To the best of my knowledge, 'Acct-Output-Octet' count does include the PPP header and IP header count. As tested in the lab, when we downloaded 1455649 ATM cells (with 10-byte IP payload for each cell), total amount of bytes sent to the PPP client (including 2-byte PPP header +20-byte IP header)= (2+20+10) * 1455649 = 46580768
Result shown in Radius accounting record: Acct-Output-Octet: 46581817 which shows that we are correct.