## 5 Further Research Opportunities of the SDN-Enabled Smart Grid  
SDN is an emerging and promising future networking solution, but it needs to be optimized for use in a smart grid environment. Traditional SDN is designed for rich resource systems that are generally considered for backhaul IP networks. From a network design perspective, SDN-enabled smart grid systems require an additional network processing server, which must consider the end-to-end route between the SDN controller and each smart grid device in the region. Therefore, the communication overhead between the SDN controller and devices needs to be optimized in order to reduce network congestion, bottlenecks and computational delay problems. Challenging issues pertaining to the SDN-enabled smart grid system are as below:  
* Reducing SDN control message overhead: In spite of the different benefits, such as simplified network and system management, SDN may not be directly utilized in resource-constrained AMI networks because of the control message and end-to-end session-management overhead. Generally, the SDN controller and all clients periodically exchange control messages for service provisioning and monitoring via a secure path. These control messages use secure control paths that are logically isolated from the data path using SSL or virtual private networks (VPNs). However, the MAC and physical perspective of the communication interface control and data packet use the same physical interface. Further, SSL-based data communication requires end-to-end session management control packets that are periodically exchanged between the SDN controller and all devices in the network. From a security perspective, control and data channel isolation are acceptable methods, but the end-to-end transmission reliability of the data packet can suffer from network congestion due to the SDN control traffic. In addition, when the control message between the controller and device is not stably delivered, metering traffic cannot be transmitted, even if the network capacity is sufficient. In particular, resource-constrained smart grid networks, such as wireless HANs, need to consider low-overhead message exchange protocols, such as Internet Engineering Task Force (IETF)-Constrained Application Protocol (CoAP), to reduce congestion and realize a reliable SDN-enabled smart grid [41; 45]. * Route construction between SDN controller and clients: SDN enables flow-based data forwarding that can replace the traditional distributed routing protocols. This centralized routing is a key feature of SDN, but during the network initialization stage, each client needs to find a default route path to the SDN controller [42]. To solve this problem, a distributed routing protocol, such as OSPF, is considered for simultaneous use with SDN in wired SDN, but it is not suitable for use in resource-constrained wireless and PLC-based HANs. To solve this problem, the SDN-enabled smart grid must use the default routing function, which provides a dynamic configuration to legacy routing protocols, or discover the routing path by using an SDN routing application. In HANs, the routes between the SDN controller and a device are constructed using a link discovery procedure. Generally, the SDN controller uses a link discovery application that enables it to find the adjacent one-hop neighbor SDN controller of the device. Meanwhile, the device that is connected to the SDN controller can respond to the link discovery request with SDN connection flags. This connection information can be sequentially flooded to the entirenetwork, and the default routes between the SDN controller and the devices are established without additional routing protocols. * SDN controller and function distribution: In a large-scale smart grid network, the flow information request messages from all clients are concentrated at an SDN controller. There is the potential problem of network congestion, queue overflow, large processing delay and service instability of the SDN.