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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track December 27, 2009
5 Expires: June 30, 2010
7 Application of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Header Fields
9 draft-reschke-rfc2231-in-http-07
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 June 30, 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. Document History and Future Plans (to be removed
101 by RFC Editor before publication) . . . . . . . . . . 11
102 Appendix B. Change Log (to be removed by RFC Editor before
103 publication) . . . . . . . . . . . . . . . . . . . . 12
104 B.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 12
105 B.2. Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 12
106 B.3. Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 12
107 B.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 12
108 B.5. Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 12
109 B.6. Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 12
110 B.7. Since draft-reschke-rfc2231-in-http-06 . . . . . . . . . . 12
111 Appendix C. Open issues (to be removed by RFC Editor prior to
112 publication) . . . . . . . . . . . . . . . . . . . . 13
113 C.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
114 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
116 1. Introduction
118 By default, message header field parameters in HTTP ([RFC2616])
119 messages can not carry characters outside the ISO-8859-1 character
120 set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping
121 mechanism for use in MIME headers. This document specifies a profile
122 of that encoding for use in HTTP.
124 2. Notational Conventions
126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
128 document are to be interpreted as described in [RFC2119].
130 This specification uses the ABNF (Augmented Backus-Naur Form)
131 notation defined in [RFC5234]. The following core rules are included
132 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
133 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
134 (linear white space).
136 Note that this specification uses the term "character set" for
137 consistency with other IETF specifications such as RFC 2277 (see
138 [RFC2277], Section 3). A more accurate term would be "character
139 encoding" (a mapping of code points to octet sequences).
141 3. A Profile of RFC 2231 for Use in HTTP
143 RFC 2231 defines several extensions to MIME. The sections below
144 discuss if and how they apply to HTTP.
146 In short:
148 o Parameter Continuations aren't needed (Section 3.1),
150 o Character Set and Language Information are useful, therefore a
151 simple subset is specified (Section 3.2), and
153 o Language Specifications in Encoded Words aren't needed
154 (Section 3.3).
156 3.1. Parameter Continuations
158 Section 3 of [RFC2231] defines a mechanism that deals with the length
159 limitations that apply to MIME headers. These limitations do not
160 apply to HTTP ([RFC2616], Section 19.4.7).
162 Thus in HTTP, senders MUST NOT use parameter continuations, and
163 therefore recipients do not need to support them.
165 3.2. Parameter Value Character Set and Language Information
167 Section 4 of [RFC2231] specifies how to embed language information
168 into parameter values, and also how to encode non-ASCII characters,
169 dealing with restrictions both in MIME and HTTP header parameters.
171 However, RFC 2231 does not specify a mandatory-to-implement character
172 set, making it hard for senders to decide which character set to use.
173 Thus, recipients implementing this specification MUST support the
174 character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
176 Furthermore, RFC 2231 allows leaving out the character set
177 information. The profile defined by this specification does not
178 allow that.
180 The syntax for parameters is defined in Section 3.6 of [RFC2616]
181 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
183 parameter = attribute LWSP "=" LWSP value
185 attribute = token
186 value = token / quoted-string
188 quoted-string =
189 token =
191 This specification extends the grammar to:
193 parameter = reg-parameter / ext-parameter
195 reg-parameter = attribute LWSP "=" LWSP value
197 ext-parameter = attribute "*" LWSP "=" LWSP ext-value
199 ext-value = charset "'" [ language ] "'" value-chars
200 ; extended-initial-value,
201 ; defined in [RFC2231], Section 7
203 charset = "UTF-8" / "ISO-8859-1" / mime-charset
205 mime-charset = 1*mime-charsetc
206 mime-charsetc = ALPHA / DIGIT
207 / "!" / "#" / "$" / "%" / "&"
208 / "+" / "-" / "^" / "_" / "`"
209 / "{" / "}" / "~"
210 ; as in Section 2.3 of [RFC2978]
211 ; except that the single quote is not included
213 language =
215 value-chars = *( pct-encoded / attr-char )
217 pct-encoded = "%" HEXDIG HEXDIG
218 ; see [RFC3986], Section 2.1
220 attr-char = ALPHA / DIGIT
221 / "-" / "." / "_" / "~" / ":"
222 / "!" / "$" / "&" / "+"
224 Thus, a parameter is either regular parameter (reg-parameter), as
225 previously defined in Section 3.6 of [RFC2616], or an extended
226 parameter (ext-parameter).
228 Extended parameters are those where the left hand side of the
229 assignment ends with an asterisk character.
231 The value part of an extended parameter (ext-value) is a token that
232 consists of three parts: the REQUIRED character set name (charset),
233 the OPTIONAL language information (language), and a character
234 sequence representing the actual value (value-chars), separated by
235 single quote characters. Note that both character set names and
236 language tags are restricted to the US-ASCII character set, and are
237 matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646],
238 Section 2.1.1).
240 Inside the value part, characters not contained in attr-char are
241 encoded into an octet sequence using the specified character set.
242 That octet sequence then is percent-encoded as specified in Section
243 2.1 of [RFC3986].
245 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
246 or "ISO-8859-1" ([ISO-8859-1]). Extension character sets (ext-
247 charset) are reserved for future use.
249 Note: recipients should be prepared to handle encoding errors,
250 such as malformed or incomplete percent escape sequences, or non-
251 decodable octet sequences, in a robust manner. This specification
252 does not mandate any specific behavior, for instance the following
253 strategies are all acceptable:
255 * ignoring the parameter,
257 * stripping a non-decodable octet sequence,
259 * substituting a non-decodable octet sequence by a replacement
260 character, such as the Unicode character U+FFFD (Replacement
261 Character).
263 Note: the ABNF defined here differs from the one in
264 Section 2.3 of [RFC2978] in that it does not allow the single
265 quote character (see also RFC Editor Errata ID 1912 [3]). In
266 practice, no character set names using that character have been
267 registered at the time of this writing.
269 3.2.1. Examples
271 Non-extended notation, using "token":
273 foo: bar; title=Economy
275 Non-extended notation, using "quoted-string":
277 foo: bar; title="US-$ rates"
279 Extended notation, using the unicode character U+00A3 (POUND SIGN):
281 foo: bar; title*=iso-8859-1'en'%A3%20rates
283 Note: the Unicode pound sign character U+00A3 was encoded using ISO-
284 8859-1 into the single octet A3, then percent-encoded. Also note
285 that the space character was encoded as %20, as it is not contained
286 in attr-char.
288 Extended notation, using the unicode characters U+00A3 (POUND SIGN)
289 and U+20AC (EURO SIGN):
291 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
293 Note: the unicode pound sign character U+00A3 was encoded using UTF-8
294 into the octet sequence C2 A3, then percent-encoded. Likewise, the
295 unicode euro sign character U+20AC was encoded into the octet
296 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
297 allows both lower-case and upper-case character, so recipients must
298 understand both, and that the language information is optional, while
299 the character set is not.
301 3.3. Language specification in Encoded Words
303 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
304 also support language specification in encoded words. Although the
305 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
306 2.2), it's not clear to which header field exactly it applies, and
307 whether it is implemented in practice (see
308 for details).
310 Thus, the RFC 2231 profile defined by this specification does not
311 include this feature.
313 4. Guidelines for Usage in HTTP Header Field Definitions
315 Specifications of HTTP header fields that use the extensions defined
316 in Section 3.2 should clearly state that. A simple way to achieve
317 this is to normatively reference this specification, and to include
318 the ext-value production into the ABNF for that header field.
320 For instance:
322 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
323 title-param = "title" LWSP "=" LWSP value
324 / "title*" LWSP "=" LWSP ext-value
325 ext-value =
327 [[rfcno: Note to RFC Editor: in the figure above, please replace
328 "xxxx" by the RFC number assigned to this specification.]]
330 4.1. When to Use the Extension
332 Section 4.2 of [RFC2277] requires that protocol elements containing
333 text can carry language information. Thus, the ext-value production
334 should always be used when the parameter value is of textual nature.
336 Furthermore, the extension should also be used whenever the parameter
337 value needs to carry characters not present in the US-ASCII
338 ([USASCII]) character set (note that it would be unacceptable to
339 define a new parameter that would be restricted to a subset of the
340 Unicode character set).
342 4.2. Error Handling
344 Header specifications that include parameters should also specify
345 whether same-named parameters can occur multiple times. If
346 repetitions are not allowed (and this is believed to be the common
347 case), the specification should state whether regular or the extended
348 syntax takes precedence. In the latter case, this could be used by
349 producers to use both formats without breaking recipients that do not
350 understand the syntax.
352 Example:
354 foo: bar; title="EURO exchange rates";
355 title*=utf-8''%e2%82%ac%20exchange%20rates
357 In this case, the sender provides an ASCII version of the title for
358 legacy recipients, but also includes an internationalized version for
359 recipients understanding this specification -- the latter obviously
360 should prefer the new syntax over the old one.
362 Note: at the time of this writing, many implementations failed to
363 ignore the form they do not understand, or prioritize the ASCII
364 form although the extended syntax was present.
366 4.3. Using Multiple Instances for Internationalization
368 It is expected that in many cases, internationalization of parameters
369 in response headers is implemented using server driven content
370 negotiation ([RFC2616], Section 12.1) using the Accept-Language
371 header ([RFC2616], Section 14.4). However, the format described in
372 this specification also allows to use multiple instances providing
373 multiple languages in a single header. Specifications that want to
374 take advantage of this should clearly specify the expected processing
375 by the recipient.
377 Example:
379 foo: bar; title*=utf-8'en'Document%20Title;
380 title*=utf-8'de'Titel%20des%20Dokuments
382 5. Security Considerations
384 This document does not discuss security issues and is not believed to
385 raise any security issues not already endemic in HTTP.
387 6. IANA Considerations
389 There are no IANA Considerations related to this specification.
391 7. Acknowledgements
393 Thanks to Martin Duerst and Frank Ellermann for help figuring out
394 ABNF details, and to Roar Lauritzsen for implementer's feedback.
396 8. References
398 8.1. Normative References
400 [ISO-8859-1]
401 International Organization for Standardization,
402 "Information technology -- 8-bit single-byte coded graphic
403 character sets -- Part 1: Latin alphabet No. 1", ISO/
404 IEC 8859-1:1998, 1998.
406 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
407 Requirement Levels", BCP 14, RFC 2119, March 1997.
409 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
410 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
411 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
413 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
414 10646", RFC 3629, STD 63, November 2003.
416 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
417 Specifications: ABNF", STD 68, RFC 5234, January 2008.
419 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
420 Languages", BCP 47, RFC 5646, September 2009.
422 8.2. Informative References
424 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
425 Part Three: Message Header Extensions for Non-ASCII Text",
426 RFC 2047, November 1996.
428 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
429 Word Extensions:
430 Character Sets, Languages, and Continuations", RFC 2231,
431 November 1997.
433 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
434 Languages", BCP 18, RFC 2277, January 1998.
436 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
437 Procedures", BCP 19, RFC 2978, October 2000.
439 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
440 Resource Identifier (URI): Generic Syntax", RFC 3986,
441 STD 66, January 2005.
443 [USASCII] American National Standards Institute, "Coded Character
444 Set -- 7-bit American Standard Code for Information
445 Interchange", ANSI X3.4, 1986.
447 URIs
449 [1]
451 [2]
453 [3]
455 Appendix A. Document History and Future Plans (to be removed by RFC
456 Editor before publication)
458 Problems with the internationalization of the HTTP Content-
459 Disposition header field have been known for many years (see test
460 cases at ).
462 During IETF 72
463 (), the
464 HTTPbis Working Group shortly discussed how to deal with the
465 underspecification of (1) Content-Disposition, and its (2)
466 internationalization aspects. Back then, there was rough consensus
467 in the room to move the definition into a separate draft.
469 This specification addresses problem (2), by defining a simple subset
470 of the encoding format defined in RFC 2231. A separate
471 specification, draft-reschke-rfc2183-in-http, is planned to address
472 problem (1). Note that this approach was chosen because Content-
473 Disposition is just an example for an HTTP header field using this
474 kind of encoding. Another example is the currently proposed Link
475 header field (draft-nottingham-http-link-header).
477 This document is planned to be published on the IETF Standards Track,
478 so that other standards-track level documents can depend on it, such
479 as the new specification of Content-Disposition, or potentially
480 future revisions of the HTTP Link Header specification.
482 Also note that this document specifies a proper subset of the
483 extensions defined in RFC 2231, but does not normatively refer to it.
484 Thus, RFC 2231 can be revised separately, should the email community
485 decide to.
487 Appendix B. Change Log (to be removed by RFC Editor before publication)
489 B.1. Since draft-reschke-rfc2231-in-http-00
491 Use RFC5234-style ABNF, closer to the one used in RFC 2231.
493 Make RFC 2231 dependency informative, so this specification can
494 evolve independently.
496 Explain the ABNF in prose.
498 B.2. Since draft-reschke-rfc2231-in-http-01
500 Remove unneeded RFC5137 notation (code point vs character).
502 B.3. Since draft-reschke-rfc2231-in-http-02
504 And and resolve issues "charset", "repeats" and "rfc4646".
506 B.4. Since draft-reschke-rfc2231-in-http-03
508 And and resolve issue "charsetmatch".
510 B.5. Since draft-reschke-rfc2231-in-http-04
512 Add and resolve issues "badseq" and "tokenquotcharset".
514 B.6. Since draft-reschke-rfc2231-in-http-05
516 Say "header field" instead of "header" in the context of HTTP.
518 B.7. Since draft-reschke-rfc2231-in-http-06
520 Add an appendix discussing document history and future plans, to be
521 removed before publication.
523 Appendix C. Open issues (to be removed by RFC Editor prior to
524 publication)
526 C.1. edit
528 Type: edit
530 julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
531 editorial fixes/enhancements.
533 Author's Address
535 Julian F. Reschke
536 greenbytes GmbH
537 Hafenweg 16
538 Muenster, NW 48155
539 Germany
541 Email: julian.reschke@greenbytes.de
542 URI: http://greenbytes.de/tech/webdav/