A method for transmitting a plurality of data packets is disclosed. The method includes: receiving one or more first medium access control (MAC) protocol data units (PDUs) which include a plurality of radio link control (RLC) PDUs intended for a destination UE from a source UE; demultiplexing the first MAC PDUs to retrieve the RLC PDUs based on a header field of each first MAC PDU; generating a plurality of RLC PDU segments from one or more of the RLC PDUs or a plurality of RRLC (Relay RLC) PDUs from all of the RLC PDUs according to one or more MAC PDU sizes, wherein the RRLC is a protocol layer between the MAC layer and the RLC layer; multiplexing the RLC PDUs and the RLC PDU segments or the RRLC PDUs into one or more second MAC PDUs; and transmitting the second MAC PDUs to the destination UE.