A connection progresses through a series of states during its lifetime.
The states are:
- LISTEN,
- SYN-SENT,
- SYN-RECEIVED,
- ESTABLISHED,
- FIN-WAIT-1,
- FIN-WAIT-2,
- CLOSE-WAIT,
- CLOSING,
- LAST-ACK,
- TIME-WAIT,
- CLOSED.
CLOSED is fictional because it represents the state when there is no TCB, and therefore, no connection.
Briefly the meanings of the states are:
-
LISTEN : represents waiting for a connection request from any remote TCP and port.
-
SYN-SENT : represents waiting for a matching connection request after having sent a connection request.
-
SYN-RECEIVED : represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.
-
ESTABLISHED : represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
-
FIN-WAIT-1 : represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.
-
FIN-WAIT-2 : represents waiting for a connection termination request from the remote TCP.
-
CLOSE-WAIT : represents waiting for a connection termination request from the local user(app).
-
CLOSING : represents waiting for a connection termination request acknowledgment from the remote TCP.
-
LAST-ACK : represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request).
-
TIME-WAIT : represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request.
-
CLOSED : represents no connection state at all.
A TCP connection progresses from one state to another in response to events.
The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, ABORT, and STATUS; the incoming segments, particularly those containing the SYN, ACK, RST and FIN flags; and timeouts.