Huawei GGSN not charging multicast/broadcast traffic, why?

[zefs]

Hello to everybody.

Recently we encountered this situation with a customer in roaming, so the partners SGSN counted the traffic, but our Home GGSN did not.
I know that the GGSN acts more like a router and does not forward multicast/broadcast traffic through Gi interface, ok, but why not counting it?

Can anyone point me to the right 3GPP spec regarding charging multicast/broadcast traffic?

Thanks a lot.