Java Network Programming (3rd ed) [Electronic resources] نسخه متنی

Chapter 14. Multicast Sockets

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
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 your 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 possibilitytrue
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 to the destinations. Internet multicasting is built on top of
UDP. Multicasting in Java uses the
DatagramPacket class introduced in Chapter 13, along with a new
MulticastSocket class.