### Leveraging Software-Defined Networking (SDNs) in the Design of IIoT-Edge Networks  
The term _\"Software-defined networking\"_ was coined by [PERSON] in _MIT Technology Review_ in 2009 when describing the newly created OpenFlow specifications [72] as _\"a networking approach that utilizes software-based controllers or application programming interfaces (APIs) to manage hardware infrastructure and direct network traffic\"_. Other researchers have adopted the term to represent their ideas and work around OpenFlow [73; 74; 75]. Fundamentally, a software-defined network (SDN) decouples the control plane from the data plane, making the network devices (in the data plane) programmable through standardized application interfaces. It differs from traditional networks where the control and data planes are bundled in a single device. The control plane makes the forwarding decisions, packages them as flow instructions, and sends them asynchronously to the data plane that comprises ethernet switches known as forwarding devices (FDs) [75; 76]. These FDs forward the packet from source to destination. Figure 5 illustrates the SDN architecture, comprising four planes: (i) data plane, (ii) control plane, (iii) application plane, and (iv) management plane [18; 77]:  
**Data Plane:** The data plane, at the bottom of the architecture, comprises the FDs that forward ethernet packets or frames from source to destination. These FDs in the data plane rely on the controllers in the control plane to determine their forwarding behavior. As such, the data plane FDs interact with the control plane's controllers via the Southbound Interface (SBI) using the OpenFlow protocol [73]. At the SBI, the other protocols used besides OpenFlow protocol are Open vSwitch Database (OVSB) [78], Protocol-Oblivious Forwarding (POF) [79], CISCO OpFLEX [80], OpenState [81], Revised Open-Flow Library (ROFL) [80], Hardware Abstraction Layer (HAL) [73], programmable abstraction of data path (PAD) [82], and Forwarding and Control Element Separation (ForCES) [83]. **Control Plane:** To ensure that the control plane is not a single point of failure in the architecture, \((n+1)\) controllers are deployed as either a flat distributed peer-to-peer or a hierarchical-based cluster. They communicate with each other, sharing the network state of the FDs in the data plane under their administrative domain through an east/westbound interface [80; 84]. The controller's core functions include, but are not limited to, managing the network state of the network. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Features** & **Traditional Networks** & **Software-Defined Networks** \\ \hline Architectural Design & Distributed design with control plane and data plane coupled in a single device & Centralized design with decoupled control plane and data plane \\ \hline Programmability & Non-programmable; Difficult to replace existing program as per use & Programmable; Easy to update existing program per use \\ \hline Configuring and Managing & Supports static and manual configuration Difficult to troubleshoot and report in a distributed control design & Supports reactive/proactive automated configuration \\ \hline Cost implications & High CAPEX and OPEX & High CAPEX and low OPEX \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A feature-based comparison between the traditional communication networks and the software-defined networks. Figure 4: Comparing the traditional networks with the software-defined networks applicable to both switch and router networks. ing the network topology, link discovery, collecting network-based statistics, managing queues, and traffic flow [85]. Currently, there exists a wide array of SDN controllers such as OpenDayLight (ODL) [86], Open Network Operating System (ONOS) [87], Python-based SDN Openflow eXperimentation platform (POX) [88], Floodlight [89], Network Operating System (NOX) [90], Ryu [91], Beacon [92], Maestro [92], Iris [93], MUL [84], Runox [84], SEL-5056 SDN flow controller [94], and Lib-Fluid [84].