# Energy-Efficient Controller Placement in Software-Defined Satellite-Terrestrial Integrated Network
## 2 Related Work
### Controller Placement in the Software-Defined STIN  
In the software-defined STIN architectures, the location of controllers determines the system for control configuration and management policies. Many related works study the software-defined STIN architectures, in which the location of controllers decides the control mode of the system. The controllers are often deployed on different locations such as on satellites and ground. In this paper, according to the location of the controllers, the controller placement schemes can be classified into three categories: ground only, satellite only, and cross-segment placement scheme. Ground networks have strong computing power and storage capacity. The satellite gateways are responsible for exchanging information. [PERSON] et al. [25] firstly investigated the joint placement of ground controllers and satellite gateways for maximum STIN reliability while satisfying the latency constraint. [PERSON] et al. [26] presented a composite STIN architecture that the SDN controller deployed on the terrestrial domain to collect network information. [PERSON] et al. [27] targeted the optimization of the load average difference rate in the STIN to solve the joint placement problem of gateways and controllers. However, the controllers on the ground are static, which causes large energy consumption and time delay, so it is hard to satisfy the dynamic traffic demands. As satellite networks provide relatively low latency, controllers can be hosted on GEO or LEO satellites. [PERSON] et al. [28] put forward an OpenSAN architecture with SDN controllers on GEO satellites, which have wide coverage and are responsible for translating the rules and monitoring the network status. [PERSON] et al. [29] implemented a lightweight prototype control framework in a single-layer LEO satellite network, which avoided the bottleneck caused by GEO satellites in [28]. [PERSON] et al. [30] proposed a dynamic controller placement scheme in the LEO satellite network to minimize the average flow setup time considering the dynamic changes in traffic demands. This paper mainly aims to solve the minimization of communication costs of counties along the Belt and Road, which requires green economic development. To maximize the information transmission rate, we only consider the architecture in which controllers are deployed on the LEO satellites. In the cross-segment distributed control mode, the characteristics of different network segments are fully used to reduce the control overhead. [PERSON] et al. [31] discussed the multi-controller deployment strategy based on a genetic algorithm (GA) to optimize network delay and controller load in a hierarchical STIN network. [PERSON] et al. [32] designed a multi-layer control architecture with GEO satellites as the master controller and some LEO satellites as slave controllers. They proposed a dynamic deployment method of LEO controllers with an approximation algorithm (AA), which can meet the network coverage requirements of emergency satellites and optimize the network delay at the same time. [PERSON] et al. [33] proposed a multi-layer control scheme composed of GEO, Medium Earth Orbit (MEO), and LEO satellites, in which multiple optimization objectives such as delay, link flow, reliability, and controller load were considered to determine LEO controller placement, which is solved by a Particle Swarm Optimization (PSO) algorithm. [PERSON] et al. [34] investigated the multi-controller deployment strategy of an SDN-based 6G STIN and proposed a switch migration strategy considering the imbalance load of SDN controllers. Table 1 shows the comparison of the proposed scheme with state-of-the-art methods in the STIN architecture. In this paper, we notice the effect of global traffic demands on network energy consumption. The control message mainly comes from the user demands, and the link belonging to the control network should always be working to quickly reply to the control message.