Abstract: A method for setting a header, having a header extension type field, of a protocol data unit in a radio link control layer of a wireless communications system includes configuring a “use of the special value of the header extension type field” mode, and setting the header extension type field to indicate that a last octet of the protocol data unit is a last octet of a service data unit when the last octet of the protocol data unit is the last octet of the service data unit and there is no concatenation of service data units inside the protocol data unit.