Border Gateway Multicast Protocol (BGMP) is a protocol for inter-domain multicast routing. BGMP natively supports "source-specific multicast" (SSM). To also support "any-source multicast" (ASM), BGMP builds shared trees for active multicast groups, and allows domains to build source-specific, inter-domain, distribution branches where needed. Building upon concepts from PIM-SM and CBT, BGMP requires that each global multicast group be associated with a single root. However, in BGMP, the root is an entire exchange or domain, rather than a single router.

For non-source-specific groups, BGMP assumes that ranges of the multicast address space have been associated with selected domains. Each such domain then becomes the root of the shared domain-trees for all groups in its range. An address allocator will generally achieve better distribution trees if it takes its multicast addresses from its own domain"s part of the space, thereby causing the root domain to be local.

BGMP uses TCP as its transport protocol. This eliminates the need to implement message fragmentation, retransmission, acknowledgement, and sequencing. BGMP uses TCP port 264 for establishing its connections. This port is distinct from BGP"s port to provide protocol independence, and to facilitate distinguishing between protocol packets.

Two BGMP peers form a TCP connection between one another, and exchange messages to open and confirm the connection parameters. They then send incremental Join/Prune Updates as group memberships change. BGMP does not require periodic refresh of individual entries. KeepAlive messages are sent periodically to ensure the liveness of the connection. Notification messages are sent in response to errors or special conditions. If a connection encounters an error condition, a notification message is sent and the connection is closed if the error is a fatal one.

Protocol Structure

Length - The total length of the message including the header in octets. It allows one to locate in the transport-level stream the start of the next message.

Type - The type code of the message. The following type codes are available:

• 1 OPEN;
• 2 UPDATE;
• 3 NOTIFICATION;
• 4 KEEPALIVE

After a transport protocol connection is established, the first message sent by each side is an OPEN message. If the OPEN message is acceptable, a KEEPALIVE message confirming the OPEN is sent back. Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION messages may be exchanged.

The format of each message type is different.

Related Terms: IP, IPv6, TCP, BGP, PIM-SM

Sponsor Source: BGMP is discussed in IETF (http://www.ietf.org) with draft documents currently.

Reference: http://www.javvin.com/protocol/ietf-bgmp-spec05.pdf : Border Gateway Multicast Protocol (BGMP): Protocol Specification <draft-ietf-bgmp-spec-05.txt>