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2 Network Working Group J. Reschke
3 Internet-Draft greenbytes
4 Intended status: Standards Track August 15, 2008
5 Expires: February 16, 2009
7 Applicability of RFC 2231 Encoding to
8 Hypertext Transfer Protocol (HTTP) Headers
9 draft-reschke-rfc2231-in-http-00
11 Status of this Memo
13 By submitting this Internet-Draft, each author represents that any
14 applicable patent or other IPR claims of which he or she is aware
15 have been or will be disclosed, and any of which he or she becomes
16 aware will be disclosed, in accordance with Section 6 of BCP 79.
18 Internet-Drafts are working documents of the Internet Engineering
19 Task Force (IETF), its areas, and its working groups. Note that
20 other groups may also distribute working documents as Internet-
21 Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six months
24 and may be updated, replaced, or obsoleted by other documents at any
25 time. It is inappropriate to use Internet-Drafts as reference
26 material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at
29 http://www.ietf.org/ietf/1id-abstracts.txt.
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html.
34 This Internet-Draft will expire on February 16, 2009.
36 Abstract
38 By default, message header parameters in Hypertext Transfer Protocol
39 (HTTP) messages can not carry characters outside the ISO-8859-1
40 character set. RFC 2231 defines an escaping mechanism for use in
41 Multipurpose Internet Mail Extensions (MIME) headers. This document
42 specifies a profile of that encoding suitable for use in HTTP.
44 Editorial Note (To be removed by RFC Editor before publication)
46 There are multiple HTTP headers that already use RFC 2231 encoding in
47 practice (Content-Disposition) or might use it in the future (Link).
48 The purpose of this document is to provide a single place where the
49 generic aspects of RFC 2231 encoding in HTTP headers are defined.
51 Distribution of this document is unlimited. Although this is not a
52 work item of the HTTPbis Working Group, comments should be sent to
53 the Hypertext Transfer Protocol (HTTP) mailing list at
54 ietf-http-wg@w3.org [1], which may be joined by sending a message
55 with subject "subscribe" to ietf-http-wg-request@w3.org [2].
57 Discussions of the HTTPbis Working Group are archived at
58 .
60 XML versions, latest edits and the issues list for this document are
61 available from
62 .
64 Table of Contents
66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
67 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
68 3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 3
69 3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 4
70 3.2. Parameter Value Character Set and Language Information . . 4
71 3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 5
72 3.3. Language specification in Encoded Words . . . . . . . . . 6
73 4. Guidelines for Usage in HTTP Header Definitions . . . . . . . 6
74 4.1. When to Use the Extension . . . . . . . . . . . . . . . . 6
75 4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 7
76 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
77 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
78 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
79 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
80 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
81 8.2. Informative References . . . . . . . . . . . . . . . . . . 8
82 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
83 Intellectual Property and Copyright Statements . . . . . . . . . . 10
85 1. Introduction
87 By default, message header parameters in HTTP ([RFC2616]) messages
88 can not carry characters outside the ISO-8859-1 character set
89 ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping mechanism
90 for use in MIME headers. This document specifies a profile of that
91 encoding for use in HTTP.
93 2. Notational Conventions
95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
97 document are to be interpreted as described in [RFC2119].
99 This specification uses the augmented BNF notation defined in Section
100 2.1 of [RFC2616], including its rules for linear whitespace (LWS).
101 [[LWS: This needs to be checked.]]
103 Non-ASCII characters used in prose for examples are encoded using the
104 format "Backslash-U with Delimiters", defined in Section 5.1 of
105 [RFC5137].
107 Note that this specification uses the term "character set" for
108 consistency with other IETF specifications such as RFC 2277 (see
109 [RFC2277], Section 3). A more accurate term would be "character
110 encoding" (a mapping of code points to octet sequences).
112 3. A Profile of RFC 2231 for Use in HTTP
114 RFC 2231 defines several extensions to MIME. The sections below
115 discuss if and how they apply to HTTP.
117 In short:
119 o Parameter Continuations aren't needed (Section 3.1),
121 o Character Set and Language Information are useful, therefore a
122 simple subset is specified (Section 3.2), and
124 o Language Specifications in Encoded Words aren't needed
125 (Section 3.3).
127 3.1. Parameter Continuations
129 Section 3 of [RFC2231] defines a mechanism that deals with the length
130 limitations that apply to MIME headers. These limitations do not
131 apply to HTTP ([RFC2616], Section 19.4.7).
133 Thus in HTTP, senders MUST NOT use parameter continuations, and
134 therefore recipients do not need to support them.
136 3.2. Parameter Value Character Set and Language Information
138 Section 4 of [RFC2231] specifies how to embed language information
139 into parameter values, and also how to encode non-ASCII characters,
140 dealing with restrictions both in MIME and HTTP header parameters.
142 However, RFC 2231 does not specify mandatory-to-implement character
143 encoding, making it hard for senders to decide which character set to
144 use. Thus, recipients implementing this specification MUST support
145 the character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
147 Furthermore, RFC 2231 allows leaving out the character encoding
148 information. The profile defined by this specification does not
149 allow that.
151 The syntax for parameters is defined in Section 3.6 of [RFC2616]:
153 parameter = attribute "=" value
155 attribute = token
156 value = token | quoted-string
158 quoted-string =
159 token =
161 This specification extends the grammar to:
163 parameter = reg-parameter | ext-parameter
165 reg-parameter = attribute "=" value
166 ext-parameter = attribute "*=" ext-value
168 ext-value = charset "'" [ language ] "'" value-chars
170 charset = "UTF-8" | "ISO-8859-1" | ext-charset
171 ; NOTE: case-insensitive
173 ext-charset = token ; see IANA charset registry
174 ; ()
176 language =
178 value-chars = *( pct-encoded | attr-char )
180 pct-encoded = "%" HEXDIG HEXDIG
182 attr-char = ALPHA | DIGIT
183 | "-" | "." | "_" | "~" | ":"
184 | "!" | "$" | "&" | "+"
186 ALPHA = %x41-5A | %x61-7A
187 ; A-Z | a-z
188 DIGIT = %x30-39
189 ; any US-ASCII digit "0".."9"
190 HEXDIG = DIGIT | "A" | "B" | "C" | "D" | "E" | "F"
191 ; NOTE: case-insensitive
193 3.2.1. Examples
195 Non-extended notation, using "token":
197 foo: bar; title=Economy
199 Non-extended notation, using "quoted-string":
201 foo: bar; title="US-$ rates"
203 Extended notation, using the unicode character \u'00A3' (POUND SIGN):
205 foo: bar; title*=iso-8859-1'en'%A3%20rates
207 Note: the Unicode pound sign character \u'00A3' was encoded using
208 ISO-8859-1 into the single octet A3, then percent-encoded. Also note
209 that the space character was encoded as %20, as attr-char does not
210 contain it.
212 Extended notation, using the unicode characters \u'00A3' (POUND SIGN)
213 and \u'20AC' (EURO SIGN):
215 foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
217 Note: the unicode pound sign character \u'00A3' was encoded using
218 UTF-8 into the octet sequence C2 A3, then percent-encoded. Likewise,
219 the unicode euro sign character \u'20AC' was encoded into the octet
220 sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
221 allows both lower-case and upper-case character, so recipients must
222 understand both, and that the language information is optional, while
223 the character set is not.
225 3.3. Language specification in Encoded Words
227 Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
228 also support language specification in encoded words. Although the
229 HTTP/1.1 does refer to RFC 2047 ([RFC2616], Section 2.2), it's not
230 clear to which header field exactly it applies, and whether it is
231 implemented in practice (see
232 for details).
234 Thus, the RFC 2231 profile defined by this specification does not
235 include this feature.
237 4. Guidelines for Usage in HTTP Header Definitions
239 Specifications of HTTP headers that use the extensions defined in
240 Section 3.2 should clearly state that. The best way to achieve this
241 is to normatively reference this specification, and to include the
242 ext-parameter production into the ABNF for that header.
244 4.1. When to Use the Extension
246 Section 4.2 of [RFC2277] requires that protocol elements containing
247 text can carry language information. Thus, the ext-parameter
248 production should always be used when the parameter value is of
249 textual nature.
251 Furthermore, the extension should also be used whenever the parameter
252 value needs to carry characters not present in the US-ASCII
253 ([USASCII]) character set (note that it would be unacceptable to
254 define a new header that would be restricted to a subset of the
255 Unicode character set).
257 4.2. Error Handling
259 Header specifications that include parameters should also specify
260 whether same-named parameters can occur multiple times. If
261 repetitions are not allowed (and this is believed to be the common
262 case), the specification should state whether regular or the extended
263 syntax takes precedence. In the latter case, this could be used by
264 senders to use both formats without breaking recipients that do not
265 understand the syntax.
267 Example:
269 foo: bar; title="EURO exchange rates";
270 title*=utf-8''%e2%82%ac%20exchange%20rates
272 In this case, the sender provides an ASCII version of the title for
273 legacy recipient, but also includes an internationalized version for
274 recipients understanding this specification -- the latter obviously
275 should prefer the new syntax over the old one.
277 5. Security Considerations
279 This document does not discuss security issues and is not believed to
280 raise any security issues not already endemic in HTTP.
282 6. IANA Considerations
284 There are no IANA Considerations related to this specification.
286 7. Acknowledgements
288 Thanks to Frank Ellermann for help figuring out BNF details.
290 8. References
292 8.1. Normative References
294 [ISO-8859-1]
295 International Organization for Standardization,
296 "Information technology -- 8-bit single-byte coded graphic
297 character sets -- Part 1: Latin alphabet No. 1", ISO/
298 IEC 8859-1:1998, 1998.
300 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
301 Requirement Levels", BCP 14, RFC 2119, March 1997.
303 [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
304 Word Extensions:
305 Character Sets, Languages, and Continuations", RFC 2231,
306 November 1997.
308 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
309 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
310 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
312 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
313 10646", RFC 3629, STD 63, November 2003.
315 [RFC4646] Phillips, A. and M. Davis, "Tags for Identifying
316 Languages", BCP 47, RFC 4646, September 2006.
318 8.2. Informative References
320 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
321 Part Three: Message Header Extensions for Non-ASCII Text",
322 RFC 2047, November 1996.
324 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
325 Languages", BCP 18, RFC 2277, January 1998.
327 [RFC5137] Klensin, J., "ASCII Escaping of Unicode Characters",
328 BCP 137, RFC 5137, February 2008.
330 [USASCII] American National Standards Institute, "Coded Character
331 Set -- 7-bit American Standard Code for Information
332 Interchange", ANSI X3.4, 1986.
334 URIs
336 [1]
338 [2]
340 Author's Address
342 Julian F. Reschke
343 greenbytes GmbH
344 Hafenweg 16
345 Muenster, NW 48155
346 Germany
348 Email: julian.reschke@greenbytes.de
349 URI: http://greenbytes.de/tech/webdav/
351 Full Copyright Statement
353 Copyright (C) The IETF Trust (2008).
355 This document is subject to the rights, licenses and restrictions
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