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A heterogeneous wireless network integrating IEEE 802.11 WLAN and 3G/UMTS mobile phone system will be a common scenario in future mobile networking. The multi-homing feature of SCTP and the dynamic address configuration extension had made seamless vertical handover possible in heterogeneous mobile wireless networks. However, due to the switching of radio interfaces and the attenuation of radio signal, the SACK could get lost during the handover process. This will result in unnecessary retransmission after the handover and lead to degradation in network utilization. A solution to this problem will be presented in this article. The proposed approach introduces a new parameter, named Received Last TSN (RLT), to the standard ASCONF control message. The RLT keeps track the sequence number of lastly received packet from the old interface. The mobile node reports the RLT to its corresponding node right after the handover. With this information, the corresponding node can then proceed with effective transmission and avoid the unnecessary retransmission caused by the loss of SACK. Simulation results reveal that the proposed scheme is a feasible and effective measure for the reduction of redundant retransmission during vertical handover in heterogeneous mobile wireless networks.