# DRL-Based Dynamic SFC Orchestration Algorithm for LEO Satellite Networks
## 1 Introduction  
Satellite communications have emerged as a focal point of academic research and industrial development, driven by their distinctive advantages including extensive coverage, long-distance transmission capabilities, high capacity, operational flexibility, scalability, and resilience to terrestrial natural disasters [1]. Within this domain, Low-Earth-Orbit (LEO) satellites have garnered particular attention for their reduced latency, minimal transmission losses, high data rates, and cost-effectiveness, positioning them as crucial enablers for expanding network coverage and enhancing communication efficiency. Low-Earth-Orbit (LEO) satellite networks operate through constellations comprising anywhere from dozens to tens of thousands of satellites working in synchronized coordination. Positioned at altitudes between 500 and 2000 km, these networks achieve comprehensive global coverage without blind spots [2]. Their significance extends beyond mere coverage, playing a pivotal role in delivering connectivity to sparsely populated regions while simultaneously serving as network extensions in densely populated areas, thereby alleviating the burden on terrestrial infrastructure. This dual functionality has established LEO satellite networks as an integral component of 5G network architecture [3], bridging critical connectivity gaps and enhancing overall network resilience. The communication architecture of low-orbit satellites can be divided into two types [4]. The \"bent-pipe\" architecture, widely adopted due to its simplicity and cost-effectiveness, positions satellites as basic relay stations that forward signals between ground stations without signal processing. This configuration currently dominates satellite deployment practices. In contrast, the \"space-based\" architecture represents a more sophisticated approach, with each satellite equipped with signal processing and routing capabilities. These satellites communicate directly through inter-satellite links, fostering a more efficient network topology. This advanced architecture demonstrates superior flexibility by eliminating the need for extensive ground station infrastructure, consequently offering broader application possibilities and enhanced operational capabilities. Like traditional terrestrial networks, LEO satellite networks face significant challenges, particularly in hardware-network function coupling and structural rigidity. The introduction of software-defined networking (SDN) and network function virtualization (NFV) technologies in 2012 revolutionized terrestrial network management and resource optimization [5; 6]. Service function chaining (SFC) technology enables sequential traffic routing through interconnected network functions to deliver user services [7]. By integrating NFV and SDN capabilities, SFC empowers operators to provide flexible, customized services. However, satellite network development has traditionally lagged behind terrestrial networks, primarily due to the complex implementation of network technology and protocol hardware in onboard processors, coupled with satellite manufacturing and launch constraints. To address these challenges in satellite network management, flexibility, and rigidity, researchers have explored the viability of SDN/NFV technologies in satellite networks [8; 9], identifying potential applications and challenges. Subsequently, reference [10] proposed SDSD technology to separate satellite network control and data layers, enabling flexible network configuration. The advancement of SDS/SDSN technology, combined with enhanced onboard processing capabilities, has now made SFC-based in-orbit service orchestration in LEO satellite networks feasible. The deployment of SFCs encompasses the placement of VNFs on physical nodes and the mapping of virtual links onto physical network paths. This process faces multiple challenges due to constraints inherent in both physical network infrastructure and SFC request characteristics.