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During vertical handoff, some undesirable phenomena may mistakenly trigger TCP congestion control operations and thus degrade TCP performance. In this paper, we propose an approach that can quickly estimate available bandwidth when a mobile node (MN) handoff occurs. A sender updates the adaptive slow-start threshold (ssthresh) and congestion window size (cwnd) to improve TCP performance during soft vertical handoff in hybrid mobile networks. When handoff is impending, the receiver sends a handoff notification from another link during soft vertical handoff. The receiver uses the handoff notification option field in the TCP header to notify the sender. The receiver will send this ACK from the new link. When the sender receives this ACK, it will halt some timeout timer parameters and freeze data transmission. TCP gets the value of the round trip time (RTT) when it receives this ACK in a new network environment. Base stations send advertisement messages at a regular interval to help an MN to detect link changes. Then TCP readjusts the proper ssthresh using the exponential weight moving average (EWMA). Our scheme requires only minor modifications of the transport layer of the end hosts. Simulation results show that our scheme effectively improves the TCP performance.