Multicast: From Theory to Practice

As the Internet grows up, new communication needs arise. First, e-mail and FTP were enough for most people. Then the WWW arrived and people wanted to see graphics, not just plaintext. Now, even static graphics are not enough; real-time video and audio are demanded.

As communication needs evolve, communication paradigms originally designed to deal with e-mail and FTP need to evolve too. A new one that has developed is “multicast”.

Imagine transmitting an event over the Internet (perhaps a Linus Torvalds conference), and multicast is not available. A single source of information, which could be a computer connected to both the Internet and the video cameras and microphones Linus is talking to, is transmitting multimedia streams to several hosts dispersed over the Internet. Of course, traffic should be sent as efficiently as possible—the less bandwidth used, the better.

With pre-multicast technology, two communication paradigms are available, both of which are inadequate.

The first one is Unicast. TELNET, FTP, SMTP and HTTP are unicast-based protocols with one source and one destination. To send to multiple destinations, different communication paths are needed between the source and each of the destinations (see Figure 1.a). Therefore, a copy of each audio and video stream would need to be made and sent separately to each receiver. Clearly, this is not affordable. Even if you are quick enough in copying real-time audio and video streams, both your network and the Internet would collapse. Audio and video are extremely bandwidth expensive. Obviously, TCP cannot be used in multicast applications, as it is clearly unicast-oriented.

Figure 1.

The second choice is Broadcast. The broadcast paradigm (see Figure 1.b) saves a lot of bandwidth compared to unicast. If you want to send something to all computers on your LAN, you don't need a separate copy for each. On the contrary, only one copy is sent to the wire, and all computers connected to it receive the copy. This solution is better for our problem but is still insufficient, as we probably need to broadcast to only some of our computers, not all. Even worse, it is almost certain that many hosts interested in your conference will be outside your LAN. While broadcast performs well inside a LAN, problems arise when broadcast packets are routed across different LANs. Thus, broadcast is good for applications and protocols that don't need to cross LAN limits (such as ARP, BOOTP, DHCP and even routed), but it is not good enough for our problem. Finally, it is very likely people want to have more than one video conference at a time, when only one broadcast address is available.

After having looked at the problem, it is apparent we need a solution that provides the following:

The third point relates well to radio or TV channels (not cable TV). If you are interested in a particular channel, you tune your receiver to listen to a particular range of frequencies and discard the rest.

The solution that meets all three requirements is multicast. Figure 1.c shows that host 1 sends data only once (i.e., no per-receiver copies are made) and only hosts interested in this data (hosts 3, 5 and 6) receive it.

The radio/TV example will remain a good starting point for the rest of the article. In the same way that multiple frequencies ease the process of recognizing and isolating different TV channels, multiple multicast addresses ease the process of recognizing the multicast traffic of interest.

The range of IP addresses is divided into classes, based on the higher order bits of the IP address. Multicast addresses are class D addresses: those starting with the first three bits set to 1 and the fourth set to 0. This means multicast addresses range from 224.0.0.0 to 239.255.255.255. This is the range of “frequencies” in which you can transmit or listen for traffic. Each “frequency” identifies a different and specific multicast group.

Some of these multicast addresses are well-known, reserved for a specific purpose. For instance, 224.0.0.1 is the all-hosts group. Just “ping” this address, and all multicast-capable hosts on the network should answer. Any multicast-capable host must join this group at start-up on all its multicast capable interfaces. 224.0.0.2 is the all-routers group, and so on. In any case, your multicast applications should never send datagrams to addresses 224.0.0.0 through 224.0.0.255, as they won't be forwarded across multicast routers. Similarly, you should avoid groups 239.0.0.0 through 239.255.255.255 as they are reserved for administrative scoping. See the “Multicast over TCP/IP HOWTO” (included in the Linux Documentation Project) for further details.