Which protocol detects path up/down, measures loss/latency/jitter, and evaluates IPsec tunnel MTU quality?

BFD (Bidirectional Forwarding Detection) is the correct answer because it is specifically designed for rapid detection of forwarding path failures between two systems. It measures loss, latency, and jitter by sending lightweight BFD packets at a configurable interval. Additionally, BFD can be used to evaluate the MTU quality of IPsec tunnels by detecting fragmentation or black holes. DTLS is a security protocol used for control plane encryption, not path monitoring. OMP (Overlay Management Protocol) is the routing protocol for the SD-WAN overlay and doesn't perform active path quality measurements. IP-SLA is a Cisco feature for measuring network performance but is not the primary protocol used by SD-WAN for real-time path quality and tunnel MTU evaluation.