I understand that certain PIM messages such as Join/Prune and Graft have a field called 'Upstream Neighbour Address' used to indicate the recipient of the message, which is the upstream router for a particular multicast traffic stream.
Say I have a network segment with 4 routers, and one of them is forwarding multicast traffic on the said segment, traffic sourced in another network. If one of the other 3 routers would send a Prune message towards the forwarding router, how does it know from which router is the multicast traffic incoming? Is it based on the reverse route to the source? Or something simpler, such as the source MAC of the multicast traffic?