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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track December 30, 2008
5 Expires: July 3, 2009
7 Application of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Headers
9 draft-reschke-rfc2231-in-http-01
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 July 3, 2009.
34 Copyright Notice
36 Copyright (c) 2008 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
41 (http://trustee.ietf.org/license-info) in effect on the date of
42 publication of this document. Please review these documents
43 carefully, as they describe your rights and restrictions with respect
44 to this document.
46 Abstract
48 By default, message header parameters in Hypertext Transfer Protocol
49 (HTTP) messages can not carry characters outside the ISO-8859-1
50 character set. RFC 2231 defines an escaping mechanism for use in
51 Multipurpose Internet Mail Extensions (MIME) headers. This document
52 specifies a profile of that encoding suitable for use in HTTP.
54 Editorial Note (To be removed by RFC Editor before publication)
56 There are multiple HTTP headers that already use RFC 2231 encoding in
57 practice (Content-Disposition) or might use it in the future (Link).
58 The purpose of this document is to provide a single place where the
59 generic aspects of RFC 2231 encoding in HTTP headers are defined.
61 Distribution of this document is unlimited. Although this is not a
62 work item of the HTTPbis Working Group, comments should be sent to
63 the Hypertext Transfer Protocol (HTTP) mailing list at
64 ietf-http-wg@w3.org [1], which may be joined by sending a message
65 with subject "subscribe" to ietf-http-wg-request@w3.org [2].
67 Discussions of the HTTPbis Working Group are archived at
68 .
70 XML versions, latest edits and the issues list for this document are
71 available from
72 . A
73 collection of test cases is available at
74 .
76 Table of Contents
78 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
79 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
80 3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4
81 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 5
82 3.2. Parameter Value Character Set and Language Information . . 5
83 3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
84 3.3. Language specification in Encoded Words . . . . . . . . . 7
85 4. Guidelines for Usage in HTTP Header Definitions . . . . . . . 8
86 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 8
87 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 8
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) . . . . . . . . . . . . . . . . . . . . 10
96 A.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 10
97 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
99 1. Introduction
101 By default, message header parameters in HTTP ([RFC2616]) messages
102 can not carry characters outside the ISO-8859-1 character set
103 ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping mechanism
104 for use in MIME headers. This document specifies a profile of that
105 encoding for use in HTTP.
107 2. Notational Conventions
109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
111 document are to be interpreted as described in [RFC2119].
113 This specification uses the ABNF (Augmented Backus-Naur Form)
114 notation defined in [RFC5234]. The following core rules are included
115 by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
116 DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
117 (linear white space).
119 Non-ASCII characters used in prose for examples are encoded using the
120 format "Backslash-U with Delimiters", defined in Section 5.1 of
121 [RFC5137].
123 Note that this specification uses the term "character set" for
124 consistency with other IETF specifications such as RFC 2277 (see
125 [RFC2277], Section 3). A more accurate term would be "character
126 encoding" (a mapping of code points to octet sequences).
128 3. A Profile of RFC 2231 for Use in HTTP
130 RFC 2231 defines several extensions to MIME. The sections below
131 discuss if and how they apply to HTTP.
133 In short:
135 o Parameter Continuations aren't needed (Section 3.1),
137 o Character Set and Language Information are useful, therefore a
138 simple subset is specified (Section 3.2), and
140 o Language Specifications in Encoded Words aren't needed
141 (Section 3.3).
143 3.1. Parameter Continuations
145 Section 3 of [RFC2231] defines a mechanism that deals with the length
146 limitations that apply to MIME headers. These limitations do not
147 apply to HTTP ([RFC2616], Section 19.4.7).
149 Thus in HTTP, senders MUST NOT use parameter continuations, and
150 therefore recipients do not need to support them.
152 3.2. Parameter Value Character Set and Language Information
154 Section 4 of [RFC2231] specifies how to embed language information
155 into parameter values, and also how to encode non-ASCII characters,
156 dealing with restrictions both in MIME and HTTP header parameters.
158 However, RFC 2231 does not specify a mandatory-to-implement character
159 encoding, making it hard for senders to decide which character set to
160 use. Thus, recipients implementing this specification MUST support
161 the character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
163 Furthermore, RFC 2231 allows leaving out the character encoding
164 information. The profile defined by this specification does not
165 allow that.
167 The syntax for parameters is defined in Section 3.6 of [RFC2616]
168 (with RFC 2616 implied LWS translated to RFC 5234 LWSP):
170 parameter = attribute LWSP "=" LWSP value
172 attribute = token
173 value = token / quoted-string
175 quoted-string =
176 token =
178 This specification extends the grammar to:
180 parameter = reg-parameter / ext-parameter
182 reg-parameter = attribute LWSP "=" LWSP value
184 ext-parameter = attribute "*" LWSP "=" LWSP ext-value
186 ext-value = charset "'" [ language ] "'" value-chars
187 ; extended-initial-value,
188 ; defined in [RFC2231], Section 7
190 charset = %x55.54.46.2D.38 ; "UTF-8"
191 / %x49.53.4F.2D.38.38.35.39.2D.31 ; "ISO-8859-1"
192 / ext-charset
194 ext-charset = token ; see IANA charset registry
195 ; ()
197 language =
199 value-chars = *( pct-encoded / attr-char )
201 pct-encoded = "%" HEXDIG HEXDIG
202 ; see [RFC3986], Section 2.1
204 attr-char = ALPHA / DIGIT
205 / "-" / "." / "_" / "~" / ":"
206 / "!" / "$" / "&" / "+"
208 Thus, a parameter is either regular parameter (reg-parameter), as
209 previously defined in Section 3.6 of [RFC2616], or an extended
210 parameter (ext-parameter).
212 Extended parameters are those where the left hand side of the
213 assignment ends with an asterisk character.
215 The value part of an extended parameter (ext-value) is a token that
216 consists of three parts: the REQUIRED character set name (charset),
217 the OPTIONAL language information (language), and a a character
218 sequence representing the actual value (value-chars), separated by
219 single quote characters.
221 Inside the value part, characters not contained in attr-char are
222 encoded into an octet sequence using the specified character set.
223 That octet sequence then is percent-encoded as specified in Section
224 2.1 of [RFC3986].
226 Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
227 or ISO-8859-1 ([ISO-8859-1]). Extension character sets (ext-charset)
228 are reserved for future use.
230 3.2.1. Examples
232 Non-extended notation, using "token":
234 foo: bar; title=Economy
236 Non-extended notation, using "quoted-string":
238 foo: bar; title="US-$ rates"
240 Extended notation, using the unicode character \u'00A3' (POUND SIGN):
242 foo: bar; title*=iso-8859-1'en'%A3%20rates
244 Note: the Unicode pound sign character \u'00A3' was encoded using
245 ISO-8859-1 into the single octet A3, then percent-encoded. Also note
246 that the space character was encoded as %20, as it is not contained
247 in attr-char.
249 Extended notation, using the unicode characters \u'00A3' (POUND SIGN)
250 and \u'20AC' (EURO SIGN):
252 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
254 Note: the unicode pound sign character \u'00A3' was encoded using
255 UTF-8 into the octet sequence C2 A3, then percent-encoded. Likewise,
256 the unicode euro sign character \u'20AC' was encoded into the octet
257 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
258 allows both lower-case and upper-case character, so recipients must
259 understand both, and that the language information is optional, while
260 the character set is not.
262 3.3. Language specification in Encoded Words
264 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
265 also support language specification in encoded words. Although the
266 HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
267 2.2), it's not clear to which header field exactly it applies, and
268 whether it is implemented in practice (see
269 for details).
271 Thus, the RFC 2231 profile defined by this specification does not
272 include this feature.
274 4. Guidelines for Usage in HTTP Header Definitions
276 Specifications of HTTP headers that use the extensions defined in
277 Section 3.2 should clearly state that. A simple way to achieve this
278 is to normatively reference this specification, and to include the
279 ext-value production into the ABNF for that header.
281 For instance:
283 foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
284 title-param = "title" LWSP "=" LWSP value
285 / "title*" LWSP "=" LWSP ext-value
286 ext-value =
288 [[rfcno: Note to RFC Editor: in the figure above, please replace
289 "xxxx" by the RFC number assigned to this specification.]]
291 4.1. When to Use the Extension
293 Section 4.2 of [RFC2277] requires that protocol elements containing
294 text can carry language information. Thus, the ext-value production
295 should always be used when the parameter value is of textual nature.
297 Furthermore, the extension should also be used whenever the parameter
298 value needs to carry characters not present in the US-ASCII
299 ([USASCII]) character set (note that it would be unacceptable to
300 define a new parameter that would be restricted to a subset of the
301 Unicode character set).
303 4.2. Error Handling
305 Header specifications that include parameters should also specify
306 whether same-named parameters can occur multiple times. If
307 repetitions are not allowed (and this is believed to be the common
308 case), the specification should state whether regular or the extended
309 syntax takes precedence. In the latter case, this could be used by
310 producers to use both formats without breaking recipients that do not
311 understand the syntax. [[anchor7: Does not work as expected, see
312 and
313 .]]
315 Example:
317 foo: bar; title="EURO exchange rates";
318 title*=utf-8''%e2%82%ac%20exchange%20rates
320 In this case, the sender provides an ASCII version of the title for
321 legacy recipients, but also includes an internationalized version for
322 recipients understanding this specification -- the latter obviously
323 should prefer the new syntax over the old one.
325 5. Security Considerations
327 This document does not discuss security issues and is not believed to
328 raise any security issues not already endemic in HTTP.
330 6. IANA Considerations
332 There are no IANA Considerations related to this specification.
334 7. Acknowledgements
336 Thanks to Frank Ellermann for help figuring out ABNF details.
338 8. References
340 8.1. Normative References
342 [ISO-8859-1]
343 International Organization for Standardization,
344 "Information technology -- 8-bit single-byte coded graphic
345 character sets -- Part 1: Latin alphabet No. 1", ISO/
346 IEC 8859-1:1998, 1998.
348 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
349 Requirement Levels", BCP 14, RFC 2119, March 1997.
351 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
352 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
353 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
355 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
356 10646", RFC 3629, STD 63, November 2003.
358 [RFC4646] Phillips, A. and M. Davis, "Tags for Identifying
359 Languages", BCP 47, RFC 4646, September 2006.
361 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
362 Specifications: ABNF", STD 68, RFC 5234, January 2008.
364 8.2. Informative References
366 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
367 Part Three: Message Header Extensions for Non-ASCII Text",
368 RFC 2047, November 1996.
370 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
371 Word Extensions:
372 Character Sets, Languages, and Continuations", RFC 2231,
373 November 1997.
375 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
376 Languages", BCP 18, RFC 2277, January 1998.
378 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
379 Resource Identifier (URI): Generic Syntax", RFC 3986,
380 STD 66, January 2005.
382 [RFC5137] Klensin, J., "ASCII Escaping of Unicode Characters",
383 BCP 137, RFC 5137, February 2008.
385 [USASCII] American National Standards Institute, "Coded Character
386 Set -- 7-bit American Standard Code for Information
387 Interchange", ANSI X3.4, 1986.
389 URIs
391 [1]
393 [2]
395 Appendix A. Change Log (to be removed by RFC Editor before publication)
397 A.1. Since draft-reschke-rfc2231-in-http-00
399 Use RFC5234-style ABNF, closer to the one used in RFC 2231.
401 Make RFC 2231 dependency informative, so this specification can
402 evolve independantly.
404 Explain the ABNF in prose.
406 Author's Address
408 Julian F. Reschke
409 greenbytes GmbH
410 Hafenweg 16
411 Muenster, NW 48155
412 Germany
414 Email: julian.reschke@greenbytes.de
415 URI: http://greenbytes.de/tech/webdav/