Editing Keepalive (section)

==Description==

Once a TCP connection has been established, that connection is defined to be valid until one side closes it. Once the connection has entered the connected state, it will remain connected indefinitely. But, in reality, the connection will not last indefinitely. Many firewall or NAT systems will close a connection if there has been no activity in some time period. The Keep Alive signal can be used to trick intermediate hosts to not close the connection due to inactivity. It is also possible that one host is no longer listening (e.g. application or system crash). In this case, the connection is closed, but no <code>FIN</code> was ever sent. In this case, a KeepAlive packet can be used to interrogate a connection to check if it is still intact.

A keepalive signal is often sent at predefined intervals, and plays an important role on the [[Internet]]. After a signal is sent, if no reply is received, the [[Data link|link]] is assumed to be down and future data will be routed via another path until the link is up again. A keepalive signal can also be used to indicate to Internet infrastructure that the connection should be preserved. Without a keepalive signal, intermediate [[Network address translation|NAT-enabled routers]] can drop the connection after timeout.

Since the only purpose is to find links that do not work or to indicate connections that should be preserved, keepalive messages tend to be short and not take much [[Bandwidth (computing)|bandwidth]]. However, their precise format and usage terms depend on the communication protocol.