IP multicast
A technique for distributing bandwidth-intensive data across a WAN.
IP multicast is an ideal alternative to IP broadcast and unicast transmission for these applications. Consider a specialized ticker that transmits packets to 100 traders. Unicasting would require the periodic transmission of 100 identical packets, with many packets traversing the same links. Using broadcasting, all network end stations would receive the packets, regardless of the small recipient group. But multicasting sends the same information only once and only to the intended recipients. The resulting bandwidth savings and scalability inherent in multicast provide a major benefit to network operators.
IP multicasting relies on two mechanisms: a group management protocol to establish and maintain multicast groups, and multicast routing protocols to route packets efficiently. The availability of better versions of these protocols is the primary driver behind multicast scalability.
Internet Group Management Protocol (IGMP) manages packet communication between endstations and their local multicast router, letting them join or leave groups. The most recent version, IGMP Version 3, supports source filtering, which ensures that endstations only receive packets from specified multicast sources, rather than all multicast sources.
When group membership is established, multicast packets, identified through a group address in the destination field of the IP header, are forwarded between routers using multicast routing protocols.
Multicast routing protocols construct distribution trees through the network and perform multicast forwarding. Distribution trees define the path that multicast traffic will take through the network to group members. These paths are based on source trees or shared trees.
From IP multicasting comes of age, Network World Tech Update, 05/20/02.
Additional resources
IP Multicast audio primer
Listen to a more detailed explanation.
IP Multicasting: Concepts, Algorithms, and Protocols
Includes definitions of the various protocols and techniques used in IP multicast.
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