# Time-Sensitive Networking Mechanism Aided by Multilevel Cyclic Queues in LEO Satellite Networks
## 1 Introduction  
Low-Earth-orbit satellite networking technology has developed tremendously in recent years. Like the Starlink, Kuiper, and OneWeb constellations, they comparable provide network service capabilities to those of terrestrial networks [1]. However, due to the initial stage, existing LEO satellite networks still need to improve in terms of quality-of-service (QoS) guarantee and differentiated service capability [2]. How to offer time-sensitive services is still an unsolved problem in LEO satellite networks. However, the satellite network faces multifaceted challenges when performing time-sensitive services. On the one hand, the satellite networks' spatial scale range is large, and dynamic changes in the network topology lead to the unstable channel quality of intersatellite links (ISLs) [3; 4; 5]. On the other hand, onboard resources such as computing, storage, and bandwidth are constrained [6; 7; 8]. Therefore, applying existing TSN technology directly to satellite networks takes time. Traditional terrestrial TSN technologies, including time-trigger Ethernet (TTE), and cyclic queue and forwarding (CQF), distribute time-sensitive traffic to other queues, away from regular traffic for forwarding. According to IEEE 802.1 Qch, this mechanism of split forwarding gives deterministic capability to traditional Ethernet switches, but introduces some new problems. On the one hand, the size limitation of the queue constrains the bandwidth of both regular and time-sensitive traffic [9]. Burst traffic may cause network congestion or bandwidth waste. On the other hand, the existing CQF mechanism cannot provide differentiated services for multilevel time-sensitive traffic. Due to high-performance Ethernet devices, these two shortcomings are not apparent in terrestrial networks [10]. However, thisis not tolerable in satellite networks, since the propagation delay of satellite networks is in the tens of milliseconds, resulting in a considerable time limitation scale for time-sensitive services. Therefore, a conventional time-sensitive forwarding strategy leads to the time-out of many flows for high-level time-sensitive services. Many studies have been conducted to improve the reliability and delay characteristics of LEO satellite networks. Some researchers focused on routing computation and produced some exciting work [11; 12; 13]. Furthermore, queuing delay is an essential factor affecting time-sensitive services, which enhances the routing problem into a trade-off between the hop number and queuing delay of paths [14; 15]. Some researchers focused on the joint routing and queuing scheduling for time-sensitive services [9; 16]. However, most existing models focus on latency optimization, but neglect the transmission capacity of time-sensitive flows (TSFs) with limited onboard resources. Due to the limited resources for LEO satellites, terrestrial-traffic scheduling schemes do not fully use onboard resources for TSFs. The motivation of this paper is to facilitate LEO satellite networks in offering differentiated forwarding actions for varied time-sensitive services. As satellite resources are limited, service latency requirements exhibit a high degree of differentiation. However, current network architectures and algorithms do not furnish differential forwarding capabilities for time-sensitive services, which hinders ensuring the determinism of numerous high-priority services. Therefore, this paper addresses the existing gap in a network's forwarding capabilities and enhances the efficiency of LEO satellite networks in providing differentiated forwarding actions for diverse time-sensitive services. This paper seeks to provide different forwarding mechanisms for multilevel TSF to improve the network throughput. First, departing from the priority-based forwarding mechanism of Ethernet, we propose a delay-urgency forwarding mechanism that assigns delay-urgency levels to traffic in satellite networks. This mechanism queues all traffic flows of different levels into different delay-urgency queues.