congestion avoidance (CA)

Congestion Avoidance (CA) is an overload control for avoiding congestion situations in routers. If the queues of routers are full, they normally discard pending data packets.

In the TCP protocol,packet losses first lead to a reduction in the transmission rate. This is increased again as soon as no more packet losses are reported. The algorithm developed to reduce the transmission rate in congestion situations is also used if data packets are discarded simultaneously on several TCP connections. The result is that all affected senders reduce their data rate, but immediately increase it again as soon as no more packet losses occur. This phenomenon is called "global synchronization" and has the effect that an overload of links due to the congestion situation is alternately followed by an underload.

To compensate for this phenomenon, various congestion detection and elimination techniques have been developed by randomly or selectively discarding data packets: Random Early Detection( RED) and Weighted Random Early Detection(WRED).