Finding upstream node

[Christos Papadopoulos]

In my simulation, I want to have packets sent upstream towards the
source of a multicast tree.

Say I have a multicast packet at node N which is addressed to <S, G>.
The packet may have arrived from any downstream link. I want to forward
this packet upstream towards the source. What is the best way to find
the upstream link (or node) given that I know N and <S, G>?  I 'd like
to do this from within C++.

Thanks,

Christos.