TCP states - explained

akashtp · ‎08-03-2015

CLOSED: There is no connection.
LISTEN: The local end-point is waiting for a connection request from a remote end-point i.e. a passive open was performed.
ESTABLISHED: The third step of the three-way connection handshake was performed. The connection is open.
FIN-WAIT-1: The first step of an active close (four-way handshake) was performed. The local end-point has sent a connection termination request to the remote end-point.
CLOSE-WAIT: The local end-point has received a connection termination request and acknowledged it e.g. a passive close has been performed and the local end-point needs to perform an active close to leave this state.
FIN-WAIT-2: The remote end-point has sent an acknowledgement for the previously sent connection termination request. The local end-point waits for an active connection termination request from the remote end-point.
LAST-ACK: The local end-point has performed a passive close and has initiated an active close by sending a connection termination request to the remote end-point.
CLOSING: The local end-point is waiting for an acknowledgement for a connection termination request before going to the TIME-WAIT state.
TIME-WAIT: The local end-point waits for twice the maximum segment lifetime (MSL) to pass before going to CLOSED to be sure that the remote end-point received the acknowledgement.

FIN_WAIT_2 seems to occur when the server has an active connection with a client and wants to shut down the TCP connection (probably in response to a normal application layer "exit"). The server sends the client a packet with a "FIN" bit set. At this point, the server is in FIN_WAIT_1 state. The client gets the FIN packet and goes into CLOSE_WAIT state, and sends an acknowledgment packet back to the server. When the server gets that packet, it goes into FIN_WAIT_2 state. From the server's perspective, the connection is now closed, and the 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 the server TCP implementation should ACK. The server should close after a period of time defined by the Maximum Segment Lifetime (MSL).

    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

Report Inappropriate Content · ‎09-27-2019

Hello,

I had issues, when socket used to hang at LAST_ACK - it was a bug reported by me and later fixed by FreeBSD-SA-15:13.tcp. For more information about how to treat LAST_ACK: what can cause it to hang on the IIS webserver service.