AIMD

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The additive-increase-multiplicative-decrease (AIMD) algorithm is a feedback control algorithm and used in TCP Congestion Avoidance. Basically, AIMD represents a linear growth of the congestion window, combined to an exponential reduction when a congestion takes place.

The approach taken is to increase the transmission rate (window size), probing for usable bandwidth, until loss occurs. The policy of additive increase basically says to increase the congestion window by 1 MSS ( Maximum_segment_size ) every RTT until a loss is detected.

When loss is detected, the policy is changed to be one of multiplicative decrease which is to cut the congestion window in half after loss.

The result is a saw tooth behaviour that represents the probe for bandwidth.

A loss event is generally described to be either a timeout or the event of receiving 3 duplicate acks. Also related to TCP congestion control is the slow start mechanism.

Also see the other policies or algorithms for fairness in congestion control: AIAD, MIAD and MIMD.

Also related: TCP Reno, TCP Tahoe, TCP Vegas.