Inter-RAT (radio access technology) reestablishment enhancements in multi-RAT dual connectivity (MR-DC)

1. A method for a target node, in a radio access network (RAN), to establish a connection with a user equipment (UE) operating in dual connectivity (DC) with a master cell group (MCG) comprising a primary cell (PCell) served by a master node (MN), and with a secondary cell group (SCG) comprising a primary secondary cell (PSCell) served by a secondary node (SN), the method comprising:

receiving, from the UE, a re-establishment request message comprising:

a plurality of parameters associated with the PSCell,

a first message authentication code (MAC), and

an indication of the UE's selection of a target cell served by the target node as a replacement for the PCell:

the PCell utilizes utilizing a first radio access technology (RAT), and

the PSCell and the target cell utilizing a second RAT different from the first RAT;

verifying integrity of the re-establishment request message based on the first MAC;

determining a full UE context for the UE based on:

the plurality of parameters associated with the PSCell, and

a successful verification of the integrity of the re-establishment request message; and

establishing a connection with the UE based on the full UE context, whereby the target cell served by the target node will serve as a PCell for DC; and

the determining the full UE context comprises:

based on the plurality of parameters associated with the PSCell, determining that the target node is different than the SN;

sending, to the SN, a context request message including the plurality of parameters associated with the PSCell; and

receiving, from the MN, a context response message including the full UE context.

2. The method of claim 1 , wherein the MAC included in the re-establishment request message is based on a plurality of parameters associated with the PSCell and on an identifier of the target cell.

3. The method of claim 2 , wherein the plurality of parameters associated with the PSCell include an identifier of the PSCell, an identifier of the UE in relation to the PSCell, and integrity protection information associated with the SN.

4. The method of claim 1 , wherein determining the full UE context comprises:

performing the following operations based on the plurality of parameters associated with the PSCell:

determining that the target node is the same as the SN; and

retrieving a UE context associated with the SN, wherein the UE context associated with the SN includes a first identifier of the UE in relation to the MN;

sending, to the MN, a context request message including the first identifier of the UE in relation to the MN; and

receiving, from the MN, a context response message including the full UE context.

5. The method of claim 4 , wherein:

the UE context associated with the SN includes integrity protection information associated with the SN; and

verifying the integrity of the re-establishment request message comprises:

calculating a second MAC based on an identifier of the target cell, the plurality of parameters associated with the PSCell, and the integrity protection information associated with the SN; and

comparing the first MAC and the second MAC to determine if there is a match between the first MAC and the second MAC, wherein a match indicates successful verification of the integrity.

6. The method of claim 5 , wherein the identifier of the target cell is included in the re-establishment request message.

7. The method of claim 5 , wherein the context request message is sent to the MN based on determining that there is a match between the first MAC and the second MAC.

8. The method of claim 1 , wherein verifying the integrity of the re-establishment request message comprises sending the first MAC in the context request message for verification by the SN.

9. A network node, of a radio access network, RAN, configured to operate in dual connectivity towards a user equipment, UE together with a further network node in the RAN to establish a connection with a user equipment (UE) operating in dual connectivity (DC) with a master cell group (MCG) comprising a primary cell (PCell) served by a master node (MN), and with a secondary cell group (SCG) comprising a primary secondary cell (PSCell) served by a secondary node (SN), the network node comprising:

interface circuitry operable to communicate with the UE and with at least the further network node in the RAN; and

processing circuitry operably coupled to the interface circuitry, whereby the processing circuitry and the interface circuitry are configured to receive, from the UE, a re-establishment request message comprising:

a plurality of parameters associated with the PSCell,

a first message authentication code (MAC), and

an indication of the UE's selection of a target cell served by a target node as a replacement for the PCell:

the PCell utilizes a first radio access technology (RAT), and

the PSCell and the target cell utilize a second RAT different from the first RAT; and

verify integrity of the re-establishment request message based on the first MAC;

determine a full UE context for the UE based on:

the plurality of parameters associated with the PSCell, and

a successful verification of the integrity of the re-establishment request message;

establish a connection with the UE based on the full UE context, whereby the target cell served by the target node will serve as a PCell for DC; and

the determining the full UE context comprises:

based on the plurality of parameters associated with the PSCell, determining that the target node is different than the SN;

sending, to the SN, a context request message including the plurality of parameters associated with the PSCell; and

receiving, from the MN, a context response message including the full UE context.

10. The network node of claim 9 , wherein the MAC included in the re-establishment request message is based on a plurality of parameters associated with the PSCell and on an identifier of the target cell.

11. The network node of claim 10 , wherein the plurality of parameters associated with the PSCell include an identifier of the PSCell, an identifier of the UE in relation to the PSCell, and integrity protection information associated with the SN.

12. The network node of claim 9 , wherein determining the full UE context comprises:

performing the following operations based on the plurality of parameters associated with the PSCell:

determining that the target node is the same as the SN; and

retrieving a UE context associated with the SN, wherein the UE context associated with the SN includes a first identifier of the UE in relation to the MN;

sending, to the MN, a context request message including the first identifier of the UE in relation to the MN; and

receiving, from the MN, a context response message including the full UE context.

13. The network node of claim 12 , wherein:

the UE context associated with the SN includes integrity protection information associated with the SN; and

verifying the integrity of the re-establishment request message comprises:

calculating a second MAC based on an identifier of the target cell, the plurality of parameters associated with the PSCell, and the integrity protection information associated with the SN; and

comparing the first MAC and the second MAC to determine if there is a match between the first MAC and the second MAC, wherein a match indicates successful verification of the integrity.

14. The network node of claim 13 , wherein the identifier of the target cell is included in the re-establishment request message.

15. The network node of claim 13 , wherein the context request message is sent to the MN based on determining that there is a match between the first MAC and the second MAC.

16. The network node of claim 9 , wherein verifying the integrity of the re-establishment request message comprises sending the first MAC in the context request message for verification by the SN.