Internet Core Protocols: The Definitive Guide

Distributed Multicasting

One of the big differences between IP multicasts and IP broadcasts is that multicast datagrams can be sent to remote networks, while broadcasts are normally restricted to the local network. By being able to forward multicast feeds across network boundaries, multicast routers can get multicast datagrams to hosts around the world, allowing the hosts to participate in global multicasting groups, with multicast datagrams going to every network that has a host that is participating in that particular multicast group.

This process is achieved by using multicasting routers that are particularly aware of multicast traffic and the routing issues associated with that type of traffic. This model involves a couple of theories that are somewhat different from traditional routing.

For one thing, the local multicast router must be listening for multicast packets. The router has to actively monitor the network for IP traffic that has a destination address of 224.0.0.1 through 239.255.255.255, and then forward that data on to any other multicast-enabled network segments that are attached to the router (or any tunnels to remote multicast networks that may be configured). Traditional routers do not monitor the network looking for packets that need to be forwarded, but instead wait for data that needs to be sent to the router (for forwarding) explicitly.

Also, since the destination address is a multicast address and not a unicast address, the multicast router has to forward any ...