While all these industries are making great—and highly publicized—plans for future (inter)national digital video on demand, the Internet community has been quietly implementing its own digital multimedia system, MBone (Multicast Backbone). In this section we will give a brief overview of what it is and how it works.
MBone can be thought of as Internet television. Unlike video on demand, where the emphasis is on calling up and viewing precompressed movies stored on a server, MBone is used for broadcasting live video in digital form all over the world via the Internet. It has been operational since early 1992. Many scientific conferences, especially IETF meetings, have been broadcast, as well as newsworthy scientific events, such as space shuttle launches. A Rolling Stones concert
was once broadcast over MBone as were portions of the Cannes Film Festival. Whether this qualifies as a newsworthy scientific event is arguable. Technically, MBone is a virtual overlay network on top of the Internet. It consists of multicast-capable islands connected by tunnels,
In this figure, MBone consists of six islands, A through F, connected by seven tunnels. Each island (typically a LAN or group of interconnected LANs) supports hardware multicast to its hosts. The tunnels propagate MBone packets between the islands. Some day in the future, when all the routers are capable of handling multicast traffic directly, this superstructure will no longer be needed, but for the moment, it does the job.
Each island contains one or more special routers called mrouters (multicast routers). Some of these are actually normal routers, but most are just UNIX workstations running special user-level software (but as the root). The mrouters are logically connected by tunnels. MBone packets are encapsulated within IP packets and sent as regular unicast packets to the destination mrouter’s IP address. Tunnels are configured manually. Usually, a tunnel runs above a path for
which a physical connection exists, but this is not a requirement. If, by accident, the physical path underlying a tunnel goes down, the mrouters using the tunnel will not even notice it, since the Internet will automatically reroute all the IP traffic between them via other lines. When a new island appears and wishes to join MBone, such as G in Fig. 7-25, its administrator sends a message announcing its existence to the MBone mailing list. The administrators of nearby sites then contact him to arrange to set up tunnels. Sometimes existing tunnels are reshuffled to take advantage of the new island to optimize the topology. After all, tunnels have no physical existence. They are defined by tables in the mrouters and can be added, deleted, or moved simply by changing these tables. Typically, each country on MBone has a backbone,
with regional islands attached to it. Normally, MBone is configured with one or two tunnels crossing the Atlantic and Pacific oceans, making MBone global in scale.
