ORA-609 , ORA-12170 and TNS-12537 in 11g Alert.log file

The ORA-609 error is thrown when a client connection of any kind failed to complete or aborted the connection process before the connection/authentication process was complete.

Other error may occurred due to this paramter
ORA-12170: TNS:Connect timeout occurred error message to the sqlnet.log file.
ORA-12547: TNS:lost contact
ORA-12637: Packet receive failed error message.

Solution:
Increase value of INBOUND_CONNECT_TIMEOUT parameter in both SQLNET and LISTENER file.
Inbound_connect_timeout parameter is the time in Seconds used for client to connect with the database and within time provide authentication to connection.

SQLNET.ORA : SQLNET.INBOUND_CONNECT_TIMEOUT=180
Listener.ora: INBOUND_CONNECT_TIMEOUT_listener_name=120

In Listener, Oracle Recommends to set this parameter in combination for protect both the listener and DB Server. Set the Listener parameter to lower value than sqlnet parameter.If client not able to make connection and got above error then increased the above parameter as needed.
You need to mention this parameter with listener name and reload the listener after changes.

lsnrctl reload

If linux system is used , change the value in /etc/sysctl.conf file

net.ipv4.ip_forward = 1

If the issue is not resolved from it, check with oracle support.