IETF Policy on Character Sets and Languages

Network Working Group                                     H. Alvestrand
Request for Comments: 2277                                      UNINETT
BCP: 18                                                    January 1998
Category: Best Current Practice


              IETF Policy on Character Sets and Languages

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

1.  Introduction

   The Internet is international.

   With the international Internet follows an absolute requirement to
   interchange data in a multiplicity of languages, which in turn
   utilize a bewildering number of characters.

   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.

   The document is very much based upon the recommendations of the IAB
   Character Set Workshop of February 29-March 1, 1996, which is
   documented in RFC 2130 [WR].  This document attempts to be concise,
   explicit and clear; people wanting more background are encouraged to
   read RFC 2130.

   The document uses the terms 'MUST', 'SHOULD' and 'MAY', and their
   negatives, in the way described in [RFC 2119].  In this case, 'the
   specification' as used by RFC 2119 refers to the processing of
   protocols being submitted to the IETF standards process.

2.  Where to do internationalization

   Internationalization is for humans. This means that protocols are not
   subject to internationalization; text strings are. Where protocol
   elements look like text tokens, such as in many IETF application
   layer protocols, protocols MUST specify which parts are protocol and
   which are text. [WR 2.2.1.1]

   Names are a problem, because people feel strongly about them, many of
   them are mostly for local usage, and all of them tend to leak out of
   the local context at times. RFC 1958 [RFC 1958] recommends US-ASCII
   for all globally visible names.

   This document does not mandate a policy on name internationalization,
   but requires that all protocols describe whether names are
   internationalized or US-ASCII.

   NOTE: In the protocol stack for any given application, there is
   usually one or a few layers that need to address these problems.

   It would, for instance, not be appropriate to define language tags
   for Ethernet frames. But it is the responsibility of the WGs to
   ensure that whenever responsibility for internationalization is left
   to "another layer", those responsible for that layer are in fact
   aware that they HAVE that responsibility.

3.  Definition of Terms

   This document uses the term "charset" to mean a set of rules for
   mapping from a sequence of octets to a sequence of characters, such
   as the combination of a coded character set and a character encoding
   scheme; this is also what is used as an identifier in MIME "charset="
   parameters, and registered in the IANA charset registry [REG].  (Note
   that this is NOT a term used by other standards bodies, such as ISO).

   For a definition of the term "coded character set", refer to the
   workshop report.

   A "name" is an identifier such as a person's name, a hostname, a
   domainname, a filename or an E-mail address; it is often treated as
   an identifier rather than as a piece of text, and is often used in
   protocols as an identifier for entities, without surrounding text.

3.1.  What charset to use

   All protocols MUST identify, for all character data, which charset is
   in use.

   Protocols MUST be able to use the UTF-8 charset, which consists of
   the ISO 10646 coded character set combined with the UTF-8 character
   encoding scheme, as defined in [10646] Annex R (published in
   Amendment 2), for all text.

   Protocols MAY specify, in addition, how to use other charsets or
   other character encoding schemes for ISO 10646, such as UTF-16, but
   lack of an ability to use UTF-8 is a violation of this policy; such a
   violation would need a variance procedure ([BCP9] section 9) with
   clear and solid justification in the protocol specification document
   before being entered into or advanced upon the standards track.

   For existing protocols or protocols that move data from existing
   datastores, support of other charsets, or even using a default other
   than UTF-8, may be a requirement. This is acceptable, but UTF-8
   support MUST be possible.

   When using other charsets than UTF-8, these MUST be registered in the
   IANA charset registry, if necessary by registering them when the
   protocol is published.

   (Note: ISO 10646 calls the UTF-8 CES a "Transformation Format" rather
   than a "character encoding scheme", but it fits the charset workshop
   report definition of a character encoding scheme).

3.2.  How to decide a charset

   When the protocol allows a choice of multiple charsets, someone must
   make a decision on which charset to use.

   In some cases, like HTTP, there is direct or semi-direct
   communication between the producer and the consumer of data
   containing text. In such cases, it may make sense to negotiate a
   charset before sending data.

   In other cases, like E-mail or stored data, there is no such
   communication, and the best one can do is to make sure the charset is
   clearly identified with the stored data, and choosing a charset that
   is as widely known as possible.

   Note that a charset is an absolute; text that is encoded in a charset
   cannot be rendered comprehensibly without supporting that charset.

   (This also applies to English texts; charsets like EBCDIC do NOT have
   ASCII as a proper subset)
   Negotiating a charset may be regarded as an interim mechanism that is
   to be supported until support for interchange of UTF-8 is prevalent;
   however, the timeframe of "interim" may be at least 50 years, so
   there is every reason to think of it as permanent in practice.

4.  Languages

4.1.  The need for language information

   All human-readable text has a language.

   Many operations, including high quality formatting, text-to-speech
   synthesis, searching, hyphenation, spellchecking and so on benefit
   greatly from access to information about the language of a piece of
   text. [WC 3.1.1.4].

   Humans have some tolerance for foreign languages, but are generally
   very unhappy with being presented text in a language they do not
   understand; this is why negotiation of language is needed.

   In most cases, machines will not be able to deduce the language of a
   transmitted text by themselves; the protocol must specify how to
   transfer the language information if it is to be available at all.

   The interaction between language and processing is complex; for
   instance, if I compare "name-of-thing(lang=en)" to "name-of-
   thing(lang=no)" for equality, I will generally expect a match, while
   the word "ask(no)" is a kind of tree, and is hardly useful as a
   command verb.

4.2.  Requirement for language tagging

   Protocols that transfer text MUST provide for carrying information
   about the language of that text.

   Protocols SHOULD also provide for carrying information about the
   language of names, where appropriate.

   Note that this does NOT mean that such information must always be
   present; the requirement is that if the sender of information wishes
   to send information about the language of a text, the protocol
   provides a well-defined way to carry this information.

4.3.  How to identify a language

   The RFC 1766 language tag is at the moment the most flexible tool
   available for identifying a language; protocols SHOULD use this, or
   provide clear and solid justification for doing otherwise in the
   document.

   Note also that a language is distinct from a POSIX locale; a POSIX
   locale identifies a set of cultural conventions, which may imply a
   language (the POSIX or "C" locale of course do not), while a language
   tag as described in RFC 1766 identifies only a language.

4.4.  Considerations for language negotiation

   Protocols where users have text presented to them in response to user
   actions MUST provide for support of multiple languages.

   How this is done will vary between protocols; for instance, in some
   cases, a negotiation where the client proposes a set of languages and
   the server replies with one is appropriate; in other cases, a server
   may choose to send multiple variants of a text and let the client
   pick which one to display.

   Negotiation is useful in the case where one side of the protocol
   exchange is able to present text in multiple languages to the other
   side, and the other side has a preference for one of these; the most
   common example is the text part of error responses, or Web pages that
   are available in multiple languages.

   Negotiating a language should be regarded as a permanent requirement
   of the protocol that will not go away at any time in the future.

   In many cases, it should be possible to include it as part of the
   connection establishment, together with authentication and other
   preferences negotiation.

4.5.  Default Language

   When human-readable text must be presented in a context where the
   sender has no knowledge of the recipient's language preferences (such
   as login failures or E-mailed warnings, or prior to language
   negotiation), text SHOULD be presented in Default Language.

   Default Language is assigned the tag "i-default" according to the
   procedures of RFC 1766. It is not a specific language, but rather
   identifies the condition where the language preferences of the user
   cannot be established.

   Messages in Default Language MUST be understandable by an English-
   speaking person, since English is the language which, worldwide, the
   greatest number of people will be able to get adequate help in
   interpreting when working with computers.

   Note that negotiating English is NOT the same as Default Language;
   Default Language is an emergency measure in otherwise unmanageable
   situations.

   In many cases, using only English text is reasonable; in some cases,
   the English text may be augumented by text in other languages.

5.  Locale

   The POSIX standard [POSIX] defines a concept called a "locale", which
   includes a lot of information about collating order for sorting, date
   format, currency format and so on.

   In some cases, and especially with text where the user is expected to
   do processing on the text, locale information may be usefully
   attached to the text; this would identify the sender's opinion about
   appropriate rules to follow when processing the document, which the
   recipient may choose to agree with or ignore.

   This document does not require the communication of locale
   information on all text, but encourages its inclusion when
   appropriate.

   Note that language and character set information will often be
   present as parts of a locale tag (such as no_NO.iso-8859-1; the
   language is before the underscore and the character set is after the
   dot); care must be taken to define precisely which specification of
   character set and language applies to any one text item.

   The default locale is the "POSIX" locale.

6.  Documenting internationalization decisions

   In documents that deal with internationalization issues at all, a
   synopsis of the approaches chosen for internationalization SHOULD be
   collected into a section called "Internationalization
   considerations", and placed next to the Security Considerations
   section.

   This provides an easy reference for those who are looking for advice
   on these issues when implementing the protocol.

7.  Security Considerations

   Apart from the fact that security warnings in a foreign language may
   cause inappropriate behaviour from the user, and the fact that
   multilingual systems usually have problems with consistency between
   language variants, no security considerations relevant have been
   identified.

8.  References

   [10646]
        ISO/IEC, Information Technology - Universal Multiple-Octet Coded
        Character Set (UCS) - Part 1: Architecture and Basic
        Multilingual Plane, May 1993, with amendments

   [RFC 2119]
        Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [WR] Weider, C., Preston, C., Simonsen, K., Alvestrand, H,
        Atkinson, R., Crispin, M., and P. Svanberg, "The Report of the
        IAB Character Set Workshop held 29 February - 1 March, 1996",
        RFC 2130, April 1997.

   [RFC 1958]
        Carpenter, B., "Architectural Principles of the Internet", RFC
        1958, June 1996.

   [POSIX]
        ISO/IEC 9945-2:1993 Information technology -- Portable Operating
        System Interface (POSIX) -- Part 2: Shell and Utilities

   [REG]
        Freed, N., and J. Postel, "IANA Charset Registration
        Procedures", BCP 19, RFC 2278, January 1998.

   [UTF-8]
        Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [BCP9]
        Bradner, S., "The Internet Standards Process -- Revision 3," BCP
        9, RFC 2026, October 1996.

9.  Author's Address

   Harald Tveit Alvestrand
   UNINETT
   P.O.Box 6883 Elgeseter
   N-7002 TRONDHEIM
   NORWAY

   Phone: +47 73 59 70 94
   EMail: Harald.T.Alvestrand@uninett.no

10.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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