from WLAN to UMTS (upward handoff) usually happen when the MNs move out of the WLAN coverage areas and are therefore compulsory for the purpose of maintaining the ongoing connections. Horizontal handoffs are usually triggered by comparing the radio signal strength (RSS) or signal-to- interference ratio (SIR) from different RANs. However, this method cannot be directly applied to vertical handoffs because it is difficult to directly compare the RSS/SIR of heterogeneous RANs. Several schemes have been proposed for vertical handoff management. Their operation scope varies from network layer to application layer. Generally, net- work layer solutions are based on Mobile IP (MIP) developed by the IETF [7], which requires some mobility-supporting agents to be incorporated in the var- ious networks involved, while the transport layer solutions and the SIP-based application layer solutions are agnostic to the underlying network layer and thus easier to be adopted in the current IP-based network environment. More- over, the timely transmissions of signalling messages is crucial for realizing seamless handoffs; however, this is a challenge since network environments during handoff periods may be unstable. Currently, SIP messages are deliv- ered using User Datagram Protocol (UDP), or Transmission Control Protocol (TCP) in cases where the message size is large. While UDP provides an unre- liable best-effort data delivery, TCP provides a reliable data delivery service without regard to latency. These two extremes in data delivery methods are not flexible enough for reliable and timely delivery of SIP signalling messages over unsteady network environments. To address this challenge, a transport layer protocol known as Stream Control Transmission Protocol (SCTP) [8] that was recently standardized by the IETF can be an attractive candidate due to its partial-reliability option for data delivery. In addition, the multi- homing feature of SCTP also enables it to support vertical handoffs over the transport layer. The objective of t