Multicast Source Discovery Protocol (MSDP) is a Protocol Independent Multicast (PIM) family multicast routing protocol defined by Experimental RFC 3618. MSDP interconnects multiple IPv4 PIM Sparse-Mode (PIM-SM) domains which enables PIM-SM to have Rendezvous Point (RP) redundancy and inter-domain multicasting RFC 4611.
MSDP uses TCP as its transport protocol. Each multicast tree has to have its own RP. All of the RP's are peers (directly or through other MSDP peers). Messages contain Source of Data, Group Address The Data Source Sends To (S,G). If an RP on its own domain receives a message it determines if there are group members on this domain interested in a multicast. If someone is interested it triggers a join towards the data source (into the source domain) in the way of (S, G). In a peering relationship, one MSDP peer listens for new TCP connections on the well-known port 639. MSDP is the protocol for IPv4 and IPv6 Multicast.