 TU-RES REQ-TU Error OK200 Message Message INVITE Proceeding Proceeding INVITE Server Temp1xx STATE Proceeding Proceeding Proceeding INVITE 1' 'NEW TEMP-T TEMP-T-ERR TEMP-T-OK CMP-CFM Temp1xx Error Completed OK200 Completed Confirmed Error Completed Timer G Completed Confirmed STATE STATE Confirmed Terminated +5000 Error Completed R-Re-INVITE-T Timer I Completed Terminated +32000 Terminated Timer H Terminated Destroy_IS INVITE Terminated STATE Receive Message Message FIGURE 16.9 INVITE server model. structure. In [12], a SIP-based dynamic discovery protocol, modeling also relies on the SIP transaction structure of client and server. No matter the model in the section, or the large models shown in other papers, such as [10,12], the whole SIP modeling is complex, which would even become the resistance of analysis. For example, if we would like to model the interaction between SIP and H.323, a modeling interaction based on a whole SIP model would become onerous and the state space would be too large for analysis. So, for specific analysis purposes, a simplified modeling method would be more suitable. 16.3.2.2 Simplified SIP Modeling In order to simplify the modeling procedure and analysis of the model, it is always useful to pay particular attention to those that are of interest to the study [15] and select the necessary hierarchies and functions for modeling. That is modeling on-the-fly. Although SIP is a simple protocol, transaction of layers, interaction of different protocols, and security problems are complex to model integrally. Even if the whole system is modeled, it is too large for further analysis. Simplified SIP modeling helps to solve the problem. As an example of the method, Figure 16.10 shows a simple BYE attack with interactions between SIP and RTP. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security (A) Top level model (B) Intruder model (C) UA1 model (D) UA2 model In the illustration, the SIP model is simplified from models in Figure 16.8 and Figure 16.9. RTP her