A network engineer is troubleshooting a Cisco Catalyst SD-WAN deployment. Two WAN Edge (cEdge) routers at different branch sites have successfully registered with the controllers and have exchanged OMP routes. However, users report that traffic between the two branches is failing even though routes are present in the routing table. The engineer needs to verify the component that is responsible for detecting the liveness and quality of the direct site-to-site data plane tunnels. Which mechanism should the engineer investigate?