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Session initiation protocol (SIP), which is widely used for Internet telephony, is an application-layer protocol to handle sessions between two points. SIP is implemented on the top of user datagram protocol (UDP) or transmission control protocol (TCP). SIP messages are transmitted by UDP or TCP. This study is on the performance of SIP signaling network using UDP and TCP. A network simulator (ns-2) is used to evaluate throughput and call setup delay as performance measures. We have implemented new agents that act as SIP elements in the ns-2 and a significant performance difference was found to exist. In the case of SIP over UDP, retransmissions of SIP messages decrease throughput. On the other hand, in the case of SIP over TCP, the window control of TCP enlarges a call setup delay.