cuments (RFCs) containing language-specific modeling [15] and graphical- specific modeling. The other puts SIP into a network environment, tests the whole network (such as Voice-over-Internet Protocol (VoIP) network, wireless mobile network, etc.), tests the interactions between different protocols, and evaluates performances. Specific network simulators are used for this complex modeling, e.g., NS-2 [16], OPNET [17], and QualNet [18]. Not only do they provide standard SIP modules but also performance evaluations, statistic col- lection, and analysis functions. The key to modeling is the ability to closely match the generated model map to the real system. And for SIP, standard RFCs are the benchmark of the purpose. Simulation has been defined in various ways. In this chapter, simulation is considered to be the running of a model by use of simulation toolkits. Simulation is the basis for making decisions. Decisions are formulated based on the information resulting from the simulation [19], which based on the level of understanding of the problem, the correctness of the model and the interpretation of running results. Simulation and modeling can be developed by the same simulator which provides such functions. After modeling and setting the characteristics and properties of the system, simulation is simplified as running tracing, data collection, and result analysis. However, complex network architectures and topologies cause the burden of "trial and error." 16.1.3 Our Work Based on our previous modeling and simulation trial for SIP problems, we focused on general formal methodologies and general network simulators for modeling and simulation of SIP, which are separately discussed in Section 16.2 and Section 16.4. As one of our research projects is about the security problems of SIP, some of the examples are illustrated by such problems. The main C 2009 by Taylor & Francis Group, LLC SIP Modeling and Simulation method for the modeling and simulation work of SIP is timed Hierarchial Coloured Petri Nets (HCPN), which