TIME_WAIT vs CLOSE_WAIT

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CLOSE_WAIT vs TIMEWAIT

Some snippets from the blog:
TIME_WAIT and CLOSE_WAIT are the most confusing two among TCP’s 11 states (CLOSED, LISTEN, SYN_SENT, SYN_RECV, ESTABLISHED, CLOSE_WAIT, LAST_ACK, FIN_WAIT1, FIN_WAIT2, CLOSING, and TIME_WAIT), which are displayed by netstat(1).

Active Open: An end sends a SYN segment to the other end by calling connect(2). This end is usually called a client.
Passive Open: An end issues a passive open by calling socket(2), bind(2), and listen(2) so that it can accept(2) the clients’ connections. This end is usually called a server.
Active Close: An end performs active close when it calls close(2) first. It results a FIN segment being sent.
Passive Close: The other end receives the FIN segment performs the passive close.

The end that performs the active close goes through the TIME_WAIT state; while the end performs the passive close enters the CLOSE_WAIT state. Note that either end (the server or the client) can perform the active close!

What’s the TIME_WAIT state for?
1) When the final ACK being sent by the end that performed the active close was lost, the other end would resend a FIN. TCP maintains the information of the connection, hence it would resend the lost ACK rather than respond with an RST (interpreted as error).
2) To allow wandering duplicates to expire in the network, so that the duplicate segments would not corrupt later connections.

The value of TIME_WAIT delay is 2MSL, which is TCP implementation dependent and is generally about 1-4 minutes.

CLOSE_WAIT happens when an end has received a FIN segment from the other end, to wait for the program to close the socket. Long duration of CLOSE_WAIT suggests that there might be a bug in your program. In fact, a TCP connection can stay in the CLOSE_WAIT state forever unless it is explicitly closed. It is the application’s responsibility to close its socket after use, to release the resource of the connection.
MSL: Maximum Segment Lifetime, the maximum time a segment can live in the network before being discarded.