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A Survey on Space-Air-Ground-Sea Integrated Network Security in 6G
6G空地海一体化网络安全研究综述

Hongzhi Guo , Member, IEEE, Jingyi , Graduate Student Member, IEEE,
郭 宏志 ， IEEE会员， 静怡 ， IEEE研究生会员，Jiajia Liu , Senior Member, IEEE, Na Tian, Graduate Student Member, IEEE, and Nei Kato , Fellow, IEEE
刘佳佳 ，IEEE高级会员，田娜，研究生会员， IEEE和内加藤 ，研究员， IEEE

Abstract 抽象

Space-air-ground-sea integrated network (SAGSIN), which integrates satellite communication networks, aerial networks, terrestrial networks, and marine communication networks, has been widely envisioned as a promising network architecture for . In consideration of its cooperation characteristics of multi-layer networks, open communication environment, and time-varying topologies, SAGSIN faces many unprecedented security challenges, and there have been a number of researches related to SAGSIN security performed over the past few years. Based on such observation, we provide in this paper a detailed survey of recent progress and ongoing research works on SAGSIN security in the aspects of security threats, attack methodologies, and defense countermeasures. To the best of our knowledge, we are the first to present the state-of-the-art of security for SAGSIN, since existing surveys focused either on a certain segment or on several segments of the integrated network, and little can be found on the full coverage network. In addition to reviewing existing works on SAGSIN security, we also present some discussions on cross-layer attacks and security countermeasure in SAGSIN, and identify new challenges ahead and future research directions.
天-空-地-海一体化网络（SAGSIN）是集卫星通信网络、航空网络、地面网络和海洋通信网络于一体的，被广泛认为是一种很有前途的 网络架构。考虑到多层网络、开放通信环境、时变拓扑等协同特点，SAGSIN面临着许多前所未有的安全挑战，近年来开展了大量与SAGSIN安全相关的研究。基于上述观察，本文从安全威胁、攻击方法和防御对策等方面对SAGSIN安全的最新进展和研究工作进行了详细的调查。据我们所知，我们是第一个为SAGSIN提供最新安全技术的公司，因为现有的调查要么集中在集成网络的某个部分，要么集中在集成网络的几个部分，而在全覆盖网络上几乎找不到。除了对已有的SAGSIN安全研究成果进行回顾外，本文还对SAGSIN中的跨层攻击和安全对策进行了一些讨论，并确定了未来的新挑战和未来的研究方向。

Index Terms-Space-air-ground-sea integrated network, B5G, 6G, security threats, attack methodologies, defense countermeasures.
索引术语-天空地海一体化网络，B5G，6G，安全威胁，攻击方法，防御对策。

I. INTRODUCTION 一、引言

O VER the past few years, with the development of mobile communication technology, especially the commercialization of in many countries around the world, traditional terrestrial networks have experienced an explosive growth in terms of throughput and user access amount. Nevertheless, with the advance of the Internet of Things (IoT), relying
在过去的几年里，随着移动通信技术的发展，特别是世界许多国家的商业化 ，传统的地面网络在吞吐量和用户接入量方面都经历了爆炸式的增长。然而，随着物联网 （IoT） 的进步，依靠
Manuscript received May 26, 2021; revised October 9, 2021; accepted November 2, 2021. Date of publication November 30, 2021; date of current version February 24, 2022. This work was supported in part by the National Natural Science Foundation of China under Grant 62001393; in part by the Natural Science Basic Research Program of Shaanxi under Grant 2020JC-15; in part by the Fundamental Research Funds for the Central Universities under Grant D5000210817; in part by Xi'an Unmanned System Security and Intelligent Communications ISTC Center; and in part by Special Funds for Central Universities Construction of WorldClass Universities (Disciplines) and Special Development Guidance under Grant 0639021GH0201024. (Corresponding author: Jiajia Liu.)
稿件于2021年5月26日收到;2021 年 10 月 9 日修订;2021年11月2日接受。发布日期：2021 年 11 月 30 日;当前版本的日期为 2022 年 2 月 24 日。这项工作得到了中国国家自然科学基金62001393资助的部分支持;部分由陕西省自然科学基础研究计划资助2020JC-15;部分由D5000210817资助下的中央大学基础研究基金提供;部分由习无人系统安全与智能通信ISTC中心提供;部分由中央大学专项资金、世界一流大学（学科）建设和特别发展指导资助 0639021GH0201024 提供。（通讯作者：刘佳佳）
Hongzhi Guo, Jiajia Liu, and Na Tian are with the National Engineering Laboratory for Integrated Aero-Space-Ground-Ocean Big Data Application Technology, School of Cybersecurity, Northwestern Polytechnical University, Xi'an 710072, Shaanxi, China (e-mail: liujiajia@nwpu.edu.cn).
郭洪志、刘佳佳、田娜就职于陕西省习安710072西北工业大学网络安全学院空天地海洋一体化大数据应用技术国家工程实验室（电子邮件：liujiajia@nwpu.edu.cn）。
Jingyi Li is with the School of Cyber Engineering, Xidian University, Xi'an 710071, China
李静怡就职于西安电子科技大学网络工程学院，中国习安710071
Nei Kato is with the Graduate School of Information Sciences, Tohoku University, Sendai 9808579, Japan.
加藤内就职于日本仙台9808579东北大学信息科学研究生院。
Digital Object Identifier 10.1109/COMST.2021.3131332 solely on the ground network can no longer meet the exploding requirements on high-speed and reliable network access at anytime and anywhere on the earth, in consideration of its limited coverage and network capacity. This has aroused widespread concern in the academia and industry on the air-ground coordination and the space-air-ground integrated network (SAGIN) [1], [2]. In particular, many telecommunications companies around the world have released their plans for SAGIN development in the coming years. For example, in 2020, China Unicom launched their SAGIN construction plan and began to promote the maturity of the SAGIN industry chain. In academia, Liu et al. [1] reviewed recent works on SAGIN ranging from network design and resource allocation to performance analysis and optimization, and presented some challenges and future research directions. Kato et al. [3] focused on the utilization of artificial intelligence (AI) in optimizing the performance of SAGIN including intelligent traffic control, intelligent Resource Allocation, Smart Anomaly Detection, etc. After that, Cheng et al. [4] developed a SAGIN simulation platform, which adopts centralized and decentralized controllers to optimize network functions, such as access control and resource scheduling.
数字对象标识符 10.1109/COMST.2021.3131332 考虑到其有限的覆盖范围和网络容量，仅靠地面网络已无法满足地球上随时随地高速可靠网络接入的爆炸性要求。这引起了学术界和工业界对空地协调和空空地一体化网络（SAGIN）的广泛关注[1]，[2]。特别是，全球许多电信公司已经发布了未来几年的SAGIN发展计划。例如，2020年，中国联通启动了SAGIN建设计划，开始推动SAGIN产业链的成熟。在学术界，Liu等[1]回顾了SAGIN的最新研究，从网络设计和资源分配到性能分析和优化，并提出了一些挑战和未来的研究方向。Kato等[3]重点研究了人工智能（AI）在优化SAGIN性能方面的应用，包括智能交通控制、智能资源分配、智能异常检测等。之后，Cheng等[4]开发了SAGIN仿真平台，采用集中式和分散式控制器来优化网络功能，如访问控制和资源调度。
Since 5G commercialization in 2019, the researchers around the world and the leading communication technology service providers like Huawei, Ericsson, etc., have turned their attention to and . In the following years, various potential 6G technologies including SAGSIN, THz communication, orbital angular momentum (OAM), reconfigurable intelligent surface (RIS), visible light communication (VLC), quantum communication (QC), etc., have been proposed [5]-[8]. Among these many candidate technologies, SAGSIN has been widely envisioned to be a promising development direction in future 6G [9]. Specifically, Zhang et al. [10] focused on conducting efficient implementation of intelligent device-to-device (D2D) in future 6G communication systems, mainly describing D2D-enhanced mobile edge computing, D2D-enabled intelligent network slicing, non-orthogonal multiple access (NOMA), and D2D based cognitive networking. Yang et al. [11] proposed an AI-enabled intelligent architecture for networks to realize knowledge discovery, smart resource management, automatic network adjustment, and intelligent service provisioning, furthermore, discussed the application of in networks and how to use AI to optimize network performance effectively. After that, Pang et al. [12] proposed SAGIN-assisted maritime communication network by adopting mobile edge computing, and
自 2019 年 5G 商用以来，全球研究人员以及华为、爱立信等领先的通信技术服务提供商将目光转向 了 和 .在接下来的几年里，各种潜在的6G技术被提出，包括SAGSIN、太赫兹通信、轨道角动量（OAM）、可重构智能表面（RIS）、可见光通信（VLC）、量子通信（QC）等[5]-[8]。在这些候选技术中，SAGSIN被广泛认为是未来6G的一个有前途的发展方向[9]。具体而言，Zhang等[10]重点研究了未来6G通信系统中智能设备到设备（D2D）的高效实现，主要描述了D2D增强的移动边缘计算、D2D驱动的智能网络切片、非正交多址（NOMA）和基于D2D的认知网络。Yang等[11]提出了一种基于AI的 网络智能架构，以实现知识发现、智能资源管理、网络自动调整和智能业务发放，并讨论了 人工智能在网络中的 应用以及如何利用人工智能有效地优化网络性能。之后，Pang等[12]通过采用移动边缘计算提出了SAGIN辅助的海上通信网络，并提出了SAGIN辅助的海上通信网络。
explained how to overcome the limitations of implementing satellite networks for maritime communications.
解释了如何克服实施卫星网络进行海上通信的局限性。
At the same time, SAGSIN security has attracted a lot of attention, and a number of research works have been published recently, which separately focused on secure communications, intrusion detection, side channel attack, GPS spoofing attack, network eavesdropping, message modification/injection, etc. [13]-[15]. These works related to SAGSIN security were separately carried out on a certain segment (e.g., satellite network, underwater acoustic network) [16], [17] or several segments (e.g., space-ground network, air-ground network) [18], [19] of the integrated network. In particular, some of them focused on analyzing existing security threats [20], [21], some presented their attack methodologies [14], [22], and others concentrated more on SAGSIN security design or designing workable defense countermeasures [23]-[25].
同时，SAGSIN安全也备受关注，近期发表了多篇研究著作，分别关注安全通信、入侵检测、侧信道攻击、GPS欺骗攻击、网络窃听、消息修改/注入等[13]-[15]。这些与SAGSIN安全相关的工作分别在集成网络的某个部分（例如，卫星网络，水声网络）[16]，[17]或多个部分（例如，空地网络，空地网络）[18]，[19]上进行。特别是，其中一些专注于分析现有的安全威胁[20]，[21]，一些介绍了他们的攻击方法[14]，[22]，而另一些则更侧重于SAGSIN安全设计或设计可行的防御对策[23]-[25]。

A. Existing Surveys A. 现有调查

Until now, there have been several survey papers on general aspects of SAGSIN security, with their emphasis on wireless networks, networks, underwater networks, space information networks, and so on [26], [29]. Firstly, some of them were conducted on single networks. Teng et al. [30] studied the resource allocation methods in ultra-dense networks and gave a classified of them. Li et al. [41] presented a comprehensive investigation of unmanned aerial vehicle (UAV) communication oriented to wireless network, and conducted relevant studies on the challenges faced by the network. Moreover, UAVs are usually implemented for commercial use, and civilian UAVs are also very important for air traffic [27], in which understanding the network characteristics of UAVs is very necessary for multiple UAVs cooperation in a reliable and safe manner. Furthermore, there are also some researchers focusing on satellite communications. Kodheli et al. [28] studied space networks and analyzed possible future challenges, where a space-based information network was proposed to extend the range of observations and transmit data in real time.
到目前为止，已经有几篇关于SAGSIN安全一般方面的调查论文，重点是无线网络、 网络、水下网络、空间信息网络等[26]，[29]。首先，其中一些是在单一网络上进行的。Teng等[30]研究了超高密度网络中的资源分配方法，并对其进行了分类。Li等[41]对面向 无线网络的无人机通信进行了全面研究，并对网络面临的挑战进行了相关研究。此外，无人机通常用于商业用途，民用无人机对于空中交通也非常重要[27]，其中了解无人机的网络特性对于多架无人机以可靠和安全的方式进行合作非常必要。此外，还有一些研究人员专注于卫星通信。Kodheli等[28]研究了空间网络并分析了未来可能面临的挑战，其中提出了天基信息网络以扩大观测范围并实时传输数据。
Secondly, some works focused on the integration of different kinds of networks, e.g., air-ground network, satellite-ground network and non-terrestrial network. In view of the characteristics of easy deployment and high mobility of UAV, Zhang et al. [32] proposed an architecture for air-ground integrated mobile edge network, and discussed the communication, computing and caching in UAV edge computing network. An air-to-ground channel propagation model was presented [33], which can be used in the design and evaluation of UAV communication links for non-payload and payload data transmission. Regarding the integration of satellite and ground networks, Wang et al. [35] made full use of the inherent advantages and multicast/broadcast characteristics of satellite communication system, and described the current situation and key application fields of satellite-ground network standardization. Niephaus et al. [34] presented some technical challenges in the integration of satellite and ground networks. In addition, non-terrestrial network system [36] is also very important in the field of wireless communications, since it can provide wide area coverage to meet ubiquitous connectivity all the time. In consideration of its wide coverage and large communication range, Wang et al. [37] stated that satellite-airground hybrid networks can provide effective services in the case of natural disasters or large-scale emergencies.
其次，一些工作侧重于不同类型网络的整合，例如空地网络、卫星地网络和非地面网络。针对无人机易于部署、高机动性的特点，Zhang等[32]提出了一种空地一体化移动边缘网络架构，并讨论了无人机边缘计算网络中的通信、计算和缓存。提出了一种空对地信道传播模型[33]，可用于设计和评估无人机非有效载荷和有效载荷数据传输的通信链路。关于卫星与地面网络的融合，Wang等[35]充分利用了卫星通信系统的固有优势和组播/广播特性，描述了卫星地网标准化的现状和关键应用领域。Niephaus等[34]在卫星和地面网络的整合中提出了一些技术挑战。此外，非地面网络系统[36]在无线通信领域也非常重要，因为它可以提供广域覆盖，满足无处不在的连接。Wang等[37]考虑到其覆盖范围广、通信范围大等特点，指出卫星空地混合网络在发生自然灾害或大规模突发事件时可以提供有效的服务。
Besides, with the emergence of multi-tiered and diversified networks, some security issues have come along. In particular, Sharma and Rawat [31] introduced the most advanced cognitive radio network (CRN) security research results, and summarized the main CRN attacks and countermeasures in recent years. In 2020, both Khan et al. [38] and Cao et al. [39] reviewed the security issues that existed in communication networks. The former presented the core technologies for building a 5G security model, network software security, physical layer security, and 5G privacy issues, etc. The latter focused on the network architecture and security functions in 3GPP 5G network, and introduced the challenges brought by the technologies such as network slicing and vehicle to everything communication. Moreover, Ahmad et al. [40] pointed out the existence of security vulnerabilities in 5G technology, and gave the direction of B5G wireless network security. Cao et al. [29] summarized the security functions of LTE and LTE-A networks, and discussed the security vulnerabilities in the architecture design of them. Besides, Jiang [20] presented underwater acoustic networks (UWAN) security structures, and gave a comprehensive review of security threats faced by UWAN and corresponding solutions. Table I summaries existing surveys related to SAGSIN security.
此外，随着多层次和多样化网络的出现，也出现了一些安全问题。特别是Sharma和Rawat[31]介绍了最先进的认知无线电网络（CRN）安全研究成果，并总结了近年来CRN的主要攻击和对策。2020年，Khan等[38]和Cao等[39]都对 通信网络中存在的安全问题进行了综述。前者介绍了构建5G安全模型、网络软件安全、物理层安全、5G隐私问题等核心技术。后者聚焦3GPP 5G网络的网络架构和安全功能，并介绍了网络切片、车联网通信等技术带来的挑战。此外，Ahmad等[40]指出了5G技术中存在的安全漏洞，并给出了B5G无线网络安全的方向。Cao等[29]总结了LTE和LTE-A网络的安全功能，并讨论了它们在架构设计中的安全漏洞。此外，江[20]介绍了水声网络（UWAN）安全结构，并全面回顾了UWAN面临的安全威胁及其解决方案。表一总结了与SAGSIN安全有关的现有调查。

B. Motivation B. 动机

Although existing surveys have presented security issues related to satellite networks [42], unmanned aerial systems, underwater acoustic network, and satellite-ground coordination [43], they all have common limitations. In particular, these works either focused on a single network segment in space or in the air, or on the integration of two-tiered segments like airground network, satellite-terrestrial network, etc. Compared to single/two-tiered networks, UAVs can be used as a relay for cross-layer data transmission between the ground network layer and the satellite layer in SAGIN/SAGSIN. This way can reduce the radio jamming on the ground-satellite link, and improve the communication performance, while it is possible to attack satellites or UAVs through these cross-layer links, which were previously targeted only at the ground layer.
尽管现有的调查提出了与卫星网络[42]、无人机系统、水声网络和卫星-地面协调[43]相关的安全问题，但它们都有共同的局限性。特别是，这些工作要么集中在太空或空中的单个网段，要么集中在空地网、卫星-地面网络等两层网段的整合上。与单层/两层网络相比，无人机可以作为SAGIN/SAGSIN中地面网络层和卫星层之间跨层数据传输的中继器。这种方式可以减少地星链路上的无线电干扰，提高通信性能，同时可以通过这些以前只针对地面层的跨层链路攻击卫星或无人机。
Moreover, in consideration of the openness of UAV links, the jamming and eavesdropping attacks aimed at UAVs can also be transmitted to satellite communication through the uplinks, so that the satellite may receive false commands, resulting in the space-ground communications being interfered or interrupted. To address these new issues, Internet Protocol Security (IPsec) or Secure Sockets Layer (SSL), which were originally designed for terrestrial networks, provide us a potential solution. However, introducing these protocols will inevitably bring about corresponding performance penalties in satellite network. Therefore, we need to consider the uniqueness of hybrid network, and design new secure group communication protocols.
此外，考虑到无人机链路的开放性，针对无人机的干扰和窃听攻击也可以通过上行链路传输到卫星通信中，使卫星可能接收到错误指令，导致空地通信受到干扰或中断。为了解决这些新问题，最初为地面网络设计的互联网协议安全 （IPsec） 或安全套接字层 （SSL） 为我们提供了一个潜在的解决方案。然而，引入这些协议将不可避免地给卫星网络带来相应的性能损失。因此，我们需要考虑混合网络的独特性，并设计新的安全组通信协议。
TABLE I 表一
SUMmary of SURVEYS RELATEd to SAGSIN SECURITY
与SAGSIN安全相关的调查总和
Network Type Reference Features Main Contributions 主要贡献
Space information 空间信息
network
[26] Single Reviewed the research challenges associated with physical layer security in space network.
综述了空间网络物理层安全相关的研究挑战。
UAV network [27] Reviewed the characteristics of UAV networks and civilian application requirements.
回顾了无人机网络的特点和民用应用要求。
Satellite network 卫星网络 [28]
Reviewed the characteristics of non-terrestrial networks and introduced the applications of ,
综述了非地面网络的特点，并介绍了非地面网络的 应用，
including space communication, earth observation, aviation, maritime tracking, etc.
包括空间通信、对地观测、航空、海上跟踪等。
Underwater acoustic 水下声学
network
[20]
Reviewed UWAN security threats from the physical layer to the transport layer, and
审查了从物理层到传输层的 UWAN 安全威胁，以及
corresponding defense schemes.
相应的防御方案。
Wireless network 无线网络 [29] LTE/LTE-A Reviewed the security features and vulnerabilities of LTE/LTE-A networks.
回顾了LTE/LTE-A网络的安全特性和漏洞。
[30] Ultra-dense Reviewed resource allocation methods, explained existing problems and some novel techniques.
回顾了资源分配方法，解释了存在的问题和一些新技术。
Cognitive radio network 认知无线电网络 - Reviewed existential security threats and countermeasures in CRN.
回顾了 CRN 中存在的安全威胁和对策。
Space-air-ground network
空地网		 Integrated
Reviewed existing surveys and discussed the challenges of SAGIN-assisted maritime
审查了现有的调查，并讨论了SAGIN协助海事的挑战
communication. 通信。
Air-ground network 空地网络
Reviewed the characteristics and composition of UAVs, and introduced the applications and
回顾了无人机的特点和组成，并介绍了无人机的应用和
challenges of UAV-assisted MEC networks.
无人机辅助MEC网络的挑战。
[33]
Reviewed existing air-to-ground communication channel models and their limitations,
审查了现有的空对地通信信道模型及其局限性，
discussed future research directions in UAV communication scenarios.
讨论了无人机通信场景的未来研究方向。
Satellite-terrestrial network
卫星-地面网络
Reviewed the security issues and related technical challenges associated with the convergence
审查了与融合相关的安全问题和相关技术挑战
of satellite and ground networks.
卫星和地面网络。
Reviewed the issues in satellite-ground fusion, and summarized the current situation of
回顾了卫星-地核聚变中存在的问题，总结了卫星-地核聚变的现状
standardization of network.
网络标准化。
Non-terrestrial network 非地面网络 [36]
Reviewed the importance and security of non-terrestrial networks in the field of wireless
回顾了非地面网络在无线领域的重要性和安全性
communications, and indicated future issues and research directions.
通信，并指出了未来的问题和研究方向。
Satellite-aerial-terrestrial
卫星-天线-地面
network
[37]
Studied the hybrid networks to provide effective services in emergency situations, and
研究了混合网络以在紧急情况下提供有效服务，以及
explained the security issues encountered in the network.
解释了网络中遇到的安全问题。
Mobile network 移动网络
Reviewed the core technology and implementations for building 5G security model, and
回顾了构建5G安全模型的核心技术和实现，以及
described security issues of the network.
描述了网络的安全问题。
Reviewed the network architecture and security functions of 3GPP 5G network, and discussed
回顾了3GPP 5G网络的网络架构和安全功能，并讨论了
the security characteristics of new features, technologies and solutions.
新功能、技术和解决方案的安全特性。
Reviewed the network threat, security vulnerabilities in , and discussed future directions.
回顾了 网络 威胁、安全漏洞，并讨论了未来的发展方向。
5G/B5G
Studied UAV communications oriented to wireless network, and discussed the related
研究了面向 无线网络的无人机通信，并讨论了相关的
research challenges. 研究挑战。
6G
Reviewed some potential D2D solutions related to , including mobile edge computing,
回顾了一些潜在的 D2D 解决方案 ，包括移动边缘计算、
network slicing, and NOMA cognitive networks.
网络切片和 NOMA 认知网络。
Reviewed the application of artificial intelligence technology in network, and proposed a
回顾了人工智能技术在网络中的 应用，并提出了
network intelligence architecture.
网络智能架构。
In addition, the surface network can directly communicate with the coastal base, satellite, and UAV in SAGSIN, and this may bring about malicious attacks at sea nodes and threats to communication channels, attacks at other network layers through signal transmission, causing new network security issues. Moreover, in order to improve the quality of service (QoS) of multi-tiered networks, traffic routing and gateway placement are needed, resulting in more security threats and cross-layer attack portal in SAGIN/SGASIN. This new converged network architecture also prevents the original security defense measures for single-layer networks from being properly applied in SAGSIN, especially in view of the full coverage SAGSIN. Therefore, it is important and necessary to study SAGSIN security in the future 6G. However, little survey can be found on SAGSIN security. Toward this end, we provide in this paper a comprehensive survey of SAGSIN security, and as far as we know, this is the first detailed survey of existing researches related to SAGSIN security.
此外，地面网络可以直接与SAGSIN中的沿海基地、卫星和无人机进行通信，这可能会带来对海上节点的恶意攻击和对通信信道的威胁，通过信号传输对其他网络层进行攻击，造成新的网络安全问题。此外，为了提高多层网络的服务质量（QoS），需要流量路由和网关布局，导致SAGIN/SGASIN中更多的安全威胁和跨层攻击门户。这种新的融合网络架构也阻止了原有的单层网络安全防御措施在SAGSIN中得到正确应用，特别是考虑到SAGSIN的全覆盖。因此，研究未来6G的SAGSIN安全性是重要且必要的。然而，关于SAGSIN安全性的调查很少。为此，本文对SAGSIN安全性进行了全面调查，据我们所知，这是对SAGSIN安全性相关现有研究的首次详细调查。

C. Contributions C. 会费

In particular, our main contributions are summarized as follows.
具体而言，我们的主要贡献总结如下。
  1. We survey existing SAGSIN systems and their architecture, and then discuss the characteristics of SAGSIN and potential challenges in its development.
    我们调查了现有的SAGSIN系统及其架构，然后讨论了SAGSIN的特点及其开发中的潜在挑战。
  2. The security requirements of SAGSIN are presented, with our emphasis on the differences among typical networks.
    本文介绍了SAGSIN的安全要求，并重点介绍了典型网络之间的差异。
  3. We detail the security threats, attack methodologies, and defense countermeasures for SAGSIN, where new security issues, attack and defense methods brought by SAGSIN, i.e., a multi-tiered and diversified network, are highlighted.
    本文详细介绍了SAGSIN的安全威胁、攻击方法和防御对策，重点介绍了SAGSIN带来的新的安全问题、攻击和防御方法，即多层次、多样化的网络。
  4. Based on our analysis of SAGSIN, some security challenges ahead and new research trends are presented.
    基于我们对SAGSIN的分析，提出了未来的一些安全挑战和新的研究趋势。
The rest of this paper is organized as follows, as shown in Fig. 1. We give the architecture and characteristics of SAGSIN in Section II. Section III introduces security requirements in SAGSIN. We discuss security threats, attack methods, and defense countermeasures in Sections IV, V, and VI, respectively, and introduces a comprehensive overview of related aspects. Then, we point out some technical challenges and future directions in Section VII. Finally, Section VIII concludes the whole paper. For convenience, the abbreviations used in this paper are listed in the Appendix.
本文的其余部分组织如下，如图 1 所示。我们在第二节中给出了SAGSIN的架构和特性。第三节介绍了SAGSIN中的安全要求。本文分别在第四、第五和第六部分讨论了安全威胁、攻击方法和防御对策，并对相关方面进行了全面概述。然后，我们在第七节中指出了一些技术挑战和未来方向。最后，第八节结束了整篇论文。为方便起见，本文中使用的缩写列在附录中。

II. BACKGROUND 二、背景

In this section, we first provide a brief introduction of existing integrated systems, and present the composition of the integrated system architecture. Then, we give the characteristics of SAGSIN and the communication technologies used in
在本节中，我们首先对现有的集成系统进行简要介绍，并介绍集成系统架构的构成。然后，我们给出了SAGSIN的特性和用于的通信技术
Fig. 1. Organization of this paper.
图 1.本文的组织。
SAGSIN. Finally, some existing challenges and open issues in SAGSIN are discussed.
粟子。最后，讨论了SAGSIN中存在的一些挑战和悬而未决的问题。

A. Existing Systems A. 现有系统

In the past decades, many kinds of SAGSIN systems have been built and applied to wireless communications, e.g., Global Information Grid (GIG). Specifically, a variety of satellite communication systems have been proposed according to different orbital altitudes, including geostationary orbit (GEO) satellite systems like Mobile Satellite (MSAT) and Inmarsat, Odyssey medium earth orbit (MEO) satellite system, low earth orbit (LEO) satellite systems like Iridium and Global star, maritime satellite communication systems like Global Maritime Distress and safety System (GMDSS), Navigational Telex (Navtex), etc.
在过去的几十年中，已经建立了多种SAGSIN系统并将其应用于无线通信，例如全球信息网格（GIG）。具体来说，已经根据不同的轨道高度提出了各种卫星通信系统，包括地球静止轨道（GEO）卫星系统，如移动卫星（MSAT）和国际海事卫星组织，奥德赛中地球轨道（MEO）卫星系统，低地球轨道（LEO）卫星系统，如铱星和全球之星，海上卫星通信系统，如全球海上遇险和安全系统（GMDSS）， 导航电传（Navtex）等
  1. GIG: GIG refers to the global information network connected by the classified and unclassified computing networks, which is provided by the U.S. Military for soldiers, sailors, pilots, marines and decision makers. In particular, GIG is mainly composed of four layers, i.e., ground layer, space layer, near-space layer, and satellite layer. It builds a grid of information networks by means of information transmission capability, which can be linked to any two or more points around the world. Moreover, after more than ten years of development, GIG has become the most representative model with the highest technical maturity in network information system construction.
    GIG：GIG是指由美国军方为士兵、水手、飞行员、海军陆战队员和决策者提供的机密和非机密计算网络连接的全球信息网络。特别是，GIG主要由地面层、空间层、近空间层和卫星层四层组成。它通过信息传输能力构建信息网络网格，可以链接到世界上任何两个或多个点。而且，经过十余年的发展，GIG已成为网络信息系统建设中最具代表性、技术成熟度最高的模型。
  2. MSAT: MSAT is the first regional satellite mobile communication system of the world, known as the North American Satellite Mobile Communication System. MSAT was the first star operated in Canada, and the system can be used for both public and private communications.
    MSAT：MSAT是世界上第一个区域卫星移动通信系统，被称为北美卫星移动通信系统。MSAT是第一个在加拿大运营的明星，该系统可用于公共和私人通信。
  3. Inmarsat: Inmarsat is an intergovernmental international cooperation that operates global Inmarsat communications, providing maritime salvage security communications and commercial communications to maritime users. The Inmarsat system consists of ship station, shore station, network coordination station, and satellite. With the development of the world network system, the Inmarsat-P terminal will eventually provide transocean global hand-held satellite voice communications.
    国际海事卫星组织（Inmarsat）：国际海事卫星组织（Inmarsat）是一个政府间国际合作组织，负责运营全球国际海事卫星组织（Inmarsat）通信，为海事用户提供海上救助安全通信和商业通信。Inmarsat系统由船舶站、岸站、网络协调站和卫星组成。随着世界网络系统的发展，Inmarsat-P终端最终将提供跨洋全球手持卫星语音通信。
  4. Odyssey: Odyssey is a mid-orbit system launched by TRW company, which consists of three parts, i.e., space segment, ground segment and user unit. It can be used as complement and extension to land cellular mobile communication systems, supporting dynamic, reliable, automatic, and user transparent services.
    奥德赛：奥德赛是天合公司推出的中轨系统，由空间段、地面段和用户单元三部分组成。可作为陆上蜂窝移动通信系统的补充和延伸，支持动态、可靠、自动化、用户透明的服务。
  5. Iridium: The Iridium system, which was developed by Motorola company, uses low-orbit satellites for global satellite mobile communications. It mainly consists of satellite constellations as well as ground control facilities, gateway stations and user terminals. Different from the ground mobile communication system, the cross-zone swap of the Iridium system is the movement of cells across users, rather than the movement of users across cells.
    铱星：由摩托罗拉公司开发的铱星系统使用低轨道卫星进行全球卫星移动通信。它主要由卫星星座以及地面控制设施、网关站和用户终端组成。与地面移动通信系统不同，铱星系统的跨区域交换是小区在用户之间的移动，而不是用户在小区之间的移动。
  6. Globalstar: The Globalstar system, which was developed by LQSS company, is a mobile satellite communication system with continuous global coverage using LEO satellites. It differs from the Iridium system in both structural design and technology. The Globalstar system is
    Globalstar：由LQSS公司开发的Globalstar系统是一种使用LEO卫星连续覆盖全球的移动卫星通信系统。它在结构设计和技术上都与铱星系统不同。Globalstar系统是
Fig. 2. System architecture for space-air-ground-sea integrated network in 6G.
图 2.6G空地海一体化网络系统架构
non-circuitous and does not have a separate network, and its purpose is to ensure that any user around the world can access the ground public network through the system at any time. Moreover, the connection interface of Globalstar is located at the gateway, and it is an extension of ground cellular mobile communication systems and other mobile communication systems, which is to provide voice, data, facsimile, or radio-location services to all parts of the world.
不迂回，没有单独的网络，其目的是确保世界各地的任何用户都可以随时通过系统访问地面公共网络。此外，Globalstar的连接接口位于网关处，是地面蜂窝移动通信系统和其他移动通信系统的延伸，旨在向世界各地提供语音、数据、传真或无线电定位服务。
  1. VSAT: VSAT is a new satellite communication system developed in the mid-1980s using modern technology. VSAT satellite communication network, which adopts a star structure, consists of VSAT small station, master station, and satellite repeater. With the help of VSAT, user data terminals can directly use satellite channel and remote computer network to complete data transmission, file exchange or remote processing, so as to get rid of the ground relay problem at local areas.
    VSAT：VSAT是1980年代中期使用现代技术开发的新型卫星通信系统。VSAT卫星通信网络采用星形结构，由VSAT小站、主站和卫星中继站组成。借助VSAT，用户数据终端可以直接使用卫星信道和远程计算机网络完成数据传输、文件交换或远程处理，从而摆脱局部的地面中继问题。
  2. GMDSS: GMDSS is a global communications network designed to maximize the safety of life and property at sea. It is mainly shore-based as the core. Once there is a shipwreck, GMDSS can timely and effectively search and rescue ships in distress. In particular, GMDSS system is mainly composed of four parts, i.e., Inmarsat, Ground radio communication system, Maritime Safety Information (MSI) broadcasting system, and COSPAS-SARSAT.
    GMDSS：GMDSS是一个全球通信网络，旨在最大限度地提高海上生命和财产安全。它主要以岸基为核心。一旦发生海难，GMDSS可以及时有效地搜救遇险船舶。具体而言，GMDSS系统主要由国际海事卫星组织（Inmarsat）、地面无线电通信系统（Ground Radio Communication system）、海上安全信息（MSI）广播系统和COSPAS-SARSAT四部分组成。
  3. Navtex: Navtex is a maritime communications system that provides seafarers with weather forecasts, navigation information, emergencies, safety and work area information at sea. Navies use the Navtex system to warn against entering these areas before conducting training and exercises at sea.
    Navtex：Navtex 是一种海上通信系统，为海员提供海上天气预报、导航信息、紧急情况、安全和工作区域信息。海军在进行海上训练和演习之前使用Navtex系统警告不要进入这些区域。

B. System Architecture B. 系统架构

As shown in Fig. 2, the space-air-ground-sea network consists of four segments, including space network, air network, ground network, and sea network, respectively. These segments process data independently or transmit information cooperatively with each other, which can achieve high-quality data communication and provide users with flexible end-to-end services.
如图2所示，空地海网由空间网、空网、地网和海网4个部分组成。这些分段独立处理数据或相互协作传输信息，可实现高质量的数据通信，为用户提供灵活的端到端服务。
  1. Space Network: The space network consists of diverse types of satellites, constellations, and the corresponding ground infrastructures, including the ground stations and the control centers. According to their different altitudes, satellites can be divided into GEO, MEO, LEO [21], as well as very low earth orbit (VLEO) satellites [44]. GEO satellites are used for television broadcasts, overseas telegrams, and microwave communication, etc. MEO and LEO satellites are close to the ground, which remotely take pictures of objects with high resolution. They are usually applied to topographic reconnaissance, resource detection, and meteorological monitoring. Since VLEO is the lowest-orbiting satellite, it is expected to provide high-rate data services and precise positioning.
    空间网络：空间网络由不同类型的卫星、星座和相应的地面基础设施组成，包括地面站和控制中心。根据高度的不同，卫星可分为GEO、MEO、LEO[21]和甚低地球轨道（VLEO）卫星[44]。GEO卫星用于电视广播、海外电报、微波通信等，MEO和LEO卫星靠近地面，可以远程拍摄高分辨率的物体照片。它们通常应用于地形勘察、资源探测和气象监测。由于VLEO是轨道最低的卫星，因此有望提供高速数据服务和精确定位。
Although satellites have a high position, long transmission links, and large propagation latencies, it is difficult to provide users with global mobile communication. Therefore, the American company came up with Iridium to provide mobile users with global voice and data connections, by adopting loworbit satellites to provide mobile Internet access and dynamic networks for ground vehicles. However, in consideration of the large signal propagation delay of satellite-ground link, it is easy to be attacked and destroyed by malicious nodes in
卫星虽然位置高，传输链路长，传播时延大，但很难为用户提供全球移动通信。因此，这家美国公司提出了铱星，通过采用低轨道卫星为地面车辆提供移动互联网接入和动态网络，为移动用户提供全球语音和数据连接。但考虑到星地链路信号传播时延较大，容易被恶意节点攻击破坏
TABLE II 表二
SUMmary of THE COMPosition of SEA NEtwork IN 6G
6G中SEA NEtwork的组成总结
Sea Network Entity Communication techniques
沟通技巧		 Advantages Disadvantages
Surface network 地面网络 Ships
Underwater acoustic 水下声学
communication
Wide communication coverage,
广通信覆盖，
whole sea coverage, 全海覆盖，
relatively stable channel state
相对稳定的通道状态
Restricted
communication range 通信范围
Buoys
USVs
Underwater acoustic communication,
水声通信，
satellite communication 卫星通信
Underwater
observation network 观测网络
Optical fiber 光纤
Optical cable communication,
光缆通信，
submarine cable communication
海底电缆通信
Good transmission quality, high
传输质量好，高
reliability, and long service life
可靠性高，使用寿命长
High construction cost, 建设成本高，
maintenance difficulty 维护困难
Shore-based
station
Underwater
self-organizing network 自组织网络
AUVs Acoustic waves 声波
Distributed and low-cost, and used for
分布式和低成本，用于
marine monitoring, pollution monitoring,
海洋监测、污染监测、
and shipwreck search and rescue
和沉船搜救
Limited bandwidth, 带宽有限，
long delay time 延迟时间长
ROVs
UUVs
the transmission process, which makes it difficult to guarantee the QoS of real-time interactive applications. As a key technology of future 6G, SAGSIN will deploy a large number of VLEO satellites in space to provide broad geographical coverage, sufficient capacity, and a wide range of broadband communication services for residential and commercial institutions, governments and professional customers around the world.
传输过程，难以保证实时交互应用的QoS。作为未来6G的关键技术，SAGSIN将在太空部署大量VLEO卫星，为全球住宅和商业机构、政府和专业客户提供广阔的地理覆盖、充足的容量和广泛的宽带通信服务。
  1. Air Network: The air network is made up of aircraft, UAVs, airships, and balloons [45]. It can provide automatic routing of data between aircraft and ground station, aircraft and sea network, and can exchange data information with satellite layer. Among them, the high-altitude platform (HAP) is a common air network located in the stratosphere 20 kilometers from the ground. Compared with satellite communication platforms, HAPs have advantages of good mobility, short communication response time and low cost, and they are commonly used for broadcast/multicast services, emergency communications, disaster relief activities, and large-scale temporary events.
    航空网络：航空网络由飞机、无人机、飞艇和气球组成[45]。可提供飞机与地面站、飞机与海网之间的数据自动路由，并可与卫星层交换数据信息。其中，高空平台（HAP）是位于距地面20公里的平流层的通用空中网络。与卫星通信平台相比，HAP具有移动性好、通信响应时间短、成本低等优点，常用于广播/组播业务、应急通信、救灾活动、大型临时活动等。
As the UAV communication platform is close to the ground, it has the characteristics of short response time, high throughput, high line-of-sight transmission reliability, high mobility and flexibility, etc. Therefore, multiple UAVs can be used to form an aerial subnet to assist ground vehicles in transmitting road information. Moreover, the camera and GPS information on the UAVs can be used to detect obstacles and navigate. However, in view of the rapid change of its network topology and link channel, it is necessary to design an effective coordination mechanism so as to ensure the network security, providing high throughput and low latency network access for ground users [46].
由于无人机通信平台靠近地面，具有响应时间短、吞吐量高、视距传输可靠性高、机动性和灵活性高等特点。因此，可以使用多架无人机组成空中子网，以协助地面车辆传输道路信息。此外，无人机上的摄像头和GPS信息可用于检测障碍物和导航。然而，鉴于其网络拓扑结构和链路信道的快速变化，有必要设计一种有效的协调机制，以保证网络安全，为地面用户提供高吞吐量和低延迟的网络接入[46]。
  1. Ground Network: The ground network is composed of many sub-networks such as cellular network, ad hoc network, wireless local area network (WLAN) [47], worldwide interoperability for microwave access (WiMAX) [48], coastal base stations, and so on. In general, with the rapid development of the global mobile communication system, ground network technologies have been quite mature [49].
    地面网络：地面网络由蜂窝网络、自组织网络、无线局域网（WLAN）[47]、全球微波接入互操作性（WiMAX）[48]、沿海基站等许多子网组成。总的来说，随着全球移动通信系统的快速发展，地面网络技术已经相当成熟[49]。
Compared to other segments in SAGSIN, the ground network can provide users with a high data transmission rate and high throughput, nevertheless, the network coverage in remote areas is limited. Besides, it is vulnerable to natural disasters and man-made infrastructure damages, so that only relying on the ground network cannot catch up with the increasing demands of high-quality services in the future.
与SAGSIN的其他细分市场相比，地面网络可以为用户提供高数据传输速率和高吞吐量，但偏远地区的网络覆盖范围有限。此外，它容易受到自然灾害和人为基础设施破坏的影响，因此仅依靠地面网络无法赶上未来日益增长的高质量服务需求。
4) Sea Network: The sea network is mainly composed of two parts, i.e., the surface network and the underwater network. Table II presents some characteristics of the sea network.
4）海网：海网主要由水面网和水下网两部分组成。表二列举了海网的一些特点。
Surface network: The surface network includes ships, buoys, and unmanned surface vessels (USV) [12]. It is obvious that, the mobility of the USV is stronger than that of a buoy. Ships, buoys, and USVs nodes have communication capabilities and can provide edge services. Compared with buoys and USVs, ships provide more computing power. Note that the computing performance of these nodes is much lower than that of remote cloud servers, due to the limited communication conditions of maritime remote services, limited bandwidth and unstable channel quality. Thus, we need to make full use of the computing resources of these nodes so as to complete the delay-sensitive tasks in the sea.
水面网络：水面网络包括船舶、浮标和无人水面舰艇（USV）[12]。很明显，USV的机动性比浮标强。舰船、浮标、USV节点具有通信能力，可以提供边缘服务。与浮标和USV相比，船舶提供了更多的计算能力。需要注意的是，由于海事远程业务的通信条件有限，带宽有限，信道质量不稳定，这些节点的计算性能远低于远端云服务器。因此，我们需要充分利用这些节点的计算资源，以完成海上对时延敏感的任务。
Underwater network: Generally, underwater network consists of an underwater observation network and an underwater self-organizing network [50]. The underwater observation network communicates by laying optical cables and submarine cables. However, due to the harsh marine environment, the exorbitant cost of construction and maintenance, it is difficult to deploy underwater observation network on a large scale. The underwater self-organizing network is an extension of the underwater network in wireless communications, which is mainly composed of fixed sensor nodes and mobile nodes such as autonomous underwater vehicles (AUV), remotely operated vehicles (ROV), unmanned underwater vehicles (UUV), etc. It usually uses acoustic waves for communication, and has the disadvantages of limited bandwidth and long delay.
水下网络：水下网络通常由水下观测网络和水下自组织网络组成[50]。水下观测网络通过铺设光缆和海底光缆进行通信。然而，由于海洋环境恶劣，建造和维护成本高昂，难以大规模部署水下观测网络。水下自组织网络是无线通信中水下网络的延伸，主要由自主水下航行器（AUV）、遥控航行器（ROV）、无人水下航行器（UUV）等固定传感器节点和移动节点组成。它通常使用声波进行通信，并具有带宽有限和延迟长的缺点。
At present, maritime means of communication have low efficiency and narrow bandwidth, which cannot meet the growing needs of economic activities and life on the sea. Therefore, using coastal base drones and building new communication systems are apparently necessary and significant. Through the establishment of SAGSIN, global sea coverage is likely to be achieved by adopting GMDSS and Inmarsat.
目前，海上通信手段效率低下，带宽窄，无法满足日益增长的经济活动和海上生活需求。因此，使用沿海基地无人机和建立新的通信系统显然是必要和重要的。通过建立SAGSIN，采用GMDSS和Inmarsat有可能实现全球海洋覆盖。
SAGSIN is mainly based on the ground network and extended by the space network. It carries all kinds of network services in the space, air, ground, and sea, and can provide information guarantee for all kinds of users' activities. SAGSIN has the characteristics of super-large scale, threedimensional multi-layer topology, high heterogeneity, and supporting wide variety of businesses. In particular, the space network can make full use of the features of long satellite transmission distance and flexible and efficient networking,
SAGSIN主要基于地面网络，并由空间网络扩展。承载天、空、陆、海各类网络服务，可为各类用户活动提供信息保障。SAGSIN具有超大规模、立体多层拓扑、高异质性、支持多种业务等特点。特别是空间网络可以充分利用卫星传输距离长、组网灵活高效等特点，
and realizes efficient and reliable communication for the whole world through networking and interconnection among mobile satellite nodes, ground nodes, and air nodes. Air network communication is mainly carried by stratospheric airship, stratospheric high-altitude balloon, UAV, helicopter and other high altitude platforms. It can significantly expand the communication distance, extend the coverage of the ground network, and complete the forwarding and exchange of information in the air, and thus it can be conveniently adopted in maritime affairs. Specifically, ground-based communication is used for emergency communication in near shore emergencies, and its main equipment is deployed at fixed stations along the coast or on islands and reefs. When special situations occur in the near shore waters and emergency rescue is needed, the fixed location of the existing stations and the strong support capability of the platform can be utilized to ensure the emergency communication access of the coast or the surrounding islands. Furthermore, sea-based communication adopts ships and buoys as platforms to quickly establish communication support and to carry out rescue missions in case of maritime emergencies or natural disasters.
通过卫星移动节点、地面节点、空中节点之间的组网互联，实现全球高效可靠的通信。空中网络通信主要由平流层飞艇、平流层高空气球、无人机、直升机等高空平台承载。它可以显著扩大通信距离，扩大地面网络的覆盖范围，完成空中信息的转发和交换，从而可以方便地应用于海洋事务。具体而言，地基通信用于近岸突发事件下的应急通信，其主要设备部署在沿海或岛礁上的固定站点。当近岸水域发生特殊情况，需要紧急救援时，可利用现有站点的固定位置和平台强大的支撑能力，确保海岸或周边岛屿的应急通信接入。此外，海基通信以船舶和浮标为平台，在海上紧急情况或自然灾害发生时快速建立通信支持并执行救援任务。

C. Communication Technology of SAGSIN
C. SAGSIN的通信技术

  1. Mm-Wave Communication: Generally, millimeter-wave (mm-wave) refers to the frequency band between 30 and 300 GHz. Compared with traditional RF technology, it significantly widens the available bandwidth. Mm-wave is used to achieve data rates up to several hundred meters in gigabits per second levels at a bandwidth of several gigahertzes. At the same time, the shortest wavelength in mm-wave leads to smaller antenna size, which can improve the portability and integration of equipment, and it is conducive to specific applications such as detection radar and physical layer security. In addition, the wide band and high transmission power of mm-wave will lead to serious nonlinear signal distortion, which may bring more security issues. At the same time, due to the effective transmission range of mm-wave, it is usually dominated by line of sight (LoS) path. At present, the standardization of mm-wave field mainly focuses on the band for indoor use.
    毫米波通信：通常毫米波（mm-wave）是指30至300GHz之间的频段，与传统射频技术相比，它大大拓宽了可用带宽。毫米波用于在几千兆赫兹的带宽下实现高达几百米的数据速率，以千兆比特/秒为单位。同时，毫米波波中最短的波长导致天线尺寸更小，可以提高设备的便携性和集成度，有利于探测雷达和物理层安全等特定应用。此外，毫米波的宽带和高传输功率将导致严重的非线性信号失真，这可能会带来更多的安全问题。同时，由于毫米波的有效传输范围，通常以视距（LoS）路径为主。目前，毫米波场的标准化主要集中在室内使用的 频段上。
  2. Terahertz Communication: Terahertz communication is envisioned as a driving technology for the 6G Internet of Things, requiring a data rate of Gbps and a delay less than . It is expected to meet the future needs of 6G IoT applications. Compared with mm-wave, the terahertz spectrum can address the spectrum shortage in wireless communications and significantly enhance the capability of wireless systems. Moreover, terahertz communication can also provide high broadband and throughput, supporting ultra-broadband applications like virtual reality. However, how to effectively use integrated circuits to modulate baseband signals to high frequency carriers remains the most critical challenge in the practical implementation of terahertz technology.
    太赫兹通信：太赫兹通信被设想为6G物联网的驱动技术，需要Gbps的数据 速率和小于 的延迟。有望满足未来6G物联网应用需求。与毫米波相比，太赫兹频谱可以解决无线通信频谱不足的问题，显著增强无线系统的能力。此外，太赫兹通信还可以提供高宽带和吞吐量，支持虚拟现实等超宽带应用。然而，如何有效地利用集成电路对高频载波的基带信号进行调制，仍然是太赫兹技术实际实施中最关键的挑战。
  3. Visible Light Communication: Visible light communication (VLC) is an optical wireless technology that operates in the frequency ranging from 400 terahertz to 800 terahertz. Compared with radio frequency system, VLC has the advantages of high data rate, large available spectrum, strong anti-interference ability, inherent safety line, etc., which has been widely concerned by people. Compared to other wireless communication technologies, VLC systems can provide a higher level of security. However, due to their broadcast nature and LoS propagation, VLC systems are also vulnerable to eavesdropping by unauthorized nodes located in the transmitter coverage area, which means higher security risks.
    可见光通信：可见光通信 （VLC） 是一种光学无线技术，工作频率范围为 400 太赫兹至 800 太赫兹。与射频系统相比，VLC具有数据速率高、可用频谱大、抗干扰能力强、固有安全线等优点，受到人们的广泛关注。与其他无线通信技术相比，VLC系统可以提供更高级别的安全性。然而，由于其广播性质和LoS传播，VLC系统也容易受到位于发射机覆盖区域内的未经授权的节点的窃听，这意味着更高的安全风险。
  4. Optical Wireless Communication: Optical wireless band refers to the electromagnetic spectrum with the carrier frequency of infrared, visible and ultraviolet light, and the wavelength range is and , respectively. They can be used for wireless communications in indoor, outdoor, underground and underwater scenarios. Optical wireless communication is a promising complementary technology to traditional wireless communications in RF band. The main difference between it and traditional frequency band is that there is no multipath fading, Doppler Effect and bandwidth adjustment. Moreover, optical bands can provide almost unlimited bandwidth without permission from regulators around the world. Due to the availability of optical emitters and detectors, it can be used to achieve low-cost high-speed access, and can provide high capacity intersatellite links for satellite constellations. Recently, research interest in ultraviolet communication has been gradually increasing.
    光无线通信：光无线频段是指与红外光、可见光和紫外光的载波频率相符的电磁频谱，波长范围分别为 和 。它们可用于室内、室外、地下和水下场景的无线通信。光无线通信是射频频段传统无线通信的一种很有前途的补充技术。它与传统频段的主要区别在于没有多径衰落、多普勒效应和带宽调整。此外，光频段可以提供几乎无限的带宽，而无需获得世界各地监管机构的许可。由于光发射器和探测器的可用性，可用于实现低成本的高速接入，并可为卫星星座提供高容量的星间链路。近年来，人们对紫外线通信的研究兴趣逐渐增加。
  5. Underwater Acoustic Communication: Acoustic wave is the main carrier of underwater information, which has been widely used in underwater communication, sensing, detection, navigation, positioning and other fields. Acoustic wave belongs to mechanical wave (longitudinal wave). The signal attenuation in underwater transmission is small (i.e., its attenuation rate is of electromagnetic wave.), and the transmission distance is long. Its communication range can extend from several hundred meters to dozens of kilometers, and thus acoustic wave is very suitable for deep-water communication with stable temperature. Moreover, underwater acoustic channel is a very complex multi-path transmission channel with high ambient noise, narrow band width, low applicable carrier frequency and large transmission delay. In order to overcome these disadvantages and to improve its bandwidth utilization efficiency as much as possible, further research on new technical solutions is needed, including multi-carrier modulation, multi-input multi-output technology, etc.
    水声通信：声波是水下信息的主要载体，已广泛应用于水下通信、传感、探测、导航、定位等领域。声波属于机械波（纵波）。水下传输中的信号衰减小（即其衰减率为 电磁波），传输距离长。它的通信范围可以从几百米延伸到几十公里，因此声波非常适合温度稳定的深水通信。此外，水声信道是一种非常复杂的多径传输信道，具有环境噪声高、带宽窄、适用载波频率低、传输时延大等特点。为了克服这些缺点，尽可能提高其带宽利用效率，需要进一步研究新的技术方案，包括多载波调制、多输入多输出技术等。

D. Characteristics of SAGSIN
D. SAGSIN的特性

SAGSIN not only has all the inherent characteristics, but also includes large coverage, high throughput, and good resilience of the four segments. Meanwhile, with the integration of these networks, SAGSIN also has its new characteristics, such as heterogeneity, self-organization, time-variability, etc.
SAGSIN不仅具有所有固有特性，而且包括覆盖面广、通量高、弹性好的四个段。同时，随着这些网络的整合，SAGSIN也具有了异质性、自组织性、时变性等新特性。
  1. Heterogeneity: Heterogeneity refers to that the network comprises multiple wireless communications systems using different access techniques [51]. SAGSIN covers a wide range, spanning space, air, ground, and sea. Each layer consists of several subnets, and each subnet has diverse communication modes and devices, which will inevitably affect network delay,
    异构性：异构性是指网络由使用不同接入技术的多个无线通信系统组成[51]。SAGSIN覆盖范围广，跨越太空、空中、地面和海洋。每一层由多个子网组成，每个子网都有不同的通信模式和设备，这不可避免地会影响网络时延，
    transmission data rate, and so on. In order to satisfy the needs of communication services, it is an effective method to fully exploit the advantages of existing wireless communications systems utilizing system integration. Normally, diverse protocols of each segment supports different functions or applications, resulting that the four-layer heterogeneous network can provide various packet transfers. Furthermore, according to users' requirements, SAGSIN can select the appropriate network access technique for them and provide better QoS [52].
    传输数据速率等。为了满足通信业务的需求，充分利用现有无线通信系统利用系统集成的优势，是一种有效的方法。通常，每个网段的不同协议支持不同的功能或应用，导致四层异构网络可以提供各种数据包传输。此外，根据用户的需求，SAGSIN可以为他们选择合适的网络接入技术，并提供更好的QoS[52]。
  2. Self-Organization: The self-organization of SAGSIN enables it to use the routing and forwarding function to communicate in an infrastructure environment [53]. It integrates multi-layer networks and accesses various types of mobile devices. These devices are always in high-speed motion and frequently switch between multiple subnets. In SAGSIN, the nodes can join or leave the network at anytime, and the failure of any node may not affect the operation of the whole network, which has a strong destruction resistance. Based on this characteristic, the network can be used for military communications, emergency services, and disaster recovery [54], [55]. Specifically, various military equipments carry out information exchange coordinately to complete combat tasks, and it can establish temporary networks quickly, thereby reducing the rescue time and the harm caused by the disaster.
    自组织：SAGSIN的自组织使其能够使用路由和转发功能在基础设施环境中进行通信[53]。它集成了多层网络并访问各种类型的移动设备。这些设备始终处于高速运动状态，并经常在多个子网之间切换。在SAGSIN中，节点可以随时加入或离开网络，任何节点的故障都可能不影响整个网络的运行，具有很强的抗破坏能力。基于这一特性，该网络可用于军事通信、应急服务和灾难恢复[54]，[55]。具体来说，各种军事装备协调进行信息交换，完成作战任务，可以快速建立临时网络，从而减少救援时间和灾难造成的伤害。
  3. Time-Variability: The space network is organized by constellation and established by inter-satellite links. The design of the constellation determines the particularity of satellite network topology. The establishment of an inter-satellite link not only reduces the dependence of the satellite network on the ground network, but also increases the design difficulty and manufacturing cost of the system. The high mobility of nodes and dynamic change of topology structure further lead to the time-varying feature of SAGSIN. The time-varying network can be divided into two cases. The first is the change of relative distance between nodes, which mainly affects the transmission delay. This change is slow and can be ignored under certain conditions. The second is the links handover between nodes, which can change the network structure and affect the routing of local nodes, thus having an impact on the performance of all aspects of the network.
    时变性：空间网络按星座组织，并由卫星间链路建立。星座的设计决定了卫星网络拓扑的特殊性。星间链路的建立不仅降低了卫星网络对地面网络的依赖，而且增加了系统的设计难度和制造成本。节点的高迁移率和拓扑结构的动态变化进一步导致了SAGSIN的时变特性。时变网络可以分为两种情况。首先是节点之间相对距离的变化，主要影响传输时延。此更改很慢，在某些情况下可以忽略。二是节点之间的链路切换，可以改变网络结构，影响本地节点的路由，从而对网络各个方面的性能产生影响。

E. Challenges E. 挑战

Besides the benefits for practical services and applications, SAGSIN is also facing many unprecedented challenges due to its unique characteristics, as shown in Fig. 3. The traditional network need consider collaborative control and management, cooperative data transmission, inter-connection, and inter-communication. As for SAGSIN, the heterogeneity and self-organization characteristics may bring new challenging issues like cross-layer design and optimization, network security, and load balancing, while the time-variability can affect propagation channel modeling, mobility management, traffic distribution, and routing mechanism. The following summarizes several significant technical challenges and future directions.
如图3所示，SAGSIN除了具有实用服务和应用的优势外，还面临着许多前所未有的挑战。传统网络需要考虑协同控制和管理、协同数据传输、互联互通等。对于SAGSIN来说，异质性和自组织特性可能会带来跨层设计与优化、网络安全和负载均衡等新的挑战性问题，而时变性会影响传播信道建模、移动性管理、流量分布和路由机制。下面总结了几个重大的技术挑战和未来方向。
  1. Network Management: Network Management is the premise to ensure the normal operation of the network,
    网络管理：网络管理是保证网络正常运行的前提，
Fig. 3. Challenges brought by SAGSIN's characteristics.
图 3.SAGSIN的特点带来的挑战。
especially for SAGSIN, which has a wide distribution of nodes and a complex network environment. In addition to the traditional configuration, fault, performance and security management functions, SAGSIN focuses more on the management of wireless resources and mobile users in the network. The high cost of satellite wireless resources and the uneven distribution of satellite users make it difficult to manage network resources effectively. And the dynamic changes of satellite nodes usually waste additional resources to maintain the balance of communication. Therefore, how to adopt appropriate wireless resource allocation technology to improve the utilization rate of satellite resources and reduce costs, this may become a research hot spot.
特别是对于节点分布广泛、网络环境复杂的SAGSIN。除了传统的配置、故障、性能和安全管理功能外，SAGSIN更注重网络中无线资源和移动用户的管理。卫星无线资源成本高，卫星用户分布不均，难以有效管理网络资源。而卫星节点的动态变化通常会浪费额外的资源来维持通信的平衡。因此，如何采用适当的无线资源分配技术来提高卫星资源的利用率和降低成本，这可能成为研究的热点。
Not only that, the communication environment of the subnets of SAGSIN are very different, let alone the business needs. At the same time, each subnet supports different application scenarios, resulting in uneven transmission loads. Thus, solving the problem of unified management and reasonable distribution of network resources is a top priority. Moreover, mobile management technology supports the seamless communication of mobile terminals in the whole network coverage, and guarantees that a mobile terminal can continue to receive services from the network. With the movement of terminal users and satellites, the network access points and channels change dynamically. To ensure the continuous and effective communication capacity of user terminals in this mobile environment, the network needs to find out appropriate management strategies to track mobile terminal users constantly and provide services for them.
不仅如此，SAGSIN子网的通信环境也大不相同，更不用说业务需求了。同时，每个子网支持不同的应用场景，导致传输负载不均匀。因此，解决网络资源的统一管理和合理分配问题是当务之急。此外，移动管理技术支持移动终端在全网覆盖下的无缝通信，保证移动终端能够持续接收网络业务。随着终端用户和卫星的移动，网络接入点和信道会动态变化。为了保证用户终端在这种移动环境中的持续有效的通信能力，网络需要找出合适的管理策略，持续跟踪移动终端用户并为他们提供服务。
  1. QoS Guarantee: With the rapid development of highspeed data transmission and interactive communication, SAGSIN needs to provide a strict end-to-end QoS guarantee. First of all, since the satellites are distributed in a wide space, the transmission latency between these nodes is much larger than that between the nodes in the ground network. Therefore, the node transmission between satellites becomes the main component of SAGSIN communication delay. This kind of high latency increases the TCP end-to-end delay, resulting in slow feeding back of the confirmation information to the sending nodes. Moreover, it reduces network throughput and brings about a large number of packets staying on the satellite communication channel, increasing the probability of packet loss, which may cause network congestion and unavailability.
    QoS保障：随着高速数据传输和交互通信的快速发展，SAGSIN需要提供严格的端到端QoS保障。首先，由于卫星分布在广阔的空间中，这些节点之间的传输时延远大于地面网络中节点之间的传输时延。因此，卫星之间的节点传输成为SAGSIN通信时延的主要组成部分。这种高延迟增加了 TCP 端到端的延迟，导致确认信息反馈到发送节点的速度变慢。此外，降低网络吞吐量，导致大量报文滞留在卫星通信信道上，增加了丢包概率，可能导致网络拥塞和不可用。
Furthermore, underwater wireless networks [56] generally use acoustic waves as data carriers. According to the transmission speed of the medium and the distance between networks,
此外，水下无线网络[56]通常使用声波作为数据载体。根据介质的传输速度和网络之间的距离，
Fig. 4. Summary of Security Requirements.
图 4.安全要求摘要。
this may cause long delay between the sea surface and the ground network, the sea surface and the air network. In order to establish a real-time network, it is necessary to improve the satellite network and sea surface network organization, and design new data transmission methods. The main problem of multi-service integrated transmission in the network is transmission efficiency, and how to ensure that all kinds of business data can be effectively transmitted in the network is an urgent issue to be solved.
这可能会导致海面与地网、海面与空网之间的长时间延迟。为了建立实时网络，需要完善卫星网络和海面网络组织，并设计新的数据传输方式。网络中多业务集成传输的主要问题是传输效率，如何保证各类业务数据在网络中有效传输是亟待解决的问题。
  1. Gateway Placement and Selection: In SAGSIN, satellite gateways play an important role in transmitting traffic from the ground network to the space network. Different satellite gateway locations have an impact on network performance. Many factors like poor weather conditions, can lead network nodes and links to fail. Therefore, on the premise of meeting the limitation of satellite link capacity, how to select the appropriate gateways and place them in the optimal position is worth further analysis. Generally speaking, the gateway selection must take inter-segment influences into account in SAGSIN. Especially for the air network segment, as the relay of ground and satellite communication, how to choose the appropriate number of routing nodes is a challenging problem. Selecting the gateway not only the traffic distribution within the segments, but also the total traffic from the ground segment and the capacity of the air link should be considered [57]. Regarding determining the locations of satellite gateways, we should pay more attention to the geographical location and the network topology, as well as the overall traffic distribution [58].
    网关布局和选择：在SAGSIN中，卫星网关在将流量从地面网络传输到空间网络方面发挥着重要作用。不同的卫星网关位置会影响网络性能。许多因素（如恶劣的天气条件）都可能导致网络节点和链路出现故障。因此，在满足卫星链路容量限制的前提下，如何选择合适的网关并将其放置在最佳位置值得进一步分析。一般来说，网关选择必须考虑SAGSIN中的网段间影响。特别是对于空中网络段，作为地面和卫星通信的中继，如何选择适当数量的路由节点是一个具有挑战性的问题。选择网关时，不仅要考虑网段内的流量分布，还要考虑地面网段的总流量和空中链路的容量[57]。在确定卫星网关的位置时，应更加关注地理位置和网络拓扑结构，以及整体流量分布[58]。
Although cables, optical cables, and other wired techniques can be used for underwater communication, the nodes cannot be moved in these methods, and the applicable objects are extremely limited. Moveover, the attenuation of electromagnetic waves in the water is very large. If we want to transmit a long distance in the water, we have to adopt very low frequencies, which requires very high transmission energy and a long antenna, and it is usually difficult to be achieved. Therefore, underwater communication mainly include underwater acoustic communication, long-wave communication, and underwater laser communication. In particular, the underwater acoustic communication network [59] can be divided into a centralized network and a distributed peer-to-peer network. When using the underwater acoustic communication network, the topology of network determines the routing method, energy loss, network capacity, and reliability of the network. Thus, the information such as the network topology of the sea surface and ground base stations should be firstly considered before confirming the nodes of the underwater acoustic network.
虽然可以使用电缆、光缆和其他有线技术进行水下通信，但这些方法无法移动节点，适用对象极为有限。移动时，电磁波在水中的衰减非常大。如果我们想在水中长距离传输，就必须采用非常低的频率，这需要非常高的传输能量和很长的天线，通常很难实现。因此，水下通信主要包括水声通信、长波通信和水下激光通信。特别是，水声通信网络[59]可分为集中式网络和分布式点对点网络。使用水声通信网络时，网络的拓扑结构决定了网络的路由方式、能量损耗、网络容量和可靠性。因此，在确认水声网络节点之前，应首先考虑海面和地面基站的网络拓扑等信息。
  1. Security: As a four-layer heterogeneous network with open links and dynamic topologies, SAGSIN faces many new security challenges. Like other communication networks, it is vulnerable to denial of service (DoS) attacks, jamming attacks, spoofing attacks, unauthorized access, malware, etc [60]. Specifically, SAGSIN chronically in a severe natural environment results that the inter-satellite and satellite-ground links suffer from the electromagnetic signal interference, and these links may be eavesdropped by malicious users. The satellite nodes directly exposed to space orbit are easily subjected to illegal interception. Moreover, the integration of various heterogeneous networks leads to the higher security requirements on traditional routing, network access, and handover strategies, and thus, it is urgent to implement the interconnection control and to ensure multi-level security. Besides, considering that the scattering and refraction path is small while the signal is transmitted at sea, if the antenna height of the buoy is low, the curvature of the earth and the obstruction of the waves may cause the signal transmission to be interrupted. Overall, SAGSIN has been being widely used in military and civilian communications, and how to ensure the reliability and security of the private data should be well accounted for.
    安全性：作为具有开放链路和动态拓扑结构的四层异构网络，SAGSIN面临着许多新的安全挑战。与其他通信网络一样，它容易受到拒绝服务（DoS）攻击、干扰攻击、欺骗攻击、未经授权的访问、恶意软件等[60]。具体而言，SAGSIN长期处于恶劣的自然环境中，导致星间和卫星-地面链路受到电磁信号干扰，这些链路可能被恶意用户窃听。直接暴露在空间轨道上的卫星节点很容易受到非法拦截。此外，各种异构网络的融合导致对传统路由、网络接入和切换策略的安全要求更高，因此，实施互联控制并确保多级安全迫在眉睫。此外，考虑到信号在海上传输时散射和折射路径较小，如果浮标的天线高度较低，地球的曲率和波浪的阻碍可能会导致信号传输中断。总体而言，SAGSIN在军民通信中得到了广泛的应用，如何确保私人数据的可靠性和安全性应该得到很好的考虑。

III. Security Requirements
三、安全要求

The security requirements are specified for the sake of protecting the wireless transmissions. Satisfying the basic security requirements is an indispensable condition for the successful operation of SAGSIN. In general, secure wireless communications should meet the requirements of confidentiality, integrity, and availability, which are discussed detailedly in the following, as shown in Fig. 4.
为了保护无线传输而指定了安全要求。满足基本的安全要求是SAGSIN成功运行的必要条件。一般来说，安全的无线通信应满足机密性、完整性和可用性的要求，下文将详细讨论这些要求，如图 4 所示。

A. Confidentiality A. 保密

Confidentiality is to ensure that only the designated users have the authority to access data while preventing information from being leaked to unauthorized entities. Because the transmission data contains relevant information of the user behaviors, malicious attackers can infer sensitive information indirectly by utilizing available information that is unintentionally disclosed.
保密性是为了确保只有指定的用户才有权访问数据，同时防止信息泄露给未经授权的实体。由于传输数据包含用户行为的相关信息，恶意攻击者可以利用无意中泄露的可用信息间接推断敏感信息。
Possible confidentiality violations in SAGSIN: In SAGSIN, the network effectively organizes the system nodes through inter-satellite links, satellite-ground links and satellite-sea links. In this process, there may be a breach of system confidentiality. For example, in the communication process of UAVs, an eavesdropper can secretly monitor the data information of UAVs by eavesdropping on the communications between UAVs and other nodes. As a result, the information in UAV network may be stolen and the confidentiality of UAVs is breached. In addition, when taking plaintext and a cryptographic key to produce ciphertext, the implementation of an encryption algorithm usually outputs unexpected information as a by-product of the actual calculation [61]. Attackers can exploit this accidental information leaks to bypass or undermine the protection mechanisms of the network. Because of this attack, physical layer security [62] was proposed as an excellent measure to prevent eavesdropping and protect the confidentiality of wireless transmission.
SAGSIN中可能存在的保密违规行为：在SAGSIN中，网络通过卫星间链路、卫星-地面链路和卫星-海链路有效地组织系统节点。在此过程中，可能会违反系统机密性。例如，在无人机的通信过程中，窃听者可以通过窃听无人机与其他节点之间的通信来秘密监控无人机的数据信息。因此，无人机网络中的信息可能被盗，无人机的机密性被破坏。此外，当采用明文和加密密钥来生成密文时，加密算法的实现通常会输出意外信息作为实际计算的副产品[61]。攻击者可以利用这种意外的信息泄漏来绕过或破坏网络的保护机制。由于这种攻击，物理层安全[62]被提出作为防止窃听和保护无线传输机密性的极好措施。

B. Integrity B. 诚信

Integrity refers to preventing unauthorized users or systems from falsifying or modifying information, guaranteeing the received information be the same as the sent information in the message transmission process. The integrity of data transmitted over wireless networks may be compromised by internal attacks. Unlike the attacks targeting confidentiality, the attack on data integrity can be regarded as a less violent but more sophisticated attack. In other words, this attack attempts to modify the original information in the system and disrupt the normal exchange of information on the network. More specifically, if a legitimate node is modified and destroyed by an opponent, the destroyed node may cause damage to data integrity by launching malicious attacks (including message injection, false routing, data modification, etc.) [63].
完整性是指防止未经授权的用户或系统伪造或修改信息，保证接收到的信息与消息传输过程中发送的信息相同。通过无线网络传输的数据的完整性可能会受到内部攻击的影响。与针对机密性的攻击不同，对数据完整性的攻击可以被视为一种不那么暴力但更复杂的攻击。换句话说，这种攻击试图修改系统中的原始信息并破坏网络上的正常信息交换。更具体地说，如果一个合法节点被对手修改和破坏，被破坏的节点可能会通过发起恶意攻击（包括消息注入、错误路由、数据修改等）对数据完整性造成损害。[63].
Possible integrity violations in SAGSIN: In SAGSIN, integrity is compromised by attempting to manipulate and abuse content transmitted internally and externally to and from the core network through jamming attacks, DoS attacks, and other attack methods. In SAGSIN, more channels are exposed to the adversary than in other typical networks, and it is easy to establish links in the network, adding more threat vectors and violating its integrity. For example, in the satellite-ground link, legitimate nodes are illegally modified to malicious nodes, so that the data information in the link is tampered with. In this way, the damaged nodes may launch network attacks, resulting in the destruction of data integrity in satellite-ground transmission. Generally, it is difficult to detect attacks on network nodes because the attacked nodes running malicious codes still have valid identities, which is a thorny issue to be solved urgently. Therefore, improving the intrusion detection technology is of significant importance [64].
SAGSIN 中可能存在的完整性违规：在 SAGSIN 中，通过干扰攻击、DoS 攻击和其他攻击方法，试图操纵和滥用内部和外部传输到核心网络和从核心网络传输的内容，从而损害完整性。在SAGSIN中，与其他典型网络相比，暴露给对手的信道更多，并且很容易在网络中建立链接，从而增加更多的威胁向量并破坏其完整性。例如，在星地链路中，合法节点被非法修改为恶意节点，使链路中的数据信息被篡改。这样一来，受损节点就可能发起网络攻击，导致星地传输中数据完整性的破坏。一般来说，由于运行恶意代码的攻击节点仍然具有有效身份，因此很难检测到对网络节点的攻击，这是一个亟待解决的棘手问题。因此，改进入侵检测技术具有重要意义[64]。

C. Availability C. 可用性

Availability means that authorized users can access the wireless network based on the requests at anytime and anywhere. The network should provide effective service when necessary, even if it is being attacked [60]. Malicious attacks on availability can be considered as a kind of DoS attacks, which attempt to delay, block, or even disrupt the information transmission, thereby making the network resources unavailable to the communication nodes. For instance, any unauthorized node can launch DoS attacks at the physical layer, which maliciously interferes with the communication between legitimate users, also known as jamming attacks [65]. Existing wireless systems usually adopt spread spectrum technology to resist jamming attacks, where the signal uses an unrelated code sequence to extend the spectrum so that its bandwidth far exceeds the required minimum bandwidth. Specific contents about jamming attacks will be described in Section V.
可用性意味着授权用户可以随时随地根据请求访问无线网络。网络在必要时应提供有效的服务，即使它受到攻击[60]。对可用性的恶意攻击可以看作是一种DoS攻击，它试图延迟、阻塞甚至破坏信息传输，从而使网络资源对通信节点不可用。例如，任何未经授权的节点都可以在物理层发起DoS攻击，恶意干扰合法用户之间的通信，也称为干扰攻击[65]。现有的无线系统通常采用扩频技术来抵抗干扰攻击，其中信号使用不相关的代码序列来扩展频谱，使其带宽远远超过所需的最小带宽。有关干扰攻击的具体内容将在第五节中描述。
Possible availability violations in SAGSIN: The main availability violation in SAGSIN is communication channel interruption caused by DoS attacks, which prevent authorized users from accessing the wireless network, resulting in bad quality of experience ( . For example, the users need to access the wireless network at anytime and anywhere, nevertheless, when malicious attacks occur, the transmission of data in the network will be interrupted, thus interfering with the communication among users. In , the network service architecture needs to have sufficient capacity to handle the requirements of users, and the failures to integrate these requirements into the new network can compromise network flow and result in inaccessibility. Recently, a large number of robots have been used for distributed attacks, and these botnettype attacks limit the accessibility of legitimate users and harm their availability.
SAGSIN 中可能存在的可用性冲突：SAGSIN 中的主要可用性冲突是由 DoS 攻击引起的通信信道中断，这会阻止授权用户访问无线网络，从而导致体验质量不佳 （ .例如，用户需要随时随地访问无线网络，但是，当恶意攻击发生时，网络中的数据传输会中断，从而干扰用户之间的通信。在 中 ，网络服务架构需要有足够的容量来处理用户的需求，如果无法将这些需求集成到新网络中，可能会影响网络流量并导致无法访问。最近，大量的机器人被用于分布式攻击，这些僵尸网络类型的攻击限制了合法用户的可访问性并损害了他们的可用性。
Remarks: In SAGSIN, nodes are linked to each other to improve the flexibility, mobility and intelligence of the network. Although confidentiality, integrity and availability mentioned above are of paramount importance, authenticity and non-repudiation of network information should also be ensured. In particular, authentication is the process of authenticating the identities of all parties involved in communication or resource access. These mechanisms are executed either individually or with each other through extended scenarios. Non-repudiation means that the communication parties are sure of the true identity of the participants themselves and the information provided by them in the process of information exchange, that is, all participants cannot deny or deny their true identity. In SAGSIN, these security requirements should be satisfied as much as possible, so as to realize the holographic and ubiquitous connections of network communications.
备注：在SAGSIN中，节点相互链接，以提高网络的灵活性、移动性和智能性。虽然上述机密性、完整性和可用性至关重要，但也应确保网络信息的真实性和不可否认性。具体而言，身份验证是对通信或资源访问中涉及的所有各方的身份进行身份验证的过程。这些机制可以单独执行，也可以通过扩展方案相互执行。不可否认是指通信各方确定参与者本人的真实身份以及他们在信息交换过程中提供的信息，即所有参与者都不能否认或否认其真实身份。在SAGSIN中，应尽可能满足这些安全要求，从而实现网络通信的全息和泛在连接。

IV. Security Threats in SAGSIN
四、SAGSIN的安全威胁

According to the actual security situations and requirements, the threats in SAGSIN can be generally classified into four types, i.e., physical threats, operational threats, network threats, and data/information threats, as shown in Fig. 5, and the details are given as follows.
根据实际的安全情况和要求，SAGSIN中的威胁大致可分为物理威胁、操作威胁、网络威胁和数据/信息威胁四种类型，如图5所示，具体如下。

A. Physical Threats A. 人身威胁

In terms of physical threats, they come down to the malicious destruction of physical infrastructures caused by human beings as well as the harsh natural environment.
就物理威胁而言，它们归结为人类对物理基础设施的恶意破坏以及恶劣的自然环境。
Fig. 5. Organization of security threats in SAGSIN.
图 5.SAGSIN中的安全威胁组织。
  1. Natural Occurrences and Environment Factors: Even without the threats posed by human activities, irresistible environmental factors and natural events may destroy the communication links in SAGSIN. Natural disasters, such as earthquakes, floods, sandstorms, tornadoes, heavy rain or snow, thunder, and lightning, etc., may damage ground stations and cause service interruptions. Satellite links are susceptible to hostile space-based environments, such as solar and cosmic radiation, atmospheric disturbances, meteoroids, asteroids, etc [66]. In the "satellite eclipse" season, the sun is blocked by the earth and the satellite solar cannot get sunlight, and the onboard battery only maintains the satellite rotation, but cannot support the transponder to work properly. During the period of "sun outage", the sun, the satellite, and the ground station happen to be in a straight line. Strong electromagnetic radiation from the sun will be looked upon as interference to the downlinks signal in satellite, inevitably causing communication interruption. Moreover, satellites will also face the threat of space junk floating in orbit, which come from disused spacecraft and invalid satellites, thus increasing the possibility of collisions.
    自然事件和环境因素：即使没有人类活动带来的威胁，不可抗拒的环境因素和自然事件也可能破坏SAGSIN的通信链路。地震、洪水、沙尘暴、龙卷风、大雨或大雪、雷电等自然灾害可能会损坏地面站并导致业务中断。卫星链路容易受到恶劣的天基环境的影响，如太阳和宇宙辐射、大气扰动、流星体、小行星等[66]。在“卫星日食”季节，太阳被地球挡住，卫星太阳无法获得阳光，机载电池只能维持卫星自转，但无法支持应答器正常工作。在“停电”期间，太阳、卫星和地面站恰好在一条直线上。来自太阳的强电磁辐射将被视为对卫星下行链路信号的干扰，不可避免地导致通信中断。此外，卫星还将面临漂浮在轨道上的太空垃圾的威胁，这些垃圾来自废弃的航天器和无效的卫星，从而增加了碰撞的可能性。
Besides, due to the influence of external environmental disturbances such as wind, tides, and air currents, the sea surface is usually uneven and fluctuates randomly over time. When the acoustic signal propagates in the sea, scattering and reflection will occur if it encounters the uneven sea boundary. The scattering effect of the sea boundary causes the multipath propagation of the acoustic signal [20]. In longdistance communication, the receiving end may be interfered by the multipath signal caused by the scattering of the sea boundary. In addition, the underwater acoustic channels may be affected by temperature, salinity, and depth, which can lead to dynamic changes in the propagation speed of acoustic waves in seawater.
此外，由于风、潮汐、气流等外部环境干扰的影响，海面通常不平坦，随时间随机波动。当声信号在海中传播时，如果遇到不平坦的海界，就会发生散射和反射。海边界的散射效应导致声信号的多径传播[20]。在远距离通信中，接收端可能会受到海界散射引起的多径信号的干扰。此外，水声通道还可能受到温度、盐度和深度的影响，从而导致声波在海水中传播速度的动态变化。
  1. Artificial Destruction: Network infrastructure is often threatened by human errors or a lack of security awareness, which inadvertently creates opportunities for criminals to steal or sabotage equipment. In particular, satellites are potential targets for interceptors such as space mines and anti-satellite orbiting missiles. The use of directed energy weapons, e.g., high-power laser beams, laser weapons, can destroy satellite systems and services. Moreover, stations, links, and communication networks are all vulnerable to cyberattacks similar to those used in computer networks and the Internet. Potential cyber-attacks include DoS, eavesdropping, and data interception. An attacker can inject false information to gain unauthorized access to the database. Faulty commands in telemetry, which controls spacecraft can cause satellites to deviate from designated orbits and even self-destruct. Viruses can also be injected into the ground computer networks associated with space systems, leading to the loss of spacecraft. Furthermore, collisions between ships and the destruction of coastal base stations may affect the secure communication of the underwater acoustic network. If the attacker's channel is linked to a special node, such as a base station, access point, or gateway, a successful interference attack may destroy the entire network.
    人为破坏：网络基础设施经常受到人为错误或缺乏安全意识的威胁，这无意中为犯罪分子窃取或破坏设备创造了机会。特别是，卫星是太空地雷和反卫星轨道导弹等拦截器的潜在目标。使用定向能武器，例如高功率激光束、激光武器，可以摧毁卫星系统和服务。此外，电台、链路和通信网络都容易受到类似于计算机网络和互联网中使用的网络攻击。潜在的网络攻击包括 DoS、窃听和数据拦截。攻击者可以注入虚假信息，以获得对数据库的未经授权的访问。控制航天器的遥测错误命令可能导致卫星偏离指定轨道甚至自毁。病毒也可以注入与空间系统相关的地面计算机网络，导致航天器丢失。此外，船舶之间的碰撞和沿海基站的破坏可能会影响水声网络的安全通信。如果攻击者的信道链接到一个特殊的节点，如基站、接入点、网关，成功的干扰攻击可能会破坏整个网络。
  2. Interference: Nowadays, an increasing number of satellites put forward higher requirements on certain frequencies and orbits, resulting in orbital or spectral crowding. This congestion may cause unintended disruption to satellite services. Even when operating in the optimal geographical areas where satellite links are successfully achieved, natural phenomena or human interference can disrupt satellite communications systems and seriously disable civilian or military satellite communications systems. The interference of satellite communication systems is mainly posed by uplink and downlink interferences [21], among which uplink interference includes ground-based interference that requires large antennas to achieve power matching with the blocked original link signals. Downlink, which is usually easier to block than uplink, can be disrupted by low-orbit satellites and aircraft, and they can inject signals directly into terminal receivers on the ground with more serious and dangerous consequences directly.
    干扰：如今，越来越多的卫星对某些频率和轨道提出了更高的要求，导致轨道或频谱拥挤。这种拥塞可能会对卫星服务造成意外中断。即使在成功实现卫星链路的最佳地理区域运行，自然现象或人为干扰也会破坏卫星通信系统，并严重瘫痪民用或军用卫星通信系统。卫星通信系统的干扰主要由上行链路和下行链路干扰构成[21]，其中上行链路干扰包括需要大型天线才能与被阻塞的原始链路信号进行功率匹配的地基干扰。下行链路通常比上行链路更容易被阻塞，可以被低轨道卫星和飞机破坏，它们可以将信号直接注入地面终端接收器，直接造成更严重和危险的后果。
The Lack of corresponding security defense measures will lead to serious social consequences if the signal interference cause the interruption of social services. For telemedicine, the interruption of a satellite connection to pre-diagnose and operate may endanger lives. Moreover, tampering with satellite television signals also has undesirable social consequences. Besides, the main interference in the underwater acoustic network is channel interference and link interference [20]. Channel interference refers to that one or several attackers transmit many signals to a channel, and interferes with the reception of legitimate nodes to paralyze normal communication. As reliable physical communication is the basis for the operation of the entire network, this kind of attack can cause great damage to the underwater acoustic network, since such interference will affect the acquisition of marine data and the exploration of submarine natural resources.
如果信号干扰导致社会服务中断，缺乏相应的安全防御措施将导致严重的社会后果。对于远程医疗，卫星连接中断以进行预诊断和操作可能会危及生命。此外，篡改卫星电视信号也会产生不良的社会后果。此外，水声网络中的主要干扰是信道干扰和链路干扰[20]。信道干扰是指一个或多个攻击者向一个信道发送多个信号，并干扰合法节点的接收，使正常通信瘫痪。由于可靠的物理通信是整个网络运行的基础，这种攻击会对水声网络造成很大的破坏，因为这种干扰会影响海洋数据的获取和海底自然资源的勘探。

B. Operation Threats B. 行动威胁

During the network operation, malicious softwares can implement control by destroying the software process, which reduces or even loses the availability of the network, leading to the paralysis of the entire network. Therefore, SAGSIN has extremely high requirements for large-scale safe operations. While ensuring the safe operation and reliability of the system, it should also ensure the successful access of remote users. However, only in the authorization and authentication process of the access terminals, the network systems are threatened by unauthorized access, which will undoubtedly affect the security of wireless access.
在网络运行过程中，恶意软件可以通过破坏软件进程来实现控制，从而降低甚至失去网络的可用性，导致整个网络瘫痪。因此，SAGSIN对大规模安全作业的要求极高。在保证系统安全运行和可靠性的同时，也要保证远程用户的成功接入。然而，只有在接入终端的授权和认证过程中，网络系统才会受到未经授权的访问的威胁，这无疑会影响无线接入的安全性。
  1. Malware: Malware is malicious, intrusive program code, which is designed to take control of the devices by destroying processes without the knowledge of user. It is often distributed as spam in malicious attachments or links to an infected network, which can evade feature detection by dynamically changing the attack codes. Specifically, malware can be classified into the following major categories.
    恶意软件：恶意软件是恶意的侵入性程序代码，旨在通过在用户不知情的情况下破坏进程来控制设备。它通常以垃圾邮件的形式分发到指向受感染网络的恶意附件或链接中，这些网络可以通过动态更改攻击代码来逃避功能检测。具体来说，恶意软件可分为以下几大类。
  • Trojan horse: A Trojan horse is a remote unauthorized control program hidden in the system using client/server structure. It provides a backdoor for unauthorized access to privileged functions of the system [67]. Since Trojan horse program can open system permissions, leak user information, or even filch entire management access without the noticing of the administrator, it has become one of the most commonly used tools by hackers. In SAGSIN, once a terminal node is perceived to be compromised, an attacker can gain unauthorized access by simply analyzing the confidential information stored in the terminal or network node via the connection of a communication link. This leads to non-terrestrial network nodes to receive false information, resulting in serious security threats. In consideration of the complexity of the signals, Trojans are very difficult to detect through visual or other forms of manual inspection [68].
    特洛伊木马：特洛伊木马是使用客户端/服务器结构隐藏在系统中的远程未经授权的控制程序。它为未经授权访问系统的特权功能提供了后门[67]。由于特洛伊木马程序可以在管理员不注意的情况下打开系统权限、泄露用户信息，甚至破坏整个管理访问权限，因此它已成为黑客最常用的工具之一。在SAGSIN中，一旦感知到终端节点遭到入侵，攻击者只需通过连接通信链路分析终端或网络节点中存储的机密信息，即可获得未经授权的访问。这会导致非地面网络节点接收虚假信息，从而造成严重的安全威胁。考虑到信号的复杂性，特洛伊木马很难通过目视或其他形式的人工检查来检测[68]。
  • Worms: Worms are malicious programs that can spread without human intervention. They replicate themselves and run independently out of user control. The worm mainly includes three modules, namely the transmission module, the hidden module, and the function module. In particular, the transmission module is only used for rapid spread, and the hidden module is to hide the worm programs just copied to the network in case they are spotted. The function module mainly realizes the control, monitoring, and destruction of the network. By making use of some vulnerabilities existing in the computer network, worms continuously obtain access rights from the network to spread widely. When worms invade and take full control of one system, they continue to infect other systems exponentially using the recursive method.
    蠕虫：蠕虫是恶意程序，无需人工干预即可传播。它们自我复制并在用户控制之外独立运行。蠕虫主要包括三个模块，即传输模块、隐藏模块和功能模块。特别是传输模块仅用于快速传播，隐藏模块是隐藏刚刚复制到网络上的蠕虫程序，以防被发现。功能模块主要实现网络的控制、监控、破坏。蠕虫利用计算机网络中存在的一些漏洞，不断从网络获取访问权限，广泛传播。当蠕虫入侵并完全控制一个系统时，它们会继续使用递归方法以指数方式感染其他系统。
  • Virus: Computer viruses are a set of instructions or code inserted into programs or files, which can replicate itself, and has the characteristics of concealment, parasitism, transmissibility, and destructiveness. They can destroy the system data and affect the normal operation of software and hardware, leading to data loss and system crashes in severe cases, and system efficiency reduction. Generally, viruses are parasitic in other executable programs and will be activated once conditions are met. After a virus determines the target, it replicates itself to infect other programs. For instance, when files attached with viruses are copied or transferred from one user to another, viruses rapidly diffuse along with the files.
    病毒：计算机病毒是插入程序或文件中的一组指令或代码，可以自我复制，具有隐蔽性、寄生性、传播性和破坏性等特点。它们会破坏系统数据，影响软硬件的正常运行，严重时会导致数据丢失和系统崩溃，系统效率降低。通常，病毒寄生在其他可执行程序中，一旦满足条件就会被激活。病毒确定目标后，它会自我复制以感染其他程序。例如，当带有病毒的文件从一个用户复制或传输到另一个用户时，病毒会随着文件迅速扩散。
  • Backdoor programs: Backdoor programs generally means that bypass security mechanisms to gain access to different systems. In the phase of software development, programmers often create backdoors in case of defects in the design, which are convenient to test and enhance module functions. However, if the backdoor programs are acquired by someone with ulterior motives, or the backdoor is not removed before the software is released, there may be potential risks, and it is easy for an attacker to use it.
    后门程序：后门程序通常意味着绕过安全机制来访问不同的系统。在软件开发阶段，程序员经常在设计出现缺陷的情况下创建后门，方便测试和增强模块功能。但是，如果后门程序被别有用心的人获取，或者后门在软件发布前没有被移除，则可能存在潜在风险，并且很容易被攻击者使用。
  1. Unauthorized Access: Unauthorized access refers to the unauthorized use of network resources or using network resources in an unauthorized manner. It mainly includes unauthorized users accessing the network to perform illegal operations, and legitimate users conducting unauthorized operations. The openness of SAGSIN provides illegal intruders with more opportunities for unauthorized access. Unauthorized intruders can bypass the access control mechanism, abnormally make use of network equipment and resources, and arbitrarily extend their access rights. One concrete example is hijacking channel information of commercial drones by illegally accessing the network and launching anonymous attacks. This kind of threat will lead to the users' important information being stolen and the system security being destroyed, resulting in seriously breaking the security order of the network. To avoid it, we should strengthen the access control mechanism, flexibly set the types and quantities of access control according to the different security levels and network environment.
    未经授权的访问：未经授权的访问是指未经授权使用网络资源或以未经授权的方式使用网络资源。主要包括未经授权用户访问网络进行非法操作，以及合法用户进行未经授权操作。SAGSIN的开放性为非法入侵者提供了更多未经授权的访问机会。未经授权的入侵者可以绕过访问控制机制，异常使用网络设备和资源，任意扩展其访问权限。一个具体的例子就是通过非法访问网络和发起匿名攻击来劫持商用无人机的频道信息。这种威胁会导致用户的重要信息被盗用，系统安全被破坏，导致网络安全秩序严重中断。为了避免这种情况，我们应该加强访问控制机制，根据不同的安全级别和网络环境灵活设置访问控制的类型和数量。

C. Network Threats C. 网络威胁

The dynamic change of nodes has very adverse effects on the performance of the network, and it is susceptible to accidental or malicious damage. Therefore, higher requirements
节点的动态变化对网络性能有非常不利的影响，容易受到意外或恶意破坏。因此，更高的要求
are put forward on the routing and protocol mechanisms of the connection-oriented communication links in SAGSIN.
对SAGSIN中面向连接的通信链路的路由和协议机制提出了建议。
  1. Insecure Routing: The router automatically selects and forwards packets according to the actual operation situations of adjacent nodes, and tries to deliver packets in the optimal path and at the minimum cost. However, the routing node lacks physical protection in SAGSIN, and the node with flexibility and mobility inevitably limits its resources and computing power to apply complex cryptographic algorithms. The vulnerability of each node makes it easy to be controlled or captured by the enemy.
    不安全路由：路由器根据相邻节点的实际运行情况自动选择转发报文，尝试以最优路径、最低成本投递报文。然而，路由节点在SAGSIN中缺乏物理保护，具有灵活性和移动性的节点不可避免地限制了其应用复杂密码算法的资源和计算能力。每个节点的脆弱性使其很容易被敌人控制或俘虏。
Theoretically, attackers can launch an attack from any location on the network. Once the routing is broken by hackers, they can do anything in the whole network. For example, an attacker can forge the shortest path information of a target node to attract messages to its node and selectively discard partial messages according to a certain strategy. Reliable routing can also be hidden in special ways so that network communication flows to the nodes controlled by the attackers.
从理论上讲，攻击者可以从网络上的任何位置发起攻击。一旦路由被黑客破坏，他们就可以在整个网络中做任何事情。例如，攻击者可以伪造目标节点的最短路径信息，将消息吸引到其节点，并根据一定的策略有选择地丢弃部分消息。可靠的路由也可以以特殊方式隐藏，以便网络通信流向攻击者控制的节点。
More seriously, malicious nodes can frequently send unnecessary routing request information, resulting in network congestion and making other nodes unable to access network resources normally. Take a black hole attack an instance, the attacker broadcasts the cheapest or shortest forged path to the destination. The receiving node chooses the path that passes through the attacker, and then the attacker can analyze or even discard these data packets at will. Due to the broadcast nature of the communication media and loose topology control, this kind of attack becomes easier in wireless ad hoc networks. More cleverly, an attacker may drop packets for a certain period or within a certain percentage to make it difficult to detect. Considering these cases, it is necessary to construct secure and effective routing algorithms and protocols to ensure the normal network operation.
更严重的是，恶意节点会频繁发送不必要的路由请求信息，导致网络拥塞，使其他节点无法正常访问网络资源。以黑洞攻击为例，攻击者广播最便宜或最短的伪造路径到达目的地。接收节点选择通过攻击者的路径，然后攻击者可以随意分析甚至丢弃这些数据包。由于通信媒体的广播性质和松散的拓扑控制，这种攻击在无线自组网中变得更容易。更巧妙的是，攻击者可能会在一定时间段内或一定百分比内丢弃数据包，使其难以检测。考虑到这些情况，有必要构建安全有效的路由算法和协议，以保证网络的正常运行。
  1. Insecure Protocol: TCP is originally designed for terrestrial wired network. However, different from terrestrial wired network, satellite network has long link propagation and bandwidth delay, high bit error rate, and asymmetric link, resulting that the performance of standard TCP may reduce in satellite network. Moreover, frequent handover in the network organization process will significantly affect the transmission performance of the network. Furthermore, traditional IP networking technology based on fixed topology structure cannot be directly applied to satellite networks, so that it is necessary and important to design new network protocol suitable for SAGSIN's topological dynamic environment.
    不安全协议：TCP 最初是为地面有线网络设计的。然而，与地面有线网络不同，卫星网络具有链路传播长、带宽延延长、误码率高、链路不对称等特点，导致标准TCP在卫星网络中的性能可能会降低。此外，网络组织过程中频繁的交接会显著影响网络的传输性能。此外，传统的基于固定拓扑结构的IP组网技术无法直接应用于卫星网络，因此设计适合SAGSIN拓扑动态环境的新型网络协议是必要且重要的。
Besides, geographic routing protocols are more popular in underwater acoustic networks, since the data packets are forwarded based on the location information (e.g., depth) of the nodes without a dedicated route discovery process. However, this kind of protocol is particularly vulnerable to location/neighbor spoofing, considering that the broadcast-based information exchange process is vulnerable to attacks.
此外，地理路由协议在水声网络中更受欢迎，因为数据包是根据节点的位置信息（例如深度）转发的，没有专门的路线发现过程。然而，考虑到基于广播的信息交换过程容易受到攻击，这种协议特别容易受到位置/邻居欺骗。

D. Data/Information Threats
D. 数据/信息威胁

Due to the openness of data links, the information resources are subject to various threats in SAGSIN. The abuse of data, the leakage of private data, and the interception of data packets are aggravating the severity of security issues. Thus, it is urgent to establish protective mechanisms to ensure that the data transmitted and exchanged over the network are not modified, lost, and leaked.
由于数据链路的开放性，信息资源在SAGSIN中受到各种威胁。数据滥用、私人数据泄露、数据包被拦截等行为加剧了安全问题的严重性。因此，迫切需要建立保护机制，以确保通过网络传输和交换的数据不被修改、丢失和泄露。
  1. Data Misuse: Data misuse occurs when privileged insiders use legitimate ability to access sensitive data for inappropriate purposes. Abuse by insiders is not necessarily malicious, and it is possible that some insiders accidentally misuse the system by mistake. In the open environment of SAGSIN, sharing confidential and sensitive information is inevitable. Therefore, organizations must make significant efforts to protect confidential information and detect incidents of data misuse, and effective security mechanisms are needed to monitor the usage of sensitive information. To avoid internal data misuse, it is necessary to properly organize and systematically analyze the identity and authorization data of legitimate users involved in the system. These approaches include collecting identity data that propagates between different applications, systematically cleaning up user account data, grouping privileges and access rights of users, etc [15].
    数据滥用：当特权内部人员使用合法能力出于不当目的访问敏感数据时，就会发生数据滥用。内部人员的滥用不一定是恶意的，也有可能是一些内部人员不小心误用了系统。在SAGSIN的开放环境中，共享机密和敏感信息是不可避免的。因此，组织必须做出重大努力来保护机密信息并检测数据滥用事件，并且需要有效的安全机制来监控敏感信息的使用情况。为避免内部数据滥用，有必要对系统中涉及的合法用户的身份和授权数据进行适当的组织和系统分析。这些方法包括收集在不同应用程序之间传播的身份数据，系统地清理用户帐户数据，对用户的权限和访问权限进行分组等[15]。
  2. Data Leakage: Data leakage mainly include the damage caused by operation faults and the interception via network monitoring during transmission, and the attackers can obtain private data through application-level vulnerabilities or malware. Once they have access to the network nodes, they can bypass the monitor and steal data directly at the operating system layer. Facing with these severe situations in SAGSIN, traditional security measures are difficult to prevent data leakage independently, and strengthening the data security is imminent.
    数据泄露：数据泄露主要包括传输过程中操作故障和网络监控拦截造成的破坏，攻击者可以通过应用级漏洞或恶意软件获取私有数据。一旦他们能够访问网络节点，他们就可以绕过监视器并直接在操作系统层窃取数据。面对SAGSIN的这些严峻情况，传统的安全措施难以独立防止数据泄露，加强数据安全迫在眉睫。
  3. Packet Interception: Packet interception means that a damaged node, which may be a router, intercepts the packets passing through it, and selectively discards them or takes other actions. For example, the attacker returns an acknowledgment to the source node if it is the target node, but discards the corresponding data packet. In this case, the target node cannot receive the content which was sent to it. The attacker or damaged node can even inject other data packets into the target node based on the behavior of the source node. In a man-in-the-middle attack, the attacker may secretly relay the communication between a pair of nodes and make them believe that they are directly communicating with each other. By monitoring the network and locating nodes, attackers can improve attack power by attacking key nodes (e.g., the root node of a tree topology network). Although multipath forwarding can be used to defend against packet interception, it requires more bandwidth and energy consumption, and new defense technologies against packet interception, especially combined with SAGSIN's characteristics, are needed.
    数据包拦截：数据包拦截是指损坏的节点（可能是路由器）拦截通过它的数据包，并有选择地丢弃它们或采取其他操作。例如，如果源节点是目标节点，攻击者会向源节点返回确认，但会丢弃相应的数据包。在这种情况下，目标节点无法接收发送给它的内容。攻击者或受损节点甚至可以根据源节点的行为将其他数据包注入目标节点。在中间人攻击中，攻击者可能会秘密中继一对节点之间的通信，并使它们相信它们正在直接相互通信。通过监控网络和定位节点，攻击者可以通过攻击关键节点（例如树形拓扑网络的根节点）来提高攻击能力。虽然多径转发可用于防御数据包拦截，但它需要更多的带宽和能耗，并且需要新的数据包拦截防御技术，特别是结合SAGSIN的特性。
Remarks: In addition to the security threats described above, SAGSIN may risk many other threats types. In particular, the attackers try to alter the exchanged data, gain authentication, corrupt some privacy requirement, or acquire authorization by modifying packet transmission through the network or inserting false packets into the data stream. Moreover, in a passive attack, the attacker does not alter the operation of a routing protocol, but he tries to gain valuable information by listening
备注：除上述安全威胁外，SAGSIN还可能面临许多其他威胁类型。特别是，攻击者试图通过修改通过网络的数据包传输或将虚假数据包插入数据流来更改交换的数据、获得身份验证、破坏某些隐私要求或获得授权。此外，在被动攻击中，攻击者不会改变路由协议的操作，而是试图通过监听来获取有价值的信息
TABLE III 表三
SUMmary of RELATED WORKS ON JAMMING ATTACKS IN SAGSIN
关于SAGSIN干扰攻击的相关著作综多
Network structure 网络结构 Reference Protocol Layer 协议层 Features/Advantages 特点/优势 Proposed algorithm/scheme
建议的算法/方案
Wireless
sensor network 传感器网络
[69] MAC layer
Investigated various possible jamming
研究了各种可能的干扰
attacks against wireless sensor networks
针对无线传感器网络的攻击
Spectral evasion (channel surfing) or
光谱规避（信道冲浪）或
spatial evasion (spatial retreats)
空间回避（空间撤退）
Transport layer 传输层
Considered the case of a complex
考虑了复合体的情况
jammer blocking a region in a
干扰器阻止
single-channel wireless sensor network
单通道无线传感器网络
An intuitive heuristic jamming strategy
直观的启发式干扰策略
Wireless ad hoc 无线自组织
network
Generated adversarial network
生成的对抗网络
An adversarial 对抗性
machine learning approach
机器学习方法
Wireless network 无线网络
Used Reduction of Quality (RoQ) attacks
使用的质量降低 （RoQ） 攻击
to reduce wireless TCP throughput
降低无线 TCP 吞吐量
A RoQ attack model,
一种 RoQ 攻击模型，
a CTS interference method
CTS干扰法
Underwater
acoustic networks 声学网络
Physical layer 物理层
Evaluated a malicious node
评估恶意节点
attempting to perform a jamming attack
尝试执行干扰攻击
A zero-sum game approach
零和博弈方法
Considered the half-duplex
被认为是半双工
characteristics of the underwater transceiver
水下收发器的特性
and the block transmission structure
和块传输结构
A half duplex jamming protocol
半双工干扰协议
Underwater
sensor network 传感器网络
Formulated the interaction between
制定了两者之间的相互作用
underwater sensors and interference as an
水下传感器和干扰作为
underwater interference game
水下干扰游戏
An anti-jamming method for UWSN
UWSN的抗干扰方法
based on reinforcement learning
基于强化学习
MAC layer,
Network layer 网络层
Used distributed location algorithm nodes
使用的分布式位置算法节点
A Time Synchronization 时间同步
High Latency (TSHL) protocol
高延迟 （TSHL） 协议
MAC layer,
Physical layer 物理层
Adopted cross-layer design to
采用跨层设计，以
alleviate reactive power interference
减轻无功干扰
A cross-layer UWSN 跨层UWSN
channel allocation model
通道分配模型
Fig. 6. Organization of attack methodologies in SAGSIN.
图 6.SAGSIN中攻击方法的组织。
to the routing traffic, which makes this attack very difficult to be detected. Furthermore, an adversary may attempt to lower performance and cause economic damage by creating unwarranted safety concerns. The adversary is capable of external cyber physical exploits, such as active manipulation of data, node impersonation, and internal exploits like compromising cyber elements. Note that, with recent development of B5G and technologies, some security threats that SAGSIN may encounter have become a hot topic for many researchers, e.g., system infection caused by malware, intentional DoS against critical safety systems, etc., and these may become our future research directions.
到路由流量，这使得这种攻击很难被检测到。此外，攻击者可能会试图通过制造不必要的安全问题来降低性能并造成经济损失。攻击者能够进行外部网络物理攻击，例如主动操作数据、节点模拟，以及破坏网络元素等内部攻击。需要注意的是，随着B5G和技术 的不断发展，SAGSIN可能遇到的一些安全威胁已成为许多研究人员的热点话题，例如恶意软件导致的系统感染、针对关键安全系统的故意DoS等，这些都可能成为我们未来的研究方向。

V. Attack Methodologies in SAGSiN
五、SAGSiN的攻击方法

In SAGSIN, exposed nodes and open communication links make it vulnerable to both active and passive attacks. Active attacks mainly include DoS attack, distributed DoS (DDoS)
在 SAGSIN 中，暴露的节点和开放的通信链路使其容易受到主动和被动攻击。主动攻击主要包括DoS攻击、分布式DoS（DDoS）
Fig. 7. An example of the jammer and mutual interference in UAV-Satellite communication,
图 7.无人机-卫星通信中的干扰器和相互干扰的一个例子，
attack, replay attack, information tampering, camouflage, and so on. In active attacks, malicious entities may actively modify data streams or generate erroneous data streams to fool users. By contrast, passive attacks aim to acquire or collect information transmitted without notification from legitimate visitors, which are done by observing ongoing communications without taking any steps to interfere with the privacy of legitimate entities. Passive attacks include sniffing, eavesdropping, etc. Moreover, active attacks are easy to detect for the purpose of destroying data and services. However, passive attacks are difficult to detect without the goal of destroying data, and thus prevention is important. In this section, we focus on several common and representative attacks in SAGSIN, as shown in Fig. 6, and present a review of existing literature.
攻击、回放攻击、信息篡改、伪装等。在主动攻击中，恶意实体可能会主动修改数据流或生成错误的数据流来欺骗用户。相比之下，被动攻击旨在获取或收集未经合法访问者通知而传输的信息，这些信息是通过观察正在进行的通信而不采取任何步骤干扰合法实体的隐私来实现的。被动攻击包括嗅探、窃听等。此外，主动攻击很容易被检测到，目的是破坏数据和服务。然而，如果没有破坏数据的目标，被动攻击就很难被发现，因此预防很重要。在本节中，我们将重点介绍SAGSIN中几种常见且具有代表性的攻击，如图6所示，并对现有文献进行了综述。

A. Jamming Attack A. 干扰攻击

Jamming attack is a kind of active attacks. It occupies the communication channel of network nodes by sending highpower signals or packets, resulting in signal-to-noise ratio (SINR) be reduced, the failure of normal data communication with external nodes, and the loss of availability. This paper summarizes the existing work in Table III. Fig. 7 gives an example of jamming attack in UAV-Satellite communication. Jamming attack can be present at local receivers (downlink
干扰攻击是一种主动攻击。它通过发送高功率信号或数据包来占用网络节点的通信信道，导致信噪比（SINR）降低，与外部节点的正常数据通信失败，可用性损失。本文总结了表三中的现有工作。图7给出了无人机-卫星通信中的干扰攻击示例。干扰攻击可能存在于本地接收器（下行链路
jamming) or directed at the satellite mixed with an effective carrier (uplink jamming). Uplink jamming is mainly transmitting strong signals from the ground to the satellite, which may interfere with some channels of the satellite transponder, so that the channel signals will be interrupted. Uplink jamming is easy to implement, but it is difficult to detect and deal with. This is because that the jammer is mobile and it can be located anywhere, and it is not feasible to locate the threat jammer during broadcast. Downlink jamming is mainly information flowing from satellites to the ground or mobile stations. Various jammers can be used to interfere with downlink communications, but their impact is only on specific ground or mobile receiving stations [21].
干扰）或针对与有效载波混合的卫星（上行链路干扰）。上行干扰主要是从地面向卫星发射强信号，可能会干扰卫星转发器的某些信道，使信道信号中断。上行链路干扰容易实现，但检测和处理难度大。这是因为干扰器是移动的，可以位于任何地方，并且在广播过程中定位威胁干扰器是不可行的。下行链路干扰主要是从卫星流向地面或移动站的信息。各种干扰器可用于干扰下行链路通信，但它们的影响仅限于特定的地面或移动接收站[21]。
The broadcast characteristics of satellite communication make data transmission between nodes vulnerable to jamming attacks in SAGSIN. In particular, the satellite receiver may be disabled under jamming messages sent by an adversary, potentially cutting off vital military communications at critical moments, forcing them to communicate with another satellite, another transponder, or different parts of the same transponder. In jamming attacks, most of the message carriers are radio frequency attacks, which have been used to disturb radar systems of aircrafts and missiles. In view of the limited transmission power and strong anti-attack capability of wireless sensor network [69], it is extremely vulnerable to RF attacks.
卫星通信的广播特性使得节点间的数据传输容易受到SAGSIN的干扰攻击。特别是，卫星接收机可能会在对手发送的干扰信息下被禁用，从而可能在关键时刻切断重要的军事通信，迫使它们与另一颗卫星、另一个转发器或同一转发器的不同部分进行通信。在干扰攻击中，大多数消息载体是射频攻击，用于干扰飞机和导弹的雷达系统。鉴于无线传感器网络的传输功率有限，抗攻击能力强[69]，极易受到射频攻击。
Moreover, in the underwater acoustic network, due to the energy limitation of the nodes, the jammer has two goals, including interrupting the communication, and shortening life cycle of the victim by making the victim send more redundancy [73]. Underwater communication may be hindered by the attenuation of electromagnetic waves, and the propagation delay may be longer than the signal duration [76], especially in remote production scenarios. In this case, long-distance nodes are vulnerable to the harm or attack of interference technology, which delays or hinders the communication of underwater wireless sensor networks, and reduces network performance [77].
此外，在水声网络中，由于节点的能量限制，干扰器有两个目标，包括中断通信，以及通过使受害者发送更多的冗余来缩短受害者的生命周期[73]。水下通信可能受到电磁波衰减的阻碍，传播延迟可能长于信号持续时间[76]，特别是在远程生产场景中。在这种情况下，远距离节点容易受到干扰技术的伤害或攻击，从而延迟或阻碍水下无线传感器网络的通信，并降低网络性能[77]。
During the Iraq war, Iraqi armies used six simple jammers to prompt some American cruise missiles hitting into Turkey [78]. Since then, the problem of jamming attacks on global positioning system (GPS) satellite receivers has attracted great attention from the military and industry. Jammer refers to the entity that intentionally interferes with the physical transmission and reception of wireless communications [72]. In the physical layer, jammer usually destroys the communication link or transmission signal with high power in the areas to interfere with the network. At the transport/network layer, the jammer can inject false messages or dispose of legitimate routing control packets so that other nodes cannot access the channel, resulting in low throughput and providing delays of satellite services.
在伊拉克战争期间，伊拉克军队使用六种简单的干扰器促使一些美国巡航导弹击中土耳其[78]。从那时起，对全球定位系统（GPS）卫星接收机的干扰攻击问题引起了军方和工业界的极大关注。干扰是指故意干扰无线通信的物理传输和接收的实体[72]。在物理层中，干扰器通常会破坏区域内高功率的通信链路或传输信号，从而干扰网络。在传输/网络层，干扰器可以注入虚假消息或处理合法的路由控制数据包，使其他节点无法访问信道，从而导致吞吐量低并提供卫星业务延迟。
Brown et al. [79] focused on the problem of an attacker disrupting an encrypted wireless ad hoc network via jamming at the transport/network layer. They developed a packet classifier, which adapts to variations across networks and across different encryption models, to make victim networks less vulnerable to these kinds of attacks. Li et al. [70] analyzed a scenario where a complicated jammer blocks the running area of a single-channel random-access-based wireless sensor network and proposed a meaningful heuristic algorithm for an efficient jamming strategy. Moreover, Erpek et al. [71] introduced an adversarial machine learning (ML) approach to launch jamming attacks on wireless communications, where both the transmitter and the jammer used ML to learn the spectrum. A jammer collects channel status and ACKs to build a deep learning classifier that reliably predicts the next successful transmissions and effectively jams them.
Brown等[79]研究了攻击者通过传输/网络层的干扰来破坏加密的无线自组织网络的问题。他们开发了一种数据包分类器，可以适应网络和不同加密模型的变化，使受害者网络不易受到此类攻击。Li等[70]分析了复杂干扰器阻塞基于单通道随机接入的无线传感器网络运行区域的场景，并提出了一种有意义的启发式算法，用于高效的干扰策略。此外，Erpek等[71]引入了一种对抗性机器学习（ML）方法来对无线通信发起干扰攻击，其中发射机和干扰机都使用ML来学习频谱。干扰器收集信道状态和 ACK 以构建深度学习分类器，该分类器可以可靠地预测下一次成功的传输并有效地干扰它们。
With the development of 6G, many researchers begin to study the interference that might be encountered in underwater acoustic network, in order to better improve the security of SAGSIN. In underwater acoustic network, Huang et al. [74] introduced packet transmission methods to make it secure, and adopted the half-duplex characteristics of underwater transceiver and large propagation delay to generate interference in the eavesdropper. Specifically, the receiver sends jamming packets to the malicious node during the data block transmission, which keeps the jamming transducer in the receiving state and prevents it from sending malicious signals. Xiao et al. [75] conducted a game theory study on jamming attacks in underwater sensor networks, where the interaction between sensors and jammers in networks was formulated as a kind of jamming game. This allows each sensor to select its transmitted power in the presence of an interfering signal, maximizing its utility with SINR and transmission cost based on the legitimate signal of the ground receiver.
随着6G的发展，许多研究人员开始研究水声网络中可能遇到的干扰，以更好地提高SAGSIN的安全性。在水声网络中，Huang等[74]引入了分组传输方法使其安全，并采用水下收发器的半双工特性和较大的传播延迟对窃听者产生干扰。具体来说，接收方在数据块传输过程中向恶意节点发送干扰数据包，使干扰换能器保持在接收状态，防止其发送恶意信号。Xiao等[75]对水下传感器网络中的干扰攻击进行了博弈论研究，将网络中传感器与干扰器之间的相互作用表述为一种干扰博弈。这允许每个传感器在存在干扰信号的情况下选择其发射功率，从而根据地面接收机的合法信号最大限度地提高其 SINR 和传输成本的效用。

B. Eavesdropping Attack B. 窃听攻击

Eavesdropping is to access the self-built information exchange of nodes through silent eavesdropping without interrupting the network. The eavesdroppers can use the collected data to infer confidential information. This practice is widely considered illegal in network security, so it is necessary to improve the security of SAGSIN to prevent eavesdropping attacks. Here, we will give some specific attacks scenarios in the following, and Table IV summarizes existing works on eavesdropping attacks.
窃听是通过无声窃听的方式，在不中断网络的情况下，访问节点的自建信息交换。窃听者可以使用收集到的数据来推断机密信息。这种做法在网络安全方面被广泛认为是非法的，因此有必要提高SAGSIN的安全性以防止窃听攻击。下面我们将给出一些具体的攻击场景，表四总结了现有的窃听攻击工作。
An et al. [80] considered a cognitive satellite-terrestrial network, where an eavesdropper illegally wants to overhear the transmitted signal in the process of the satellite and the base station transmitting signals intended for the primary user and secondary user. The authors derived the analytical expressions for the secrecy outage probability, and the average secrecy rate demonstrated the impacts of various parameters on the secrecy performance of the satellite network. Bankey and Upadhyay [18] investigated the physical layer security of a downlink multi-user multi-relay hybrid satellite-terrestrial relay network in the presence of multiple eavesdroppers. Lei et al. [82] studied a security scenario, where an eavesdropper is located outside/inside the satellite coverage in the multi-beam satellite communication system, and obtained suboptimal beamforming weights by eliminating the channel interference and nulled the eavesdroppers signal simultaneously.
A等[80]认为这是一种认知卫星-地面网络，其中窃听者非法希望在卫星和基站向主要用户和次要用户发送信号的过程中偷听传输的信号。作者推导了保密中断概率的解析表达式，平均保密率证明了各种参数对卫星网络保密性能的影响。Bankey和Upadhyay[18]研究了存在多个窃听者的情况下下行链路多用户多中继混合星地中继网络的物理层安全性。Lei等[82]研究了在多波束卫星通信系统中，窃听者位于卫星覆盖范围之外/内部的安全场景，通过消除信道干扰获得次优波束成形权重，同时消除窃听者信号。
In SAGSIN, eavesdropping activities are accompanied by many malicious attacks, and detecting eavesdroppers is
在SAGSIN中，窃听活动伴随着许多恶意攻击，而检测窃听者是
TABLE IV 表四
SUMMARY OF RELATED WORKS ON EAVESDROPPING ATTACKS IN SAGSIN
SAGSIN窃听攻击相关著作综述
Protocol Layer 协议层 Reference Network structure 网络结构 Features/Advantages 特点/优势 Proposed algorithm/scheme
建议的算法/方案
Physical layer 物理层 [80]
Satellite-
terrestrial network 地面网络
Adopted multi-antenna BS
采用多天线BS		 Two beamforming algorithms
两种波束成形算法
Considered user relay selection criteria
考虑的用户继电器选择标准
Amplify and forward protocol,
放大和转发协议，
decode and forward protocol
解码和转发协议
Considered a multi-antenna array
被认为是多天线阵列
A beamforming scheme for
波束成形方案
cooperative and secure transmission
协作安全传输
Satellite network 卫星网络
Studied multi-beam 研究多波束
satellite secure communication
卫星安全通信
An iterative algorithm to obtain an
一种迭代算法，用于获取
optimal power allocation strategy
最佳功率分配策略
[83] Considered multiple beams
考虑多光束
A partial zero forcing method for obtaining
一种获得的部分零强迫方法
low-complexity sub-optimal solutions
低复杂度次优解决方案
UAV-terrestrial 无人机-地面
network
Considered UAVs with friendly jammers
被认为是带有友好干扰器的无人机		 An intercept probability security region
拦截概率安全区域
Network layer 网络层
Underwater
acoustic networks 声学网络
Eliminated the need to deploy
无需部署
additional key distribution centers
其他主要配送中心
Multiple RSS-based secret
多个基于 RSS 的密钥
key generation methods 密钥生成方法
Underwater
sensor network 传感器网络
Utilized the serial transmission protocol
使用串行传输协议
and the broadcast characteristics of the
以及
underwater acoustic medium
水声介质
A localization scheme 本地化方案
Transport layer 传输层
Considered the signal in the
考虑了
beam with the highest energy
具有最高能量的光束
An active sonar localization method
一种主动声呐定位方法
based on passive sonar array
基于被动声呐阵列
Underwater
acoustic network 声学网络
Considered orthogonal frequency
考虑正交频率
division multiplexing 分频复用
as the modulation technique
作为调制技术
A CoMP transmission strategy
CoMP 传输策略
Considered the randomness of information
考虑信息的随机性
A method for dynamically generating keys
一种动态生成密钥的方法
using related channel measurement
使用相关通道测量
[87]
Underwater
acoustic sensor 声学传感器
networks
Used linear array hydrophones
二手线性阵列水听器
and isotropic hydrophones
和各向同性水听器
An analytical framework, two different
一个分析框架，两个不同的
scenarios: IUSN and AUSN
方案：IUSN 和 AUSN
Fig. 8. Eavesdropping attack.
图 8.窃听攻击。
difficult since the eavesdroppers silently tap the underwater channel without revealing themselves. As shown in Fig. 8, Wang et al. [87] derived an analysis framework for calculating the probability of eavesdropping attacks in UAV systems. Carroll et al. [14] developed an underwater positioning technology that can be used for system users positioning. If an eavesdropper is a passive adversary, passive target location techniques, such as distance and orientation estimation [85] and target tracking [88] in passive sonar applications, can be used to estimate the location of eavesdroppers.
很难，因为窃听者在不暴露自己的情况下默默地窃听水下通道。如图8所示，Wang等[87]推导了用于计算无人机系统窃听攻击概率的分析框架。Carroll等[14]开发了一种水下定位技术，可用于系统用户定位。如果窃听者是被动对手，则可以使用被动目标定位技术，例如距离和方向估计[85]以及被动声纳应用中的目标跟踪[88]，来估计窃听者的位置。
Due to the powerful computational ability of potential eavesdroppers, traditional cryptographic protocols cannot guarantee secure satellite communication. The idea of designing artificial noise-assisted scheme is another appropriate solution that legitimates transmitters inject artificial noise into their transmission signals to confuse malicious eavesdroppers. By introducing artificial noise, the quality of eavesdropping channels is not as good as that of legal channels, so that eavesdroppers cannot extract useful information from the signals even if they can obtain them. Zheng et al. [83] introduced physical layer security techniques for fixed legitimate receivers dispersed throughout multiple beams, which are possibly surrounded by multiple eavesdroppers, and explored the usage of artificial noise as an additional degree-of-freedom for protection against eavesdroppers.
由于潜在窃听者的强大计算能力，传统的加密协议无法保证卫星通信的安全。设计人工噪声辅助方案的想法是另一种合适的解决方案，它使发射机合法化，将人工噪声注入其传输信号中以迷惑恶意窃听者。通过引入人工噪声，窃听信道的质量不如合法信道，使窃听者即使能够获得信号，也无法从信号中提取有用的信息。Zheng等[83]介绍了分散在多个波束中的固定合法接收器的物理层安全技术，这些接收器可能被多个窃听者包围，并探索了使用人工噪声作为防止窃听者的额外自由度。
Zhou et al. [19] described an attack scenario that a source communicates with a legitimate receiver, while an eavesdropper attempts to overhear the legitimate transmission within a given target area. By employing a UAV equipped with an air-to-ground friendly jammer, they can realize jamming at unknown eavesdropper location to enhance the secrecy performance. Moreover, Du et al. [81] focused on a security scenario, where the satellite communicates with fixed-satellite service (FSS) terminals distributed within its coverage and equipped with antennas. An eavesdropper located inside the satellite coverage intends to wiretap the confidential message transmitted to one FSS terminal, named eavesdropped FSS terminal. In the established security scenario, a cooperative secure transmission beamforming scheme was designed to ensure the security of satellite communications, which was realized through the satellite's adaptive beamforming, artificial noise, and cooperative beamforming of BSs implemented by ground BSs.
周等[19]描述了一种攻击场景，即源与合法接收者通信，而窃听者试图偷听给定目标区域内的合法传输。通过使用配备空对地友好干扰器的无人机，可以在未知的窃听位置实现干扰，以提高保密性能。此外，Du等[81]关注了一种安全场景，即卫星与分布在其覆盖范围内并配备天线的卫星固定服务（FSS）终端进行通信。位于卫星覆盖范围内的窃听者打算窃听传输到一个 FSS 终端的机密信息，称为窃听 FSS 终端。在已建立的安全场景中，通过地面基站实现的卫星自适应波束成形、人工噪声和基站协同波束成形，设计了一种协同安全传输波束成形方案来保证卫星通信的安全。
With the emergence of SAGSIN, related works on eavesdropping attacks in underwater acoustic network have also appeared. Regarding underwater acoustic network
随着SAGSIN的出现，水声网络窃听攻击的相关著作也层出不穷。关于水声网络
TABLE V 表五
SUMmary of Related WORKS ON SPOOFING ATTACKS IN SAGSIN
SAGSIN中欺骗攻击的相关著作综述
Types Reference Network structure 网络结构 Features/Advantages 特点/优势 Proposed algorithm/scheme
建议的算法/方案
GPS spoofing
Global navigation 全球导航
satellite system 卫星系统
Utilized the relationship between
利用了
deception interference and particle weight
欺骗干扰和颗粒重量
A maximum particle weight
最大颗粒重量
monitoring scheme based on particle
基于粒子的监测方案
filter
Utilized positioning error
利用的定位误差
correction and M-estimation theory
校正和M估计理论
A GPS anti-spoofing algorithm
GPS反欺骗算法
based on improved particle filter
基于改进的颗粒过滤器
Considered a system that
被认为是一个系统
provides open and encrypted
提供开放和加密
services at the same frequency
相同频率的服务
A new authentication scheme based on
一种新的身份验证方案，基于
signal authentication sequence
信号认证序列
UAV system
Used different features such as pseudo-
使用了不同的功能，例如伪
range, doppler frequency shift, SINR
量程、多普勒频移、SINR
to classify GPS signals
对 GPS 信号进行分类
A supervised ML method based on an
一种基于
artificial neural network
人工神经网络
Global positioning 全球定位
system
Considered GPS carrier- 被认为是GPS运营商-
to-noise ratio  噪声比
A cross-layer detection mechanism
一种跨层检测机制
Cyber-physical 网络物理
system
Studied the impact of GPS time
研究了GPS时间的影响
spoofing attacks on pseudorange,
伪范围欺骗攻击，
receiver position and time error
接收机位置和时间误差
A method to deceive GPS time by
一种欺骗GPS时间的方法
manipulating GPS time stamp or the
操纵 GPS 时间戳或
signal propagation time of GPS
GPS的信号传播时间
satellite signals 卫星信号
Neighbor spoofing 邻居欺骗
Underwater
sensor network 传感器网络
Reduced location and neighbor
减少位置和邻居
spoofing attacks in UASN
UASN 中的欺骗攻击
An anti-spoofing (a-spoof) algorithm
一种反欺骗（a-spoof）算法
Ground and
underwater spoofing 水下欺骗
Considered the interaction between
考虑了两者之间的相互作用
the surface observatory and
地面天文台和
the underwater deceiver 水下骗子
A Q-learning-based deception
基于Q学习的欺骗
jamming detection method for
一种干扰检测方法
dynamic underwater sensor networks
动态水下传感器网络
Replay spoofing 重播欺骗 [96]
Explained the statistical characteristics
解释了统计特征
of replay spoofing attacks
重放欺骗攻击
A spoofing attack detection strategy
欺骗攻击检测策略
for encrypted and secure GNSS signals
用于加密和安全的 GNSS 信号
communication, Wang and Wang [86] stated that the existence of eavesdropping attacks is mainly caused by the physical security of coordinated multipoint (CoMP) transmission of underwater acoustics. They studied the countermeasures against eavesdropping attacks of legal underwater users in coordinated multipoint transmission, and a signal alignment for transmission security was proposed by utilizing the low sound velocity in the water and the spatial diversity of distributed antenna elements.
Wang和Wang[86]指出，窃听攻击的存在主要是由于水声协调多点（CoMP）传输的物理安全性造成的。研究了协同多点传输中合法水下用户窃听攻击的对策，利用水中低声速和分布式天线元件的空间多样性，提出了传输安全的信号对准。
In order to improve the RSS-based key generation rate in underwater acoustic networks, Luo et al. [84] proposed a strategy to divide the communication frequency band into multiple independent sub-channels and to perform multi-channel key generation, and a smoothing filter was proposed to reduce random fluctuations so as to improve the probability of key agreement. Huang et al. [17] studied a key generation method based on the randomness of underwater acoustic network channels. Based on the channel frequency response observed at each user, a predefined linear block code (e.g., BCH code) was used for key bit extraction. To alleviate the difference in channel observation between two users brought by noise or channel asymmetry, syndrome information at one user was sent to the other user and used for key coordination at the other user.
为了提高水声网络中基于RSS的密钥生成率，Luo等[84]提出了一种将通信频段划分为多个独立子信道并进行多信道密钥生成的策略，并提出了一种平滑滤波器来减少随机波动，从而提高密钥一致性的概率。Huang等[17]研究了一种基于水声网络信道随机性的密钥生成方法。根据在每个用户处观察到的信道频率响应，使用预定义的线性块代码（例如，BCH代码）进行密钥位提取。为了缓解噪声或信道不对称带来的两个用户之间的信道观察差异，将一个用户的综合征信息发送给另一个用户，并用于另一个用户的键协调。

C. Spoofing Attack C. 欺骗攻击

In essence, spoofing is an attack method of impersonating identity and obtaining trust through authentication. The attacker disguises himself as a trusted party to communicate with the victim based on the defects of authentication mechanism, and finally grabs information or carries out further attacks. For satellite system, it usually has very high positioning accuracy, but lacks immunity to spoofing attacks. The spoofing signals are generated to be similar to real signals, and the receiver tracks them unaware. They provide false pseudodistances and satellite positions for calculation, which misleads receiver positioning and increases the signal propagation delay. In the early days, encrypted spread spectrum codes were used to counter spoofing in the military, where passwords are known only to authorized users, so that spoofers cannot simulate real signals. However, this encryption scheme cannot effectively prevent replay spoofing. Table V summarizes existing works on spoofing attacks in SAGSIN.
从本质上讲，欺骗是一种冒充身份并通过身份验证获得信任的攻击方法。攻击者伪装成可信方，基于认证机制的缺陷与受害者进行通信，最终获取信息或进行进一步攻击。对于卫星系统，它通常具有非常高的定位精度，但缺乏对欺骗攻击的免疫力。欺骗信号的生成与真实信号相似，接收器在不知不觉中跟踪它们。它们提供虚假的伪距和卫星位置进行计算，从而误导接收机定位并增加信号传播延迟。在早期，加密扩频码用于打击军队中的欺骗行为，其中只有授权用户知道密码，因此欺骗者无法模拟真实信号。但是，这种加密方案无法有效防止重播欺骗。表五总结了SAGSIN中关于欺骗攻击的现有工作。
At present, using particle filter (PF) against spoofing has become the mainstream of research. PF is a statistic process method based on monte carlo simulations, and it offers an approximate optimal Bayesian estimation for any non-linear or non-gaussian state space. Han et al. [89] proposed a novel antispoofing method based on PF for global navigation satellite system (GNSS), i.e., a maximum particle weight monitoring scheme for spoofing detection, and an improved robust estimation scheme for spoofing suppression. Nevertheless, the method only redesigned the positioning calculation module of the GNSS receiver without any extra hardware or signal modification. Li et al. [13] introduced the GPS anti-spoofing algorithm based on improved PF, which includes two steps, that is spoofing jamming detection and spoofing jamming suppression. After detecting the spoofing jamming, the improved algorithm is used to combine with the robust statistical theory of M-estimation to revise the process of the additional correction of pseudo-range particle update, and then eliminate the effect of spoofing. The results of the simulation verified the effectiveness and superiority of the proposed method.
目前，使用粒子过滤器（PF）对抗欺骗已成为研究的主流。PF是一种基于蒙特卡罗模拟的统计过程方法，它为任何非线性或非高斯状态空间提供了近似最优贝叶斯估计。Han等[89]提出了一种基于PF的全球导航卫星系统（GNSS）反欺骗方法，即用于欺骗检测的最大粒子量监测方案和改进的欺骗抑制鲁棒估计方案。然而，该方法仅重新设计了GNSS接收机的定位计算模块，而没有进行任何额外的硬件或信号修改。Li等[13]介绍了基于改进PF的GPS反欺骗算法，该算法包括欺骗干扰检测和欺骗干扰抑制两个步骤。在检测到欺骗干扰后，利用改进 算法结合M估计的鲁棒统计理论，修正伪距粒子更新的附加校正过程，进而消除欺骗效应。仿真结果验证了所提方法的有效性和优越性。
Spoofing attacks can be mainly divided into location spoofing and neighbor spoofing. In location spoofing, malicious nodes obtain the trust of the source (victim) by forwarding false locations. However, in neighbor spoofing, malicious nodes spoof the real next hop id to interrupt the routing of data [94]. Because of the characteristics of underwater acoustic
欺骗攻击主要可分为位置欺骗和邻居欺骗。在位置欺骗中，恶意节点通过转发虚假位置来获得源（受害者）的信任。然而，在邻居欺骗中，恶意节点欺骗真实的下一跳ID以中断数据的路由[94]。由于水声的特性
TABLE VI 表六
SUMmary of ReLATED WORKS ON DoS ATTACKS IN SAGSIN
SUMmary of ReLATED 在 SAGSIN 中对 DoS 攻击进行研究
Types Reference Network structure 网络结构 Features/Advantages 特点/优势 Proposed algorithm/scheme
建议的算法/方案
DoS attack [98] Satellite network 卫星网络
Discussed the vulnerability of low-orbit
讨论了低轨道的脆弱性
satellite network systems to DoS attacks
卫星网络系统受到 DoS 攻击
A method for early detection of DoS attacks
一种早期检测DoS攻击的方法
near the source of flooding and before
洪水源附近和之前
system breakdown 系统故障
Satellite network 卫星网络
Allowed satellite servers to
允许卫星服务器
quickly discard false requests
快速丢弃虚假请求
A general mechanism to prevent DoS attacks
防止 DoS 攻击的一般机制
in the control plane protocol
在控制平面协议中
[99] Satellite network 卫星网络
Disabled communication between the
禁用了
server and its legitimate clients
服务器及其合法客户端
A scheme for detecting and
一种用于检测和
defending separate DoS attacks
防御单独的 DoS 攻击
DDoS attack
Wireless ad hoc 无线自组织
network
Used secure transmission
使用安全传输
protocol communication technology
协议通信技术
A symmetric key exchange and
对称密钥交换和
password hiding method in the UDP package
UDP 包中的密码隐藏方法
Satellite network 卫星网络 Reduced network energy consumption
降低网络能耗
An improved network 改进的网络
topology generation algorithm and
拓扑生成算法和
DDOS attack mitigation strategy
DDOS攻击缓解策略
SDoS attack [101]
Underwater sensor 水下传感器
network
Detected and resisted 检测和抵抗
SDoS attacks and common IFA attacks
SDoS 攻击和常见的 IFA 攻击
A defense method based on carefully
一种基于谨慎的防御方法
designed adaptive thresholds, burst traffic
设计自适应阈值，突发流量
detection, and attacker identification
检测和攻击者识别
Fig. 9. GPS spoofing attack.
图 9.GPS欺骗攻击。
channel in the water sensor network, Domingo [97] showed that spoofing attacks can be detected by encrypting all data packets. Li et al. [95] proposed an authentication scheme which applied hypothesis testing to detect spoofing attacks in wireless sensor networks. The interaction between underwater spoofers and surface receivers was formulated as a zero-sum game, using reinforcement learning-based method of spoofing detection.
Domingo[97]表明，通过加密所有数据包，可以检测欺骗攻击。Li等[95]提出了一种认证方案，该方案应用假设检验来检测无线传感器网络中的欺骗攻击。使用基于强化学习的欺骗检测方法，将水下欺骗者和水面接收者之间的交互表述为零和博弈。
GPS provides accurate location information for military and civilian users around the world. To calculate the precise location of the user, GPS needs to synchronize the time on the device with satellite time. GPS spoofing attack is the process of producing a fake GPS signal to interfere with navigation and time synchronization of the receiver, as shown in Fig. 9. In unmanned aerial system (UAS), it is vulnerable to GPS spoofing attacks. In this attack, a malicious user sends a false signal to the GPS receiver of the UAS. Fake signals will not only mislead the aircraft, but also mislead air traffic controllers, causing serious problems,for example, from hijacking to collisions and casualties.
GPS为世界各地的军事和民用用户提供准确的位置信息。为了计算用户的精确位置，GPS需要将设备上的时间与卫星时间同步。GPS欺骗攻击是产生假GPS信号以干扰接收机的导航和时间同步的过程，如图9所示。在无人机系统（UAS）中，它容易受到GPS欺骗攻击。在这种攻击中，恶意用户向 UAS 的 GPS 接收器发送错误信号。虚假信号不仅会误导飞机，还会误导空中交通管制员，造成严重的问题，例如从劫持到碰撞和人员伤亡。
Manesh et al. [91] proposed a supervised ML method based on artificial neural networks to detect GPS spoofing signals, where different features such as pseudo-range, Doppler shift, and SINR were adopted to classify GPS signals. Pozzobon et al. [90] discussed the open GNSS signal authentication, where a new authentication scheme was proposed based on signal authentication sequences integrated with GNSS. The method can be performed in the systems that provide open and encrypted service on the same frequency, and the proposed scheme would bring minimum impact to the system.
Manesh等[91]提出了一种基于人工神经网络的监督ML方法检测GPS欺骗信号，采用伪距、多普勒频移和SINR等不同特征对GPS信号进行分类。Pozzobon等[90]讨论了开放GNSS信号认证，提出了一种基于与GNSS集成的信号认证序列的新认证方案。该方法可以在提供相同频率的开放和加密服务的系统中执行，所提出的方案对系统的影响最小。
Moreover, Humphreys [96] presented a strategy for detecting spoofing attacks against cryptographically-secured GNSS signals, and the strategy was based on a model that captured the essential features of a replay-type spoofing attack. Their strategy can be applied not only to military GPS signals but also to the proposed security-enhanced civil GNSS signals. Wei and Sikdar [93] focused on GPS time spoofing attacks, which are implemented by manipulating the two variables of GPS time stamp and signal propagation time, and the relationship between pseudo-range error, positioning error, and receiver time error in different spoofing scenarios were studied.
此外，Humphreys[96]提出了一种针对加密安全的GNSS信号检测欺骗攻击的策略，该策略基于一个模型，该模型捕获了重放型欺骗攻击的基本特征。他们的策略不仅可以应用于军用GPS信号，还可以应用于拟议的安全增强型民用GNSS信号。Wei和Sikdar[93]通过操纵GPS时间戳和信号传播时间两个变量来实现GPS时间欺骗攻击，研究了不同欺骗场景下伪距误差、定位误差和接收机时间误差之间的关系。

D. Denial of Service Attack
D. 拒绝服务攻击

DoS attacks are intended to make network services unavailable by denying legitimate users access to specific network resources, and they may lead to system overloads and prevent some or all legitimate requests from being satisfied. In SAGSIN, different opponents can send a large number of false messages to the satellite, so that the satellite spends a lot of computing cycles to deal with false messages. Due to the limited processing capacity of the satellite, it will be easily overwhelmed and result in system DoS. Furthermore, some malicious opponents intentionally send false disconnect requests for the server, and thus disrupting communication between legitimate clients and the server. Table VI summarizes existing work on DoS attacks in SAGSIN.
DoS攻击旨在通过拒绝合法用户访问特定网络资源来使网络服务不可用，并可能导致系统过载，并阻止部分或全部合法请求得到满足。在SAGSIN中，不同的对手可以向卫星发送大量的虚假信息，使卫星花费大量的计算周期来处理虚假信息。由于卫星的处理能力有限，很容易不堪重负，导致系统DoS。此外，一些恶意攻击者故意向服务器发送虚假的断开连接请求，从而中断合法客户端与服务器之间的通信。表VI总结了SAGSIN中关于DoS攻击的现有工作。
DoS attacks can exist in all protocol layers, and it is in the form of interference in the physical layer, as shown in Fig. 10. The MAC layer is also vulnerable to DoS attacks, including conflicts, queries, and packet replay. To protect the MAC layer from DoS attacks, powerful end-to-end authentication and anti-replay capabilities are required. At the network layer, DoS attacks include spoofing, replays, and changing routing traffic. At the transport layer, the purpose of TCP SYN flooding is to consume connection buffer resources, and we can protect the transport layer from flooding by using SYN cookies which encode information from client TCP SYN messages. Moreover, application layer security issues include prevention, malicious nodes, and virus detection, and packet
DoS 攻击可以存在于所有协议层中，并且以物理层干扰的形式存在，如图 10 所示。MAC 层也容易受到 DoS 攻击，包括冲突、查询和数据包重放。为了保护 MAC 层免受 DoS 攻击，需要强大的端到端身份验证和防重放功能。在网络层，DoS 攻击包括欺骗、重放和更改路由流量。在传输层，TCP SYN 泛洪的目的是消耗连接缓冲区资源，我们可以通过使用 SYN cookie 来保护传输层免受泛洪，这些 SYN cookie 对来自客户端 TCP SYN 消息的信息进行编码。此外，应用层安全问题包括预防、恶意节点、病毒检测和数据包
Fig. 10. DoS attacks on wireless communication networks in SAGSIN.
图 10.对 SAGSIN 无线通信网络的 DoS 攻击。
authentication and anti-replay techniques are usually used to prevent opponents from overwhelming the network.
身份验证和防重放技术通常用于防止对手压倒网络。
To prevent DoS attacks against satellite networks, Taleb et al. [98] presented a round-trip time-based prevention technique to prevent TCP-based bandwidth attacks over satellite networks. This method is less in computationally expensive than encryption, but it was not a perfect defense against DoS attacks. Specifically, if an intruder launches a DoS attack and the bandwidth consumption does not exceed a predefined threshold, the satellite system will not take any protective measure.
为了防止针对卫星网络的DoS攻击，Taleb等[98]提出了一种基于往返时间的预防技术，以防止卫星网络上基于TCP的带宽攻击。这种方法的计算成本低于加密，但它并不是对 DoS 攻击的完美防御。具体来说，如果入侵者发起DoS攻击，并且带宽消耗不超过预定义的阈值，卫星系统将不会采取任何保护措施。
An efficient protocol was designed to prevent DoS attacks in [22], which includes two-step validation. First, the sequence number retrieved from the message should be equal to a nonce value. Then, the monitoring system used its private key to compare the calculated MAC address value with the address value retrieved from the header for a match. Finally, the network control center further verified the message integrity.
在[22]中设计了一种有效的协议来防止DoS攻击，其中包括两步验证。首先，从消息中检索到的序列号应等于随机数值。然后，监控系统使用其私钥将计算出的 MAC 地址值与从标头中检索到的地址值进行比较以进行匹配。最后，网络控制中心进一步验证了消息的完整性。
However, the sequence number of each message is not protected by any information hiding technology. As a result, the messages are vulnerable to DoS attacks. Regarding the problem, Ma et al. [99] proposed an enhanced algorithm to protect the sequence number of each message, which was based on the characteristics of the one-way Rabin function. It employed the Rabin function to encrypt the sequence number to improve the security of the sequence number, and can effectively defend the satellite networks against DoS attacks.
但是，每条消息的序列号不受任何信息隐藏技术的保护。因此，这些消息容易受到 DoS 攻击。关于这个问题，马等[99]提出了一种增强算法来保护每条消息的序列号，该算法基于单向Rabin函数的特征。它采用Rabin功能对序列号进行加密，以提高序列号的安全性，并能有效保护卫星网络免受DoS攻击。
DDoS attacks are also one of the main security threats that current networks (especially satellite networks with limited resources) need to deal with [102]. Ghavidel and Issac [100] proposed a technical solution to asymmetric key exchange and hid a secret code in the user datagram protocol (UDP) packets to secure transport protocol communication in the case of DDoS attacks. In the software-defined satellite network (SDSN), abnormal traffic generated by DDoS attacks occupies satellite node processing resources, while also having a great impact on the storage and forwarding of normal traffic. This not only reduces the overall utilization of the satellite network but also increases the energy consumption of the network. Therefore, it is necessary to find an intelligent mitigation strategy to suppress the abnormal traffic generated by DDoS attacks, while ensuring the forwarding of normal traffic, improving the overall security and resource utilization of the satellite network.
DDoS攻击也是当前网络（尤其是资源有限的卫星网络）需要应对的主要安全威胁之一[102]。Ghavidel和Issac[100]提出了一种非对称密钥交换的技术解决方案，并在用户数据报协议（UDP）数据包中隐藏了密码，以在DDoS攻击的情况下保护传输协议通信。在软件定义卫星网络（SDSN）中，DDoS攻击产生的异常流量占用了卫星节点的处理资源，同时也对正常流量的存储和转发产生了很大影响。这不仅降低了卫星网络的整体利用率，还增加了网络的能耗。因此，有必要找到一种智能的缓解策略来抑制DDoS攻击产生的异常流量，同时保证正常流量的转发，提高卫星网络的整体安全性和资源利用率。
Tu et al. [16] proposed an intelligent mitigation strategy based on deep reinforcement learning, which can effectively alleviate the abnormal traffic of SDSN caused by DDoS attacks. Through training and verification, it was proved that the proposed mitigation strategy can greatly reduce the energy consumption of satellite nodes. In recent years, although underwater named data networking has performed well in terms of data transmission, it also faces some security threats, such as DOS attacks caused by the flood of interest attacks. Li et al. [101] proposed a new type of DOS attack, called synergetic DOS (SDoS) attack, where the attackers automatically form an attack network and respond to malicious interests in turn with the greatest delay before the attack expires.
Tu等[16]提出了一种基于深度强化学习的智能缓解策略，可有效缓解DDoS攻击导致的SDSN异常流量。通过训练和验证，证明所提出的缓解策略可以大大降低卫星节点的能耗。近年来，水下命名数据网络虽然在数据传输方面表现良好，但也面临着一些安全威胁，如DOS攻击引发的兴趣泛滥攻击。Li等[101]提出了一种新型的DOS攻击，称为协同DOS（SDoS）攻击，攻击者在攻击结束前自动形成攻击网络，以最大的延迟依次响应恶意利益。
Remarks: In addition to the four attack methods mentioned above, SAGSIN may also be attacked by other methods, such as side channel attack, false data injection attack, protocol manipulation attack, active attacks on handover, etc. Specifically, side-channel attack is an important source of software vulnerabilities. It means that a malicious adversary can gain information of private data by measuring processing time of a software system, power usage, or memory. False data injection attack, which uses loop holes of networks to attack supervisory control and data acquisition communication and control centers, can temper data measurements and hinder the normal operation of state estimator. As for protocol manipulation attack, instead of only relying on implementation bugs in protocols, adversaries can exploit incomplete knowledge by misrepresenting network conditions to honest participants. It can induce undesirable behaviors in them. Active attacks on handover mainly include impersonation attack techniques for cross-layer authentication, theft, tampering, replay attacks on switched messages, spoofing and colluding attacks to switch predictive messages. Moreover, in recent years, with the wide application of AI and deep learning technology, the attack methods based AI techniques have emerged to be a hot spot.
备注：除了上述四种攻击方式外，SAGSIN还可能受到其他方式的攻击，如侧信道攻击、虚假数据注入攻击、协议操纵攻击、主动切换攻击等。具体来说，侧信道攻击是软件漏洞的重要来源。这意味着恶意攻击者可以通过测量软件系统、功耗或内存的处理时间来获取私有数据的信息。虚假数据注入攻击利用网络漏洞攻击监控和数据采集通信控制中心，可以调节数据测量，阻碍状态估计器的正常运行。至于协议操纵攻击，攻击者不仅可以依赖协议中的实现错误，还可以通过向诚实的参与者歪曲网络条件来利用不完整的知识。它可以在他们身上诱发不良行为。主动切换攻击主要包括跨层认证的冒充攻击技术、盗窃、篡改、对切换消息的重放攻击、欺骗和串通攻击切换预测消息。此外，近年来，随着人工智能和深度学习技术的广泛应用，基于攻击方法的人工智能技术已成为一个热点。

VI. Security Countermeasures for SAGSIN
六、SAGSIN的安全对策

In this section, we focus on common security defense countermeasures for SAGSIN, as shown in Fig. 11, and review the existing works.
在本节中，我们将重点介绍SAGSIN常见的安全防御对策，如图11所示，并回顾现有工作。

A. Anti-Jamming Techniques
A. 抗干扰技术

Along with the development of network technology, SAGSIN faces jamming attacks at different layers. For these jamming attacks, some researchers have proposed antijamming techniques, including spread spectrum, game theory, reinforcement learning, directional antenna in the satellite system, etc. Table VII summarizes the related works on anti-jamming techniques.
随着网络技术的发展，SAGSIN面临着不同层的干扰攻击。针对这些干扰攻击，一些研究人员提出了抗干扰技术，包括扩频、博弈论、强化学习、卫星系统中的定向天线等。表七总结了抗干扰技术的相关工作。
  1. Spread Spectrum: To deal with jamming attacks, a variety of technologies and strategies have been proposed. Traditional physical layer techniques against jamming attacks rely on spread spectrum [118], including direct sequence spread spectrum and frequency-hopping sequence spread spectrum, and these techniques are widely used in the military field. Spread spectrum communication consumes more frequency resources, which adopts pseudo-random code with a high
    扩频：为了应对干扰攻击，已经提出了各种技术和策略。传统的物理层技术依赖于扩频[118]，包括直接序列扩频和跳频序列扩频，这些技术在军事领域得到广泛应用。扩频通信消耗较多的频率资源，采用伪随机码，高
Fig. 11. Organization of security defense countermeasures for SAGSIN.
图 11.组织SAGSIN的安全防御对策。
TABLE VII 表七
SUMmARY OF RELATED WORKS ON ANTI-JAMming TECHNIQUeS FOR SAGSIN
SAGSIN抗JAMming TECHNIQUeS相关著作综述
Techniques Reference
Communications
network
Proposed algorithm/scheme
建议的算法/方案		 Contributions Results
Spread spectrum 扩频
Satellite
communication
A time-frequency diversity
时频分集
strategy at the link layer, and
链路层的策略，以及
a deep interleaving strategy
深度交错策略
at the physical layer
在物理层
Time-frequency classification
时频分类
of cross-layer design strategy
跨层设计策略
Reduced the probability of
降低了
jamming
An uncoordinated spread 不协调的传播
spectrum technology 频谱技术
Achieved anti-jamming in the
在
absence of shared secrets
没有共享密钥
Reduced latency and 减少延迟和
improved system security
提高系统安全性
UAV
communication
A practical local reaction
实际的局部反应
anti-jamming scheme 抗干扰方案
Applied to UAV swarms
应用于无人机群
Reduced the packet 减少了数据包
transmission delay, improved
传输延迟，改善
the anti-jamming ability of
抗干扰能力
delay-sensitive UAVs 对延迟敏感的无人机
Underwater
communication
An incongruously direct 一个不协调的直接
sequence spread spectrum
序列扩频
technique
Considered multi-channel
被认为是多渠道的
underwater communication
水下通信
jamming
Reduced bit error rate and
降低误码率和
SINR
Wireless
communication
A direct spread spectrum
直接扩频
system based on variable
基于变量的系统
spread spectrum sequence
扩频序列
Used a shared key and an
使用共享密钥和
appropriate password 适当的密码
Had good system 有良好的系统
synchronization and bit error
同步和位错误
rate performance 评价性能
Game theory
reinforcement
learning
A zero-sum Markov game
零和马尔可夫博弈
scheme
Considered the interaction
考虑了交互
between legitimate 在合法之间
transmitters and jammers
发射器和干扰器
Improved average throughput,
提高平均吞吐量，
strong anti-jamming ability
抗干扰能力强
An algorithm for dynamic
一种动态算法
execution of the game
游戏的执行
Used Markov chains 二手马尔可夫链 Improved the system security
提高了系统安全性
UAV
communication
A Q-learning based 基于Q-learning的
anti-jamming scheme 抗干扰方案
Considered static and 被认为是静态的，并且
dynamic games 动态游戏
Had higher average utility and
具有更高的平均效用和
SINR
A Stackelberg game method
斯塔克尔伯格博弈法
Considered the impact of
考虑了
transmission costs on users
用户的传输成本
and smart jammers 和智能干扰器
Improved the channel gain,
改进了通道增益，
ensured the security of
确保安全
communication
Underwater
communication
An underwater anti-jamming
水下抗干扰
transmission frame 传动架
Took advantage of the
利用了
mobility of the transducer
换能器的移动性
Had faster learning speeds,
学习速度更快，
better SINR, lower bit error
更好的 SINR，更低的位误差
rate, higher overall utilization
率，更高的整体利用率
An anti-jamming relay scheme
一种抗干扰继电器方案
using reinforcement learning
使用强化学习
Considered relay performance
考虑继电器性能
of computing nodes 计算节点数
Reduced bit error rate, saved
降低误码率，保存
energy consumption, 能量消耗
improved utilization rate
提高利用率
Satellite
communication
A two-layer asymmetric 两层不对称
zero-sum game framework 零和博弈框架
Considered the damage effect
考虑了损害效果
of FH spread spectrum
FH扩频
Got higher returns and
获得了更高的回报和
improved system security
提高系统安全性
Directive antenna 定向天线
A new space-time 新的时空
adaptive processor based
基于自适应处理器
on least meansquare digital
在最小均方数字上
beamforming algorithm 波束成形算法
Took into account the
考虑到
radiation pattern deviation
辐射方向图偏差
Improved the omnidirectional
改进了全向性
array property and the ability
数组属性和能力
to suppress wideband jamming
抑制宽带干扰
A space-time interference
时空干扰
suppression model 抑制模型
Took into account downlink
考虑下行链路
jamming
Improved the SINR of system
改进了系统的SINR
output, had a strong
输出，具有很强的
mechanism to jamming signal
干扰信号的机制
An efficient compensation
高效的补偿
algorithm for anti-jamming
抗干扰算法
antenna phase center change
天线相位中心变化
Used for GPS system
用于GPS系统
Reduced carrier phase 减少载流子相位
residuals
A dual system anti-jamming
双系统抗干扰
satellite navigation receiver
卫星导航接收机
scheme
Used adaptive filtering 使用的自适应筛选
method
Improved the anti-jamming
改进了抗干扰性能
ability, ensured the system
能力，保证了系统
security
Wireless
communication
A method using Kalman filter
一种使用卡尔曼滤波的方法
Considered a closed form of
被认为是
beamforming weights 波束成形权重
Had good performance, low
性能好，低
computational complexity
计算复杂度
data rate to extend the spectrum of the baseband signal. It has a good anti-jamming performance and advantages of secrecy.
数据速率，以扩展基带信号的频谱。具有良好的抗干扰性能和保密性优点。
Munir and Maud [107] proposed a direct sequence spread spectrum system using variable spreading sequence, which adopted a shared secret key with appropriate ciphers
Munir和Maud[107]提出了一种使用可变扩散序列的直接序列扩频系统，该系统采用具有适当密码的共享密钥
(e.g., AES) for selection of variable spreading sequence. Each bit was spread in a different spreading sequence resulting in jamming immunity, and the synchronization and bit error rate performance was well improved. Rong and Ruimin [103] studied satellite frequency-hopping (FH) communication for anti-jamming, and a time-frequency diversity strategy was presented to improve the anti-jamming performance of the system on the measurement of link frame throughput. Lower complexity and shorter delay can be achieved by adopting their strategy.
（例如，AES）用于选择可变扩散序列。每个比特都以不同的扩频顺序进行扩展，从而实现抗干扰，同步和误码率性能得到了很好的提高。Rong和Ruimin[103]研究了卫星跳频（FH）通信的抗干扰性能，提出了一种时频分集策略，以提高系统在链路帧吞吐量测量方面的抗干扰性能。通过采用他们的策略，可以实现更低的复杂性和更短的延迟。
Moreover, in UAV communication, Li et al. [105] took into account the ability of UAV swarms to withstand a hostile jamming attack locally, and they proposed a local reactive anti-jamming scheme to reduce packet transmission delay. When the blocked nodes change dynamically, the blocked nodes and single-hop neighbors exchange their corresponding information. The information of the frequency status determines their node type on each frequency (channel), and adaptive frequency hopping (AFH) was performed accordingly.
此外，在无人机通信方面，Li等[105]考虑了无人机群在局部抵御敌方干扰攻击的能力，提出了一种局部反应抗干扰方案，以降低数据包传输延迟。当被阻止的节点动态变化时，被阻止的节点和单跳邻居交换相应的信息。频率状态的信息决定了它们在每个频率（信道）上的节点类型，并相应地执行自适应跳频（AFH）。
Furthermore, some researchers proposed to adopt uncoordinated spread spectrum technology for anti-jamming attacks. Popper et al. [104] utilized uncoordinated spread spectrum techniques to enable anti-jamming communication between senders and receivers, which cannot share any secret key. Kalita and Sahu [106] presented an uncoordinated direct sequence spread spectrum technology to handle an unlimited number of receivers for anti-interference multi-channel underwater communications, and the bit error rates were reduced.
此外，一些研究人员建议采用不协调的扩频技术进行抗干扰攻击。Popper等[104]利用不协调扩频技术在发送方和接收方之间实现抗干扰通信，而发送方和接收方不能共享任何密钥。Kalita和Sahu[106]提出了一种不协调的直接序列扩频技术，用于处理无限数量的接收机，用于抗干扰多通道水下通信，并降低了误码率。
  1. Game Theory and Reinforcement Learning: Although the spread spectrum technology mentioned above can resist jamming attacks, considering the limitations of network bandwidth, energy consumption and computing capacity, traditional physical layer technology is not suitable for the practical application of SGASIN. In view of this, scholars have put forward various defense methods of jamming attacks, and it has been verified that game theory and reinforcement learning (RL) are effective methods to study jamming and anti-jamming in wireless networks.
    博弈论与强化学习：虽然上述扩频技术可以抵御干扰攻击，但考虑到网络带宽、能耗和计算能力的限制，传统的物理层技术并不适合SGASIN的实际应用。有鉴于此，学者们提出了多种干扰攻击防御方法，并验证了博弈论和强化学习（RL）是研究无线网络干扰和抗干扰的有效方法。
Regarding wireless communication, in order to provide better anti-jamming capabilities, Hanawal et al. [108] proposed to model the interaction between the legal transmitter and the jammer as a constrained zero-sum Markov game in the presence of jammers. Li et al. [109] developed an integrated game theory framework to investigate the interactive decisionmaking process between a sensor node and an attacker. A constraint-relaxed problem formulation and the corresponding closed-form expression was provided, and the computation complexity was significantly reduced.
在无线通信方面，为了提供更好的抗干扰能力，Hanawal等[108]提出将合法发射机和干扰机之间的相互作用建模为干扰机存在下的约束零和马尔可夫博弈。Li等[109]开发了一个集成博弈论框架来研究传感器节点和攻击者之间的交互决策过程。提供了约束宽松的问题公式和相应的闭式表达式，计算复杂度显著降低。
For UAV network, Lv et al. [110] analyzed the anti-jamming issues in both static and dynamic games, and proposed an optimal strategy based on multi-slot learning to achieve relatively high average utility and signal interference plus noise ratio. Considering malicious external jamming and UAV jamming with each other, Xu et al. [111] designed an antijamming Stackelberg game to deduce the optimal transmitting power of user and intelligent jamming. Their experiments validated the influence of transmitting cost on user and intelligent jammer. With the development of satellite communications, Wang et al. [23] minimized the destructive effects of satellite jamming attacks, and proposed an asymmetric zero-sum game framework, so as to determine the optimal strategy in each case through the analysis of the participants.
针对无人机网络，Lv等[110]分析了静态和动态博弈中的抗干扰问题，提出了一种基于多时隙学习的最优策略，以实现相对较高的平均效用和信号干扰噪声比。Xu等[111]考虑了外部恶意干扰和无人机干扰之间的相互影响，设计了一种抗干扰Stackelberg博弈，推导了用户和智能干扰的最佳发射功率。他们的实验验证了传输成本对用户和智能干扰器的影响。随着卫星通信的发展，Wang等[23]将卫星干扰攻击的破坏性影响降到最低，提出了一种非对称零和博弈框架，通过对参与者的分析，确定每种情况下的最优策略。
In underwater sensor networks, the power constraints of underwater sensors bring about lots of challenges on antijamming attacks. To address these issues, Xiao et al. [112] proposed an underwater anti-jamming transmission scheme using RL to select sensors, transmit power and receiver location to solve static and nearsighted jamming. After that, They [113] further gave an RL based anti-jamming relay scheme for ultra wide band wireless sensor networks. Experiments conducted in non-pools equipped with underwater sensors verified the analysis results against intelligent jamming attacks, and the relay performance can be improved.
在水下传感器网络中，水下传感器的功率限制给抗干扰攻击带来了许多挑战。针对这些问题，Xiao等[112]提出了一种水下抗干扰传输方案，利用RL选择传感器、发射功率和接收机位置，以解决静态和近视干扰问题。之后，他们[113]进一步给出了一种基于RL的超宽带无线传感器网络抗干扰中继方案。在非水下传感器池中进行的实验验证了针对智能干扰攻击的分析结果，并提高了中继性能。
  1. Directive Antenna: In satellite communication, directional antenna and signal processing technology can effectively prevent jamming attacks. Yin-Ting et al. [114] combined the digital beamforming technique and the space-time adaptive anti-jamming algorithm to minimize the deviation of the radiation pattern of the anti-jamming antenna array, and the jamming cancellation capability was improved. Wang et al. [115] established a space-time jamming suppression model, and adopted the linearly constrained minimum variance (LCMV)proportional-integral jamming suppression algorithm, which can form a deeper zero in the direction of the interference signal and increase the output system SINR.
    定向天线：在卫星通信中，定向天线和信号处理技术可以有效防止干扰攻击。Yin-Ting等[114]将数字波束成形技术和时空自适应抗干扰算法相结合，将抗干扰天线阵列辐射方向图的偏差降至最低，提高了干扰消除能力。Wang等[115]建立了时空干扰抑制模型，并采用了线性约束最小方差（LCMV）比例积分干扰抑制算法，该算法可以在干扰信号方向上形成更深的零点，提高输出系统的SINR。
Sadr et al. [117] designed a closed-form beamforming weight, maximizing the achievable rate while preserving the total relay power transmission below a given threshold level. The proposed method showed efficient performance for different destruction powers of the jammer. Cao et al. [116] proposed an anti-jamming antenna azimuth antenna phase center change compensation algorithm, so that the anti-jamming antenna can realize real-time high-precision applications. Moreover, Xiao and Zhong [78] presented a dual-system antijamming satellite navigation receiver design scheme, with LCMV as the optimal criterion, and powered reversal weighting to enhance the receivers' anti-jamming capability.
Sadr等[117]设计了一种封闭形式的波束成形权重，在将继电器总功率传输保持在给定阈值水平以下的同时，最大限度地提高了可实现的速率。所提方法在干扰机的不同破坏能力下表现出高效的性能。Cao等[116]提出了一种抗干扰天线方位角天线相位偏心补偿算法，使抗干扰天线能够实现实时高精度应用。此外，Xiao和Zhong[78]提出了一种以LCMV为最优准则的双系统抗干扰卫星导航接收机设计方案，并采用动力反转加权来增强接收机的抗干扰能力。
In addition to the methods mentioned above, adopting reliable and effective coding can also improve the anti-jamming ability in data transmission process, and great progress has been made in solving the issues of jamming robustness, especially in the application of cognitive radio technology [131]. Martinelli et al. [132] concentrated on the utilization of channel coding to enhance the anti-jamming capabilities, and compared the performance with different types of jammers to identify a solution to providing a good trade-off between robustness to jamming and latency. Lin et al. [133] combined the joint approximate diagonalization (JAD) of the fourth order cumulant matrix and post position wavelet denoising, and solved the problem of blind separation interference in satellite communication signal and noisy environment.
除上述方法外，采用可靠有效的编码还可以提高数据传输过程中的抗干扰能力，在解决干扰鲁棒性问题方面取得了很大进展，特别是在认知无线电技术的应用方面[131]。Martinelli等[132]专注于利用信道编码来增强抗干扰能力，并比较了不同类型干扰器的性能，以确定在干扰鲁棒性和延迟之间提供良好权衡的解决方案。Lin等[133]将四阶累积矩阵的联合近似对角化（JAD）与后位小波去噪相结合，解决了卫星通信信号和噪声环境中的盲分离干扰问题。
Besides, the security threats caused by malicious UAVs make anti-UAV systems inevitable. Jin et al. [134] proposed an anti-UAV system consisting of a surveillance system to detect drones and a countermeasure system to prevent drone behavior. It adopted the UAV location information detected by
此外，恶意无人机造成的安全威胁使得反无人机系统不可避免。Jin等[134]提出了一种反无人机系统，该系统由探测无人机的监视系统和防止无人机行为的对抗系统组成。它采用了无人机检测到的位置信息
TABLE VIII 表八
SUMmary of Related WORKS ON SeCURE RoUTING IN SAGSIN
SAGSIN中SeCURE轮盘的相关工作总结
Network Reference Proposed algorithm/scheme
建议的算法/方案		 Features Results
Space information 空间信息
network
A semi-deterministic space-time
半确定性时空
graph model
Timeliness-aware data 时效性感知数据
mining and time-space 采矿和时空
graph
Reduced the total link overhead, ensured
降低总链路开销，确保
the connectivity and reliability of the path
路径的连通性和可靠性
Satellite network 卫星网络 A location-based routing algorithm
基于位置的路由算法		 Distributed, packet routing
分布式数据包路由
Had high efficiency in signal cost and
信号成本效率高，且
control cost
A new survivable routing protocol
一种新的可生存路由协议
Two-layer structure, 双层结构，
centralized/distributed 集中式/分布式
Reduced the minimum routing delay
降低了最小路由延迟
A new periodic time-varying ISL
一种新的周期性时变 ISL
subnet routing scheme 子网路由方案
Dynamic virtual topology
动态虚拟拓扑
routing
Reduced the minimum delay jitter
降低了最小延迟抖动
A new link allocation (topology
新的链路分配（拓扑
design) framework design） 框架
Static/dynamic routing 静态/动态路由
Reduced the blocking probability and the
降低了阻塞概率和
link allocation, achieved optimal
链路分配，实现最优
performance
A TDMA-based scheme 基于TDMA的方案 Cross-layer
Improved the QoS, ensured the system
改进QoS，确保系统
security
An active distributed QoS routing
活动分布式 QoS 路由
policy
Active, distributed 主动式、分布式 Had better CBP and CDP performance
具有更好的 CBP 和 CDP 性能
A network coding based routing
基于网络编码的路由
protocol
Multipath cooperative 多路径合作
routing
Improved throughput and TCP
提高吞吐量和 TCP
performance, reduced end-to-end latency
性能，减少端到端延迟
Underwater acoustic 水下声学
network
A secure multi-user network protocol
安全的多用户网络协议		 Flooding routing protocol
泛洪路由协议		 Improved packet transfer rate
提高数据包传输速率
Underwater wireless 水下无线
network
A channel-aware routing protocol
信道感知路由协议		 Cross-layer routing 跨层路由
Improved throughput, improved end-to-end
提高吞吐量，改善端到端
latency
UWSN A SEECR based secure protocol
基于SEECR的安全协议		 Cooperative routing protocol
协作路由协议
Increased number of live nodes, improved
增加了活动节点的数量，改进了
throughput, reduced transmission losses,
吞吐量，降低传输损耗，
energy taxes, and end-to-end latency
能源税和端到端延迟
An enhanced version of channel
频道的增强版本
aware routing protocol 感知路由协议
Packet forwarding 数据包转发 Improved relay node selection strategy
改进了中继节点选择策略
An energy efficient routing protocol
高能效路由协议
Reduced communication cost, improved
降低通信成本，提高
network performance to a certain extent
网络性能在一定程度上
the monitoring system to radiate directional interference signals to the malicious UAV, thereby reducing the interference performance.
监控系统向恶意无人机辐射定向干扰信号，从而降低干扰性能。

B. Secure Routing B. 安全路由

Secure routing mainly addresses the availability, integrity, and reliability of routing information, to ensure unblocked communication. Compared with other attack methods, the attacks on routing mechanism are more destructive and covert in SAGSIN. Attackers cannot only easily intercept or discard packets, but also increase the cost of routing control, and even cause the whole network to crash. Therefore, how to construct a robust routing protocol is a challenging issue. Table VIII summarizes existing works on secure routing in SAGSIN.
安全路由主要解决路由信息的可用性、完整性和可靠性问题，以确保通信畅通无阻。与其他攻击方式相比，SAGSIN中路由机制的攻击更具破坏性和隐蔽性。攻击者不仅轻易拦截或丢弃数据包，还增加了路由控制的成本，甚至导致整个网络崩溃。因此，如何构建鲁棒的路由协议是一个具有挑战性的问题。表VIII总结了SAGSIN中有关安全路由的现有工作。
In consideration of the complex topology of satellite network, it is impossible to use the routing protocol of ground network directly. Hence, in order to adapt to the dynamic change of satellite network, the development of special routing algorithm based on interstellar link has become a research hot spot in the field of communication. With the development of satellite network structure from single layer to multilayer, scholars have proposed a variety of routing algorithms. Specifically, Hou et al. [119] presented a highly reliable routing algorithm, which used a time-aware data mining algorithm to predict the dynamic changes of each node connection and used time-space graph model to draw topology, which not only reduced the total link overhead but also ensured the reliability of routing. Lu et al. [120] designed a kind of routing topology that can guarantee the consistency of the double layer satellite network topology control strategy. Based on this general combination of centralized and distributed routing strategies, the proposed routing protocol has stronger robustness, which strengthened the independence of routing, improved the ability of the routing security, and reduced the routing time delay.
考虑到卫星网络拓扑结构复杂，无法直接使用地面网络的路由协议。因此，为了适应卫星网络的动态变化，基于星际链路的专用路由算法的发展成为通信领域的研究热点。随着卫星网络结构从单层向多层的发展，学者们提出了多种路由算法。具体而言，Hou等[119]提出了一种高度可靠的路由算法，该算法使用时间感知数据挖掘算法预测每个节点连接的动态变化，并利用时空图模型绘制拓扑结构，不仅降低了链路总开销，而且保证了路由的可靠性。Lu等[120]设计了一种能够保证双层卫星网络拓扑控制策略一致性的路由拓扑结构。基于这种集中式和分布式路由策略的通用组合，所提出的路由协议具有更强的鲁棒性，增强了路由的独立性，提高了路由安全性，降低了路由时延。
Not only that, every time a satellite fails, all its fixed topologies need to be updated. For many discrete topologies, centralized routing schemes have large computation and storage overhead, and slow response to random satellite faults. The centralized routing algorithm based on virtual topology [121] adopted discrete-time network model to deal with time-varying topology. Due to the usage of interstellar links in LEO satellite networks, Chang et al. [122] proposed a new framework for the topological design problem. In the proposed framework, an LEO satellite network as a finite-state automaton (FSA) was modeled, and this FSA-based framework allowed the link assignment problem in LEO satellite networks to be treated as a set of link assignment problems in fixed topology networks. In the network architecture of the ground and space parts of the satellite network, the space segment proposal was geared toward a time division multiple access (TDMA)-based satellite network comprising many nongeostationary satellites, while the ground segment proposal did not put any restriction on the satellite network. Mauger and Rosenberg [123] believed that it will play a major role in providing an efficient way to deliver multimedia services over asynchronous transfer mode (ATM) networks. The space segment proposal aimed at providing a robust easily managed and controlled architecture, so as to guarantee QoS by using a combination of connectionless and connection-oriented techniques.
不仅如此，每当卫星发生故障时，其所有固定拓扑都需要更新。对于许多离散拓扑，集中式路由方案具有较大的计算和存储开销，并且对随机卫星故障的响应速度较慢。基于虚拟拓扑的集中式路由算法[121]采用离散时间网络模型处理时变拓扑。由于在LEO卫星网络中使用了星际链路，Chang等[122]提出了拓扑设计问题的新框架。在所提出的框架中，将LEO卫星网络建模为有限状态自动机（FSA），该基于FSA的框架允许将LEO卫星网络中的链路分配问题视为固定拓扑网络中的一组链路分配问题。在卫星网络地面和空间部分的网络结构中，空间段提案面向由许多非对地静止卫星组成的基于时分多址（TDMA）的卫星网络，而地面段提案则没有对卫星网络施加任何限制。Mauger和Rosenberg[123]认为，它将在提供一种通过异步传输模式（ATM）网络提供多媒体服务的有效方式方面发挥重要作用。空间段提案旨在提供一个强大的易于管理和控制的架构，以便通过使用无连接和面向连接的技术的组合来保证QoS。
Due to the short orbital period, fast network topology change and small coverage of LEO satellite network (compared with GEO), its routing algorithm is representative, which lays a foundation for studying routing algorithms of other types of satellite network. Although distributed routing may incur nominal overhead, its fault-tolerance ability is limited due to the lack of global topology information. et al. [124] presented an active distributed QoS routing strategy, which used three types of mobile agents to accomplish the routing process, and adopted traffic prediction to avoid congestion and make the routing decision. Finally, link duration was introduced into a cost function to find an optimum path that simultaneously satisfies multiple QoS constraints. Ekici et al. [24] extended the data gram routing algorithm for LEO satellite networks, which generated minimum propagation delay paths between source and destination. The routing decisions were made independently for each packet, and the packets were routed between the logical locations, which were embodied by the closest satellites. The algorithm brought no overhead since the satellites do not exchange any topology information, and it also avoided congestions and failures on making local decision at a low cost. Tang et al. [125] formulated the multi-path cooperative routing problem, and then proposed a network coding based multipath cooperative routing protocol for LEO satellite networks to improve the throughput.
由于LEO卫星网络的轨道周期短、网络拓扑变化快、覆盖范围小（与GEO相比），其路由算法具有代表性，为研究其他类型卫星网络的路由算法奠定了基础。尽管分布式路由可能会产生名义开销，但由于缺乏全局拓扑信息，其容错能力受到限制。 等[124]提出了一种主动分布式QoS路由策略，该策略使用三种类型的移动代理来完成路由过程，并采用流量预测来避免拥塞并做出路由决策。最后，将链路持续时间引入成本函数，以找到同时满足多个QoS约束的最优路径。Ekici等[24]扩展了LEO卫星网络的数据报路由算法，该算法在源和目标之间产生了最小的传播延迟路径。每个数据包的路由决策是独立做出的，数据包在逻辑位置之间路由，这些位置由最近的卫星表示。该算法没有带来开销，因为卫星不交换任何拓扑信息，而且还避免了以低成本做出本地决策的拥塞和失败。Tang等[125]提出了多路径协同路由问题，并提出了一种基于网络编码的LEO卫星网络多径协同路由协议，以提高吞吐量。
Furthermore, with the development of the underwater IoT, a large number of smart objects are deployed underwater to form an underwater wireless sensor network (UWSN) to facilitate the discovery of the vast undeveloped ocean. Routing protocols are not expensive in terms of data packet forwarding and energy consumption, and are the basis of sensory data collection and transmission in wireless sensor networks, however, considering that the underwater acoustic channel is easily affected by the marine environment, acoustic waves, etc., traditional routing protocols, such as the AODV protocol, cannot work well. Therefore, in view of the inaccuracy of underwater acoustic communication, Ghannadrezaii and Bousquet [126] proposed a secure multi-user network protocol that used the Janus standard in a hybrid cellular/self-organizing topology. In underwater networks, the usage of optimized flooding routing protocols to relay packets increases the packet transfer rate and reduces delay. Taking into account the resource-constrained UWSN environment, Saeed et al. [128] presented a secure and energy-saving collaborative routing protocol suitable for wireless sensor networks, which adopted cooperative routing to improve network performance. In their work, the minimum amount of calculation was used to achieve security, so that it remained suitable for the underwater environment.
此外，随着水下物联网的发展，大量的智能物体被部署在水下，形成水下无线传感器网络（UWSN），以促进发现广阔的未开发海洋。路由协议在数据包转发和能耗方面并不昂贵，是无线传感器网络中感知数据采集和传输的基础，但考虑到水声信道容易受到海洋环境、声波等的影响，传统的路由协议（如AODV协议）无法很好地工作。因此，鉴于水声通信的不准确性，Ghannadrezaii和Bousquet[126]提出了一种安全的多用户网络协议，该协议在混合蜂窝/自组织拓扑中使用Janus标准。在水下网络中，使用优化的泛洪路由协议来中继数据包可以提高数据包传输速率并减少延迟。考虑到UWSN环境资源受限，Saeed等[128]提出了一种适用于无线传感器网络的安全节能协同路由协议，该协议采用协同路由来提高网络性能。在他们的工作中，使用最少的计算量来实现安全性，使其仍然适用于水下环境。
In underwater sensor network, since most sensor nodes drift with hydrodynamics, this makes the task of locating sensor nodes very expensive and error-prone. This means that routing data packets in an end-to-end manner may not be energy efficient, and greedy hop-by-hop routing is considered a more appropriate strategy. In particular, Yao et al. [129] proposed a channel aware routing protocol without location constraints and energy saving, where data packets were forwarded from the source sensor node to the sink node in a hop-by-hop manner. Zhou et al. [130] presented an enhanced channel aware routing protocol (E-CARP) to implement a location and greedy hop-by-hop packet forwarding strategy, in order to select the most appropriate relay node at each point in time. Their experiments verified that this technology can significantly reduce communication costs and increase network capacity to a certain extent. Besides, Basagni et al. [127] designed a new cross-layer routing protocol for underwater wireless sensor networks, which adopted link quality information for cross-layer relay determination, and achieved significant performance in terms of end-to-end packet delay and energy consumption.
在水下传感器网络中，由于大多数传感器节点随流体动力学漂移，这使得定位传感器节点的任务非常昂贵且容易出错。这意味着以端到端方式路由数据包可能不节能，贪婪的逐跳路由被认为是更合适的策略。特别是，Yao等[129]提出了一种没有位置约束和节能的信道感知路由协议，其中数据包以逐跳方式从源传感器节点转发到接收器节点。周等[130]提出了一种增强型信道感知路由协议（E-CARP），以实现位置和贪婪逐跳数据包转发策略，以便在每个时间点选择最合适的中继节点。他们的实验验证了该技术可以显著降低通信成本，在一定程度上增加网络容量。此外，Basagni等[127]设计了一种新的水下无线传感器网络跨层路由协议，该协议采用链路质量信息进行跨层中继判定，在端到端分组时延和能耗方面取得了显著的性能。

C. Secure Handover Schemes
C. 安全移交方案

Handover refers to the process of transferring an ongoing call or data session from one logical/physical channel to another while minimizing service loss or interruption. Due to the high-speed mobility, a single satellite has limited coverage and short communication time. To ensure uninterrupted communication between nodes in the satellite network, it is inevitable to handover frequently within the signal range covered by the satellites. The handover schemes of the LEO satellites can be divided into two categories, i.e., inter-satellite handover and in-satellite (beam) handover. Inter-satellite handover occurs when the ground terminal changes from the coverage area of one satellite to another satellite, and beam handover refers to the ground terminal switching in different beam coverage areas of a multi-beam satellite. Table IX summarizes existing works on secure handover schemes.
切换是指将正在进行的呼叫或数据会话从一个逻辑/物理通道转移到另一个逻辑/物理通道的过程，同时最大限度地减少服务丢失或中断。由于高速移动，单颗卫星的覆盖范围有限，通信时间短。为了保证卫星网络中节点之间的通信不间断，不可避免地要在卫星覆盖的信号范围内频繁切换。低轨卫星的切换方案可分为两类，即星间切换和星内（波束）切换。当地面终端从一颗卫星的覆盖区域变为另一颗卫星时，就会发生星间切换，波束切换是指在多波束卫星的不同波束覆盖区域内进行地面终端切换。表九总结了关于安全移交计划的现有工作。
Up to now, the researchers have proposed a number of solutions to the problem of link layer handover. Specifically, Korcak and Alagoz [136] presented a virtual node handover algorithm, aiming to reduce latency and data loss. To achieve soft handover, the number of satellites per orbit is increased, and the improved system can also be represented by multistate virtual network. Wu et al. [138] described a graph-based satellite handover framework, which can support a variety of satellite handover strategies by setting the link weight according to different handover criterion. In the directed graph with satellite coverage cycle as node, the link that can be switched between two overlapping cycles was calculated in advance, and the process of satellite switching was regarded as path finding in the directed graph. Moreover, Zhao et al. [137] took into account the predictability of satellite orbits, and proposed a new seamless handover scheme based on preset-satellitechain (PSC) to provide uninterrupted satellite communication services. Through the preset base station controller and seamless satellite chain handover, service interruption caused by the periodic disconnection of inter-satellite links can be eliminated.
截至目前，研究人员已经针对链路层切换问题提出了许多解决方案。具体来说，Korcak和Alagoz[136]提出了一种虚拟节点切换算法，旨在减少延迟和数据丢失。为了实现软切换，增加了每个轨道的卫星数量，改进后的系统也可以用多状态虚拟网络来表示。Wu等[138]描述了一种基于图的卫星切换框架，该框架可以根据不同的切换标准设置链路权重，从而支持多种卫星切换策略。在以卫星覆盖周期为节点的有向图中，预先计算了两个重叠周期之间可切换的链路，并将卫星切换过程视为有向图中的寻路。此外，Zhao等[137]考虑了卫星轨道的可预测性，提出了一种新的基于预置卫星链（PSC）的无缝切换方案，以提供不间断的卫星通信服务。通过预置的基站控制器和无缝的卫星链切换，可以消除星间链路周期性断线造成的业务中断。
Note that the handover of satellite nodes will inevitably affect the communication security. To alleviate the out-oforder packets caused by delay variation, Hu et al. [140] introduced a packet cache-forward method based on improved Bayesian outlier detection. The method used an access node satellite to cache all received packets in a short time when handover occurred and forward them out in order, and their
需要注意的是，卫星节点的移交将不可避免地影响通信安全。为了缓解延迟变化引起的无序数据包，胡等[140]引入了一种基于改进贝叶斯异常值检测的数据包缓存转发方法。该方法使用接入节点卫星在发生切换时在短时间内缓存所有接收到的数据包，并按顺序转发出去，并且它们的
TABLE IX 表九
Summary of Related Works on Secure Handover SchEmes for SAGSIN
SAGSIN安全切换SchEmes相关工作总结
Network structure 网络结构 Reference Target Proposed algorithm/scheme
建议的算法/方案		 Results
SAGIN [25] Handover delay 交接延迟
A handover scheme based on
基于以下因素的移交方案
pre-authentication and security context
预身份验证和安全上下文
transmission
Improved the security and
改进了安全性和
quality of communication,
沟通质量，
reduced the handover delay
减少了交接延迟
Satellite network 卫星网络 [135] A mobile management scheme
移动管理方案
Reduced service interruptions
减少服务中断
during satellite handover
卫星切换期间
[136] Link-layer handover 链路层切换 A virtual node based handover algorithm
一种基于虚拟节点的切换算法		 Reduced latency and data loss
减少延迟和数据丢失
A preset-satellite-chain-based
基于预设卫星链的
seamless handover scheme
无缝交接方案
Eliminated the problem of service
消除了服务问题
interruption, improved the quality of
中断，提高质量
satellite communication 卫星通信
Handover path 交接路径
A graph-based satellite 基于图形的卫星
handover framework 移交框架
Better flexibility, verified the
更好的灵活性，验证了
effectiveness of the handover framework
移交框架的效力
Resource reservation 资源预留
A strategy based on probability adaptive
一种基于概率自适应的策略
reservation
Reduced the connection blocking
减少了连接阻塞
probability, provided satisfactory
概率，提供令人满意
service quality for users
为用户提供优质服务
Packet out of order
数据包乱序
A packet cache forwarding method based
一种基于数据包缓存转发的方法
on improved Bayesian outlier detection
改进的贝叶斯异常值检测
Eliminated packet disorder, improved
消除包紊乱，改善
TCP performance, reduced the error rate
TCP性能，降低错误率
Frequent handover 频繁交接
An ultra-dense handover scheme
超密集交接方案
based on user center
基于用户中心
Improved user communication quality,
提高用户通信质量，
reduced handover delay 减少交接延迟
Resource allocation 资源配置 An adaptive handover scheme
自适应切换方案
Improved the overall system
改进了整个系统
performance, reduced the blocking
性能，减少阻塞
probability of handover connections
切换连接的概率
Resource optimization 资源优化
A dynamic allocation method based on
一种基于
handover prediction 移交预测
Improved system gain, provided better
提高系统增益，提供更好的
performance in terms of throughput
在吞吐量方面的表现
capacity and packet loss rate
容量和丢包率
Handover time 交接时间
scheduling
An active group handover scheme adopting
采用积极的集团交接方案
a non-cooperative game method
一种非合作的博弈方法
Reduced congestion 减少拥堵
probability during handover
移交期间的概率
[145] Handover priority 移交优先权
An ocean current prediction model
洋流预测模型
based on ML
Reduced the switching delay,
降低了开关延迟，
improved the communication quality
提高通信质量
Underwater network 水下网络 [146] Handover prediction 切换预测
A zero-scan intelligent handover prediction
零扫描智能切换预测
scheme based on ML
基于ML的方案
Reduced the overhead of channel
降低通道开销
prediction and power consumption
预测和功耗
experimental results verified that the proposed solution can improve the performance of TCP drastically. Ding et al. [25] presented a new handover scheme based on pre-authentication and security context transfer, which optimized the selection of satellite nodes. The scheme not only ensured fast and safe node handover, but also improved the security and quality of communication. Moreover, Gaber et al. [135] proposed a mobility management solution, which can reduce signaling overhead and minimize service interruption during satellite handover. From the perspective of adaptive probability retention, Chen et al. [139] designed a switching management scheme. In the strategy, the bandwidth reservation decision was made based on user location information and handover probability. Furthermore, to realize the precise adaptive reservation of the handover connection, their system can adjust the reserved bandwidth according to the current situation of the network to cope with the sudden traffic.
实验结果验证了所提方案能够显著提高TCP的性能。Ding等[25]提出了一种基于预认证和安全上下文传输的新型切换方案，优化了卫星节点的选择。该方案不仅保证了节点的快速安全切换，还提高了通信的安全性和质量。此外，Gaber等[135]提出了一种移动管理解决方案，该解决方案可以减少信令开销并最大限度地减少卫星切换期间的服务中断。Chen等[139]从自适应概率保留的角度设计了一种切换管理方案。在策略中，带宽预留决策是基于用户位置信息和切换概率做出的。此外，为了实现切换连接的精确自适应预留，他们的系统可以根据网络的当前情况调整预留带宽，以应对突发流量。
Besides, some scholars studied the security handover schemes in the low orbit satellite network, and improved the system security considering the problems between the satellite and the ground user. In particular, Li et al. [141] proposed a user-centric ultra-high-density low-orbit satellite handover scheme. Through utilizing the storage capacity of satellites to improve the communication quality of users, and buffering downlink data of users in multiple satellites at the same time, the ground users can realize seamless handover and access satellites always with the best link quality. Rahman et al. [142] designed an adaptive handover scheme to address handover issues in LEO satellite networks. The scheme provides users with improved QoS by keeping connection blocking probability and connection dropping probability at an acceptable level, which can improve the overall system performance significantly.
此外，有学者研究了低轨道卫星网络的安全切换方案，并考虑了卫星与地面用户之间的问题，提高了系统安全性。特别是，Li等[141]提出了一种以用户为中心的超高密度低轨道卫星切换方案。通过利用卫星的存储容量提高用户的通信质量，同时缓冲多颗卫星用户的下行链路数据，地面用户可以始终以最佳链路质量实现卫星的无缝切换和接入。Rahman等[142]设计了一种自适应切换方案来解决LEO卫星网络中的切换问题。该方案通过将连接阻塞概率和连接丢弃概率保持在可接受的水平，为用户提供了改进的 QoS，从而可以显着提高整体系统性能。
Li et al. [143] studied the optimization of multi-service resources in low orbit satellite downlink. Combining the advantages of positioning technology and conventional satellite orbit, a new dynamic beam allocation method based on handover prediction was proposed, which maximized the utilization of intensive beam resources and improved the system gain. Zhu et al. [144] proposed a new active group handover scheme for low-orbit satellite networks, which divided the users with similar patterns into multiple groups. Furthermore, the authors investigated the correlation between groups to make active handover decisions, and the handover time scheduling problem was then modeled as a non-cooperative game aiming to minimize the switching delay under transmission capacity constraints.
Li等[143]研究了低轨道卫星下行链路中多业务资源的优化。结合定位技术与常规卫星轨道的优势，提出了一种基于切换预测的动态波束分配方法，最大限度地利用了集约化波束资源，提高了系统增益。Zhu等[144]提出了一种新的低轨道卫星网络主动分组切换方案，将具有相似模式的用户分成多个分组。此外，作者研究了组间做出主动切换决策的相关性，然后将切换时间调度问题建模为一个非合作博弈，旨在最小化传输容量约束下的切换延迟。
For UWSNs, some nodes move by themselves like underwater robots (e.g., AUV), and some nodes move passively due to water currents and other environmental influences. For passive nodes, there are usually difficulties in estimating the location of the nodes. Therefore, it is necessary to study the switching technology of nodes. Park et al. [145] proposed an ocean current prediction method based on ML, and built a predictive model by collecting ocean data and predicted the movement of nodes that passively move following the tide in the underwater environment. Considering that sensor nodes may be affected by tides or other environmental factors, Park and Jo [146] presented a handover priority decision method suitable for the Internet of Underwater Things network, where the problem was solved by adopting location probability and zero scanning.
对于UWSN，一些节点像水下机器人（例如AUV）一样自行移动，而一些节点由于水流和其他环境影响而被动移动。对于被动节点，通常难以估计节点的位置。因此，有必要研究节点的交换技术。Park等[145]提出了一种基于ML的洋流预测方法，通过收集海洋数据构建了预测模型，预测了水下环境中随潮汐被动移动的节点的运动。考虑到传感器节点可能受到潮汐或其他环境因素的影响，Park和Jo[146]提出了一种适用于水下物联网网络的切换优先级决策方法，该方法通过采用位置概率和零扫描来解决该问题。
TABLE X 表十
Summary of Related Works on SECURE Key Management for SAGSIN
SAGSIN安全密钥管理相关工作总结
Network structure 网络结构 Reference Ways Proposed algorithm/scheme
建议的算法/方案		 Advantages Results
Satellite network 卫星网络 Hierarchy
A hierarchy multicast key
层次结构多播键
management method 管理方法
Suitable for hybrid network,
适用于混合网络，
carried out packet communication
进行分组通信
Enhanced the security 增强了安全性
of communication data 通信数据
in the network 在网络中
Dynamics
A multi-layer Internet 多层互联网
protocol security and LKH
协议安全和LKH
interconnection solution
互连解决方案
Considered the life-cycle
考虑生命周期
key management costs 密钥管理成本
of multicast connections
组播连接数
Improved network utilization,
提高网络利用率，
connected with satellite links
通过卫星链路连接
Group
A centralized identity-based
基于身份的集中式
key management scheme 密钥管理方案
Applied to space network, can
应用于空间网络，可以
resist quantum attack 抵抗量子攻击
Improved the efficiency of
提高了效率
group key distribution 组密钥分发
in noisy channel, had good
在嘈杂的频道里，有很好的
security and reliability
安全可靠
A threshold value- 阈值 -
based group key 基于组密钥
management scheme 管理方案
Set up group key sharing
设置组密钥共享
distributed center 分布式中心
Saved system resources, had
节省了系统资源，有
stronger security, lower computing
更强的安全性，更低的计算量
and communication costs 和通信成本
A based key management
基于密钥管理
unit, satellite terminal
单元， 卫星终端
certificate algorithm 证书算法
Applied to tactical satellite
应用于战术卫星
networks
Improved key distribution speed,
提高密钥分发速度，
had high performance 具有高性能
A based on near-space
A 基于近空间
network hierarchy and 网络层次结构和
domain structure scheme 域结构方案
Avoided the problem of single
避免了单身的问题
points of failure 故障点
Reduced the computing 减少了计算量
overhead caused by the
由
handover of high-speed nodes
高速节点交接
A lightweight group key
轻量级组密钥
protocol, a secure switching
协议，安全交换
authentication scheme based
基于身份验证方案
on group key
Considered the highly dynamic
被认为是高度动态的
network topology 网络拓扑
Reduced computation and 减少计算量和
communication overhead, 通信开销，
improved security 提高安全性
multi-
group
A key management diagram
密钥管理图
scheme
Applied to satellite multi-group
应用于卫星多组
communication with large number
与大量通信
of dynamic users 动态用户数
Reduced the overhead of
减少了
satellite storage and 卫星存储和
communication
QKD
A new routing and key
新的路由和密钥
allocation algorithm 分配算法
Applied to double layer quantum
应用于双层量子
satellite network 卫星网络
Improved the success rate of the
提高了
key relay service 密钥中继服务
A routing and key allocation
路由和密钥分配
algorithm for secure multicast
安全组播算法
services
Applied to two-layer quantum
应用于双层量子
satellite network, key distribution
卫星网络，密钥分发
is carried out by relay tree
由中继树执行
Saved more key resources,
节省了更多关键资源，
obtained higher security
获得更高的安全性
probability
Underwater network 水下网络
A based on air-water channel
基于空气-水通道的A
system model
Discussed the influence of wind
讨论了风的影响
speed and incident angle on
速度和入射角开启
systematic error 系统误差
In general, reduced the error rate
一般来说，降低了错误率
A multi-hop quantum key
多跳量子密钥
distribution scheme 分配方案
Used for underwater turbulence
用于水下湍流
channels
Improved the realization
提高实现率
distance, mitigated turbulence
距离，减轻湍流
induced fading 诱导衰落
Multi-
party
A multi-party key generation
多方密钥生成
scheme against active attack
针对主动攻击的方案
Obtained the uncertainty of
获得的不确定性
source sequences from legitimate
来自合法的源序列
and eavesdropping nodes 和窃听节点
Had high the key security
有高的钥匙安全性
- A new key generation system
新的密钥生成系统
Used channel properties to
将通道属性用于
generate keys 生成密钥
Had high computing and
具有高计算能力和
communication efficiency
通信效率

D. Secure Key Management
D. 安全密钥管理

Key management means implementing security management of user secret keys during the whole life cycle. The purpose of key management is to ensure the authenticity and validity of the keys and then to guarantee the safety of the data encryption system. In the research of modern cryptography, encryption and decryption algorithms are generally public. When the cryptography algorithm is determined, the confidentiality degree of the cryptography system completely depends on the level of confidentiality of the key. Therefore, key management occupies an important position in the process of data privacy. If the key is not properly protected and managed, no matter how sophisticated the algorithm is designed, the network is fragile. Table summarizes existing works on secure key management for SAGSIN.
密钥管理是指对用户密钥进行全生命周期的安全管理。密钥管理的目的是保证密钥的真实性和有效性，进而保证数据加密系统的安全。在现代密码学的研究中，加密和解密算法通常是公开的。在确定密码算法时，密码系统的保密程度完全取决于密钥的保密程度。因此，密钥管理在数据隐私过程中占有重要地位。如果密钥没有得到适当的保护和管理，无论算法设计得多么复杂，网络都是脆弱的。表 总结了SAGSIN安全密钥管理的现有工作。
In recent years, there have been many related works on key management. Liu et al. [149] proposed a centralized identity-based key management scheme, which uses PKC public key cryptosystem to improve the efficiency of group key distribution on noisy channels. Xue et al. [153] designed a lightweight group key agreement protocol suitable for specific secure networks to ensure security and applicability. Moreover, considering the high dynamics of the network topology, a secure handover authentication scheme based on group keys was designed to reduce the overhead of handover authentication.
近年来，关于密钥管理的相关著作不少。Liu等[149]提出了一种基于身份的集中式密钥管理方案，该方案使用PKC公钥密码系统来提高嘈杂信道上群密钥分发的效率。Xue等[153]设计了一种适用于特定安全网络的轻量级组密钥协议，以确保安全性和适用性。此外，考虑到网络拓扑的高动态性，设计了一种基于组密钥的安全切换认证方案，以降低切换认证的开销。
In the satellite network, Howarth et al. [148] proposed an interworking solution between multi-layer IPsec and LKH for encrypted multi-cast traffic transmitted via satellites, with the target to reduce the key management traffic and to minimize satellite resource usage. Roy-Chowdhury et al. [147] studied the protocol for secure group communication in a hybrid satellite network, and presented a hierarchical approach to group key management. The proposed approach was verified to be robust, scalable, and suitable for the characteristic topology of the hybrid network. Jiao et al. [150] designed a group key management scheme of satellite network based on ellipse curve cipher and threshold value technique, which has the function of authentication and does not need a secure channel, and the system resources were saved.
在卫星网络方面，Howarth等[148]提出了一种多层IPsec和LKH之间的互通解决方案，用于通过卫星传输的加密组播流量，目标是减少密钥管理流量并最大限度地减少卫星资源使用。Roy-Chowdhury等[147]研究了混合卫星网络中用于安全群组通信的协议，并提出了一种分层的群密钥管理方法。所提出的方法被验证为鲁棒性、可扩展性，并且适用于混合网络的特征拓扑结构。Jiao等[150]设计了一种基于椭圆曲线密码和阈值技术的卫星网络群密钥管理方案，该方案具有认证功能，不需要安全信道，节省了系统资源。
Moreover, Elmasri et al. [151] proposed three novel mechanisms, including the authentication of satellite terminal operator by adopting key management unit to prevent illegitimate users from using the terminal, the certificates sharing verification key management for satellite terminals, and new group key management algorithm. Wang et al. [152] designed a group key management scheme based on the hierarchical and domain structure of Near-Space Network, which can efficaciously reduce the rekeying overhead caused by switch actions of high-speed nodes. Sun and Ma [154] proposed a new scheme for satellite multi-group key management, in which subgroups are established according to the access ability of user. Their scheme fulfilled forward and backward security and has good scalability, and reduced the consumption of the satellite in rekeying.
此外，Elmasri等[151]提出了三种新的机制，包括通过采用密钥管理单元防止非法用户使用终端对卫星终端运营商进行认证，卫星终端的证书共享验证密钥管理，以及新的组密钥管理算法。Wang等[152]设计了一种基于近空网络的分层和域结构的群密钥管理方案，可以有效降低高速节点切换操作带来的重密钥开销。Sun和马[154]提出了一种新的卫星多组密钥管理方案，其中根据用户的访问能力建立子组。他们的方案实现了前向和后向安全性，并具有良好的可扩展性，并减少了卫星在重新密钥中的消耗。
Furthermore, for secure multicast service in two-layer quantum satellite networks, He et al. [156] proposed a routing and key distribution algorithm, which generates a key relay tree for key forwarding. Experimental results validated that the algorithm can reduce the waste of key resources, and has better performance in terms of safety probability. Huang et al. [155] presented a new double-layer quantum satellite network architecture based on a trusted repeater, and implemented a quantum key pool (QKP) to relay the key of the ground station to maximize the generation of secret keys. This scheme was showed to be able to improve the success probability of key relay services significantly.
此外，针对两层量子卫星网络中的安全组播服务，He等[156]提出了一种路由和密钥分发算法，该算法为密钥转发生成密钥中继树。实验结果验证了该算法能够减少关键资源的浪费，在安全概率方面具有更好的性能。Huang等[155]提出了一种基于可信中继器的新型双层量子卫星网络架构，并实现了量子密钥池（QKP）来中继地面站的密钥，以最大限度地生成密钥。该方案能够显著提高关键中继业务的成功概率。
With the increasing communication requirements of underwater systems (such as underwater sensor network, submarine and various underwater vehicles, etc.), QKD based on airwater channel is crucial to ensure the absolute security of underwater systems, and QKD system model was established by some researchers [157]. Specifically, Raouf et al. [158] studied a multi-diving underwater , in which intermediate nodes help the key distribution between the source node and the destination node. They considered deploying passive relays, simply redirecting the qubits to the next relay node or receiver without any measurement. Moreover, aiming at the problem of how to extract highly secret keys for any number of legitimate nodes in the underwater acoustic environment with multi-path and Doppler effects, Xu et al. [159] proposed a multi-party key generation scheme for underwater acoustic channels against active attacks. The authors accurately described the -order renyi entropy under multipath and Doppler effects, and obtained the uncertainty of source sequences of legitimate nodes and eavesdropping nodes. Considering that the demand for the security of underwater acoustic communication is growing rapidly, Liu et al. [160] designed a new key generation system, which is easy to implement, and has high calculation. Besides, the communication efficiency in this system is robust to the changing marine environment in time and space.
随着水下系统（如水下传感器网络、潜艇和各种水下航行器等）通信需求的增加，基于气水通道的QKD对于保证水下系统的绝对安全至关重要，一些研究者建立了QKD系统模型[157]。具体来说，Raouf等[158]研究了水 下多潜水，其中中间节点有助于源节点和目标节点之间的密钥分配。他们考虑部署无源中继，只需将量子比特重定向到下一个中继节点或接收器，而无需进行任何测量。此外，针对水声环境中多路径和多普勒效应下任意数量的合法节点如何提取高度机密密钥的问题，Xu等[159]提出了一种针对主动攻击的水声信道的多方密钥生成方案。作者准确地描述了多径和多普勒效应下的 -阶雷尼熵，得到了合法节点和窃听节点源序列的不确定性。考虑到水声通信安全需求快速增长，Liu等[160]设计了一种新的密钥生成系统，该系统易于实现，计算能力高。此外，该系统的通信效率对不断变化的海洋环境具有鲁棒性。

E. Intrusion Detection System
E. 入侵检测系统

Intrusion detection system (IDS) [167] is a kind of network security equipment that monitors the network transmission in real-time. It sends out alarms or takes active response
入侵检测系统（IDS）[167]是一种实时监控网络传输的网络安全设备。它发出警报或采取主动响应
Fig. 12. Classification of intrusion detection algorithms.
图 12.入侵检测算法的分类。
measures when the suspicious transmission is found. This can effectively protect the network from internal and external intruders. The intrusion detection technology can be generally divided into misuse detection and anomaly detection. Misuse detection observes the situation of resource utilization through predefined intrusion patterns, so that it is not possible to identify attacks that have not yet occurred. While abnormal detection adopts neural networks, support vector machines (SVM), and other learning algorithms to detect abnormal behaviors of monitored nodes, where the unusual behaviors of users will be also tracked. Although IDS techniques can detect new attacks, its computation cost is extremely expensive. Table XI summarizes existing works on IDS for SAGSIN.
在发现可疑传播时采取措施。这样可以有效地保护网络免受内部和外部入侵者的侵害。入侵检测技术一般可分为误用检测和异常检测。滥用检测通过预定义的入侵模式观察资源利用情况，因此无法识别尚未发生的攻击。异常检测采用神经网络、支持向量机（SVM）等学习算法来检测被监控节点的异常行为，同时对用户的异常行为进行跟踪。尽管IDS技术可以检测新的攻击，但其计算成本非常昂贵。表十一总结了关于SAGSIN的IDS的现有工作。
Traditional IDS is based on the centralized model, where IDS sends the network state information collected by the detectors to the control center for analysis and processing, as shown in Fig. 12. However, regarding large-scale heterogeneous network environment and distributed coordinated attacks, the model is inefficient and often has the bottleneck of single fault location. Analyzing the characteristic of the satellite network, Wen-Bo et al. [161] proposed a hierarchical distributed IDS model for satellite network, and designed the cooperation mechanism of satellite intrusion detection agent within and between security domains. Simulation results showed that the IDS model and cooperation mechanism were suitable for satellite network.
传统的IDS基于集中式模型，IDS将检测器采集到的网络状态信息发送到控制中心进行分析和处理，如图12所示。然而，对于大规模异构网络环境和分布式协同攻击，该模型效率低下，往往存在单故障定位的瓶颈。温等[161]分析了卫星网络的特点，提出了卫星网络的分层分布式IDS模型，并设计了安全域内和安全域间卫星入侵检测代理的协作机制。仿真结果表明，IDS模型和配合机制适用于卫星网络。
To solve the problem of limited satellite network resources and high privacy requirements, Li et al. [162] presented a distributed federated learning (FL) based IDS in satelliteterrestrial integrated networks, where FL was adopted to reasonably allocate resources in each domain and malicious traffic was analyzed and blocked, especially DDoS attacks. The results showed that this method is superior to traditional deep learning based intrusion detection methods in recognition rate of malicious traffic, packet loss rate, CPU utilization, etc. Mover, regarding UAV-ground networks, Sedjelmaci et al. [163] designed a novel IDS that operated at the UAV and ground station levels. The normal behaviors of a UAV was modeled with a set of rules, where the IDS agent wades all the rules and exchange them with its neighbors to detect malicious anomalies that threaten the network.
针对卫星网络资源有限、隐私要求高的问题，Li等[162]提出了一种基于分布式联邦学习（FL）的卫星地面集成网络IDS，采用FL在各域合理分配资源，分析和阻断恶意流量，特别是DDoS攻击。结果表明，该方法在恶意流量识别率、丢包率、CPU利用率等方面优于传统的基于深度学习的入侵检测方法。关于无人机地面网络，Sedjelmaci等[163]设计了一种新型IDS，可在无人机和地面站级别运行。无人机的正常行为是用一组规则建模的，其中IDS代理涉足所有规则并与邻居交换它们，以检测威胁网络的恶意异常。
Underwater acoustic networks are widely used in fields such as climate change monitoring, pollution control and tracking,
水声网络广泛应用于气候变化监测、污染控制和跟踪等领域。
TABLE XI 表十一
SUMmary of RELATED WORKS ON INTRUSION DETECTION SYSTEM fOR SAGSIN
入侵检测系统fOR SAGSIN相关著作综述
Network structure 网络结构 Reference
Proposed
algorithm/scheme 算法/方案
Advantages Contributions Results
Satellite network 卫星网络
A cooperative 合作社
mechanism for satellite 卫星机制
intrusion detection 入侵检测
Adopted a security domain
采用安全域
partition strategy based on
基于分区策略
delayed testing 延迟测试
Hierarchical distributed
分层分布式
intrusion detection model
入侵检测模型
Implemented cooperative 实施合作社
intrusion detection among
入侵检测
agents on satellite nodes
卫星节点上的代理
Satellite-terrestrial 卫星-地面
integrated network 集成网络
A distributed IDS, an
分布式 IDS、
adaptive algorithm 自适应算法
Usources in each domain,
每个域中的资源，
optimized satellite network
优化的卫星网络
topology
Solved the problem of
解决了以下问题
limited resources, high 资源有限，高
privacy requirements, 隐私要求，
reduced handover of links
减少链接的交接
Improved performance in 改进了性能
malicious traffic, 恶意流量，
recognition rate, packet
识别率、数据包
loss rate, CPU utilization
丢失率、CPU 使用率
UAV-ground
network
A new intrusion 新的入侵
detection and response 检测和响应
scheme
Used layered intrusion 使用的分层入侵
detection, supported vector
检测，支持的载体
anomaly detection 异常检测
UAV and ground, detected
无人机和地面，检测到
cyber attacks on UAV
对无人机的网络攻击
Higher detection rate, lower
检出率更高，更低
false positive rate, lower
误报率，更低
communication overhead 通信开销
Underwater network 水下网络
tactical surveillance, and offshore exploration. Recently autonomous underwater robots have been increasingly used in marine applications to obtain underwater information, and under the guidance of the intelligent topology control algorithm, the robots can effectively adjust its position, to realize the ideal UWSN configuration. To detect any hostile or accidental underwater intrusion, a suitable application is the utilization of automated submarines in surveillance tasks, which are evenly distributed around port entrances or large civilian or military maritime vessels (such as aircraft carriers).
战术监视和海上勘探。近年来，自主水下机器人越来越多地用于海洋应用中获取水下信息，并在智能拓扑控制算法的指导下，机器人可以有效地调整其位置，实现理想的UWSN配置。为了检测任何敌对或意外的水下入侵，一个合适的应用是在监视任务中使用自动潜艇，这些潜艇均匀分布在港口入口或大型民用或军用海上船只（如航空母舰）周围。
Zou et al. [164] proposed a topology control mechanism based on a particle swarm optimization algorithm, i.e., 3D particle swarm optimization algorithm, which was used for underwater robots in unknown 3D underwater space. Moreover, in the underwater sensor network, Sun and Shi [165] presented a new deployment architecture, which uses intrusion detection sensors to construct an underwater sensor barrier near the underwater channel that the target may pass through. After that, a method to maximize the coverage of the entire monitoring area was given using a limited number of sensor nodes, and the Voronoi diagram is used to determine which sensor nodes at the same depth are redundant in the water.
Zou等[164]提出了一种基于粒子群优化算法的拓扑控制机制，即三维粒子群优化算法，用于未知三维水下空间的水下机器人。此外，在水下传感器网络中，Sun和Shi[165]提出了一种新的部署架构，该架构使用入侵检测传感器在目标可能通过的水下通道附近构建水下传感器屏障。之后，给出了一种使用有限数量的传感器节点来最大化整个监测区域覆盖范围的方法，并使用Voronoi图来确定相同深度的哪些传感器节点在水中是多余的。
Furthermore, in underwater combat communication networks, underwater acoustic communication is easily affected by underwater processing noise and external interference. The external intrusion feature is entanglement with the underwater acoustic interference. Considering that traditional detection methods took the pattern threshold matching method, once entanglement feature causes distortion, it is hard to detect the intrusion feature of underwater combat communication networks effectively. Tian-Jie [166] proposed an improved entangled intrusion signal detection method based on fuzzy SVM algorithm, where the principal component analysis method was used to reduce the complexity of the data, and to delete the redundant data. Their work showed that he proposed algorithm can effectively detect the entangled intrusion signal in the underwater combat communication network, the error detection rate is lower than that of traditional methods, and the detection accuracy is greatly improved.
此外，在水下作战通信网络中，水声通信容易受到水下加工噪声和外界干扰的影响。外部侵入特征是与水声干扰的纠缠。考虑到传统的检测方法采用模式阈值匹配方法，一旦纠缠特征导致失真，就很难有效检测水下作战通信网络的入侵特征。Tian-Jie[166]提出了一种基于模糊支持向量机算法的改进纠缠入侵信号检测方法，采用主成分分析方法降低数据复杂度，去除冗余数据。他们的工作表明，他提出的算法能够有效地检测水下作战通信网络中的纠缠入侵信号，错误检测率低于传统方法，检测精度大大提高。
Remarks: In addition to the defensive measures discussed above, the attacks that exist in SAGSIN can be resolved in other ways. Specifically, backup and restore can be used to solve the issues of data security causing by the physical threats [168]. Authentication service is to verify an aircraft or ground identity, and to assure the recipient that the message is from the source as it claims to be. It may involve confirming the identity of a person by validating their identity documents, verifying the authenticity of a website with a digital certificate. Network structure optimization is mainly carried out from three dimensions, including coverage, capacity and frequency. Moreover, there are also many secure transport protocols including SSL/TLS protocol, TinySec protocol, C-Sec protocol, etc. Among them, SSL/TLS protocol is used for securing communication on the network by ensuring data confidentiality, data integrity, and authenticity between the communicating party. Besides, in recent years, the rapid development of deep learning refreshes human beings' realization of AI technique, and the utilization of deep learning technology to improve the performance of SAGSIN network has emerged as a promising direction.
备注：除了上面讨论的防御措施外，SAGSIN中存在的攻击可以通过其他方式解决。具体来说，备份和恢复可用于解决物理威胁引起的数据安全问题[168]。身份验证服务用于验证飞机或地面身份，并向收件人保证消息来自其声称的来源。它可能涉及通过验证身份证件来确认一个人的身份，使用数字证书验证网站的真实性。网络结构优化主要从覆盖、容量和频率三个维度进行。此外，还有许多安全传输协议，包括SSL/TLS协议、TinySec协议、C-Sec协议等。其中，SSL/TLS协议用于通过保证通信方之间的数据机密性、数据完整性和真实性来保护网络上的通信。此外，近年来，深度学习的快速发展刷新了人类对AI技术的认识，利用深度学习技术提高SAGSIN网络的性能已成为一个有前途的方向。

VII. Discussions & Challenges Ahead
VII. 未来的讨论和挑战

A. Cross-Layer Attacks in SAGSIN
A. SAGSIN中的跨层攻击

As the core network architecture of 6G, SAGSIN, which has the inherent advantages of wide coverage, large capacity, etc., can be used in many fields including defense missions, maritime transportation, emergency rescue, and so on. Nevertheless, many new security issues will continuously emerge in its development. In actual SAGSIN applications, malicious attackers will not only attack a single protocol layer but use cross-layer attacks and hybrid attack strategies to cause greater threats and damage, making it more difficult
作为6G的核心网络架构，SAGSIN具有覆盖范围广、容量大等先天优势，可应用于国防任务、海上运输、应急救援等多个领域。然而，在其发展过程中，许多新的安全问题将不断出现。在实际的SAGSIN应用中，恶意攻击者不仅会攻击单个协议层，还会使用跨层攻击和混合攻击策略造成更大的威胁和破坏，使其更加困难
to be detected. For example, wormhole attacks that seriously threaten underwater sensor networks can be combined with misdirection attacks and drop attacks to modify or drop data packets. A mixed attack of the two is more difficult to detect and prevent.
待检测。例如，严重威胁水下传感器网络的虫洞攻击可以与误导攻击和丢弃攻击相结合，以修改或丢弃数据包。两者的混合攻击更难检测和预防。
Existing research on network attacks mostly focused on a single network layer, while cross-layer attacks have appeared more and more in recent years. In particular, Hossain and Xie [169] proposed a cross-layer attack in cognitive radiobased networks, called an off-sensing and route manipulation attack, where the attack is launched at a lower layer as an attack point but the ultimate goal is to manipulate the traffic around the attacker. Nagireddygari and Thomas [170] presented a MAC-TCP cross-layer attack, in which the attacker regularly preempts himself to use the shared channel, and affects TCP performance by creating large round-trip time (RTT) changes. Moreover, an interference-assisted cross-layer attack was designed by Zhang and Melodia by adopting a multi-hop infrastructure-free network [171].
现有的网络攻击研究大多集中在单个网络层，而跨层攻击近年来越来越多。特别是，Hossain和Xie[169]提出了一种基于认知无线电的网络中的跨层攻击，称为脱敏和路由操纵攻击，其中攻击在较低层作为攻击点发起，但最终目标是操纵攻击者周围的流量。Nagireddygari和Thomas[170]提出了一种MAC-TCP跨层攻击，其中攻击者经常抢占自己使用共享信道，并通过创建较大的往返时间（RTT）变化来影响TCP性能。此外，Zhang和Melodia通过采用多跳无基础设施网络设计了一种干扰辅助的跨层攻击[171]。
Regarding hybrid satellite networks, they are vulnerable to a series of attacks, including jamming attacks, eavesdropping attacks, etc., appearing in the network at the same time, which reduces the security of data in the network and makes data vulnerable to destruction. Ahmed et al. [172] proposed a stealth interference attack, that is, malicious nodes periodically generate false traffic to trigger false routing interruption, which can seriously reduce the network performance.
关于混合卫星网络，它们容易受到一系列攻击，包括干扰攻击、窃听攻击等，同时出现在网络中，降低了网络中数据的安全性，使数据容易受到破坏。Ahmed等[172]提出了一种隐性干扰攻击，即恶意节点周期性地产生虚假流量，触发虚假路由中断，严重降低网络性能。
In the future hierarchical SAGSIN, mixed attacks and crosslayer attacks between different layers are more likely to occur frequently, and they are becoming more and more destructive to the network, and malicious nodes in the network are getting harder to find. From single attack to hybrid attack, the attack methods are constantly being strengthened, leading to more processing and transmission delays in the network, which may have a more serious impact on the real-time communication in SAGSIN.
在未来的分层SAGSIN中，不同层之间的混合攻击和跨层攻击更有可能频繁发生，并且对网络的破坏力越来越大，网络中的恶意节点越来越难发现。从单一攻击到混合攻击，攻击手段不断强化，导致网络处理和传输时延较多，可能对SAGSIN的实时通信产生更严重的影响。

B. Cross-Layer Defense Countermeasures in SAGSIN
B. SAGSIN的跨层防御对策

To cope with cross-layer attacks and hybrid attacks, security defenses measures should also be cross-layer and hybrid, and traditional layered security strategies cannot defend against cross-layer and hybrid attacks perfectly. For cross-layer attacks and cross-layer network security, it is inappropriate to consider the problem from the perspective of only one layer, since we cannot specifically limit it to a certain layer many times. Moreover, traditional single-layer IDS may not achieve an optimal secure routing path and the detection of multi-layer security attacks.
应对跨层攻击和混合攻击，安全防御措施也应该是跨层、混合的，传统的分层安全策略无法完美防御跨层、混合攻击。对于跨层攻击和跨层网络安全，只从一层的角度来考虑是不合适的，因为我们不能多次明确地将其限制在某一层。此外，传统的单层IDS可能无法实现最优的安全路由路径和多层安全攻击的检测。
In wireless networks, a cross-layer-based IDS was designed, where the information between different layers can be used to effectively detect multi-layer hybrid attacks and cross-layer attacks with complex nature. Moreover, the switching behavior between attacks can also been detected by adopted this system [173], [174]. For hybrid attacks, malicious nodes can easily interrupt routing and access communications without authentication. To deal with this, Jothilakshmi et al. [175] proposed a cross-layer IDS, which used the available information between different layers of the protocol stack to detect malicious nodes and different kinds of DoS attacks.
在无线网络中，设计了一种基于跨层的IDS，利用不同层之间的信息，有效检测多层混合攻击和复杂性质的跨层攻击。此外，通过采用该系统[173]，[174]也可以检测攻击之间的切换行为。对于混合攻击，恶意节点可以很容易地中断路由并访问通信，而无需身份验证。为了解决这个问题，Jothilakshmi等[175]提出了一种跨层IDS，它利用协议栈不同层之间的可用信息来检测恶意节点和不同类型的DoS攻击。
In order to deal with the destructive hybrid attacks in the network. Aryai and Binnu [176] proposed a cross-layer method that adopted mobile agents to detect and prevent sinkhole attacks. In consideration of the information optimization between network layers and nodes, Xu et al. [177] established a cross-layer security framework and introduced keys to establish a scheme so as to improve the network communication security. Vinayagam et al. [178] proposed a cross-layer method to detect malicious nodes in the network, where the information of malicious nodes are broadcasted to other nodes in the network. This method was proved to have a better performance compared with traditional methods against cross-layer attacks. Moreover, To improve the security of the physical layer and reduce the interference in SAGSIN, Ward and Younis [179] proposed a cross-layer anonymous enhancement technique. According to their work, a cross-layer implementation of the distributed beam can successfully mislead the attacker's situation and make the attack far away from the base station.
为了应对网络中的破坏性混合攻击。Aryai和Binnu[176]提出了一种跨层方法，采用移动代理来检测和防止天坑攻击。考虑到网络层和节点之间的信息优化，Xu等[177]建立了跨层安全框架，并引入了密钥来建立方案，以提高网络通信安全性。Vinayagam等[178]提出了一种跨层方法来检测网络中的恶意节点，其中恶意节点的信息被广播到网络中的其他节点。与传统方法相比，该方法在对抗跨层攻击方面具有更好的性能。此外，为了提高物理层的安全性并减少对SAGSIN的干扰，Ward和Younis[179]提出了一种跨层匿名增强技术。根据他们的工作，分布式波束的跨层实现可以成功地误导攻击者的情况，并使攻击远离基站。
UWSN is a new type of underwater network system, which is vulnerable to malicious attacks, and the layered security schemes can no longer protect UWSN from hybrid attacks [180]. To cope with this, Ma [181] proposed a cross-layer trust model and node selection algorithm for UWSN, and their experimental results showed that the cross-layer trust model can effectively improve the system security and network performance, and defend against the attacks from malicious nodes.
UWSN是一种新型的水下网络系统，容易受到恶意攻击，分层安全方案已无法保护UWSN免受混合攻击[180]。针对这一问题，马[181]提出了一种针对UWSN的跨层信任模型和节点选择算法，实验结果表明，跨层信任模型可以有效提高系统安全性和网络性能，抵御恶意节点的攻击。
To provide strong security measures against various internal and external hybrid attacks, as well as cross-layer attacks in the SAGSIN, a trust model is needed for hybrid attack modes, which put forward efficient and reliable security mechanisms to maintain normal operation in SAGSIN. For hybrid attacks in SAGSIN, a cross-layer method adopting covert channel detection mechanism provides us a solution to ensuring the integrity, availability and confidentiality of the network as much as possible.
为了提供强大的安全措施，以应对SAGSIN中的各种内外部混合攻击以及跨层攻击，需要混合攻击模式的信任模型，该模型提出了高效可靠的安全机制来维持SAGSIN的正常运行。对于SAGSIN中的混合攻击，采用隐蔽信道检测机制的跨层方法为我们提供了一种解决方案，以尽可能保证网络的完整性、可用性和机密性。

C. Challenges Ahead C. 今后的挑战

After reviewing existing works, we elaborate on some emerging challenges in SAGSIN security in this part, with the emphasis on emerging security issues along with the multi-layer integrated network, including potential security issues related to AI techniques, SAGSIN security by design, tradeoff between data security, user privacy protection and QoS, lightweight authentication and fast re-authentication, quantum-based communication security issues, etc.
在回顾了已有的工作之后，我们在这一部分详细阐述了SAGSIN安全中的一些新挑战，重点关注了多层集成网络中出现的安全问题，包括与AI技术相关的潜在安全问题、SAGSIN安全设计、数据安全之间的权衡、用户隐私保护和QoS、轻量级认证和快速再认证、基于量子的通信安全问题。 等。
Emerging AI security issues in SAGSIN: It is widely regarded that will play an important role in the future development of 6G, in which SAGSIN is expected to be the promising network architecture. In particular, AI or ML methodologies will be implemented throughout many aspects of , including network control, spectrum management, energy management, routing and handover management, etc [3], [182]. In view of this, AI security comes to be a critical issue in SAGSIN, which includes not only the security
SAGSIN中新出现的AI安全问题：人们普遍认为 ，SAGSIN将在6G的未来发展中发挥重要作用，其中SAGSIN有望成为有前途的网络架构。特别是，人工智能或机器学习方法将在许多方面实施 ，包括网络控制、频谱管理、能源管理、路由和切换管理等[3]，[182]。有鉴于此，人工智能安全成为SAGSIN的一个关键问题，其中不仅包括安全性
of AI itself, such as ML models, training datasets, but also recently emerging attack methodologies/tools by adopting AI techniques, e.g., Deep Locker [183], and Malware-GAN [184].
人工智能本身，如机器学习模型、训练数据集，以及最近通过采用人工智能技术（例如Deep Locker[183]和Malware-GAN [184]）出现的攻击方法/工具。
In order to address these AI security issues, on the one hand, we are looking forward to further development of AI technologies so as to give a clear explanation of the effects of training models and datasets on the results. For example, FL is also an emerging AI foundational technology, and it can conduct data usage and ML modeling while meeting the requirements of user privacy protection, data privacy security, and government regulations. Furthermore, we can use FL to solve the security issues in SAGSIN. On the other hand, designing security defense countermeasures by adopting AI techniques may be a more anticipated development trend in the future [185], e.g., smart anomaly detection and intrusion detection by adopting deep learning methods [186].
为了解决这些人工智能安全问题，一方面，我们期待人工智能技术的进一步发展，以便清楚地解释训练模型和数据集对结果的影响。例如，FL也是一种新兴的AI基础技术，它可以在满足用户隐私保护、数据隐私安全和政府法规要求的同时进行数据使用和ML建模。此外，我们可以使用 FL 来解决 SAGSIN 中的安全问题。另一方面，采用人工智能技术设计安全防御对策可能是未来更值得期待的发展趋势[185]，例如采用深度学习方法进行智能异常检测和入侵检测[186]。
SAGSIN security by design: As an integrated network, the network design for each layer of SAGSIN was initially done independently, where security was often not a primary concern. During the fusion of different segments, many security threats and cross-layer attacks have come up due to its open links and dynamic topologies. Recently, the academic and business have begun to pay attention to an overall network design of SAGSIN. For example, China Mobile proposes to build a ubiquitous communication network integrating ground, air, space, and sea networks in the future [187]. Nevertheless, they focused more on communication efficiency, network coverage, subscriber access amount, etc.
SAGSIN安全设计：作为一个集成网络，SAGSIN的每一层网络设计最初是独立完成的，安全性通常不是主要问题。在不同段的融合过程中，由于其开放链路和动态拓扑结构，出现了许多安全威胁和跨层攻击。最近，学术界和企业界都开始关注SAGSIN的整体网络设计。例如，中国移动提出未来构建陆、空、空、海三网的泛在通信网络[187]。然而，他们更关注通信效率、网络覆盖、用户访问量等。
Although we all know the importance of cyber security and privacy protection, they are seldom a primary concern in the beginning of many network design. To address these issues, SAGSIN security by design (SbD), which means that security should be built into SAGSIN at the initial design stage instead of being added later, provides us a promising solution [188]. Furthermore, this SbD principle appears to be particularly important in the era of SAGSIN, since just adding security measures and services in a certain layer may not be able to solve cross-layer security issues in SAGSIN.
尽管我们都知道网络安全和隐私保护的重要性，但在许多网络设计开始时，它们很少成为主要关注点。为了解决这些问题，SAGSIN安全设计（SbD）为我们提供了一个很有前途的解决方案[188]。此外，这种SbD原则在SAGSIN时代显得尤为重要，因为仅仅在某一层添加安全措施和服务可能无法解决SAGSIN中的跨层安全问题。
Tradeoff between data security, user privacy protection and QoS: As mentioned above, in consideration of its cooperation characteristics of multi-layer networks, open communication environment, and time-varying topologies, there exists a tradeoff between SAGSIN QoS guarantee and data security, user privacy protection [189]. For instance, as an important application, location-based service (LBS) is sensitive to mobility and volatility, and LBS providers have to collect users' privacy information through frequent location information requests. During this process, a large amount of personal privacy may be exposed to the external attackers.
数据安全、用户隐私保护与QoS之间的权衡：如上所述，考虑到SAGSIN的多层网络、开放通信环境和时变拓扑的协同特性，SAGSIN QoS保证与数据安全、用户隐私保护之间存在权衡[189]。例如，基于位置的服务（LBS）作为一项重要的应用，对移动性和波动性很敏感，LBS提供商必须通过频繁的位置信息请求来收集用户的隐私信息。在此过程中，大量的个人隐私可能会暴露给外部攻击者。
Moreover, data security in distributed learning poses a significant challenge in the era of AI-driven SAGSIN. To address these kind of problems, blockchain, FL, and cloudedge-end collaboration have been envisioned as potential techniques [190]-[192]. Nevertheless, how to balance the tradeoff between computation efficiency and data security is still a chronic problem, where optimization and game theories may continue to play an important role.
此外，分布式学习中的数据安全在人工智能驱动的SAGSIN时代提出了重大挑战。为了解决这类问题，区块链、联邦学习和云端协作被认为是潜在的技术[190]-[192]。然而，如何平衡计算效率和数据安全之间的权衡仍然是一个长期存在的问题，其中优化和博弈论可能会继续发挥重要作用。
Lightweight authentication and fast re-authentication: In view of the limited resources in space and air network nodes, including diverse satellites, UAVs, etc., and many IoT scenarios, traditional encryption and authentication algorithms might not be applicable. To address these issues, lightweight encryption and authentication theories and algorithms should be one of the key research points. In order to ensure the quality of SGASIN communication, a corresponding handover authentication protocol is required. To further improve authentication efficiency and ensure link security, a new protocol should be designed based on existing encryption algorithms. In other words, on the basis of shared key, we should consider designing a secure and lightweight authentication scheme.
轻量级认证和快速重认证：针对包括卫星、无人机等在内的空空网络节点资源有限，以及众多物联网场景，传统的加密认证算法可能不适用。为了解决这些问题，轻量级加密和认证理论和算法应该是研究的重点之一。为了保证SGASIN通信的质量，需要相应的切换认证协议。为了进一步提高认证效率，保证链路安全，应在现有加密算法的基础上设计新的协议。也就是说，在共享密钥的基础上，我们应该考虑设计一个安全、轻量级的认证方案。
Moreover, due to the velocity of LEO and VLEO satellites, UAVs, and airplanes, frequent handovers can be performed intra- and inter-satellites/UAVs/airplanes. Therefore, a fast reauthentication mechanism should be considered in the access authentication scheme to improve the handover efficiency of network communication. Then, fast re-authentication emerges to be another challenging trend [193].
此外，由于LEO和VLEO卫星、无人机和飞机的速度，可以进行星内和星间/无人机/飞机的频繁切换。因此，在访问认证方案中应考虑快速重认证机制，以提高网络通信的切换效率。然后，快速重新认证成为另一个具有挑战性的趋势[193]。
Quantum-based communication security issues: As two promising techniques in beyond or , SAGSIN and quantum communications will play important roles in the future development. Regarding cyber security, quantum methods may provide us another choice while designing physical security and cyber security schemes, thus, the security of the 6G communication link is ensured. Quantum computing is expected to be used in 6G communication networks to detect, mitigation, and prevent security vulnerabilities. Quantum computationassisted communication is a new research field, which may study the possibility of replacing quantum channels with noiseless classical communication channels to achieve extremely high reliability in 6G.
基于量子的通信安全问题：SAGSIN和量子通信作为未来 发展中具有前途的两种技术。在网络安全方面，量子方法可以为我们在设计物理安全和网络安全方案时提供另一种选择，从而确保6G通信链路的安全性。量子计算有望用于 6G 通信网络，以检测、缓解和预防安全漏洞。量子计算辅助通信是一个新兴的研究领域，可以研究用无噪声经典通信信道取代量子信道的可能性，以实现6G的极高可靠性。
The introduction of ML-based network security and quantum encryption technology into the communication link of network may open a new research era. The improvement of quantum ML algorithm in supervised and unsupervised learning of clustering and classification tasks may improve the security and privacy of communication network. The potential of applications in the application of quantum security mechanisms advances widely, and among them, quantum cryptography and post quantum cryptography have attracted more and more attention in recent years [194], [195].
将基于ML的网络安全和量子加密技术引入 网络通信链路，或将开启一个新的研究时代。量子ML算法在聚类和分类任务的监督和无监督学习中的改进可以提高通信网络的安全性和隐私性。量子安全机制应用的潜力 不断扩大，其中量子密码学和后量子密码学近年来越来越受到关注[194]，[195]。

VIII. CONCLUSION 八、结语

In this paper, we have presented a comprehensive survey of recent research works related to SAGSIN security, with the emphasis on existing security threats, available attack methodologies, and security defense countermeasures. In particular, we first gave a brief introduction of the multi-layer integrated network, and pointed out several challenging issues in SAGSIN, including network management, QoS guarantee, gateway selection, security, etc. Next, we reviewed basic security requirements in wireless communications and presented some specific discussions on SAGSIN. Then, we summarized existing security threats in SAGSIN, and classified them into four categories, i.e., physical threats, operation
本文对近期与SAGSIN安全相关的研究工作进行了全面综述，重点介绍了现有的安全威胁、可用的攻击方法和安全防御对策。特别是，我们首先对多层集成网络进行了简要介绍，并指出了SAGSIN中的几个挑战性问题，包括网络管理、QoS保障、网关选择、安全性等。接下来，我们回顾了无线通信的基本安全要求，并就SAGSIN进行了一些具体讨论。然后，我们总结了SAGSIN中现有的安全威胁，并将它们分为四类，即物理威胁、操作
threats, network threats, and data/information threats. After that, we provided a survey of available attack methodologies in SAGSIN including jamming attack, eavesdropping attack, DoS attack, as well as spoofing attack, and reviewed recently proposed security defense countermeasures, such as anti-jamming techniques, secure routing algorithms, secure handover schemes, secure key management methods, IDS, etc. Finally, we presented some further discussions on crosslayer mixed attacks and countermeasures, and outlined some challenges ahead which are believed to deserve further explorations.
威胁、网络威胁和数据/信息威胁。之后，我们对SAGSIN中可用的攻击方法进行了调查，包括干扰攻击、窃听攻击、DoS攻击以及欺骗攻击，并回顾了最近提出的安全防御对策，如抗干扰技术、安全路由算法、安全切换方案、安全密钥管理方法、IDS等。最后，我们进一步讨论了跨层混合攻击和对策，并概述了一些值得进一步探讨的挑战。

APPENDIX ACRONYMS 附录缩略语

AFH Adaptive Frequency-Hopping
自适应跳频
AHS Adaptive Handover Scheme
自适应切换方案
AI Artificial Intelligence 人工智能
APRS Adaptive Probability Reservation Strategy
自适应概率保留策略
ATM Asynchronous Transfer Mode
异步传输模式
AUV Autonomous Underwater Vehicle
自主水下航行器
B5G Beyond 5G
CARP Channel Aware Routing Protocol
信道感知路由协议
CoMP Coordinated Multipoint 协调多点
CRN Cognitive Radio Network 认知无线电网络
D2D Device-to-Device 设备到设备
DDoS Distributed Denial of Service
分布式拒绝服务
DoS Denial of Service 拒绝服务
DSRC Dedicated Short Range Communication
专用短程通信
DSSS Direct Sequence Spread Spectrum
直接序列扩频
DTC Decision Tree Classifier
决策树分类器
E-CARP Enhanced Channel Aware Routing Protocol
增强型信道感知路由协议
FH Frequency-Hopping 跳频
FL Federated Learning 联邦学习
FSA Finite State Automaton 有限状态自动机
FSS Fixed Satellite Service 卫星固定服务
GEO Geostationary Orbit 地球静止轨道
GIG Global Information Grid 全球信息网格
GNSS Global Navigation Satellite System
全球卫星导航系统
GMDSS Global Maritime Distress and safety System
全球海上遇险与安全系统
GPS Global Positioning System
全球定位系统
HAP High-Altitude Platform 高空平台
IDS Intrusion Detection System
入侵检测系统
IoT Internet of Things 物联网
IPsec Internet Protocol Security
互联网协议安全
JAD Joint Approximate Diagonalization
关节近似对角化
LBS Location-Based Service 基于位置的服务
LCMV Linearly Constrained Minimum Variance
线性约束最小方差
LEO Low Earth Orbit 近地轨道
LoS Line of Sight 视线
MAC Media Access Control 媒体访问控制
MEO Medium Earth Orbit 中地球轨道
ML Machine Learning 机器学习
MSI Maritime Safety Information
海上安全信息
Navtex Navigational Telex 导航电传
NOMA Non-orthogonal Multiple Access
非正交多址
OAM Orbital Angular Momentum
轨道角动量
PCV Phase Center Variation 相位中心变化
PF Particle Filter 颗粒过滤器
PMU Phasor Measurement Unit 相量测量单元
PSC Preset Satellite Chain 预设卫星链
QKP Quantum Key Pool 量子密钥池
QKD Quantum Key Distribution
量子密钥分发
QoE Quality of Experience 体验质量
QoS Quality of Service 服务质量
RIS Reconfigurable Intelligent Surface
可重新配置的智能表面
RL Reinforcement Learning 强化学习
ROV Remotely Operated Vehicle
遥控车辆
RTT Round-Trip Time 往返时间
SAGIN Space-Air-Ground Integrated Network
空空地一体化网络
SAGSIN Space-Air-Ground-Sea Integrated Network
天空地海一体化网络
SbD Security by Design 安全设计
SDN Software-Defined Network
软件定义网络
SDoS Synergetic Denial of Service
协同拒绝服务
SDSN Software-Defined Satellite Network
软件定义卫星网络
SIN Space Information Network
空间信息网
SINR Signal to Interference plus Noise Ratio
信干比加噪声比
SSL Secure Sockets Layer 安全套接字层
SVM Support Vector Machine 支持向量机
TCP Transmission Control Protocol
传输控制协议
TDMA Time Division Multiple Access
时分多址
UAS Unmanned Aerial System 无人机系统
UAV Unmanned Aerial Vehicle 无人机
UDP User Datagram Protocol 用户数据报协议
USV Unmanned Surface Vessel 无人水面舰艇
UUV Unmanned Underwater Vehicle
无人水下航行器
UWAN Underwater Acoustic Network
水声网络
UWSN Underwater Wireless Sensor Network
水下无线传感器网络
VANET Vehicular Ad-Hoc Network
车载自组网
VLC Visible Light Communication
可见光通信
VLEO Very Low Earth Orbit
极低地球轨道
VN-HO Virtual Node Handover 虚拟节点切换
WLAN Wireless Local Area Network
无线局域网
WiMAX Worldwide Interoperability for Microwave
微波的全球互操作性
Access.

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Hongzhi Guo (Member, IEEE) received the B.S., M.S., and Ph.D. degrees in computer science and technology from the Harbin Institute of Technology in 2004, 2006, and 2011, respectively. He is currently an Associate Professor with the School of Cybersecurity, Northwestern Polytechnical University. He has published more than 40 peer-reviewed papers in many prestigious IEEE journals and conferences. His research interests cover edge computing, SAGSIN, IoT security, AI security, and security. He was the recipient of the WiMob Best Paper Award 2019. He currently serves as an Editor for IEEE TRaNSACTIONS ON VEHICUlar TECHNOLOGY.
郭洪志（IEEE会员）分别于2004年、2006年和2011年获得哈尔滨工业大学计算机科学与技术学士、硕士和博士学位。他目前是西北工业大学网络安全学院的副教授。他在许多著名的IEEE期刊和会议上发表了40多篇同行评审论文。他的研究兴趣包括边缘计算、SAGSIN、物联网安全、人工智能安全和安全 。他是 2019 年 WiMob 最佳论文奖的获得者。他目前担任IEEE TRaNSACTIONS ON VEHICUlar TECHNOLOGY的编辑。
Jingyi Li (Graduate Student Member, IEEE) received the B.S. degree in electronic information science and technology from Henan Agricultural University in 2018. She is currently pursuing the Ph.D. degree with the School of Cyber Engineering, Xidian University. Her research interests include aerial-ground communications, non-convex optimization, and intelligent reflecting surface.
李静毅（IEEE研究生会员）于2018年获得河南农业大学电子信息科学与技术学士学位。目前在西安电子科技大学网络工程学院攻读博士学位。她的研究兴趣包括天对地通信、非凸优化和智能反射面。
Jiajia Liu (Senior Member, IEEE) is a Full Professor (Vice Dean) with the School of Cybersecurity, Northwestern Polytechnical University. He has published more than 180 peer-reviewed papers in many high quality publications, including prestigious IEEE journals and conferences. His research interests cover a wide range of areas including intelligent and connected vehicles, mobile/edge/cloud computing and storage, IoT security, wireless and mobile ad hoc networks, and SAGIN. He received the IEEE VTS Early Career Award in 2019, the IEEE ComSoc Asia-Pacific Outstanding Young Researcher Award in 2017, the IEEE ComSoc AsiaPacific Outstanding Paper Award in 2019. He has been actively joining the society activities, such as serving as an Associate Editor for IEEE TRansactions ON Wireless Communications (since, May 2018), IEEE Transactions on Vehicular Technology (since, January 2016) and IEEE Transactions ON COMPUTERs (October 2015-June 2017), and an Editor for IEEE Network (since July 2015), and IEEE Transactions on Cognitive Communications and Networking (since, January 2019). He is the Vice Chair of IEEE IOT-AHSN TC, and is a Distinguished Lecturer of IEEE Communications Society and Vehicular Technology Society.
刘佳佳（IEEE高级会员）是西北工业大学网络安全学院的正教授（副院长）。他在许多高质量的出版物上发表了180多篇同行评审论文，包括著名的IEEE期刊和会议。他的研究兴趣涵盖广泛的领域，包括智能和互联汽车、移动/边缘/云计算和存储、物联网安全、无线和移动自组网以及 SAGIN。他于2019年获得IEEE VTS早期职业奖，2017年获得IEEE ComSoc亚太区杰出青年研究员奖，2019年获得IEEE ComSoc亚太区杰出论文奖。他一直积极参与学会活动，例如担任IEEE TRansactions ON Wireless Communications（自2018年5月起）、IEEE Transactions on Vehicular Technology（自2016年1月起）和IEEE Transactions ON COMPUTERs（2015年10月至2017年6月）的副主编，以及IEEE Network（自2015年7月起）和IEEE Transactions on Cognitive Communications and Networking（自2015年7月起）的编辑。 2019 年 1 月）。他是IEEE IOT-AHSN TC的副主席，也是IEEE通信学会和车辆技术学会的杰出讲师。
Na Tian (Graduate Student Member, IEEE) received the B.S. degree in computer science and technology from the Taiyuan Institute of Technology in 2019. She is currently pursuing the master's degree with the School of Cybersecurity, Northwestern Polytechnical University. Her research interests include Internet of Vehicles and aerial-ground collaborative computing.
田娜（IEEE研究生会员）于2019年获得太原理工大学计算机科学与技术学士学位。她目前正在西北工业大学网络安全学院攻读硕士学位。她的研究兴趣包括车联网和空中地面协同计算。
Nei Kato (Fellow, IEEE) is a Full Professor and the Dean with the Graduate School of Information Sciences and was the Director of Research Organization of Electrical Communication from 2015 to 2019 and the Strategic Adviser to the President, Tohoku University in 2013. He has published more than 450 papers in prestigious peer-reviewed journals and conferences. He has researched on computer networking, wireless mobile communications, satellite communications, ad hoc & sensor & mesh networks, UAV networks, smart grid, AI, IoT, big data, and pattern recognition. He is the Vice-President (Member & Global Activities) of IEEE Communications Society in 2018, and the Editorin-Chief of IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY in 2017. He is a Fellow of the Engineering Academy of Japan and IEICE.
内加藤（IEEE院士）是信息科学研究生院的正教授和院长，2015年至2019年担任电气通信研究机构主任，2013年担任东北大学校长战略顾问。他在著名的同行评审期刊和会议上发表了 450 多篇论文。他研究过计算机网络、无线移动通信、卫星通信、自组织、传感器和网状网络、无人机网络、智能电网、人工智能、物联网、大数据和模式识别。他于2018年担任IEEE通信学会副主席（会员和全球活动），并于2017年担任IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY主编。他是日本工程院和IEICE的院士。