TCP has an attribute called " State " that shows what kind of situation the host is in.
For example, a "state" in which TCP connection is established and data communication is possible is called Established .
The figure below is a state transition diagram of TCP.
The host that sends SYN first, the server that first receives SYN, the host that sends FIN first is Active Close host, and the host that receives FIN first is called Passive Close host.(It is said that FIN can be sent first from either client or server.)
Confirm TCP status
To check the status of TCP, use the command prompt for Windows, or the netstat command for shell on Linux.The following is an example of Windows.
For studying TCP / UDP, the following books are recommended.