1. An apparatus comprising:at least one processor; and
at least one memory including computer program code, the at least one memory and the computer program code configured, with the at least one processor, to cause the apparatus to perform at least the following:
performing, in a communication system where a primary cell is controlled by a first base station and a secondary cell is controlled by a second, different base station, flow control between the primary cell and the secondary cell for data for a radio link control layer;
communicating, according to the flow control, the data for the radio link control layer between the first and second base stations using a link between the first and second base stations;
estimating by the secondary cell a time lag between an original packet data unit request sent over the link and a corresponding response message received over the link, the response message comprising a packet data unit, and wherein the measured time lag is an estimate of round trip time; and
estimating by the secondary cell an approximate length of time that queued data for the radio link control layer will last for a certain user equipment.