TimeWait

TIMEWAIT is a state that a TCP connection enters after it has been closed by the local end. This state is necessary to ensure that any delayed segments from the previous incarnation of the connection can be identified and discarded, preventing them from being misinterpreted as belonging to a new connection. When a TCP connection is closed by the local side, it sends a FIN segment and then enters the TIMEWAIT state. It remains in this state for a period known as the 2MSL (two times the maximum segment lifetime) timer. The 2MSL is the maximum amount of time a segment is expected to exist on the network.

During the TIMEWAIT state, the local host waits to receive any segments that might still be in transit. If a segment arrives with the same source IP address, source port, destination IP address, and destination port as the connection in TIMEWAIT, and its sequence number is within the acceptable window, it is considered a delayed segment. Such segments are acknowledged and then discarded. This prevents a new connection from being established with the same parameters from accidentally accepting old data. Once the 2MSL timer expires, the connection resources are released, and the port becomes available for reuse. The TIMEWAIT state is a crucial part of TCP's reliability mechanisms.