This document defines a framework and requirements for multi-party usage of SIP. To enable discussion of multi-party features and applications we define an abstract call model for describing the media relationships required by many of these. The model and actions described here are specifically chosen to be independent of the SIP signaling and/or mixing approach chosen to actually setup the media relationships. In addition to its dialog manipulation aspect, this framework includes requirements for communicating related information and events such as conference and session state, and session history. This framework also describes other goals which embody the spirit of SIP applications as used on the Internet.