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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track November 22, 2009
5 Expires: May 26, 2010
7 Application of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Header Fields
9 draft-reschke-rfc2231-in-http-06
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.
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 is 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 Status of this Memo
45 This Internet-Draft is submitted to IETF in full conformance with the
46 provisions of BCP 78 and BCP 79.
48 Internet-Drafts are working documents of the Internet Engineering
49 Task Force (IETF), its areas, and its working groups. Note that
50 other groups may also distribute working documents as Internet-
51 Drafts.
53 Internet-Drafts are draft documents valid for a maximum of six months
54 and may be updated, replaced, or obsoleted by other documents at any
55 time. It is inappropriate to use Internet-Drafts as reference
56 material or to cite them other than as "work in progress."
58 The list of current Internet-Drafts can be accessed at
59 http://www.ietf.org/ietf/1id-abstracts.txt.
61 The list of Internet-Draft Shadow Directories can be accessed at
62 http://www.ietf.org/shadow.html.
64 This Internet-Draft will expire on May 26, 2010.
66 Copyright Notice
68 Copyright (c) 2009 IETF Trust and the persons identified as the
69 document authors. All rights reserved.
71 This document is subject to BCP 78 and the IETF Trust's Legal
72 Provisions Relating to IETF Documents
73 (http://trustee.ietf.org/license-info) in effect on the date of
74 publication of this document. Please review these documents
75 carefully, as they describe your rights and restrictions with respect
76 to this document. Code Components extracted from this document must
77 include Simplified BSD License text as described in Section 4.e of
78 the Trust Legal Provisions and are provided without warranty as
79 described in the BSD License.
81 Table of Contents
83 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
84 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
85 3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4
86 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 4
87 3.2. Parameter Value Character Set and Language Information . . 5
88 3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
89 3.3. Language specification in Encoded Words . . . . . . . . . 8
90 4. Guidelines for Usage in HTTP Header Field Definitions . . . . 8
91 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 8
92 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
93 4.3. Using Multiple Instances for Internationalization . . . . 9
94 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
95 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
96 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
97 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
98 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
99 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
100 Appendix A. Change Log (to be removed by RFC Editor before
101 publication) . . . . . . . . . . . . . . . . . . . . 11
102 A.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 11
103 A.2. Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 11
104 A.3. Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 11
105 A.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 12
106 A.5. Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 12
107 A.6. Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 12
108 Appendix B. Open issues (to be removed by RFC Editor prior to
109 publication) . . . . . . . . . . . . . . . . . . . . 12
110 B.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
111 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
113 1. Introduction
115 By default, message header field parameters in HTTP ([RFC2616])
116 messages can not carry characters outside the ISO-8859-1 character
117 set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping
118 mechanism for use in MIME headers. This document specifies a profile
119 of that encoding for use in HTTP.
121 2. Notational Conventions
123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
125 document are to be interpreted as described in [RFC2119].
127 This specification uses the ABNF (Augmented Backus-Naur Form)
128 notation defined in [RFC5234]. The following core rules are included
129 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
130 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
131 (linear white space).
133 Note that this specification uses the term "character set" for
134 consistency with other IETF specifications such as RFC 2277 (see
135 [RFC2277], Section 3). A more accurate term would be "character
136 encoding" (a mapping of code points to octet sequences).
138 3. A Profile of RFC 2231 for Use in HTTP
140 RFC 2231 defines several extensions to MIME. The sections below
141 discuss if and how they apply to HTTP.
143 In short:
145 o Parameter Continuations aren't needed (Section 3.1),
147 o Character Set and Language Information are useful, therefore a
148 simple subset is specified (Section 3.2), and
150 o Language Specifications in Encoded Words aren't needed
151 (Section 3.3).
153 3.1. Parameter Continuations
155 Section 3 of [RFC2231] defines a mechanism that deals with the length
156 limitations that apply to MIME headers. These limitations do not
157 apply to HTTP ([RFC2616], Section 19.4.7).
159 Thus in HTTP, senders MUST NOT use parameter continuations, and
160 therefore recipients do not need to support them.
162 3.2. Parameter Value Character Set and Language Information
164 Section 4 of [RFC2231] specifies how to embed language information
165 into parameter values, and also how to encode non-ASCII characters,
166 dealing with restrictions both in MIME and HTTP header parameters.
168 However, RFC 2231 does not specify a mandatory-to-implement character
169 set, making it hard for senders to decide which character set to use.
170 Thus, recipients implementing this specification MUST support the
171 character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
173 Furthermore, RFC 2231 allows leaving out the character set
174 information. The profile defined by this specification does not
175 allow that.
177 The syntax for parameters is defined in Section 3.6 of [RFC2616]
178 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
180 parameter = attribute LWSP "=" LWSP value
182 attribute = token
183 value = token / quoted-string
185 quoted-string =
186 token =
188 This specification extends the grammar to:
190 parameter = reg-parameter / ext-parameter
192 reg-parameter = attribute LWSP "=" LWSP value
194 ext-parameter = attribute "*" LWSP "=" LWSP ext-value
196 ext-value = charset "'" [ language ] "'" value-chars
197 ; extended-initial-value,
198 ; defined in [RFC2231], Section 7
200 charset = "UTF-8" / "ISO-8859-1" / mime-charset
202 mime-charset = 1*mime-charsetc
203 mime-charsetc = ALPHA / DIGIT
204 / "!" / "#" / "$" / "%" / "&"
205 / "+" / "-" / "^" / "_" / "`"
206 / "{" / "}" / "~"
207 ; as in Section 2.3 of [RFC2978]
208 ; except that the single quote is not included
210 language =
212 value-chars = *( pct-encoded / attr-char )
214 pct-encoded = "%" HEXDIG HEXDIG
215 ; see [RFC3986], Section 2.1
217 attr-char = ALPHA / DIGIT
218 / "-" / "." / "_" / "~" / ":"
219 / "!" / "$" / "&" / "+"
221 Thus, a parameter is either regular parameter (reg-parameter), as
222 previously defined in Section 3.6 of [RFC2616], or an extended
223 parameter (ext-parameter).
225 Extended parameters are those where the left hand side of the
226 assignment ends with an asterisk character.
228 The value part of an extended parameter (ext-value) is a token that
229 consists of three parts: the REQUIRED character set name (charset),
230 the OPTIONAL language information (language), and a character
231 sequence representing the actual value (value-chars), separated by
232 single quote characters. Note that both character set names and
233 language tags are restricted to the US-ASCII character set, and are
234 matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646],
235 Section 2.1.1).
237 Inside the value part, characters not contained in attr-char are
238 encoded into an octet sequence using the specified character set.
239 That octet sequence then is percent-encoded as specified in Section
240 2.1 of [RFC3986].
242 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
243 or "ISO-8859-1" ([ISO-8859-1]). Extension character sets (ext-
244 charset) are reserved for future use.
246 Note: recipients should be prepared to handle encoding errors,
247 such as malformed or incomplete percent escape sequences, or non-
248 decodable octet sequences, in a robust manner. This specification
249 does not mandate any specific behavior, for instance the following
250 strategies are all acceptable:
252 * ignoring the parameter,
254 * stripping a non-decodable octet sequence,
256 * substituting a non-decodable octet sequence by a replacement
257 character, such as the Unicode character U+FFFD (Replacement
258 Character).
260 Note: the ABNF defined here differs from the one in
261 Section 2.3 of [RFC2978] in that it does not allow the single
262 quote character (see also RFC Editor Errata ID 1912 [3]). In
263 practice, no character set names using that character have been
264 registered at the time of this writing.
266 3.2.1. Examples
268 Non-extended notation, using "token":
270 foo: bar; title=Economy
272 Non-extended notation, using "quoted-string":
274 foo: bar; title="US-$ rates"
276 Extended notation, using the unicode character U+00A3 (POUND SIGN):
278 foo: bar; title*=iso-8859-1'en'%A3%20rates
280 Note: the Unicode pound sign character U+00A3 was encoded using ISO-
281 8859-1 into the single octet A3, then percent-encoded. Also note
282 that the space character was encoded as %20, as it is not contained
283 in attr-char.
285 Extended notation, using the unicode characters U+00A3 (POUND SIGN)
286 and U+20AC (EURO SIGN):
288 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
290 Note: the unicode pound sign character U+00A3 was encoded using UTF-8
291 into the octet sequence C2 A3, then percent-encoded. Likewise, the
292 unicode euro sign character U+20AC was encoded into the octet
293 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
294 allows both lower-case and upper-case character, so recipients must
295 understand both, and that the language information is optional, while
296 the character set is not.
298 3.3. Language specification in Encoded Words
300 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
301 also support language specification in encoded words. Although the
302 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
303 2.2), it's not clear to which header field exactly it applies, and
304 whether it is implemented in practice (see
305 for details).
307 Thus, the RFC 2231 profile defined by this specification does not
308 include this feature.
310 4. Guidelines for Usage in HTTP Header Field Definitions
312 Specifications of HTTP header fields that use the extensions defined
313 in Section 3.2 should clearly state that. A simple way to achieve
314 this is to normatively reference this specification, and to include
315 the ext-value production into the ABNF for that header field.
317 For instance:
319 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
320 title-param = "title" LWSP "=" LWSP value
321 / "title*" LWSP "=" LWSP ext-value
322 ext-value =
324 [[rfcno: Note to RFC Editor: in the figure above, please replace
325 "xxxx" by the RFC number assigned to this specification.]]
327 4.1. When to Use the Extension
329 Section 4.2 of [RFC2277] requires that protocol elements containing
330 text can carry language information. Thus, the ext-value production
331 should always be used when the parameter value is of textual nature.
333 Furthermore, the extension should also be used whenever the parameter
334 value needs to carry characters not present in the US-ASCII
335 ([USASCII]) character set (note that it would be unacceptable to
336 define a new parameter that would be restricted to a subset of the
337 Unicode character set).
339 4.2. Error Handling
341 Header specifications that include parameters should also specify
342 whether same-named parameters can occur multiple times. If
343 repetitions are not allowed (and this is believed to be the common
344 case), the specification should state whether regular or the extended
345 syntax takes precedence. In the latter case, this could be used by
346 producers to use both formats without breaking recipients that do not
347 understand the syntax.
349 Example:
351 foo: bar; title="EURO exchange rates";
352 title*=utf-8''%e2%82%ac%20exchange%20rates
354 In this case, the sender provides an ASCII version of the title for
355 legacy recipients, but also includes an internationalized version for
356 recipients understanding this specification -- the latter obviously
357 should prefer the new syntax over the old one.
359 Note: at the time of this writing, many implementations failed to
360 ignore the form they do not understand, or prioritize the ASCII
361 form although the extended syntax was present.
363 4.3. Using Multiple Instances for Internationalization
365 It is expected that in many cases, internationalization of parameters
366 in response headers is implemented using server driven content
367 negotiation ([RFC2616], Section 12.1) using the Accept-Language
368 header ([RFC2616], Section 14.4). However, the format described in
369 this specification also allows to use multiple instances providing
370 multiple languages in a single header. Specifications that want to
371 take advantage of this should clearly specify the expected processing
372 by the recipient.
374 Example:
376 foo: bar; title*=utf-8'en'Document%20Title;
377 title*=utf-8'de'Titel%20des%20Dokuments
379 5. Security Considerations
381 This document does not discuss security issues and is not believed to
382 raise any security issues not already endemic in HTTP.
384 6. IANA Considerations
386 There are no IANA Considerations related to this specification.
388 7. Acknowledgements
390 Thanks to Martin Duerst and Frank Ellermann for help figuring out
391 ABNF details, and to Roar Lauritzsen for implementer's feedback.
393 8. References
395 8.1. Normative References
397 [ISO-8859-1]
398 International Organization for Standardization,
399 "Information technology -- 8-bit single-byte coded graphic
400 character sets -- Part 1: Latin alphabet No. 1", ISO/
401 IEC 8859-1:1998, 1998.
403 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
404 Requirement Levels", BCP 14, RFC 2119, March 1997.
406 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
407 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
408 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
410 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
411 10646", RFC 3629, STD 63, November 2003.
413 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
414 Specifications: ABNF", STD 68, RFC 5234, January 2008.
416 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
417 Languages", BCP 47, RFC 5646, September 2009.
419 8.2. Informative References
421 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
422 Part Three: Message Header Extensions for Non-ASCII Text",
423 RFC 2047, November 1996.
425 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
426 Word Extensions:
427 Character Sets, Languages, and Continuations", RFC 2231,
428 November 1997.
430 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
431 Languages", BCP 18, RFC 2277, January 1998.
433 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
434 Procedures", BCP 19, RFC 2978, October 2000.
436 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
437 Resource Identifier (URI): Generic Syntax", RFC 3986,
438 STD 66, January 2005.
440 [USASCII] American National Standards Institute, "Coded Character
441 Set -- 7-bit American Standard Code for Information
442 Interchange", ANSI X3.4, 1986.
444 URIs
446 [1]
448 [2]
450 [3]
452 Appendix A. Change Log (to be removed by RFC Editor before publication)
454 A.1. Since draft-reschke-rfc2231-in-http-00
456 Use RFC5234-style ABNF, closer to the one used in RFC 2231.
458 Make RFC 2231 dependency informative, so this specification can
459 evolve independently.
461 Explain the ABNF in prose.
463 A.2. Since draft-reschke-rfc2231-in-http-01
465 Remove unneeded RFC5137 notation (code point vs character).
467 A.3. Since draft-reschke-rfc2231-in-http-02
469 And and resolve issues "charset", "repeats" and "rfc4646".
471 A.4. Since draft-reschke-rfc2231-in-http-03
473 And and resolve issue "charsetmatch".
475 A.5. Since draft-reschke-rfc2231-in-http-04
477 Add and resolve issues "badseq" and "tokenquotcharset".
479 A.6. Since draft-reschke-rfc2231-in-http-05
481 Say "header field" instead of "header" in the context of HTTP.
483 Appendix B. Open issues (to be removed by RFC Editor prior to
484 publication)
486 B.1. edit
488 Type: edit
490 julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
491 editorial fixes/enhancements.
493 Author's Address
495 Julian F. Reschke
496 greenbytes GmbH
497 Hafenweg 16
498 Muenster, NW 48155
499 Germany
501 Email: julian.reschke@greenbytes.de
502 URI: http://greenbytes.de/tech/webdav/