Feature Description

The 5G Session Management procedures defined in 3GPP TS 23.502 ensure that the EPS interworking is successful when the UE moves to an LTE 4G radio after performing the initial attach to a 5G NR radio.

Support for Number of Packet Filters in NAS Message

The UE sends the Number of packet filter IE to the SMF in PDU Establishment and Modification request messages. By default, the UE sends a maximum of 16 packet filters.

The UE supports more than 16 packet filters in the following scenarios:

  • When the UE is attaching to the SMF in N1 mode.

  • When the initial attach to the SMF in S1 mode is complete and the 4G to 5G handover is ongoing.

The SMF sends the maximum filters to the PCF in PolicyCreateControl in "NumOfPackFilter" field. If the Number of packet filter IE is received from the UE in N1 mode, then the SMF uses the “Maximum number of supported filters” field in PDU establishment request. If this IE is not received from the UE in N1 mode or if the received value is lesser than 16, the SMF sends the max filters as 16. If the UE attaches to the SMF in S1 mode and the 4G to 5G or 5G to 4G handover is ongoing, the SMF sends the default value, that is, 16 packet filters.

If there is any change in the packet filter value, then the SMF sends the new value to the PCF through PolicyUpdate message along with NUM_OF_PACKET_FILTER trigger.

The SMF controls the maximum filters allowed per PDU session based on the numOfPackFilter IE. If the number of packet filters crosses the maximum allowed by the UE, the SMF caps the packet filters. This means that the SMF drops the PCC rules when the limit crosses and sends the rule report with INCOR_FLOW_INFO failure code.

Note

INCOR_FLOW_INFO is not the correct failure code for this kind of deletion. Use the appropriate failure code when available in the 3GPP specification.

Maximum supported filters are only valid for dynamic rules and not for static and predefined rules.

The "pcc_rule_report_max_supported_filter" statistics is introduced under the policy_pcc_rule_report category. This statistics is incremented if the PCC rule report is generated upon reaching the maximum supported filters.

Support for PCF ID in SmContextCreate

The AMF includes the PCF ID in the Nsmf_PDUSession_CreateSMContext Request. The PCF ID identifies the Home Policy Control Function (H-PCF) in the non-roaming case and the Visited Policy Control Function (V-PCF) in the local breakout roaming case. See the 3GPP specification 23.501, section 6.3.7.1 for more details on when the AMF forwards the PCF ID to the SMF.

When the SMF receives the PCF ID, use the following CLI configuration in the PCF network profile to control the SMF behaviour in using the PCF ID.

UseAmfProvidedPCF [True/False]

The default behaviour is to use the PCF ID provided by AMF in SmContextCreate.

If the PCF ID provided by AMF is not reachable, the SMF behaves as per the configured failure handling template. In this case, it uses the static configuration.

Support for DNN Selection Mode in SmContextCreate

The SMF uses the DNN Selection Mode for deciding whether to accept or reject the UE request.

The SMF uses the DNN Selection Mode for deciding whether to retrieve the Session Management Subscription data. In case the DNN, S-NSSAI of the HPLMN is not explicitly subscribed, the SMF uses the local configuration instead of the Session Management Subscription data.

Note

The preceding use case is not supported.

The SMF validates the IE present in SmContextCreate data. If there is a DnnSelectionMode failure due to the mismatch between DnnSelectionMode and the configured CLI, the SMF does not proceed with the registration. When the DnnSelectionMode failure is observed, the “disc_pdusetup_sm_cxt_unsupported_ie" is incremented as part of the disconnect reasons.

DnnSelectionMode Type Description

Not Present

The SMF sends the subscription request to fetch the subscription data.

Verified

The SMF sends the subscription request to fetch the subscription data.

UE_DNN_NOT_VERIFIED

If the dnn-selection-mode verified ue-provided CLI command is configured as shown in the following configuration, the SMF sends the subscription request to fetch the subscription data. Otherwise, the SMF rejects the Context Request with "Invalid DNN selection Mode" cause.

NW_DNN_NOT_VERIFIED

If the dnn-selection-mode verified network-provided CLI is configured, the SMF sends the subscription request to fetch the subscription data. Otherwise, the SMF rejects the Context Request with "Invalid DNN selection Mode" cause.

The SMF uses the following configuration to configure the DnnSelectionMode.

configure 
   profile smf profile_name 
      dnn-selection-mode [ verified ue-provided | network-provided ] 
      end 

One or more DnnSelectionMode types can be configured. By default, the DnnSelectionMode is verified.

Post the subscription request, if no subscription data is fetched from UDM, the SMF falls back to the local DNN profile for subscription data. Neither the subscription data is fetched from the UDM nor the local configuration is present, the SMF sends the SmContextCreateError with subscription failure.