Using a new technology usually carries some advantages and
disadvantages. The advantages of multicast are -I think- clear.
The main disadvantage is that hundreds of hosts and, specially,
routers don't support it yet. As a consequence, people who
started working on multicast, bought new equipment, modified
their operating systems, and built multicast islands in their
local places. Then they discovered that it was difficult to
communicate with people doing similar things because if only one
of the routers between them didn't support multicast there was
nothing to do... The solution was clear: they decided to build a
virtual multicast network in the top of the Internet. That is:
sites with multicast routers between them could communicate
directly. But sites joined across unicast routers would send
their island's multicast traffic encapsulated in unicast packets
to other multicast islands. Routers in the middle would not have
problems, as they would be dealing with unicast traffic.
Finally, in the receiving site,