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TCP Operational Overview and the TCP Finite State Machine (FSM)
(Page 3 of 3)
Finite State Machine Steps Represent the Stages of a Connection
It's important to remember that this
state machine is followed for each connection. This means
at any given time TCP may be in one state for one connection to socket
X, while in another for its connection to socket Y. Also, the typical
movement between states for the two processes in a particular connection
is not symmetric, because the roles of the devices are not symmetric:
one device initiates a connection, the other responds; one device starts
termination, the other replies. There is also an alternate path taken
for connection establishment and termination if both devices initiate
simultaneously (which is unusual, but can happen). This is shown by
the color codings in Figure 210.
Thus, for example, at the start of
connection establishment, the two devices will take different routes
to get to ESTABLISHED: one device (the server usually) will pass
through the LISTEN state while the other (the client) will go
through SYN-SENT. Similarly, one device will initiate connection
termination and take the path through FIN-WAIT-1 to get back
to CLOSED; the other will go through CLOSE-WAIT and LAST-ACK.
However, if both try to open at once, they each proceed through SYN-SENT
and SYN-RECEIVED, and if both try to close at once, they go through
FIN-WAIT-1, CLOSING and TIME-WAIT roughly simultaneously.
 Key Concept: The TCP finite state machine describes the sequence of steps taken by both devices in a TCP session as they establish, manage and close the connection. Each device may take a different path through the states since under normal circumstances the operation of the protocol is not symmetric—one device initiates either connection establishment or termination, and the other responds.
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