Generate Trace File With Required Name

As a sys user i want to trace a user session. so i am using 

SQL> EXEC DBMS_SYSTEM.set_sql_trace_in_session(sid=>123, serial#=>1234, sql_trace=>TRUE);SQL> EXEC DBMS_SYSTEM.set_sql_trace_in_session(sid=>123, serial#=>1234, sql_trace=>FALSE); 

to trace. but this trace file is mixing with another trace files in udump. did some Google and found that we can use

ALTER SESSION SET TRACEFILE_IDENTIFIER = "MY_TEST_SESSION".

but this will generate trace file for sys user account not for the desired user session. how we can define trace file name for desired user session trace.