OnOffHelper onOffApp ("ns3::TcpSocketFactory", InetSocketAddress (interfaces.GetAddress (1), 9));
  onOffApp.SetAttribute ("DataRate", StringValue ("1Mbps"));
  onOffApp.SetAttribute ("PacketSize", UintegerValue (2048));
  onOffApp.SetAttribute ("OnTime", StringValue ("ns3::ConstantRandomVariable[Constant=2.0]"));
  onOffApp.SetAttribute ("OffTime", StringValue ("ns3::UniformRandomVariable[Min=1.0|Max=3.0]"));