A literal-domain type. Sometimes a host is not known to the domain name system and communication (and, in particular, communication to report and repair the error) is blocked. To bypass this barrier, a special literal form of the address is allowed as an alternative to a domain name.
We have three kinds of literal-domain:
val extension : (string * string) t
An user-defined literal domain kind.
make kind v returns a literal-domain according RFC 5321. It should fails if
extension and value does not respect standards.