RFC 3958:Domain-Based Application Service Location...
RFC-Ref

UDP


Click on the red underlined text to get to the source

... "tcp") in a given domain. The definition of these terms implies specific things (e.g., that protocol should be one of UDP or TCP) without being precise. Restriction to UDP ...
... UDP or TCP) without being precise. Restriction to UDP and TCP is insufficient for the uses described here. ...



Google
Web
RFC-Ref