Lectures notes for 2010 - KTH

RPB + Prunes ⇒ Reverse Path Multicast
(RPM)
When source S starts a multicast transmission the first packet is propagated to all
the network nodes (i.e., flooding). Therefore all leaf nodes receive the first
multicast packet. However, if there is a leaf node that does not want to receive
further packets, it will send back a “prune” message to the router that sent it this
packet - saying effectively “don’t send further packets from source S to group G
on this interface I.”
There are two obvious drawback in the flood and prune algorithm:
• The first packet is flooded to the whole network
• The routers must keep states per group and source
When a listener joins at a leaf that was pruned, we add this leaf back by grafting.
Flood and prune was acceptable in the experimental MBONE which had only a few tens of thousands of
nodes, but for the Internet where both the number of sources and the number of groups becomes very large,
there is a risk of exhausting the memory resources in network routers.
Maguire RPB + Prunes ⇒ Reverse Path Multicast (RPM) 8: 41 of 78
maguire@kth.se 2010.03.21 Internetworking/Internetteknik