A multicast tree with the minimized cost is called a Steiner tree. Why do none of the protocols proposed in IETF RFCs try to construct a Steiner multicast tree? In general, we may think that the cost...

A multicast tree with the minimized cost is called a Steiner tree. Why do none of the protocols proposed in IETF RFCs try to construct a Steiner multicast tree?

In general, we may think that the cost of a source-based tree shall be less than that of a shared-based tree. Do you agree or not? Why?

Construct a counter example to show that the cost of a source-based tree is actually larger than that of a shared-based tree.