Zero-trust VPN mesh

Design a managed VPN / secure-tunnel service (a mesh overlay like a WireGuard-based zero-trust network) connecting 500k client devices and 50k servers across many private networks. You need encrypted point-to-point tunnels, sub-100 ms added latency, key rotation, and a control plane that programs who-can-reach-whom. How do you separate control and data planes, route packets, and handle key distribution?