# SDN-Based Secure Common Emergency Service for Railway and Road Co-Existence Scenarios
## 2 Related Work and Motivation  
Existing SDN-based work aimed at improving coexistence between rail and road environments can be divided into two categories:  
* **Work focused on a single domain:** This work aims to enable more efficient dissemination of information in either the rail or road environment. The idea is, therefore, to try to ensure global safety without implementing exchanges between road and rail servers/terminals. Most of these solutions have been developed for the road environment. Indeed, the recent and numerous studies on automated and connected vehicles have encouraged the development of proposals designed to guarantee a high level of safety. In this road context, we can cite the following as interesting examples [24; 25; 26]: they aim to manage and anticipate the mobility of cars in the best possible way to guarantee the dissemination of critical messages with minimum latency. Similar work can be found in the railway environment, such as [18]. These projects have two similar features: (1) mobility management adapted to both rail and road environments, and (2) traffic differentiation designed to guarantee faster dissemination of critical messages. However, they do not enable the interconnection of rail and road environments, and therefore, cannot ensure global safety.
* **Work seeking to propose a solution common to both domains:** The idea here is to ensure that safety is shared by both the road and rail domains, and thus, to guarantee the transmission of information between these two environments. So far, this idea seems to have received little consideration in the context of SDN-based network architectures and, as far as we know, our article [27] was the first to mention it. However, the existence of work in a non-SDN context, aimed at enabling the exchange of critical information [8], both in research articles and in European projects, seems to demonstrate the relevance of this approach. Furthermore, the existence of SDN-based applications in each of the environments also seems to be an argument in favor of implementing such solutions interconnecting both environments.  
Therefore, existing research in the field has primarily focused on narrow aspects of SDN-based common emergency services for vehicular networks and railways. Most researchers have considered only vehicular networks in which numerous network-based solutions are currently being developed. Prior to the documentation of our current practical study, no previous studies were found that explored the implementation of a unified messaging for emergency services using a centralized SDN-based networking application.  
In addition, in order to limit the risks that the interconnection of these two environments could generate, we also propose integrating elements designed to reinforce system security. Indeed, the opening of doors between the two communication systems could be used by attackers to carry out denial-of-service (DoS) attacks against the rail and road environments, with the impact maximized by the duplication of messages to both environments. An attack that floods a network or server with data or internet traffic is referred to as a DoS/DDoS attack [28]. We have, therefore, integrated a message filtering mechanism into our architecture, taking our inspiration from the solutions described in the literature for managing DoS attacks in SDN-based systems [29; 30]. On this point, our contribution consists of adapting these solutions to the environment under consideration (interconnection of heterogeneous rail-road systems) and proposing an open-source implementation that can be reused by other researchers in the future, both in rail and road environments and in other environments.