A queueing network model of a virtual circuit in a packet computer network having end to-end window flow control is considered. In the queueing model a simplified protocol is studied, including two single server queueing systems in tandem, having buffers Bx and B> of capacity L - 1 and with an external buffer BE in front. For the flow control is considered that in the two tandem systems in each moment there are at most L packets. To the system in addition to usual (positive) customers arriving to the outer buffer, there are negative customers arriving to each of the inner nodes. Modeling with a Markov process a necessary and sufficient ergodicity condition is obtained and a matrix algebraic algorithm is developed for the computation of stationary probabilities. © ASMTA 2006, All Rights Reserved.