4.23.5 PDU Session Management procedure

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

4.23.5.1 PDU Session establishment procedure

For non roaming or LBO roaming, it includes the following cases:

– If the service area of the selected SMF includes the current UE location, the UE requested PDU Session Establishment procedure is same as described in clause 4.3.2.2.1.

– If the service area of the selected SMF does not include the current UE location and the UE does not request for a MA PDU Session, the AMF selects an I-SMF that serves the area where UE camps. The UE requested PDU Session Establishment procedure for Home-routed Roaming defined in clause 4.3.2.2.2 is used to establish the PDU Session. Compared to the procedure defined in clause 4.3.2.2.2, the V-SMF and V-UPF are replaced by I-SMF and I-UPF and H-SMF and H-UPF are replaced by SMF and UPF(PSA) respectively. Also, only the S-NSSAI with the value defined by the serving PLMN is sent to the SMF. The I-SMF provides the DNAI list it supports to SMF and the SMF provides the DNAI(s) of interest for this PDU Session to I-SMF based on the DNAI list information received from I-SMF as defined in Figure 4.23.9.1-1 step 1.

This may happen e.g. at PDU Session mobility from non-3GPP access to 3GPP access as defined in clause 4.23.15.

– If the service area of the selected SMF does not include the current UE location and the UE requests a MA PDU Session, then the AMF rejects the MA PDU Session Establishment procedure.

– When the delegated discovery is used, the SCP selects the SMF as described in Annex E.

– If an I-SMF is selected and the PDU Session supports mechanisms for redundant transmission defined in clause 5.33.2.2 of TS 23.501 [2], the SMF rejects the PDU Session Establishment Request.

– If an I-SMF is selected and the PDU Session supports Time Sensitive Communications (as defined in clauses 5.27 and 5.28 of TS 23.501 [2]), or if the PDU session supports redundant transmission defined in clauses 5.33.2.1 or 5.33.2.3 of TS 23.501 [2], the SMF may, based on local policy, reject the PDU Session Establishment Request.

– If the selected SMF cannot serve the target DNAI requested by the PCC rule, the SMF issues Nsmf_PDUSession_SMContextStatusNotify to provide the target DNAI information to the AMF. The procedure of Home-routed Roaming defined in clause 4.3.2.2.2 is used to establish the PDU Session. Compared to this procedure, the V-SMF and V-UPF are replaced by I-SMF and I-UPF respectively and H-SMF and H-UPF are replaced by SMF and UPF(PSA) respectively. The AMF selects an I-SMF that serves this target DNAI in step 2. Then AMF sends a Nsmf_PDUSession_CreateSMContext Request to the I-SMF as defined in step 3a and the target DNAI received from SMF is included in the message.

– The S-NSSAI registered to UDM by SMF is always the S-NSSAI of HPLMN.

For the Home-Routed roaming case, the UE requested PDU Session Establishment procedure for Home-routed Roaming in clause 4.3.2.2.2 can be reused with the following change.

– If the service area of the selected V-SMF does not include the current UE location and the UE requests a MA PDU Session, then the AMF rejects the MA PDU Session Establishment procedure.

4.23.5.2 PDU Session Release procedure

For the non roaming or LBO roaming case, the procedure defined in clause 4.3.4.3 (UE or network requested PDU Session Release for Home-routed Roaming) is used to release the PDU Session, with the V-SMF and V-UPF are replaced by I-SMF and I-UPF and H-SMF and H-UPF are replaced by SMF and UPF(PSA) respectively. Also if the ULCL/BP is included in the data path, in step 1 the I-SMF releases the local UPF (PSA) and includes N4 information in the Nsmf_PDUSession_Release or Nsmf_PDUSession_Update Request request respectively.

For AMF triggers PDU Session Release procedure in Registration procedure (see step 17 of clause 4.2.2.2.2) and Deregistration procedure (see step 2 of clause 4.2.2.3.2), if I-SMF is used for the session, the AMF invokes PDU Session Release operation towards the I-SMF, then I-SMF releases the PDU Session towards SMF.

4.23.5.3 PDU Session modification procedure

For the non-roaming or LBO roaming case, the procedure defined in clause 4.3.3.3 (UE or network requested PDU Session Modification for Home-routed Roaming) is used to modify the PDU Session, with the V-SMF and V-UPF are replaced by I-SMF and I-UPF and H-SMF and H-UPF are replaced by SMF and UPF(PSA) respectively.

If the QoS Monitoring as defined in clause 5.33.3 of TS 23.501 [2] is triggered, the following enhancement to clause 4.3.3.3 applies:

– In step 3, if the SMF determines the need for QoS Monitoring for a QoS flow according to the information received from the PCF in step 1b, or based on SMF local policy, SMF includes QoS Monitoring indication and how frequently QoS Monitoring reporting to be performed, in Nsmf_PDUSession_Update Request message.

– In step 3, if, according to the information received from the PCF in step 1b, or based on SMF local policy, the SMF determines the need for GTP-U Path Monitoring as defined in clause 5.33.3.3 of TS 23.501 [2], the SMF includes QoS monitoring policy in Nsmf_PDUSession_Update Request message.

– In step 4b, if in step 3 the I-SMF received QoS Monitoring indication and may receive how frequently QoS Monitoring reporting to be performed in Nsmf_PDUSession_Update Request the I-SMF includes the QoS Monitoring indication and how frequently QoS Monitoring reporting to be performed in N2 SM message sent to the 5G AN.

– In step 4b, if in step 3 the I-SMF received QoS monitoring policy indication and may receive how frequently QoS Monitoring reporting to be performed in Nsmf_PDUSession_Update Request, the I-SMF includes such QoS monitoring policy also to I-UPF for GTP-U path monitoring.

– In step 15, the I-SMF sends Nsmf_PDUSession_Update Response to SMF. The SMF updates N4 session of the UPF PSA as defined for the case where is no I-SMF.

– If later on the RAN provides over N2 the QoS Monitoring Result with UL packet delay information comprising the packet delays of RAN and N3 interface, the I-SMF forwards this information to the SMF in Nsmf_PDUSession_Update Request message.

– I-SMF shall forward monitoring report (for QoS monitoring per GTP-U path) from I-UPF to SMF.

4.23.5.4 SMF triggered I-SMF selection or removal

Figure 4.23.5.4-1: SMF triggered I-SMF selection or removal

1. The PCF sends to the SMF PCC rule(s) which may include DNAI(s) for the PDU sessions by invoking Npcf_SMPolicyControl_UpdateNotify service operation.

Based on the received DNAI(s), the SMF may subscribe to UE mobility event notification from the AMF (e.g. UE moves into or out of Area of Interest corresponding to the received DNAI(s)).

The SMF may determine the target DNAI(s) which are applicable to the current UE location. Then the SMF may decide a target DNAI finally.

NOTE: It is also possible that the SMF determines that there is no any target DNAI can be used based on PCC rule(s), e.g. when the updated PCC rules removes previously provided DNAI(s).

2. The SMF invokes a Nsmf_PDUSession_SMContextStatusNotify service operation (or Nsmf_PDUSession_StatusNotify) in the following cases:

– if the SMF (or the associated old I-SMF) cannot serve the target DNAI, or

– if an I-SMF is used for the PDU Session and the SMF decides that the DNAI currently served by I-SMF is no longer be used for the PDU Session anymore hence the existing I-SMF is not needed, or

– if an I-SMF is used for the PDU Session and the SMF decides that the SMF itself can serve the target DNAI hence the existing I-SMF is not needed.

The content of the message includes the target DNAI information. The target DNAI information which indicates that the I-SMF selection or removal is expected and may include a target DNAI. This is to trigger the AMF to (re)select a suitable I-SMF, or remove the existing I-SMF (if the AMF decides that the SMF can serve the Target DNAI, or the AMF receive a target DNAI information without including target DNAI) for the PDU Session. The target DNAI is used for selecting (I-)SMF, which controls UPF connecting to that DNAI.

If there is an I-SMF serving the PDU session, the SMF invokes Nsmf_PDUSession_StatusNotify and then the I-SMF invokes Nsmf_PDUSession_SMContextStatusNotify message to send the target DNAI information for existing PDU session to AMF.

3. If the I-SMF selection or removal is expected, the AMF selects a new I-SMF which can serve the target DNAI or remove the existing I-SMF (if the AMF decides that the SMF can serve the Target DNAI, or if the AMF receive a target DNAI information without including target DNAI) for the existing PDU Session as described in clause 5.34.3 of TS 23.501 [2].

4. The AMF sends a Nsmf_PDUSession_CreateSMContext Request to the new I-SMF as described in step 3 of clause 4.23.4.3, or to the SMF as described in step 10 of clause 4.23.4.3, with the following enhancement:

The AMF includes target DNAI received from SMF in the message. When the UE is in CONNECTED state the AMF also include indication of no NG-RAN change.

5. The procedure described in clauses 4.23.4.3 (case: I-SMF insertion or I-SMF change) starting from step 4 takes place with the following difference:

In step 4a, the (target) new I-SMF sends the indication of no NG-RAN change to the old I-SMF or SMF as it is received from AMF.

In step 4b, when the old I-SMF or SMF receives indication of no NG-RAN change it include the additional Downlink Tunnel Info of NG-RAN in the SM context of the PDU Session.

In step 5, the I-SMF selects the I-UPF based on target DNAI.

In step 6, the target I-SMF should reuse the downlink Tunnel Info of serving RAN node if received from old I-SMF/SMF as described in clause 4.23.4.3.

In step 9, if the new I-SMF receives the Downlink Tunnel Info of NG-RAN, the N2 SM information includes PDU Session Resource Modification message.

The procedure described in clause 4.23.4.3 (case: I-SMF removal) starting from step 11 takes place with the following difference:

In step 11a, the SMF sends an indication of no NG-RAN change to the old I-SMF as it received from AMF.

In step 11b, when the old I-SMF receives indication of no NG-RAN change it include the additional Downlink Tunnel Info of NG-RAN in the SM context of the PDU Session.

In step 12, the SMF selects a new I-UPF based on target DNAI.

In step 16, if the SMF receives the Downlink Tunnel Info of NG-RAN, the N2 SM information includes PDU Session Resource Modification message.

If the UE is in IDLE state the step 17-21 are skipped. Steps 17a and 17b are still performed to release the old I-SMF.

6. [Conditional] In the case that I-SMF insertion or I-SMF change is performed in step 5, the PSA and UL CL/BP controlled by I-SMF is inserted as described from steps 2 to 11 in figure 4.23.9.1-1 with the following difference:

In step 2, the I-SMF selects a new PDU Session Anchor (PSA2) based on the target DNAI received in step 4.

For the case of I-SMF removal, the PSA and UL CL/BP controlled by SMF is inserted as described from steps 2 to 8 in Figure 4.3.5.4-1 with the following difference:

In step 2, the SMF selects a new PDU Session Anchor (PSA2) based on the target DNAI if received in step 4.