What is the difference between the Slow Start phase in TCP operation and the Congestion Avoidance phase? During Slow Start the initial window size is always set to one MSS unit. The window is then incremented by doubling its size with every RTT. During congestion avoidance, the window size grows by one MSS with every time increment (given in units of RTT).. During Slow Start the initial window size is half the maximum size as negotiated during the TCP handshake. The window is then incremented by doubling its size with every RTT. During Congestion Avoidance, the window size grows by one MSS with every time increment (given in units of RTT). During Slow Start the initial window size is the maximum size as negotiated during the TCP handshake. The window is then reduced by one MSS with every time increment (given in units of RTT). During Congestion Avoidance, the window size grows by one MSS with every time increment (given again in units of RTT).