|Network Working Group||J. Reschke|
|Intended status: Standards Track||December 30, 2008|
|Expires: July 3, 2009|
Application of RFC 2231 Encoding to Hypertext Transfer Protocol (HTTP) Headers
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By default, message header parameters in Hypertext Transfer Protocol (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.
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 email@example.com, which may be joined by sending a message with subject "subscribe" to firstname.lastname@example.org.
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/>.
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.
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).
RFC 2231 defines several extensions to MIME. The sections below discuss if and how they apply to HTTP.
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.
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:
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 = %x188.8.131.52D.38 ; "UTF-8" / %x49.53.4F.2D.184.108.40.206.2D.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 / "-" / "." / "_" / "~" / ":" / "!" / "$" / "&" / "+"
Thus, a parameter is either regular parameter (reg-parameter), as previously defined in Section 3.6 of [RFC2616], or an extended parameter (ext-parameter).
Extended parameters are those where the left hand side of the assignment ends with an asterisk character.
The value part of an extended parameter (ext-value) is a token that consists of three parts: the REQUIRED character set name (charset), the OPTIONAL language information (language), and a a character sequence representing the actual value (value-chars), separated by single quote characters.
Inside the value part, characters not contained in attr-char are encoded into an octet sequence using the specified character set. That octet sequence then is percent-encoded as specified in Section 2.1 of [RFC3986].
Producers MUST NOT use character sets other than "UTF-8" ([RFC3629]) or ISO-8859-1 ([ISO-8859-1]). Extension character sets (ext-charset) are reserved for future use.
Non-extended notation, using "token":
foo: bar; title=Economy
Non-extended notation, using "quoted-string":
foo: bar; title="US-$ 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.
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.
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.
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.]
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).
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. [rfc.comment.1: Does not work as expected, see <http://greenbytes.de/tech/tc2231/#attfnboth> and <http://greenbytes.de/tech/tc2231/#attfnboth2>.]
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 recipients understanding this specification -- the latter obviously should prefer the new syntax over the old one.
This document does not discuss security issues and is not believed to raise any security issues not already endemic in HTTP.
There are no IANA Considerations related to this specification.
Thanks to Frank Ellermann for help figuring out ABNF details.
|[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.|
|[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.|
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.