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Multicast Overview

IPv4 has three fundamental types of addresses: unicast, broadcast, and multicast. A unicast address is used to send a packet to a single destination. A broadcast address is used to send a datagram to an entire subnetwork. A multicast address is used to send a datagram to a set of hosts that can be on different subnetworks and that are configured as members of a multicast group.

A multicast datagram is delivered to destination group members with the same best-effort reliability as a standard unicast IP datagram. This means that multicast datagrams are not guaranteed to reach all members of a group or to arrive in the same order in which they were transmitted. The only difference between a multicast IP packet and a unicast IP packet is the presence of a group address in the IP header destination address field. Multicast addresses use the Class D address format.

Individual hosts can join or leave a multicast group at any time. There are no restrictions on the physical location or the number of members in a multicast group. A host can be a member of more than one multicast group at any time and does not have to belong to a group to send packets to members of a group.

Routers use a group membership protocol to learn about the presence of group members on directly attached subnetworks. When a host joins a multicast group, it transmits a group membership protocol message for the group or groups that it wants to receive and sets its IP process and network interface card to receive frames addressed to the multicast group.

The Internet Multicast Backbone (MBone) is an interconnected set of subnetworks and routers that support the delivery of IP multicast traffic. The MBone is a virtual network that is layered on top of sections of the physical Internet. The MBone is composed of islands of multicast routing capability that are connected to other islands by virtual point-to-point links called tunnels. The tunnels allow multicast traffic to pass undisturbed through the parts of the Internet that are not multicast-capable. Because the MBone and the Internet have different topologies, multicast routers execute a separate routing protocol to decide how to forward multicast packets.

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