In the present invention, a transmitting device receives, at a packet data convergence protocol (PDCP) layer, a PDCP service data unit (SDU) from an upper layer. The transmitting device generates, at the PDCP layer, a PDCP protocol data unit (PDU) containing the PDCP SDU. The transmitting device submits, at the PDCP layer, the PDCP PDU to a lower layer. If the PDCP SDU includes a transmission control protocol (TCP) ACK packet, the PDCP layer generates a PDCP control PDU as the PDCP PDU.