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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Obsoletes: 5987 (if approved) December 8, 2011
5 Intended status: Standards Track
6 Expires: June 10, 2012
8 Indicating Character Encoding and Language for HTTP Header Field
9 Parameters
10 draft-reschke-rfc5987bis-03
12 Abstract
14 By default, message header field parameters in Hypertext Transfer
15 Protocol (HTTP) messages cannot carry characters outside the ISO-
16 8859-1 character set. RFC 2231 defines an encoding mechanism for use
17 in Multipurpose Internet Mail Extensions (MIME) headers. This
18 document specifies an encoding suitable for use in HTTP header fields
19 that is compatible with a profile of the encoding defined in RFC
20 2231.
22 Editorial Note (To be removed by RFC Editor before publication)
24 Distribution of this document is unlimited. Although this is not a
25 work item of the HTTPbis Working Group, comments should be sent to
26 the Hypertext Transfer Protocol (HTTP) mailing list at
27 ietf-http-wg@w3.org [1], which may be joined by sending a message
28 with subject "subscribe" to ietf-http-wg-request@w3.org [2].
30 Discussions of the HTTPbis Working Group are archived at
31 .
33 XML versions, latest edits and the issues list for this document are
34 available from
35 . A
36 collection of test cases is available at
37 .
39 Status of This Memo
41 This Internet-Draft is submitted in full conformance with the
42 provisions of BCP 78 and BCP 79.
44 Internet-Drafts are working documents of the Internet Engineering
45 Task Force (IETF). Note that other groups may also distribute
46 working documents as Internet-Drafts. The list of current Internet-
47 Drafts is at http://datatracker.ietf.org/drafts/current/.
49 Internet-Drafts are draft documents valid for a maximum of six months
50 and may be updated, replaced, or obsoleted by other documents at any
51 time. It is inappropriate to use Internet-Drafts as reference
52 material or to cite them other than as "work in progress."
54 This Internet-Draft will expire on June 10, 2012.
56 Copyright Notice
58 Copyright (c) 2011 IETF Trust and the persons identified as the
59 document authors. All rights reserved.
61 This document is subject to BCP 78 and the IETF Trust's Legal
62 Provisions Relating to IETF Documents
63 (http://trustee.ietf.org/license-info) in effect on the date of
64 publication of this document. Please review these documents
65 carefully, as they describe your rights and restrictions with respect
66 to this document. Code Components extracted from this document must
67 include Simplified BSD License text as described in Section 4.e of
68 the Trust Legal Provisions and are provided without warranty as
69 described in the Simplified BSD License.
71 Table of Contents
73 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
74 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
75 3. Comparison to RFC 2231 and Definition of the Encoding . . . . 4
76 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 5
77 3.2. Parameter Value Character Encoding and Language
78 Information . . . . . . . . . . . . . . . . . . . . . . . 5
79 3.2.1. Definition . . . . . . . . . . . . . . . . . . . . . . 5
80 3.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 8
81 3.3. Language Specification in Encoded Words . . . . . . . . . 8
82 4. Guidelines for Usage in HTTP Header Field Definitions . . . . 8
83 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 9
84 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
85 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
86 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
87 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
88 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
89 7.2. Informative References . . . . . . . . . . . . . . . . . . 11
90 Appendix A. Changes from RFC 5987 . . . . . . . . . . . . . . . . 12
91 Appendix B. Implementation Report . . . . . . . . . . . . . . . . 12
92 Appendix C. Change Log (to be removed by RFC Editor before
93 publication) . . . . . . . . . . . . . . . . . . . . 13
94 C.1. Since RFC5987 . . . . . . . . . . . . . . . . . . . . . . 13
95 C.2. Since draft-reschke-rfc5987bis-00 . . . . . . . . . . . . 13
96 C.3. Since draft-reschke-rfc5987bis-01 . . . . . . . . . . . . 13
97 C.4. Since draft-reschke-rfc5987bis-02 . . . . . . . . . . . . 13
98 Appendix D. Resolved issues (to be removed by RFC Editor
99 before publication) . . . . . . . . . . . . . . . . . 13
100 D.1. terminology . . . . . . . . . . . . . . . . . . . . . . . 13
101 Appendix E. Open issues (to be removed by RFC Editor prior to
102 publication) . . . . . . . . . . . . . . . . . . . . 13
103 E.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
104 E.2. parmsyntax . . . . . . . . . . . . . . . . . . . . . . . . 14
105 E.3. valuesyntax . . . . . . . . . . . . . . . . . . . . . . . 14
106 E.4. httpbis . . . . . . . . . . . . . . . . . . . . . . . . . 14
108 1. Introduction
110 By default, message header field parameters in HTTP ([RFC2616])
111 messages cannot carry characters outside the ISO-8859-1 coded
112 character set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an
113 encoding mechanism for use in MIME headers. This document specifies
114 an encoding suitable for use in HTTP header fields that is compatible
115 with a profile of the encoding defined in RFC 2231.
117 This document obsoletes [RFC5987] and moves it to "historic" status;
118 the changes are summarized in Appendix A.
120 Note: in the remainder of this document, RFC 2231 is only
121 referenced for the purpose of explaining the choice of features
122 that were adopted; they are therefore purely informative.
124 Note: this encoding does not apply to message payloads transmitted
125 over HTTP, such as when using the media type "multipart/form-data"
126 ([RFC2388]).
128 2. Notational Conventions
130 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
131 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
132 document are to be interpreted as described in [RFC2119].
134 This specification uses the ABNF (Augmented Backus-Naur Form)
135 notation defined in [RFC5234]. The following core rules are included
136 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
137 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP
138 (linear whitespace).
140 This specification uses terminology defined in [RFC6365], namely:
141 "character encoding scheme" (below abbreviated to "character
142 encoding"), "charset" and "coded character set".
144 Note that this differs from RFC 2231, which uses the term "character
145 set" for "character encoding scheme".
147 3. Comparison to RFC 2231 and Definition of the Encoding
149 RFC 2231 defines several extensions to MIME. The sections below
150 discuss if and how they apply to HTTP header fields.
152 In short:
154 o Parameter Continuations aren't needed (Section 3.1),
155 o Character Encoding and Language Information are useful, therefore
156 a simple subset is specified (Section 3.2), and
158 o Language Specifications in Encoded Words aren't needed
159 (Section 3.3).
161 3.1. Parameter Continuations
163 Section 3 of [RFC2231] defines a mechanism that deals with the length
164 limitations that apply to MIME headers. These limitations do not
165 apply to HTTP ([RFC2616], Section 19.4.7).
167 Thus, parameter continuations are not part of the encoding defined by
168 this specification.
170 3.2. Parameter Value Character Encoding and Language Information
172 Section 4 of [RFC2231] specifies how to embed language information
173 into parameter values, and also how to encode non-ASCII characters,
174 dealing with restrictions both in MIME and HTTP header field
175 parameters.
177 However, RFC 2231 does not specify a mandatory-to-implement character
178 encoding, making it hard for senders to decide which encoding to use.
179 Thus, recipients implementing this specification MUST support the
180 "UTF-8" character encoding [RFC3629].
182 Furthermore, RFC 2231 allows the character encoding information to be
183 left out. The encoding defined by this specification does not allow
184 that.
186 3.2.1. Definition
188 The syntax for parameters is defined in Section 3.6 of [RFC2616]
189 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
191 parameter = attribute LWSP "=" LWSP value
193 attribute = token
194 value = token / quoted-string
196 quoted-string =
197 token =
199 In order to include character encoding and language information, this
200 specification modifies the RFC 2616 grammar to be:
202 parameter = reg-parameter / ext-parameter
204 reg-parameter = parmname LWSP "=" LWSP value
206 ext-parameter = parmname "*" LWSP "=" LWSP ext-value
208 parmname = 1*attr-char
210 ext-value = charset "'" [ language ] "'" value-chars
211 ; like RFC 2231's
212 ; (see [RFC2231], Section 7)
214 charset = "UTF-8" / mime-charset
216 mime-charset = 1*mime-charsetc
217 mime-charsetc = ALPHA / DIGIT
218 / "!" / "#" / "$" / "%" / "&"
219 / "+" / "-" / "^" / "_" / "`"
220 / "{" / "}" / "~"
221 ; as in Section 2.3 of [RFC2978]
222 ; except that the single quote is not included
223 ; SHOULD be registered in the IANA charset registry
225 language =
227 value-chars = *( pct-encoded / attr-char )
229 pct-encoded = "%" HEXDIG HEXDIG
230 ; see [RFC3986], Section 2.1
232 attr-char = ALPHA / DIGIT
233 / "!" / "#" / "$" / "&" / "+" / "-" / "."
234 / "^" / "_" / "`" / "|" / "~"
235 ; token except ( "*" / "'" / "%" )
237 Thus, a parameter is either a regular parameter (reg-parameter), as
238 previously defined in Section 3.6 of [RFC2616], or an extended
239 parameter (ext-parameter).
241 Extended parameters are those where the left-hand side of the
242 assignment ends with an asterisk character.
244 The value part of an extended parameter (ext-value) is a token that
245 consists of three parts: the REQUIRED character encoding name
246 (charset), the OPTIONAL language information (language), and a
247 character sequence representing the actual value (value-chars),
248 separated by single quote characters. Note that both character
249 encoding names and language tags are restricted to the US-ASCII coded
250 character set, and are matched case-insensitively (see [RFC2978],
251 Section 2.3 and [RFC5646], Section 2.1.1).
253 Inside the value part, characters not contained in attr-char are
254 encoded into an octet sequence using the specified character
255 encoding. That octet sequence is then percent-encoded as specified
256 in Section 2.1 of [RFC3986].
258 Producers MUST use the "UTF-8" ([RFC3629]) character encoding.
259 Extension character encodings (mime-charset) are reserved for future
260 use.
262 Note: recipients should be prepared to handle encoding errors,
263 such as malformed or incomplete percent escape sequences, or non-
264 decodable octet sequences, in a robust manner. This specification
265 does not mandate any specific behavior, for instance, the
266 following strategies are all acceptable:
268 * ignoring the parameter,
270 * stripping a non-decodable octet sequence,
272 * substituting a non-decodable octet sequence by a replacement
273 character, such as the Unicode character U+FFFD (Replacement
274 Character).
276 Note: the RFC 2616 token production ([RFC2616], Section 2.2)
277 differs from the production used in RFC 2231 (imported from
278 Section 5.1 of [RFC2045]) in that curly braces ("{" and "}") are
279 excluded. Thus, these two characters are excluded from the attr-
280 char production as well.
282 Note: the ABNF defined here differs from the one in
283 Section 2.3 of [RFC2978] in that it does not allow the single
284 quote character (see also RFC Errata ID 1912 [Err1912]). In
285 practice, no character encoding names using that character have
286 been registered at the time of this writing.
288 Note: [RFC5987] did require support for ISO-8859-1, too; for
289 compatibility with legacy code, recipients are encouraged to
290 support this encoding as well.
292 3.2.2. Examples
294 Non-extended notation, using "token":
296 foo: bar; title=Economy
298 Non-extended notation, using "quoted-string":
300 foo: bar; title="US-$ rates"
302 Extended notation, using the Unicode character U+00A3 (POUND SIGN):
304 foo: bar; title*=utf-8'en'%C2%A3%20rates
306 Note: the Unicode pound sign character U+00A3 was encoded into the
307 octet sequence C2 A3 using the UTF-8 character encoding, then
308 percent-encoded. Also, note that the space character was encoded as
309 %20, as it is not contained in attr-char.
311 Extended notation, using the Unicode characters U+00A3 (POUND SIGN)
312 and U+20AC (EURO SIGN):
314 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
316 Note: the Unicode pound sign character U+00A3 was encoded into the
317 octet sequence C2 A3 using the UTF-8 character encoding, then
318 percent-encoded. Likewise, the Unicode euro sign character U+20AC
319 was encoded into the octet sequence E2 82 AC, then percent-encoded.
320 Also note that HEXDIG allows both lowercase and uppercase characters,
321 so recipients must understand both, and that the language information
322 is optional, while the character encoding is not.
324 3.3. Language Specification in Encoded Words
326 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
327 also support language specification in encoded words. Although the
328 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
329 2.2), it's not clear to which header field exactly it applies, and
330 whether it is implemented in practice (see
331 for details).
333 Thus, this specification does not include this feature.
335 4. Guidelines for Usage in HTTP Header Field Definitions
337 Specifications of HTTP header fields that use the extensions defined
338 in Section 3.2 ought to clearly state that. A simple way to achieve
339 this is to normatively reference this specification, and to include
340 the ext-value production into the ABNF for that header field.
342 For instance:
344 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
345 title-param = "title" LWSP "=" LWSP value
346 / "title*" LWSP "=" LWSP ext-value
347 ext-value =
349 Note: The Parameter Value Continuation feature defined in Section
350 3 of [RFC2231] makes it impossible to have multiple instances of
351 extended parameters with identical parmname components, as the
352 processing of continuations would become ambiguous. Thus,
353 specifications using this extension are advised to disallow this
354 case for compatibility with RFC 2231.
356 4.1. When to Use the Extension
358 Section 4.2 of [RFC2277] requires that protocol elements containing
359 human-readable text are able to carry language information. Thus,
360 the ext-value production ought to be always used when the parameter
361 value is of textual nature and its language is known.
363 Furthermore, the extension ought to also be used whenever the
364 parameter value needs to carry characters not present in the US-ASCII
365 ([USASCII]) coded character set (note that it would be unacceptable
366 to define a new parameter that would be restricted to a subset of the
367 Unicode character set).
369 4.2. Error Handling
371 Header field specifications need to define whether multiple instances
372 of parameters with identical parmname components are allowed, and how
373 they should be processed. This specification suggests that a
374 parameter using the extended syntax takes precedence. This would
375 allow producers to use both formats without breaking recipients that
376 do not understand the extended syntax yet.
378 Example:
380 foo: bar; title="EURO exchange rates";
381 title*=utf-8''%e2%82%ac%20exchange%20rates
383 In this case, the sender provides an ASCII version of the title for
384 legacy recipients, but also includes an internationalized version for
385 recipients understanding this specification -- the latter obviously
386 ought to prefer the new syntax over the old one.
388 Note: at the time of this writing, many implementations failed to
389 ignore the form they do not understand, or prioritize the ASCII
390 form although the extended syntax was present.
392 5. Security Considerations
394 The format described in this document makes it possible to transport
395 non-ASCII characters, and thus enables character "spoofing"
396 scenarios, in which a displayed value appears to be something other
397 than it is.
399 Furthermore, there are known attack scenarios relating to decoding
400 UTF-8.
402 See Section 10 of [RFC3629] for more information on both topics.
404 In addition, the extension specified in this document makes it
405 possible to transport multiple language variants for a single
406 parameter, and such use might allow spoofing attacks, where different
407 language versions of the same parameter are not equivalent. Whether
408 this attack is useful as an attack depends on the parameter
409 specified.
411 6. Acknowledgements
413 Thanks to Martin Duerst and Frank Ellermann for help figuring out
414 ABNF details, to Graham Klyne and Alexey Melnikov for general review,
415 to Chris Newman for pointing out an RFC 2231 incompatibility, and to
416 Benjamin Carlyle, Roar Lauritzsen, and Eric Lawrence for
417 implementer's feedback.
419 7. References
421 7.1. Normative References
423 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
424 Requirement Levels", BCP 14, RFC 2119, March 1997.
426 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
427 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
428 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
430 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
431 Procedures", BCP 19, RFC 2978, October 2000.
433 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
434 10646", STD 63, RFC 3629, November 2003.
436 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
437 "Uniform Resource Identifier (URI): Generic Syntax",
438 STD 66, RFC 3986, January 2005.
440 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
441 Syntax Specifications: ABNF", STD 68, RFC 5234,
442 January 2008.
444 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for
445 Identifying Languages", BCP 47, RFC 5646,
446 September 2009.
448 [USASCII] American National Standards Institute, "Coded Character
449 Set -- 7-bit American Standard Code for Information
450 Interchange", ANSI X3.4, 1986.
452 7.2. Informative References
454 [Err1912] RFC Errata, "Errata ID 1912, RFC 2978",
455 .
457 [ISO-8859-1] International Organization for Standardization,
458 "Information technology -- 8-bit single-byte coded
459 graphic character sets -- Part 1: Latin alphabet No.
460 1", ISO/IEC 8859-1:1998, 1998.
462 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
463 Mail Extensions (MIME) Part One: Format of Internet
464 Message Bodies", RFC 2045, November 1996.
466 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
467 Extensions) Part Three: Message Header Extensions for
468 Non-ASCII Text", RFC 2047, November 1996.
470 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
471 Encoded Word Extensions: Character Sets, Languages, and
472 Continuations", RFC 2231, November 1997.
474 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
475 Languages", BCP 18, RFC 2277, January 1998.
477 [RFC2388] Masinter, L., "Returning Values from Forms: multipart/
478 form-data", RFC 2388, August 1998.
480 [RFC5987] Reschke, J., "Character Set and Language Encoding for
481 Hypertext Transfer Protocol (HTTP) Header Field
482 Parameters", RFC 5987, August 2010.
484 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
486 [RFC6266] Reschke, J., "Use of the Content-Disposition Header
487 Field in the Hypertext Transfer Protocol (HTTP)",
488 RFC 6266, June 2011.
490 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
491 Internationalization in the IETF", BCP 166, RFC 6365,
492 September 2011.
494 URIs
496 [1]
498 [2]
500 Appendix A. Changes from RFC 5987
502 This section summarizes the changes compared to [RFC5987]:
504 o The document title was changed to "Indicating Character Encoding
505 and Language for HTTP Header Field Parameters".
507 o The requirement to support the "ISO-8859-1" encoding was removed.
509 Appendix B. Implementation Report
511 The encoding defined in this document currently is used for two
512 different HTTP header fields:
514 o "Content-Disposition", defined in [RFC6266], and
516 o "Link", defined in [RFC5988].
518 As the encoding is a profile/clarification of the one defined in
519 [RFC2231] in 1997, many user agents already supported it for use in
520 "Content-Disposition" when [RFC5987] got published.
522 Since the publication of [RFC5987], two more popular desktop user
523 agents have added support for this encoding; see for details.
525 At this time, only one major desktop user agent (Safari) does not
526 support it.
528 Note that the implementation in Internet Explorer 9 does not support
529 the ISO-8859-1 character encoding; this document revision
530 acknowledges that UTF-8 is sufficient for expressing all code points,
531 and removes the requirement to support ISO-8859-1.
533 The "Link" header field, on the other hand, was only recently
534 specified in [RFC5988]. At the time of this writing, no User Agent
535 supported the "title*" parameter, using the encoding defined by this
536 document, but implementation for Firefox was already in progress (see
537 ).
539 Appendix C. Change Log (to be removed by RFC Editor before publication)
541 C.1. Since RFC5987
543 Only editorial changes for the purpose of starting the revision
544 process (obs5987).
546 C.2. Since draft-reschke-rfc5987bis-00
548 Resolved issues "iso-8859-1" and "title" (title simplified). Added
549 and resolved issue "historic5987".
551 C.3. Since draft-reschke-rfc5987bis-01
553 Added issues "httpbis", "parmsyntax", "terminology" and
554 "valuesyntax". Closed issue "impls".
556 C.4. Since draft-reschke-rfc5987bis-02
558 Resolved issue "terminology".
560 Appendix D. Resolved issues (to be removed by RFC Editor before
561 publication)
563 Issues that were either rejected or resolved in this version of this
564 document.
566 D.1. terminology
568 Type: edit
570 julian.reschke@greenbytes.de (2011-09-17): Try to be consistent with
571 the terminology defined in RFC 6365.
573 Resolution (2011-12-08): Done (but abbreviating "character encoding
574 scheme" to "character encoding").
576 Appendix E. Open issues (to be removed by RFC Editor prior to
577 publication)
579 E.1. edit
581 Type: edit
583 julian.reschke@greenbytes.de (2011-04-15): Umbrella issue for
584 editorial fixes/enhancements.
586 E.2. parmsyntax
588 Type: edit
590
593 James.H.Manger@team.telstra.com (2011-11-02): Noted by James Manger:
594 "Presumably RFC5987 (or its predecessors) decided it was highly
595 unlikely that any parameter names in use ended in "*" (though they
596 are valid) so it could redefine the syntax of values for such names."
597 - add a note that the notation indeed overloads parameter name syntax
598 and clarify the use.
600 E.3. valuesyntax
602 Type: edit
604
607 James.H.Manger@team.telstra.com (2011-11-02): Noted by James Manger:
608 "Curiously, RFC5987 disobeys the proposed recommendations for new
609 parameters. It allows foo*=UTF-8''coll%C3%A8gues but not foo*="UTF-
610 8''coll%C3%A8gues" That might be ok with a parser that understands
611 token, quoted-string, and RFC5987, but presumably it will cause
612 problems when RFC5987 processing is done after a "standard httpbis
613 parser" handles the token | quoted-string step. " - add a note
614 clarifying that this is indeed a shortcoming of the format, and what
615 it means for implementations.
617 E.4. httpbis
619 Type: edit
621 julian.reschke@greenbytes.de (2011-09-17): The document refers
622 normatively to RFC 2616. Should it continue to do so, or should we
623 wait for HTTPbis? This may affect edge case in the ABNF, such as the
624 definition of linear white space or the characters allowed in
625 "token".
627 Author's Address
629 Julian F. Reschke
630 greenbytes GmbH
631 Hafenweg 16
632 Muenster, NW 48155
633 Germany
635 EMail: julian.reschke@greenbytes.de
636 URI: http://greenbytes.de/tech/webdav/