Telecom Italia L. Pasteur University/SIP This document defines the Extensible Peer Protocol (XPP), a lightweight binary protocol for end-to-end sessions between peers in distributed overlay networks. One of the main goals while creating this protocol was support for nodes located behind firewalls and NATs. XPP therefore uses UDP and allows endpoints to simultaneously initiate sessions. Given the choice of the underlying protocol (UDP), XPP also defines mechanisms for message fragmentation and reliability.