Slow-start

Slow-start is part of the congestion control strategy used by TCP, the data transmission protocol used by many Internet applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network congestion. The algorithm is specified by RFC 5681.

Algorithm

Slow-start is one of the algorithms that TCP uses to control congestion inside the network. It is also known as the exponential growth phase.

Slow-start begins initially with a congestion window Size (cwnd) of 1, 2 or 10. The value of the Congestion Window will be increased with each acknowledgement (ACK) received, effectively doubling the window size each round trip time ("although it is not exactly exponential because the receiver may delay its ACKs, typically sending one ACK for every two segments that it receives"). The transmission rate will be increased with slow-start algorithm until either a loss is detected, or the receiver's advertised window (rwnd) is the limiting factor, or the slow start threshold (ssthresh) is reached. If a loss event occurs, TCP assumes that it is due to network congestion and takes steps to reduce the offered load on the network. These measurements depend on the used TCP congestion avoidance algorithm. Once ssthresh is reached, TCP changes from slow-start algorithm to the linear growth (congestion avoidance) algorithm. At this point, the window is increased by 1 segment for each RTT.