Internet Core Protocols: The Definitive Guide

Limited forwarding

The primary rule that multicast routers follow is that they will only forward multicast traffic to the networks that have expressed an interest in that specific type of data. For example, Figure 4.2 shows two subnets. When Ferret sends a multicast datagram to the 224.1.2.3 group address, Sasquatch has to forward the datagram to the 192.168.20.0 network in order for Fungi to receive it. However, Sasquatch will only do this if Fungi has explicitly stated that it was interested in data for the 224.1.2.3 multicast group.

This “statement of interest” is achieved by using IGMP messages, which are sent by the hosts when they first come onto the network, and which are also sent in response to IGMP queries issued by local multicast routers. There are lots of rules and conditions that are involved with this scenario which are discussed in more detail in Managing Group Memberships later in this chapter. However, the basis of the protocol is that hosts announce which multicast groups they want to listen to, and routers use this information to determine which networks they will forward data to. In essence, a multicast router will only forward multicast data to a network if there is an active listener for that group address on that network.

In addition, multicast routers must also forward any group registration information they have to all other multicast routers that they know about. This procedure ensures that other routers know to send multicast data to their networks. Otherwise, ...