In TCP networking, what is a FIN_WAIT state?

FIN_WAIT_2 seems to occur when the server has an active connectionwith a client and it wants to shut down the TCP connection (probably inresponse to a normal application layer "exit"). The server sends theclient a packet with a "FIN" bit set. At this point, the server is inFIN_WAIT_1 state. The client gets the FIN packet and goes intoCLOSE_WAIT state, and sends an acknowledgment packet back to theserver. When the server gets that packet, it goes into FIN_WAIT_2state. From the server's perspective, the connection is now closed, andthe server can't send any more data. However, under the TCP protocol,the client needs to shut down also by sending a FIN packet, which theserver TCP implementation should ACK. The server should close about twomilliseconds later.

The following diagram is taken from RFC 793 (defining the TCP protocol):

TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 4. (Close) TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --< CLOSED 6. (2 MSL) CLOSED

Some clients have faulty TCP implementations, and leave the server hanging in FIN_WAIT_2 state. In particular, this can cause problems
with Indiana University's Unicorn system.