P2P (Peer-to-Peer) traffic has become one of the most significant portions of the network traffic. Accurate identification of P2P traffic makes great sense for efficient network management and reasonable utility of network resources. In this paper we propose taxonomy of the existing methods, such as port-based, payload-based and transport layer or application layer feature-based methods, and we discuss the limitations of each category of methods. It puts stress on the introduction about the basic principle and the advantages and disadvantages of these methods.