Since the demand for mobile real-time application grows fast, the handoff efficiency of Mobile IP becomes extraordinarily important. Seamless handoff requires that users and applications do not suffer loss of connectivity or any noticeable interruption in traffic during handoffs. In this paper, we present a seamless handoff scheme for Mobile IP, which is especially suitable for large open indoor environments where more than one Access Points tend to overlap. By introducing a decision making entity which we term control engine, redesigning part of the Layer 3 message exchange among communicating entities and utilizing a handoff decision algorithm with pure software-based movement tracking techniques, this novel scheme is able to eliminate handoff latency and packet loss which may have been noticed by a Mobile Node during handoffs completely. Network Simulator-2 was employed to validate the feasibility of this scheme, and the result shows that the performance in terms of latency is eliminated completely. More importantly, this scheme is capable of providing handoff between foreign agents with no packet loss. While the throughput is kept at a relatively high level, the end-to-end delay from correspondent node to Mobile Node is reduced obviously.