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I recently started studying multicast and I am a bit confused by how reverse path forwarding is explained. Most of the sources describe RPF as 'moving packets away from the source' but I understand that unicast routing achieves the same thing:

  • in unicast routing, the routing table is consulted and the packets are forwarded away from the source, towards the destination host, based on the destination address of the packets.
  • in multicast routing, the RPF check is applied to the packets and if they pass, they are forwarded away from the source, towards the multicast receivers based on the information in the forwarding states, otherwise, they are discarded to prevent loops.

I understand that in multicast routing, routers need to keep track of the upstream and downstream interfaces, source IP, etc. to apply the RPF check on the packets, but since the 'source IP' is not the only piece of information needed to make a forwarding decision overall, I do not see how the phrase 'moving packets away from the source' makes a difference between unicast and multicast routing.

Conceptually, is there something more to reverse path forwarding than the RPF check?

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With unicast traffic you have a single destination, so you know exactly where you're going. In case of multicast you can go to 100 receivers, or 10, or 0. Receivers can connect as you send the multicast stream, or disconnect - so all you can say about multicast traffic is that it's moved away from the source, since it doesn't have any predefined destination(s). Generally speaking, as a new user connects and wants to get a steam, the routing table of multicast routers along the path is adjusted to send the traffic downstream while checking correctness (RPF check) from upstream.

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RPF is one tool in your toolbox to help keep your network safe. RPF is used to verify that packets are coming from the direction of a reachable source. This helps to prevent malicious packets from arriving on an interface that is not toward the source address contained in the packet header.

Malicious packets may try to be injected into your network by spoofing the source address of the packets to try to fool, e.g. a router ACL, RPF verifies the reachability of the source on the interface where packets are received. If the source address on a packet cannot be reached from the interface where it was received, the packet is dropped.

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