MBONE: Multicasting Tomorrow's Internet

How Is the MBONE Different from Multicasting?

Unfortunately, the majority of the routers on the Internet today don't know how to handle multicasting. Most routers are set up to move traditional Internet Protocol (IP) unicast packets -- information that has a single, specific destination. Although the number of routers that know how to deal with multicast are growing, those products are still in the minority.

Router manufacturers have been reluctant to create equipment that can do multicasting until there is a proven need for such equipment. But, as you might expect, it's difficult for users to try out a technology until they have a way to use it. Without the right routers, there's no multicasting. Without multicasting, there won't be the right routers. Catch-22.

What is a router?

A router is a device that connects a local area network -- such as an inter-office LAN -- to a wide area network -- such as the Internet. The router's job is to move information between the two networks.

Most routers today are unicast routers: They are designed to move information from a specific place to another specific place. However, routers that include multicasting capabilities are becoming more common.

In 1992, some bright fellows on the Internet Engineering Task Force (IETF) decided that what no one would do in hardware, they could do in software. So they created a "virtual network" -- a network that runs on top of the Internet -- and wrote software that allows multicast packets to traverse the Net. Armed with the custom software, these folks could send data to not just one Internet node, but to 2, 10, or 100 nodes. Thus, the MBONE was born.

The MBONE is called a virtual network because it shares the same physical media -- wires, routers and other equipment -- as the Internet.

The MBONE allows multicast packets to travel through routers that are set up to handle only unicast traffic. Software that utilizes the MBONE hides the multicast packets in traditional unicast packets so that unicast routers can handle the information.

The scheme of moving multicast packets by putting them in regular unicast packets is called tunneling. In the future, most commercial routers will support multicasting, eliminating the headaches of tunneling information through unicast routers.

When the multicast packets that are hidden in unicast packets reach a router that understands multicast packets, or a workstation that's running the right software, the packets are recognized and processed as the multicast packets they really are. Machines (workstations or routers) that are equipped to support multicast IP are called mrouters (multicast routers). Mrouters are either commercial routers that can handle multicasting or (more commonly) dedicated workstations running special software that works in conjunction with standard routers.

So, what's the difference between multicasting and the MBONE? Multicasting is a network routing facility -- a method of sending packets to more than one site at a time. The MBONE is a loose confederation of sites that currently implement IP multicasting.

The MBONE -- or multicast backbone -- is a fancy kludge, a hack. It is at best a temporary utility that will eventually become obsolete when multicasting is a standard feature in Internet routers. By then there will be an established base of MBONE users (which should make the router manufacturers happy). The utilities and programs that work on today's MBONE will undoubtedly work on the multicast backbone of tomorrow.

Pavel Curtis, a researcher at Xerox PARC (Palo Alto Research Center) says, "I believe that IP multicast is very likely to remain an important part of the Internet for quite a long time, and that it will be the primary audio/video transmission medium on the Net."

"On the other hand," he continues, "I think that the MBONE as an identifiable subset of machines on the Net is already beginning to fade away, as more and more router and computer vendors supply IP multicast support in their products; when multicast support is ubiquitous, the MBONE ceases to be identifiable as something other than the Net as a whole."

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