Controlling transmission rate dependent upon block ack/nak bitmask signals and receiver buffer fill levels

Streaming system, possibly using real time (control) protocols (RTP/RTCP) can adapt transmission data rates dependent upon ack/nack bitmaps detailing receipt of multiple packets. This explicit acknowledgement data is used to generate packet loss S902, packet delay S912 and retransmission delay S915 metrics, in order of decreasing priority. These are compared to thresholds S904 or used to derive trends in the metric values S914/S916 which are then used to initiate changes to the system transmission rate S906/S908/S910. The system also transmits information about the receiver buffer fill level S918, which can also be used to adjust the transmission rate S920, but which has lower priority than the acknowledgement derived metrics.