## 1 Introduction
### Influencing Factors of SDN Controller Deployment  
The SDN control plane includes one or more controllers, and the SDNs with multiple controllers can communicate with each other through the east-west protocol. For small-scale regional networks, a single controller is sufficient [9]. However, the scalability and information transmission efficiency of large-scale multi-area networks increase the demand for multiple controllers in SDN [22; 23]. The main differences between a single controller and multiple controllers are shown in Figure 3:  
As shown in Figure 3(1), the controller is responsible for controlling four routers when there is a single controller, while there are two controllers in Figure 3(2). In this case, the network state replicates and synchronizes between the two controllers. For the control plane deployment with multiple controllers, the following problems will arise: How can we better perform state synchronization? How many controllers should be placed in the SDN, and how should the controllers be deployed? When there are multiple controllers, it is essential to consider how to make it more efficient to determine the placement of the SDN control plane. The above problems further promote the research of SDN control plane placement problem, namely CPP. For the influencing factors of CPP, the following four aspects are analyzed:  
1. **Controller location.** In the SDN, the expected position of the controller is regarded as a subset of the set of switch positions. This design choice makes SDN migration easy,  
Figure 3: SDN comparison of single controller and multiple controllers.  
because traditional switches are gradually replaced by SDN switches and configured SDN controllers, thereby eliminating changes to the existing network layout.
2. **Number of control devices.** The key to studying CPP is to find the number of controllers, but the number of controllers is usually regarded as an input. In most of the existing research literature, the number of controllers is obtained by analysis ([24,25]) or experience ([13,26]).
3. **Control command load.** The load modeling generated by SDN switch control commands is different for single-area and multi-area networks. For single-area networks, the load from the SDN switch includes the load control packets it generates. However, for multi-area networks, the SDN switch control load is regarded as the sum of the load packets generated by it and the load from adjacent switches belonging to different regions [27]. In the SDVN, it is necessary to analyze and predict the network traffic, and then perform load balancing.
4. **Controller processing delay.** Compared with the propagation delay of switches, most studies ignore the processing time of SDN controllers. However, the control delay experienced by SDN switches is a key design consideration for the placement of SDN controllers, which directly affects the information transmission efficiency of SDVNs. In large-scale SDVNs, delay is positively correlated with distance, that is, the farther the distance, the greater the delay.  
The organizational structure of this article is as follows. Section 2 focuses on the methodology, that is communication delay parameters between vehicles and RSUs, mathematical model construction of control plane deployment, and the deterministic annealing clustering algorithm. Section 3 explores the SDVN traffic analysis and prediction in detail, and has presented the results of the experiment. Finally, in Section 4, a conclusion is drawn.