http://www.itpub.net/490412.html
http://www.adp-gmbh.ch/ora/network/sqlnet_ora.html#sqlnet_expire_time
sqlnet.expire_time
Enables dead connection detection, that is, after the
specifed time (in minutes) the server checks if the client is still
connected. If not, the server process exits.
This parameter must be set on the server.