IP Multicast Terminology
Multicast has its own particular set of terms and acronyms that apply to IP multicast routers and networks. Figure 1 shows a general view of some of the terms commonly used in an IP multicast network.
In a multicast network, the key component is the router, able to replicate packets and therefore multicast-capable. The routers in the IP multicast network, which has exactly the same topology as the unicast network it is based on, use a multicast routing protocol to build a distribution tree that connects receivers (preferred to the multimedia implications of listeners, but listeners is also used) to sources. In multicast terminology, the distribution tree is rooted at the source (the root of the distribution tree is the source). The interface on the router leading toward the source is the upstream interface, although the less precise terms incoming or inbound interface are used as well. To keep bandwidth use to a minimum, it is best for only one upstream interface on the router to receive multicast packets. The interface on the router leading toward the receivers is the downstream interface, although the less precise terms outgoing or outbound interface are used as well. There can be 0 to N–1 downstream interfaces on a router, where N is the number of logical interfaces on the router. To prevent looping, the upstream interface must never receive copies of downstream multicast packets.
Figure 1: Multicast Terminology in an IP Network
Routing loops are disastrous in multicast networks because of the risk of repeatedly replicated packets. One of the complexities of modern multicast routing protocols is the need to avoid routing loops, packet by packet, much more rigorously than in unicast routing protocols.
Multicast terminology includes two more complex concepts:
