Slow Start

Slow start is a mechanism devised by Van Jacobson for a TCP connection to real equilibirum. The proposed mechanism is used whenever starting of restarting a connection after a loss, ie it is used when the connection wants to establish or re-establish a flow of packets in transit.

By using slow start, the sender quickly, but gradually and reassuringly increases the rate in which data is sent out.

The entire mechanism is controlled through a congestion window (cwnd) that starts of with a initial value of 1 segment. Upon reciept of an acknowledgement for this packet in slow start, it increases the size of the cwnd by one (StandardTCP). This is equivalent to an exponent increase in the cwnd size with every ack as recieving the ack results in sending another packet plus a new packet into the network.

The increase of cwnd, as expected is quite aggressive. As such, it is only used to get TCP to a stable point for Congestion Avoidance to enable a stable flow of packets out of the sender. We wish TCP to reach this equilibrium quickly which is why Slow Start is aggressive.

Several studies [J. Hoe; Improving the slow start behavior of Congestion Control Scheme for TCP, K. Fall and S. Floyd; Simulation-based Comparisions of Tahoe, Reno and SACK TCP]

Wed, 23 July, 2003 13:07