This chapter explains three sequences of state transitions during TCP connection teardown. There is a fourth possible sequence, which traverses an additional arc (not shown in Figure 5.7) from FIN_WAIT_1 to TIME_WAIT and labeled FIN+ACK/ACK. Explain the circumstances that result in this fourth teardown sequence.     Notes from the book to help: There is actually a fourth, although rare, sequence of transitions that leads to the CLOSED state; it follows the arc from FIN WAIT 1 to TIME WAIT. We leave it as an exercise for you to figure out what combination of circumstances leads to this fourth possibility. The main thing to recognize about connection teardown is that a connection in the TIME WAIT state cannot move to the CLOSED state until it has waited for two times the maximum amount of time an IP datagram might live in the Internet (i.e., 120 seconds). The reason for this is that, while the local side of the connection has sent an ACK in response to the other side’s FIN segment, it does not know that the ACK was successfully delivered. As a consequence, the other side might retransmit its FIN segment, and this second FIN segment might be delayed in the network. If the connection were allowed to move directly to the CLOSED state, then another pair of application processes might come along and open the same connection (i.e., use the same pair of port numbers), and the delayed FIN segment from the earlier incarnation of the connection would immediately initiate the termination of the later incarnation of that connection.

This chapter explains three sequences of state transitions during TCP connection teardown. There is a fourth possible sequence, which traverses an additional arc (not shown in Figure 5.7) from FIN_WAIT_1 to TIME_WAIT and labeled FIN+ACK/ACK. Explain the circumstances that result in this fourth teardown sequence.

Notes from the book to help: There is actually a fourth, although rare, sequence of transitions that leads to the CLOSED state; it follows the arc from FIN WAIT 1 to TIME WAIT. We leave it as an exercise for you to figure out what combination of circumstances leads to this fourth possibility. The main thing to recognize about connection teardown is that a connection in the TIME WAIT state cannot move to the CLOSED state until it has waited for two times the maximum amount of time an IP datagram might live in the Internet (i.e., 120 seconds). The reason for this is that, while the local side of the connection has sent an ACK in response to the other side’s FIN segment, it does not know that the ACK was successfully delivered. As a consequence, the other side might retransmit its FIN segment, and this second FIN segment might be delayed in the network. If the connection were allowed to move directly to the CLOSED state, then another pair of application processes might come along and open the same connection (i.e., use the same pair of port numbers), and the delayed FIN segment from the earlier incarnation of the connection would immediately initiate the termination of the later incarnation of that connection.