service tcp-keepalives-in

[Steve]

Hiii all,
i've seen this recently:

R1(config)#line vty 0 4
R1(config-line)#exec-timeout 3
R1(config-line)#exit
R1(config)#service tcp-keepalives-in

I know that the exec-timeout command prevents an idle session from consuming the VTY indefinitely, but i couldn't understand what does the service tcp-keepalives-in mean?? could someone plz explain to me?
much appreciated

[gabrielshorn]

The "service tcp-keepalives-in|out" commands are to enable a Cisco device to send and receive TCP keepalive messages. These messages that are part of the TCP protocol are sent when a TCP connection has been completely idle for a pre-determined amount of time. When the idle timer for a connection hits zero, the TCP protocol sends an empty TCP segment with just the ACK flag turned on. If the remote connection is still active, it replies with an empty reply with just the ACK bit set as well. At this point, the initiator of the keepalive resets its idle timer for that connection to its original value and starts counting down again. If the initiator doesn't receive an ACK from the remote host, it assumes the connection has died and closes the connection.

[Steve]

thank u for the clarification :)