In message <rmi3bt9is0s.fsf@fnord.ir.bbn.com>,
>Greg Troxel writes:
>  It does that EACH time it gets a request. So if we have ten different 
>  groups, we have ten different sockets.
>
>Probably OK until you hit the file descriptor limit.
>
>  Is it reasonable to have the same socket join ten different multicast 
>  groups? I _think_ that would be the easiest thing to do, but wanted to 
>  check.
>
>You almost certainly know this, but for v4 the answer would be 10 yes,
>21 no.

Mmmm. I was going to say "yes". But, UTSLing, perhaps a PF_INET socket
can *join* as many multicast groups as it wants, but only send on (at
most) 20?  That seems to match the use of ip_moptions in ip_output().

More ULTSing shows a comment that all 20 ``must fit in one mbuf''; but
I don't recall what the reason for that constraint was (if indeed it
dates back to Steve Deering's code).

Kernels are now bigger than physical RAM on the machines I was
compiling kernels on, back in those days. Seems high time to remove
the 20-group limit, if anyone's game.

Bill: does that buy you anything, if other OSes have a similar limit?