# SDN-Based Survivability Analysis for V2I Communications
## 2 SDN-Based V2I Network Communications  
The SDN-based vehicle network architecture is shown in Figure 1, which is divided into three layers: the data layer, control layer, and application layer. For the data layer, there are three elements: the vehicle, infrastructure, and switching route. The switching between communication links in V2I networks is completed through switches and/or routers. For the control layer, the SDN controller serves as a network center management part, separating the network control and the network topology of the vehicle networks, where the original network architecture extracts from the hardware control, instead of a software programmable design to centralize the dynamic management of various network parts. By using the Open Flow protocol at the south interface, it can be connected to various switching or routing devices in the data layer. Particularly, its north interface connects each application service and coordinates and controls each application service program to carry on effectively. On the other hand, for the application service layer, it mainly includes applications of the vehicle network: the security application service of the vehicle network, web service, media service of watching or downloading video, mobile information service of querying the electronic map in real time, real-time communication service, and so on. Each of these applications is controlled by the SDN controller.  
To analyze and study the survivability of SDN-based V2I vehicle networks, we establish a V2I network communication model based on the vehicle network architecture, which is shown in Figure 2. In this V2I network communication model, each vehicle equips three V2I communication links in three directions, namely the vehicle-to-RSU connection, vehicle-to-Wi-Fi connection, and vehicle-to-BS  
Figure 1: SDN-based vehicle network architecture.  
(such as LTE/4G/5G/6G) connection. In addition, the connection between the vehicle and RSU is carried out through the IEEE 802.11P protocol, which guarantees short-range communication between vehicle and vehicle. Moreover, vehicles and Wi-Fi IEEE802.11 standards are adopted to connect the vehicle network interface via the wireless access point (AP). For the MPTCP application, to improve the quality of network services, it is assumed that each vehicle involves multiple network interfaces. Each of them switches through MPTCP ports. The transfer between the TCP1, TCP2, and TCP3 interfaces in the MPTCP is controlled and switched by the Open Flow protocol in SDN central controller. In the Figure 2, due to the high speed movement of vehicles, there are some uncertain abnormal factors, such as the malicious vehicle intrusion or denial of service attack, channel fading, man-made accidents, natural disasters, and so on. All of these events can possibly lead to a breakdown for the V2I network and communication link. When one of the V2I links fails, MPTCP immediately responds by switching to one of the other two idle links to continue communication until the fault link is repaired in time. If either of the other two links fail, multiple repairs should be performed until the data is restored for transmission. For our proposed analysis and study of the survivability of V2I networks, we consider the fault state, fault type, and ability of the network to continue to complete the transmission task when the failure is repaired in time. The fault state includes three V2I link faults (vehicle and RSU, vehicle and base station, and vehicle and Wi-Fi) and the failure of the vehicle node. We consider that the fault types include link failures and node failures. In addition, in order to further study the ability of data transmission after the network failure, we also define the V2I network survivability according to the vehicle network communication model and then establish the survivability mode of the V2I network.  
Figure 2: SDN-based V2I network communication model.