TCP slow start

TCP slow start helps buildup transmission speeds to the network's capabilities. It does this without initially knowing what those capabilities are and without creating congestion. TCP slow start is an algorithm used to detect the available bandwidth for packet transmission, and balances the speed of a network connection. It prevents the appearance of network congestion whose capabilities are initially unknown, and slowly increases the volume of information diffused until the network's maximum capacity is found.

To implement TCP slow start, the congestion window (cwnd) sets an upper limit on the amount of data a source can transmit over the network before receiving an acknowledgment (ACK). The slow start threshold (ssthresh) determines the (de)activation of slow start. When a new connection is made, cwnd is initialized to one TCP data or acknowledgment packet, and waits for an acknowledgement, or ACK. When that ACK is received, the congestion window is incremented until the cwnd is less than ssthresh. Slow start also terminates when congestion is experienced.