# Multi-Controller Deployment in SDN-Enabled 6G Space-Air-Ground Integrated Network
## 1 Introduction
Therefore, this paper proposes a multi-controller deployment strategy based on a simulated annealing algorithm and a switch migration strategy oriented toward load balancing for the SDN-based 6G space-air-ground integrated network. Unlike previous work, satellite gateways and controllers are jointly deployed, or all controllers are deployed on satellites. The optimization goal is singular, and the deployment strategy does not consider network topology dynamics. This paper hierarchically deploys controllers on the ground network: the main controller is deployed at the network operation controller center, and the domain controller is deployed near the ground station to minimize the time delay loss from the ground station to the controller. The deployment strategy jointly optimizes the network propagation delay and controller load. The switch migration strategy oriented toward load balancing solves the problem of load imbalance caused by the network topology changes dynamically. In short, the main contributions of this article are as follows:  
* Firstly, based on the hierarchical SDN-enabled 6G SAGIN model, the controller hierarchical network design scheme is adopted, and the delay model of the network and the load model of the controller are constructed. The loss value, as an important basis for network division, is defined by combining the delay model and the load model. * Then, in order to obtain the global optimal solution and reduce the time complexity of the algorithm, a multi-controller deployment strategy based on simulated annealing is proposed, and a concrete deployment scheme is given according to different delay and load requirements. * Finally, considering the dynamic nature of the network topology, a switch migration strategy is proposed, in order to achieve load balance among the controllers, and some switches within the jurisdiction of the high-load controller are selected to be transferred to the low-load controller, improving the utilization of network resources. Due to the need to implement this strategy at certain intervals to avoid load imbalance between the controllers, resulting in a sharp decline in network performance, the time complexity of the switch migration algorithm cannot be too high. The rest of this paper is organized as follows: Sections 1.1 and 1.2 introduce the related work of the SDN-enabled 6G SAGIN and multi-controller deployment strategy. Section 2 introduces the hierarchical and domain SDN-based 6G SAGIN architecture. The multi-controller deployment problem SDN-based 6G SAGIN is described as a mathematical model and a hierarchical and domain multi-controller deployment strategy. Section 3 compares and analyzes the performance of this strategy and gives a feasible deployment plan. Section 4 summarizes this article. With the proposal of the SDN-enabled SAGIN, more and more studies are focused on how to design the hierarchical structure of the network and how to deploy the SDN controller reasonably in order to solve the problem of network delay, link throughput and other network performance degradation problems caused dynamic changes in SAGIN, the unbalanced distribution of large-scale nodes and strong heterogeneity.