#10 nsdr needs smarter interval calculations

[Lloyd Lim]

There are two problems:

(1) On a network with multicast only partially working,
as in Greg Cook's case, nsdr will transmit at the
maximum allowed rate if it can't see the other
announcements.

(2) On a heavily loaded group, nsdr may back off to an
unreasonably low rate. This is undesirable if the
announcement is for a live event that users are tuning
in to see.

To solve these problems, I suggest:

(1) nsdr should back off even if it can't see any other
announcements. There's no reason to transmit at the
maximum rate. It's too fast. We could set a maximum
rate of 15 seconds, for example.

(2) We could implement an option that sets the minimum
rate.

We probably need to think about this a bit more.