This paper proposes the BUC (Buffer Utilization Control) algorithm, a congestion avoidance algorithm executed exclusively in a gateway we call the “BUC gateway”. The BUC gateway maintains separate queues per transport-level-conversation at each output-port. These “per-conversationqueues” are served in a round-robin manner. To control the data-flow of conversations, the BUC algorithm updates a window in well-defined time-intervals called “epochs”. At the beginning of each epoch a conversation’s window is computed as a function of the per-conversation-queue-size and the window-size of the preceding epoch. Additionally, we propose a novel signaling mechanism called RFCN Reverse Feedback Congestion Notification. RFCN is applicable to transport protocols using sliding window flow control, e.g. TCP. Window flow control requires that the receiver transmits it ́s available buffersize to the sender in a window-field in the ACK-header. The BUC algorithm may update the credit value in this window field to its computed window to control the transmission rate of a datasender. As only the common sliding window flow control mechanism has to be provided by the transport protocol, RFCN has the ability of controlling the data flow of transport entities without needing cooperative transport protocols. This allows deployment of the BUC algorithm in existing networks without having to upgrade (TCP) transport entities. Simulations are performed to evaluate the convergence behavior and the performance of the BUC algorithm.