Overview of TCP congestion control

TCP implements a window based flow control mechanism. Roughly speaking, a window based protocol means that the so called current window size defines a strict upper bound on the amount of unacknowledged data that can be in transit between a give sender-receiver pair. Originally TCP’s flow control was governed simply by the maximum allowed window size advertised by the receiver and the policy that allowed the sender to send new packets only after receiving the acknowledgement for the previous packet.

After the occurrence of the so called congestion collapse in the Internet in the late 80’s it was realised, however, that special congestion control algorithms would be required to prevent the TCP senders from overrunning the resources of the network. In 1988, Tahoe TCP was released including three congestion control algorithms: slow start, congestion avoidance and fast retransmit. In 1990 Reno TCP, providing one more algorithm called fast recovery, was released.

Besides the receiver’s advertised window, awnd, TCP’s congestion control introduced two new variables for the connection: the congestion window, cwnd, and the slowstart threshold, ssthresh. The window size of the sender, w, was defined to be

w = min(cwnd, awnd),

instead of being equal to awnd. The congestion window can be thought of as being a counterpart to advertised window. Whereas awnd is used to prevent the sender from overrunning the resources of the receiver, the purpose of cwnd is to prevent the sender from sending more data than the network can accommodate in the current load conditions.

The idea is to modify cwnd adaptively to reflect the current load of the network. In practice, this is done through detection of lost packets. A packet loss can basically be detected either via a time-out mechanism or via duplicate ACKs.

If a packet has been lost, the receiver keeps sending acknowledgements but does not modify the sequence number field in the ACK packets. When the sender observes several ACKs acknowledging the same packet, it concludes that a packet has been lost.