TCP MSS and window size

MSS is found in the options fields of the TCP header in the SYN packet and is defined during the 3-way handshake. It cannot be changed during a session. It has a maximum value of 65535 (2 byte field).

The default TCP Maximum Segment Size is for IPv4 is 536. For IPv6 it is 1220 but it can be changed as mentioned above.

The MSS can be adjusted at the beginning of a session using the TCP MSS feature on Cisco devices.

Window size has a maximum value of 65535 (2 byte field) and is found in the main TCP header. It can be renegotiated during a session.