# Multi-Topology Routing Algorithms in SDN-Based Space Information Networks
## 1 Introduction  
The space information networks (SIN) is a network system that acquires, transmits, and processes spatial information in real time using various spatial platforms (such as synchronous satellites or medium and low-orbiting satellites, stratospheric balloons and manned or unmanned aerial vehicles) as carriers [1]. Because of its unique spatial location advantages, compared with the ground network, SIN plays an irreplaceable role in earth observation, emergency communications, air transport, space measurement and control and the expansion of national strategic interests, and has gradually become a high frontier of national strategic interests [2]. Because of the wide coverage of the SIN, remote users often cannot communicate directly and need a multi-hop relay to transmit data. Therefore, in order to improve the efficiency of the use of spatial information resources and complete different types of spatial tasks, it is necessary to design efficient routing strategies. The SIN mainly has the following problems in the design of routing algorithms: (1) Medium and low-orbit satellites move very fast, which leads to frequent dynamic changes of the SIN topology and inter-satellite links. (2) Compared with the terrestrial network, the propagation delay of the space network part is large, and the bandwidth resources are very limited. (3) Because the design of satellite-based equipment is different from that of ground-based equipment, the power, size, and weight of satellite-based equipment are strictly limited, which affects the operation and storage capacity of satellite-based equipment. Therefore, the on-board routing algorithm of the SIN should be relatively simple. The difficulties of routing algorithms in the SIN mainly come from the spatial network, and the routing algorithms in terrestrial networks have been relatively mature [3]. At present, there are few researches on routing algorithms for the SIN. According to different solutions, these algorithms can be divided into two categories, spatial virtualization method and temporal virtualization method [4]. The method of spatial virtualization is to virtualize the area covered by the whole satellite network or to virtualize the constellation of satellites by using the structure characteristics of the satellite network itself. The idea of spatial virtualization routing algorithm is to divide the ground area covered by satellite into several blocks, and then assign each region an independent logical address. The logical location of the satellite is determined by the logical address. With the operation of the satellite, the logical location is updated periodically. Such methods require relatively high on-satellite processing capacity, and may not be able to obtain optimal routing because of the use of local information for routing calculations [5]. Time virtualization mainly uses the predictability and periodicity of satellite motion to divide the whole operation period of the SIN into several time slices in a certain way and virtualize the topology of SIN from time. Therefore, the topology of the network can be approximately considered to be fixed in each time segment, so static topology can be used for routing calculation in advance. The snapshot sequence (SSS) algorithm proposed by [PERSON] et al. [6], the Virtual Path (VP) algorithm proposed by [PERSON], and the Finite State Machine Algorithm (FSA) algorithm proposed by [PERSON] et al. [7] are typical routing algorithm under the time virtualization idea [8]. Among them, the SSS algorithm proposed by [PERSON] et al. is the most feasible and simple routing algorithm for the SIN. The algorithm divides the dynamic topology of the SIN into several discrete static topologies in a period, each of which is called a snapshot. When the link of the SIN changes, that is, when the network topology changes, a new snapshot is produced, then the periodic topology of the SIN can be transformed into a set of topology snapshot cycle, and the cycle is the operation cycle of SIN.