A method for transmitting packet data units PDUs at a PDU transmitter in a communication system, the method comprising: transmitting a first PDU comprising a PDU header (706 and/or 707) and a data portion (710 or 715); receiving negative acknowledgement NACK information for at least a portion of the first PDU; segmenting the first PDU to form at least two PDU segments (720, 733) if an available PDU size at a retransmission time is smaller than a size of the first PDU, wherein each of the at least two PDU segments (720, 733) comprises a PDU segment header (723, 725, 730; 735, 740, 745) and a data field; and transmitting the at least two PDU segments to a PDU receiver, wherein the PDU segment header comprises a) one bit of a last segment indicator LSI field indicating whether or not a last portion of a corresponding PDU segment corresponds to a last portion of the first PDU, b) a length indicator LI field indicating a length of the data field of a corresponding PDU segment, c) a sequence number SN field indicating a sequence number of the first PDU, and d) a sub sequence number SN field different from the sequence number SN field and indicating a sequence number of the corresponding PDU segment at a segment level, and wherein the last segment indicator LSI field and the sub sequence number SN field are comprised in a sub SN Header and wherein a number of sub SN Headers comprised in the PDU segment header is equal to a number of times the first PDU is segmented. transmitting a first PDU comprising a PDU header (706 and/or 707) and a data portion (710 or 715); receiving negative acknowledgement NACK information for at least a portion of the first PDU; segmenting the first PDU to form at least two PDU segments (720, 733) if an available PDU size at a retransmission time is smaller than a size of the first PDU, wherein each of the at least two PDU segments (720, 733) comprises a PDU segment header (723, 725, 730; 735, 740, 745) and a data field; and transmitting the at least two PDU segments to a PDU receiver, wherein the PDU segment header comprises a) one bit of a last segment indicator LSI field indicating whether or not a last portion of a corresponding PDU segment corresponds to a last portion of the first PDU, b) a length indicator LI field indicating a length of the data field of a corresponding PDU segment, c) a sequence number SN field indicating a sequence number of the first PDU, and d) a sub sequence number SN field different from the sequence number SN field and indicating a sequence number of the corresponding PDU segment at a segment level, and wherein a) one bit of a last segment indicator LSI field indicating whether or not a last portion of a corresponding PDU segment corresponds to a last portion of the first PDU, b) a length indicator LI field indicating a length of the data field of a corresponding PDU segment, c) a sequence number SN field indicating a sequence number of the first PDU, and d) a sub sequence number SN field different from the sequence number SN field and indicating a sequence number of the corresponding PDU segment at a segment level, and wherein the last segment indicator LSI field and the sub sequence number SN field are comprised in a sub SN Header and wherein a number of sub SN Headers comprised in the PDU segment header is equal to a number of times the first PDU is segmented.