Imagine a scenario where a professor wants to conduct a real-time class with 50 students participating through the network. If the multimedia application for the conferencing employs unicasting, the professor's computer repeatedly sends out 50 audio streams to the student's computers. Unicasting wastes bandwidth because it sends 50 duplicate copies over the network, and causes a significant delay before the last student hears the professor. The audio stream could also flood every corner of the network and possibly bring the network down.
Multicasting comes to the rescue by allowing the multicast host to send out only one copy of the information, and only those hosts that are part of that group receive it. In the class example, the professor's computer sends only one audio stream to the network, and only the targeted 50 students receive the stream. The information utilizes the minimum required network bandwidth and arrives at every student's computer without any noticeable delay.
This application is an example of the practical use of multicast in everyday life. The same is true for other applications like audio/video conferencing, multiplayer online gaming, online/offline video distribution, news and so on.Even if there are only three receivers of a multimedia application, the bandwidth utilization between routers can be roughly reduced up to one-third if we use multicasting.
The concept of multicast was introduced by Steve Deering in the '80's. Adding multicast to the internet does not alter the basic model of the network. Any host can send multicast data, but with a new type of address called a host group address. IPv4 has reserved class D addresses to support multicasting. A user can dynamically subscribe to the group to receive multicast traffic by informing a local router that it is interested in a particular multicast group. However, it is not necessary to belong to a group to send multicast. The delivery of multicast traffic in the internet is accomplished by creating a multicast tree, wit all of its leaf nodes as recipients.
