I. Introduction
Network Function Virtualization (NFV) [1] is a research trend when the current network scale develops rapidly [2]. It realizes hardware and software decoupling and functional abstraction through virtualization technology. NFV transforms functions that must be provided by hardware devices, such as firewall, intrusion detection system, and load balancing, into virtualized network functions (VNF) which can be quickly deployed and expanded, thus network functions no longer depend on dedicated hardware. NFV enables flexible sharing of resources, rapid development and deployment of new services.