Abstract: Software defined networking (SDN) decouples the control plane from the switch in the data plane, which forms the SDN controller. This paradigm introduces many benefits, e.g., openness, management simplicity, etc. Nevertheless, the separation of the SDN switch and the controller also leads to great communication overhead between them due to controlling the network (the number of the control message and Table-Miss packets), and the overhead becomes the major bottleneck of SDN. On the one hand, each Table-Miss event can produce multiple Flow-Mod messages which add extra bandwidth overhead as well as delay to the southbound interface. On the other hand, controller has no awareness of flow characteristic information behind the Flow-Mod messages which make the overhead worse. This paper proposes a new architecture uFlow (split up Flow) to mitigate the overhead at the controller side based on the flow volume characteristics. We implemented the prototype of uFlow system both in software-based platform mininet and hardware-based platform ONetSwitch. Experimental results driven by the real traffic show that uFlow can significantly reduce the communication overhead between control plane and data plane, the number of the control message has a decrease of 70% off on average, eliminate redundant update of flow entries in switch and reduce the transmission delay of packets.