What powerful new byproduct of the Internet will be the next to knock the world on its ear? Nobody's making predictions, but it could be the MBone.
MBone stands for Multicast Backbone-or the Virtual Internet Backbone for Multicast IP. It allows simultaneous video, audio and graphic communication between Internet sites, and even conferences among a number of parties. An MBone node has been described as "a multimedia television station that you can use to broadcast from your desktop to the world"-an intriguing tool that relatively few Internet users yet have the facilities and the knowledge to use.
The MBone depends on an addressing scheme in the Internet Protocol (IP) originally implemented by Steve Deering, a member of the research staff at the Xerox Palo Alto Research Center (Xerox PARC) in California. Deering explained the status of the MBone in a recent meeting of the Software Entrepreneurs' Forum/UniForum Open Systems SIG in Palo Alto.
First conceived 10 years ago, the MBone didn't become real until 1992, following a meeting of the Internet Engineering Task Force (IETF) where the specifications were adopted. Beginning with 40 subnets in four countries within the Internet, the technology has spread gradually across the world among knowledgeable researchers, academics, and others with the available resources. Today it includes about 1,500 subnets in 25 countries on all continents.
The MBone is used for a variety of conferences, including U.S. House of Representatives and Senate meetings, as well as for scientific data dissemination by entities like the Woods Hole Oceanographic Institution in Massachusetts. Conferences have been held with 600 to 700 participants, Deering reported.
Deering's presentation was itself a live demonstration of the MBone's capabilities, featuring Deering as the principal speaker in an MBone conference. A dozen or so nodes logged onto the conference as Deering's video image (at a speed of one to three frames per second) appeared on a projected screen. Now and then, other MBone authorities, from as far away as Australia, joined in the discussion, with their live video images in separate windows on the screen. Other windows were reserved for conference management-such as monitoring attendees and the audio level-and for a shared electronic whiteboard that could be used for typing or drawing.
As enticing as it sounds, the MBone is still an experiment, and likely to remain so for the immediate future. The reasons have to do with infrastructure-mainly bandwidth. The MBone is a bandwidth hog, and the traffic expected during an IETF multicast is from 100 to 300 kilobits/sec, with 500 kbs as the current design bandwidth. Most sites connected to the MBone are 1.5 megabits/sec T1 communication links, although restricted traffic can be carried over slower-speed lines. Most Internet sites, on the other hand, have a link providing 56 kbs or less, and even ISDN lines support only 128 kbs.
In addition, multicasting requires special routing. Multicasting communication packets have to be transmitted between routers with a multicasting function-a capability that most routers on the Internet don't have. Therefore, to allow transmission to many sites at once, IETF multicasting employs a tunneling technique-encapsulating a multicast packet inside a unicast IP packet so that it can be sent through unicast routers to wherever the next multicast router happens to be. Deering said MBone researchers hope that the tunnels will be replaced with actual multicasting. Then the MBone would cease to be a virtual network, layered on top of portions of the physical Internet, and become one with it. "It's certainly my hope that this will become an accepted part of the Internet," Deering said. "I don't plan to quit any time soon."
Since the infrastructure for widespread use isn't available yet, MBone products have not appeared on the market. IP multicast host extensions are being added to some vendors' operating systems, however, which is one of the first steps before popularization is possible. However, no network provider is currently offering production IP multicast service.
Those with enough interest and resources may sign up as network-provider participants in the MBone, each of which provides one or more IP multicast routers to connect with tunnels to other participants. In addition, most sites use workstations running the mrouted program that supports the multicast transmissions. The person supporting a network's participation in the MBone should have the skills of a network engineer. To set up and run an mrouted machine requires the knowledge to build and install operating system kernels.
The hardware and software needed to support the mrouted program is fairly restricted. Machines with the capability include Sun Sparcstation; DEC Vax, MicroVax, or DECstation 3100 or 5000; and all Silicon Graphics machines. Silicon Graphics includes IP multicast as a standard part of its operating system.
Audio and video applications can be run point-to-point between two hosts using normal unicast addresses and routing. But to join conferences with multiple hosts, each host must run an operating system kernel with IP multicast support. A user can pick up free IP multicast software and add it to AIX 3.2, HP-UX, SunOS 4.1.x, and DEC ULTRIX. PC machines running DOS or Windows can get a package from FTP Software that includes IP multicast support. No IP multicast support is available for Macintosh or Next.
For further information on the MBone, use the following addresses: