Principles for Unicode Code Point Inclusion in Labels in the DNS

Internationalized Domain Names in Applications (IDNA) makes available to DNS zone administrators a very wide range of Unicode code points. Most operators of zones should probably not permit registration of U-labels using the entire range.  This is especially true of zones that accept registrations across organizational boundaries, such as top-level domains and, most importantly, the root.  It is unfortunately not possible to generate algorithms to determine whether permitting a code point presents a low risk.  This memo presents a set of principles that can be used to guide the decision of whether a Unicode code point may be wisely included in the repertoire of permissible code points in a U-label in a zone.