by-day to solve existing problems and to provide new services. From the academic research of SIP ideas to SIP-based software development, a complex network environ- ment, cost inefficiency, and impossibility or impracticality of direct implemen- tation are reasons for the utilization of modeling and simulation methods and tools to simulate new network architectures, to solve security problems, and to improve performance of the SIP scenario, etc. Modeling and simulation ideas are necessary, and already many methods are proposed to deal with the realistic problems of SIP. The following list contains some popular and hot SIP research topics and related modeling and simulation methods. SIP in wireless network-NS-2 [1] Interactions between SIP and H.323, PSTN, etc.-FSM [2,3] Performance evaluation of SIP such as SIP traffic, call delay, mobility, etc.-FSM [4], NS-2 [5], HSPA [6] Optimizing extensions for next generation services with SIP-FSM [7], OPNET [8], NS-2 [9] © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security SIP problems such as SIP firewall traversal, security problems, etc.- CPN [10], EFSM [11] SIP services or application development, e.g., VoIP, Instant Messag- ing and Presence, IP Multimedia Subsystem (IMS), multiparty confer- encing-CPN [12], UML [13] P2PSIP-P2PSim [14] 16.1.2 Modeling and Simulation Modeling and simulation are always work done prior to system implemen- tation. Modeling provides architects and others with the ability to visualize entire systems, assess different options, and communicate designs more clearly before taking on the risks-technical, financial, or otherwise-of actual con- struction [1]. SIP is a protocol and SIP modeling can be classified as two types: one is the modeling of the protocol itself, which focuses on call flow- ing, states while running, timer mechanisms, and some SIP extensions. Some formal modeling methodologies are suitable for this modeling, such as finite state machine (FSM) used in request for comments Standard IETF RFC Do