Design, performance management, and capacity planning of client/server applications in the commercial enterprise depends on the ability to model these distributed applications at design time as well as during normal operations. This paper specifies the functional requirements of performance modeling of the class of applications based on Remote Procedure Calls (RPC). These specifications include the need to model transactions consisting of multiple nested, synchronous and concurrent RPC's using tractable techniques. Examples of distributed application models are presented based on discrete event simulations. Suggestions for future work are included.