Independent This document specifies the Agent Registration and Discovery Protocol (ARDP), a lightweight protocol for registering, discovering, and reaching autonomous software agents in distributed and federated environments. ARDP provides stable agent identities, dynamic endpoint resolution, capability advertisement (including protocol selection among MCP, A2A, HTTP, and gRPC), minimal presence signaling, and a security-first discovery control plane. ARDP is transport-agnostic and complementary to existing agent interaction protocols.