Section: 10.4.2 MR-DC with 5GC
Content: The SN Release procedure may be initiated either by the MN or by the SN and is used to initiate the release of the UE context and relevant resources at the SN. The recipient node of this request can reject it, e.g., if an SN change procedure is triggered by the SN. In case of CPA, inter-SN CPC or inter-SN subsequent CPAC, this procedure may be initiated either by the MN or the candidate SN, and it is used to cancel all the prepared PSCells at the candidate SN and initiate the release of related UE context at the candidate SN. MN initiated SN Release Figure 10.4.2-1: SN release procedure - MN initiated Figure 10.4.2-1 shows an example signalling flow for the MN initiated SN Release procedure. 1. The MN initiates the procedure by sending the SN Release Request message. 2. The SN confirms SN Release by sending the SN Release Request Acknowledge message. If appropriate, the SN may reject SN Release, e.g., if the SN change procedure is triggered by the SN. NOTE 00: If CPA or inter-SN CPC is configured, upon reception of the SN Release Request Acknowledge message the MN cancels all CPAC with the target candidate SN(s). NOTE 00a: If subsequent CPAC is configured, upon reception of the SN Release Acknowledge message from the source SN, the MN may retain the subsequent CPAC configuration or cancel the subsequent CPAC configuration. If the MN maintains the subsequent CPAC configuration, it should provide suitable execution conditions for the evaluation of the subsequent CPAC. 2a. When applicable, the MN provides forwarding address information to the SN. NOTE 0: The MN may send the Xn-U Address Indication message to provide forwarding address information before step 2. 3/4. If required, the MN indicates in the MN RRC reconfiguration message towards the UE that the UE shall release the entire SCG configuration. In case the UE is unable to comply with (part of) the configuration included in the MN RRC reconfiguration message, it performs the reconfiguration failure procedure. NOTE 1: If data forwarding is applied, timely coordination between steps 1 and 2 may minimize gaps in service provision, this is however regarded to be an implementation matter. 5. If PDCP termination point is changed to the MN for bearers using RLC AM, the SN sends the SN Status Transfer message. 6. Data forwarding from the SN to the MN may start. 7. The SN sends the Secondary RAT Data Usage Report message to the MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2. NOTE 1a: If data forwarding is applied, the order the SN sends the Secondary RAT Data Usage Report message and starts data forwarding with MN is not defined i.e., step 7 can take place before step 6. The SN does not need to wait for the end of data forwarding to send the Secondary RAT Data Usage Report message. 8. If applicable, the PDU Session path update procedure is initiated. 9. Upon reception of the UE Context Release message, the SN releases radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue. SN initiated SN Release Figure 10.4.2-2: SN release procedure - SN initiated Figure 10.4.2-2 shows an example signalling flow for the SN initiated SN Release procedure. 1. The SN initiates the procedure by sending the SN Release Required message which may contain inter-node message to support delta configuration. 2. If data forwarding is requested, the MN provides data forwarding addresses to the SN in the SN Release Confirm message. The SN may start data forwarding and stop providing user data to the UE as early as it receives the SN Release Confirm message. NOTE 1b: If CPA or inter-SN CPC is configured, upon reception of the SN Release Required message the MN cancels all CPAC with the target candidate SN(s). NOTE 1c: If subsequent CPAC is configured, upon reception of the SN Release Required message from the source SN, the MN may retain the subsequent CPAC configuration or cancel the subsequent CPAC configuration. If the MN maintains the subsequent CPAC configuration, it should provide suitable execution conditions for the evaluation of the subsequent CPAC. 3/4. If required, the MN indicates in the MN RRC reconfiguration message towards the UE that the UE shall release the entire SCG configuration. In case the UE is unable to comply with (part of) the configuration included in the MN RRC reconfiguration message, it performs the reconfiguration failure procedure. NOTE 2: If data forwarding is applied, timely coordination between steps 2 and 3 may minimize gaps in service provision. This is however regarded to be an implementation matter. 5. If PDCP termination point is changed to the MN for bearers using RLC AM, the SN sends the SN Status Transfer message. 6. Data forwarding from the SN to the MN may start. 7. The SN sends the Secondary RAT Data Usage Report message to the MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2. NOTE 3: If data forwarding is applied, the order the SN sends the Secondary RAT Data Usage Report message and starts data forwarding with MN is not defined i.e., step 7 can take place before step 6. The SN does not need to wait for the end of data forwarding to send the Secondary RAT Data Usage Report message. 8. If applicable, the PDU Session path update procedure is initiated. 9. Upon reception of the UE Context Release message, the SN releases radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue.