This paper studies the traffic flow in a virtual circuit of a computer communication network with window flow control. Due to finite buffer capacity, overflow of data packets is possible. Lost packets have to be retransmitted. To maintain the original order of the packets, subsequently sent packets also have to be retransmitted, causing a deterioriation of throughput. An ap_proximation method, based on a queueing network model, is developed to analyse the throughput behaviour. This method leads to exact results tor the single-hop network. For the multi-hop circuit, it yields very accurate approx1mat1ons, as 1s illustrated by simulation.