## 2 Principles and Methods  
### Principles of SDN-Based ISTNs  
It is challenging to design an efficient networking solution for space networks due to the complexity and dynamical changes of network topology. Moreover, some studies have been working on optimizing the space network management, but they do not combine space networks and terrestrial networks together. Currently, some constellation satellite systems, such as the Starlink project, have been deployed, trying to compete with or even replace the existing terrestrial core networks. On the other hand, the integration of satellite networks and terrestrial networks can substantially increase the coverage and capacity of the Internet, but many challenges have not been addressed yet. In this paper, we propose a basic SDN-based ISTN system, as illustrated in Figure 1, that mainly includes a satellite optical network, satellite access users, and terrestrial user access networks with gateways. Here, the satellite optical network is the key part of the backbone network and comprises a number of high earth orbit (HEO) satellites, e.g., geostationary orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites, and middle earth orbit (MEO) satellites. FSO is used to transmit data between two satellites in the satellite optical network. Each satellite in the satellite optical network can be considered as a label switching router with its own link-state database (LSDB). Different space access users, such as airplanes, unmanned aerial vehicles (UAVs), and LEO satellites, are interconnected together via the satellite optical network to achieve positioning, navigation, data transmission, and so on. Note that LEO satellites can also be part of the satellite optical network. In this case, each LEO satellite is viewed as a label switching router with its own LSDB. Each HEO satellite is able to communicate with a terrestrial access network via a terrestrial gateway, which plays a vital role in realizing the integration between satellite and terrestrial networks through protocol conversion, e.g., between the Transmission Control Protocol (TCP)/IP protocol (used in the terrestrial access network) and the label-based AOS protocol (used in the satellite optical network). Note that only HEO satellites but not MEO satellites can communicate with terrestrial gateways due to the stability of HEO satellites, i.e., the locations of HEO satellites are more stable than MEO satellites in terms of maintaining FSO links that connect to terrestrial gateways. An SDN controller is deployedin the terrestrial network to control the behavior of satellites in the satellite optical network via the corresponding gateway to achieve constellation topology control and management, and allocate data channels for terminals in the user access network. In general, the purpose of the proposed SDN-based ISTN system is to interconnect space access users and terrestrial access networks, and achieve efficient inter-satellite routing and network operation administration and maintenance (OAM). This novel network architecture will pave the way to realizing an information-centric and/or content-centric networking Internet that offers network providers and users secure, high-performance, adaptable, and resilient services. Establishing and maintaining a satellite network is costly since (1) although the capital cost of the satellite decreases, launching a satellite to space is still expensive, and (2) repairing and changing the hardware settings of satellites is difficult owing to the harsh space environment. Hence, building a semi-physical satellite network platform to emulate various network services is necessary to ensure the correct network configurations before deploying a satellite network in space. Here, a semi-physical satellite network platform is a hardware and software co-designed simulator, where different FPGA boards are used to emulate the behaviors of satellites in a satellite optical network, and the whole communication network is built in a simulation environment.