# SDN-Based Secure Common Emergency Service for Railway and Road Co-Existence Scenarios
## 1 Introduction
Indeed, the centralized SDN controller introduces a vulnerability as it becomes a single point of failure for the entire communication network. To mitigate this risk, the deployment of multiple controllers, as suggested in [12; 13; 14; 15], could be an effective solution. This approach enhances the reliability and robustness of the network by eliminating the single point of failure. Figure 1: Railway Emergency Call [7]. The SDN technology has been widely applied in the road environment, as demonstrated by [16; 17], but also in the rail environment [18]. This demonstrates the potential of this solution to guarantee the implementation of a solution that (1) guarantees the independence of potential multiple network/infrastructure operators, (2) guarantees efficient message distribution, and (3) reinforces network security. However, such work has given little consideration to the idea of common services and network interconnections, even though such solutions have already been proposed and implemented in other environments [19]. That is why in this article we propose an SDN-based solution for implementing common emergency services for road and rail environments. In the complex interplay of road and rail systems, numerous scenarios necessitate the intervention of common emergency services. These include, but are not limited to, road accidents in proximity to railway tracks, infrastructure failures, and public-safety threats. A dangerous situation occurs frequently when a car is stacked across the railway track in a level-crossing area. If the car could send an emergency message not just to a car emergency system, but also to a rail emergency system, the approaching train could be alerted in time to prevent a potential collision. This is the case considered and presented in this paper. The aim of the proposed architecture and mechanisms, leveraging SDN capabilities, is to enable (1) coordinated management of emergency messages for the rail and road environments; (2) isolation of services intended solely for one of the two environments; (3) prevention of attacks that might seek to take advantage of this sharing of services by relying on the effective management of attacks that SDN is likely to enable [20; 21; 22]; and finally, (4) the applicability of the proposed solution in a real-life environment. The code for the proposed SDN application is available online [23], and this article aims to present the mechanisms implemented for traffic differentiation and malicious behavior detection, as well as to demonstrate how the implemented solution works. This paper is organized as follows: Section 2 gives information about some related work. Section 3 presents an overview of the mechanism to implement the secure and common emergency service. Section 4 provides information about the proof of concept (PoC) to implement the SDN application. Validation of the developed system to distribute the common emergency messages is presented in Section 5 and tackling DDoS attacks is presented in Section 6. Finally, Section 7 concludes the paper with a critical analysis.