PDU Session Modification Call Flow
This section describes the PDU session modification procedure as specified in 3GPP TS 23.502, section 4.3.3.2.

Step | Description |
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1a |
The UE initiates the UE Requested PDU Session Modification procedure by transmitting a NAS message containing a PDU Session Modification Request within the N1 SM container. The PDU Session Modification Request includes a PDU session ID, Packet Filters, Operation, Requested QoS, Segregation, and 5GSM Core Network Capability. |
1b |
(SMF-requested modification) The PCF performs a PCF-initiated SM Policy Association Modification procedure to notify the SMF about the modification of policies. The policy decision or upon AF requests, for example, Application Function influence on traffic routing, triggers this procedure. |
1c |
(SMF-requested modification) The UDM updates the subscription data of SMF by Nudm_SDM_Notification (SUPI, Session Management Subscription Data). The SMF updates the Session Management Subscription Data and acknowledges the UDM by returning an Ack with (SUPI). |
1d |
(SMF-requested modification) The SMF decides to modify PDU session. This procedure is also triggered based on locally configured policy or triggered from the (R)AN. If the SMF receives one of the triggers in step 1b to 1d, the SMF starts SMF-requested PDU Session Modification procedure. |
1e |
(AN-initiated modification) (R)AN indicates to the SMF when the AN resources onto which a QoS Flow is mapped are released irrespective of whether notification control is configured. (R)AN sends the N2 message (PDU Session ID, N2 SM information) to the AMF. The N2 SM information in the smf_PDU_Session_UpdateContext includes the following information:
The SMF supports AN-initiated modification to release the QFI from RAN. For details on this support, see the following section. |
2 |
The SMF reports the subscribed event to the PCF by performing an SMF-initiated SM Policy Association Modification procedure. The SMF skips this step if the PDU Session Modification procedure is triggered by step 1b or 1d. If the dynamic PCC is not deployed, the SMF may apply local policy to decide whether to change the QoS profile. The SMF does not invoke the steps 3 to 7 when the PDU Session Modification requires only action at a UPF (for example, gating). |
3a |
For UE or AN-initiated modification, the SMF responds to the AMF through Nsmf_PDUSession_UpdateSMContext including N2 SM information and N1 SM container. The N2 SM information carries information that the AMF provides to the (R)AN. It includes the QoS profiles and the corresponding QFIs to notify the (R)AN that one or more QoS flows were added, or modified. It includes only QFI(s) to notify the (R)AN that one or more QoS flows were removed. The N1 SM container carries the PDU Session Modification Command that the AMF provides to the UE. It includes the QoS rule(s), QoS rule operation, QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s), and Session-AMBR. |
3b |
For SMF-requested modification, the SMF invokes Namf_Communication_N1N2MessageTransfer including N2 SM information and N1 SM container. If the UE is in CM-IDLE state and an Asynchronous type communication (ATC) is activated, the AMF updates and stores the UE context based on the Namf_Communication_N1N2MessageTransfer, and skips the steps 4, 5, 6 and 7. When the UE is reachable, that is, when the UE enters CM-CONNECTED state, the AMF forwards the N1 message to synchronize the UE context with the UE. |
4 |
The AMF sends N2 PDU Session Request (N2 SM information received from the SMF, NAS message (PDU Session ID, N1 SM container (PDU Session Modification Command))) Message to the (R)AN. |
5 |
The (R)AN issues AN-specific signalling exchange with the UE that is related with the information received from the SMF. For example, in an NG-RAN, an RRC Connection Reconfiguration takes place with the UE modifying the necessary (R)AN resources related to the PDU session. |
6 |
The (R)AN acknowledges N2 PDU Session Request by sending a N2 PDU Session Ack Message to the AMF. |
7 |
The AMF forwards the N2 SM information and the User location Information from the AN to the SMF via Nsmf_PDUSession_UpdateSMContext service operation. The SMF sends Nsmf_PDUSession_UpdateSMContext Response. If the (R)AN rejects QFI(s), the SMF updates the QoS rules and QoS Flow level QoS parameters if needed for the QoS Flow(s) associated with the QoS rule(s) in the UE accordingly. |
8 |
The SMF updates N4 session of the UPF(s) that are involved by the PDU Session Modification by sending N4 Session Modification Request message to the UPF. |
9 |
The UE acknowledges the PDU Session Modification Command by sending a NAS message (PDU Session ID, N1 SM container (PDU Session Modification Command Ack)). |
10 |
The (R)AN forwards the NAS message to the AMF. |
11 |
The AMF forwards the N1 SM container (PDU Session Modification Command Ack) and User Location Information from the AN to the SMF through Nsmf_PDUSession_UpdateSMContext service operation. The SMF sends Nsmf_PDUSession_UpdateSMContext Response. |
12 |
The SMF updates N4 session of the UPF(s) that are involved by the PDU Session Modification by sending N4 Session Modification Request (N4 Session ID) message to the UPF. For a PDU Session of Ethernet PDU Session Type, the SMF notifies the UPF to add or remove Ethernet Packet Filter Set(s) and forwarding rule(s). |
13 |
If the SMF interacts with the PCF in step 1b or 2, the SMF notifies the PCF whether the PCC decision is enforced or not by performing an SMF-initiated SM Policy Association Modification procedure. The SMF notifies any entity that has subscribed to User Location Information related with PDU Session change. If the step 1b is triggered to perform Application Function influence on traffic routing, the SMF reconfigures the User Plane of the PDU session. |
Releasing QFI During AN-initiated Modification Procedure
For the SMF to support AN-initiated modification to release the QFIs, perform the following steps:
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If the EPS Interworking Indication is enabled for a given PDU session, the SMF initiates the EBI release towards the AMF.
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The SMF sends N4 Modification to the UPF to delete the Packet Detection Rule (PDR), QoS Enforcement Rule (QER), and Usage Reporting Rule (URR) related to the flows being released.
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The SMF initiates N1N2TransferMessage containing N1 PDU Session Modification command. This message includes information about the deleted flows, Mapped EPS Bearer Context.
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Then, the SMF interacts with the PCF to report the flows released for the rules if “RES_RELEASE” trigger is set.
Note | The "policy_pdu_flows_total" statistics is available to check the released flows. |
EPS Interworking Indication in PDU Session Modification
The EpsInternetworkingIndication field denotes the possibility of handover between EPS and 5GC. This field holds the following values:
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NONE: The PDU session cannot be moved to EPS.
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WITH_N26: The PDU session is moved to EPS, with N26 interface supported during EPS interworking procedures.
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WITHOUT_N26: The PDU session is moved to EPS, without N26 interface supported during EPS interworking procedures.
The SMF allows the 4G to 5G handover and vice-versa only if the EpsInternetworkingIndication value is set to WITH_N26. For other values of EpsInternetworkingIndication, the SMF rejects the handovers.
During 4G and 5G PDU session establishment, if the EPS internetworking indication is received from the AMF, the SMF includes PGW-C+SMF FQDN for S5/S8 interface in the UDM Registration request.
With the EPS Interworking Indication Support Enabled:
If the EpsInternetworkingIndication value changes from NONE or WITHOUT_N26 to WITH_N26 for a created PDU session, follow these steps to support the EPS Interworking Indication change in the PDU modification procedure.
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The AMF invokes the Nsmf_PDUSession_UpdateSMContext request with the changed EpsInterworkingIndication value.
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The SMF receives the Nsmf_PDUSession_UpdateSMContext request from the AMF, and initiates the Namf_Communication_EbiAssignmentRequest. This request includes the PDU Session ID and Allocation/Retention Priority (ARP) List.
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The AMF sends Namf_Communication_EbiAssignmentResponse to the SMF. The AMF sends the following through the response:
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assignedEbiList containing the successfully assigned EBIs.
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failedArpList containing the failed ARPs for which the EBI assignment failed.
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4XX/5XX error along with AssignEbiError representing the EBI assignment failure.
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The SMF sends N1N2MessageTransfer request message if the EBIs are created successfully. This request includes the following:
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N1:PDU SESSION MODIFICATION COMMAND ([Mapped EPS Bearer Contexts,Create])
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N2:N2_PDU_SESSION_RESOURCE_MODIFY_REQUEST_TRANSFER (QoS Flow Add or Modify Request Item with EPS Radio Access Bearer (E-RAB) ID and QoS Flow ID)
NoteIf the UE is in Idle mode, the SMF skips sending the N2 message.
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The SMF informs mapped EPS bearer context in the UE using N1 message. The SMF waits for N1: PDU SESSION MODIFICATION COMPLETE message.
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The SMF informs EBI to QoS Flow Identifier (QFI) mapping to gNodeB using N2 message. The SMF waits for N2: PDU SESSION RESOURCE MODIFY RESPONSE TRANSFER message.
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The SMF completes the PDU Session Modification procedure.
With the EPS Interworking Indication Support Disabled:
If the EpsInternetworkingIndication value changes from WITH_N26 to NONE or WITHOUT_N26 for a created PDU session, follow these steps to support the EPS Interworking Indication change in the PDU modification procedure.
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The SMF receives the Nsmf_PDUSession_UpdateSMContext request with the changed EpsInterworkingIndication value from the AMF.
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The SMF sends N1N2MessageTransfer request message. This request includes the following:
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N1:PDU SESSION MODIFICATION COMMAND ([Mapped EPS Bearer Contexts,Delete])
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N2:N2_PDU_SESSION_RESOURCE_MODIFY_REQUEST_TRANSFER
NoteIf the UE is in Idle mode, the SMF skips sending the N2 message.
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The SMF deletes Mapped EPS bearer context in UE using N1 message. The SMF waits for N1: PDU SESSION MODIFICATION COMPLETE message.
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The SMF deletes EBI to QFI mapping to gNodeB using N2 message. The SMF waits for N2: PDU SESSION RESOURCE MODIFY RESPONSE TRANSFER message.
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The SMF completes the PDU Session Modification procedure.
Use the show subscriber command to determine the EPS interworking status of the PDU session, and the EBI mapping for the QoS flows.