Multimedia doesn't do anything for me. Since we moved into our new house in 1993, I haven't gotten around to hooking up the TV. There's always been something more interesting to do--for example, fiddling with my computer or working on multicast over the Internet.
Multicast and multimedia are closely related. To understand why multicast is important (and what it is), remember that multimedia of all kinds means lots and lots of bits, like the 12MB preview of the last Star Trek movie. Multimedia usually means moving the bits from one place to another on the Internet.
On the Air
There are two problems involved in moving enough data around to allow thousands of people to see Star Trek captains Kirk and Picard in multimedia motion. First, even though the backbone of the Internet is composed of very high-speed links, it's wasteful to send the same data repeatedly on the same expensive shared resource. Why keep moving the same preview from Los Angeles to New York, on the same lines, over and over again? When the lines are between North America and Europe, the problem is more acute because bandwidth is expensive. It would be a better use of scarce resources to avoid sending the same data over the backbone more than once.
The second problem is more subtle. The access line between the provider of the data and the Internet itself becomes a major bottleneck when many users are retrieving data at the same time. For example, if a data provider had a 56-Kbps access line and a dozen people tried to retrieve a file at the same time, each user would obtain throughput not much better than that found using a 2,400-bps modem.
Multicasting is a network technology that must be mastered to solve the dual problems of wasted bandwidth and slow access lines. You probably know the term broadcasting. In the world of networks, broadcasting means sending a message, addressed to no one in particular, to every station on the network. The opposite of broadcasting is unicasting, in which a message is addressed to a single user. Multicasting is halfway between the two. A multicast message is sent to some subset of the stations on the network, called the multicast group.
The Internet has no real concept of multicasting. When a data provider wants to send the same file to 10 people at the same time, it has to send 10 copies over the Internet. If the Internet had multicasting, the data provider could send one copy, but it would be addressed to a particular multicast group. It would be up to the Internet to pick the most efficient way to distribute the information to that group.
We already use some primitive forms of multicasting on the Internet. For example, many popular FTP sites have mirrors that keep a complete copy of their contents, which are updated daily. The idea is that you should connect to the mirror closest to you and avoid transmitting the same data the long way around. Mirrors are used heavily in Europe--where the shortage of transatlantic bandwidth makes it difficult to download information from North America--Australia, and New Zealand.
The Prodigy online service works in a similar way, with frequently requested information being cached on local computers, rather than at Prodigy's main complex in White Plains, N.Y. This saves both time and bandwidth, as users in California often don't need to wait for data to come all the way across the country.
Sticks, Stones, and MBones
A more interesting application of multicasting is the MBone--the multicasting backbone--built on top of the regular Internet backbone (see "Stretching the MBone" on p. 38). The MBone was designed to experiment with multicast technology on the Internet, and it is used primarily for multimedia transmissions such as video and audio multicasts of Internet Engineering Task Force meetings. The MBone uses special software in Unix workstations to build a virtual network over the Internet that multicasts information efficiently. To participate in an MBone conference, you connect to the nearest MBone node and run the appropriate multimedia display software.
Although the MBone does stress the Internet backbone when a full video transmission is occurring, the load is a small fraction of what would be encountered if there were no MBone. In fact, videoconferencing with more than two or three sites would be impossible because of the bottleneck at each site's access line.
Last November's audio broadcast of a Rolling Stones concert introduced multicast technology to a new audience over the StoneBone. MBone conferences are becoming extremely popular. For several months in 1994, traffic over the MBone comprised almost 10 percent of the total traffic over the Internet.
Internet protocol architects are so concerned about the impact of multimedia transmission over the Internet that they are building support for multicast into the next generation of the Internet Protocol (IP). Although the shortage of address space is most frequently named as a reason for a new version of IP (called IPng), support for multimedia applications is a big reason the protocol looks the way it does.
Multimedia Citizenship
Right now, it's probably not possible for you to participate in an MBone video- or audioconference unless your workstation is connected to the Internet at a university or research lab. The day is coming, however, when technologies like Internet multicasting will be able to bring you multimedia video and audio without causing a meltdown. In the meantime, here's some advice from the protocol architect side of the Internet on how to be a good multimedia citizen:
Joel Snyder (jms@opus1.com) is a senior partner at Opus One, specializing in telecommunications and information technology.
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