When the application is not used for a significant amount of time, the firewall setting between database & application server closes physical connection between application & database.
This will invalidate the connection in connection pool. To detect such dead connections we can make use of EXPIRE_TIME parameter.
It is enabled on the server by defining SQLNET.EXPIRE_TIME in $ORACLE_HOME/network/admin/sqlnet.ora file
This parameter determines the time period between probe requests, to verify the connection between client and server.
This parameter is defined in minutes and can have any value between 1 and an infinite number.
So if the value of this parameter is specified as 1, sqlnet will send a probe after every 1 minute to check the connection validity. Oracle recommends setting the value of this parameter to a non-zero value.
This will also help to close all open connections, in case of abnormal client termination.
If it is not defined, Dead Connection Detection is not used.