AMF supports the UE Context Transfer message at source and target AMF. The following CLI configurations are added:

• allow-interplmn-supi-transfer

• horizontal-key-derivation

• use-source-key

• use-source-pcf

UE Context Transfer at Source AMF:

• Sends UE Context with SUPI value to target AMF as per the CLI configuration, when source AMF and target AMF are in different PLMN

• Uses either existing keys or generates new keys, and sends the keys to target AMF during context transfer as per the CLI configuration

• Starts context-transfer-guard timer (configured with greater than zero (0)), when UeRegStatusUpdateReqData contains transfer status as TRANSFERRED

  On expiry of the context-transfer-guard timer, source AMF performs the following:

  • Triggers the UDM Deregistration internally to clear the local ueContext

  • When the UE Context Transfer reason is INIT_REG, it updates the SMF to release the PDU context

• It releases PDU sessions in the toReleaseSessionList

• The UE-validation reason is handled as follows:

  • Without registration request

  • By omitting integrity check

  • Responding with appropriate data to target AMF

• Clears PCF association, when target AMF sends pcfReselectedInd in transfer update

• Handles reject indication received from target AMF

• Performs horizontal key derivation as per the CLI configuration

• Transfers URI with SUPI as ueContextId to target AMF

• Sends DRX, GMM capability IEs to target AMF

• Increments transfer failure counters including NOT_TRANSFERRED counters

• Doesn't send SeafData in transfer response in MOBI_REG_UE_VALIDATED when the Individual ueContext is identified with SUPI

UE Context Transfer handling at Target AMF:

• Sends Reject Indication to source AMF through StatusUpdate message when authentication or security fails

  The security algorithm mismatch is handled as follows:

  • Authenticates when integrity check fails

  • Recomputes the keys as per the algorithm received from AUSF

  • Regenerates all the keys and ignores the keys received from source AMF.

• Sends failure to source AMF when authentication or security check fails

• The SUPI as UeContextID is handled as follows:

  • Sends Identity request to UE when message integrity check fails

  • Performs UE authentication with obtained SUPI from UE

  • Sends SUPI as UeContextId, and UE-validated in UeContextTansferReq to source AMF

• Ignores the PCF information obtained from the source AMF and selects the new PCF based on the CLI configuration. Informs the selection of new PCF using pcfReselectedInd to source AMF in UeRegStatusUpdateReq.

This section describes how this feature works.

To configure this feature, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
            policy ue-ctx-transfer 
                allow-interplmn-supi-transfer { true | false } 
                horizontal-key-derivation { true | false } 
                use-source-key { true | false } 
                use-source-pcf { true | false } 
                exit 
            timers 
                context-transfer-guard value guard_time_value 
                end 

NOTES:

• allow-interplmn-supi-transfer { true | false } —Specify true or false. If configured true, the source AMF sends UE context with SUPI. The default value is false .

• horizontal-key-derivation { true | false } —If configured true, the source AMF generates a new key every time. The default value is false

• use-source-key { true | false } —If configured true, the target AMF uses a key received from the source AMF. The default value is true

• use-source-pcf { true | false } —If configured false, the target AMF sends pcfReselectedInd as true in TransferUpdate and the source AMF clears the PCF association. The default value is true

• context-transfer-guard guard_time_value —Specify the context transfer guard timer value in seconds. The AMF starts this timer on receiving the TransferUpdate. On expiry, AMF clears the PDUs locally.

  context-transfer-guard value must be an integer in the range of 0—35712000. The default value is zero (0).