From OpenHome
Songcast OHM/OHU Protocol Specification (Version 1.0)
Introduction
OHM, OHU, together with OHZ, which is discussed elsewhere, form a family of OpenHome protocols for the transport of audio around a home network.
OHM provides large scale one to many communication using multicast UDP. OHU provides small scale one to many communication using unicast UDP.
Message Format
The OHM and OHU protocols share the same message format.