Consideration is given to the computational modeling of distributed computer systems. For that, the "Network calculus" method relying on the mini (maxi) plus algebra is used. The modeling focuses on the area of network systems with a variable flow of data between the system components. The simulation is based on the extended slope (Legendre) transform to calculate the envelope of the system output flow. An example of modeling an integrated service (IntServ) type system with a summation of the two flows where one stream is that with an envelope of the type rt + b is presented. Discussed is the service function with a delay element and its impact on the output flow envelope.