A method is described for controlling a data unit receiver or a data unit sender. The data unit receiver or sender comprise a gap response procedure for responding to gaps in the sequence of data units received at the receiver. A reordering detection procedure S12 is provided for detecting a reordering indication indicative of a potential re-ordering of data units in the course of a transmission from sender to receiver, and the gap response procedure is adapted in response to detecting a re-ordering indication.