Issues with Multicast and Unicast

Reliable transport:: The basic service provided by IP (multicast/unicast) is an unreliable datagram service.
This is appropriate for applications where concern with performance supercedes reliability. However, vendors providing IPC^[1] tools develop the reliability layer that gives application guarantees of atomicity and reliability. For example, the Multicast Transport Protocol would be useful for developing applications such as distributed databases which need to be certain that all members of a multicast group agree on which packets have been received.

If you have information that should be transmitted to various (but usually not all) hosts over an internet, then Multicast is the answer. One common situation in which it is used is when distributing real time audio and video to the set of hosts which have joined a distributed conference. Multicast is much like radio or TV in the sense that only those who have tuned their receivers (by selecting a particular frequency they are interested in) receive the information. In other words, you hear the channel you are interested in, but not the others.

Marc Andreessen started working on the Mosaic web browser in 1992. He was a computer science student at the University of Illinois, but he spent a lot more time at the NCSA, the University's supercomputing center, than he did in the classroom. Working at (NCSA) National Center for Supercomputing Applications was fascinating, says Andreessen. The super-computing program in the mid-1980's originated because the government thought it was crucial that a powerful computing infrastructure be provided for scientists. An interesting thing happened around 1990: all the supercomputing centers essentially stopped running supercomputers.
They shut down the Crays. They could no longer afford to keep them running, especially with the advent of more cost efficient microprocessors. Since it no longer made sense to run a supercomputer, what do you do? And so networking quickly became a very big part of NCSA, the theory being that the next stage would be to link together everything, especially scientists and educators who up until that point had been using the Crays. Networking had arrived so fast, and everything was happening so rapidly, that people just had not yet gotten around to making it accessible.

[1] IPC: Inter-process communication allows one process to exchange data with another process. The processes can be running on the same computer or on different computers connected through a network. An example is a socket.