EE 281 Lecture Notes - Lecture 7: Data Link, Constant Bitrate, Forwarding Plane
Document Summary
If we start with a congestion window of one mss and increase it linearly, it can take a long time before we reach an effective transmission rate. Tcp slow start prevents this slow ramp at startup by increasing the cwnd size exponentially. The congestion window starts at one mss and increases by one mss with each received ack, causing it to double every rtt. Slow start starts off slowly but speeds up quickly. It continues to increase until cwnd reaches a threshold level, called ssthresh (slow start threshold). Initially, ssthresh is effectively not set (set to a maximum value), so the rate of transmission continues to increase exponentially until a transmission times out waiting for an ack. At this time, the protocol sets the threshold, ssthresh, to one half of the window size that resulted in the rto and restarts the slow start process.
