'''Multicast Source Discovery Protocol''' ('''MSDP''') is a [[Protocol Independent Multicast]] (PIM) family [[multicast]][[routing protocol]] defined by Experimental RFC 3618.<ref name="RFC3618">{{cite webIETF |last1=Meyer |last2=Fenner |title=RFC3618 - Multicast Source Discovery Protocol (MSDP) |urlrfc=https://tools.ietf.org/html/rfc36183618 |websitedate=tools.ietf.orgOctober |publisher=IETF2003 |accessdateaccess-date=17 August 2018 |language=en}}</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 webIETF |last1=McBride |last2=Meylor |last3=Meyer |title=RFC4611 - Multicast Source Discovery Protocol (MSDP) Deployment Scenarios |urlrfc=https://tools.ietf.org/html/rfc46114611 |websitedate=IETFAugust 2006 |accessdateaccess-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 needs to have its own RP. All of the RPs are peers (directly or through other MSDP peers). Messages contain the sender (source) address and the multicast group address (S,G). If an RP on its own ___domain receives a message, it determines if there are listeners for this group. If listeners exist, it triggers a PIM join into the source ___domain towards the data 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 webIETF |last1=Pekka |first1=Savola |last2=Brian |first2=Haberman |title=Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address |urlrfc=https://tools.ietf.org/html/rfc39563956 |websitedate=tools.ietf.orgNovember 2004 |publisher=IETF |accessdateaccess-date=17 August 2018 |language=en}}</ref>, for a number of reasons, including that protocols such as [[Single Source-Specific Multicast]] (SSM), [[Bi-directional Multicast]] and IPv6 Embedded Rendezvous Points would mean there was less requirement for MSDP.
