###### Abstract
These centrally managed data flow rules are easier to understand and manage for OT network users, since they view the data paths from a high-level, network-wide perspective. In parallel, the diffusion of wireless sensor networks, embedded systems, and low-cost sensors has promoted the rise of the industrial IoT (IIoT) [6]. Nonetheless, it should be noted that there are some subtle differences between the concepts of the IIoT and the IoT, since the first one needs more communication bandwidth, less latency, and high resilience to effectively handle real-time big data transmissions. For these reasons, the IIoT platform for Industry 4.0 has been widely studied and employed in various scenarios [7]. Basically, the IIoT concept provides an integrated platform for gathering data from different kinds of sensors, transmitting it to the edge or to the cloud, and updating the system state in the form of a closed-loop system [8]. In this way, the IIoT is capable of effectively detecting failures and triggering maintenance processes. Nevertheless, there are some challenges which need to be addressed, and the most important challenge is securing industrial infrastructure and its components. Cyber-attacks on critical infrastructure and industries raise concerns, as the losses incurred due to such attacks are very huge. The IIoT is sometimes referred to as the integration of IT and the OT layers, where IT comprises the enterprise network and OT involves components interacting directly with the physical machines and extracting data from them. These two components have different security requirements which need to be considered to prevent compromising the IIoT infrastructure [9]. The OT layer involves components interacting directly with the physical machines and extracting data from them. Security here is a key requirement, because malicious intruders can cause extensive damage to people and machinery. On the contrary, the IT layer can have much more relaxed constraints than the OT layer because it contains only some services for data analysis and does not involve physical machines. However, despite the benefits deriving from the integration of SDN with the IIoT [10], to the best of the authors' knowledge, there are not many works that propose an SDN-based architecture for IT/OT convergence. Therefore, this paper aims to fill that gap by proposing an architectural solution based on the SDN paradigm that aims to merge the IT and OT layers. The proposed architecture is composed of three layers: the machine layer that involves physical machines directly and the OT layer, the control layer that contains the SDN controller functionalities and the IT layer, and the application layer that contains the business applications. Moreover, the proposed architecture is built upon the SIRDAM4.0 support infrastructure for reliable data acquisition and management within IT/OT convergent networks [11], developed within the framework of the SBDIOI40 project. Then, focusing on security aspects that are central issues for industrial networks, especially in the OT layer, we qualitatively analyze the proposed architecture under cyber-attacks. Indeed, in this field, a qualitative investigation into the causes and possible effects of cyber-attacks is still missing. In fact, the SDN-enabled IIoT networks are vulnerable to different types of attacks on each level; in other words, the application plane, data plane, and above all the control plane. In general, control system vendors and OT networks have several delays in IT when it comes to advancing cyber-security controls, since such environments have typically been physically separated and rely on specialist hardware and software [12]. From a security standpoint, the ability to secure centralized controllers, espoused as a primary risk to SDNs, becomes a possibility in which the benefits of SDNs and convergence seem to outweigh the negatives.