In the present invention, the transmitting device attaches a sequence number (SN) for each of service data unit (SDU) elements to generate a protocol data unit (PDU) containing the SDU elements. Each of the SDU elements is a SDU or SDU segment. The PDU includes the SDU elements and respective SNs for the SDU elements. The PDU includes an indicator for each of the SDU elements, and the indicator indicating whether a corresponding SDU element is a SDU or SDU segment.