This document defines concepts and terminology for use of the Session Initiation Protocol in a peer-to-peer environment where the traditional proxy-registrar and message routing functions are replaced by a distributed mechanism that might be implemented using a distributed hash table or other distributed data mechanism with similar external properties. This document includes a high-level view of the functional relationships between the network elements defined herein, a conceptual model of operations, and an outline of the related open problems being addressed by the P2PSIP working group. As this document matures, it is expected to define the general framework for P2PSIP.