# An Overview on Denial-of-Service Attacks in Control Systems: Attack Models and Security Analyses
## 1 Introduction  
Many industrial control systems rely on information and communication technologies for their operation. In particular, wireless networks and the Internet are becoming key components of control systems, since they can be utilized in the transmission of measurement and control data to remote locations. As the Internet of Things is becoming more popular, the use of wireless technologies in control systems is expected to increase even more. These new developments are bringing efficiency to control systems, but they are also expected to introduce several vulnerabilities. A major concern is that cyber-attackers may be able to exploit the vulnerabilities in control systems that are utilized in power grids, transportation, water distribution, and many other services that are important for the society. To prevent financial losses and environmental damages that may be caused by disruption of those services, it is critical to assess and improve the security of existing control systems and develop new systems that are resilient against cyber attacks. Various cyber-security issues of networked control systems and detection/mitigation approaches for those issues have been discussed in [1; 2; 3; 4; 5; 6; 7; 8]. As pointed out in those works, attackers targeting vulnerable networked control systems may be able to change the contents of measurement and control packets. There may even be cases where attackers can inject false data into the system without beingdetected. These attacks require the attacker to be knowledgeable on the communication protocol as well as the system dynamics. On the other hand, attackers who have limited information on the control system may resort to denial-of-service (DoS) attacks that prevent delivery of control and measurement data packets. Networked control system under DoS attacks can face severe performance issues. Our goal in this paper is to provide an overview of attack-modeling and security analysis approaches in recent works that explore networked control systems subject to DoS attacks. To this end, we look at the control, estimation, and consensus problems and discuss different DoS attack models utilized by researchers for addressing those problems. DoS attacks can take different forms in different network settings. In this paper, we focus on packet drop attacks by malicious nodes in multi-hop networks, and jamming attacks in wireless channels. In multi-hop networks such as wireless ad hoc networks, remotely located nodes can transmit data packets to each other with the help of intermediate nodes that act as routers. It is typically assumed that all nodes obey the routing protocols in the network; however, in reality, a network can face packet drops by malicious nodes [9; 10]. For instance, in _blackhole_ DoS attacks, a malicious node first falsely introduces itself to other nodes as if it is part of the shortest path to a set of remote nodes. Then, many unsuspecting nodes in the network attempt to send all packets addressed to those remote nodes through the malicious node, which in fact drops all packets instead of forwarding them. Furthermore, in _graylhole_ attacks, malicious nodes act normal and follow the standard routing protocols for certain periods of time to avoid being detected. The authors in [9; 10; 11; 12; 13] discussed both malicious and non-malicious packet dropping issues in ad hoc networks and presented several attack detection and mitigation approaches. In addition to packet-dropping attacks, networked controls systems may also suffer from DoS attacks in the form of jamming of wireless transmissions. Specifically, a jamming attacker can prevent transmission of packets by emitting strong interference signals to a wireless channel [14; 15]. It is mentioned in [15] that jamming devices can target various wireless technologies including GPS, mobile communications, and Wi-Fi. Jamming attacks can become a major concern for control systems, since they are easy to launch.