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Applications and Real-Time HTTP HTTP header field parameter internationalization 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. Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . Working Group information can be found at and ; source code and issues list for this draft can be found at . The changes in this draft are summarized in .

By default, message header field parameters in HTTP () messages cannot carry characters outside the ISO-8859-1 coded 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 that is compatible with a profile of the encoding defined in RFC 2231. This document obsoletes and moves it to “historic” status; the changes are summarized in . 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 whitespace). This specification uses terminology defined in , namely: “character encoding scheme” (below abbreviated to “character encoding“), “charset” and “coded character set“. Note that this differs from RFC 2231, which uses the term “character set” for “character encoding scheme”.

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 Encoding 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 field parameters. However, RFC 2231 does not specify a mandatory-to-implement character encoding, making it hard for senders to decide which encoding to use. Thus, recipients implementing this specification &MUST; support the “UTF-8” character encoding . Furthermore, RFC 2231 allows the character encoding 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, see > token = <token, see >

In order to include character encoding 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” / mime-charset mime-charset = 1*mime-charsetc mime-charsetc = ALPHA / DIGIT / “!” / “#” / “$” / “%” / “&” / “+” / “-” / “^” / “_” / “`” / “{” / “}” / “~” ; as <mime-charset> in ; except that the single quote is not included ; &SHOULD; be registered in the IANA charset registry language = <Language-Tag, see > value-chars = *( pct-encoded / attr-char ) pct-encoded = “%” HEXDIG HEXDIG ; see attr-char = ALPHA / DIGIT / “!” / “#” / “$” / “&” / “+” / “-” / “.” / “^” / “_” / “`” / “|” / “~” ; 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 encoding 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 encoding names and language tags are restricted to the US-ASCII coded 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 encoding. That octet sequence is then percent-encoded as specified in . Producers &MUST; use the “UTF-8” () character encoding. Extension character encodings (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).

The RFC 7230 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. The <mime-charset> ABNF defined here differs from the one in in that it does not allow the single quote character (see also RFC Errata ID 1912 ). In practice, no character encoding names using that character have been registered at the time of this writing. For backwards compatibility with RFC 2231, the encoding defined by this specification deviates from common parameter syntax in that the quoted-string notation is not allowed. Implementations using generic parser components might not be able to detect the use of quoted-string notation and thus might accept that format, although invalid, as well. did require support for ISO-8859-1, too; for compatibility with legacy code, recipients are encouraged to support this encoding as well.

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 encoding is not.

extends the encoding defined in to also support language specification in encoded words. RFC 2616, the now-obsolete HTTP/1.1 specification, did refer to RFC 2047 (). However, it wasn’t clear to which header field it applied. Consequently, the current revision of the HTTP/1.1 specification has deprecated use of the encoding forms defined in RFC 2047 (see ). 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 RFC 5987, >

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. Note: This specification does not automatically assign a new interpretration to parameter names ending in an asterisk. As pointed out above, it’s up to the specification for the non-extended parameter to “opt in” to the syntax defined here. That being said, some existing implementations are known to automatically switch to the use of this notation when a parameter name ends with an asterisk, thus using parameter names ending in an asterisk for something else is likely to cause interoperability problems.

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 () coded 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.

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.

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This section summarizes the changes compared to : The document title was changed to “Indicating Character Encoding and Language for HTTP Header Field Parameters”. The requirement to support the “ISO-8859-1” encoding was removed.

The encoding defined in this document currently is used for two different HTTP header fields: “Content-Disposition”, defined in , and “Link”, defined in . As the encoding is a profile/clarification of the one defined in in 1997, many user agents already supported it for use in “Content-Disposition” when got published. Since the publication of , three more popular desktop user agents have added support for this encoding; see for details. At this time, the current versions of all major desktop user agents support it. Note that the implementation in Internet Explorer 9 does not support the ISO-8859-1 character 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 . At the time of this writing, no shipping User Agent except Firefox supported the “title*” parameter (starting with release 15).

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”.

Resolved issue “terminology”.

In , pull historical notes into a separate subsection. Resolved issues “valuesyntax” and “parmsyntax”.

Update status of Firefox support in HTTP Link Header field.

Update status of Firefox support in HTTP Link Header field.

Update status with respect to Safari 6. Started work on update with respect to RFC 723x.

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, Eric Lawrence, and James Manger for implementer’s feedback.