# A Disruption Tolerant Distributed Routing Algorithm in LEO Satellite Networks
## 1 Introduction
Satellite networks encounter high propagation delay and cannot tolerate higher queuing delay, and due to the large intersatellite link bandwidth, the increase in queuing delay means that a large amount of buffer size needs to be prepared, which requires a higher performance for the space router [12]. Therefore, it is more reasonable to conduct research on this type of algorithm from the perspective of real-time link load. There is currently no accepted solution to the problem of intersatellite link failure and interruption. FSA [8], a centralized routing algorithm based on virtual topology, utilizes predictable topology changes and service demand changes to dynamically allocate intersatellite link bandwidth resources, but it cannot handle unexpected link failure events. According to the distributed algorithms AODV and OLSR commonly used in terrestrial wireless ad hoc networks, both the LAOR algorithm [9] and the DODR algorithm [13] for link failure scenarios are proposed in the satellite network, and the orbit speaker strategy is proposed in CEMR [14]. These algorithms mainly save routing information overhead and unnecessary on-board routing calculation, while at the same time, they can support on-board real-time autonomous calculation of routing, and have certain load balancing capabilities. However, satellite networks have higher requirements for network service quality. When a link fails, these algorithms cannot provide an instant and reliable transmission path, which is prone to packet loss. DTDR, on the other hand, utilizes the mesh characteristics and periodic rules of the network topology of the space segment, and combines static routing with dynamic algorithms, which greatly improves the efficiency of the routing calculation. The distributed multi-path routing algorithm such as ELB [10] is suitable for solving the problems of real-time link load jitter or link failure, avoiding link overload caused by burst traffic and resulting in packet loss, and the response speed is fast. However, this kind of algorithm can only achieve local load balancing, and routing loops and cascading congestion problems are easily formed. In addition to propagating link failure information to neighboring nodes, DTDR also floods the information to neighboring nodes within a specified number of hops to achieve load balancing in the area of a failed link. At the same time, a loop avoidance mechanism is formulated in combination with the network topology characteristics of the space segment. OPSPF [15] generates an instantaneous routing table through periodic routing calculation and uses an on-demand dynamic routing mechanism to deal with the impact of link failures. The algorithm can save the storage space of the network topology, but the routing information overhead is large when the link fails, and the convergence is slow in large satellite constellations, wrong paths are easily formed. DTDR only floods the failure information to the neighboring nodes in a certain area, which reduces the routing overhead, and the algorithm converges quickly. For LEO satellite constellation routing, the immediacy and reliability of the path is a major challenge. The bandwidth of the laser link is large and the resources on the satellite are limited. There is no large-scale buffer to support the buffering of data packets [12]. The buffer can only meet the storage requirements of a small amount of data packets during the pre-switching of the intersatellite link [16; 17]. Therefore, when a link is interrupted, if there is no timely route switching strategy, a large amount of packet loss will occur. When a link interruption event occurs, there must be a backup path that can be replaced immediately, and the path should be reliable, that is, to ensure that the data is accessible under certain severe conditions. The advantage of the routing algorithm based on the link state is that it can perceive the topology of the entire network in real time, but it is more vulnerable when the link is interrupted.