Congestion window

(TCP) uses a network congestion-avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, with other schemes such as slow-start and congestion window to achieve congestion avoidance. The TCP congestion-avoidance algorithm is the primary basis for congestion control in the Internet. There are several variations and versions of the algorithm implemented and in use on the internet.

To avoid congestive collapse, TCP uses a multi-faceted congestion-control strategy. For each connection, TCP maintains a congestion window, limiting the total number of unacknowledged packets that may be in transit end-to-end. This is somewhat analogous to TCP's sliding window used for . TCP uses a mechanism called slow start to increase the congestion window after a connection is initialized or after a timeout. It starts with a window of two times the maximum segment size (MSS). Although the initial rate is low, the rate of increase is very rapid; for every packet acknowledged, the congestion window increases by 1 MSS so that the congestion window effectively doubles for every round-trip time (RTT).

When the congestion window exceeds the ssthresh threshold, the algorithm enters a new state, called congestion avoidance. In congestion avoidance state as long as non-duplicate ACKs are received, the congestion window is additively increased by one MSS every round-trip time.

In TCP, the congestion window is one of the factors that determines the number of bytes that can be outstanding at any time. The congestion window is maintained by the sender. Note that this is not to be confused with the TCP window size which is maintained by the receiver. The congestion window is a means of stopping a link between the sender and the receiver from becoming overloaded with too much traffic. It is calculated by estimating how much congestion there is on the link.

When a connection is set up, the congestion window, a value maintained independently at each host, is set to a small multiple of the MSS allowed on that connection. Further variance in the congestion window is dictated by an AIMD approach. This means that if all segments are received and the acknowledgments reach the sender on time, some constant is added to the window size. The window keeps growing exponentially until a timeout occurs or the sender reaches its limit (a threshold value "ssthresh"). If the sender reaches this threshold, the congestion window increases linearly at the rate of 1/(congestion window) segment on each new acknowledgement received.

...
Wikipedia