CLOSE Call

    CLOSED STATE (i.e., TCB does not exist)

      If the user does not have access to such a connection, return
      "error:  connection illegal for this process".

      Otherwise, return "error:  connection does not exist".

    LISTEN STATE

      Any outstanding RECEIVEs are returned with "error:  closing"
      responses.  Delete TCB, enter CLOSED state, and return.

    SYN-SENT STATE

      Delete the TCB and return "error:  closing" responses to any
      queued SENDs, or RECEIVEs.

    SYN-RECEIVED STATE

      If no SENDs have been issued and there is no pending data to send,
      then form a FIN segment and send it, and enter FIN-WAIT-1 state;
      otherwise queue for processing after entering ESTABLISHED state.

    ESTABLISHED STATE

      Queue this until all preceding SENDs have been segmentized, then
      form a FIN segment and send it.  In any case, enter FIN-WAIT-1
      state.

    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE

      Strictly speaking, this is an error and should receive a "error:
      connection closing" response.  An "ok" response would be
      acceptable, too, as long as a second FIN is not emitted (the first
      FIN may be retransmitted though).

    CLOSE-WAIT STATE

      Queue this request until all preceding SENDs have been
      segmentized; then send a FIN segment, enter CLOSING state.

    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE

      Respond with "error:  connection closing".