FINWAIT1

FINWAIT1 is a transient state in the TCP (Transmission Control Protocol) connection lifecycle. It occurs when a host has actively closed a connection by sending a FIN segment and is waiting for a response from the peer. The state is part of the TCP finite state machine used to manage the orderly shutdown of a connection.

In FINWAIT1, the local endpoint has already transmitted its FIN and is awaiting further action. The two possible events determining the next step are: (1) the arrival of an acknowledgment (ACK) for the local FIN, which moves the connection to FINWAIT2; or (2) the arrival of a FIN from the remote peer, which indicates a simultaneous or peer-initiated close. If a FIN is received, the local host responds with an ACK and transitions to the CLOSING state, where it will wait for the final acknowledgment of the peer’s FIN.

From FINWAIT1, receipt of an ACK for the local FIN leads to FINWAIT2, while receipt of the remote FIN (followed by an ACK) leads to CLOSING and eventually to TIME_WAIT after the final ACK is exchanged. The exact transition depends on the order in which the FIN and ACK messages are exchanged between the hosts.

FINWAIT1 is a temporary state; its duration is determined by network delays and the peer’s behavior. It exists on both ends of a TCP connection, representing the period after one side has initiated closure and before the final teardown is completed. It is one step in the broader sequence of states that include FINWAIT2, CLOSING, TIME_WAIT, and others described in the TCP state machine (RFC 793).