18.9 Simultaneous Close

We said earlier that one side (often, but not always, the client) performs the active close, causing the first FIN to be sent. It's also possible for both sides to perform an active close, and the TCP protocol allows for this simultaneous close.

In terms of Figure 18.12, both ends go from ESTABLISHED to FIN_WAIT_1 when the application issues the close. This causes both FINs to be sent, and they probably pass each other somewhere in the network. When the FIN is received, each end transitions from FIN_WAIT_1 to the CLOSING state, and each state sends its final ACK. When each end receives the final ACK, the state changes to TIME_WAIT. Figure 18.19 summarizes these state changes.

Figure 18.19. Segments exchanged during simultaneous close.

With a simultaneous close the same number of segments are exchanged as in the normal close.