Abstract: In order to solve the problems that P2P traffic occupies most Internet bandwidth, and ISPs are difficult to execute management, and P2P system lacked self-optimization mechanism, the paper put forward a dynamic P2P traffic optimizing model based on ISP-join. In time sequence, this model describes a series of new methods, including autonomous domain division and management, network measurement and topology-aware, bandwidth and traffic feedback. Furthermore, a prototype system whose core is called DDP(domain discovery protocol) is developed. All of these methods can maintain autonomous domains and inter-domain information, improve inter-domain connection, and keep overlay network topology consistency. According to analysis and experiment validation, this model can achieve P2P traffic dynamic optimizing based on ISP-join.