## 1 Introduction
### Related Works
The 4G/5G interface is exclusively activated when the OBU is outside the coverage of RSUs. This design ensures that compared to traditional SDVN systems, communication costs are significantly reduced by leveraging cellular connectivity only when necessary. In general, the OBU can obtain information about the nearby controller through the Basic Safety Message (BSM) broadcast by the RSU. When the vehicle is within the coverage of the RSU, the OBU is connected to the RSU controller through the V2I and V2V links, and the same link will also be used for data communication, which is called in-band control. When the vehicle moves between RSUs under different controllers, a simple switching mechanism based on signal strength is adopted. When a vehicle exits the coverage of an RSU (Road-Side Unit), the OBU (On-Board Unit) aboard leverages a dedicated 4G/5G communication channel to establish a connection with the base station. Since this channel is exclusively allocated for control and data plane communication, this specific mechanism is termed out-of-band communication. SDVN requires additional periodic collection of network information, which serves as the foundation for the control plane to enable precise and intelligent decision-making. Most existing SDVN (Software-Defined Vehicular Network) systems leverage cellular links to acquire such network information, which will generate additional communication overhead and cost. In the proposed SDVN, the communication overhead and cost pressure can be shared by the DSRC communication mode in the RSU. In SDVN, the RSU listens to regular BSM broadcasts to collect network information such as vehicle location and speed. Based on the retrieved information, the estimated distance of the vehicle, and the knowledge of the local road network, a local topology is generated. Initially, the topology information is transmitted to the nearest local controller. Subsequently, all controllers synchronize and integrate all the information received from the Road-Side Unit (RSU) to create an aggregated area network diagram. For On-Board Units (OBUs) outside the RSU coverage area, they are required to adhere to the existing state update mechanism. They send beacon messages to the nearest controller via the 4G/5G interface. In this scenario, the frequency of network information exchange is set to be consistent with the Basic Safety Message (BSM) frequency, because the network topology changes only when a new BSM arrives. In SDVN, [PERSON] et al. [21] investigated hybrid SDN architectures that integrate SDN with traditional distributed network models, presenting use cases that demonstrate how these hybrid approaches address the shortcomings of both models and provide compelling reasons for a partial shift to SDN. When the sender wants to send information to the receiver, the process is as follows: (1) When the packet arrives as the first packet of the data stream from the sender to the SDVN switch, for this packet, the SDVN switch checks the flow rules by matching it with the flow table entries in the SDVN cache. (2) If  
Figure 2: Software-Defined Vehicular Network paradigm. a matching entry is detected in the flow table of the SDVN switch, the instruction associated with a specific flow entry is executed, such as 'packet/matching field', 'update counter', etc. After that, the data packet is directed to the relevant receiver. If there is no matching item in the flow table of the switch, the data packet is directed to the controller through the secure channel. (3) In the data transmission phase, when the OBU receives a packet from an unknown destination port, the OBU sends a PACKET-IN message to the nearest controller. After that, the RSU controller analyzes the source IP address and the target IP address in the packet according to the request type and the availability of the information, and updates the switch flow table entries in the southbound API, the OpenFlow protocol path, to calculate the path required from the source node to the destination node.