Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track December 30, 2008
Expires: July 3, 2009
Application of RFC 2231 Encoding to
Hypertext Transfer Protocol (HTTP) Headers
draft-reschke-rfc2231-in-http-01
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Abstract
By default, message header parameters in Hypertext Transfer Protocol
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(HTTP) messages can not carry characters outside the ISO-8859-1
character set. RFC 2231 defines an escaping mechanism for use in
Multipurpose Internet Mail Extensions (MIME) headers. This document
specifies a profile of that encoding suitable for use in HTTP.
Editorial Note (To be removed by RFC Editor before publication)
There are multiple HTTP headers that already use RFC 2231 encoding in
practice (Content-Disposition) or might use it in the future (Link).
The purpose of this document is to provide a single place where the
generic aspects of RFC 2231 encoding in HTTP headers are defined.
Distribution of this document is unlimited. Although this is not a
work item of the HTTPbis Working Group, comments should be sent to
the Hypertext Transfer Protocol (HTTP) mailing list at
ietf-http-wg@w3.org [1], which may be joined by sending a message
with subject "subscribe" to ietf-http-wg-request@w3.org [2].
Discussions of the HTTPbis Working Group are archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
XML versions, latest edits and the issues list for this document are
available from
<http://greenbytes.de/tech/webdav/#draft-reschke-rfc2231-in-http>. A
collection of test cases is available at
<http://greenbytes.de/tech/tc2231/>.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4
3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 5
3.2. Parameter Value Character Set and Language Information . . 5
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Language specification in Encoded Words . . . . . . . . . 7
4. Guidelines for Usage in HTTP Header Definitions . . . . . . . 8
4.1. When to Use the Extension . . . . . . . . . . . . . . . . 8
4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 10
A.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
By default, message header parameters in HTTP ([RFC2616]) messages
can not carry characters outside the ISO-8859-1 character set
([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an escaping mechanism
for use in MIME headers. This document specifies a profile of that
encoding for use in HTTP.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses the ABNF (Augmented Backus-Naur Form)
notation defined in [RFC5234]. The following core rules are included
by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
(linear white space).
Non-ASCII characters used in prose for examples are encoded using the
format "Backslash-U with Delimiters", defined in Section 5.1 of
[RFC5137].
Note that this specification uses the term "character set" for
consistency with other IETF specifications such as RFC 2277 (see
[RFC2277], Section 3). A more accurate term would be "character
encoding" (a mapping of code points to octet sequences).
3. A Profile of RFC 2231 for Use in HTTP
RFC 2231 defines several extensions to MIME. The sections below
discuss if and how they apply to HTTP.
In short:
o Parameter Continuations aren't needed (Section 3.1),
o Character Set and Language Information are useful, therefore a
simple subset is specified (Section 3.2), and
o Language Specifications in Encoded Words aren't needed
(Section 3.3).
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3.1. Parameter Continuations
Section 3 of [RFC2231] defines a mechanism that deals with the length
limitations that apply to MIME headers. These limitations do not
apply to HTTP ([RFC2616], Section 19.4.7).
Thus in HTTP, senders MUST NOT use parameter continuations, and
therefore recipients do not need to support them.
3.2. Parameter Value Character Set and Language Information
Section 4 of [RFC2231] specifies how to embed language information
into parameter values, and also how to encode non-ASCII characters,
dealing with restrictions both in MIME and HTTP header parameters.
However, RFC 2231 does not specify a mandatory-to-implement character
encoding, making it hard for senders to decide which character set to
use. Thus, recipients implementing this specification MUST support
the character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
Furthermore, RFC 2231 allows leaving out the character encoding
information. The profile defined by this specification does not
allow that.
The syntax for parameters is defined in Section 3.6 of [RFC2616]
(with RFC 2616 implied LWS translated to RFC 5234 LWSP):
parameter = attribute LWSP "=" LWSP value
attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
token = <token, defined in [RFC2616], Section 2.2>
This specification extends the grammar to:
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parameter = reg-parameter / ext-parameter
reg-parameter = attribute LWSP "=" LWSP value
ext-parameter = attribute "*" LWSP "=" LWSP ext-value
ext-value = charset "'" [ language ] "'" value-chars
; extended-initial-value,
; defined in [RFC2231], Section 7
charset = %x55.54.46.2D.38 ; "UTF-8"
/ %x49.53.4F.2D.38.38.35.39.2D.31 ; "ISO-8859-1"
/ ext-charset
ext-charset = token ; see IANA charset registry
; (<http://www.iana.org/assignments/character-sets>)
language = <Language-Tag, defined in [RFC4646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "-" / "." / "_" / "~" / ":"
/ "!" / "$" / "&" / "+"
Thus, a parameter is either regular parameter (reg-parameter), as
previously defined in Section 3.6 of [RFC2616], or an extended
parameter (ext-parameter).
Extended parameters are those where the left hand side of the
assignment ends with an asterisk character.
The value part of an extended parameter (ext-value) is a token that
consists of three parts: the REQUIRED character set name (charset),
the OPTIONAL language information (language), and a a character
sequence representing the actual value (value-chars), separated by
single quote characters.
Inside the value part, characters not contained in attr-char are
encoded into an octet sequence using the specified character set.
That octet sequence then is percent-encoded as specified in Section
2.1 of [RFC3986].
Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
or ISO-8859-1 ([ISO-8859-1]). Extension character sets (ext-charset)
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are reserved for future use.
3.2.1. Examples
Non-extended notation, using "token":
foo: bar; title=Economy
Non-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"
Extended notation, using the unicode character \u'00A3' (POUND SIGN):
foo: bar; title*=iso-8859-1'en'%A3%20rates
Note: the Unicode pound sign character \u'00A3' was encoded using
ISO-8859-1 into the single octet A3, then percent-encoded. Also note
that the space character was encoded as %20, as it is not contained
in attr-char.
Extended notation, using the unicode characters \u'00A3' (POUND SIGN)
and \u'20AC' (EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
Note: the unicode pound sign character \u'00A3' was encoded using
UTF-8 into the octet sequence C2 A3, then percent-encoded. Likewise,
the unicode euro sign character \u'20AC' was encoded into the octet
sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
allows both lower-case and upper-case character, so recipients must
understand both, and that the language information is optional, while
the character set is not.
3.3. Language specification in Encoded Words
Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
also support language specification in encoded words. Although the
HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
2.2), it's not clear to which header field exactly it applies, and
whether it is implemented in practice (see
<http://tools.ietf.org/wg/httpbis/trac/ticket/111> for details).
Thus, the RFC 2231 profile defined by this specification does not
include this feature.
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4. Guidelines for Usage in HTTP Header Definitions
Specifications of HTTP headers that use the extensions defined in
Section 3.2 should clearly state that. A simple way to achieve this
is to normatively reference this specification, and to include the
ext-value production into the ABNF for that header.
For instance:
foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFCxxxx, Section 3.2>
[[rfcno: Note to RFC Editor: in the figure above, please replace
"xxxx" by the RFC number assigned to this specification.]]
4.1. When to Use the Extension
Section 4.2 of [RFC2277] requires that protocol elements containing
text can carry language information. Thus, the ext-value production
should always be used when the parameter value is of textual nature.
Furthermore, the extension should also be used whenever the parameter
value needs to carry characters not present in the US-ASCII
([USASCII]) character set (note that it would be unacceptable to
define a new parameter that would be restricted to a subset of the
Unicode character set).
4.2. Error Handling
Header specifications that include parameters should also specify
whether same-named parameters can occur multiple times. If
repetitions are not allowed (and this is believed to be the common
case), the specification should state whether regular or the extended
syntax takes precedence. In the latter case, this could be used by
producers to use both formats without breaking recipients that do not
understand the syntax. [[anchor7: Does not work as expected, see
<http://greenbytes.de/tech/tc2231/#attfnboth> and
<http://greenbytes.de/tech/tc2231/#attfnboth2>.]]
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20rates
In this case, the sender provides an ASCII version of the title for
legacy recipients, but also includes an internationalized version for
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recipients understanding this specification -- the latter obviously
should prefer the new syntax over the old one.
5. Security Considerations
This document does not discuss security issues and is not believed to
raise any security issues not already endemic in HTTP.
6. IANA Considerations
There are no IANA Considerations related to this specification.
7. Acknowledgements
Thanks to Frank Ellermann for help figuring out ABNF details.
8. References
8.1. Normative References
[ISO-8859-1]
International Organization for Standardization,
"Information technology -- 8-bit single-byte coded graphic
character sets -- Part 1: Latin alphabet No. 1", ISO/
IEC 8859-1:1998, 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, STD 63, November 2003.
[RFC4646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 4646, September 2006.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
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8.2. Informative References
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
STD 66, January 2005.
[RFC5137] Klensin, J., "ASCII Escaping of Unicode Characters",
BCP 137, RFC 5137, February 2008.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
URIs
[1] <mailto:ietf-http-wg@w3.org>
[2] <mailto:ietf-http-wg-request@w3.org?subject=subscribe>
Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-reschke-rfc2231-in-http-00
Use RFC5234-style ABNF, closer to the one used in RFC 2231.
Make RFC 2231 dependency informative, so this specification can
evolve independantly.
Explain the ABNF in prose.
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Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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