Current TCP flow control depends on packet losses to find the workload that a network can support. Packet drops not only reduce TCP performance, but also add large transfer delay to the packets. Instead of dropping the overflowed packets, Explicit Congestion Notification (ECN) detects incipient congestion and notifies the sources to reduce their windows when the queue length exceeds a threshold. In this paper, we derive closed-form formulae for requirements on threshold and buffer size to achieve zero packet loss and full link utilization. Simulation results that verify our analysis are presented. The impacts of average queue length and Random Early Detection are also discussed.

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