The Multicast Source Discovery Protocol (MSDP) describes a mechanism to connect multiple PIM Sparse-Mode (PIM-SM) domains together. Each PIM-SM domain uses its own independent RP(s) and does not have to depend on RPs in other domains. Advantages of this approach include:

• No Third-party resource dependencies on a domain"s RP PIM-SM domains can rely on their own RPs only.
• Receiver only Domains: Domains with only receivers get data without globally advertising group membership.

MSDP may be used with protocols other than PIM-SM.

MSDP-speaking routers in a PIM-SM domain have a MSDP peering relationship with MSDP peers in another domain. The peering relationship is made up of a TCP connection in which control information is exchanged. Each domain has one or more connections to this virtual topology.

The purpose of this topology is to allow domains to discover multicast sources from other domains. If the multicast sources are of interest to a domain which has receivers, the normal source-tree building mechanism in PIM-SM will be used to deliver multicast data over an inter-domain distribution tree.

Protocol Structure

MSDP TLV format

Type - Describes the format of the Value field. The following TLV Types are defined:

Length - Length of Type, Length, and Value fields in octets. Minimum length required is 4 octets, except for Keepalive messages. The maximum TLV length is 9192.

Value (variable length) - Format is based on the Type value. The length of the value field is Length field minus 3. All reserved fields in the Value field MUST be transmitted as zeros and ignored on receipt.

Related Terms: IP, TCP, BGP, PIM-SM, PIM-DM

Sponsor Source: MSDP is circulated by IETF (http://www.ietf.org) as an experimental protocol.

Reference: http://www.javvin.com/protocol/rfc3618.pdf: Multicast Source Discovery Protocol.