# Multi-Topology Routing Algorithms in SDN-Based Space Information Networks
## 2 Related Work  
SDN is a network architecture that decouples the control functions in the network from the physical forwarding device. Its control plane implements logical centralized control of the data forwarding device by introducing a centralized controller, while the data forwarding device is only responsible for performance data processing and forwarding. At the same time, its application layer can shield the difference of the underlying physical network hardware devices, and realize the network architecture of specific needs through application programming [11]. The design architecture of SDN mainly includes a data forwarding layer, control layer, an application layer, and the interface between adjacent planes [12]. The SDN structure is shown in Figure 1.  
Data forwarding layer. Data forwarding layer is composed of the underlying forwarding device of the network. Data forwarding layer is responsible for data processing, forwarding and status collection based on the traffic flow table, and executes the fast forwarding of data according to the instructions issued by the control layer. Control layer. Control layer is responsible for dealing with the abstract information of the data forwarding plane resources. On the one hand, using the southbound interface, the control layer can control the forwarding behavior of the underlying network devices, abstract the resources of the underlying network devices, obtain the information of the underlying network devices, and then generate a global network Abstract view. On the other hand, a control layer provides the abstract model of the underlying network to the upper network application through the northward interface. Figure 1: Software-defined Networking (SDN) structure. Application layer. Application layer mainly includes network applications for various network services in the control layer. It is mainly responsible for managing and controlling the network forwarding/processing strategies for applications [13]. At present, some scholars at home and abroad have proposed to apply SDN technology to the SIN, mainly focusing on the architecture and controller deployment strategy. Document [14] proposes an OpenSAN model, which is a typical SDN-based satellite network structure. Document [15] presents a space integrated multi-layer satellite communication network based on software definition network and virtualization of network functions. Documents [16; 17] focus on the controller deployment of the SIN based on SDN. Current research lacks specific analysis of the SIN routing algorithm based on SDN. Applying SDN to the SIN has the following advantages:  
1. The operation of the SIN has periodicity and regularity. SDN controller can make full use of these information to calculate and manage the network. 2. Satellite nodes in the SIN run fast, so their topology is highly dynamic. According to this topological information, the SDN controller can perform real-time and efficient operation in order to make the network run efficiently and increase the stability of the network. 3. SDN network architecture is a centralized network, and SDN controller monitors and manages the whole network uniformly. Therefore, combined with the idea of SDN, the ground SDN controller has the information and real-time data flow of all satellite nodes in the whole network. In this way, the SDN controller can find the optimal transmission path according to the data flow distribution of the whole network, which is conducive to saving the bandwidth resources of satellite communication and avoiding the cost. 4. In a traditional distributed network, when a new network protocol is deployed or a new algorithm is verified, each server needs to be updated, so that the labor cycle and human resources are enormous. However, combined with the SDN idea, only need to carry out protocol deployment or algorithm migration in the control layer of the network, and then unified calculation by the controller.