Multicast Protocols Overview

IP Version 4 (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 given 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.

Multicast host group addresses are defined to be the IP addresses whose high-order four bits are 1110 , giving an address range from 224.0.0.0 through 239.255.255.255 , or simply, 224.0.0.0/4 . (These addresses also are referred to as Class D addresses.)

The Internet Assigned Numbers Authority (IANA) maintains a list of registered IP multicast groups. The base address 224.0.0.0 is reserved and cannot be assigned to any group. The block of multicast addresses from 224.0.0.1 through 224.0.0.255 is reserved for local wire use. Groups in this range are assigned for various uses, including routing protocols and local discovery mechanisms.

The range 239.0.0.0 through 239.255.255.255 is reserved for administratively scoped addresses. Because packets addressed to administratively scoped multicast addresses do not cross configured admin-istrative boundaries, and because administratively scoped multicast addresses are locally assigned, these addresses do not need to be unique across administrative boundaries.