Note that the destination of source agent must be set before calling []create-session.
SessionSim instproc create-session { node agent } {
$self instvar session_
set nid [$node id]
set dst [$agent set dst_]
set session_($nid:$dst) [new Classifier/Replicator/Demuxer]
$agent target $session_($nid:$dst) # attach the replicator to the source;
return $session_($nid:$dst) # keep the replicator in the SessionSim instance variable array {\tt session\_};
}