Feedback mechanisms are integral components of network protocols and traffic control algorithms. Their performance evaluation is hard due to intricate time correlations introduced by feedback. Network calculus has been successfully applied for the analysis of feedback mechanisms in deterministic systems. However, an extension to random systems has remained an open problem for more than a decade. We present a stochastic network calculus analysis of a random system with feedback, specifically, a window flow control system with random service and fixed feedback delay. We quantify the service impediment due to the feedback mechanism by deriving statistical lower bounds on the available service, and obtain complementary upper bounds. We also discover special cases where an exact description of the service is feasible.