# Multi-Controller Deployment in SDN-Enabled 6G Space-Air-Ground Integrated Network
## 1 Introduction  
With the development of science and technology, 5G has entered the stage of commercial application, and more studies have begun to focus on the development of 6G. The vision of 6G communication in the future is the intelligent connection of all things, breaking through the limitations of terrain and surface, expanding to natural space such as space, sky, land, and sea, and truly realizing the universal ubiquitous connection [1, 2]. The space-air-ground integrated network (SAGIN), which combines the airspace network, sky domain network, and ground network, has become a trend to realize the vision of \"6G intelligent connection with everything\" [3, 4]. The SAGIN architecture is considered as an important framework for the future communication network because it has the characteristics of a space-based network and satellite-based network, such as wide coverage, high flexibility, strong cloud-edge cooperation [5], and the high transmission rate of a ground network [3]. SAGIN is a layered network architecture consisting of satellite networks, space-based networks, and ground-based networks. As shown in Figure 1, a geosynchronous orbit (GEO) satellite, medium-earth orbit (MEO) satellite and low-earth orbit (LEO) satellite constitute the satellite network communication system. The space-based network communication system is composed of a high-altitude platform (HAP) and low-altitude platform (LAP), such as unmanned aerial vehicle (UAV) formation, space aerostats, airships and hydrogen balloons. The ground-based network consists of a cellular wireless network, satellite ground station, ground data processing center, ground stations, ground Internet of Vehicles, etc. [6; 7; 8; 9; 10; 11]. So, a large-scale network composition model will inevitably face many problems, such as the solidification of the functions of the traditional satellite networking model, and extremely complex upgrades and maintenance. The traditional flat management-intensive network nodes lead to a sharp drop in network delay and throughput, and load imbalances between nodes are prone to appearing. The network is highly heterogeneous, and different network dimensions require different communication protocols and communication links, resulting in increased network management complexity and increased operating network costs [12; 13]. To realize the global optimization of resources and the cooperation of heterogeneous networks in such a large-scale network, a current trend is to introduce the idea of SDN centralized perception and data/control decoupled network design into the design of SGAIN [14; 15; 16; 17; 18]. SDN uses the layering to separate the data plane of the network from the control plane, and the control plane controls the network equipment of the data plane centrally through the unified and open south interface; it also uses network function virtualization (NFV) technology to abstract the physical network and device components, decouples the physical device layer and the business logic layer, collects the state informa  
Figure 1: Hierarchical 6G space–air–ground integrated network architecture. tion of the whole network from the control plane, and creates an efficient resource allocation strategy. The data plane is only responsible for forwarding the resource according to the command of the control plane. The centralized control feature of SDN enhances the resource utilization of space-based networks, and supports the global information acquisition of network resources and the global allocation and optimization of resources according to service requirements in [19; 20]. The SDN open programming interface, namely the north and south interface, makes an application layer and control layer; the control layer and network layer are seamlessly connected through a custom application to make SAGIN more efficient in [21]. An SDN-enabled SAGIN can simplify the complexity of the network and improve the network performance, but it needs an efficient network design scheme, that is, the SDN-enabled multi-controller deployment strategy.