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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track October 4, 2009
5 Expires: April 7, 2010
7 Application of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Headers
9 draft-reschke-rfc2231-in-http-04
11 Status of this Memo
13 This Internet-Draft is submitted to IETF in full conformance with the
14 provisions of BCP 78 and BCP 79.
16 Internet-Drafts are working documents of the Internet Engineering
17 Task Force (IETF), its areas, and its working groups. Note that
18 other groups may also distribute working documents as Internet-
19 Drafts.
21 Internet-Drafts are draft documents valid for a maximum of six months
22 and may be updated, replaced, or obsoleted by other documents at any
23 time. It is inappropriate to use Internet-Drafts as reference
24 material or to cite them other than as "work in progress."
26 The list of current Internet-Drafts can be accessed at
27 http://www.ietf.org/ietf/1id-abstracts.txt.
29 The list of Internet-Draft Shadow Directories can be accessed at
30 http://www.ietf.org/shadow.html.
32 This Internet-Draft will expire on April 7, 2010.
34 Copyright Notice
36 Copyright (c) 2009 IETF Trust and the persons identified as the
37 document authors. All rights reserved.
39 This document is subject to BCP 78 and the IETF Trust's Legal
40 Provisions Relating to IETF Documents in effect on the date of
41 publication of this document (http://trustee.ietf.org/license-info).
42 Please review these documents carefully, as they describe your rights
43 and restrictions with respect to this document.
45 Abstract
47 By default, message header parameters in Hypertext Transfer Protocol
48 (HTTP) messages can not carry characters outside the ISO-8859-1
49 character set. RFC 2231 defines an escaping mechanism for use in
50 Multipurpose Internet Mail Extensions (MIME) headers. This document
51 specifies a profile of that encoding suitable for use in HTTP.
53 Editorial Note (To be removed by RFC Editor before publication)
55 There are multiple HTTP headers that already use RFC 2231 encoding in
56 practice (Content-Disposition) or might use it in the future (Link).
57 The purpose of this document is to provide a single place where the
58 generic aspects of RFC 2231 encoding in HTTP headers are defined.
60 Distribution of this document is unlimited. Although this is not a
61 work item of the HTTPbis Working Group, comments should be sent to
62 the Hypertext Transfer Protocol (HTTP) mailing list at
63 ietf-http-wg@w3.org [1], which may be joined by sending a message
64 with subject "subscribe" to ietf-http-wg-request@w3.org [2].
66 Discussions of the HTTPbis Working Group are archived at
67 .
69 XML versions, latest edits and the issues list for this document are
70 available from
71 . A
72 collection of test cases is available at
73 .
75 Table of Contents
77 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
78 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
79 3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4
80 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 4
81 3.2. Parameter Value Character Set and Language Information . . 5
82 3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
83 3.3. Language specification in Encoded Words . . . . . . . . . 7
84 4. Guidelines for Usage in HTTP Header Definitions . . . . . . . 8
85 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 8
86 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 8
87 4.3. Using Multiple Instances for Internationalization . . . . 9
88 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
89 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
90 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
91 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
92 8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
93 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
94 Appendix A. Change Log (to be removed by RFC Editor before
95 publication) . . . . . . . . . . . . . . . . . . . . 11
96 A.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 11
97 A.2. Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 11
98 A.3. Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 11
99 A.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 11
100 Appendix B. Resolved issues (to be removed by RFC Editor
101 before publication) . . . . . . . . . . . . . . . . . 11
102 B.1. charsetmatch . . . . . . . . . . . . . . . . . . . . . . . 11
103 Appendix C. Open issues (to be removed by RFC Editor prior to
104 publication) . . . . . . . . . . . . . . . . . . . . 11
105 C.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
106 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
108 1. Introduction
110 By default, message header parameters in HTTP ([RFC2616]) messages
111 can not carry characters outside the ISO-8859-1 character set
112 ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping mechanism
113 for use in MIME headers. This document specifies a profile of that
114 encoding for use in HTTP.
116 2. Notational Conventions
118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
120 document are to be interpreted as described in [RFC2119].
122 This specification uses the ABNF (Augmented Backus-Naur Form)
123 notation defined in [RFC5234]. The following core rules are included
124 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
125 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
126 (linear white space).
128 Note that this specification uses the term "character set" for
129 consistency with other IETF specifications such as RFC 2277 (see
130 [RFC2277], Section 3). A more accurate term would be "character
131 encoding" (a mapping of code points to octet sequences).
133 3. A Profile of RFC 2231 for Use in HTTP
135 RFC 2231 defines several extensions to MIME. The sections below
136 discuss if and how they apply to HTTP.
138 In short:
140 o Parameter Continuations aren't needed (Section 3.1),
142 o Character Set and Language Information are useful, therefore a
143 simple subset is specified (Section 3.2), and
145 o Language Specifications in Encoded Words aren't needed
146 (Section 3.3).
148 3.1. Parameter Continuations
150 Section 3 of [RFC2231] defines a mechanism that deals with the length
151 limitations that apply to MIME headers. These limitations do not
152 apply to HTTP ([RFC2616], Section 19.4.7).
154 Thus in HTTP, senders MUST NOT use parameter continuations, and
155 therefore recipients do not need to support them.
157 3.2. Parameter Value Character Set and Language Information
159 Section 4 of [RFC2231] specifies how to embed language information
160 into parameter values, and also how to encode non-ASCII characters,
161 dealing with restrictions both in MIME and HTTP header parameters.
163 However, RFC 2231 does not specify a mandatory-to-implement character
164 set, making it hard for senders to decide which character set to use.
165 Thus, recipients implementing this specification MUST support the
166 character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
168 Furthermore, RFC 2231 allows leaving out the character set
169 information. The profile defined by this specification does not
170 allow that.
172 The syntax for parameters is defined in Section 3.6 of [RFC2616]
173 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
175 parameter = attribute LWSP "=" LWSP value
177 attribute = token
178 value = token / quoted-string
180 quoted-string =
181 token =
183 This specification extends the grammar to:
185 parameter = reg-parameter / ext-parameter
187 reg-parameter = attribute LWSP "=" LWSP value
189 ext-parameter = attribute "*" LWSP "=" LWSP ext-value
191 ext-value = charset "'" [ language ] "'" value-chars
192 ; extended-initial-value,
193 ; defined in [RFC2231], Section 7
195 charset = "UTF-8" / "ISO-8859-1" / ext-charset
197 ext-charset = token ; see IANA charset registry
198 ; ()
200 language =
202 value-chars = *( pct-encoded / attr-char )
204 pct-encoded = "%" HEXDIG HEXDIG
205 ; see [RFC3986], Section 2.1
207 attr-char = ALPHA / DIGIT
208 / "-" / "." / "_" / "~" / ":"
209 / "!" / "$" / "&" / "+"
211 Thus, a parameter is either regular parameter (reg-parameter), as
212 previously defined in Section 3.6 of [RFC2616], or an extended
213 parameter (ext-parameter).
215 Extended parameters are those where the left hand side of the
216 assignment ends with an asterisk character.
218 The value part of an extended parameter (ext-value) is a token that
219 consists of three parts: the REQUIRED character set name (charset),
220 the OPTIONAL language information (language), and a character
221 sequence representing the actual value (value-chars), separated by
222 single quote characters. Note that both character set names and
223 language tags are restricted to the US-ASCII character set, and are
224 matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646],
225 Section 2.1.1).
227 Inside the value part, characters not contained in attr-char are
228 encoded into an octet sequence using the specified character set.
229 That octet sequence then is percent-encoded as specified in Section
230 2.1 of [RFC3986].
232 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
233 or ISO-8859-1 ([ISO-8859-1]). Extension character sets (ext-charset)
234 are reserved for future use.
236 3.2.1. Examples
238 Non-extended notation, using "token":
240 foo: bar; title=Economy
242 Non-extended notation, using "quoted-string":
244 foo: bar; title="US-$ rates"
246 Extended notation, using the unicode character U+00A3 (POUND SIGN):
248 foo: bar; title*=iso-8859-1'en'%A3%20rates
250 Note: the Unicode pound sign character U+00A3 was encoded using ISO-
251 8859-1 into the single octet A3, then percent-encoded. Also note
252 that the space character was encoded as %20, as it is not contained
253 in attr-char.
255 Extended notation, using the unicode characters U+00A3 (POUND SIGN)
256 and U+20AC (EURO SIGN):
258 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
260 Note: the unicode pound sign character U+00A3 was encoded using UTF-8
261 into the octet sequence C2 A3, then percent-encoded. Likewise, the
262 unicode euro sign character U+20AC was encoded into the octet
263 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
264 allows both lower-case and upper-case character, so recipients must
265 understand both, and that the language information is optional, while
266 the character set is not.
268 3.3. Language specification in Encoded Words
270 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
271 also support language specification in encoded words. Although the
272 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
273 2.2), it's not clear to which header field exactly it applies, and
274 whether it is implemented in practice (see
275 for details).
277 Thus, the RFC 2231 profile defined by this specification does not
278 include this feature.
280 4. Guidelines for Usage in HTTP Header Definitions
282 Specifications of HTTP headers that use the extensions defined in
283 Section 3.2 should clearly state that. A simple way to achieve this
284 is to normatively reference this specification, and to include the
285 ext-value production into the ABNF for that header.
287 For instance:
289 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
290 title-param = "title" LWSP "=" LWSP value
291 / "title*" LWSP "=" LWSP ext-value
292 ext-value =
294 [[rfcno: Note to RFC Editor: in the figure above, please replace
295 "xxxx" by the RFC number assigned to this specification.]]
297 4.1. When to Use the Extension
299 Section 4.2 of [RFC2277] requires that protocol elements containing
300 text can carry language information. Thus, the ext-value production
301 should always be used when the parameter value is of textual nature.
303 Furthermore, the extension should also be used whenever the parameter
304 value needs to carry characters not present in the US-ASCII
305 ([USASCII]) character set (note that it would be unacceptable to
306 define a new parameter that would be restricted to a subset of the
307 Unicode character set).
309 4.2. Error Handling
311 Header specifications that include parameters should also specify
312 whether same-named parameters can occur multiple times. If
313 repetitions are not allowed (and this is believed to be the common
314 case), the specification should state whether regular or the extended
315 syntax takes precedence. In the latter case, this could be used by
316 producers to use both formats without breaking recipients that do not
317 understand the syntax.
319 Example:
321 foo: bar; title="EURO exchange rates";
322 title*=utf-8''%e2%82%ac%20exchange%20rates
324 In this case, the sender provides an ASCII version of the title for
325 legacy recipients, but also includes an internationalized version for
326 recipients understanding this specification -- the latter obviously
327 should prefer the new syntax over the old one.
329 Note: at the time of this writing, many implementations failed to
330 ignore the form they do not understand, or prioritize the ASCII
331 form although the extended syntax was present.
333 4.3. Using Multiple Instances for Internationalization
335 It is expected that in many cases, internationalization of parameters
336 in response headers is implemented using server driven content
337 negotiation ([RFC2616], Section 12.1) using the Accept-Language
338 header ([RFC2616], Section 14.4). However, the format described in
339 this specification also allows to use multiple instances providing
340 multiple languages in a single header. Specifications that want to
341 take advantage of this should clearly specify the expected processing
342 by the recipient.
344 Example:
346 foo: bar; title*=utf-8'en'Document%20Title;
347 title*=utf-8'de'Titel%20des%20Dokuments
349 5. Security Considerations
351 This document does not discuss security issues and is not believed to
352 raise any security issues not already endemic in HTTP.
354 6. IANA Considerations
356 There are no IANA Considerations related to this specification.
358 7. Acknowledgements
360 Thanks to Frank Ellermann for help figuring out ABNF details, and to
361 Roar Lauritzsen for implementer's feedback.
363 8. References
365 8.1. Normative References
367 [ISO-8859-1]
368 International Organization for Standardization,
369 "Information technology -- 8-bit single-byte coded graphic
370 character sets -- Part 1: Latin alphabet No. 1", ISO/
371 IEC 8859-1:1998, 1998.
373 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
374 Requirement Levels", BCP 14, RFC 2119, March 1997.
376 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
377 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
378 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
380 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
381 10646", RFC 3629, STD 63, November 2003.
383 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
384 Specifications: ABNF", STD 68, RFC 5234, January 2008.
386 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
387 Languages", BCP 47, RFC 5646, September 2009.
389 8.2. Informative References
391 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
392 Part Three: Message Header Extensions for Non-ASCII Text",
393 RFC 2047, November 1996.
395 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
396 Word Extensions:
397 Character Sets, Languages, and Continuations", RFC 2231,
398 November 1997.
400 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
401 Languages", BCP 18, RFC 2277, January 1998.
403 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
404 Procedures", BCP 19, RFC 2978, October 2000.
406 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
407 Resource Identifier (URI): Generic Syntax", RFC 3986,
408 STD 66, January 2005.
410 [USASCII] American National Standards Institute, "Coded Character
411 Set -- 7-bit American Standard Code for Information
412 Interchange", ANSI X3.4, 1986.
414 URIs
416 [1]
418 [2]
420 Appendix A. Change Log (to be removed by RFC Editor before publication)
422 A.1. Since draft-reschke-rfc2231-in-http-00
424 Use RFC5234-style ABNF, closer to the one used in RFC 2231.
426 Make RFC 2231 dependency informative, so this specification can
427 evolve independantly.
429 Explain the ABNF in prose.
431 A.2. Since draft-reschke-rfc2231-in-http-01
433 Remove unneeded RFC5137 notation (code point vs character).
435 A.3. Since draft-reschke-rfc2231-in-http-02
437 And and resolve issues "charset", "repeats" and "rfc4646".
439 A.4. Since draft-reschke-rfc2231-in-http-03
441 And and resolve issue "charsetmatch".
443 Appendix B. Resolved issues (to be removed by RFC Editor before
444 publication)
446 Issues that were either rejected or resolved in this version of this
447 document.
449 B.1. charsetmatch
451 In Section 3.2:
453 Type: change
455 julian.reschke@greenbytes.de (2009-10-03): Is the character set name
456 matched case-sensitively?
458 Resolution (2009-10-04): Be consistent with
459 http://www.iana.org/assignments/character-sets and match case-
460 insensitively.
462 Appendix C. Open issues (to be removed by RFC Editor prior to
463 publication)
465 C.1. edit
467 Type: edit
469 julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
470 editorial fixes/enhancements.
472 Author's Address
474 Julian F. Reschke
475 greenbytes GmbH
476 Hafenweg 16
477 Muenster, NW 48155
478 Germany
480 Email: julian.reschke@greenbytes.de
481 URI: http://greenbytes.de/tech/webdav/