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losses by abruptly slowing their transmissions, a response that further degrades the performance of active connections. We have identiied the factors that contribute to this performance degradation and have quantiied their effects in detail. We have shown how waits for retransmis-sion timeouts cause pauses in communication at least 650 milliseconds longer than the underlying network-level interruption. These pauses are readily noticed by interactive users and signiicantly reduce throughput. We have also described a fast retransmission scheme that can reduce the pauses in communication to 50 milliseconds past the moment when transport-level communication resumes. Fast retransmissions thus reduce interactive delays to acceptable levels and regain much of the lost through-put. The fast retransmission approach is attractive because it calls for only minimal changes to end systems, relies on no special support from the underlying network or intermediate routers, follows established congestion avoidance procedures, and preserves end-to-end reliability semantics. The approach is thus applicable to a large and varied in-ternetwork like the Internet. Our work makes clear the need for reliable transport protocols to diierentiate between motion-related and congestion-related packet losses. Our results can be used to adapt TCP to mobile computing environments. They also apply to other reliable transport protocols that must cope with both mobility and congestion. Acknowledgments Dan Duchamp and John Ioannidis provided the Mach 2.5 version of the Mobile IP software. Chuck Lewis helped to set up and maintain the testbed. Greg Minshall provided useful comments on an earlier draft of this paper. Between 1985 and 1987 he designed and developed several data communications products for Pyramid Technology Corporation. Between 1992 and 1994 he investigated networking and operating systems issues in mobile computing at Matsushita Information Technology Laboratory. In 1994 he joined AT&T Bell Laboratories , where he is pursuing research in wireless networking and mobile computing. He is a member of ACM, CPSR, and IEEE. research interests include distributed shared memory, operating systems, and mobile computing.