## 2 Related Works  
[PERSON] et al. [3] first highlighted the opportunities behind the ISTNs and discussed the potentials and solutions of the interworking operations between the satellite and terrestrial domains. Subsequent studies analyzed the challenges of satisfying the QoS requirements for seamless and converged ISTNs [4; 25]. [PERSON] et al. [26] investigated the advantages of incorporating network programmability and virtualization using SDN/NFV in satellite networks and demonstrated that integrating these approaches will benefit communication services over satellite networks. Besides, [PERSON] et al. [27] proposed an SDN-based seamless handover mechanism to provide flexible broadband satellite services for satisfying diverse QoS requirements over satellite networks. Then, [PERSON] [28] addressed the role of satellites in 5G and summarized key challenges of the ISTNs towards 5G. In terms of the heterogeneous network convergence aspects, [PERSON] et al. [5] proposed an SDN/NFV-based flexible network architecture, HetNet, to enable efficient integration of ISTNs. When it comes to the control plane of SDN, [PERSON] et al. [29] first proposed the Controller Placement Problem (CPP) to determine the number and locations of required controllers, which directly affects the entire network performance. Considering the heterogeneity and interconnections of controllers, [PERSON] et al. [30] studied the optimal model for CPP, which was merely practical for the small-scale networks. Heuristic analysis was adopted in [31] to addresses the controller placement problem with respect to various important metrics, including latency, resilience, and load balancing. Subsequent works concentrated on various performance metrics, such as reliability [32], controller capacity [33], latency [34], load balancing [22], and efficient placement algorithms [35]. In particular, [PERSON] et al. [32] not only proved the NP-hardness of the reliability-aware controller placement problem, but also proved the effectiveness of the Simulated Annealing Algorithm (SAA). Besides, [PERSON] et al. [36] firstly showed that the network reliability was principally determined by the network topology structure, especially the density of the internal nodes. The authors analyzed the impact of controller number on reliability, as well as the trade-off between reliability and latency. Considering the reliability and capacity of controllers, another work [34] proposed a controller placement strategy planning ahead for controller failures to make the networks more reliable and more resilient. Furthermore, high-quality and cost-saving solutions for resilient capacitated CPP were derived from [22], which took both the switch-controller/inter-controller latency and the capacity of the controllers into consideration. In addition, the authors in [37] addressed the dynamic CPP, which took the dynamic traffic load of switches into account to bound the communication latency. Meanwhile, since data in the integrated networks is transmitted from switches to satellites via satellite gateways, the Gateway Placement Problem (GPP) also affects the network performance. Previous works [38; 39] studied GPP in ISTNs to maximize network reliability with capacitated gateways and a propagation latency constraint. Available studies showed the recent progress of CPP or GPP in terrestrial networks, without considering the joint gateway and controller placement simultaneously. [PERSON] et al. [24] was the first work to provide a detailed overview of the joint placement of controllers and gateways in ISTNs. Moreover, it also proved that the joint placement problem is a multi-object problem, which is totally distinguished from either CPP or GPP in terrestrial networks. Unlike the above-mentioned papers, we took the intrinsic properties of the given topologies into account, i.e., the structure and the density of the internal nodes, and hybridized the network partition scheme to solve the joint placement problem.