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Network CongestionIn data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. Network protocols which use aggressive retries to compensate for packet loss tend to keep systems in a state of network congestion even after the initial load has been reduced to a level which would not normally have induced network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as congestion collapse, a term which was first defined by John Nagle in RFC 896, written in 1984, when he identified problems with early implementations of the TCP network protocol over parts of the very early Internet. Congestion collapse is a condition where a system such as a data network has settled under load into a state where traffic demand is high but little useful throughput is available, with high levels of packet loss, delay, and delay variation. Experience of widespread congestion collapse in the Internet in 1987 led to further improvements of the TCP congestion control mechanisms. Modern networks use congestion control and network congestion avoidance techniques to try to avoid congestion collapse. These include exponential backoff in protocols such as TCP and Ethernet, and fair queueing in devices such as routers. RFC 2914 addresses the subject of congestion control in detail. See also: External links * Sally Floyd, "On the Evolution of End-to-end Congestion Control in the Internet: An Idiosyncratic View", IMA Workshop on Scaling Phenomena in Communication Networks, October 1999
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