Keepalive

TCP Keepalive Probes Test Idle Connections

A TCP connection can remain established while carrying no application data. That is valid behavior: an open connection does not need a continuous stream of packets to remain a TCP connection. Silence creates a practical problem when one endpoint disappears without completing the normal close sequence. A machine can lose power, a network path can fail, or state in an intermediate device can vanish. If the surviving endpoint has no data to send, ordinary retransmission logic has nothing to act on.