SQLNET.INBOUND_CONNECT_TIMEOUT has been introduced in version 9i.This has to be configured in sqlnet.ora file specifies the time,in seconds, for a client to connect with the database server and provide the necessary authentication information.
If there are network latencies and the client fails to establish a connection and complete authentication in the time specified, then the database server terminates the connection logs the IP address of the client and an ORA-12170: TNS:Connect timeout occurred error message to the sqlnet.log file.
The client receives either an ORA-12547: TNS:lost contact or an ORA-12637: Packet receive failed error message. In version 10g and higher, the ORA-3136 errors may appear in the alert.log.
To protect both the database server and the listener for denial of service attacks
Oracle Corporation recommends setting this parameter in combination with the INBOUND_CONNECT_TIMEOUT_listener_name parameter in the listener.ora file.
In 10gR2, the default setting for these parameters is 60 seconds.
So set the parameters to at least that value in pre-10gR2 releases because there is no default setting for this versions.
Set on sqlnet.ora
Set on listener.ora for each listener