Slow start sets a lower limit on the amount of TCP data which may be sent instead of limiting the amount solely on the remote host's window size. When slow start goes into effect, the amount of TCP data which may be sent is the maximum segment size. Each time the data is acknowledged, the amount of data which may be sent is increased by the maximum segment size. Slow start goes into effect when a connection is first established or when a ICMP Source Quench message is received and can go into effect by the TCP Congestion Avoidance algorithm.

Left TCP

TCP Nagle Algorithm

Down TCP Congestion Avoidance