3Com 10014299 manual IP Multicast

33 IP MULTICAST

This chapter covers the following topics:

■IP Multicast Overview

■IP Multicast Addresses

■IP Multicast Features

■IP Multicast Routing Protocols

■IP Multicast Packet Forwarding

■IP Multicast Application

IP Multicast Overview When the destination addresses carrying information (data, voice, and video) transmit with only a few subscribers in the network, multiple transmission methods such as unicast and broadcast can be employed. Unicast transmission means establishing a separate data transmission channel for each subscriber, while broadcast transmissionmeans sending the message to all the subscribers in the network no matter whether they need it or not. If 200 subscribers in network require receiving the same message, traditionally there are two solutions for this. One is to send such message 200 times to ensure that all the subscribers are able to get it. The other one is to transmit the data within the whole network to enable subscribers to get the necessary data directly from the network by adopting the broadcast method.

Using the unicast method to transmit to 200 subscribers results in wasted bandwidth. Using the broadcast method risks information security and confidentiality. IP multicast technology solves both of these problems. The multicast source sends the information only once. The transmitted information is duplicated and distributed continuously at key network nodes. In this way, the information can be sent accurately and efficiently to each subscriber who requires it.

In simple terms, IP multicast is a bandwidth-saving technology. It sends a single information flow to several receivers simultaneously to reduce network traffic. In case a router does not support multicast in the network, the router can employ the tunnel method to encapsulate the multicast packets in the unicast packets, and send them to the adjacent multicast router. Adjacent multicast routers drop the unicast IP header, and then continue the multicast transmission to avoid causing a change to the network structure.

IP Multicast Addresses IP multicasting uses Class D addressing. Each multicast address stands for a multicast group, not for a host. Because the maximum four-digit number of a