*N (22.2) Let the number of active sessions be given by the expression N=D*T (22.3) and the duration T of one session is T = Np(Ton+Toff) (22.4) The data flow rate has an average value L given by L=N*d (22.5) The session data transfer rate can therefore be estimated with the expression: (22.6) C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Apart from the level of activity in a network which determines the return trip time (RTT), the rate at which packet acknowledgments is performed impacts the average rate of a TCP connection. The approach used by Hassan [1] is adopted here. It considers that acknowledgment is provided for every two packets received. This leads to the expression: T(Q) = RTT * log Q (22.7) When long TCP connections are involved, this expression should be substi- tuted with the maximum of TCP rate permitted by the source. In the following sections we discuss the QoS performance of SIP from various delay perspectives. 22.2.2 QoS and SIP Quality of service is a term that has been loosely used in telecommunications to refer to network [22] parameters that impact the performance of the sys- tem. In telecommunications, QoS refers mostly to the network performance variables such as delay, BER, packet loss rate, interference, bandwidth, system capacity and throughput. In a dynamic network environment the definition includes the velocity of the nodes, the length of the links between nodes and the characteristics of the links. In a multimedia situation, it includes the qual- ity of the data such as the quality of the audio content, the mean opinion scores in VoIP systems and for IPTV where video is involved to the quality of the video (images), compression quality and the frame rate. Thus, the definition of QoS includes quite a lot of system parameters and is dependent on the application in question. In this chapter, a subset of these system parameters that typically affect SIP performance is considered. This includes delay in its various forms in SIP networks inclu