-std::map<std::string, SgFlow*> flow_from_sock; // ns3::sock -> SgFlow
-std::map<std::string, ns3::ApplicationContainer> sink_from_sock; // ns3::sock -> ns3::PacketSink
+std::map<std::string, SgFlow*, std::less<>> flow_from_sock; // ns3::sock -> SgFlow
+std::map<std::string, ns3::ApplicationContainer, std::less<>> sink_from_sock; // ns3::sock -> ns3::PacketSink