Reverse-path forwarding

Reverse Path Forwarding (RPF) is a technique used in multicast routing. It is utilized to build source-specific forwarding paths (SPT, shortest path tree), amongst which the traffic can flow more efficiently. Source specific multicast makes immediate use of this. Any source multicast usually switches from a centralized tree to the SPT on certain predefined conditions and for each source individually.

This functionality is achieved by issuing source-specific joins towards the source, using the source address to look up an unicast / MRIB routing table entry. This continues router by router until the source is reached. The source and the routers inbetween now start forwarding the traffic towards the direction the original join came from. The result is that the traffic is forwarded along the reverse path from the source back to the listener.

While with symmetric routing, the reverse path is the same as the forward path, this is not neccessarily true with asymmetric routing. Because the Internet is routed asymmetrically, the paths show significant differences quite often.