Network Working Group H. Alvestrand Request for Comments: 3282 Cisco Systems Obsoletes: 1766 May 2002 Category: Standards Track Content Language Headers Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved.
AbstractThis document defines a "Content-language:" header, for use in cases where one desires to indicate the language of something that has RFC 822-like headers, like MIME body parts or Web documents, and an "Accept-Language:" header for use in cases where one wishes to indicate one's preferences with regard to language.
A prerequisite for any such function is a means of labelling the information content with an identifier for the language that is used in this information content, such as is defined by [TAGS]. This document specifies a protocol element for use with protocols that use RFC 822-like headers for carrying language tags as defined in [TAGS]. The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119]. RFC 822-like headers, such as MIME body parts or Web documents. The RFC 822 EBNF of the Content-Language header is: Content-Language = "Content-Language" ":" 1#Language-tag In the more strict RFC 2234 ABNF: Content-Language = "Content-Language" ":" [CFWS] Language-List Language-List = Language-Tag [CFWS] *("," [CFWS] Language-Tag [CFWS]) The Content-Language header may list several languages in a comma- separated list. The CFWS construct is intended to function like the whitespace convention in RFC 822, which means also that one can place parenthesized comments anywhere in the language sequence, or use continuation lines. A formal definition is given in RFC 2822 [RFC2822]. In keeping with the tradition of RFC 2822, a more liberal "obsolete" grammar is also given: obs-content-language = "Content-Language" *WSP ":" [CFWS] Language-List Like RFC 2822, this specification says that conforming implementations MUST accept the obs-content-language syntax, but MUST NOT generate it; all generated headers MUST conform to the Content- Language syntax.
RFC 822-like headers, such as MIME body parts or Web documents, are used. The RFC 822 EBNF of the Accept-Language header is: Accept-Language = "Accept-Language" ":" 1#( language-range [ ";" "q" "=" qvalue ] ) A slightly more restrictive RFC 2234 ABNF definition is: Accept-Language = "Accept-Language:" [CFWS] language-q *( "," [CFWS] language-q ) language-q = language-range [";" [CFWS] "q=" qvalue ] [CFWS] qvalue = ( "0" [ "." 0*3DIGIT ] ) / ( "1" [ "." 0*3("0") ] )
A more liberal RFC 2234 ABNF definition is: Obs-accept-language = "Accept-Language" *WSP ":" [CFWS] obs-language-q *( "," [CFWS] obs-language-q ) [CFWS] obs-language-q = language-range [ [CFWS] ";" [CFWS] "q" [CFWS] "=" qvalue ] Like RFC 2822, this specification says that conforming implementations MUST accept the obs-accept-language syntax, but MUST NOT generate it; all generated messages MUST conform to the Accept- Language syntax. The syntax and semantics of language-range is defined in [TAGS]. The Accept-Language header may list several language-ranges in a comma- separated list, and each may include a quality value Q. If no Q values are given, the language-ranges are given in priority order, with the leftmost language-range being the most preferred language; this is an extension to the HTTP/1.1 rules, but matches current practice. If Q values are given, refer to HTTP/1.1 [RFC 2616] for the details on how to evaluate it. RFC 1766, which stated that "Security issues are believed to be irrelevant to this memo", is a concern with language ranges used in content negotiation - that they may be used to infer the nationality of the sender, and thus identify potential targets for surveillance. This is a special case of the general problem that anything you send is visible to the receiving party; it is useful to be aware that such concerns can exist in some cases. The exact magnitude of the threat, and any possible countermeasures, is left to each application protocol. TAGS].
RFC 1766, and has provided a great deal of input to this revision. Bruce Lilly did a special job of reading and commenting on my ABNF definitions. [TAGS] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066 [ISO 639] ISO 639:1988 (E/F) - Code for the representation of names of languages - The International Organization for Standardization, 1st edition, 1988-04-01 Prepared by ISO/TC 37 - Terminology (principles and coordination). Note that a new version (ISO 639-1:2000) is in preparation at the time of this writing. [ISO 639-2] ISO 639-2:1998 - Codes for the representation of names of languages -- Part 2: Alpha-3 code - edition 1, 1998-11- 01, 66 pages, prepared by ISO/TC 37/SC 2 [ISO 3166] ISO 3166:1988 (E/F) - Codes for the representation of names of countries - The International Organization for Standardization, 3rd edition, 1988-08-15. [ISO 15924] ISO/DIS 15924 - Codes for the representation of names of scripts (under development by ISO TC46/SC2)
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC 2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, November 1996. [RFC 2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", RFC 2048, November 1996. [RFC 2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, November 1996. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC 2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
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