greenbytes GmbH

Hafenweg 16 MünsterNW48155 Germany julian.reschke@greenbytes.de http://greenbytes.de/tech/webdav/

Applications and Real-Time HTTP Working Group HTTP header field parameter internationalization By default, header field values in Hypertext Transfer Protocol (HTTP) messages cannot easily carry characters outside the US-ASCII-coded character set. RFC 2231 defines an encoding mechanism for use in parameters inside Multipurpose Internet Mail Extensions (MIME) header field values. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231. This document obsoletes RFC 5987.

Use of characters outside the US-ASCII-coded character set () in HTTP header fields () is non-trivial: The HTTP specification discourages use of non-US-ASCII characters in field values, placing them into the "obs-text" Augmented Backus-Naur Form (ABNF) production (, Section 3.2). Furthermore, the specification is silent about default character encoding schemes for field values, so any use of non-US-ASCII characters would need to be specific to the field definition or would require some other kind of out-of-band information. Finally, some APIs assume a default character encoding scheme in order to map from the octet sequences (obtained from the HTTP message) to character sequences: for instance, the XMLHttpRequest API () uses the Interface Definition Language type "ByteString", effectively resulting in the ISO-8859-1 character encoding scheme being used. On the other hand, RFC 2231 defines an encoding mechanism for parameters inside MIME header fields (), which, as opposed to HTTP messages, do need to be sent over non-binary transports. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a simplified profile of the encoding defined in RFC 2231. It can be applied to any HTTP header field that uses the common "parameter" ("name=value") syntax. 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; therefore, they are 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", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This specification uses the ABNF notation defined in . The following core rules are included by reference, as defined in , Appendix B.1: 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" (abbreviated to "character encoding" below), "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 ().

Section 3 of defines a mechanism that deals with the length limitations that apply to MIME headers. These limitations do not apply to HTTP (, Appendix A.6). Thus, parameter continuations are not part of the encoding defined by this specification.

Section 4 of 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 presence of extended parameter values is usually indicated by a parameter name ending in an asterisk character. Note, however, that this is just a convention, and that it needs to be explicitly specified in the definition of the header field using this extension (see ). The ABNF for extended parameter values is specified below:

; (see [RFC2231], Section 7) charset = "UTF-8" / mime-charset mime-charset = 1*mime-charsetc mime-charsetc = ALPHA / DIGIT / "!" / "#" / "$" / "%" / "&" / "+" / "-" / "^" / "_" / "`" / "{" / "}" / "~" ; as in Section 2.3 of [RFC2978] ; except that the single quote is not included ; SHOULD be registered in the IANA charset registry language = value-chars = *( pct-encoded / attr-char ) pct-encoded = "%" HEXDIG HEXDIG ; see [RFC3986], Section 2.1 attr-char = ALPHA / DIGIT / "!" / "#" / "$" / "&" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" ; token except ( "*" / "'" / "%" ) ]]>

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 Section 2.3 of and Section 2.1.1 of ). 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 Section 2.1 of . 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, and substituting a non-decodable octet sequence by a replacement character, such as the Unicode character U+FFFD (Replacement Character).

The RFC 7230 token production (, Section 3.2.6) differs from the production used in RFC 2231 (imported from Section 5.1 of ) in that curly braces (i.e., "{" 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 Section 2.3 of 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":

Non-extended notation, using "quoted-string":

Extended notation, using the Unicode character U+00A3 ("£", POUND SIGN): Note: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and 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): Note: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and then percent-encoded. Likewise, the Unicode euro sign character U+20AC was encoded into the octet sequence E2 82 AC, and 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.

test section: Czech: Č, č, Ď, ď, Ě, ě, Ň, ň, Ř, ř, Š, š, Ť, ť, Ů, ů, Ž, ž French: Œ, œ, and the very rare Ÿ the very rare Ÿ Guarani: Ẽ, ẽ, Ĩ, ĩ, Ũ, ũ, Ỹ, ỹ, G̃, g̃ g̃ Hungarian: Ő, ő, Ű, ű Māori: Ā, ā, Ē, ē, Ī, ī, Ō, ō, Ū, ū Romanian: Ă, ă, Ș, ș, Ț, ț and older Ţ, ţ with cedilla Japanese example: UNIX での日本語文字コードを扱うために使用されている従来の EUC は次のような ものでした。 This means "The conventional EUC encoding used to handle Japanese character codes on Unix was as follows." Chinese: simplified Chinese: 汉语; traditional Chinese: 漢語; Pinyin: Hànyǔ; simplified Chinese: 华语; traditional Chinese: 華語; Chinese: 中文; Greek: Ή Έ Ό Ώ ξ ώ ϴ Arabic: ص ط ٸ ڛ ݵ Thai: ๓ แ ใ ๗ (๎ THAI CHARACTER YAMAKKAN (U+0E4E)??) (็ THAI CHARACTER MAITAIKHU (U+0E47) Tibetan: couldn't copy and paste these characters. Examples of tested characters (perhaps had trouble with just subjoined letters??): ྱྱྱTIBETAN SUBJOINED LETTER YA (U+0FB1) ླTIBETAN SUBJOINED LETTER LA (U+0FB3) ྞTIBETAN SUBJOINED LETTER NNA (U+0F9E) ࿇ TIBETAN SYMBOL RDO RJE RGYA GRAM (U+0FC7) -- ok ࿐ TIBETAN MARK BSKA- SHOG GI MGO RGYAN (U+0FD0) -- ok Hangul: ᄵ ᅒ ᆂᆂᇀ (couldn't get all chars: HANGUL JUNGSEONG O-O (U+1182), ᆃ HANGUL JUNGSEONG O-U (U+1183), ᆜHANGUL JUNGSEONG I-EU (U+119C)), HANGUL JONGSEONG THIEUTH (U+11C0) Symbols: œ ™ ¡

Section 5 of 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 (, Section 2.2). 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 Section 3.2.4 of ). 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 specific header field parameters.

For instance: ]]>

Note: The parameter value continuation feature defined in Section 3 of makes it impossible to have multiple instances of extended parameters with identical names, 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 interpretation 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 using 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.

Section 4.2 of requires that protocol elements containing human-readable text be able to carry language information. Thus, the ext-value production ought to always be used when the parameter value is of a 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 names 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: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 related to decoding UTF-8. See Section 10 of 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.

This document does not require any IANA actions.

The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations. The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. RFC Errata International Organization for Standardization This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] This particular document is the third document in the series. It describes extensions to RFC 822 to allow non-US-ASCII text data in Internet mail header fields. [STANDARDS-TRACK] This memo defines extensions to the RFC 2045 media type and RFC 2183 disposition parameter value mechanisms. This memo also defines an extension to the encoded words defined in RFC 2047 to allow the specification of the language to be used for display as well as the character set. [STANDARDS-TRACK] This document is the current policies being applied by the Internet Engineering Steering Group (IESG) towards the standardization efforts in the Internet Engineering Task Force (IETF) in order to help Internet protocols fulfill these requirements. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068. [STANDARDS-TRACK] 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. [STANDARDS-TRACK] This document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header field. [STANDARDS-TRACK] RFC 2616 defines the Content-Disposition response header field, but points out that it is not part of the HTTP/1.1 Standard. This specification takes over the definition and registration of Content-Disposition, as used in HTTP, and clarifies internationalization aspects. [STANDARDS-TRACK] This document provides a list of terms used in the IETF when discussing internationalization. The purpose is to help frame discussions of internationalization in the various areas of the IETF and to help introduce the main concepts to IETF participants. This memo documents an Internet Best Current Practice. This specification defines the multipart/form-data media type, which can be used by a wide variety of applications and transported by a wide variety of protocols as a way of returning a set of values as the result of a user filling out a form. This document obsoletes RFC 2388. The Hypertext Transfer Protocol (HTTP) provides a simple challenge- response authentication mechanism that may be used by a server to challenge a client request and by a client to provide authentication information. This document defines the HTTP Digest Authentication scheme that can be used with the HTTP authentication mechanism. This document specifies extensions for the HTTP authentication framework for interactive clients. Currently, fundamental features of HTTP-level authentication are insufficient for complex requirements of various Web-based applications. This forces these applications to implement their own authentication frameworks by means such as HTML forms, which becomes one of the hurdles against introducing secure authentication mechanisms handled jointly by servers and user agents. The extended framework fills gaps between Web application requirements and HTTP authentication provisions to solve the above problems, while maintaining compatibility with existing Web and non-Web uses of HTTP authentication. WhatWG

This section summarizes the changes compared to : The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters". The introduction was rewritten to better explain the issues around non-ASCII characters in field values. The requirement to support the "ISO-8859-1" encoding was removed. This document does not attempt to re-define a generic "parameter" ABNF (it turned out that there really isn't a generic definition of parameters in HTTP; for instance, there are subtle differences with respect to whitespace handling). A note about defects in error handling in current implementations was removed, as it was no longer accurate.

The encoding defined in this document is currently used in four different HTTP header fields: "Authentication-Control", defined in , "Authorization" (as used in HTTP Digest Authentication, defined in ), "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 more recently specified in . At the time of this writing, no User Agent except Firefox supported the "title*" parameter (starting with release 15). Section 3.4 of defines the "username*" parameter for use in HTTP Digest Authentication. At the time of writing, no User Agent implemented this extension.

Thanks to Martin Dürst 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 implementers feedback. Furthermore, thanks to the members of the IETF HTTP Working Group for the feedback specific to this update of RFC 5987.