It’s an error that often gave me constant migraines every time I saw it crop up in my application logs. This error appears when you are trying to connect to an oracle database and the connection receives a time out from the database. ORA-03135 may also be as a result of your firewall settings.
Now, one of the ways you may explore in resolving this is error is by modifying the sqlnet.ora file. This file is located on $ORACLE_HOME/network/admin/sqlnet.ora. The value of thesqlnet.ora:expire_time parameter in the sqlnet.ora file may have to be increased:
SQLNET.INBOUND_CONNECT_TIMEOUT = 300
SQLNET.SEND_TIMEOUT = 300
SQLNET.RECV_TIMEOUT = 300
To check a solution for the ORA-03135 error, start by checking to see if the OS PID still exists, using the: ps –ef|grep syntax [We are assuming that your database is running on Unix platform, Duh!]