Volume of transfert for PPP/PPPoE

Hello,

I want to know the volume of transfert on my xDSL line, like it's
count by my provider. Therefore, I need the volume with the PPP
(PPPoE ?) overhead included !

In the ip-down script called by pppd, I receive only the volume at the
IP level.

Where can I find this information ? (statistics of my ethx is not the
best way to do, there can be other traffics...)

Thx