ORA-03135 is also a server process problem. As I said in solving ORA-03113:
No matter the database is down or session is lost, for external connections, the cause is that the server process is vanished.
On the other side, for local connections or through OS authentication, the symptom is the same, but we saw ORA-03135 instead.
SQL> conn hr/hr
SQL> select first_name from employees where last_name = 'Chen';
select first_name from employees where last_name = 'Chen'
ERROR at line 1:
ORA-03135: connection lost contact
Process ID: 4616
Session ID: 143 Serial number: 115
As you can see, I didn't specify the destination in the connection string, then the connection automatically went for the local database.
In the first place, the connection is fine, but the following statement failed with ORA-03135, which means the server process was vanished for some reason.
ORA-03135 means that the local connection has lost contact with the local database, you should check its health and status. Possible causes have been talked in previous post: How to Resolve ORA-03113: end-of-file on communication channel.