When routers become congested to the point in which their incoming packet buffers fill and they begin dropping packets, the effect on the network is reduced bandwidth, an impact on loss or time-sensitive traffic flows, and possibly link idle time after the congestion occurs. Explicit Congestion Notification (ECN) for TCP/IP provides a way for routers to inform Transmission Control Protocol (TCP) peers that their buffers are filling due to congestion in the network. In response, TCP peers slow their transmission of data to help prevent packet losses. This article describes the need for ECN, how it works, and how ECN is supported in the Release Candidate 1 release of Microsoft® Windows Vista™.
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