Aalyria Technologies This document specifies the DTN Peering Protocol (DPP), an inter- domain routing protocol for the Delay-Tolerant Networking (DTN) ecosystem. DPP facilitates the exchange of reachability information between distinct Administrative Domains (ADs), enabling inter-domain routing across the Solar System DTN. DPP separates the control plane from the data plane: a DPP speaker need not be a gateway that forwards bundles, allowing centralized route controllers or orchestration systems to participate in peering on behalf of the gateways they manage. DPP harmonizes the two DTN addressing schemes -- ipn (integer-based) and dtn (URI-based) -- into a unified routing framework. It leverages DNS for identity verification and supports both reactive routing and scheduled contact windows for deep-space networks.