Content for  TS 38.401  Word version:  18.4.0

The Source gNB-CU-CP sends MC BEARER CONTEXT MODIFICATION REQUEST message towards the Source gNB-CU-UP, to request the last delivered MRB Progress of an MBS Session related to a Multicast F1-U Context Reference E1.

The Source gNB-CU-UP responds to the gNB-CU-CP in the MC BEARER CONTEXT MODIFICATION RESPONSE message, providing the last delievered MRB Progress.

The source gNB-CU-CP sends the HANDOVER REQUEST message to the target gNB-CU-CP with the last delievered MRB progress for the Multicast sessions the UE has joined.

In case the Multicast session which the UE has joined was not established in the target cell, the target gNB-CU-CP triggers the Multicast MBS Session Context Establishment procedure as defined in clause 8.15.1.2.

The target gNB-CU-CP triggers MC BEARER CONTET SETUP REQUEST message towards the target gNB-CU-UP.

The target gNB-CU-UP responds to the target gNB-CU-CP by sending the MC BEARER CONTET SETUP RESPONSE message.

The target gNB-CU-CP sends UE CONTEXT SETUP REQUEST towards the target gNB-DU. If the target gNB-CU-CP has decided to request a PTP Forwarding tunnel, the message contains the PTP Forwarding Tunnel Required indication and the last delivered MRB Progress for the MRBs requested to be configured.

The target gNB-DU sends UE CONTEXT SETUP RESPONSE towards the target gNB-CU-CP.

If a PTP Forwarding Tunnel was requested in step 7, the target gNB-DU shall, if supported, trigger the F1-U tunnel establishment via MULTICAST DISTRIBUTION SETUP REQUEST, including the F1-U DL UP TNL information and the last delievered MRB Progress for the each of the MRB.

The target gNB-CU-CP sends the MC BEARER CONTEXT MODIFICATION REQUEST towards the target gNB-CU-UP, including for each MRB a request for the oldest availiable MRB Progress information, the last delivered MRB Progress, and the F1-U DL UP TNL information.

Based on the information received in step 10, the target gNB-CU-UP determines whether the Xn-U data forwarding is needed or not, and in case it is needed, provide for each MRB the oldest available MRB Progress and the assigned XN-U DL Forwarding UP TNL Info to the target gNB-CU-CP in the MC BEARER CONTEXT MODIFICATION RESPONSE message.

In case step 9 was triggerred, the target gNB-CU-CP sends the MULTICAST DISTRIBUTION SETUP RESPONSE to the target gNB-DU.

The target gNB-CU-CP sends HANDOVER REQUEST ACKNOWLEDGE to the source gNB-CU-CP, including the XN-U DL Forwarding UP TNL Info and the oldest available MRB Progress for each MRB.

The source gNB-CU-CP forwards the XN-U DL Forwarding UP TNL Info and the oldest available MRB Progress for each MRB to the source gNB-CU-UP via MC BEARER CONTEXT MODIFICATION REQUEST message.

The source gNB-CU-UP sends the MC BEARER CONTEXT MODIFICATION RESPONSE message to the source gNB-CU-CP.

The source gNB-CU-UP starts to forward MBS data towards the target gNB-CU-UP.

If a PTP Forwarding Tunnel was established, the target gNB-CU-UP sends the forwarding data to the target gNB-DU.