[c-nsp] SSM Multicast backwards compatibility with an ASM core: What is the definition of "last hop router"?

[P C]

I have a need to run Source Specific Multicast (PIM-SSM) over a network
which otherwise only supports Any Source Multicast (PIM-SM).

I understand it's only required that the "last hop router" actually be SSM
aware.  What's unclear to me however is what the last hop router means.
Does this mean, given the diagram below only the router closest to the
receiver <Router 1> would need to understand a (S,G) join directly from the
host and everyone in the middle wouldn't care -- or must <Router 3> also
understand SSM?

I would think router 3 would just think it's an ASM SPT join and this would
work, but I'm not sure.

Multicast traffic will only ever flow one direction, from sender to
receiver.

<Receiver>--<Router1>--<Router2>--<Router3>--<Sender>


Thanks!