Cause Description and Scenarios

This section provides information on the causes that SMF receives from AMF through N11 interface messages and the relevant scenarios of those causes.

REL_DUE_TO_HO

The following table describes the release due to handover cause and scenario.

Release due to Handover Cause and Scenario
Cause REL_DUE_TO_HO
Cause Description from 3GPP TS 29.502 Release due to handover
Scenario of occurrence Handover from 5GS to EPG or ePDG during roaming
Message Used vsmfUpdateData
Message Direction H-SMF to V-SMF
Comments and Specification References

3GPP TS 29.502

  • 5.2.2.8.3 Update service operation towards V-SMF

  • 5.2.2.8.3.4 Handover between 3GPP and untrusted non-3GPP access, from 5GC-N3IWF to EPS or from 5GS to EPC/ePDG

If the request indication in the request is configured to NW_REQ_PDU_SES_REL and if the Cause IE indicates the release due to handover cause, then the V-SMF initiates the release of RAN resources reserved for the PDU session, if any. However, SMF doesn't send a PDU session release command to the UE.

The V-SMF doesn't release the SM context for the PDU session.

Note
  • SMF doesn't support the roaming feature for this cause.

  • This cause is available in the SmContext release request after the N2 handover. SMF supports this scenario.

EPS_FALLBACK

The following table describes the mobility due to EPS fallback for IP Multimedia Subsystem (IMS) voice cause and the scenario of occurrence of the cause:

Release due to EPS Fallback Cause and Scenario

Cause

EPS_FALLBACK

Cause description from 3GPP TS 29.502

Mobility due to ongoing EPS fallback for IMS voice.

Scenario of occurrence

IMS voice configuration in roaming scenario

Message used

VsmfUpdatedData

This message is used in the qosFlowsFailedtoAddModList attribute, which is the Cause IE of QosFlowItem.

Message direction V-SMF to H-SMF
Comments and Specification References

SMF supports the following scenarios for this cause as per the specification:

  • 3GPP TS 23.502, Section 4.13.6.1 for EPS fallback for IMS voice.

    The PDU Session Response message towards the SMF receives the QoS flow for IMS voice through AMF. For roaming scenario, this message is sent towards H-SMF through V-SMF. NG-RAN rejects the PDU Session modification to configure the QoS flow for IMS voice indicating the ongoing mobility due to fallback for IMS voice.

  • 3GPP TS 23.502

    If the NG-RAN rejects the establishment of a voice QoS flow due to EPS Fallback for IMS voice, as defined in 3GPP TS 23.502 [3], clause 4.13, the V-SMF returns the cause. V-SMF indicates the cause as ongoing mobility due to EPS fallback for IMS voice for the corresponding flow in the qosFlowsFailedtoAddModifyList IE.

Note

This scenario doesn't support roaming.

REL_DUE_TO_UP_SEC

The following table describes the release due to unfulfilled security requirements from User Plane cause and the scenario of occurrence of the cause:

Release due to User Plane Cause and Scenario
Cause REL_DUE_TO_UP_SEC
Cause description from 3GPP TS 29.502 Release due to unfulfilled User Plane security requirements.
Scenario of occurrence

AMF-initiated release when the NG-RAN is unable to fulfill the required User Plane security enforcement.

Message used Release SM Context service operation
Message direction AMF to SMF
Comments or Specification References

3GPP 29.502, Section 5.2.2.4, Release SM Context service operation

The REL_DUE_TO_UP_SEC cause is available in SM Context Release Request when NG-RAN is unable to fulfill the required User Plane security enforcement.

DNN_CONGESTION

The following table describes the release due to the DNN-based congestion control cause and the scenario of occurrence of the cause:

Release due to DNN Congestion Cause and Scenario
Cause DNN_CONGESTION
Cause Description from 3GPP TS 29.502 Release due to the DNN-based congestion control.
Scenario of occurrence

SMF detects congestion for the requested DNN and performs an overload control for the DNN that restricts the establishment of the PDU session.

Message Used SM Context Create Error and SM Context Update Error
Message Direction

SMF to AMF

Comments or Specification References

Not supported.

S_NSSAI_CONGESTION

The following table describes the release due to the S-NSSAI-based congestion control cause and the scenario of occurrence of the cause:

Release due to S NSSAI Cause and Scenario
Cause S_NSSAI_CONGESTION
Cause description from 3GPP TS 29.502 Release due to the S-NSSAI-based congestion control.
Scenario of occurrence

SMF detects congestion for the requested S-NSSAI and performs overload control for the S-NSSAI that restricts the establishment of the PDU session.

Message used

SM Context Create Error and SM Context Update Error

Message direction

SMF to AMF

Comments or specification references

Not supported.

REL_DUE_TO_REACTIVATION

The following table describes the release due to PDU session reactivation cause and scenario of its occurrence:

Release due to Reactivation Cause and Scenario
Cause REL_DUE_TO_REACTIVATION
Cause Description from 3GPP TS 29.502 Release due to PDU session reactivation.
Scenario of occurrence

3GPP TS 29.502, Section 5.2.2.3.10, P-CSCF Restoration Procedure via AMF.

The POST request contains the release IE configured to True and the cause IE configured to REL_DUE_TO_REACTIVATION.

Message used Update SM Context service operation
Message direction AMF to SMF
Comments or specification references

After receiving the cause from AMF, SMF sends the 5GSM cause as Reactivation Required towards UE.

5G_AN_NOT_RESPONDING

The following table describes the cause when 5G access network (AN) doesn't respond to network-initiated request and the scenario of occurrence of the cause:

Release due to 5G AN Not Responding Cause and Scenario
Cause 5G_AN_NOT_RESPONDING
Cause Description from 3GPP TS 29.502 The 5G AN doesn't respond to the network-initiated request.
Scenario of occurrence None.
Message Used SM Context Status Notification or Status Notification
Message Direction SMF to AMF
Comments or Specification References

SMF supports the following scenarios for this cause:

  • When UE is activated on network, SMF sends the SM Context Status Notification or Status Notification message in the statusInfo cause during UE or network-initiated PDU session release.

  • While the activation of UE PDU session from a deactivated state, SMF waits for the PDU RES STP RES from gNB and if GNB doesn’t respond to AMF or SMF, AMF sends the SM Context Update with UP CXT State as DEACTIVATED with this cause. AMF sends the Update SM Context service operation to SMF.

REL_DUE_TO_SLICE_NOT_AVAILABLE

The following table describes the release due to unavailability of the associated S-NSSAI cause and the scenarios of the occurrence of the cause:

Release due to Slice not Available Cause and Scenario
Cause REL_DUE_TO_SLICE_NOT_AVAILABLE
Cause Description from 3GPP TS 29.502 Release due to the associated S-NSSAI is unavailable.
Scenario of occurrence

The following are the scenarios of the occurrence of the cause:

  • Scenario 1—UDM-initiated slice information change notification to AMF when PDU is activated.

  • Scenario 2—UDM-initiated slice information change notification to AMF when PDU is deactivated.

Message Used

The following are the messages used for these scenarios:

  • Scenario 1—Update SM Context service operation.

  • Scenario 2—Release SM Context service operation.

Message Direction AMF to SMF
Comments or Specification References

SMF supports the following scenarios for this cause as per the specification:

  • 3GPP TS 29.502, Section 5.2.2.3.12 AMF requested PDU Session Release due to slice not available.

    The POST request includes the release IE configured to True and the the cause IE configured to REL_DUE_TO_SLICE_NOT_AVAILABLE.

  • 3GPP TS 29.502, Section 5.2.2.4, Release SM Context service operation.

    As defined in 3GPP TS 23.501 [2], clause 5.15.5.2.2, a change of the set of network slices occur for a UE where a network slice instance is unavailable and the PDU session isn't activated.

REL_DUE_TO_DUPLICATE_SESSION_ID

The following table describes the release due to UE request for new PDU session establishment cause and the scenario of the occurrence of the cause:

Release due to Duplicate Session ID Cause and Scenario
Cause REL_DUE_TO_DUPLICATE_SESSION_ID
Cause Description from 3GPP TS 29.502 Release due to a UE request to establish a new PDU session with an identical PDU session ID.
Scenario of occurrence AMF-requested PDU Session Release due to duplicate PDU Session ID.
Message Used Update SM Context service operation
Message Direction AMF to SMF
Comments or Specification References

SMF supports the following scenario:

As defined in 3GPP TS 24.501 [7], clause 5.4.5.2.5, when the AMF receives an initial request with the existing PDU Session ID in the PDU session context of the UE, AMF requests the SMF to release the existing PDU Session. After receiving the SM context status notification indicating that the deletion of the SM context in the SMF, the AMF releases the stored context for the PDU session. Then, the AMF sends the initial request with the PDU Session ID.

The POST request includes the release IE configured to True and the cause IE configured to REL_DUE_TO_DUPLICATE_SESSION_ID.

Note

SMF doesn't send the NAS signaling to UE for the PDU session release in this procedure.

PDU_SESSION_STATUS_MISMATCH

The following table describes the release due mismatch of PDU session status between UE and AMF cause and the scenario of the occurrence of the cause:

Release due to PDU Session Status Mismatch Cause and Scenario
Cause PDU_SESSION_STATUS_MISMATCH
Cause Description from 3GPP TS 29.502 Release due to mismatch of PDU Session status between UE and AMF.
Scenario of occurrence UE service request procedure.
Message Used SM Context Release Data
Message Direction AMF to SMF
Comments or Specification References

SMF supports the following scenario:

As defined in 3GPP TS 24.501, Section 5.2.2.4, Release SM Context service operation, in case of mismatch of the PDU session status between the UE and the AMF, the AMF starts Release operation towards SMF to release the PDU context from network.

HO_FAILURE

The following table describes the handover preparation failure cause and the scenario of the occurrence of the cause:

Release due to HO Failure Cause and Scenario
Cause HO_FAILURE
Cause Description from 3GPP TS 29.502 Handover preparation failure.
Scenario of occurrence 5GS to EPS handover over N26 interface and if no resources can be assigned in EPS for any attempted PDU session to be handed over.
Message Used SM Context Update
Message Direction AMF to SMF
Comments or Specification References

SMF supports the following scenario:

AMF updates the SMF with the information that the handover preparation failed by sending a POST request with the cause attribute configured to HO_FAILURE and with an empty list of EPS bearer contexts. This procedure doesn't include the dataForwarding IE. Then, SMF releases the resources prepared for the handover and proceeds with the PDU session.

INSUFFICIENT_UP_RESOURCES

The following table describes the activation failure for User Plane connection due to insufficient resources cause and the scenario of the occurrence of the cause:

Release due to Insufficient UP Resources Cause and Scenario
Cause INSUFFICIENT_UP_RESOURCES
Cause Description from 3GPP TS 29.502 Failure to activate the User Plane connection of a PDU session due to insufficient user plane resources.
Scenario of occurrence During an idle mode exit procedure.
Message Used SM Context Updated Data
Message Direction SMF to AMF
Comments or Specification References

3GPP TS 129.502 , Section 5.2.2.3.2.2, Activation of User Plane connectivity of a PDU session

SMF supports the following scenario:

As defined in 3GPP TS 38.413 [9], clause 9.3.4.16 5G-AN sends the N2 SM information to SMF including the cause of the failure or if the resources failed to establish the PDU session.

After SMF receives this information, SMF considers that the activation of the User Plane connection has failed and configures the upCnxState attribute to DEACTIVATED.

In case the activation of the User Plane connection fails due to insufficient resources, the cause is included in the problem details response and configured to INSUFFICIENT_UP_RESOURCES with status code as 500.

PDU_SESSION_HANDED_OVER

The following table describes the handover of PDU session cause and the scenario of the occurrence of the cause:

Release due to PDU Session Handed Over Cause and Scenario
Cause PDU_SESSION_HANDED_OVER
Cause Description from 3GPP TS 29.502 The PDU session is handed over to another system or access.
Scenario of occurrence

5GC to EPS mobility without N26 interface

Handover from 5GS to EPC or ePDG

Message Used SM Context Status Notification
Message Direction SMF to AMF
Comments or Specification References

SMF supports the following specification for this cause:

  • As defined in 3GPP TS 23.502, SMF supports Section 4.11.2.2 5GC to EPS mobility without N26 interface and 4.11.4.2 Handover from 5GS to EPC or ePDG

  • As defined in 3GPP TS 29.502, Section 5.2.2.5 Notify SM Context Status service operation, SMF sends a POST request to the SM Context Status callback reference that the NF Service Consumer provides during the subscription of this notification. The payload body of the POST request contains the notification payload. If the PDU session handover triggers the notification, the notification payload contains the Cause IE with the PDU_SESSION_HANDED_OVER value.

Note
  • SMF doesn't support the 5GC to EPS mobility without N26 interface

  • SMF supports sending of SM Context Status Notification with this cause during handover from 5GS to EPCor ePDG.