'''Multicast Source Discovery Protocol''' ('''MSDP''') is a [[Protocol Independent Multicast|Protocol Independent Multicast]] (PIM) family [[multicast]][[routing protocol]] defined by Experimental RFC 3618.<ref name="RFC3618">{{cite IETF |last1=Meyer |last2=Fenner |title=Multicast Source Discovery Protocol (MSDP) |rfc=3618 |date=October 2003 |access-date=17 August 2018}}</ref> Despite becoming the IPv4 [[de facto standard]] for inter-___domain multicast, development of the protocol stopped in 2006 and it was decided by the authors not to proceed with making it a proposed standard.<ref name="RFC4611">{{cite IETF |last1=McBride |last2=Meylor |last3=Meyer |title=Multicast Source Discovery Protocol (MSDP) Deployment Scenarios |rfc=4611 |date=August 2006 |access-date=17 August 2018}}</ref> MSDP interconnects multiple [[IPv4]] [[Sparse multicast|PIM Sparse-Mode]] (PIM-SM) [[Domain name|domains]] which enables PIM-SM to have Rendezvous Point (RP) [[redundancy (engineering)|redundancy]] and [[inter-___domain]] multicasting RFC 4611.
MSDP uses [[Transmission Control Protocol|TCP]] as its transport protocol. Each multicast tree hasneeds to have its own RP. All of the RPs are peers (directly or through other MSDP peers). Messages contain Sourcethe ofsender Data,(source) Groupaddress Addressand Thethe Datamulticast Source Sendsgroup Toaddress (S,G). If an RP on its own ___domain receives a message, it determines if there are grouplisteners members onfor this ___domain interested in a multicastgroup. If someonelisteners is interestedexist, it triggers a PIM join towards the data source (into the source ___domain)intowards the waydata of (S, G)source. In a peering relationship, one MSDP peer listens for new TCP connections on the well-known port 639.
MSDP is deliberately unspecified for IPv6,<ref name="RFC4611" /><ref name="RFC3956">{{cite IETF |last1=Pekka |first1=Savola |last2=Brian |first2=Haberman |title=Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address |rfc=3956 |date=November 2004 |publisher=IETF |access-date=17 August 2018}}</ref> for a number of reasons, including that protocols such as [[Source-Specific Multicast]] (SSM), [[Bi-directional Multicast]] and IPv6 Embedded Rendezvous Points would mean there was less requirement for MSDP.
