WARNING: inbound connection timed out (ORA-3136)

WARNING: inbound connection timed out (ORA-3136)

When you see the error in the alert.log file

NOTE: It is normal warnings message , can be ignored.
Solution: 
set the following parameter in the sqlnet.ora
Both parameter will be set in the 'DB SERVER' side.
In the sqlnet.ora
SQLNET.INBOUND_CONNECT_TIMEOUT = 120
In the listener.ora
INBOUND_CONNECT_TIMEOUT_LISTENER = 120

Also refer below Metalink note : 465043.1