Undo/Rollback Segment used

How do you know How much Undo/Roll back Segment each session consuming ?

COLUMN USERNAME FORMAT a10
COLUMN status FORMAT a8
set linesize 100
set pagesize 15

Rem # rbs_used.sql
set feedback on
set linesize 132
set echo off
column RBS_NAME format a10
TTITLE 'Total Blocks used by a user in a rbs'
SELECT s.username,
s.sid,
t.used_ublk*8192/1024/1024 "RbsUsed(MB)",
t.used_urec,
t.log_io,
t.phy_io,
-- t.xidusn,
r.usn ,
r.name "RBS_NAME",
t.start_uext,
t.start_time,
t.status
FROM v$transaction t, v$session s, v$rollname r
WHERE t.addr = s.taddr
and r.usn = t.xidusn
order by s.sid
/