KeepAlive socket in 12c listener

A not uncommon issue with firewalls and listeners are timeouts. Your production database may be behind a firewall, you may connect from a remote location, even your Windows workstation may have some firewall activated, possibly you use ssh tunnels or TCPS. All those occasionally lead to timeouts and connection abortion, for instance ORA-03113 end-of-file on communication channel, ORA-03135: connection lost contact, TNS-12547 Lost contact.

The good news is that Oracle 12c now implements Socket Options (see man setsockopt), as documented in Net admin new features and more nicely in Note 1591874.1
dcd visualized

I made until now a positive experience with this keepalive behavior, especially with SSL listener. The default value for SQLNET.EXPIRE_TIME is 0, so you must set it to a non-zero value first, the recommended value is 10 (minutes).