discussing document history and future plans, to be
534 removed before publication.
536 B.8. Since draft-reschke-rfc2231-in-http-07
538 Add and resolve issues "impl" and "rel-2388".
540 Appendix C. Resolved issues (to be removed by RFC Editor before
541 publication)
543 Issues that were either rejected or resolved in this version of this
544 document.
546 C.1. edit
548 Type: edit
550 julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
551 editorial fixes/enhancements.
553 C.2. impl
555 Type: edit
557 julian.reschke@greenbytes.de (2010-01-16): Report on current
558 implementations.
560 C.3. rel-2388
562 Type: edit
564 julian.reschke@greenbytes.de (2010-01-07): Note the non-applicability
565 to the use of RFC 2231 encoding in multipart/form-data.
567 Resolution (2010-01-07): Done.
569 Author's Address
571 Julian F. Reschke
572 greenbytes GmbH
573 Hafenweg 16
574 Muenster, NW 48155
575 Germany
577 Email: julian.reschke@greenbytes.de
578 URI: http://greenbytes.de/tech/webdav/