 should be configured, such as the Maximum Simultaneous Calls, Proxy Type, Area Name of Proxy Server and the Proxy Server Connect Timeout, Domain Name, and Current Domain parameters of UAC. After configuration of agent parameters and establish- ment of topology, the SIP scenario is run for statistics collection, performance evaluation, and result analysis. QualNet network simulation software has been developed by Scalable Net- work Technologies [31], which provides a comprehensive set of tools with many components for custom network modeling and simulation. QualNet has a range of wired and wiresless models. However, the main strength of QualNet is in the wireless area. SIP Module is defined in the Multimedia & Enterprise Models section in Standard QualNet Model Libraries which consists of H323, SIP, VoIP Traffic Generator, and VoIP Jitter Buffer. SIP is enabled as the sig- naling protocol after setting SIP configurations, which recommend all nodes of a simulated network are SIP intelligible [32]. Although SIP is independent of the underlying transport layer protocol and can run over TCP, UDP, and SCTP, only TCP is used in current implementation with two types of call model-Direct and Proxy-routed. During simulation, a set of statistics about UAC, UAS, proxy and overall data can be collected and printed for further analysis, such as total number of INVITE, ACK, and BYE requests sent in UAC and Trying, Ringing, and OK responses received by a SIP node. C 2009 by Taylor & Francis Group, LLC SIP Modeling and Simulation 16.4.2.2 P2P Simulators for P2P-SIP Currently, an increasing number of people are beginning to pay attention to P2P-SIP (peer-to-peer SIP). Therefore, the P2P simulators are also considered in this section. The concept behind P2P-SIP is to leverage the distributed nature of P2P to allow for distributed resource discovery in a SIP network, eliminating (or at least reducing) the need for centralized servers [33]. P2P can reach huge dimensions like millions of nodes joining and leaving continuously, which 