When the source for the group sends a packet, the first hop router attached to that source is required to register the source with the rendezvous point. The rendezvous point is also requested to build a tree back towards the first hop router.
The first hop route encapsulates multicast data from the source into a special PIM sparse mode registration message, and that data is unicasted towards the rendezvous point.
When the rendezvous point receives the registration message, it will decapsulate the multicast data from inside the register message. If there are no active shared trees due to there being no interested receivers in that multicast stream, the rendezvous point will send a message to the first hop router requesting that the register messages are stopped directly, without making use of the encapsulation through a PIM tunnel.
If there is an activate shared tree a already for the group, it will forward the multicast packet down the shared tree, and a (S, G) is sent back towards the source to create a (S, G) shortest path tree.
If there are multiple hops along this path between the rendezvous point and the source of the multicast, each router will create a (S, G) state along the branch, including the rendezvous point.
Once a shortest path tree is built, multicast traffic will flow between the source of the multicast and the rendezvous point.
With the flow of data beginning between the source of the multicast and the rendezvous point, a register stop message is sent to the sources first hop router to indicate that the unicast register messages no longer need to be sent to the rendezvous point. Multicast traffic is flowing down the shortest path tree towards the rendezvous point and onwards down the shared tree to the receiver.
The PIM register tunnel from the first hop router to the rendezvous point will always remain in an active up/up state even if there are no multicast streams; it will remain active as long as there is a valid reverse path forward for the rendezvous point