## 1 Introduction
### Challenges in Space Communication  
The satellite network maintains three types of links [10]. Each kind of link has issues that affect the performance of the satellite networks:  
1. Inter-satellite links (ISLs): links between satellites in the same layer; for example, satellites in LEO have four ISLs to connect with four neighbors on the same orbit. Satellites in MEO connect with their immediate neighbors in their orbit. 2. Inter-orbital links (IOLs): satellites in different orbits communicate through IO; for example, communication links between GEO and MEO, GEO and LEO, and MEO and LEO. 3. User data links (UDLs): communication links between satellites and ground stations, also known as feeder links. A satellite can maintain several UDLs to multiple ground stations, and a ground station can directly connect to many satellites in any orbit. The data transmission from satellites at the lower Earth orbits suffer from the short contact windows to ground stations in each pass and are subject to highly dynamic topological changes due to the high speed of satellites. High-frequency free space optics channels in the feeder links between satellites and ground stations help to deliver data with high throughput. However, free-space optics channels are susceptible to attenuation due to unpredictable weather conditions [11]. Site diversity in ground stations helps alleviate problems due to short contact windows, but installing and maintaining multiple ground stations at several geographical locations involve huge investments [6; 12; 13]. The introduction of ISLs provides a promising solution to solve packet loss rates and long delays due to atmospheric attenuation and the short contact windows associated with direct satellite-to-ground station communication by utilizing satellite hand-overs. The higher-bandwidth ISLs between satellites on the same orbits and other orbits established by laser and radio terminals help to transmit data between satellites at high rates [6]. However, ISLs suffer from dynamic topological changes due to the inter-plane ISLs between satellites in different orbits shutting down and reestablishing (on-off switching) in and out of polar regions [14; 15]. Besides, satellites in the 0 th and (N\(-\)1)th planes across the seam cause the absence of ISLs and need to detour to access satellites on the other sides, which causes longer delays [16]. Another issue with ISLs is that the traffic load on the ISLs varies according to the demographic distribution of the coverage areas on the Earth [17]. The non-uniform traffic load on the celestial network is due to population dispersion, economic status, and technology penetration status over different geographical areas [18]. The IOLs have line-of-sight issues due to the movement of satellites in their orbits and suffer longer delays due to the distances between various orbits [19]. When multiple satellites try to relay data through a single satellite in a different layer to use the shortest path, this causes congestion and leads to packet losses [19]. Hence, it is highly challenging to design routing strategies adaptable to the link dynamics of ISLs, IOLs, and UDLs in delivering data from satellites to ground stations with an optimized delay and packet loss rate to fulfill the performance requirements of the applications they serve. The existing works on satellite networks with multi-QoS optimization routing goals use a linear combination of the metrics such as the delay, packet loss rate, and throughput with weights assigned to each. They try to optimize any one metric according to the requirement of the application by giving more importance through the weight. Hence, the goal of simultaneously minimizing both the delay and packet loss will be a challenging problem, for which we have yet to find a solution. Besides, the routing approaches that optimize delay may use paths that give short delays due to the loss of packets and suffer reduced throughput.