First the CBT protocol was created to use shared tree solutions because
DVMRP and the other dense mode protocols werent scalable. there were
many problems with CBT (which is bidirectional) so PIM-SM was cretaed
which provide some switching (between shared tree and source tree). and
after that there is some discussions about the bidirectional PIM, which
is like CBT.
Are we in circle here or what ??
If there were some progress somewhere, you might be able say "circle".
But, in IETF, no proposal makes any sense that there is absolutely
Here, you are on a point behind a start line.
SSM is no different.
The real difficulty of multicast is in the various relationships
to resource reservation.
I've heard that ISPs, which were expecting to receive special charge
for multicast service, are now bitterly recognizing one aspect of the
relationship that, even if multicast is free, customers do not bother
to use multicast if they pay flat rate to receive any amount of unicast
Those serving the customers simply increase the number of capacity
of servers, of course.