    Network Working Group                                            M. Ohta
    Request For Comments: 1815                 Tokyo Institute of Technology
    Category: Informational                                        July 1995

                   Character Sets ISO-10646 and ISO-10646-J-1

    Status of this Memo

       This memo provides information for the Internet community.  This memo
       does not specify an Internet standard of any kind.  Distribution of
       this memo is unlimited.

    Abstract

       Though the ISO character set standard of ISO 10646 is specified
       reasonably well about European characters, it is not so useful in an
       fully internationalized environment.

       For the practical use of ISO 10646, a lot of external profiling such
       as restriction of characters, restriction of combination of
       characters and addition of language information is necessary.

       This memo provides information on such profiling, along with charset
       names to each profiled instance.

       Though all the effort is done to make the resulting charset as useful
       10646 based charset as possible, the result is not so good.  So, the
       charsets defined in this memo are only for reference purpose and its
       use for practical purpose is strongly discouraged.

    Introduction

       This memo describes two text encoding schemes based on ISO 10646
       [10646].

       As ISO 10646 specifies too little about how text is visualized, to
       practically use ISO 10646, it is necessary to restrict the standard
       minimally and then add some amount of profiling information.

       For ISO 2022 [ISO2022] based national standards, sufficient profiling
       information is provided by national standardization bodies, but, for
       ISO 10646, such a profiling is not yet provided.

       As the profiling of ISO 10646 largely affects which character or
       combination of characters could be properly displayed, changes of
       profiling of ISO 10646 are as significant as additions of new
       character sets of ISO 2022.

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    RFC 1815       Character Sets ISO-10646 and ISO-10646-J-1      July 1995

       That is, it's impractical to support the entirety of ISO 10646 (new
       restriction or profiling can always be added), so a client needs to
       know whether some restriction or profiling is being used before it
       can decide whether to display the body part. Thus, it is necessary to
       provide multiple charset names to each variation of ISO 10646.

       For example, in Japan with Japanese windows NT, only those Han
       characters already supported by MS Kanji code (mostly equivalent to
       JIS X 0208 [JISX0208]) can be displayed, because no other font
       pattern is commonly provided.

       The other problem of ISO 10646 for Han characters is that, to display
       them in quality required for daily plain text processing in
       China/Japan/Korea, it is necessary to add profiling information on
       which one of Chinese/Japanese/Korean the text is using.  It should be
       noted that this feature makes multilingual mixed
       Chinese/Japanese/Korean text with ISO 10646 impractical.

       Also, just as [RFC1521] was unclear about how bi-directionality
       should be supported with "ISO-8859-6" and "ISO-8859-8" which was
       corrected by [RFC1556], it is also unclear how bi-directionality
       could be supported with ISO 10646.  There are too much ways to
       support bi- directionality.  So, until some bi-directionality
       mechanism(s) becomes widely supported, it is necessary to exclude
       characters for languages which requires bi-directionality support
       from the minimal variation.  It should be noted that, though ISO
       10646 is intended to be free from long term states, save for some
       profiling information, introduction of bi-directionality with ISO
       10646 do requires the long term states.

       Combining characters also cause problems. In many countries where
       combining characters based on [ISO2022] is used, there are
       restrictions on how combining characters are ordered [TIS].  Without
       such restriction, the result of combination is completely meaningless
       which is the current state of ISO 10646.  That is, if some
       combination is allowed in some implementation while the other does
       not support it, communication between them is difficult unless ISO
       10646 is profiled to be least common set of widely supported
       combinations.  So, again, until combination restriction will be
       developed for each language, it is necessary to exclude characters
       for such languages from the minimal variation.

       Conjoining characters also, may or may not be supported, which
       requires another profiling.

       According to those considerations, this memo defines two variations
       of ISO 10646. They are "ISO-10646" as the minimal basic variation and
       "ISO-10646-J-1" as the variation which could be useful in Japan.

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    RFC 1815       Character Sets ISO-10646 and ISO-10646-J-1      July 1995

       Finally, this memo, by no means, promotes the use of ISO 10646 on the
       Internet.  It's use is strongly discouraged, when there are other
       charsets which can encode the same information, Families of ISO 10646
       based charsets, like ISO 2022 based charsets, only forms set of
       mutually incompatible encoding systems and, unlike ISO 2022 based
       charsets [2022INT], they can not be merged together to be the single
       world wide charset.

    Description of "ISO-10646"

       ISO-10646 is profiled to be the most basic part of the family of
       encodings based on ISO 10646 and contains the following minimal
       graphic characters:

          collection number and name      positions      further restriction
          ------------------------------------------------------------------
          1 BASIC LATIN                   0020-007E
          2 LATIN-1 SUPPLEMENT            00A0-00FF

       C0 and C1 control characters may also be used as specified in the
       section 16 of ISO 10646.

       The text with "ISO-10646" encodes text in 16 bit big endian form.

       As no combining characters are included, "ISO-10646" can be used with
       applications at implementation level 1.

       Left-to-right directionality should be used.

       The encoding is implemented by Windows/NT.

       For practical communication, use of "ISO-10646" is discouraged.
       "ISO-8859-1" [RFC1345] should be used instead.

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    RFC 1815       Character Sets ISO-10646 and ISO-10646-J-1      July 1995

    Description of "ISO-10646-J-1"

       ISO-10646-J-1 is profiled to be useful for Japanese PC users who use
       Japanese version of Windows/NT and contains the following graphic
       characters:

          collection number and name         positions  further restrictions
          ------------------------------------------------------------------
          1 BASIC LATIN                      0020-007E
          2 LATIN-1 SUPPLEMENT               00A0-00FF
          8 BASIC GREEK                      0370-03CF
          10 CYRILLIC                        0400-04FF
          32 GENERAL PUNCTUATION             2000-206F  See note 1, below.
          39 MATHEMATICAL OPERATORS          2200-22FF  See note 1, below.
          44 BOX DRAWING                     2500-257F
          49 CJK SYMBOLS AND PUNCTUATION     3000-303F  See note 1, below.
          50 HIRAGANA                        3040-309F
          51 KATAKANA                        30A0-30FF
          60 CJK UNIFIED IDEOGRAPHS          4E00-9FFF  See note 1, below.
          62 CJK COMPATIBILITY IDEOGRAPHS    F900-FAFF  See note 1, below.
          66 CJK COMPATIBILITY FORMS         FE30-FE4F
          69 HALFWIDTH AND FULLWIDTH FORMS   FF00-FFEF

       Note 1: Most of the characters are excluded.  That is, only those
       characters of JIS X 0208 [JISX0208] are included. The reason is that
       the Japanese version of Windows/NT have fonts for them only and most
       of the users can not read messages which contains other characters.

       C0 and C1 control characters may also be used as specified in the
       section 16 of ISO 10646.

       The text with "ISO-10646-J-1" encodes text in 16 bit big endian form.

       Shapes of Han characters should be of Japanese Han, that is, those of
       column "J" in section 26 of ISO 10646.

       As no combining characters are included, "ISO-10646-J-1" can be used
       with applications at implementation level 1.

       Characters in "HALFWIDTH AND FULLWIDTH FORMS" compared to be
       different characters to the normal width characters.

       When text is displayed horizontally, left-to-right directionality
       should be used.

       For practical communication, use of "ISO-10646-J-1" is discouraged.
       ISO-2022-JP" [2022JP] should be used instead.

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    RFC 1815       Character Sets ISO-10646 and ISO-10646-J-1      July 1995

    MIME Considerations

       The names given to the character encoding methods described in this
       memo are, respectively, "ISO-10646" and "ISO-10646-J-1".  This name
       is intended to be used in MIME messages as follows:

                    Content-Type: text/plain; charset=iso-10646

       The ISO-10646 and ISO-10646-J-1 encoding are in 16-bit form, so it is
       often necessary to use a Content-Transfer-Encoding header.  Base64
       should be useful.

       The ISO-10646 and ISO-10646-J-1 may also be used in MIME Part 2
       headers [RFC1522].  The "B" encoding should be used with them.

    References

       [10646]     International Organization for Standardization (ISO),
                   "Universal Multiple-Octet Coded Character Set (UCS)",
                   International Standard, Ref. No. ISO/IEC 10646-1:1993
                   (E).

       [2022INT]   (An Internet Draft "draft-ohta-text-encoding-*.txt" may
                   be available).

       [2022JP]    Murai, J., Crispin, M., and E. van der Poel, "Japanese
                   Character Encoding for Internet Messages", RFC 1468, June
                   1993.

       [ISO2022]   International Organization for Standardization (ISO),
                   "Information processing -- ISO 7-bit and 8-bit coded
                   character sets -- Code extension techniques",
                   International Standard, Ref. No. ISO 2022-1986 (E).

       [JISX0208]  Japanese Standards Association, "Code of the Japanese
                   graphic character set for information interchange", JIS X
                   0208-1990.

       [RFC1345]   Simonsen, K., "Character Mnemonics & Character Sets",
                   RFC-1345, Rationel Almen Planlaegning, June 1992.

       [RFC1521]   Borenstein, N., and Freed, N., "MIME  (Multipurpose
                   Internet Mail Extensions) Part One: Mechanisms for
                   Specifying and Describing the Format of Internet Message
                   Bodies", RFC 1521, September 1993.

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    RFC 1815       Character Sets ISO-10646 and ISO-10646-J-1      July 1995

       [RFC1522]   Moore, K., "MIME (Multipurpose Internet Mail Extensions)
                   Part Two: Message Header Extensions for Non-ASCII Text",
                   RFC 1522, September 1993.

       [RFC1556]   Nussbacher, H., "Handling of Bi-directional Texts in
                   MIME" RFC 1556, Israeli Inter-University Computer Center,
                   December 1993.

       [TIS]       Thai Industrial Standard for Thai Character Code for
                   Computer, TIS 620-2533:1990.

    Security Considerations

       Security issues are not discussed in this memo.

    Author's Address

       Masataka Ohta
       Tokyo Institute of Technology
       2-12-1, O-okayama, Meguro-ku,
       Tokyo 152, JAPAN

       Phone: +81-3-5499-7084
       Fax: +81-3-3729-1940
       EMail: mohta@cc.titech.ac.jp

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