[ns] multicast

[Will Shaver <will@digitalfountain.com>]

 Is there a way to get the nodes on a multicast tree to NOT forward
packets if no-one downstream wants them. As I have it set up the packets
just keep going node after node even if there is nothing that wants them
downstream.

...

set node0 [$ns node]
set node1 [$ns node] 
set node2 [$ns node]
set node3 [$ns node]
set node4 [$ns node]
$ns duplex-link $node0 $node1 1Mb 20ms DropTail 
$ns duplex-link $node0 $node3 1Mb 20ms DropTail 
$ns duplex-link $node1 $node2 1Mb 20ms DropTail
$ns duplex-link $node3 $node4 1Mb 20ms DropTail
set mproto DM
set mrthandle [$ns mrtproto $mproto ]

set $MulticastServer [new Agent/MulticastServer]
$MulticastServer start
set MulticastClient [new Agent/MulticastClient]
$node0 attach $MulticastServer 	
$node2 attach $MulticastClient

$ns at 2.0 "$MulticastClient start"
$ns at 20.0 "finish"

...

And yet nodes 3 and 4 clearly (in NAM) are recieving and forwarding
packets generated by the server.
 -Will