FortiGate Infrastructure 7.2 Study Guide (p.41): "The RPF check is a mechanism that protects FortiGate and your network from IP spoofing attacks by checking for a return path to the source in the routing table." "FortiGate performs an RPF check only on the first packet of a new session. That is, after the first packet passes the RPF check and FortiGate accepts the session, FortiGate doesn't perform any additional RPF checks on that session." A) The RPF check is a mechanism that protects FortiGate and the network from IP spoofing attacks.
This is true because the RPF check verifies that the source IP address of an incoming packet matches the reverse route for that address, meaning that the packet came from a legitimate source and not from an attacker who is trying to impersonate another host. This prevents IP spoofing attacks, where an attacker sends packets with a forged source IP address to bypass security policies or launch denial-of-service attacks1 C) The RPF check is run on the first sent packet of any new session.
This is true because the RPF check is performed only once per session, on the first packet sent by either the client or the server, depending on the direction of the session initiation. This reduces the processing overhead and improves performance2