Dead Connection Detection (DCD) attempts to detect the unexpected loss of an Oracle connection where the client does not properly disconnect itself from the session.
DCD can also be used to keep database connections alive accross a firewall where the firewall is configured to terminate idle connections.
DCD is enabled by setting the sqlnet.ora parameter sqlnet.expire_time on the server.
With DCD, the server sends probe packets in the intervals specified by the sqlnet.expire_time value.