Thunderbolt Bridge Mode: compatible to rtp midi/ multicast udp
The Thunderbolt Bridge is not a standard Ethernet connection, but macOS provides you with an IP interface where you can send IP packets between two computers.
RTP-MIDI is a protocol that encapsulates MIDI messages within RTP packets that are exchanged over the UDP protocol. The UDP protocol runs on top of the IP protocol, and as such RTP-MIDI is fully compatible with Thunderbolt Bridge connections.
Similarly multicast UDP runs on top of the IP protocol and as such is fully compatible with Thunderbolt Bridge connections. Obviously its usefulness is not quite the same here, as you normally use multicasting to send the same data to many hosts on the network at the same time. As a Thunderbolt Bridge connection is always between two computers, and two computers only, the multicast element is not really the same.