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HTTP header field parameter internationalization 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 encoding mechanism for use in Multipurpose Internet Mail Extensions (MIME) headers. This document specifies an encoding suitable for use in HTTP header fields whichthat is compatible towith a profile of the encoding defined in RFC 2231. 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 . XML versions, latest edits and the issues list for this document are available from . A collection of test cases is available at . Note: as of February 2010, there were at least three independent implementations of the encoding defined in : Konqueror (starting with 4.4.1), Mozilla Firefox, and Opera. Umbrella issue for changes made during the RFC Editor’s AUTH48 period.

By default, message header field parameters in HTTP () messages can not carry characters outside the ISO-8859-1 character set (). RFC 2231 () defines an encoding mechanism for use in MIME headers. This document specifies an encoding suitable for use in HTTP header fields whichthat is compatible towith a profile of the encoding defined in RFC 2231. Note: in the remainder of this document, RFC 2231 is only referenced for the purpose of explaining the choice of features that were adopted; they are therefore purely informative. Note: this encoding does not apply to message payloads transmitted over HTTP, such as when using the media type “multipart/form-data” ().

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 . This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in . The following core rules are included by reference, as defined in : ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP (linear white space). Note that this specification uses the term “character set” for consistency with other IETF specifications such as RFC 2277 (see ). 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: Parameter Continuations aren’t needed (), Character Set and Language Information are useful, therefore a simple subset is specified (), and Language Specifications in Encoded Words aren’t needed ().

defines a mechanism that deals with the length limitations that apply to MIME headers. These limitations do not apply to HTTP (). Thus, parameter continuations are not part of the encoding defined by this specification.

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” and “UTF-8” . Furthermore, RFC 2231 allows leaving out the character set informationthe character set information to be left out. The encoding defined by this specification does not allow that.

The syntax for parameters is defined in (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 > token = <token, defined in >

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 ) charset = “UTF-8” / “ISO-8859-1” / mime-charset mime-charset = 1*mime-charsetc mime-charsetc = ALPHA / DIGIT / “!” / “#” / “$” / “%” / “&” / “+” / “-” / “^” / “_” / “`” / “{” / “}” / “~” ; as <mime-charset> in ; except that the single quote is not included ; &SHOULD; be registered in the IANA charset registry language = <Language-Tag, defined in > value-chars = *( pct-encoded / attr-char ) pct-encoded = “%” HEXDIG HEXDIG ; see attr-char = ALPHA / DIGIT / “!” / “#” / “$” / “&” / “+” / “-” / “.” / “^” / “_” / “`” / “|” / “~” ; token except ( “*” / “‘” / “%” )

Thus, a parameter is either a regular parameter (reg-parameter), as previously defined in , 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 and ). 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 isis then percent-encoded as specified in . Producers &MUST; use either the “UTF-8” () or the “ISO-8859-1” () character set. Extension character sets (mime-charset) are reserved for future use. Note: recipients should be prepared to handle encoding errors, such as malformed or incomplete percent escape sequences, or non-decodable octet sequences, in a robust manner. This specification does not mandate any specific behavior, for instance, the following strategies are all acceptable: ignoring the parameter, stripping a non-decodable octet sequence, substituting a non-decodable octet sequence by a replacement character, such as the Unicode character U+FFFD (Replacement Character). Note: the RFC 2616 token production () differs from the production used in RFC 2231 (imported from ) in that curly braces (“{” and “}”) are excluded. Thus, these two characters are excluded from the attr-char production as well. Note: the <mime-charset> ABNF defined here differs from the one in in that it does not allow the single quote character (see also RFC Editor Errata ID 1912RFC Errata ID 1912 ). In practice, no character set names using that character have been registered at the time of this writing.

Non-extended notation, using “token”: foo: bar; title=Economy

Non-extended notation, using “quoted-string”: foo: bar; title=”US-$ rates”

Extended notation, using the uUnicode 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 A3into the single octet A3 using the ISO-8859-1 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 uUnicode 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 uUnicode pound sign character U+00A3 was encoded using UTF-8 into the octet sequence C2 A3into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Likewise, the uUnicode 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 characters, so recipients must understand both, and that the language information is optional, while the character set is not.

extends the encoding defined in to also support language specification in encoded words. Although the HTTP/1.1 specification does refer to RFC 2047 (), it’s not clear to which header field exactly it applies, and whether it is implemented in practice (see for details). Thus, this specification does not include this feature.

Specifications of HTTP header fields that use the extensions defined in ought to 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 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 RFCxxxx 5987, >

Note to RFC Editor: in the figure above, please replace “xxxx” by the RFC number assigned to this specification. Note: The Parameter Value Continuation feature defined in makes it impossible to have multiple instances of extended parameters with identical parmname components, as the processing of continuations would become ambiguous. Thus, specifications using this extension are advised to disallow this case for compatibility with RFC 2231.

requires that protocol elements containing human-readable text are able to carry language information. Thus, the ext-value 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 () 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 could be used bywould 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.

Note: at the time of this writing, many implementations failed to ignore the form they do not understand, or prioritize the ASCII form although the extended syntax was present.

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 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, to Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle and Roar Lauritzsen for implementer’s feedback.

Harvard University

sob@harvard.edu

University of California, Irvine

fielding@ics.uci.edu

W3C

jg@w3.org

Compaq Computer Corporation

mogul@wrl.dec.com

MIT Laboratory for Computer Science

frystyk@w3.org

Xerox Corporation

masinter@parc.xerox.com

Microsoft Corporation

paulle@microsoft.com

W3C

timbl@w3.org

Alis Technologies

fyergeau@alis.com

World Wide Web Consortium

timbl@w3.org http://www.w3.org/People/Berners-Lee/

Day Software

fielding@gbiv.com http://roy.gbiv.com/

Adobe Systems Incorporated

LMM@acm.org http://larry.masinter.net/

Lab126

addison@inter-locale.com

Google

mark.davis@google.com

Brandenburg InternetWorking

+1.408.246.8253 dcrocker@bbiw.net

THUS plc.

paul.overell@thus.net

International Organization for Standardization American National Standards Institute RFC Errata Innosoft International, Inc.

ned@innosoft.com

First Virtual Holdings

nsb@nsb.fv.com

University of Tennessee

moore@cs.utk.edu

Innosoft International, Inc.

ned.freed@innosoft.com

University of Tennessee

moore@cs.utk.edu

UNINETT

Harald.T.Alvestrand@uninett.no

Xerox Palo Alto Research Center

masinter@parc.xerox.com

Problems with the internationalization of the HTTP Content-Disposition header field have been known for many years (see test cases at ). During IETF 72 (), 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 , “ext-charset” -> “mime-charset”. In , avoid the use of “should” and “recommended”. In 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 “nonorm2231”).

Include RFC-Editor’s AUTH48 changes.