A node implementing an RLC (Radio Link Control) entity and being for use in a mobile communications system transmits a sequence of RLC SDUs (SDU=Service Data Unit) towards a peer node. As a result of re-establishment of an RLC entity not all SDUs may have been received. The peer notifies the node which is the next SDU that the peer expects to receive (by transmitting the next SDU number to the node). The node resumes transmission from that SDU onwards. This may or may not lead to re-transmission of SDUs transmitted before the RLC re-establishment. As an alternative, the peer does not notify the node of the next SDU that it expects to receive, but instead the node re-transmits any unacknowledged SDUs, together with the SDU number of the first re-transmitted SDU. The peer then discards any duplicate SDUs. Both variants enable lossless data transfer whilst avoiding duplication.