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\\_};
}