# Dynamic Routings in Satellite Networks: An Overview
## 1 Introduction  
In traditional terrestrial networks, routers calculate routing tables from a routing information database established from network topology information. While updating the routing information base relies mainly on changing the link-state, such routing updates require the entire network to share topology information, resulting in greater network overhead, slow convergence, and other drawbacks of the routing protocols [1]. Since the topology of the terrestrial network varies less frequently, updating the routing table is no longer necessary once the network reaches a stable state. Meanwhile, the computing power and storage capacity of the router are sufficient to meet the calculation requirements of the routing table, so the existing routing protocols can be better adapted to the terrestrial network.  
Compared to terrestrial networks, satellite networks can provide global coverage and efficient communication services without the constraints of geography and infrastructure [2; 3]. The satellite network topology is highly dynamic and time-varying, and satellites have limited onboard computing and storage due to their size and power consumption. In particular, the relatively long inter-satellite distance and link transmission delay are the crucial factors affecting the routing performance of satellite networks [4]. Additionally, terrestrial network routing algorithms are not directly applicable to satellite networks due to their high complexity and processing requirements [5; 6]. Therefore, both academia and industry must develop new routing algorithms depending on the characteristics of the satellite network itself [7]. This paper provides a brief overview of the current dynamicrouting algorithms available in the satellite networks and discusses potential techniques and research directions.  
The routing algorithm must be capable of adapting to the dynamic satellite network topology and sensing changes such as network load traffic changes and link congestion states in a timely manner. In light of this, this paper proposes solutions for single-layer satellite network routing algorithms from different perspectives. However, the performanceof a multi-layer satellite network comprised of low-Earth orbit (LEO), medium-Earth orbit (MEO), and Geosynchronous Earth orbit (GEO) is superior to that of a single-layer satellite network. Therefore, this paper presents the most recent algorithm for dynamic routing in multi-layer satellite networks. These algorithms can solve link congestion and load traffic balancing problems and adjust quality of services (QoS) parameters based on network conditions. In addition to having a high throughput, this class of algorithms guarantees a reasonable end-to-end delay. The contributions of this paper are summarized as follows:  
* The satellite networks are introduced in this paper, and the merits and drawbacks of the LEO, MEO, and GEO networks are discussed in this paper. The issues of dynamic routings in satellite networks are analyzed according to the dynamic network topology.
* The latest satellite network routing works are summarized, including single-layer and multi-layer dynamic routings. Meanwhile, the applicability and virtues in solving the problem of satellite networks are analyzed in this paper.
* Potential technologies and future directions in dynamic satellite routings are provided and discussed, including machine learning, mobile edge Computing, digital twin, multiscale information awareness and computing in a complex environment, intelligent satellite, and intelligent routing.  
The remainder of this paper is structured as follows: Section 2 briefly describes the model, characteristics, and several key technologies of satellite networks. Four hotspots in satellite routing technologies are introduced emphatically in Section 3. Section 4 prospects future existing work directions and potential routing technologies. Finally, conclusions are in drawn Section 5. The structure diagram of the paper is shown in Figure 1.