Java Network Programming, Second Edition

Chapter 14. Multicast Sockets

All the sockets you’ve seen in the previous chapters have been unicast: they provided point-to-point communication. That is, 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 in 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 quite 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 tens 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 ...