# Advanced Persistent Threats and Their Defense Methods in Industrial Internet of Things: A Survey
## 4 APT Analysis Model
### Network Simulation Model  
Network simulation is a method of simulating network operations that can help us test, evaluate, optimize network performance, predict network behavior, and validate new network solutions [107]. Commonly used network simulation tools include NS-3 (Network Simulator 3), OPNET (optimized network engineering tool), OMNeT++, MATLAB, Simulink, GNS3, Riverbed Modeler, NetSim, QualNet, and Cisco Packet Tracer [108; 109].  
NS-3 is an open-source and highly modular network simulator that provides modules for many network protocols and supports advanced network models and topologies. OPNET is a commercial network simulation tool that provides multiple simulation capabilities, including network design, performance analysis and optimization, network security, and more. OMNeT++ is an open-source network simulator that supports various network technologies, including wireless networks, mobile networks, and the Internet. MATLAB Simulink is a commercial simulation tool that can be used for network system design, control system design, signal processing, and more. GNS3 is a free and open-source network simulation tool that can simulate various types of network equipment such as routers and switches and supports the simulation of multiple network protocols. Riverbed Modeler is a commercial network simulation tool that can be used for network design, performance optimization, traffic analysis, and more. NetSim is a commercial network simulation tool that can be used for network design, performance evaluation, fault diagnosis, and more. QualNet is a commercial network simulation tool that can be used for simulating wireless networks, satellite networks, mobile networks, sensor networks, and more. Cisco Packet Tracer is a free network simulation tool that can be used for simulating network topology and configuring network equipment.  
Network simulation models can provide a secure, controllable, and repeatable experimental environment for simulating various stages and techniques of APT attacks, as well as defensive measures and response strategies for the target network [110; 111]. Network simulation models can help network security personnel and researchers gain a deeper understanding of the principles, characteristics, behavior, and impact of APT attacks, as well as assess the level of threat and risk evaluation posed by APT attacks to the target network [112; 113]. Network simulation models can help network security personnel and researchers design and validate new APT detection, prevention, response, and traceability methods, as well as evaluate their effectiveness and performance [114; 115]. Network simulation models can also help network security personnel and researchers conduct education and training on APT attacks to improve network security awareness and capability [116; 117].  
To summarize, network simulation models are based on simulating industrial IoT systems and simulating the impact and propagation of different advanced persistent threats on the system to evaluate the security performance and vulnerability of the system. By simulating and analyzing different attack scenarios, enterprises can understand the threat propagation mechanisms, scope of impact, and damage caused by APTs and optimize the security defense of the system.