The present invention relates to a transmission device implementing a flow control mechanism for aggregate trunks. The transmission device can be implemented as a router that includes an input for receiving aggregate traffic streams, an output for releasing the aggregate traffic streams to a destination point and a control unit capable to regulate the rate of release of packets from the output. Specifically, the flow control operation effected by the control unit is dependent on receipt of acknowledgement messages issued at the destination point, each acknowledgement message confirming the receipt of one or more particular packets at the destination point. The control unit will continuously increase the packet sending rate until a packet is lost in the network between the transmission device and the destination point. On detection of packet loss, based on the lack of one or more corresponding acknowledgement messages from the destination point, the control unit will reduce the packet sending rate.