Content for  TS 38.423  Word version:  17.3.0

This procedure is used to establish necessary resources in an NG-RAN node for an incoming handover. If the procedure concerns a conditional handover, parallel transactions are allowed. Possible parallel requests are identified by the target cell ID when the source UE AP IDs are the same. The procedure uses UE-associated signalling.

The source NG-RAN node initiates the procedure by sending the HANDOVER REQUEST message to the target NG-RAN node. When the source NG-RAN node sends the HANDOVER REQUEST message, it shall start the timer TXn_RELOCprep. If the Conditional Handover Information Request IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall consider that the request concerns a conditional handover and shall include the Conditional Handover Information Acknowledge IE in the HANDOVER REQUEST ACKNOWLEDGE message. If the Target NG-RAN node UE XnAP ID IE is contained in the Conditional Handover Information Request IE included in the HANDOVER REQUEST message, then the target NG-RAN node shall remove the existing prepared conditional HO identified by the Target NG-RAN node UE XnAP ID IE and the Target Cell Global ID IE. It is up to the implementation of the target NG-RAN node when to remove the HO information. Upon reception of the HANDOVER REQUEST ACKNOWLEDGE message, the source NG-RAN node shall stop the timer TXn_RELOCprep and terminate the Handover Preparation procedure. If the procedure was initiated for an immediate handover, the source NG-RAN node shall start the timer TXn_RELOCoverall. The source NG-RAN node is then defined to have a Prepared Handover for that Xn UE-associated signalling. For each E-RAB ID IE included in the QoS Flow To Be Setup List IE in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store the content of the IE in the UE context and use it for subsequent inter-system handover. If the Masked IMEISV IE is contained in the HANDOVER REQUEST message the target NG-RAN node shall, if supported, use it to determine the characteristics of the UE for subsequent handling. At reception of the HANDOVER REQUEST message the target NG-RAN node shall prepare the configuration of the AS security relation between the UE and the target NG-RAN node by using the information in the UE Security Capabilities IE and the AS Security Information IE in the UE Context Information IE, as specified in TS 33.501. Upon reception of the PDU Session Resource Setup List IE, contained in the HANDOVER REQUEST message, the target NG-RAN node shall behave the same as specified in TS 38.413 for the PDU Session Resource Setup procedure. The target NG-RAN node shall report in the HANDOVER REQUEST ACKNOWLEDGE message the successful establishment of the result for all the requested PDU session resources. When the target NG-RAN node reports the unsuccessful establishment of a PDU session resource, the cause value should be precise enough to enable the source NG-RAN node to know the reason for the unsuccessful establishment. For each PDU session if the PDU Session Aggregate Maximum Bit Rate IE is included in the PDU Session Resources To Be Setup List IE contained in the HANDOVER REQUEST message, the target NG-RAN node shall store the received PDU Session Aggregate Maximum Bit Rate in the UE context and use it when enforcing traffic policing for Non-GBR QoS flows for the concerned UE as specified in TS 23.501. For each QoS flow for which the source NG-RAN node proposes to perform forwarding of downlink data, the source NG-RAN node shall include the DL Forwarding IE set to "DL forwarding proposed" within the Data Forwarding and Offloading Info from source NG-RAN node IE in the PDU Session Resources To Be Setup List IE in the HANDOVER REQUEST message. The source NG-RAN node shall include the DL Forwarding IE set to "DL forwarding proposed" for all the QoS flows mapped to a DRB, if it requests a DAPS handover for that DRB. For each PDU session for which the target NG-RAN node decides to admit the data forwarding for at least one QoS flow, the target NG-RAN node may include the PDU Session level DL data forwarding UP TNL Information IE within the Data Forwarding Info from target NG-RAN node IE in the PDU Session Resource Admitted Info IE contained in the PDU Session Resources Admitted List IE in the HANDOVER REQUEST ACKNOWLEDGE message. For each QoS flow for which the source NG-RAN node has not yet received the SDAP end marker packet if QoS flow re-mapping happened before handover, the source NG-RAN node shall include the UL Forwarding Proposal IE within the Data Forwarding and Offloading Info from source NG-RAN node IE in the HANDOVER REQUEST message, and if the target NG-RAN node decides to admit uplink data forwarding for at least one QoS flow, the target NG-RAN node may include the PDU Session Level UL Data Forwarding UP TNL Information IE in the Data Forwarding Info from target NG-RAN node IE in the PDU Session Resources Admitted Item IE contained in the PDU Session Resources Admitted List IE in the HANDOVER REQUEST ACKNOWLEDGE message to indicate that it accepts the uplink data forwarding. For each PDU session resource successfully setup at the target NG-RAN, the target NG-RAN node may allocate resources for additional Xn-U PDU session resource GTP-U tunnels, indicated in the Secondary Data Forwarding Info from target NG-RAN node List IE. For each PDU session in the HANDOVER REQUEST message, if the Alternative QoS Parameters Set List IE is included in the GBR QoS Flow Information IE in the PDU Session Resources To Be Setup List IE, the target NG-RAN node may accept the setup of the involved QoS flow when notification control has been enabled if the requested QoS parameters set or at least one of the alternative QoS parameters sets can be fulfilled at the time of handover as specified in TS 23.501. In case the target NG-RAN node accepts the handover fulfilling one of the alternative QoS parameters it shall indicate the alternative QoS parameters set which it can currently fulfil in the Current QoS Parameters Set Index IE within the PDU Session Resources Admitted List IE of the HANDOVER REQUEST ACKNOWLEDGE message while setting the QoS parameters towards the UE according to the requested QoS parameters set as specified in TS 23.501. For each DRB for which the source NG-RAN node proposes to perform forwarding of downlink data, the source NG-RAN node shall include the DRB ID IE and the mapped QoS Flows List IE within the Source DRB to QoS Flow Mapping List IE contained in the PDU Session Resources To Be Setup List IE in the HANDOVER REQUEST message. The source NG-RAN node may include the QoS Flow Mapping Indication IE in the Source DRB to QoS Flow Mapping List IE to indicate that only the uplink or downlink QoS flow is mapped to the DRB. If the target NG-RAN node decides to use the same DRB configuration and to map the same QoS flows as the source NG-RAN node, the target NG-RAN node includes the DL Forwarding GTP Tunnel Endpoint IE within the Data Forwarding Response DRB List IE in the HANDOVER REQUEST ACKNOWLEDGE message to indicate that it accepts the proposed forwarding of downlink data for this DRB. The target NG-RAN node may additionally include the Redundant DL Forwarding UP TNL Information IE if at least one of the QoS flow mapped to the DRB is eligible to the redundant transmission feature as indicated in the Redundant QoS Flow Indicator IE within the PDU Session Resource To Be Setup List IE received in the HANDOVER REQUEST message for the QoS flow. If the HANDOVER REQUEST ACKNOWLEDGE message contains the UL Forwarding UP TNL Information IE for a given DRB in the Data Forwarding Response DRB List IE within Data Forwarding Info from target NG-RAN node IE in the PDU Session Resources Admitted List IE and the source NG-RAN node accepts the data forwarding proposed by the target NG-RAN node, the source NG-RAN node shall perform forwarding of uplink data for the DRB. If the HANDOVER REQUEST includes PDU session resources for PDU sessions associated to S-NSSAIs not supported by target NG-RAN, the target NG-RAN node shall reject such PDU session resources. In this case, and if at least one PDU Session Resource To Be Setup Item IE is admitted, the target NG-RAN node shall send the HANDOVER REQUEST ACKNOWLEDGE message including the PDU Session Resources Not Admitted List IE listing corresponding PDU sessions rejected at the target NG-RAN. If the Mobility Restriction List IE is

• contained in the HANDOVER REQUEST message, the target NG-RAN node shall
  • store the information received in the Mobility Restriction List IE in the UE context;
  • use this information to determine a target for the UE during subsequent mobility action for which the NG-RAN node provides information about the target of the mobility action towards the UE, except when one of the PDU sessions has a particular ARP value (TS 23.501) in which case the information shall not apply;
  • use this information to select a proper SCG during dual connectivity operation.
  • use this information to select proper RNA(s) for the UE when moving the UE to RRC_INACTIVE.
• not contained in the HANDOVER REQUEST message, the target NG-RAN node shall
  • consider that no roaming and no access restriction apply to the UE except for the PNI-NPN mobility as described in TS 23.501.

The target NG-RAN node shall consider that roaming or access to CAG cells is only allowed if the Allowed PNI-NPN ID List IE is contained in the HANDOVER REQUEST message, as described in TS 23.501. If the Trace Activation IE is included in the HANDOVER REQUEST message the target NG-RAN node shall, if supported, initiate the requested trace function as specified in TS 32.422. If the Index to RAT/Frequency Selection Priority IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall store this information and use it as defined in TS 23.501. If the UE Context Reference at the S-NG-RAN IE is contained in the HANDOVER REQUEST message the target NG-RAN node may use it as specified in TS 37.340. In this case, the source NG-RAN node may expect the target NG-RAN node to include the UE Context Kept Indicator IE set to "True" in the HANDOVER REQUEST ACKNOWLEDGE message, which shall use this information as specified in TS 37.340. For each PDU session, if the Network Instance IE is included in the PDU Session Resource To Be Setup List IE and the Common Network Instance IE is not present, the target NG-RAN node shall, if supported, use it when selecting transport network resource as specified in TS 23.501. Redundant transmission:

• For each PDU session, if the Redundant UL NG-U UP TNL Information at UPF IE is included in the PDU Session Resource To Be Setup List IE, the target NG-RAN node shall, if supported, use it as the uplink termination point for the user plane data for the redundant transmission for the concerned PDU session.
• For each PDU session, if the Additional Redundant UL NG-U UP TNL Information at UPF List IE is included in the PDU Session Resource To Be Setup List IE, the target NG-RAN node shall, if supported, use them as the uplink termination points for the user plane data for the redundant transmission for the concerned PDU session.
• For each PDU session, if the Redundant Common Network Instance IE is included in the PDU Session Resource To Be Setup List IE, the target NG-RAN node shall, if supported, use it when selecting transport network resource for the redundant transmission as specified in TS 23.501.
• For each PDU session, if the Redundant PDU Session Information IE is included in the PDU Session Resource To Be Setup List IE contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store the received information in the UE context and set up the redundant user plane for the concerned PDU session, as specified in TS 23.501. If the PDU Session Pair ID IE is included in the Redundant PDU Session Information IE, the target NG-RAN node may store and use it to identify the paired PDU sessions.

If the TSC Traffic Characteristics IE is included in the QoS Flows To Be Setup List in the PDU Session Resource To Be Setup List IE, the target NG-RAN node shall, if supported, use it as specified in TS 23.501. For each PDU session, if the Common Network Instance IE is included in the PDU Session Resource To Be Setup List IE or in the Additional UL NG-U UP TNL Information at UPF List IE, or in the Additional Redundant UL NG-U UP TNL Information at UPF List IE, the target NG-RAN node shall, if supported, use it when selecting transport network resource for the concerned NG-U transport bearer as specified in TS 23.501. For each PDU session for which the Security Indication IE is included in the PDU Session Resource To Be Setup List IE and the Integrity Protection Indication IE or Confidentiality Protection Indication IE is set to "required", the target NG-RAN node shall perform user plane integrity protection or ciphering, respectively. If the NG-RAN node is not able to perform the user plane integrity protection or ciphering, it shall reject the setup of the PDU Session Resources with an appropriate cause value. If the NG-RAN node is an ng-eNB, it shall reject all PDU sessions for which the Integrity Protection Indication IE is set to "required". For each PDU session for which the Security Indication IE is included in the PDU Session Resource To Be Setup List IE and the Integrity Protection Indication IE or the Confidentiality Protection Indication IE is set to "preferred", the target NG-RAN node should, if supported, perform user plane integrity protection or ciphering, respectively and shall notify the SMF whether it succeeded the user plane integrity protection or ciphering or not for the concerned security policy. For each PDU session for which the Maximum Integrity Protected Data Rate IE is included in the Security Indication IE in the PDU Session Resources To Be Setup List IE, the NG-RAN node shall store the respective information and, if integrity protection is to be performed for the PDU session, it shall enforce the traffic corresponding to the received Maximum Integrity Protected Data Rate IE, for the concerned PDU session and concerned UE, as specified in TS 23.501. For each PDU session for which the Security Indication IE is included in the PDU Session Resource To Be Setup List IE and the Integrity Protection Indication IE or Confidentiality Protection Indication IE is set to "not needed", the target NG-RAN node shall not perform user plane integrity protection or ciphering, respectively, for the concerned PDU session. For each PDU session, if the Additional UL NG-U UP TNL Information List IE is included in the PDU Session Resources To Be Setup List IE contained in the HANDOVER REQUEST message, the target NG-RAN node may forward the UP transport layer information to the target S-NG-RAN node as the uplink termination point for the user plane data for this PDU session split in different tunnel. If the Location Reporting Information IE is included in the HANDOVER REQUEST message, then the target NG-RAN node should initiate the requested location reporting functionality as defined in TS 38.413. Upon reception of UE History Information IE in the HANDOVER REQUEST message, the target NG-RAN node shall collect the information defined as mandatory in the UE History Information IE and shall, if supported, collect the information defined as optional in the UE History Information IE, for as long as the UE stays in one of its cells, and store the collected information to be used for future handover preparations. If the Trace Activation IE is included in the HANDOVER REQUEST message which includes

• the MDT Activation IE set to "Immediate MDT and Trace", then the target NG-RAN node shall if supported, initiate the requested trace session and MDT session as described in TS 32.422.
• the MDT Activation IE set to "Immediate MDT Only" or "Logged MDT only", the target NG-RAN node shall, if supported, initiate the requested MDT session as described in TS 32.422 and the target NG-RAN node shall ignore the Interfaces To Trace IE, and the Trace Depth IE.
• the MDT Location Information IE, within the MDT Configuration IE, the target NG-RAN node shall, if supported, store this information and take it into account in the requested MDT session.
• the MDT Activation IE set to "Immediate MDT Only" or "Logged MDT only", and if the Signalling based MDT PLMN List IE is included in the MDT Configuration IE, the target NG-RAN node may use it to propagate the MDT Configuration as described in TS 37.320.
• the Bluetooth Measurement Configuration IE, within the MDT Configuration IE, the target NG-RAN node shall, if supported, take it into account for MDT Configuration as described in TS 37.320.
• the WLAN Measurement Configuration IE, within the MDT Configuration IE, the target NG-RAN node shall, if supported, take it into account for MDT Configuration as described in TS 37.320.
• the Sensor Measurement Configuration IE, within the MDT Configuration IE, the target NG-RAN node shall take it into account for MDT Configuration as described in TS 37.320.
• the MDT Configuration IE and if the target NG-RAN node is a gNB receiving a MDT Configuration-EUTRA IE, or the target NG-RAN node is a ng-eNB receiving a MDT Configuration-NR IE, the target NG-RAN node shall store it as part of the UE context, and use it as described in TS 37.320.

If the Management Based MDT PLMN List IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store the received information in the UE context, and use this information to allow subsequent selection of the UE for management based MDT defined in TS 32.422. If the HANDOVER REQUEST message includes the Management Based MDT PLMN List IE, the target NG-RAN node shall, if supported, store it in the UE context, and take it into account if it includes information regarding the PLMN serving the UE in the target NG-RAN node. If the Mobility Information IE is provided in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store this information. The target NG-RAN shall, if supported, store the C-RNTI assigned at the source cell as received in the HANDOVER REQUEST message. Upon reception of the UE History Information from the UE IE in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store the collected information and use it for future handover preparations. For each QoS flow which has been successfully established in the target NG-RAN node, if the QoS Monitoring Request IE was included in the QoS Flow Level QoS Parameters IE contained in the HANDOVER REQUEST message, the target NG-RAN node shall store this information, and shall, if supported, perform delay measurement and QoS monitoring, as specified in TS 23.501. If the QoS Monitoring Reporting Frequency IE was included in the QoS Flow Level QoS Parameters IE contained in the HANDOVER REQUEST message, the target NG-RAN node shall store this information, and shall, if supported, use it for RAN part delay reporting. If the 5GC Mobility Restriction List Container IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store this information in the UE context and use it as specified in TS 38.300. V2X:

• If the NR V2X Services Authorized IE is included in the HANDOVER REQUEST message and it contains one or more IEs set to "authorized", the target NG-RAN node shall, if supported, consider that the UE is authorized for the relevant service(s).
• If the LTE V2X Services Authorized IE is included in the HANDOVER REQUEST message and it contains one or more IEs set to "authorized", the target NG-RAN node shall, if supported, consider that the UE is authorized for the relevant service(s).
• If the NR UE Sidelink Aggregate Maximum Bit Rate IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use the received value for the concerned UE's sidelink communication in network scheduled mode for NR V2X services.
• If the LTE UE Sidelink Aggregate Maximum Bit Rate IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use the received value for the concerned UE's sidelink communication in network scheduled mode for LTE V2X services.

5G ProSe:

• If the 5G ProSe Authorized IE is included in the HANDOVER REQUEST message and it contains one or more IEs set to "authorized", the target NG-RAN node shall, if supported, consider that the UE is authorized for the relevant service(s).
• If the 5G ProSe UE PC5 Aggregate Maximum Bit Rate IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use the received value for the concerned UE's sidelink communication in network scheduled mode for 5G ProSe services.
• If the 5G ProSe PC5 QoS Parameters IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use it as defined in TS 23.304.

If the PC5 QoS Parameters IE is included in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use it as defined in TS 23.287. If the DAPS Request Information IE is included for a given DRB in the HANDOVER REQUEST message, the target NG-RAN node shall consider that the request concerns a DAPS handover for that DRB, as described in TS 38.300. Accordingly, the target NG-RAN node shall include the DAPS Response Information IE in the HANDOVER REQUEST ACKNOWLEDGE message. If the Maximum Number of CHO Preparations IE is included in the Conditional Handover Information Acknowledge IE contained in the HANDOVER REQUEST ACKNOWLEDGE message, then the source NG-RAN node should not prepare more candidate target cells for a CHO for the same UE towards the target NG-RAN node than the number indicated in the IE. If the Estimated Arrival Probability IE is contained in the Conditional Handover Information Request IE included in the HANDOVER REQUEST message, then the target NG-RAN node may use the information to allocate necessary resources for the incoming CHO. If the IAB Node Indication IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, consider that the handover is for an IAB node. In addition:

• If the No PDU Session Indication IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, consider the UE as an IAB-node which does not have any PDU sessions activated, and ignore the PDU Session Resources To Be Setup List IE, and shall not take any action with respect to PDU session setup. Subsequently, the source NG-RAN node shall, if supported, ignore the PDU Session Resources Admitted To Be Added List IE in the HANDOVER REQUEST ACKNOWLEDGE message.

If the UE Radio Capability ID IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store this information in the UE context and use it as defined in TS 23.501 and TS 23.502. If for a given QoS Flow the Source DL Forwarding IP Address IE is included within the Data Forwarding and Offloading Info from source NG-RAN node IE in the PDU Session Resources To Be Setup List IE contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store this information and use it as part of its ACL functionality configuration actions, if such ACL functionality is deployed. If the MBS Session Information List IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, establish an NG-RAN MBS session resources context as specified in TS 23.247 and TS 38.300, if applicable. If the HANDOVER REQUEST message includes the MBS Area Session ID IE, the target NG-RAN, if supported, shall use this information as an indication from which MBS Area Session ID the UE is handed over. For each MBS session for which the Active MBS Session Information IE is included in the MBS Session Information Item List IE, the target NG-RAN shall, if supported, use this information to setup respective MBS session resources. The target NG-RAN node shall, if supported, consider that the MBS sessions for which the Active MBS Session Information IE is not included are inactive. If the HANDOVER REQUEST ACKNOWLEDGE message contains in the MBS Session Information Response List IE the MBS Data Forwarding Response Info IE that the source NG-RAN node shall use the information for forwarding MBS traffic to the target NG-RAN node. If the MBS Session Associated Information List IE is included in the PDU Session Resources To Be Setup List IE in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, use the information contained in the Associated QoS Flows Information List IE as specified in TS 23.247. For each MRB indicated in the MBS Mapping and Data Forwarding Request Info from source NG-RAN node IE, the target NG-RAN node shall use the MRB ID IE and, if included, the MRB Progress Information IE which includes the highest PDCP SN of the packet which has already been delivered to the UE for the MRB, to decide whether to apply data forwarding for that MRB and to establish respective resources. The source NG-RAN shall, for each MRB in the MBS Data Forwarding Response Info from target NG-RAN node IE in the HANDOVER REQUEST ACKNOWLEDGE message, start data forwarding to the indicated DL Forwarding UP TNL Information. If the MRB Progress Information IE is included the source NG-RAN node may use the information to determine when to stop data forwarding. If the Time Synchronisation Assistance Information IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store this information in the UE context and use it as defined in TS 23.501. If the QMC Configuration Information IE is contained in the HANDOVER REQUEST message, the target NG-RAN node shall, if supported, take it into account for QoE measurements handling, as described in TS 38.300. If the UE Slice-Maximum Bit Rate List IE is contained in HANDOVER REQUEST message, the target NG-RAN node shall, if supported, store the received UE Slice Maximum Bit Rate List in the UE context, and use the received UE Slice Maximum Bit Rate value for each S-NSSAI for the concerned UE as specified in TS 23.501. Interaction with SN Status Transfer procedure: If the UE Context Kept Indicator IE set to "True" and the DRBs transferred to MN IE are included in the HANDOVER REQUEST ACKNOWLEDGE message, the source NG-RAN node shall, if supported, include the uplink/downlink PDCP SN and HFN status received from the S-NG-RAN node in the SN Status Transfer procedure towards the target NG-RAN node, as specified in TS 37.340.

If the target NG-RAN node does not admit at least one PDU session resource, or a failure occurs during the Handover Preparation, the target NG-RAN node shall send the HANDOVER PREPARATION FAILURE message to the source NG-RAN node. The message shall contain the Cause IE with an appropriate value. If the Conditional Handover Information Request IE is contained in the HANDOVER REQUEST message and the target NG-RAN node rejects the handover or a failure occurs during the Handover Preparation, the target NG-RAN node shall include the Requested Target Cell ID IE in the HANDOVER PREPARATION FAILURE message. Interactions with Handover Cancel procedure: If there is no response from the target NG-RAN node to the HANDOVER REQUEST message before timer TXn_RELOCprep expires in the source NG-RAN node, the source NG-RAN node should cancel the Handover Preparation procedure towards the target NG-RAN node by initiating the Handover Cancel procedure with the appropriate value for the Cause IE. The source NG-RAN node shall ignore any HANDOVER REQUEST ACKNOWLEDGE or HANDOVER PREPARATION FAILURE message received after the initiation of the Handover Cancel procedure and remove any reference and release any resources related to the concerned Xn UE-associated signalling.

If the supported algorithms for encryption defined in the UE Security Capabilities IE in the UE Context Information IE, plus the mandated support of the EEA0 and NEA0 algorithms in all UEs (TS 33.501), do not match any allowed algorithms defined in the configured list of allowed encryption algorithms in the NG-RAN node (TS 33.501), the NG-RAN node shall reject the procedure using the HANDOVER PREPARATION FAILURE message. If the supported algorithms for integrity defined in the UE Security Capabilities IE in the UE Context Information IE, plus the mandated support of the EIA0 and NIA0 algorithms in all UEs (TS 33.501), do not match any allowed algorithms defined in the configured list of allowed integrity protection algorithms in the NG-RAN node (TS 33.501), the NG-RAN node shall reject the procedure using the HANDOVER PREPARATION FAILURE message. If the CHO trigger IE is set to "CHO-replace" in the HANDOVER REQUEST message, but there is no CHO prepared for the included Target NG-RAN node UE XnAP ID, or the candidate cell in the Target Cell ID IE was not prepared using the same UE-associated signaling connection, the NG-RAN node shall reject the procedure using the HANDOVER PREPARATION FAILURE message. If the HANDOVER REQUEST message includes information for a PLMN not serving the UE in the target NG-RAN node in the Management Based MDT PLMN List IE, the target NG-RAN node shall ignore information for that PLMN within the Management Based MDT PLMN List.

The purpose of the SN Status Transfer procedure is to transfer the uplink PDCP SN and HFN receiver status and the downlink PDCP SN and HFN transmitter status either, from the source to the target NG-RAN node during an Xn handover, between the NG-RAN nodes involved in dual connectivity, or after retrieval of a UE context for RRC reestablishment, for each respective DRB of the source DRB configuration for which PDCP SN and HFN status preservation applies. In case that the Xn handover is a DAPS handover, the SN Status Transfer procedure may also be used to transfer the uplink PDCP SN and HFN receiver status, and the downlink PDCP SN and HFN transmitter status for a DRB associated with RLC-UM and configured with DAPS as described in TS 38.300. In case that the Xn handover is a CHO, the SN Status Transfer procedure may also be used to transfer handover related information. If the SN Status Transfer procedure is applied in the course of dual connectivity or RRC connection re-establishment in the subsequent specification text

• the behaviour of the NG-RAN node from which the DRB context is transferred, i.e. the NG-RAN node involved in dual connectivity or RRC connection re-establishment, from which data is forwarded, is specified by the behaviour of the "source NG-RAN node",
• the behaviour of the NG-RAN node to which the DRB context is transferred, i.e., the NG-RAN node involved in dual connectivity or RRC connection re-establishment, to which data is forwarded, is specified by the behaviour of the "target NG-RAN node".

The procedure uses UE-associated signalling.

The source NG-RAN node initiates the procedure by stop assigning PDCP SNs to downlink SDUs and stop delivering UL SDUs towards the 5GC and sending the SN STATUS TRANSFER message to the target NG-RAN node at the time point when it considers the transmitter/receiver status to be frozen. The target NG-RAN node using full configuration for this handover as per TS 38.300 or for the MR-DC operations as per TS 37.340 shall ignore the information received in this message. In case of MR-DC, if the target NG-RAN node performs PDCP SN length change or RLC mode change for a DRB as specified in TS 37.340, it shall ignore the information received for that DRB in this message. In case that the Xn handover is a DAPS handover, the source NG-RAN node may continue assigning PDCP SNs to downlink SDUs and delivering uplink SDUs toward the 5GC when initiating this procedure for DRBs not configured with DAPS as in TS 38.300. For each DRB in the DRBs Subject to Status Transfer List IE, the source NG-RAN node shall include the DRB ID IE, the UL COUNT Value IE and the DL COUNT Value IE. The source NG-RAN node may also include in the SN STATUS TRANSFER message the missing and the received uplink SDUs in the Receive Status of UL PDCP SDUs IE for each DRB for which the source NG-RAN node has accepted the request from the target NG-RAN node for uplink forwarding. For each DRB in the DRBs Subject to Status Transfer List IE, the target NG-RAN node shall not deliver any uplink packet which has a PDCP-SN lower than the value contained within the UL COUNT Value IE. For each DRB in the DRBs Subject to Status Transfer List IE, the target NG-RAN node shall use the value of the PDCP SN contained within the DL COUNT Value IE for the first downlink packet for which there is no PDCP-SN yet assigned. If the Receive Status of UL PDCP SDUs IE is included for at least one DRB in the SN STATUS TRANSFER message, the target NG-RAN node may use it in a Status Report message sent to the UE over the radio interface. If the SN STATUS TRANSFER message contains in the DRBs Subject To Status Transfer List IE the Old QoS Flow List - UL End Marker expected IE, the target NG-RAN node shall be prepared to receive the SDAP end marker for the QoS flow via the corresponding DRB, as specified in TS 38.300. If the CHO Configuration IE is included in the SN STATUS TRANSFER message, the target NG-RAN node shall, if supported, store this information in the UE context and use it as specified in TS 38.300. If the Mobility Information IE is included in the SN STATUS TRANSFER message, the target NG-RAN node shall, if supported, store this information in the UE context and use it as specified in TS 38.300.

Not applicable.

If the target NG-RAN node receives this message for a UE for which no prepared handover exists at the target NG-RAN node, the target NG-RAN node shall ignore the message.