TCP State

TCP sockets have numerous states. Many of these can be seen in the output of a properly timed netstat command.

Handshake states:

• LISTEN: Awaiting a SYN
• SYN-Sent: A client has sent the SYN, and is awaiting the SYNACK
• SYN-RECIEVED: A server has received and a SYN, sent a SYNACK, and is awaiting the ACK

Data states:

• ESTABLISHED: Normal data transmission occurring.
• FIN-WAIT-1: Client FIN sent, awaiting acknowledgement from Server. May still receive data.
• FIN-WAIT-2: Client FIN acknowledged by Server (half closed), awaiting Server FIN. May still receive data.
• CLOSE_WAIT: Client FIN received and acknowledged by Server. Waiting for Server to send it's own FIN (This is the server view of FIN-WAIT-1 & 2).
• LAST-ACK: Server sends FIN. Awaiting Client to acknowledge it for teardown.
• TIME-WAIT: There is a "cooldown" period before the socket can be closed for good.
• CLOSED: No more connection.

TIME-WAIT is why you occasionally get errors that a port is in use when rapidly re-running code that creates network socket.

You can use socket options to solve this.