# Modeling and Performance Analysis of Satellite Network Moving Target Defense System with Petri Nets
## 2 Background
### Active Defense Techniques  
As a new technology against cyber attacks, active defense adopts a completely different defense idea from traditional defense techniques, and overcomes the shortcomings of traditional passive defense. Typical active defense technologies include End Hopping (EH), Moving Target Defense (MTD), Mimic Security Defense (MSD), and so on.  
EH [6] technology was proposed by [PERSON] in 2008. It refers to military frequency hopping communication countermeasure technology. In the end-to-end data transmission, both sides or one party of the communication pseudorandom change the port, IP address, time slot, protocol, and other End Information according to the agreement to realize the active network defense.  
MTD [8] technology is a revolutionary \"game-changing\" technology in cyberspace proposed by Federal Networking and Information Technology Research and Development (NITRD) in 2011. Unlike prior efforts in cybersecurity research, MTD does not rely on increasing the complexity of the security system to achieve protection of the target. The core idea of MTD is to make the system dynamic, seeking to convert the fixed network into a flexible one, so as to raise the difficulty and cost for attackers and effectively restrict the vulnerabilities exposure and the opportunities for attack.  
MSD [9] technology was proposed by Academician [PERSON] in 2014 with reference to the way that mimicry octopus protects itself through morphological changes. The main idea is that, in addition to the service function and performance of the target object not being able to be hidden, the hardware and software of the system can be camouflaged by dynamic changes, so as to achieve the state that the system is controllable to the defender but unknown to the attacker, so as to achieve the purpose of active network defense to protect the system from attack.  
According to the above statement, we can clearly know that unlike traditional passive defense methods, active defense techniques are dynamic, versatile, and unpredictable, and are therefore effective in countering direct attacks and interfering with enemy information interception. Among them, MTD is the most representative technology in the active defense system and is a key development direction in the field of future network security. Through the implementation of multi-level, dynamic, and continuous transfer of the attack surface of the protected system, the attacker will have to face as much uncertainty as the defender today, thus reducing the success rate of the intrusion into the system until the attacker is forced to give up the attack. Here, the attack surface can be understood as the set of system resources that can be exploited and attacked in the system. MTD is precisely through the defender to continuously change the resources on the attack surface to achieve changes in the attack surface, so as to confuse or mislead the attacker, prompting the attacker to lose the attack target. At present, the dynamic change technology of the attack surface mainly includes four categories: (1) dynamic change technology based on data attack surface [13]; (2) dynamic change technology based on software attack surface, mainly including instruction set randomization, code randomization [14], etc.; (3) dynamic change technology, based on the platform attack surface, mainly includes platform dynamic migration, virtualization techniques [15], etc.; (4) dynamic change technology based on network attack surface. The main idea is to introduce a dynamic change update mechanism. By collaboratively changing network IP addresses or ports, attackers are always unable to determine the real addresses of the communicating parties, thus undermining the sniffing attacks of attackers and achieving privacy protection for hosts. Among the above four types of attack surface dynamic change technology, the research on MTD based on the change of network attack surface is the most common and mature, and has been widely used.