Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 267
Reverse Path Forwarding (RPF)

Since the spanning tree was better but still not efficient or scaleable, many other multicast protocols were tested and experimented, but the protocol that is used with multicasting is Reverse Path Forwarding, or RPF. This algorithm provides a group-specific spanning tree. That is, for each group source (the host that is originating the multicast datagram), a separate distinct spanning tree is built between that source and all the potential host recipients.

This algorithm that you are about to learn may seem backwards, and in a way it is. We know that multicasting is based on a (source, group) pair. If a multicast datagram is received on a router’s interface, the router then determines if the interface that the datagram was received on is the shortest path back to the source. Sounds like the opposite of RIP? Well, it is. If the router determines that the interface does provide the shortest path back to the source, it forwards the received datagram on every active interface except the one on which it received the datagram. Otherwise, the router determines that the interface does not provide the shortest path back to the source, and it discards the datagram. The interface determined to be the shortest path back to the source is called the parent link. The interface on which the router forwards the multicast datagram is called the child link.

Where does the router get this information to allow it to make a decision on what is the shortest path back the source? The routing tables of the router. If you are using a link-state routing update protocol such as OSPF, each router maintains a routing table for whole network, multicast or unicast. If you are using a distance-vector protocol such as RIP or RIP2, a routing update is needed. We will explain it in further detail in our discussion of DVMRP.

RPF contains many advantages over the mechanism previously described. It is simple to implement. Multicast datagrams are forwarded over multiple links since RDF allows for building separate spanning trees for each source and group pair.

Reverse Path Forwarding (RPF)