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.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.
