TCP Reno
This is the classical Congestion control in TCP. It starts the window size to be 1 packets big. It uses AIMD with two different signals.
- If a message is waited on 3 times, it halves the congestion window in AIMD.
- If it reaches a timeout window (i.e. no message is received for a given length of time). It resets the congestion window back to 1.
This however is quite slow to start. Therefore Reno implements “slow start” doubling up to some threshold. This means that while the congestion window is below the that threshold every RTT it doubles the congestion window. Once it passes this threshold or it drops a packet it switchest back to AIMD The “slow start” also takes place if we have a timeout. In this case the threshold is set to be the congestion window before the timeout.