s some performance improvements when SIP traffic is heavy. However, transporting SIP over SCTP may not improve the performance when the traffic is light. Previous work suggested that SCTP is not recommended to be used between a SIP user agent (UA) and a SIP server due to the low traffic load, although SCTP gives other benefits, such as fast handoff over heterogeneous mobile networks [21,22]. In this section, we propose a new scheme for transporting SIP over partially-reliable SCTP (PR-SCTP) [31], which chooses the most effective retransmission mechanism between the application and transport layer protocols depending on the traffic condition, thus achieving performance improvements no matter whether the network traffic load is high or low [32]. We compare the performance of SIP over UDP, SIP over SCTP, and SIP over PR-SCTP by simulations. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security 11.4.1 SIP and SCTP Retransmission Mechanisms SIP recovers lost messages by retransmitting them in the application layer. SIP messages are classified as request and response messages. A SIP request message is considered by the sender to be lost if the corresponding response message is not received within a specified time period. SIP increases the retransmission timeout period exponentially in successive retransmissions until the expected response for the message has been received or the number of retransmissions has reached the maximum value of six. The initial SIP timeout period T1 is normally set to 500 ms, based on an estimate of the round-trip- time (RTT) of the network. According to the RTT in each specific situation, the value of T1 may be adjusted. SCTP has similar flow and congestion control mechanisms to those of TCP, although there are some differences between them. SCTP detects packet losses by checking the gaps between blocks in its selective acknowledgment (SACK) chunks. Compared to TCP, SCTP allows a larger number of SACK blocks to be reported in each SACK. SCTP considers a data