John Klensin has brought to my attention that it is currently impossible
to represent some people's names in PRECIS usernames because some of the
relevant Unicode code points are disallowed by the IdentifierClass
defined in RFC 7564 (and thus by the UsernameCaseMapped and
UsernameCasePreserved profiles defined in RFC 7613).
First, RFC 7564 disallows "default ignorable" code points in the
IdentifierClass. However, as I understand it some of these code points
are need to represent characters in names that might be desirable to
people living within communities that use Indic script and eastern
Arabic script (e.g., Persian and writing systems derived from Persian).
In particular, the Unicode Standard specifies that ZWJ and ZWNJ are
"default ignorable" and it seems that these code points are especially
important in this context.
Second, apparently some Chinese family names are typically written
(especially outside the People's Republic of China) using characters
that the Unicode Consortium assigns to non-BMP code points, or assigns
in the BMP but as compatibility decomposable characters (and thus
disallowed by RFC 7564 in the IdentifierClass).
I'm not sure whether we can solve these problems (internationalization
is messy and we've never tried to guarantee that any particular name or
preferred string could be represented in PRECIS usernames), but input
from people with a deeper understanding of these issues would be
appreciated. I have attempted to reach out to relevant experts, and will
report back to this list with any findings.
In the meantime, I plan to submit revised I-Ds addressing other issues
with the PRECIS specifications sometime this evening.
Peter
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