Java Network Programming, 4th Edition

Chapter 13. IP Multicast

The sockets in the previous chapters are unicast: they provide point-to-point communication. Unicast sockets create a connection with two well-defined endpoints; there is one sender and one receiver and, although they may switch roles, at any given time it is easy to tell which is which. However, although point-to-point communications serve many, if not most needs (people have engaged in one-on-one conversations for millennia), many tasks require a different model. For example, a television station broadcasts data from one location to every point within range of its transmitter. The signal reaches every television set, whether or not it’s turned on and whether or not it’s tuned to that particular station. Indeed, the signal even reaches homes with cable boxes instead of antennas and homes that don’t have a television. This is the classic example of broadcasting. It’s indiscriminate and quite wasteful of both the electromagnetic spectrum and power.

Videoconferencing, by contrast, sends an audio-video feed to a select group of people. Usenet news is posted at one site and distributed around the world to hundreds of thousands of people. DNS router updates travel from the site, announcing a change to many other routers. However, the sender relies on the intermediate sites to copy and relay the message to downstream sites. The sender does not address its message to every host that will eventually receive it. These are examples of multicasting, although they’re ...