MBONE: Multicasting Tomorrow's Internet
In terms of memory, there is no absolute number. My multicast router runs FreeBSD on a dedicated 90 MHz pentium machine with 20 megabytes of memory and seems to work fine. Because the MBONE is based on multicast UDP (User Datagram Protocol) traffic, and UDP has been designed to be unreliable, a packet that does not make it to the dest-ination is not retransmitted and thus not stored for later retransmission. If the MBONE was based on TCP and the physical link over which a tunnel would go experienced problems, then the packet would be stored for later retransmission (up to the maximum size of the network buffers) and a lot of memory would be taken up this way.
Now you're probably asking, "What, the MBONE is unreliable?" The answer is yes. It is and it has to be, except for the text medium (mumble), which hopefully will be based on RMP (Reliable Multicast Protocol) in future versions. The MBONE for the other media has to be unreliable because we don't want an audio packet, for example, to be retransmitted when there is a problem with the original transmission. A packet that is missed cannot be retrieved; it will just make a hole in the speech you're listening to. After a packet has been missed, the rest of the speech continues to flow, and if a retransmitted packet arrives in the middle of a word three seconds after the word it was originally part of, the result could easily be an unintelligible speech.
What is probably more important than memory is the CPU power of your machine. When I first heard about the MBONE, I decided to try it using my own workstation and was able to have mrouted run on a Sun IPX. The tunnel I had was destined to feed my machine only, which was working fine. That same machine was my personal workstation, and I was running all my processes on that machine, including X Windows and the MBONE applications, so any Pentium PC would do fine for a single user. You could even afford having a few tunnels to friends who are willing to try the MBONE. You would want no more than one or two tunnels, however, if you plan to continue working on that machine.
In terms of disk storage, you don't need much, except for running UNIX. My multicast router has a 1GB disk, which is more than enough since most of the machine's activity consists of receiving, duplicating, and sending network packets.
More important than disk space and memory is the speed of the bus and the performance of the network adapters. Fast adapters on a PCI bus will do a very good job.