Packet marking by BGP peer

I have a CCR1009 and multiple BGP peers. Some I peer with through dedicated interfaces so this is easy to traffic-account for. However, some peers I reach via bilateral-peering through an IX and I need a way to account individually for this traffic. How can this be achieved to the point of the packet marking please?

When you say that you need to “account” for it, what does that mean exactly?

Seems like you could use netflow for this purpose and then pull the data out of a netflow collector instead of adding the CPU overhead of marking every single packet

Traffic In and Traffic Out. At present I do this with Firewall / Mangle and connection marking + packet marking. But I am not clear how to do this over an IX interface with multiple bilateral peers.