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Most existing soft handoff approaches lead to plenty of out-of-order packets during downward vertical handoffs (VHOs). We have presented a soft VHO scheme, called SHORDER, to avoid packet reordering caused by downward VHOs. In this paper, we analyze the effects of our SHORDER scheme and another typical existing soft VHO method on the handoff latency and the received data size during a downward VHO for TCP applications. Then, we approximately derive the applicable conditions of the two approaches, and further propose a diagnosis-based soft vertical handoff (DSVH) mechanism which can self-adaptively deal with reordering packets. The mechanism has practical advantages of no changes to correspondent nodes and compatibility with various enhanced TCP variants. With numerical analysis and test-bed experiments, we show that the DSVH mechanism has better performance than the SHORDER scheme and the typical existing method. Furthermore, experimental and analytical results are consistent with each other.