RFC 1815 (rfc1815) - Page 2 of 6

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

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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 [RFC 1521] was unclear about how bi-directionality
   should be supported with "ISO-8859-6" and "ISO-8859-8" which was
   corrected by [RFC 1556], 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.

M. Ohta                      Informational

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