hnologies, and Security any value between 500 ms and 250 ms, which is compliant with the changes of the real RTT. In the method based on the packet loss ratio, if the packet loss ratio is less than the UPPER LIMIT, which is 10% in our simulation, T1 is set to be 500 ms. When SIP traffic is low, data loss rate is low, thus the T1 is likely to remain at 500 ms. Therefore, in the case of low SIP traffic, the dynamic T1 adjustment method according to the RTT is more effective than the method according to the packet loss ratio. 11.5 Conclusions One of the main challenges for NG wireless networks to provide "Always Best Connected" services is how to manage vertical handoffs seamlessly. Compared with horizontal handoffs, vertical handoffs require technology-independent mobility management schemes and involve more extensive procedures, thereby leading to longer delays. Vertical handoffs schemes proposed in the literature can be classified according to the layer they operate on. Network layer schemes require additional mobility supporting network entities that may hinder their wide deployment. TCP-based transport layer schemes have the drawback that they require modifications to the TCP operations in all peer nodes. MSCTP proves to be a good candidate for supporting vertical handoffs when SCTP becomes widely adopted in the future. Due to the operation at the highest layer, SIP-based schemes are easy to deploy but also incur long handoff delays. To solve this problem, we have proposed a S-SIP scheme that enables the MN to simultaneously establish two ongoing connections with the CN and realize "seamless-handoff." Moreover, with the help of tracking technologies, the MN can predict potential handoffs and establish the new connection before the current connection losses its signal. Simulation results have been presented to show that the proposed scheme can support seamless inter-domain roaming. In addition, current SIP signalling traffic is transported over UDP with loss recovery performed at the SIP layer using its own retr