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As a prevalent reliable transport protocol in the Internet, TCP uses two key functions: the additive-increase multiplicative-decrease (AIMD) congestion control and a cumulative ACK technique for guaranteeing delivery. However, these two functions lead to the inefficiency of TCP in ad hoc networks where the TCP connections have very small bandwidth-delay products (BDPs) and there are frequent packet losses in the network due to various reasons such as route breakages and radio interference. In this paper, we show that if the BDP of a path is as low as several packets and is known before the connection establishment, any AIMD-style congestion control is costly and is hence not necessary for ad hoc networks. On the contrary, a technique for guaranteeing reliable transmission and recovering packet losses plays a more critical role in the design of a transport protocol over ad hoc networks. With this basis, we propose a novel effective datagram-oriented end-to-end reliable transport protocol for ad hoc networks, which we call Datagram Transport Protocol for Ad hoc networks (DTPA). The proposed scheme incorporates two techniques: a fixed-size- window-based flow-control algorithm and a cumulative bit-vector-based selective ACK strategy. We then develop a mathematical model for evaluating the performance of DTPA based on these two techniques. An optimum transmission window is determined for an n-hop chain and is computed to be the BDP of the path plus 3. The protocol is verified using GloMoSim, and simulation results show that our proposal substantially improves the network performance in terms of throughput, round-trip time, number of retransmissions, and IP queue size.