## 5 Key Enablers
### Software Defined Networking  
Software defined networking (SDN) is another key enabler that can offer flexible architecture for edge cloud networks by decoupling controlling and forwarding functions. The decoupling of the control plane and forwarding plane of SDNs could provide enhanced network management, control, and traffic engineering. An SDN controller is capable of  
Figure 6: SDN-enabled edge cloud infrastructure.  
dynamically computing a path based on the requirement of an application [37]. A simplified architecture of an SDN network is shown in Figure 7. As we mentioned before, edge cloud is a large-scale distributed network architecture that brings storage and computing closer to the end user. Decoupling of the control and data planes can play a crucial role in edge cloud networks for better management, control, and handling of user data. By using an SDN controller in an edge cloud network, administrators can modify the characteristics of switching devices and manage the routing tables to control data flow in the network [38; 39]. Cloud providers such as Amazon, Google, and Microsoft have already started deploying SDNs in their cloud data centers [40; 41; 42]. Therefore, an SDN-enabled edge cloud architecture could provide dynamic and scalable network management capabilities to adapt with changing resource demands and availability of various applications, such as autonomous vehicles, smart cities, and industrial IoT [43].  
The seamless communication between data plane and control plane is established by southbound interface (SBI) or southbound API (SBAPI). It facilitates the installation of flow entries, push configuration, enforce policies in the network elements (NEs), and obtaining information from NEs for better management, monitoring, and telemetry. OpenFlow protocol is a standard SBI, initially proposed by open network foundation (ONF) [44; 45].  
In Table 3, we present a set of related works focusing on edge cloud and integration with SDNs and NFV. The following research papers focus on portability, offloading, load balancing, task booking, resource allocation, network control with the integration of SDN, NFV, and machine learning to reduce the latency and improve the throughput and management of bandwidth, etc.  
In the SDN-enabled edge cloud architecture, data planes and control planes are integrated in the access network to the central cloud. The data plane is responsible for forwarding data and the control plane is responsible for path selection, synchronization, resource management, and policy implementation. Moreover, the SDN controller supports centralized network management, providing network operators with a global visibility of the network and programming the controller for intelligent network service management by employing machine learning. Therefore, major advantages of SDN-enabled edge cloud networks include (1) network management, (2) flexibility and load balancing, (3) bandwidth allocation per flow, (4) multi-tenancy, (5) virtual application networks, (6) dynamically allocated workload, (7) adaptability and interoperability, (8) VM or container mobility management, (9) reduce latency, and (10) lower cost. In the next section, we present the software-defined controllers proposed by different researchers from industry and academia for edge cloud networks in detail.  
Figure 7: Simplified SDN architecture.