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Versions: 00 01 RFC 2277

draft                       Charset policy                     June 97


               IETF Policy on Character Sets and Languages

                     Fri Aug 29 10:41:03 MET DST 1997


                         Harald Tveit Alvestrand
                                 UNINETT
                      Harald.T.Alvestrand@uninett.no






    Status of this Memo

    This draft document is being circulated for comment.

    Please send comments to the author, or to the mailing list <ietf-
    charsets@innosoft.com>

    The following text is required by the Internet-draft rules:

    This document is an Internet Draft.  Internet Drafts are working
    documents of the Internet Engineering Task Force (IETF), its
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    Internet Drafts are draft documents valid for a maximum of six
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    Please check the I-D abstract listing contained in each Internet
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    The file name of this version is draft-alvestrand-charset-
    policy-01.txt









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    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 (INTENDED TO BE) the current policies being
    applied by the Internet Engineering Steering Group towards the
    standardization efforts in the Internet Engineering Task Force in
    order to help Internet protocols fulfil 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 [ARCH] 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.





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    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.





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    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

    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.










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    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.









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    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





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    negotiation), text SHOULD be presented in Default Language.

    The Default Language is English, since this is the language which
    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. It may be appropriate for application designers to
    make sure that messages in Default Language are understandable to
    people with a limited understanding of the English language.


    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





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    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]
         S. Bradner, "Key words for use in RFCs to Indicate
         Requirement Levels", 03/26/1997 - RFC 2119

    [WR] C. Weider, C. Preston, K. Simonsen, H. Alvestrand, R.
         Atkinson, M. Crispin, P. Svanberg, "The Report of the IAB
         Character Set Workshop held 29 February - 1 March, 1996",
         04/21/1997, RFC 2130

    [ARCH]
         B. Carpenter, "Architectural Principles of the Internet",
         06/06/1996, RFC 1958

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

    [REG]
         N. Freed, J. Postel: IANA Charset Registration Procedures,





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         Work In Progress (draft-freed-charset-reg-02.txt)

    [UTF-8]
         F. Yergeau:  UTF-8, a transformation format of Unicode and
         ISO 10646, Work In Progress (draft-yergeau-utf8-rev-00.txt,
         obsoletes RFC 2044)

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


    9.  Author's address

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

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



























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