4.23.7 Inter NG-RAN node N2 based handover

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.23.7.1 General

The following two scenarios are considered:

– The I-SMF is available for the PDU Session and I-SMF is not changed or removed during the inter NG-RAN node N2 based handover procedure. The procedure to support this scenario is described in clause 4.23.7.2.

– The I-SMF is inserted, changed or removed during inter NG-RAN node N2 based handover procedure. The procedure to support this scenario is described in clause 4.23.7.3.

4.23.7.2 Inter NG-RAN node N2 based handover without I-SMF change/removal

4.23.7.2.1 General

When both I-SMF and SMF are available for a PDU Session and no I-SMF change or removal is needed during inter NG-RAN node N2 handover procedure, the procedure defined in clause 4.9.1.3.2, 4.9.1.3.3 are used with the following differences.

4.23.7.2.2 Preparation phase

Compared to the procedure in clause 4.9.1.3.2, the SMF interacting with the S-UPF, T-UPF, S-AMF and T-AMF is the I-SMF. The difference is following:

– Step 3: The N14 context exchanged between S-AMF and T-AMF contains the SM Context ID at I-SMF, or SM Context ID at SMF if I-SMF is not applicable before.

– Step 4: The T-AMF determines whether Target I-SMF needs to be selected based on UE location and service area of the SMF. In this case no I-SMF change or removal is needed.

– Step 5: The I-SMF checks whether I-UPF needs to be reallocated, i.e. select a T-UPF.

4.23.7.2.3 Execution phase

Compared to the procedure for execution phase in 4.9.1.3.3, the SMF interacting with the S-UPF, T-UPF, S-AMF and T-AMF is the I-SMF. The difference is as following:

– Step 10a: If I-SMF is available for a PDU Session and the existing intermediate S-UPF is re-allocated, i.e. a new T-UPF is selected, the I-SMF invokes an Nsmf_PDUSession_Update Request (DL CN Tunnel Info of the T-UPF) service operation toward the SMF. The SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, providing DL CN Tunnel Info to the PDU Session Anchor UPF. The PDU Session Anchor sends one or more "end marker" packets for each N3/N9 tunnel on the old path immediately after switching the path, the source NG-RAN shall forward the "end marker" packets to the target NG-RAN.

– Step 10b: The SMF responds with the Nsmf_PDUSession_Update Response service operation to I-SMF once the PDU Session Anchor UPF is updated with the DL Tunnel Info of the T-UPF.

4.23.7.2.4 Handover Cancel

The home routed roaming procedure defined in clause 4.9.1.4 is applied, with V-SMF replaced by I-SMF.

4.23.7.3 Inter NG-RAN node N2 based handover with I-SMF insertion/change/removal

4.23.7.3.1 General

When I-SMF is inserted or changed or removed during inter NG-RAN node N2 handover, the procedures defined in this clause are used.

To support the EAS session continuity upon UL CL relocation, a N9 forwarding tunnel to support the EAS session continuity is established and released between the Source UL CL and Target UL CL as described in clause 4.23.9.4 or clause 4.23.9.5.

NOTE: This allows the UE to go on exchanging with the source EAS despite the fact that a new UL CL has been allocated to the PDU Session.

4.23.7.3.2 Preparation phase

Figure 4.23.7.3.2-1: Inter NG-RAN node N2 based handover, preparation phase, with I-SMF insertion/change/removal

1. Steps 1-3 in clause 4.9.1.3.2 are performed.

2. For PDU sessions in the UE context, the Target AMF determines whether a (new) Target I-SMF needs to be selected based on Target UE location and service area of the SMF or of the old I-SMF. If Target I-SMF needs to be selected, the AMF selects a Target I-SMF as described in clause 5.34.3 of TS 23.501 [2]. If the UE moves from the service area of the I-SMF to the service area of the SMF, the I-SMF will be removed.

The rest of steps are performed for PDU sessions requested to be handed over, i.e. the PDU Sessions with active UP connections.

Case: I-SMF insertion, or I-SMF change, step 3~8 are skipped for I-SMF removal case.

3. T-AMF to Target I-SMF: Nsmf_PDUSession_CreateSMContext (PDU Session ID, Target ID, T-AMF ID, SM Context ID).

The SM Context ID points to the source I-SMF in the case of I-SMF change or to SMF in the case of I-SMF insertion.

Case: I-SMF change, steps 4 are skipped for I-SMF insertion case.

4a. (I-SMF change case) Target I-SMF to Source I-SMF: Target I-SMF retrieves SM Context from the source I-SMF by invoking Nsmf_PDUSession_Context Request (SM context type, SM Context ID).

The Target I-SMF uses SM Context ID received from T-AMF for this service operation. SM context type indicates that the requested information is all SM context, i.e. PDN Connection Context and 5G SM context. The SM Context ID is used by the recipient of Nsmf_PDUSession_Context Request in order to determine the targeted PDU Session.

4b. Source I-SMF to Target I-SMF: Nsmf_PDUSession_Context Response. The source I-SMF responds with the requested SM context.

Case: I-SMF insertion, steps 5 are skipped for I-SMF change case.

5a. Target I-SMF to SMF: Target I-SMF retrieves SM Context from the SMF by invoking Nsmf_PDUSession_Context Request (SM context type, SM Context ID).

5b. Void.

5c. Void.

5d. SMF to Target I-SMF: Nsmf_PDUSession_Context Response.The SMF responds with the requested SM context.

6. The Target I-SMF selects a Target I-UPF: Based on the received SM context, e.g. S-NSSAI and UE location information, the Target I-SMF selects a Target I-UPF as described in clause 6.3.3 of TS 23.501 [2].

7a. The Target I-SMF to Target I-UPF: N4 Session Establishment Request.

An N4 Session Establishment Request message is sent to the Target I-UPF, providing Packet detection, enforcement and reporting rules to be installed on the Target I-UPF. The UL CN Tunnel Info (on N9) of UPF (PSA) for this PDU Session, which is used to setup N9 tunnel, is also provided to the Target I-UPF.

7b. Target I-UPF to Target I-SMF or SMF: N4 Session Establishment Response.

The Target I-UPF sends an N4 Session Establishment Response message to the Target I-SMF with DL CN Tunnel Info (i.e. N9 tunnel info) and UL CN Tunnel Info (i.e. N3 tunnel info).

Case: I-SMF insertion, step 7c~7f are skipped for I-SMF change case.

7c. Target I-SMF to SMF: Nsmf_PDUSession_Create Request (PDU Session ID, HO Preparation Indication).

7d. [Conditional] SMF to UPF (PSA): N4 Session Modification Request.

If different CN Tunnel Info need be used by PSA UPF, i.e. the CN Tunnel Info for N3 and N9 are different, the SMF request CN tunnel information from UPF.

7e. [Conditional] UPF(PSA) to SMF: N4 Session Modification Response.

The UPF (PSA) sends an N4 Session Modification Response message to the SMF with CN Tunnel Info (on N9).

7f. SMF to Target I-SMF: Nsmf_PDUSession_Create Response (PDU Session ID, CN Tunnel Info of UPF(PSA) for N9).

The Target I-SMF provides the CN Tunnel Info of UPF(PSA) for N9 to Target I-UPF via N4 Session Modification.

8. The Target I-SMF to T-AMF: Nsmf_PDUSession_CreateSMContext Response (PDU Session ID, N2 SM Information, Reason for non-acceptance).

If N2 handover for the PDU Session is accepted, the Target I-SMF includes in the Nsmf_PDUSession_CreateSMContext Response the N2 SM Information containing the N3 UP address and the UL CN Tunnel ID of the UPF and the QoS parameters.

Case: I-SMF removal, step 9~13 are skipped for I-SMF insertion, or I-SMF change case.

9. T-AMF to SMF: Nsmf_PDUSession_CreateSMContext (PDU Session ID, Target ID, T-AMF ID, SM Context ID). The SM Context ID points to the source I-SMF.

10. The SMF selects a Target I-UPF if the UE is not in the service area of the PDU Session Anchor UPF. The SMF selects a Target I-UPF as described in clause 6.3.3 of TS 23.501 [2].

11a. [Conditional] SMF to UPF(PSA): N4 Session Modification Request.

If the Target I-UPF was not selected (i.e. the service area of PSA covers UE location) and different CN Tunnel Info (on N3) need to be used by PSA, the SMF sends N4 Session Modification Request to UPF(PSA).

11b. [Conditional] UPF(PSA) to SMF: N4 Session Modification Response. The PSA UPF sends UL CN Tunnel Info (i.e. N3 tunnel info) to SMF.

12a: [Conditional] SMF to Target I-UPF: N4 Session Establishment Request.

If a Target I-UPF is selected by SMF in step 10, the SMF sends N4 Session Establishment Request to Target I-UPF.

12b. [Conditional]Target I-UPF to SMF: N4 Session Establishment Response. The Target I-UPF sends an N4 Session Establishment Response message to the SMF with DL CN Tunnel Info (i.e. N9 tunnel info) and UL CN Tunnel Info (i.e. N3 tunnel info).

13. SMF to T-AMF: Nsmf_PDUSession_CreateSMContext Response (PDU Session ID, N2 SM Information, Reason for non-acceptance).

14. Same as step 8-10 clause 4.9.1.3.2 are performed.

Case: I-SMF insertion, or I-SMF change, step 15~23 are skipped for I-SMF removal case.

15. T-AMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, N2 SM response received from T-RAN).

The Target I-SMF stores the N3 tunnel info of T-RAN from the N2 SM response if N2 handover is accepted by T-RAN.

16a. [Conditional]Target I-SMF to Target I-UPF: N4 Session modification request (T-RAN SM N3 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding).

Indirect forwarding may be performed via a UPF which is different from the Target I-UPF, in which case the Target I-SMF selects another UPF for indirect forwarding.

16b. [Conditional]Target I-UPF to Target I-SMF: N4 Session Modification Response (Target I-UPF N9 forwarding Information list).

The Target I-UPF allocates Tunnel Info and returns an N4 Session Modification Response message to the Target I-SMF.

The Target I-UPF SM N9 forwarding info list includes Target I-UPF N9 address, Target I-UPF N9 Tunnel identifiers for forwarding data.

Case: I-SMF change, step 17~19 are skipped for I-SMF insertion case.

17. [Conditional]Target I-SMF to Source I-SMF: Nsmf_PDUSession_UpdateSMContext Request.

Target I-SMF invokes Nsmf_PDUSession_UpdateSMContext Request (Target I-UPF SM N9 forwarding Information list, Operation type) to the source I-SMF in order to establish the indirect forwarding tunnel. The Target I-SMF uses the SM Context ID received from Target AMF for this service operation. The Operation type indicates the establishment of forwarding tunnel(s) for indirect forwarding.

18a. [Conditional]The source I-SMF initiates a N4 session modification request (Target I-UPF SM N9 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding) to the source I-UPF to establish indirect forwarding tunnel.

Indirect forwarding may be performed via a UPF which is different from the Source I-UPF.

18b. [Conditional]The source I-UPF to source I-SMF: N4 Session Modification Response (source I-UPF SM N3 forwarding Information list).

19. [Conditional]Source I-SMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext response (Source I-UPF SM N3 forwarding Information list).

Case: I-SMF insertion, step 20~22 are skipped for I-SMF change case.

20. [Conditional]Target I-SMF to SMF: Nsmf_PDU Session_UpdateSMContext.

The Target I-SMF invokes Nsmf_PDUSession_UpdateSMContext Request (Target I-UPF SM N9 forwarding Information list, Operation type) to the SMF in order to establish the indirect forwarding tunnel. The Target I-SMF uses the SM Context ID received from Target AMF for this service operation. The Operation type indicates the establishment of forwarding tunnel(s) for indirect forwarding.

21a. [Conditional]The SMF initiates a N4 session modification request (UPF SM N9 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding) to the UPF(PSA) to establish indirect forwarding tunnel.

Indirect forwarding may be performed via a UPF which is different from the UPF(PSA).

21b [Conditional] The UPF(PSA) to SMF: N4 Session Modification Response (UPF SM N3 forwarding Information list).

22. [Conditional] The SMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext response (UPF SM N3 forwarding Information list).

23. Target I-SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (N2 SM Information).

Target I-SMF creates an N2 SM information containing the DL forwarding Tunnel Info to be sent to the S-RAN by Source AMF via the Target AMF. Target I-SMF includes this information in the Nsmf_PDUSession_UpdateSMContext response. The DL forwarding Tunnel Info can be one of the following information:

– If direct forwarding applies, then Target I-SMF includes the T-RAN N3 forwarding information received in step 15.

– If the indirect forwarding tunnel is setup, then the SMF includes Source I-UPF forwarding information containing the N3 UP address and the Tunnel ID of the Source I-UPF.

Case: I-SMF removal, step 24~30 are skipped for I-SMF insertion, or I-SMF change case.

24. T-AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, N2 SM response received from T-RAN).

The SMF stores the N3 tunnel info of T-RAN from the N2 SM response if N2 handover is accepted by T-RAN.

25a. [Conditional] SMF to UPF (PSA): N4 Session modification Request.

If the Target I-UPF is not selected (i.e. the service area of PSA covers UE location), the SMF sends N4 Session modification request to UPF(PSA) to allocate DL forwarding tunnel(s).

Indirect forwarding may be performed via a UPF which is different from the UPF(PSA), in which case the SMF selects another UPF for indirect forwarding.

25b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response (UPF N9 forwarding Information list).

26a. [Conditional] SMF to Target I-UPF:

If the Target I-UPF is selected, the SMF sends N4 Session modification request to Target I-UPF to allocate DL forwarding tunnel(s) for indirect forwarding;

Indirect forwarding may be performed via a UPF which is different from the Target I-UPF, in which case the SMF selects another UPF for indirect forwarding.

26b. [Conditional] Target I-UPF to SMF: N4 Session Modification Response (Target I-UPF N9 forwarding Information list).

27. [Conditional] SMF to Source I-SMF: Nsmf_PDUSession_UpdateSMContext.

The SMF invokes Nsmf_PDUSession_UpdateSMContext Request (SM N9 forwarding Information list, Operation type) to the source I-SMF in order to establish the indirect forwarding tunnel. The SMF uses the SM Context ID received from T-AMF for this service operation. The Operation type indicates the establishment of forwarding tunnel(s) for indirect forwarding.

28a. [Conditional] Source I-SMF to Source I-UPF: N4 Session Modification Request.

The source I-SMF initiates a N4 session modification request (Target I-UPF SM N9 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding) to the source I-UPF to establish indirect forwarding tunnel.

Indirect forwarding may be performed via a UPF which is different from the Source I-UPF.

28b. [Conditional]The source I-UPF to source I-SMF: N4 Session Modification Response (source I-UPF SM N3 forwarding Information list).

29. [Conditional]The source I-SMF to SMF: Nsmf_PDUSession_UpdateSMContext response (Source I-UPF SM N3 forwarding Information list).

30. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (N2 SM Information).

The SMF creates an N2 SM information containing the DL forwarding Tunnel Info to be sent to the S-RAN by the Source AMF via the Target AMF. The DL forwarding Tunnel Info can be one of the following information:

– If direct forwarding applies, then the SMF includes the T-RAN N3 forwarding information the SMF received in step 24.

– If the indirect forwarding tunnel is setup, then the SMF includes Source I-UPF forwarding information containing the N3 UP address and the Tunnel ID of the Source I-UPF.

31. Same as step 12 in clause 4.9.1.3.2 is performed.

4.23.7.3.3 Execution phase

Figure 4.23.7.3.3-1: Inter NG-RAN node N2 based handover, execution phase, with I-SMF insertion/change/removal

1. Steps 1-6 in clause 4.9.1.3.3 are performed with the following change:

Step 6a: For PDU sessions in the UE context, if the I-SMF is either to be changed, or to be removed, the T-AMF includes an indication in Namf_Communication_N2InfoNotify to indicate the I-SMF change/removal.

Step 6c: The SMF in this step is source I-SMF in the case of I-SMF removal or change, or is SMF in the case of I-SMF insertion.

Case: I-SMF insertion, or I-SMF change, step 2~9 are skipped for I-SMF removal case.

2. T-AMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Request (Handover Complete indication, (N2 SM Information (Secondary RAT usage data))).

Handover Complete indication is sent per each PDU Session to the corresponding Target I-SMF to indicate the success of the N2 Handover.

If in step 6b of clause 4.9.1.3.3 the source AMF has provided information for secondary RAT usage reporting the T-AMF propagates this information to the Target I-SMF.

Case: I-SMF change, step 3 is skipped for I-SMF insertion.

3a. S-AMF to Source I-SMF: Nsmf_PDUSession_ReleaseSMContext Request (I-SMF only indication).

After received N2 handover notify from T-AMF, if indication of I-SMF change/removal has been received, the S-AMF invokes Nsmf_PDUSession_ReleaseSMContext Request to inform the Source I-SMF to release the SM context of the PDU Session. The I-SMF only indication is used to inform the Source I-SMF not to invoke resource release in SMF. The Source I-SMF initiates a timer to release the SM Context of the PDU Session if indirect forwarding tunnel(s) were previously established, or if the Source I-SMF has not received request from Target I-SMF to retrieve SM Context. Otherwise, the Source I-SMF immediately releases the SM Context.

3b. Source I-SMF to S-AMF: Nsmf_PDUSession_ReleaseSMContext Response.

4a. Void.

4b. Void.

5a. Target I-SMF to Target I-UPF: N4 Session Modification Request. The N4 Modification Request indicates DL AN Tunnel Info of T-RAN to UPF.

5b. The Target I-UPF to Target I-SMF: N4 Session Modification Response.

6. Target I-SMF to SMF:

In the case of I-SMF change, Nsmf_PDUSession_Update Request (PDU Session ID, DL CN Tunnel Info of Target I-UPF for N9, DNAI(s) supported by the I-SMF, Secondary RAT usage data).

In the case of I-SMF insertion, Nsmf_PDUSession_Update Request (PDU Session ID, DL CN Tunnel Info of Target I-UPF for N9, DNAI(s), Secondary RAT usage data, Handover Complete Indication). The SMF initiates a timer to release the resource, i.e. resource for indirect data forwarding tunnel.

If the T-AMF has provided information for secondary RAT usage reporting in step 2, the Target I-SMF propagates this information to the SMF.

7a. SMF to UPF (PSA): N4 Session Modification Request.

The SMF sends N4 Session Modification Request to UPF PSA, providing the DL CN Tunnel Info of Target I-UPF to the UPF PSA.

7b. UPF (PSA) to SMF: N4 Session Modification Response.

8. SMF to Target I-SMF: In the case of I-SMF change, Nsmf_PDUSession_Update Response. In the case of I-SMF insertion, Nsmf_PDUSession_Create Response. The SMF provides the DNAI(s) of interest for this PDU Session to Target I-SMF.

In the case of I-SMF insertion and the PDU session corresponds to a LADN, the SMF shall release the PDU session after the handover procedure is completed.

9. Target I-SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response.

If indirect data forwarding applies, the Target I-SMF starts an indirect data forwarding timer, to be used to release the resource of indirect data forwarding tunnel.

Case: I-SMF removal, step 10~14 are skipped for I-SMF insertion, or I-SMF change case.

10. T-AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (Handover Complete indication, (N2 SM Information (Secondary RAT usage data))).

Handover Complete indication is sent per each PDU Session to the corresponding SMF to indicate the success of the N2 Handover.

If in step 6b of clause 4.9.1.3.3 the source AMF has provided information for secondary RAT usage reporting the T-AMF propagates this information to the SMF.

11a. S-AMF to Source I-SMF: Nsmf_PDUSession_ReleaseSMContext Request I-SMF only indication.

After received N2 handover notify from T-AMF, if indication of I-SMF change/removal has been received, the S-AMF invokes Nsmf_PDUSession_ReleaseSMContext Request to inform the Source I-SMF to release the SM context of the PDU Session. I-SMF only indication is used to inform the Source I-SMF not to invoke resource release in SMF. The Source I-SMF initiates a timer to release the SM Context of the PDU Session if indirect forwarding tunnel(s) were previously established, otherwise, the Source I-SMF immediately releases the SM Context.

11b. Source I-SMF to S-AMF: Nsmf_PDUSession_ReleaseSMContext Response.

12a. [Conditional]SMF to Target I-UPF: N4 Session Modification Request.

If the Target I-UPF is selected by SMF, the SMF to Target I-UPF: N4 Session Modification Request. The N4 Modification Request indicates DL AN Tunnel Info of T-RAN to Target I-UPF.

12b. [Conditional] Target I-UPF to SMF: N4 Session Modification Response

13a. SMF to UPF (PSA): N4 Session Modification Request.

The SMF sends N4 Session Modification Request to UPF(PSA). The N4 Modification Request indicates DL AN Tunnel Info of T-RAN to UPF(PSA) if Target I-UPF is not selected by SMF. The N4 Modification Request indicates DL CN Tunnel Info of Target I-UPF if Target I-UPF is selected by SMF.

13b. UPF (PSA) to SMF: N4 Session Modification Response. PDU Session Anchor sends one or more "end marker" packets for each N3/N9 tunnel on the old path immediately after switching the path, the source NG-RAN shall forward the "end marker" packets to the target NG-RAN.

14. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (PDU Session ID).

If indirect data forwarding applies, the SMF starts an indirect data forwarding timer, to be used to release the resource of indirect data forwarding tunnel.

15. Steps 12, 14 in clause 4.9.1.3.3 are performed.

During the UE mobility registration procedure, if required, the T-AMF performs I-SMF insertion/change/removal for the PDU session which were not handed over, i.e. the PDU sessions without active UP connections. This takes place as described in clause 4.23.3 with the exception that there is no UE context retrieved from the old AMF and that steps 17a and 17b as described in clause 4.23.4.3 are not applicable.

Case: I-SMF insertion, or I-SMF change, step 16~18 are skipped for I-SMF removal case.

16a. [Conditional]Target I-SMF to Target I-UPF: N4 Session Modification Request.

After indirect data forwarding timer set in step 9 expires, the Target I-SMF sends an N4 Session Modification Request to Target I-UPF to release the indirect data forwarding resource in Target I-UPF.

16b. [Conditional]Target I-UPF to SMF: N4 Session Modification Response.

Case: I-SMF change, step 17 is skipped for I-SMF insertion.

17a. Source I-SMF to Source I-UPF: N4 Session Release Request.

Upon the timer set in step 3 expires, the Source I-SMF sends N4 Session Release Request (Release Cause) to Source I-UPF to release the resources for the PDU Session. This message is also used to release the indirect data forwarding resource in Source I-UPF.

If the Source I-UPF acts as UL CL and is not co-located with local PSA, the Source I-SMF also sends N4 Session Release Request to the local PSA to release the resources for the PDU Session.

17b. Source I-UPF to Source I-SMF: N4 Session Release Response.

The Source I-SMF releases SM Context of the PDU Session.

Case: I-SMF insertion, step 18 is skipped for I-SMF change.

18a. SMF to UPF: N4 Session Modification Request.

Upon the timer set in step 6 expires, if UPF(PSA) is used for indirect forwarding, the SMF sends an N4 Session Modification Request to UPF(PSA) to release the indirect data forwarding resource in UPF(PSA). If the UPF (PSA) uses different Tunnel Info for N3 and N9, this message is also used to release the N3 Tunnel. If I-UPF is used for indirect forwarding, the SMF sends an N4 Session Modification Request to the I-UPF to release the indirect data forwarding resource.

18b. UPF to SMF: N4 Session Modification Response.

If UPF(PSA) is used for indirect forwarding, the UPF (PSA) sends N4 Session Modification Response to SMF.

If I-UPF is used for indirect forwarding, the I-UPF sends N4 Session Modification Response to SMF.

Case: I-SMF removal, step 19~20 are skipped for I-SMF insertion, I-SMF change case.

19a. The Source I-SMF to Source I-UPF: N4 Session Release Request.

Upon the timer set in step 11 expires, the Source I-SMF sends N4 Session Release Request (Release Cause) to Source I-UPF to release the resources for the PDU Session. This message is also used to release the indirect data forwarding resource in Source I-UPF.

19b. Source I-UPF to Source I-SMF: N4 Session Release Response.

The Source I-SMF releases SM Context of the PDU Session.

20a. SMF to UPF: N4 Session Modification Request.

Upon the timer set in step 14 expires, if UPF(PSA) is used for indirect forwarding, the SMF sends an N4 Session Modification Request to UPF (PSA) to release the indirect forwarding resource in UPF (PSA). If the UPF (PSA) uses different Tunnel Info for N3 and N9, this message is also used to release the N3 Tunnel. If I-UPF is used for indirect forwarding, the SMF sends an N4 Session Modification Request to the I-UPF to release the indirect data forwarding resource.

20b. UPF to SMF: N4 Session Modification Response.

If UPF(PSA) is used for indirect forwarding, the UPF (PSA) sends N4 Session Modification Response to SMF.

If I-UPF is used for indirect forwarding, the I-UPF sends N4 Session Modification Response to SMF.

4.23.7.3.4 Handover Cancel

Figure 4.23.7.3.4-1: Handover Cancel procedure

1. Step 1~3 in clause 4.11.1.2.3 are performed.

Case: I-SMF Change, step 4~9 are skipped for I-SMF Insertion case and I-SMF Removal case.

4. T-AMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication).

The target AMF invokes Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication) to the target I-SMF to release the SM Context and all resources allocated on the target I-SMF during preparation phase.

5a. Target I-SMF to Target I-UPF: N4 Session Release Request.

The target I-SMF invokes N4 Session Release Request to target I-UPF, to release all resources allocated for the N4 session.

5b. Target I-UPF to Target I-SMF: N4 Session Release Response.

6. [Conditional] Target I-SMF to Source I-SMF: Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication).

If indirect forwarding tunnel is setup during preparation phase, the target I-SMF initiates Nsmf_PDUSession_UpdateSMContext Request to the source I-SMF, indicating the source I-SMF to delete the resources temporarily allocated for indirect forwarding tunnel.

7a. [Conditional] Source I-SMF to Source I-UPF: N4 Session Modification Request.

The source I-SMF invokes N4 Session Modification Request to source I-UPF, to delete all resources allocated for indirect forwarding tunnel.

7b. [Conditional] Source I-UPF to Source I-SMF: N4 Session Modification Response.

8. [Conditional] Source I-SMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Response.

9. Target I-SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response.

Case: I-SMF Insertion, step 10~18 are skipped for I-SMF Change case and I-SMF Removal case.

10. T-AMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication).

11a. Target I-SMF to Target I-UPF: N4 Session Release Request.

The target I-SMF invokes N4 Session Release Request to target I-UPF, to release all resources allocated for the N4 session.

11b. Target I-UPF to Target I-SMF: N4 Session Release Response.

12. Target I-SMF to SMF: Nsmf_PDUSession_Update Request (PDU Session ID, Relocation Cancel Indication).

The target I-SMF invokes Nsmf_PDUSession_Update Request (PDU Session ID, Relocation Cancel Indication) to the SMF, to release the PDU Session resources allocated during preparation phase, e.g. CN Tunnel for N9.

13a. [Conditional] SMF to UPF (PSA): N4 Session Modification Request.

If CN Tunnel for N9 is allocated during preparation phase, i.e. the CN Tunnel for N3 and N9 are different, the SMF asks UPF (PSA) to release the CN Tunnel on N9.

13b. [Conditional] UPF(PSA) to SMF: N4 Session Modification Response.

14. SMF to Target I-SMF: Nsmf_PDUSession_Update Response.

15. [Conditional] Target I-SMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (Operation Type).

If indirect forwarding tunnel is setup during preparation phase, the target I-SMF initiates Nsmf_PDUSession_UpdateSMContext Request (Operation Type) to the SMF, indicating the SMF to delete the resources temporarily allocated for indirect forwarding tunnel.

16a. [Conditional] SMF to UPF (PSA): N4 Session Modification Request.

The SMF invokes N4 Session Modification Request to UPF (PSA), to delete all resources allocated for indirect forwarding tunnel.

16b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response.

17. [Conditional] SMF to Target I-SMF: Nsmf_PDUSession_UpdateSMContext Response.

18. Target I-SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response.

Case: I-SMF Removal, step 19~24 are skipped for I-SMF Insertion case and I-SMF Change case.

19. T-AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication).

The target AMF invokes Nsmf_PDUSession_UpdateSMContext Request (Relocation Cancel Indication) to the SMF to release the SM Context and all resources allocated on the SMF during preparation phase.

20a. [Conditional] SMF to UPF (PSA): N4 Session Modification Request.

If CN Tunnel for N3 is allocated during preparation phase, i.e. the CN Tunnel for N3 and N9 are different, the SMF asks UPF (PSA) to release the CN Tunnel on N3.

20b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response.

21. [Conditional] SMF to Source I-SMF: Nsmf_PDUSession_UpdateSMContext Request (Operation Type).

If indirect forwarding tunnel is setup during preparation phase, the SMF initiates Nsmf_PDUSession_UpdateSMContext Request (Operation Type) to the source I-SMF, indicating the source I-SMF to delete the resources temporarily allocated for indirect forwarding tunnel.

22a. [Conditional] Source I-SMF to Source I-UPF: N4 Session Modification Request.

The source I-SMF invokes N4 Session Modification Request to source I-UPF, to delete all resources allocated for indirect forwarding tunnel.

22. [Conditional] Source I-UPF to SMF: N4 Session Modification Response.

23. [Conditional] Source I-SMF to SMF: Nsmf_PDUSession_UpdateSMContext Response.

24. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response.