Most research on design and implementation of multicast network systems has concentrated mainly on the capacities of network devices, without completely considering the impact from multicast sessions on network systems. This impact heavily depends on the activities during the life span of multicast sessions, such as the session initialization, communication, termination, and membership management, and on the influence within the overall architecture context of multicast sessions, such as routing devices and protocols. Based on the fundamental rules and principles of multicast sessions, this paper defines performance metrics for IP multicast sessions. The paper summarizes the metrics in four categories: latency, quality of service, group characteristics, and link bandwidth consumptions. Also presented are results from our simulations.