MCI WorldCom MCI WorldCom This document specifies the architecture, protocols and messages for SIP based IP communications between Internet domains in support of QoS and AAA. Detailed message flows and message contents are discussed and specified. AAA requirements including inter-domain and user authorization in mobile and non-mobile environments are addressed. A solution is proposed where session setup and teardown are linked with QoS and AAA signaling using AAA policy servers and clearinghouses.