Core-Based Trees (CBT)
Sparse Mode: shared tree set up before forwarding. Good scaling properties for WAN multicast, with scattered receivers
Each group has a “core router” which is dynamically discovered (bootstrapping)
A host which wants to join the group sends a JOIN_REQUEST towards the core and gets a JOIN_ACK from the nearest router already on the tree.
Forwarding cache = group, {outgoing interface list}
- Packet is forwarded onto all outgoing interfaces except the one in which it arrived
- Packet transmission can be bidirectional (“upstream” in a CBT refers to the direction towards the core, not the source)
Non-tree source employs IP-in-IP encapsulation to send packets to the core