Content Language Headers

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.

Abstract

   This 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.

1. Introduction

   There are a number of languages presently or previously used by human
   beings in this world.

   A great number of these people would prefer to have information
   presented in a language which they understand.

   In some contexts, it is possible to have information available in
   more than one language, or it might be possible to provide tools
   (such as dictionaries) to assist in the understanding of a language.

   In other cases, it may be desirable to use a computer program to
   convert information from one format (such as plaintext) into another
   (such as computer-synthesized speech, or Braille, or high-quality
   print renderings).

   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].

2. The Content-language header

   The "Content-Language" header is intended for use in the case where
   one desires to indicate the language(s) of something that has 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.

2.1 Examples of Content-language values

   Voice recording from Liverpool downtown

      Content-type: audio/basic
      Content-Language: en-scouse

   Document in Mingo, an American Indian language which does not have an
   ISO 639 code:

      Content-type: text/plain
      Content-Language: i-mingo

   A English-French dictionary

      Content-type: application/dictionary
      Content-Language: en, fr (This is a dictionary)

   An official European Commission document (in a few of its official
   languages):

      Content-type: multipart/alternative
      Content-Language: da, de, el, en, fr, it

   An excerpt from Star Trek

      Content-type: video/mpeg
      Content-Language: i-klingon

3. The Accept-Language header

   The "Accept-Language" header is intended for use in cases where a
   user or a process desires to identify the preferred language(s) when
   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.

4. Security Considerations

   The only security issue that has been raised with language tags since
   the publication of 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.

5. Character set considerations

   This document adds no new considerations beyond what is mentioned in
   [TAGS].

6. Acknowledgements

   This document has benefited from many rounds of review and comments
   in various fora of the IETF and the Internet working groups.

   Any list of contributors is bound to be incomplete; please regard the
   following as only a selection from the group of people who have
   contributed to make this document what it is today.

   In alphabetical order:

   Tim Berners-Lee, Nathaniel Borenstein, Sean M. Burke, John Clews, Jim
   Conklin, John Cowan, Dave Crocker, Martin Duerst, Michael Everson,
   Ned Freed, Tim Goodwin, Dirk-Willem van Gulik, Marion Gunn, Paul
   Hoffman, Olle Jarnefors, John Klensin, Bruce Lilly, Keith Moore,
   Chris Newman, Masataka Ohta, Keld Jorn Simonsen, Rhys Weatherley,
   Misha Wolf, Francois Yergeau and many, many others.

   Special thanks must go to Michael Everson, who has served as language
   tag reviewer for almost the entire period, since the publication of
   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.

7. References

   [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.

Appendix A: Changes from RFC 1766

   The definition of the language tags has been split, and is now RFC
   3066.  The differences parameter to multipart/alternative is no
   longer part of this standard, because no implementations of the
   function were ever found.  Consult RFC 1766 if you need the
   information.

   The ABNF for content-language has been updated to use the RFC 2234
   ABNF.

Author's Address

   Harald Tveit Alvestrand
   Cisco Systems
   Weidemanns vei 27
   7043 Trondheim
   NORWAY

   EMail: Harald@Alvestrand.no
   Phone: +47 73 50 33 52

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