This document defines the Peer-to-Peer Protocol (P2PP), an application-layer binary protocol, for creating and maintaining an overlay of participant nodes. The overlay can be created using various structured and unstructured peer-to-peer protocols such as Bamboo, Chord, Pastry, Kademlia, Gnutella, and Gia. P2PP uses a secure transport, supports an application API, has mechanisms for NAT and firewall traversal, exchanging node capabilities, and diagnostic information. P2PP is designed to support a P2P Session Initiation Protocol (SIP) network, but it can be used for other applications as well.