It is widely anticipated that beyond 3G network will be an IP-based integration of heterogeneous wireless access technologies. "Seamless mobility" in such network is much more difficult to be achieved than in traditional wireless telecommunication networks because the B3G network environments are more complex, and various IP-based services presents various QoS requirements, and single handover strategy can not meet all the situations. An integrated mobility management solution is really needed instead of the onefold IP-layer solution such as mobile IP. In fact, mobile IP is a simple routing redirection mechanism and the handover performance of mobile IP is very poor due to the inherent problems in IP such as no sensitivity on the requirements of QoS from application layer and the change of under layer service environments. This article tries to divide the task of "seamless mobility" into several sub tasks including a well-defined mobility management framework, an IP-layer signaling protocol with good extensibility to support mobility, an set of typical IP-layer handover strategies, an adaptive handover control scheme and multi-mode support in mobile nodes. Obviously, mobile IPv6 is perfect as IP-layer mobility protocol and all the handover strategies are designed based on hierarchical network-layer mobility management framework and implemented by extending of mobile IPv6. The adaptive handover control scheme is proposed to improve IP-layer handover performance and optimize the utilization of multiple network resources based on accurate handover decision on target network, handover strategy and handover time according to cross-layer information. In addition, mobile nodes should be modified to support multiple available interfaces and the network-layer mobility management functions