# Dynamic Routings in Satellite Networks: An Overview
## 4 Existing Works on Dynamic Satellite Routings
### Dynamic Routings in the Multi-Layer Satellite Networks
#### 4.2.2 QoS-Based Dynamic Routings
Since multi-layer satellite networks have limited resources, to make more appropriate use of the constrained network resources, QoS satellite routing is provided for various types of services. To compare the existing QoS routing algorithms, various types of services have different QoS requirements. If all services are processed under the same QoS conditions, the QoS requirements of the higher-priority services will be decreased. Therefore, based on priority and failure probability, the work in [86] proposed a routing algorithm for LEO-MEO satellite networks. To a certain extent, the approach can reduce the packet loss rate and service delay, improve the network throughput, and guarantee the QoS requirements of the service. Nevertheless, to ensure the QoS for high-priority services, low-priority services are flooded onto MEO satellites, causing traffic congestion. Furthermore, the algorithm considers only two QoS parameters: bandwidth and latency. In the next QoS dynamic routing study, more QoS parameters can be considered to help the routing algorithm select better routing paths based on user QoS needs. The QoS-based dynamic routing algorithm studied above mainly considers some traditional QoS parameters, including the available bandwidth, path delay, packet loss rate, and other parameters of available resource allocation. However, these routing algorithms fail to provide trusted routes when there is an attack on the network. The SDN framework has emerged as a promising QoS-assurance solution. The multi-layer satellite network model with SDN architecture meets QoS requirements and outperforms others in end-to-end delay, packet loss rate, and network throughput. In 2020, based on hte trusted resource matrix (TRM), the work in [87] presented a trusted routing (TR) model to protect normal ISTN traffic from network attacks. In the process, the SDN controller constructs the feature matrix by collecting the state information of each routing node. An entropy estimation method is used to determine if a routing node is under attack. The scheme discovers trusted transmission paths through the TR model to assign trust values to TRM routing nodes. Based on careful consideration of the available and trusted resources, a hybrid routing model (HR) was proposed. The model introduces TRM into the QoS evaluation system and selects trusted paths to avoid attack traffic. The HR algorithm combines QoS and transport requirements to provide trusted routes. In the simulation experiments, the proposed algorithm is compared with the traditional QoS-based routing (QR) algorithm and the shortest path (SP) algorithm. The results show that the TR algorithm and the HR algorithm can avoid network attack traffic better and maximize the security of normal traffic. Furthermore, the HR algorithm can utilize limited resources better than the TR algorithm. However, the hybrid routing model fails to optimize the distribution of weights between available and trusted resources. There may be an optimal weight ratio based on network states. The HR model is better for trusted path selection and packet transmission efficiency. The issue will be addressed in the following research and analysis. In wireless networks, scheduling algorithms are a prominent topic, notably on GEO satellites that employ digital video broadcasting satellite second-generation (DVB-S2). Designing a proper scheduling method is crucial for GEO satellites in a multi-layer satellite network to meet QoS limitations. Therefore, the work in [88] developed a hierarchical scheduler to support different QoS levels for user traffic while avoiding performance degradation. In the literature [89], the authors designed a cross-layer packet scheduler based on the physical and network layers. Simulation results show that this scheme maximizes