RFC 1815 (rfc1815) - Page 2 of 6
Character Sets ISO-10646 and ISO-10646-J-1
Alternative Format: Original Text Document
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
