BGP Generalized TTL Security Mechanism (GTSM) is a security feature designed to protect BGP sessions from CPU-utilization-based attacks. It requires a TTL of 255 to be configured between neighbors to ensure packets are not intercepted and altered by man-in-the-middle attacks. This ensures that packets only travel one hop. BGP GTSM can definitely be implemented between routers R2, R3, and R4 which are in the same autonomous system, and between R1 and R2 which are BGP neighbors. Additionally, BGP GTSM requires a firewall filter to discard packets with incorrect TTL. Therefore, the correct statements are: B, C, and D.