The essence of next generation network is that it is an information infrastructure which not only aims to support various information services, but also can sustain the complete informatization of politics, economy, culture, education and national defense of every country. The traditional network architecture layered only by the basic communication functions can no longer fit the requirements of next generation network. An interaction based network service architecture (INSA) is proposed in this paper, which can act as a reference model for next generation network service architecture. The whole structure and the functions of each layer of INSA are described in detail. An abstract formal model of next generation network service architecture is also proposed, which provides a new approach to describing, analyzing and verifying the service related characters of the next generation network service architecture.