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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, traffic would be
de-encapsulated, and sent to the island in the original multicast way.
Two ends converting from multicast to unicast, and then again to multicast
define what is called a multicast tunnel.
The MBone or Multicast Backbone is that virtual multicast network
based on multicast islands connected by multicast tunnels.
Several activities take place in the MBone daily, but it deserves to be
remarked the profusion of tele-conferences with real time audio and video
taking place across the whole Internet. As an example, it was recently
transmitted (live) the talk Linus Torvalds gave to the Silicon Valley Linux
Users Group.
For more information on the MBone, see:
http://www.mediadesign.co.at/newmedia/more/mbone-faq.html
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