Multicast Source Discovery Protocol (MSDP) is a Protocol Independent Multicast

Protocol Independent Multicast

Protocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...

 (PIM) family multicast

Multicast

In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...

 routing protocol

Routing protocol

A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of...

 defined by Experimental RFC 3618. MSDP interconnects multiple IPv4

IPv4

Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...

 PIM Sparse-Mode

Sparse multicast

Protocol Independent Multicast - Sparse-Mode is a protocol for efficiently routing Internet Protocol packets to multicast groups that may span wide-area and inter-domain internets...

 (PIM-SM) domains

Domain name

A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....

 which enables PIM-SM to have Rendezvous Point (RP) redundancy

Redundancy (engineering)

In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....

 and inter-domain

Inter-domain

inter-domain is a term used to describe data flow control and interaction between Primary Domain Controller computers. This type of computer uses various computer protocols and services to operate. It is most commonly used to multicast between internet domains.-Internet use:An Internet service...

multicasting.

MSDP uses TCP as its transport protocol. Each multicast tree has to have its own RP. All of the RPs 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.