is very difficult to simulate. So P2P simulators are developed with special features to solve these problems. Peersim, or Peer-to-Peer Simulator, written in Java Language, is composed of many simple extendable and pluggable components, with a flexible config- uration mechanism and a scalable simulation testbed [34]. P2PSim as part of IRIS project is a free, multi-threaded, discrete event simulator to evaluate, investigate, and explore P2P protocols [35]. They both support various P2P protocols like Chord, Accordion, Koorde, Kelips and SO on. Besides such P2P simulators, there are specific simulators, e.g., the P2P File-sharing simulator, which is designed as an implementation of "Incentive Resource Distribution in P2P Networks" scheme and the Progressive Filling (PF) algorithm [36]. Now there are extensions to SIP regarding utilization of SIP for different applica- tions, such as SIP based file-sharing service, which could be modeled by use of these simulators. 16.5 Summary In the paper, only some general formal modeling methodologies and specific simulators are enumerated and discussed through related examples. After all, no matter what kind of methodologies we used for SIP, the key point is the purpose of modeling and simulation which is the motivation, the standard to select methods, and to evaluate the results of our work. References [1] Rajaram, V.S. 2006. Session initiation protocol for wireless channels. Master's thesis, Office of Graduate Studies of Texas A&M University. [2] Wang, L., A. Agarwal, and J.W. Atwood. 2004. Modeling and verifica- tion of interworking between SIP and H.323, Computer Networks: The International Journal of Computer and Telecommunications Network- ing, Elsevier North-Holland, Inc. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security [3] Wang, L., J.W. Atwood, and A. Agarwal. 2003. Validation of SIP/H.323 Interworking Using SDL/MSC. SDL Forum, 335-51. [4] De Marco, G, G. Iacovoni, and L. Barolli. 2005. A technique to analyse session initiation pr