# 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  
Satellite networks are developing towards merging with the terrestrial networks. Therefore, more services will flood into the satellite network, especially for multi-layer satellite networks. Effective QoS dynamic routing algorithms must be designed to meet the QoS needs of multi-layer satellite networks. Meeting QoS requirements is a multi-parameter optimization problem, considered polynomial-complete. Several QoS-based dynamic satellite routing schemes have been proposed one after another. Table 9 summarizes the works related to the QoS-based dynamic routings. Based on the time slot division approach, the work in [72] proposed a new dynamical hierarchical distributed QoS routing protocol (HDRP). The algorithm utilizes QoS metric information, including delay and bandwidth, to calculate the routing table while ensuring real-time multi-media service transmission. However, the satellite group management scheme fails to guarantee the reliability of satellite data transmission. For resource-constrained, multi-layer satellite networks, finding low-cost routes that meet the QoS requirements of all users is a significant challenge. Based on the idea of ranking optimization, the work in [73] designed a new algorithm for multi-QoS path selection. Compared with the existing SPF protocol, the approach can effectively balance the network traffic load and prevent packet loss. Since the algorithm is ground-based, it avoids onboard network constraints. The experimental results show that the algorithm can effectively perform routing under all pre-defined QoS parameters and shows better performance in hierarchical satellite networks. With the application of heuristic algorithms, the path selection capability of dynamic routing for QoS has been greatly improved. By improving the virtual topology policy, the work in [74] introduced heuristic algorithms to meet the QoS requirements of MLSNs. The method uses ant colony, forbidden search, and genetic algorithms to optimize multi-layer satellite network routing. Simulation results show that the algorithm obtains \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Network Structure** & **Reference** & **Proposed Algorithm/Scheme** & **Results** \\ \hline SDN-MPTCP & [61] & A MPTCP routing scheme & Be adaptable to changing network loads, further increased throughput of the network \\ \hline SDSN & [68] & An algorithm for topology generation & Reduced network energy consumption effectively \\ \hline  & [60] & A routing strategy based on amoeba algorithm and amoeboid–triple spreading routing algorithm & Reduced communication costs of the network, improved convergence speed of routing \\ \cline{2-4} GEO-MEO-LEO & [64] & An adaptive routing algorithm & Found the shortest communication path between satellites, optimized routing in real-time based on the state of the satellite \\ \cline{2-4}  & [65] & A snapshot routing scheme & Reduced bandwidth resource loss in the channel effectively, improved efficiency in the use of the satellite network resources \\ \hline SERvICE & [62] & QoS-oriented satellite routing algorithm and QoS-oriented bandwidth allocation algorithm & Guaranteed QoS requirements for multiple users \\ \hline  & [70] & A QoS-aware routing scheme & Reduced onboard storage costs dramatically, capable of meeting various QoS requirements better \\ \cline{2-4} GEO-LEO & [71] & A new fuzzy CNN-based multi-task routing algorithm & Improved output efficiency of CNN, achieved optimal \\ \hline SDN-SGIN & [64] & An optimization algorithm for routing based on machine learning & Updated routing based on real-time link status, improved correlated satellite link sensing capability \\ \hline MEO-LEO & [69] & A multi-path routing algorithm & Reduced duplication of routes, calculated multiple QoS-compliant routing paths simultaneously, improved network security \\ \hline \hline \end{tabular}
\end{table} 
Table 8: The summary of the works related to the SDN-based dynamic routing algorithms.