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As interests in MANET (mobile ad hoc networks) have increased, routing protocols suitable for MANET have been standardized specially in IETF (Internet Engineering Task Force). In addition to routing protocols, efforts to adopt the TCP stack as a reliable end-to-end transport protocol with some modifications appropriate for MANETs have been made for the smooth integration with the fixed Internet. Unlike previous approaches to improve TCP performance in MANETs, we modified the fast retransmission technique on the standard TCP. Basically, the fast retransmission technique utilizes the third duplicate ACK to trigger retransmission. However, if TCP is served by on-demand reactive ad-hoc routing protocols like DSR (dynamic source routing), AODV (ad hoc on-demand distance vector) and ABR (associativity-based routing) which can be considered as kinds of connection-oriented routing protocols until a route breakage occurs after a route acquisition, we can improve TCP performance by advancing the retransmission time using the first duplicate ACK triggered retransmission technique, without waiting for the third duplicate ACK packet. We show the performance improvement with this approach by using GIoMoSim simulator with AODV used as its underlying routing protocol, one of standardized on-demand reactive routing protocols in IETF.