# A Disruption Tolerant Distributed Routing Algorithm in LEO Satellite Networks
## 1 Introduction  
Compared with GEO satellite networks, LEO satellite networks have the advantages of low communication delay, wide coverage, and low transmission power of communication equipment. With the growth of global communication services, LEO satellite networks have become more widely used in military, commercial, and civilian applications [1]. The LEO satellite communication system is composed of LEO satellite constellations, user terminals, and ground base stations. In the forward link, the traffic is uploaded from the ground base stations, then routed to the corresponding satellite, and finally transmitted to the user terminal. Correspondingly, in the reverse link, the traffic is uploaded from the user terminal, then routed to the corresponding satellite, and finally transmitted to the ground base station. This shows routing is an important issue in satellite communication systems. Laser terminal has been widely used in intersatellite communication, due to its advantages of intensity, high transmission rate, and strong anti-interference, such as StarLink, LeoGate, and TelSate [2]. However, the laser intersatellite communication is unstable in various events such as sudden changes in channel status, satellite operating condition conversion, and on-board equipment failure, which may cause communication interruption (hereinafter referred to as link-off). Furthermore, various failures of the entire satellite can also be abstracted into corresponding link failure events that request specific consideration for the routing algorithm design [3]. In recent years, numerous routing algorithms have been designed to deal with the routing problems of LEO satellite networks. In order to solve the topology changes caused by satellite movement, there are mainly two topology discovery strategies [4]. The firstis the virtual topology strategy [5], which is suitable for complex network structures and changeable network topologies. The network topology is stored in each satellite in fragments, avoiding a large amount of routing information interaction and routing calculations, but at the same time, the satellites are required to have a certain storage capacity. The second strategy for virtual nodes [6] uses the periodicity of topology changes to avoid storage requirements, but it is only suitable for LEO satellite networks with simple topology and good coverage. The two topology discovery strategies have their own advantages and disadvantages. The virtual topology strategy divides the feeder link switching more finely, but also requires more storage space. The virtual node strategy saves storage space, but its utilization rate of the feeder link is low. At the same time, the satellite node is required to switch the IP address synchronously. However, none of these routing algorithms considers the instability of intersatellite communication. In order to achieve network load balancing in the space segment, there are three types of solution [7]. The first type for centralized offline routing [8] solves the difficulties of real-time routing better, but it cannot solve the problem of changes in business requirements. The second type for distributed on-demand routing strategy [9] mainly saves the overhead of routing information and unnecessary on-board routing calculations. It can support on-board real-time autonomous routing calculations and has certain load balancing capabilities. This kind of scheme is suitable for sudden business access, but it is difficult to reach a global optimum. The third type for distributed multi-path routing strategy [1; 3; 10; 11] is adapted to solve real-time link load jitter or link failure problems, avoiding link overload caused by burst traffic and resulting in packet loss, while the response speed is faster. This strategy can achieve local load balancing. However, existing research mostly uses router interface buffers to measure the degree of congestion of the link, which is impractical in satellite networks.