Handling Failed QoS Flow Identifier During PDU Session Modification

The SMF supports handling of the failed QoS flows over N2 interface during the PDU session modification.

If the modification of a PDU session or a QoS flow fails, the NG-RAN node falls back to the older configuration. That is, it falls back to the configuration of the session or the flow that was available before receiving the PDU SESSION RESOURCE MODIFY REQUEST message.

The SMF receives the QoS Flow Identifier for which the flow add/modify failed during the PDU SESSION RESOURCE MODIFY REQUEST.

If the new flow addition fails, the SMF performs the following:

  • Removes the failed flow towards N1 (UE)

  • Stops sending the failed flow-related information towards N4 (UPF)

  • Stops sending the failed flow-related information towards N40 (CHF)

  • Checks if the triggers are enabled and then sends the rule report for the failed flow towards N7 (PCF).

If the modification of flow fails, the SMF performs the following:

  • Replaces the old information for the failed flow towards N1 (UE)

  • Stops sending the modified flow-related information towards N4 (UPF)

  • Stops sending the modified flow-related information towards N40 (CHF)

  • Checks if the triggers are enabled and then sends the Rule Report for the failed flow towards N7 (PCF).

The following table captures the SMF behavior for the cause values included in the PDU Session Resource Modify Unsuccessful Transfer IE. These cause values are applicable for the PDU session modification procedure.

Cause Group

Cause Value

SMF Behavior

Comment

Radio Network Layer Cause

Unspecified

General

Unknown PDU Session ID

Delete the session

N1 FiveGSM Cause reactivation requested

Unknown QoS Flow ID

Send delete details to N1

Send PCF report about rule(s)

Multiple PDU Session ID Instances

Delete the session

Multiple QoS Flow ID Instances

Delete the session

N1 FiveGSM Cause reactivation requested

Xn handover triggered

Act based on collision handling

Not supported 5QI value

Send delete details to N1

Send PCF report about rule(s)

IMS voice EPS fallback or RAT fallback triggered

Already supported

Transport Layer Cause

Transport resource unavailable

Send delete details to N1

Send PCF report about rule(s)

Unspecified

NAS Cause

Normal release

Delete the session

Authentication failure

Delete the session

Deregister

Delete the session

Unspecified

Delete the session

Protocol Cause

Transfer syntax error

N1 rollback

Error log fail procedure

Abstract syntax error (reject)

N1 rollback

Error log fail procedure

Abstract syntax error (ignore and notify)

N1 rollback

Error log fail procedure

Message not compatible with receiver state

N1 rollback

Error log fail procedure

Semantic error

N1 rollback

Error log fail procedure

Abstract syntax error (falsely constructed message)

N1 rollback

Error log fail procedure

Unspecified

N1 rollback

Error log fail procedure

Miscellaneous Cause

Control processing overload

N1 rollback

Error log fail procedure

Not enough user plane processing resources

N1 rollback

Error log fail procedure

Hardware failure

Delete the session

O&M intervention

N1 rollback

Error log fail procedure

Unknown PLMN

Delete the session

Bulk Statistics

The following statistics provide details about the failed QoS flows over the N2 interface.

  • policy_pdu_flows_total

    • total attempted

    • total succeeded

    • total failed

  • policy_pdu_flows_current

    • current attempted

    • current succeeded

    • current failed