 within a period of time, the retransmissions of obsolete SIP messages by SCTP is undesirable for the networks. (2) It gives better network performance than SIP over UDP only when SIP traffic is high. C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks When SIP traffic is low, SIP over UDP, which recovers errors at the application layer only, can be more effective as discussed above. (3) SCTP transports all SIP messages reliably regardless of needs. SIP provisional response messages, like "180 Ringing" and "100 Trying," are informative and not required to be transported reliably. In SIP over UDP, these messages are not retransmitted by SIP. However, SCTP only provides a reliable service. It treats all data packets in the same way and transports all of them reliably. Our proposed scheme of SIP over PR-SCTP can overcome the above limitations. 11.4.2 Proposed Scheme of SIP over PR-SCTP By engaging an appropriate PR-SCTP service according to the reliability requirement of a SIP message, SIP messages can be transported over PR- SCTP either with partial reliability or unreliably as appropriate. Moreover, for efficient recovery of lost packets, our proposed scheme chooses the most effective retransmission mechanisms between SIP and PR-SCTP, thus achiev- ing a better performance regardless of whether SIP traffic is high or low. Our SIP over PR-SCTP scheme adopts the "timed reliability" service of PR- SCTP. Instead of setting the lifetime of a PR-SCTP data packet to a constant value, the lifetime is changed according to the application layer timeout values of the SIP message transported. Besides applying the partial reliable transport service of PR-SCTP within the lifetime of the SIP message, our scheme uses SIP to retransmit the lost message as well. This allows our scheme to benefit from both the flexibility of PR-SCTP and the retransmission mechanism of SIP. When a SIP message is first transmitted, the lifetime of the data packet carrying this SIP message is set to T1, the