Overview

The practice of Broadcasting has long been used in the IP world as a means of informing many hosts about certain information. We saw in Chapter 10, that RIP uses broadcasts to exchange routing information. Also, many other protocols make use of this simple technique to communicate information to many hosts with a single packet. Broadcasting then can be an attractive method of transmission, simple to understand, easy to implement, and one that has been with us since the dawn of networking itself. But broadcasting is not some magical method for data delivery, it does have its associated costs. IP Broadcasts are sent as broadcasts at layer 2 which can cause hosts to consume many CPU cycles. For example, hosts that have no interest in the information being conveyed, have to service an interrupt from the lower layers and decode the data, only to discard it when they find out what the packet contains.

Multicasting is similar in that this too allows information from a single packet to be conveyed to multiple hosts simultaneously. Multicast packets though, are addressed to groups of stations at both the IP layer and at Layer 2 rather than to everyone. Thus, those hosts that are not part of the multicast group will discard the frame at Layer 2 and therefore conserve cycles within the host itself.

The Internet community is dedicated to moving to an environment where multicasting should replace broadcasting and much work is being done in this area. In this chapter...