To keep track of all the different events happening during connection establishment, connection termination, and data transfer, TCP is specified as the finite state machine shown in Figure.

The figure shows the two FSMs used by the TCP client and server combined in one diagram. The ovals represent the states. The transition from one state to another is shown using directed lines.

The dotted black lines in the figure represent the transition that a server normally goes through; the solid black lines show the transitions that a client normally goes through.

The state marked as ESTBLISHED in the FSM is in fact two different sets of states that the client and server undergo to transfer data.

The states for TCP are as follows: