A domain name is a sequence of labels separated by dots, such as `foo.example`.
Each label may contain any bytes. The length of each label may not exceed 63
charactes. The total length of a domain name is limited to 253 (byte
representation is 255), but other protocols (such as SMTP) may apply even
smaller limits. A domain name label is case preserving, comparison is done in a
case insensitive manner.
