8.7.6 MeNB initiated SgNB Modification Preparation

36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)

8.7.6.1 General

This procedure is used to enable an MeNB to request an en-gNB to modify the UE context at the en-gNB, or to query the current SCG configuration for supporting delta signalling in MeNB initiated SgNB change, or to provide the S-RLF-related information to the en-gNB.

The procedure uses UE-associated signalling.

8.7.6.2 Successful Operation

Figure 8.7.6.2-1: MeNB initiated SgNB Modification Preparation, successful operation

The MeNB initiates the procedure by sending the SGNB MODIFICATION REQUEST message to the en-gNB. When the MeNB sends the SGNB MODIFICATION REQUEST message, it shall start the timer TDCprep.

The SGNB MODIFICATION REQUEST message may contain:

– within the UE Context Information IE (if the modification of the UE context at the en-gNB is requested);

– E-RABs to be added within the E-RABs To Be Added Item IE;

– E-RABs to be modified within the E-RABs To Be Modified Item IE;

– E-RABs to be released within the E-RABs To Be Released Item IE;

– the SgNB UE Aggregate Maximum Bit Rate IE;

– the MeNB to SgNB Container IE;

– the SCG Configuration Query IE;

– the MeNB Resource Coordination Information IE;

– the Requested split SRBs IE;

– the Requested split SRBs release IE;

– the Requested fast MCG recovery via SRB3 IE;

– the Requested fast MCG recovery via SRB3 Release IE.

If the SGNB MODIFICATION REQUEST message contains the Serving PLMN IE, the en-gNB may use it for RRM purposes.

If the SGNB MODIFICATION REQUEST message contains the Handover Restriction List IE, the en-gNB shall

– replace the previously provided Handover Restriction List by the received Handover Restriction List in the UE context;

– use this information to select an appropriate NR cell.

If the SgNB UE Aggregate Maximum Bit Rate IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall:

– replace the previously provided SgNB UE Aggregate Maximum Bit Rate by the received SgNB UE Aggregate Maximum Bit Rate in the UE context;

– use the received SgNB UE Aggregate Maximum Bit Rate for non-GBR Bearers for the concerned UE as defined in TS 37.340 [32].

The allocation of resources according to the values of the QCI IE, Allocation and Retention Priority IE or GBR QoS Information IE included in the Full E-RAB Level QoS Parameters IE or in the Requested SCG E-RAB Level QoS Parameters IE shall follow the principles described for the E-RAB Setup procedure in TS 36.413 [4].

If the SGNB MODIFICATION REQUEST message contains the MeNB Resource Coordination Information IE, the en-gNB should forward it to lower layers and it may use it for the purpose of resource coordination with the MeNB, or to coordinate with sidelink resources used in the MeNB. The en-gNB shall consider the received UL Coordination Information IE value valid until reception of a new update of the IE for the same UE. The en-gNB shall consider the received DL Coordination Information IE value valid until reception of a new update of the IE for the same UE. If the MeNB Coordination Assistance Information IE is contained in the MeNB Resource Coordination Information IE, the en-gNB shall, if supported, use the information to determine further coordination of resource utilisation between the en-gNB and the MeNB.

If at least one of the requested modifications is admitted by the en-gNB, the en-gNB shall modify the related part of the UE context accordingly and send the SGNB MODIFICATION REQUEST ACKNOWLEDGE message back to the MeNB.

The en-gNB shall include the E-RABs for which resources have been either added or modified or released at the en-gNB either in the E-RABs Admitted To Be Added List IE or the E-RABs Admitted To Be Modified List IE or the E-RABs Admitted To Be Released List IE. The en-gNB shall include the E-RABs that have not been admitted in the E-RABs Not Admitted List IE with an appropriate cause value.

For each E-RAB successfully established or modified or released in the en-gNB, the en-gNB shall report to the MeNB, in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the same value in the EN-DC Resource Configuration IE as received in the SGNB MODIFICATION REQUEST message.

The en-gNB shall, if included, choose the ciphering algorithm based on the information in the NR UE Security Capabilities IE and locally configured priority list of AS encryption algorithms and apply the key indicated in the SgNB Security Key IE as specified in the TS 33.401 [18].

For each E-RAB for which allocation of the PDCP entity is requested at the en-gNB:

– if applicable, the MeNB may propose to apply forwarding of downlink data by including the DL Forwarding IE within the E-RABs To Be Added Item IE of the SGNB MODIFICATION REQUEST message. For each E-RAB that it has decided to admit, the en-gNB may include the DL Forwarding GTP Tunnel Endpoint IE within the E-RABs Admitted To Be Added Item IE of the SGNB MODIFICATION REQUEST ACKNOWLEDGE message to indicate that it accepts the proposed forwarding of downlink data for this bearer. The MeNB may also provide for an applicable E-RAB to be released the DL Forwarding GTP Tunnel Endpoint IE and the UL Forwarding GTP Tunnel Endpoint IE within the E-RABs To Be Released Item IE of the SGNB MODIFICATION REQUEST message.

– if applicable, the en-gNB may include for each bearer in the E-RABs Admitted To Be Added List IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the UL Forwarding GTP Tunnel Endpoint IE to indicate that it requests data forwarding of uplink packets to be performed for that bearer.

– if applicable, the en-gNB may include for each bearer in the E-RABs Admitted To Be Modified List IE which is configured with the SN terminated split bearer option in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the UL Configuration IE to indicate that the MCG UL configuration of the UE has changed.

– if applicable, the en-gNB may include for each bearer in the E-RABs Admitted To Be Added List IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the UL PDCP SN Length IE and the DL PDCP SN Length IE to indicate the PDCP SN length for that bearer.

– If the Bearer Type IE for the concerned E-RAB is received by the en-gNB and is set to"non IP", then the en-gNB shall, if supported, not perform IP header compression for the concerned E-RAB.

– If the Ethernet Type IE for the concerned E-RAB is received by the en-gNB and is set to "True", the en-gNB shall take this into account to perform header compression appropriately for the concerned E-RAB.

For each E-RAB configured with SCG resources and the PDCP entity is hosted by the MeNB and

– requested to be modified,

– if the SGNB MODIFICATION REQUEST message includes the MeNB UL GTP Tunnel Endpoint at PDCP IE in the E-RABs To Be Modified Item IE, the en-gNB shall act as specified in TS 37.340 [32].

– if the SGNB MODIFICATION REQUEST message contains the MeNB UL GTP Tunnel Endpoint at PDCP IE the en-gNB shall use it as the new UL X2-U address.

– the en-gNB may include in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the SgNB DL GTP Tunnel Endpoint at SCG IE.

If, dependent on the configured bearer type, the Full E-RAB Level QoS Parameters IE or the Maximum MCG admittable E-RAB Level QoS Parameters IE or the Requested SCG E-RAB level QoS Parameters IE are included in the SGNB MODIFICATION REQUEST message for an E-RAB to be modified the en-gNB shall allocate respective resources and provide corresponding radio configuration information within the SgNB to MeNB Container IE as described in TS 37.340 [32].

If the SGNB MODIFICATION REQUEST message contains, for an E-RAB to be modified which is configured with the PDCP entity in the en-gNB, the S1 UL GTP Tunnel Endpoint IE, the en-gNB shall use it as the new UL S1-U address.

If the SGNB MODIFICATION REQUEST message contains an E-RAB to be modified which is configured with the MN terminated split bearer option, the MeNB may include the UL Configuration IE to indicate that the SCG UL configuration of the UE has changed.

If the SGNB MODIFICATION REQUEST message contains for an E-RAB to be modified which is configured with the PDCP enitiy in the en-gNB and MCG resources the MeNB DL GTP Tunnel Endpoint at MCG IE the en-gNB shall use it as the DL X2-U address.

If the SGNB MODIFICATION REQUEST message contains the Subscriber Profile ID for RAT/Frequency Priority IE, the en-gNB may use it for RRM purposes.

If the SGNB MODIFICATION REQUEST message contains the Additional RRM Policy Index IE, the en-gNB may use it for RRM purposes.

For an E-RAB to be modified which is configured with the PDCP entity in the en-gNB the en-gNB may include in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the S1 DL GTP Tunnel Endpoint at the SgNB IE.

If the SGNB MODIFICATION REQUEST ACKNOWLEDGE message contains the SgNB Resource Coordination Information IE, the MeNB may use it for the purpose of resource coordination with the en-gNB. The MeNB shall consider the received UL Coordination Information IE value valid until reception of a new update of the IE for the same UE. The MeNB shall consider the received DL Coordination Information IE value valid until reception of a new update of the IE for the same UE. If the SgNB Coordination Assistance Information IE is contained in the SgNB Resource Coordination Information IE, the MeNB shall, if supported, use the information to determine further coordination of resource utilisation between the en-gNB and the MeNB.

Upon reception of the SGNB MODIFICATION REQUEST ACKNOWLEDGE message the MeNB shall stop the timer TDCprep. If the SGNB MODIFICATION REQUEST ACKNOWLEDGE message has included the SgNB to MeNB Container IE the MeNB is then defined to have a Prepared SgNB Modification for that X2 UE-associated signalling.

If the SCG Configuration Query IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall provide corresponding radio configuration information within the SgNB to MeNB Container IE as described in TS 37.340 [32].

If the SGNB MODIFICATION REQUEST message contains the Requested split SRBs IE, the en-gNB may use it to add split SRBs. If the SGNB MODIFICATION REQUEST message contains the Requested split SRBs release IE, the en-gNB may use it to release split SRBs.

If the Requested Fast MCG recovery via SRB3 IE set to "true" is included in the SGNB MODIFICATION REQUEST message and the en-gNB decides to configure fast MCG link recovery via SRB3 as specified in TS 37.340 [32], the en-gNB shall, if supported, include the Available fast MCG recovery via SRB3 IE set to "true" in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message. If the Requested Fast MCG recovery via SRB3 Release IE set to "true" is included in the SGNB MODIFICATION REQUEST message and the en-gNB decides to release fast MCG link recovery via SRB3, the en-gNB shall, if supported, include the Release fast MCG recovery via SRB3 IE set to "true" in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message.

If the en-gNB receives for an E-RAB to be setup for which the PDCP entiy is allocated at the MeNB the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE in the SGNB MODIFICATION REQUEST message, it may provide the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE and the LCID IE to the MeNB in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message if PDCP duplication is configured at the en-gNB.

If the SGNB MODIFICATION REQUEST message contains the RLC Status IE, the en-gNB shall assume that RLC has been reestablished at the MeNB and may trigger PDCP data recovery.

If the en-gNB applied a full configuration or delta configuration, e.g. as part of a mobility procedure involving a change of DU, the en-gNB shall inform the MeNB by including the RRC config indication IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message.

If SGNB MODIFICATION REQUEST message contains the UL PDCP SN Length IE and the DL PDCP SN Length IE, the en-gNB shall, if supported, store this information and use it for lower layer configuration of the concerned MN terminated bearer.

If the RLC Mode IE is included for an E-RAB within the E-RABs To be Added List IE in the SGNB MODIFICATION REQUEST message, it indicates the mode that the MeNB used for the E-RAB when it was hosted at the MeNB.

If the SGNB MODIFICATION REQUEST message contains the MeNB Cell ID IE, the en-gNB may search for the target NR cell among the NR neighbour cells of the E-UTRAN cell indicated in MeNB Cell ID IE, as specified in the TS 37.340 [32].

If the SGNB MODIFICATION REQUEST ACKNOWLEDGE message contains the RLC Status IE, the MeNB shall assume that RLC has been reestablished at the en-gNB and may trigger PDCP data recovery.

The en-gNB may include the Location Information at SgNB IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, if respective information is available at the en-gNB.

If the Location Information at en-gNB Reporting IE set to "pscell" is included in the SGNB MODIFICATION REQUEST, the SgNB shall start providing information about the current location of the UE. If the Location Information at SgNB IE is included in the SGNB MODIFICATION REQUEST ACKNOWLEDGE, the MeNB shall store the included information so that it may be transferred towards the MME.

If the Lower Layer presence status change IE set to "release lower layers" is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall act as specified in TS 37.340 [32].

If the Lower Layer presence status change IE set to "re-establish lower layers" is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall act as specified in TS 37.340 [32].

If the Lower Layer presence status change IE set to "suspend lower layers" is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall act as specified in TS 37.340 [32].

If the Lower Layer presence status change IE set to "resume lower layers" is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall act as specified in TS 37.340 [32].

If the SGNB MODIFICATION REQUEST message contains the IAB Node Indication IE, the en-gNB shall, if supported, consider that the request is for an IAB node.

For each E-RAB for which the Security Indication IE is included in the E-RABs To Be Added Item IE of the SGNB MODIFICATION REQUEST message:

– if the Integrity Protection Indication IE is set to "required", the en-gNB shall, if supported, perform user plane integrity protection for the concerned E-RAB as specified in TS 33.401 [18], and otherwise it shall reject the addition of the concerned E-RAB with an appropriate cause value.

– if the Integrity Protection Indication IE is set to "preferred", the en-gNB should perform user plane integrity protection for the concerned E-RAB as specified in TS 33.401 [18], and it shall notify the MeNB whether it performed the user plane integrity protection by including the Integrity Protection result IE in the Security Result IE of the SGNB MODIFICATION REQUEST ACKNOWLEDGE message.

– if the Integrity Protection Indication IE is set to "not needed", the en-gNB shall not perform user plane integrity protection for the concerned E-RAB.

For each requested E-RAB configured as MN-terminated split bearer/SCG bearer, if the QoS Mapping Information IE is contained in the GTP Tunnel Endpoint IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, use it to set DSCP and/or flow label fields for the downlink IP packets which are transmitted from MeNB to SgNB through the GTP tunnels indicated by the GTP Tunnel Endpoint IE.

If the PSCell History Information Retrieve IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall, if supported, use this information as specified in TS 37.340 [32].

If the UE History Information from the UE IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall, if supported, store this information.

If the CHO Information SN Modification IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall, if supported, consider that the MeNB initiated SgNB Modification Preparation procedure has been triggered as part of a conditional handover. If the Estimated Arrival Probability IE is contained in the CHO Information SN Modification IE included in the SGNB MODIFICATION REQUEST message, then the en-gNB may use the information to allocate necessary resources for the UE.

If the SCG Activation Request IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB may use it to configure SCG resources as specified in TS 37.340 [32], and shall, if supported, include the SCG Activation Status IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message.

If the Conditional PSCell Change Information Update IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall, if supported, consider that the request provides the list of PSCells prepared at the target en-gNB, as described in TS 37.340 [32].

If the Conditional PSCell Addition Information Modification Request IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB shall, if supported, consider that the request concerns an update of the previous CPAC preparation, as described in TS 37.340 [32]. Accordingly, the en-gNB shall, if supported, include the Conditional PSCell Addition Information Modification Acknowledge IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message.

If the CG-CandidateList is included in the SgNB to MeNB Container IE in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, use it for the purpose of CPAC.

If the Estimated Arrival Probability IE is contained in the Conditional PSCell Information Modification Request IE included in the SGNB MODIFICATION REQUEST message, then the candidate target en-gNB node may use the information to allocate necessary resources for the incoming CPAC procedure.

If the Source DL Forwarding IP Address IE is included in the SGNB MODIFICATION REQUEST message, the en-gNB 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 Source DL Forwarding IP Address IE is included in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall, if supported, store this information and use it as part of its ACL functionality configuration actions to identify source TNL address for data forwarding in case of subsequent handover preparation, if such ACL functionality is deployed.

Interactions with the MeNB initiated SgNB Modification procedure:

If the en-gNB provides for an E-RAB to be setup for which the PDCP entiy is allocated at the MeNB the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE and the LCID IE to the MeNB in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message and the MeNB has not provided the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE in the SGNB MODIFICATION REQUEST message, the MeNB shall trigger the MeNB initiated SgNB Modification procedure to provide the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE and the Duplication Activation IE to the SgNB.

Interactions with the SgNB Reconfiguration Completion procedure:

If the en-gNB admits a modification of the UE context requiring the MeNB to report about the success of the RRC connection reconfiguration procedure, the en-gNB shall start the timer TDCoverall when sending the SGNB MODIFICATION REQUEST ACKNOWLEDGE message to the MeNB except for a request for conditional configuration. The reception of the SGNB RECONFIGURATION COMPLETE message shall stop the timer TDCoverall if TDCoverall is running.

Interaction with the Activity Notification procedure

Upon receiving an SGNB MODIFICATION REQUEST message containing the Desired Activity Notification Level IE, the en-gNB shall, if supported, use this information to decide whether to trigger subsequent SgNB Activity Notification procedures, or stop or modify ongoing triggering of these procedures due to a previous request.

Interaction with the SgNB initiated SgNB Modification Preparation procedure:

If the MeNB receives the SGNB MODIFICATION REQUIRED message and the requested SN modification procedure needs further information from MeNB, the MeNB shall send SGNB MODIFICATION REQUEST message to en-gNB in response to a previously SgNB initiated SgNB Modification procedure.

8.7.6.3 Unsuccessful Operation

Figure 8.7.6.3-1: MeNB initiated SgNB Modification Preparation, unsuccessful operation

If the en-gNB does not admit any modification requested by the MeNB, or a failure occurs during the MeNB initiated SgNB Modfication Preparation, the en-gNB shall send the SGNB MODIFICATION REQUEST REJECT message to the MeNB. The message shall contain the Cause IE with an appropriate value.

If the en-gNB receives a SGNB MODIFICATION REQUEST message containing the MeNB to SgNB Container IE that does not include required information as specified in TS 38.331 [31], the en-gNB shall send the SGNB MODIFICATION REQUEST REJECT message to the MeNB.

8.7.6.4 Abnormal Conditions

If the en-gNB receives a SGNB MODIFICATION REQUEST message containing multiple E-RAB ID IEs (in the E-RABs To Be Added List IE and/or the E-RABs To Be Modified List IE) set to the same value, the en-gNB shall not admit the action requested for the corresponding E-RABs.

If the en-gNB receives an SGNB MODIFICATION REQUEST message containing multiple E-RAB ID IEs (in the E-RAB To Be Released List IE) set to the same value, the en-gNB shall initiate the release of one corresponding E-RAB and ignore the duplication of the instances of the selected corresponding E-RABs.

If the en-gNB receives a SGNB MODIFICATION REQUEST message containing, dependent on the configured bearer type, the Full E-RAB Level QoS Parameters IE or the Requested SCG E-RAB Level QoS Parameters IE which contains a QCI IE indicating a GBR bearer (as defined in TS 23.203 [13]), and which does not contain the GBR QoS Information IE, the en-gNB shall not admit the corresponding E-RAB.

If the supported algorithms for encryption defined in the NR Encryption Algorithms IE in the NR UE Security Capabilities IE in the UE Context Information IE, plus the mandated support of NEA0 in all UEs (TS 33.401 [18]), do not match any algorithms defined in the configured list of allowed encryption algorithms in the en-gNB (TS 33.401 [18]), the en-gNB shall reject the procedure using the SGNB MODIFICATION REQUEST REJECT message.

If the supported algorithms for integrity defined in the NR Integrity Protection Algorithms IE in the NR UE Security Capabilities IE in the UE Context Information IE do not match any algorithms defined in the configured list of allowed integrity protection algorithms in the en-gNB (TS 33.401 [18]), the en-gNB shall reject the procedure using the SGNB MODIFICATION REQUEST REJECT message.

If the timer TDCprep expires before the MeNB has received the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall regard the MeNB initiated SgNB Modification Preparation procedure as being failed and shall release the UE Context at the en-gNB.

If the MeNB has provided the en-gNB for an E-RAB to be setupr which the PDCP entiy is allocated at the MeNB the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE in the SGNB MODIFICATION REQUEST message, and the en-gNB does not provide the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE to the MeNB in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall assume that PDCP duplication was not configured at the en-gNB and releases duplication resources.

If the en-gNB provides for an E-RAB to be setup for which the PDCP entiy is allocated at the MeNB the Secondary SgNB DL GTP Tunnel Endpoint at SCG IE to the MeNB in the SGNB MODIFICATION REQUEST ACKNOWLEDGE message and the MeNB has not provided the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE in the SGNB MODIFICATION REQUEST message, and the MeNB does not trigger the MeNB initiated SgNB Modification procedure to provide the Secondary MeNB UL GTP Tunnel Endpoint at PDCP IE to the SgNB the en-gNB before the SgNB Reconfigurationi Completion procedure was triggered, the en-gNB shall trigger the release of the concerned E-RAB.

Interactions with the SgNB Reconfiguration Completion and SgNB initiated SgNB Release procedure:

If the timer TDCoverall expires before the en-gNB has received the SGNB RECONFIGURATION COMPLETE or the SGNB RELEASE REQUEST message, the en-gNB shall regard the requested modification RRC connection reconfiguration as being not applied by the UE and shall trigger the SgNB initiated SgNB Release procedure.

Interaction with the SgNB initiated SgNB Modification Preparation procedure:

If the MeNB, after having initiated the MeNB initiated SgNB Modification procedure, receives the SGNB MODIFICATION REQUIRED message, the MeNB shall refuse the SgNB initiated SgNB Modification procedure with an appropriate cause value in the Cause IE.

If the MeNB has a Prepared SgNB Modification and receives the SGNB MODIFICATION REQUIRED message, the MeNB shall respond with the SGNB MODIFICATION REFUSE message to the en-gNB with an appropriate cause value in the Cause IE.

Interactions with the MeNB initiated SgNB Release procedure:

If the timer TDCprep expires before the MeNB has received the SGNB MODIFICATION REQUEST ACKNOWLEDGE message, the MeNB shall regard the SgNB Modification Preparation procedure as being failed and may trigger the MeNB initiated SgNB Release procedure.