To prove why TCP is a three-way handshake is to prove that two is too few and four is not necessary.
What is the nature of the so-called three-way handshake? In fact, the client needs to inspect the sending and receiving capabilities of the server, and the server also needs to inspect the sending and receiving capabilities of the client.
For the first time,
Inspection of the client
Sending capability: OK
Reception capacity: No
An examination of the server side
Sending capability: No
Reception capacity: No
The second time
Inspection of the client
Sending capability: OK
Reception capacity: OK
An examination of the server side
Sending capability: OK
Reception capacity: No
The third time
Inspection of the client
Sending capability: OK
Reception capacity: OK
An examination of the server side
Sending capability: OK
Reception capacity: OK