Reverse Path Forwarding (RPF) is a feature used in IP multicast routing that ensures multicast traffic is sent back to the receiver efficiently and without looping. In the context of a content delivery network (CDN) sending video traffic, when a receiver requests traffic through an IGMP join message, the multicast routing device uses RPF to populate the multicast routing table with the correct source (S) and group (G) information.
RPF checks the source of the incoming packet against the multicast routing table and forwards the packet only if it arrives on the interface that leads back to the source. This mechanism prevents traffic loops and ensures that the multicast distribution device sends the required traffic back to the receiver along the most efficient path.
References: The explanation is based on the principles of IP multicast routing as detailed in Cisco's Data Center Core Technologies documentation, specifically the sections on "Implementing Multicast in Data Center" and "IP Multicast Technology Overview" which discuss the role of RPF in multicast distribution