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During a bulk data transfer over a high speed network, there is a high probability that the next packet received from the network by the destination host is the next packet in the transfer. An optimistic implementation of a bulk data transfer protocol takes advantage of this observation by instructing the network interface on the destination host to deposit the data of the next packet immediately into its anticipated final location. No copying of the data is required in the common case, and overhead is greatly reduced. Our optimistic implementation of the V kernel bulk data transfer protocols on SUN-3/50 workstations connected by a 10 megabit Ethernet achieves peak process-to-process data rates of 8.3 megabits per second for 1-megabyte transfers, and 6.8 megabits per second for 8-kilobyte transfers, compared to 6.1 and 5.0 megabits per second for the pessimistic implementation. When the reception of a bulk data transfer is interrupted by the arrival of unexpected packets at the destination, the worst-case performance of the optimistic implementation is only 15 percent less than that of the pessimistic implementation. Measurements and simulation indicate that for a wide range of load conditions the optimistic implementation outperforms the pessimistic implementation.