Java Reference
In-Depth Information
CHAPTER 13
IP Multicast
The sockets in the previous chapters are
unicast
: they provide point-to-point commu‐
nication. 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 millen‐
nia), 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 broad‐
casting. 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
implemented with additional application layer protocols on top of TCP or UDP. These
protocols require fairly detailed configuration and intervention by human beings. For
instance, to join Usenet you have to find a site willing to send news to you and relay
your outgoing news to the rest of the world. To add you to the Usenet feed, the news
administrator of the news relay has to specifically add your site to their news config
files. However, recent developments with the network software in most major operating
systems as well as Internet routers have opened up a new possibility—true multicasting,
in which the routers decide how to efficiently move a message to individual hosts. In
particular, the initial router sends only one copy of the message to a router near the
receiving hosts, which then makes multiple copies for different recipients at or closer
