A keepalive (KA) is a message sent by one device to another to check that the link between the two is operating, or to prevent the link from being broken. 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. Most firewall or NAT systems will close a connection if there has not been any 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 ```FIN``` 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 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 NAT-enabled routers can drop the connection after timeout. Since the only purpose is to find links that don't work or to indicate connections that should be preserved, keepalive messages tend to be short and not take much bandwidth. However, their precise format and usage terms depend on the communication protocol. TCP keepalive Transmission Control Protocol (TCP) keepalives are an optional feature, and if included must default to off. The keepalive packet contains no data. In an Ethernet network, this results in frames of minimum size (64 bytes). There are three parameters related to keepalive: Keepalive time is the duration between two keepalive transmissions in idle condition. TCP keepalive period is required to be configurable and by default is set to no less than 2 hours. Keepalive interval is the duration between two successive keepalive retransmissions, if acknowledgement to the previous keepalive transmission is not received. Keepalive retry is the number of retransmissions to be carried out before declaring that remote end is not available When two hosts are