| Network Working Group | J. F. Reschke |
| Internet-Draft | greenbytes |
| Obsoletes: 5987 (if approved) | November 21, 2011 |
| Intended status: Standards Track | |
| Expires: May 24, 2012 |
Indicating Character Encoding and Language for HTTP Header Field Parameters
draft-reschke-rfc5987bis-02
By default, message header field parameters in Hypertext Transfer Protocol (HTTP) messages cannot carry characters outside the ISO-8859-1 character set. RFC 2231 defines an encoding mechanism for use in Multipurpose Internet Mail Extensions (MIME) headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.
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-rfc5987bis. A collection of test cases is available at http://greenbytes.de/tech/tc2231/.
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By default, message header field parameters in HTTP ([RFC2616]) messages cannot carry characters outside the ISO-8859-1 character set ([ISO-8859-1]). RFC 2231 ([RFC2231]) defines an encoding mechanism for use in MIME headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.
This document obsoletes [RFC5987] and moves it to "historic" status; the changes are summarized in Appendix Appendix A.
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 whitespace).
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 header fields.
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, parameter continuations are not part of the encoding defined by this specification.
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 "UTF-8" character set [RFC3629].
Furthermore, RFC 2231 allows the character set information to be left out. The encoding 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>
In order to include character set and language information, this specification modifies the RFC 2616 grammar to be:
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" / 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 a 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 is then percent-encoded as specified in Section 2.1 of [RFC3986].
Producers MUST use the "UTF-8" ([RFC3629]) character set. Extension character sets (mime-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*=utf-8'en'%C2%A3%20rates
Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, 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 into the octet sequence C2 A3 using the UTF-8 character encoding, 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 lowercase and uppercase characters, 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, this specification does not include this feature.
Specifications of HTTP header fields that use the extensions defined in Section 3.2 ought to clearly state that. A simple way to achieve this is to normatively reference this specification, and to include the ext-value [bnf] production into the ABNF for that header field.
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 RFC 5987, Section 3.2>
Section 4.2 of [RFC2277] requires that protocol elements containing human-readable text are able to carry language information. Thus, the ext-value [bnf] production ought to be always used when the parameter value is of textual nature and its language is known.
Furthermore, the extension ought to 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 field specifications need to define whether multiple instances of parameters with identical parmname components are allowed, and how they should be processed. This specification suggests that a parameter using the extended syntax takes precedence. This would allow producers to use both formats without breaking recipients that do not understand the extended syntax yet.
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 ought to prefer the new syntax over the old one.
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.
Thanks to Martin Duerst and Frank Ellermann for help figuring out ABNF details, to Graham Klyne and Alexey Melnikov for general review, to Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle, Roar Lauritzsen, and Eric Lawrence for implementer's feedback.
This section summarizes the changes compared to [RFC5987]:
The encoding defined in this document currently is used for two different HTTP header fields:
As the encoding is a profile/clarification of the one defined in [RFC2231] in 1997, many user agents already supported it for use in "Content-Disposition" when [RFC5987] got published.
Since the publication of [RFC5987], two more popular desktop user agents have added support for this encoding; see http://purl.org/NET/http/content-disposition-tests#encoding-2231-char for details. At this time, only one major desktop user agent (Safari) does not support it.
Note that the implementation in Internet Explorer 9 does not support the ISO-8859-1 encoding; this document revision acknowledges that UTF-8 is sufficient for expressing all code points, and removes the requirement to support ISO-8859-1.
The "Link" header field, on the other hand, was only recently specified in [RFC5988]. At the time of this writing, no User Agent supported the "title*" parameter, using the encoding defined by this document, but implementation for Firefox was already in progress (see https://bugzilla.mozilla.org/show_bug.cgi?id=663057).
Only editorial changes for the purpose of starting the revision process (obs5987).
Resolved issues "iso-8859-1" and "title" (title simplified). Added and resolved issue "historic5987".
Added issues "httpbis", "parmsyntax", "terminology" and "valuesyntax". Closed issue "impls".
Issues that were either rejected or resolved in this version of this document.
Type: change
julian.reschke@greenbytes.de (2011-04-15): Add implementation report.
Type: edit
julian.reschke@greenbytes.de (2011-04-15): Umbrella issue for editorial fixes/enhancements.
Type: edit
julian.reschke@greenbytes.de (2011-09-17): Try to be consistent with the terminology defined in RFC 6365.
Type: edit
<http://lists.w3.org/Archives/Public/ietf-http-wg/2011OctDec/0159.html>
James.H.Manger@team.telstra.com (2011-11-02): Noted by James Manger: "Presumably RFC5987 (or its predecessors) decided it was highly unlikely that any parameter names in use ended in "*" (though they are valid) so it could redefine the syntax of values for such names." - add a note that the notation indeed overloads parameter name syntax and clarify the use.
Type: edit
<http://lists.w3.org/Archives/Public/ietf-http-wg/2011OctDec/0159.html>
James.H.Manger@team.telstra.com (2011-11-02): Noted by James Manger: "Curiously, RFC5987 disobeys the proposed recommendations for new parameters. It allows foo*=UTF-8''coll%C3%A8gues but not foo*="UTF-8''coll%C3%A8gues" That might be ok with a parser that understands token, quoted-string, and RFC5987, but presumably it will cause problems when RFC5987 processing is done after a "standard httpbis parser" handles the token | quoted-string step. " - add a note clarifying that this is indeed a shortcoming of the format, and what it means for implementations.
Type: edit
julian.reschke@greenbytes.de (2011-09-17): The document refers normatively to RFC 2616. Should it continue to do so, or should we wait for HTTPbis? This may affect edge case in the ABNF, such as the definition of linear white space or the characters allowed in "token".