Enhancement for multi-access PDU session release

1. A method, comprising:

establishing a protocol data unit (PDU) session with a 5G system (5GS) by a user equipment (UE);

receiving a PDU session release command message from the 5GS by the UE, wherein the PDU session release command message comprises an access type indicating a first radio access technology (RAT) access; and

determining, by the UE, whether the PDU session is a multi-access (MA) PDU session and whether the MA PDU session having user plane resources established on only the first RAT access indicated in the access type or the MA PDU session having the user plane resources established on only a second RAT access different from the first RAT access indicated in the access type; and

transmitting a PDU session release complete message or a 5G session management (5GSM) status message to the 5GS by the UE in response to the PDU session release command message comprising the access type.

2. The method of claim 1 , wherein the PDU session is the MA PDU session, and the method further comprises:

determining the MA PDU session as released in response to the MA PDU session having the user plane resources established on only the first RAT access indicated in the access type.

3. The method of claim 2 , wherein the determining of the MA PDU session as released comprises:

releasing the MA PDU session; and

deleting PDU session contexts associated with the MA PDU session.

4. The method of claim 1 , wherein the PDU session is the MA PDU session, and the method further comprising:

determining the user plane resources of the MA PDU session on the first RAT access indicated in the access type as released in response to the MA PDU session having the user plane resources established on only a second RAT access different from the first RAT access indicated in the access type or on both the first RAT access and the second RAT access.

5. The method of claim 4 , wherein the determining of the user plane resources of the MA PDU session on the first RAT access as released comprises:

keeping PDU session contexts associated with the MA PDU session.

6. The method of claim 5 , wherein the determining of the user plane resources of the MA PDU session on the first RAT access as released comprises:

releasing the user plane resources of the MA PDU session on the first RAT access in response to the MA PDU session having user plane resources established on both the first RAT access and the second RAT access.

7. The method of claim 1 , wherein the first RAT access is one of a 3rd generation partner project (3GPP) access and a non-3GPP access.

8. The method of claim 1 , wherein the 5GSM status message comprises a cause value indicating that the PDU session release command message is not valid for the PDU session.

9. The method of claim 1 , further comprising:

keeping the PDU session as it is by the UE.

10. The method of claim 1 , wherein the PDU session is not the MA PDU session, and the method further comprising:

releasing the PDU session by the UE.

11. A User Equipment (UE), comprising:

a Protocol data unit (PDU) session handling circuit that establishes a PDU session with a 5G system (5GS); and

a receiver that receives a PDU session release command message from the 5GS, wherein the PDU session release command message comprises an access type indicating a first radio access technology (RAT) access;

a processor that determines whether the PDU session is a multi-access (MA) PDU session and whether the MA PDU session having user plane resources established on only the first RAT access indicated in the access type or the MA PDU session having the user plane resources established on only a second RAT access different from the first RAT access indicated in the access type; and

a transmitter that transmits a PDU session release complete message or a 5G session management (5GSM) status message to the 5GS in response to the PDU session release command message comprising the access type.

12. The UE of claim 11 , wherein the PDU session is the MA PDU session, and the UE determines the MA PDU session as released in response to the MA PDU session having user plane resources established on only the first RAT access indicated in the access type.

13. The UE of claim 12 , wherein, in determining the MA PDU session as released, the UE releases the MA PDU session and deletes PDU session contexts associated with the MA PDU session.

14. The UE of claim 11 , wherein the PDU session is the MA PDU session, and the UE determines user plane resources of the MA PDU session on the first RAT access indicated in the access type as released in response to the MA PDU session having user plane resources established on only a second RAT access different from the first RAT access indicated in the access type or on both the first RAT access and the second RAT access.

15. The UE of claim 14 , wherein, in determining the user plane resources of the MA PDU session on the first RAT access as released, the UE keeps PDU session contexts associated with the MA PDU session.

16. The UE of claim 15 , wherein, in determining the user plane resources of the MA PDU session on the first RAT access as released, the UE releases the user plane resources of the MA PDU session on the first RAT access in response to the MA PDU session having user plane resources established on both the first RAT access and the second RAT access.

17. The UE of claim 11 , wherein the first RAT access is one of a 3rd generation partner project (3GPP) access and a non-3GPP access.

18. The UE of claim 11 , wherein the 5GSM status message comprises a cause value indicating that the PDU session release command message is not valid for the PDU session.

19. The UE of claim 11 , wherein the UE keeps the PDU session as it is.

20. The UE of claim 11 , wherein the PDU session is not the MA PDU session, and the UE releases the PDU session.