Multicast streaming may be a big win when you're only streaming the top
5 or 10 networks (for some value of 5 or 10). What's the performance
characteristics if you have 300K customers, and at any given time, 10%
are watching something from the "long tail" - what's the difference between
handling 30K unicast streams, and 30K multicast streams that each have only
one or at most 2-3 viewers?
1/2, 1/3, etc the bandwidth for each additional viewer of the same stream?
The worst case for a multicast stream is the same as the unicast stream, but the unicast stream is always the worst case.
Multicast doesn't have to be real-time. If you collect interested subscribers over a longer time period, e.g. scheduled downloads over the next hour, day, week, month, you can aggregate more multicast receivers through the same stream. TiVo collects its content using a broadcast
schedule.
A "long tail" distribution includes not only the tail, but also the head. 30K unicast streams may be the same as 30K multicast streams, but
30K multicast streams is a lot better than 300,000 unicast streams.
Although the long tail steams may have 1, 2, 3 receivers of a stream, the Parato curve also has 1, 2, 3 streams with 50K, 25K, 12K receivers.
With Source-Specific Multicast addressing there isn't a shortage of multicast addresses for the typical broadcast usage. At least not until
we also run out of IPv4 unicast addresses.
There is rarely only one way to solve a problem. There will be multiple
ways to distribute data, video, voice, etc.