TCP State Transition Diagram

The operation of TCP with regard to connection establishment and connection termination can be specified with a state transition diagram.

There are 11 different states defined for a connection and the rules of TCP dictate the transitions from one state to another, based on the current state and the segment received in that state. For example, if an application performs an active open in the CLOSED state, TCP sends a SYN and the new state is SYN_SENT. If TCP next receives a SYN with an ACK, it sends an ACK and the new state is ESTABLISHED. This final state is where most data transfer occurs.(CLOASED => SYN_SENT => ESTABLISHED)

The two arrows leading from the ESTABLISHED state deal with the termination of a connection. If an application calls close before receiving a FIN (an active close), the transition is to the FIN_WAIT_1 state. But if an application receives a FIN while in the ESTABLISHED state (a passive close), the transition is to the CLOSE_WAIT state.

We denote the normal client transitions with a darker solid line and the normal server transitions with a darker dashed line. We also note that there are two transitions that we have not talked about: a simultaneous open (when both ends send SYNs at about the same time and the SYNs cross in the network) and a simultaneous close (when both ends send FINs at the same time).

One reason for showing the state transition diagram is to show the 11 TCP states with their names. These states are displayed by netstat, which is a useful tool when debugging client/server applications. We can use netstat to monitor state changes.