Hi,
It's like when you meet someone new; you handshake at start for HELLO and handshake at the end for BYE. The same goes for TCP, where the TCP session is build via a handshake and torn down via another form oh handshake (for a graceful connection closeup). What happens in between the session being established (at start) and session being closed (at the end), it's just data transfer. Here's a close look un session establishment and session closing:
https://support.microsoft.com/en-gb/help/172983/explanation-of-the-three-way-handshake-via-tcp-ip
Think about it, if when you meet someone you'll constantly and continuously perform handshake, there will be no time to do anything else, so the 3-way handshake is what brings the session up at start, what happens after is data being transmitted back and forward, while in the end the session gets terminated by a 4-way handshake.
Regards,
Cristian Matei.
