WARNING: inbound connection timed out (ORA-3136)

The "WARNING: inbound connection timed out (ORA-3136)" in the alert log indicates that the client was not able to complete it's authentication within the period of time specified by parameter SQLNET.INBOUND_CONNECT_TIMEOUT.

You may also witness ORA-12170 without timeout error on the database server sqlnet.log file.
This entry would also have the clinet address which failed to get authenticated. Some applications or JDBC thin driver applications may not have these details.
From 10.2 onwards the default value of this parameter is 60 seconds, hence if the client is not able authenticate within 60 secs , the warning would appear in the alert log and the client connection will be terminated.
This timeout restriction was introduced to combat Denial of Service (DoS) attack whereby malicious clients attempt to flood database servers with connect requests that consumes resources.

There can be three main reasons for this error

1. Server gets a connection request from a malicious client which is not supposed to connect to the database , in which case the error thrown is the correct behavior. You can get the client address for which the error was thrown via sqlnet log file.
2. The server receives a valid client connection request but the client takes a long time to authenticate more than the default 60 seconds.
3. The DB server is heavily loaded due to which it cannot finish the client logon within the timeout specified.

The default value of 60 seconds is good enough in most conditions for the database server to authenticate a client connection. If its taking longer period, then its worth checking all the below points before going for the workadound:

1. Check whether local connection on the database server is sucessful & quick.
2. If local connections are quick ,then check for underlying network delay with the help of your network administrator.
3. Check whether your Database performance has degraded by anyway.
4. Check alert log for any critical errors for eg, ORA-600 or ORA-7445 and get them resolved first.

These critical errors might have triggered the slowness of the database server.

As a workaround to avoid only this warning messages, you can set the parameters SQLNET.INBOUND_CONNECT_TIMEOUT and INBOUND_CONNECT_TIMEOUT_listenername
to the value more than 60.
In server side sqlnet.ora file add SQLNET.INBOUND_CONNECT_TIMEOUT

SQLNET.INBOUND_CONNECT_TIMEOUT = 120

In listener.ora file INBOUND_CONNECT_TIMEOUT_ listenername

INBOUND_CONNECT_TIMEOUT_LISTENER = 110

From Oracle version 10.2.0.3 onwards the default value of INBOUND_CONNECT_TIMEOUT_ is 60 seconds. For previous releases it is zero by default.

More on meta link note 465043.1 and 345197.1