Introduction to IP Multicast Routing

The first part of this paper describes the benefits of multicasting, the Multicast Backbone (MBONE), Class D addressing, and the operation of the Internet Group Management Protocol (IGMP). The second section explores a number of different algorithms that may potentially be employed by multicast routing protocols: Flooding Spanning Trees Reverse Path Broadcasting (RPB) Truncated Reverse Path Broadcasting (TRPB) Reverse Path Multicasting (RPM) Core-Based Trees The third part contains the main body of the paper. It describes how the previous algorithms are implemented in multicast routing protocols available today. Distance Vector Multicast Routing Protocol (DVMRP) Multicast OSPF (MOSPF) Protocol-Independent Multicast (PIM) Introduction There are three fundamental types of IPv4 addresses: unicast, broadcast, and multicast. A unicast address is designed to transmit a packet to a single destination. A broadcast address is used to send a datagram to an entire subnetwork. A multicast address is designed to enable the delivery of datagrams to a set of hosts that have been configured as members of a multicast group in various scattered subnetworks. Multicasting is not connection oriented. A multicast datagram is delivered to destination group members with the same “best-effort” reliability as a standard unicast IP datagram. This means that a multicast datagram is not guaranteed to reach all members of the group, or arrive in the same order relative to the transmission of other packets. The only difference between a multicast IP packet and a unicast IP packet is the presence of a “group address” in the Destination Address field of the IP header. Instead of a Class A, B, or C IP address, multicasting employs a Class D destination address format (224.0.0.0239.255.255.255). Multicast Groups Individual hosts are free to 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 may be a member of more than one multicast group at any given time and does not have to belong to a group to send messages to members of a group. Group Membership Protocol A group membership protocol is employed by routers to learn about the presence of group members on their directly attached subnetworks. When a host joins a multicast group, it transmits a group membership protocol message for the group(s) that it wishes to receive, and sets its IP process and network interface card to receive frames addressed to the multicast group. This receiver-initiated join process has excellent scaling properties since, as the multicast group increases in size, it becomes ever more likely that a new group member will be able to locate a nearby branch of the multicast distribution tree.