TCP Keepalive timer
Optional timer which probes to see if idle connection ok.
Not part of TCP spec, but found in most implementations.
- Not necessary, because “connection” defined by endpoints. Connection can be “up”as long as source/destination “up”. Intermediate routers etc may crash and reboot multiple times
Typical use: to detect idle clients or half-open connections and deallocate server resources tied up to them. Eg: done in telnet, ftp: 900s