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3.2. Phase Two: Register-Stop

Register-encapsulation of data packets is inefficient for two reasons:

  • Encapsulation and decapsulation may be relatively expensive operations for a router to perform, depending on whether or not the router has appropriate hardware for these tasks.

  • Traveling all the way to the RP, and then back down the shared tree may result in the packets traveling a relatively long distance to reach receivers that are close to the sender. For some applications, this increased latency or bandwidth consumption is undesirable.

Although Register-encapsulation may continue indefinitely, for these reasons, the RP will normally choose to switch to native forwarding. To do this, when the RP receives a register-encapsulated data packet from source S on group G, it will normally initiate an (S,G) source-specific Join towards S. This Join message travels hop-by-hop towards S, instantiating (S,G) multicast tree state in the routers along the path. (S,G) multicast tree state is used only to forward packets for group G if those packets come from source S. Eventually the Join message reaches S's subnet or a router that already has (S,G) multicast tree state, and then packets from S start to flow following the (S,G) tree state towards the RP. These data packets may also reach routers with (*,G) state along the path towards the RP; if they do, they can shortcut onto the RP tree at this point.

While the RP is in the process of joining the source-specific tree for S, the data packets will continue being encapsulated to the RP. When packets from S also start to arrive natively at the RP, the RP will be receiving two copies of each of these packets. At this point, the RP starts to discard the encapsulated copy of these packets, and it sends a Register-Stop message back to S's DR to prevent the DR from unnecessarily encapsulating the packets.

At the end of phase two, traffic will be flowing natively from S along a source-specific tree to the RP, and from there along the shared tree to the receivers. Where the two trees intersect, traffic may transfer from the source-specific tree to the RP tree and thus avoid taking a long detour via the RP.

Note that a sender may start sending before or after a receiver joins the group, and thus phase two may happen before the shared tree to the receiver is built.