| Network Working Group | J. Reschke |
| Internet-Draft | greenbytes |
| Intended status: Standards Track | April 30, 2010 |
| Expires: November 1, 2010 |
By default, message header field parameters in Hypertext Transfer Protocol (HTTP) messages can not carry characters outside the ISO-8859-1 character set. RFC 2231 defines an <ed:replace>
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There are multiple HTTP header fields 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 header fields 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, which may be joined by sending a message with subject "subscribe" to ietf-http-wg-request@w3.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 field parameters in HTTP ([RFC2616]) messages can not carry characters outside the ISO-8859-1 character set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an <ed:replace>
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).¶
RFC 2231 defines several extensions to MIME. The sections below discuss if and how they apply to HTTP<ed:replace>
In short: ¶
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<ed:replace>
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 set, 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 set information. The <ed:replace>
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 modifies the grammar to:¶
parameter = reg-parameter / ext-parameter
reg-parameter = parmname LWSP "=" LWSP value
ext-parameter = parmname "*" LWSP "=" LWSP ext-value
parmname = 1*attr-char
ext-value = charset "'" [ language ] "'" value-chars
; extended-initial-value,
; defined in [RFC2231], Section 7
charset = "UTF-8" / "ISO-8859-1" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in Section 2.3 of [RFC2978]
; except that the single quote is not included
; SHOULD be registered in the IANA charset registry
language = <Language-Tag, defined in [RFC5646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
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 character sequence representing the actual value (value-chars), separated by single quote characters. Note that both character set names and language tags are restricted to the US-ASCII character set, and are matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646], Section 2.1.1).¶
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 <ed:replace>
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>
In order to include character set and language information, this specification modifies the RFC 2616 grammar to:¶
parameter = reg-parameter / ext-parameter
reg-parameter = parmname LWSP "=" LWSP value
ext-parameter = parmname "*" LWSP "=" LWSP ext-value
parmname = 1*attr-char
ext-value = charset "'" [ language ] "'" value-chars
; like RFC 2231's <extended-initial-value>
; (see [RFC2231], Section 7)
charset = "UTF-8" / "ISO-8859-1" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in Section 2.3 of [RFC2978]
; except that the single quote is not included
; SHOULD be registered in the IANA charset registry
language = <Language-Tag, defined in [RFC5646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
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 character sequence representing the actual value (value-chars), separated by single quote characters. Note that both character set names and language tags are restricted to the US-ASCII character set, and are matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646], Section 2.1.1).¶
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 <ed:replace>
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, <ed:replace>
Specifications of HTTP header fields that use the extensions defined in Section 3.2 <ed:replace>
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.] ¶
Section 4.2 of [RFC2277] requires that protocol elements containing <ed:replace>
Furthermore, the extension <ed:replace>
Header field specifications need to define whether multiple instances of parameters with identical parmname components are allowed, and how they should processed. <ed:replace>
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 recipients understanding this specification -- the latter obviously <ed:replace>
The format described in this document makes it possible to transport non-ASCII characters, and thus enables character "spoofing" scenarios, in which a displayed value appears to be something other than it is.¶
Furthermore, there are known attack scenarios relating to decoding UTF-8.¶
See Section 10 of [RFC3629] for more information on both topics.¶
In addition, the extension specified in this document makes it possible to transport multiple language variants for a single parameter, and such use might allow spoofing attacks, where different language versions of the same parameter are not equivalent. Whether this attack is useful as an attack depends on the parameter specified.¶
There are no IANA Considerations related to this specification.¶
Thanks to Martin Duerst and Frank Ellermann for help figuring out ABNF details, to Graham Klyne and Alexey Melnikov for general review, Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle and Roar Lauritzsen for implementer's feedback.¶
Problems with the internationalization of the HTTP Content-Disposition header field have been known for many years (see test cases at <http://greenbytes.de/tech/tc2231/>).¶
During IETF 72 (<http://tools.ietf.org/wg/httpbis/minutes?item=minutes72.html>), the HTTPbis Working Group shortly discussed how to deal with the underspecification of (1) Content-Disposition, and its (2) internationalization aspects. Back then, there was rough consensus in the room to move the definition into a separate draft.¶
This specification addresses problem (2), by defining a simple subset of the encoding format defined in RFC 2231. A separate specification, draft-reschke-rfc2183-in-http, is planned to address problem (1). Note that this approach was chosen because Content-Disposition is just an example for an HTTP header field using this kind of encoding. Another example is the currently proposed Link header field (draft-nottingham-http-link-header).¶
This document is planned to be published on the IETF Standards Track, so that other standards-track level documents can depend on it, such as the new specification of Content-Disposition, or potentially future revisions of the HTTP Link Header specification.¶
Also note that this document specifies a proper subset of the extensions defined in RFC 2231, but does not normatively refer to it. Thus, RFC 2231 can be revised separately, should the email community decide to.¶
Use RFC5234-style ABNF, closer to the one used in RFC 2231.¶
Make RFC 2231 dependency informative, so this specification can evolve independently.¶
Explain the ABNF in prose.¶
Remove unneeded RFC5137 notation (code point vs character).¶
And and resolve issues "charset", "repeats" and "rfc4646".¶
And and resolve issue "charsetmatch".¶
Add and resolve issues "badseq" and "tokenquotcharset".¶
Say "header field" instead of "header" in the context of HTTP.¶
Add an appendix discussing document history and future plans, to be removed before publication.¶
Add and resolve issues "impl" and "rel-2388".¶
Editorial improvements. Add and resolve issues "attrcharvstoken" and "tokengrammar".¶
Add issues "i18n-spoofing", "iso8859", "parameter-abnf", and "when-ext-value". Add and resolve issues "rfc2978-normative", "rfc3986-normative" and "usascii-normative".¶
Resolve issues "i18n-spoofing", "iso8859", "parameter-abnf", and "when-ext-value".¶
Add and resolve issue "charset-registered", "handling-multiple", "multiple-inst-spoofing", "repeated-param" and "value-abnf".¶
Update the KDE implementation note.¶
In the prose in Section 3.2, "ext-charset" -> "mime-charset". In Section 4, avoid the use of "should" and "recommended". In Section 4.1 clarify that the RFC 2277 requirement is about human-readable text. Clarify parts that made it look as if this spec has a normative dependency on RFC 2231 (new issue "<ed:issueref>