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2 Network Working Group J. 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/