# What will happen if congestion window size touches to zero in congestion control mechanisms in TCP?

Folowing is the Congestion Control Window graph of TCP. What will happen if Congestion Window reaches to zero when timeout occurs ?

• On timeout, the TCP protocol executes the following three steps: (1) congestion window is reset to 1 MSS, (2) ssthresh is set to half the congestion window size before the timeout, and (3) slow start is initiated. You can see that according to this protocol, congestion window never reaches $0$ after a timeout. – zdm Sep 3 '18 at 22:14