Claims:1. A method of transmitting data in a device to device communication system, the method comprising:
determining whether a security feature is applied to a packet data convergence protocol (PDCP) service data unit (SDU);
configuring a PDCP protocol data unit (PDU) based on the determined result; and
transmitting the PDCP PDU to a receiving user equipment (UE),
wherein a PDCP header of the PDCP PDU includes an SDU type, a PDCP sequence number (SN), least significant bits of a pro-se group key (PGK) identification (ID) and a pro-se traffic key (PTK) ID, and
wherein the SDU type is set to a value indicating a protocol type of the PDCP SDU.

2. The method of claim 1, wherein, if the security feature is not applied to the PDCP SDU, the PDCP SN, the least significant bits of the PGK ID and the PTK ID are set to a zero value in the PDCP header.

3. The method of claim 1, wherein the determining of whether the security feature is applied to the PDCP SDU is based on a pro-se function in a network.

4. The method of claim 1, wherein the configuring the PDCP PDU based on the determined result is based on a pro-se key management function in a network.

5. The method of claim 1, wherein the configuring of the PDCP PDU if the security feature is applied to the PDCP SDU comprises:
including the SDU type and the PDCP SN in the PDCP header of the PDCP PDU, wherein the PDCP SN is set to one of values associated with the PDCP SDU;
including the least significant bits of the PGK ID in the PDCP header of the PDCP PDU, wherein the least significant bits of the PGK ID identifies a PGK used to generate a PTK for securing the PDCP SDU; and
including PTK ID in the PDCP header of the PDCP PDU, wherein the PTK ID identifies the PTK used to generate a pro-se encryption key (PEK) for securing the PDCP SDU.

6. A method of receiving data in a device to device communication system, the method comprising:
receiving a packet data convergence protocol (PDCP) protocol data unit (PDU); and
processing the PDCP PDU configured based on whether a security feature is applied to a PDCP service data unit (SDU),
wherein a PDCP header of the PDCP PDU includes an SDU type, a PDCP sequence number (SN), least significant bits of a pro-se group key (PGK) identification (ID) and a pro-se traffic key (PTK) ID, and
wherein the SDU type is set to a value indicating a protocol type of the PDCP SDU.

7. The method of claim 6, wherein whether the security feature is applied to the PDCP SDU is determined based on a pro-se function in a network.

8. The method of claim 6, wherein the PDCP PDU is configured based on a pro-se key management function in a network.

9. The method of claim 6, wherein the processing of the PDCP PDU comprises:
determining the security feature applied status on the PDCP SDU based on configuration information from a pro-se function;
determining a PGK and a PTK for securing the PDCP SDU based on the PTK ID and the PGK ID in the PDCP header by a PDCP entity if the security feature is applied to the PDCP SDU;
decrypting the received PDCP SDU; and
sending the decrypted PDCP SDU to an upper layer.

10. The method of claim 6, wherein the processing of the PDCP PDU comprises:
determining the security feature applied status on the PDCP SDU based on configuration information from a pro-se function; and
sending the PDCP SDU to an upper layer without decryption if the security feature is not applied to the PDCP SDU.

11. The method of claim 6, wherein the processing of the PDCP PDU comprises:
determining the security feature applied on the PDCP SDU if the zero value is not assigned to the PDCP SN, the PGK ID, and the PTK ID;
determining a PGK and a PTK for securing the PDCP SDU based on at least one of the PDCP SN, the PGK ID, or the PTK ID in the PDCP header if the security feature is applied to the PDCP SDU;
decrypting the received PDCP SDU; and
sending the decrypted PDCP SDU to an upper layer.

12. The method of claim 6, wherein the processing of the PDCP PDU comprises:
determining the security feature is not applied on the PDCP SDU if the zero value is assigned to the PDCP SN, the PGK ID, and the PTK ID; and
sending the received PDCP SDU to an upper layer without decryption.

13. An apparatus for transmitting data in a device to device communication system, the apparatus comprising:
at least one processor configured to:
determine whether a security feature is applied to a packet data convergence protocol (PDCP) service data unit (SDU), and
configure a PDCP protocol data unit (PDU) based on the determined result; and
a transmitter configured to transmit the PDCP PDU to a receiving user equipment (UE),
wherein a PDCP header of the PDCP PDU includes an SDU type, a PDCP sequence number (SN), least significant bits of a pro-se group key (PGK) identification (ID) and a pro-se traffic key (PTK) ID, and
wherein the SDU type is set to a value indicating a protocol type of the PDCP SDU.

14. The apparatus of claim 13, wherein, if the security feature is not applied to the PDCP SDU, the PDCP SN, the least significant bits of the PGK ID and the PTK ID are set to a zero value in the PDCP header.

15. The apparatus of claim 13, wherein, if the security feature is applied to the PDCP SDU:
the PDCP header of the PDCP PDU comprises the SDU type and the PDCP SN, wherein the PDCP SN is set to one of values associated with the PDCP SDU;
the PDCP header of the PDCP PDU comprises the least significant bits of the PGK ID, wherein the least significant bits of the PGK ID identify a PGK used to generate a PTK for securing the PDCP SDU; and
the PDCP header of the PDCP PDU comprises PTK ID, wherein the PTK ID identifies the PTK used to generate a pro-se encryption key (PEK) for securing the PDCP SDU.

16. An apparatus of receiving data in a device to device communication system, the apparatus comprising:
a receiver for receiving a packet data convergence protocol (PDCP) protocol data unit (PDU); and
at least one processor configured to process the PDCP PDU configured based on whether a security feature is applied to a PDCP service data unit (SDU),
wherein a PDCP header of the PDCP PDU includes an SDU type, a PDCP sequence number (SN), least significant bits of a pro-se group key (PGK) identification (ID) and a pro-se traffic key (PTK) ID, and
wherein the SDU type is set to a value indicating a protocol type of the PDCP SDU.

17. The apparatus of claim 16, wherein the at least one processor is further configured to:
determine the security feature applied status on the PDCP SDU based on configuration information from a pro-se function;
determine a PGK and a PTK for securing the PDCP SDU based on the PTK ID and the PGK ID in the PDCP header by a PDCP entity if the security feature is applied to the PDCP SDU;
decrypt the received PDCP SDU; and
send the decrypted PDCP SDU to an upper layer.

18. The apparatus of claim 16, wherein the at least one processor is further configured to:
determine the security feature applied status on the PDCP SDU based on configuration information from a pro-se function; and
send the PDCP SDU to an upper layer without decryption if the security feature is not applied to the PDCP SDU.

19. The apparatus of claim 16, wherein the at least one processor is further configured to:
determine the security feature applied on the PDCP SDU if the zero value is not assigned to the PDCP SN, the PGK ID, and the PTK ID;
determine a PGK and a PTK for securing the PDCP SDU based on at least one of the PDCP SN, the PGK ID, or the PTK ID in the PDCP header if the security feature is applied to the PDCP SDU;
decrypt the received PDCP SDU; and
send the decrypted PDCP SDU to an upper layer.

20. The apparatus of claim 16, wherein the at least one processor is further configured to:
determine the security feature is not applied on the PDCP SDU if the zero value is assigned to the PDCP SN, the PGK ID, and the PTK ID; and
send the received PDCP SDU to an upper layer without decryption.
, Description:as attached