PFCP Session Modification Procedure

The SMF uses the PFCP Session Modification procedure to modify an existing PFCP session on the UPF. For instance, configuring a new rule, modifying an existing rule, or deleting an existing rule, and so on. The SMF sends the Create URR IE, Update URR IE (to update the trigger or threshold) and Remove URR IE (to remove an existing URR created earlier by SMF during Session Establishment Procedure) in the same message.

The following call flow depicts the PFCP Session Modification procedure.

StepDescription
1The SMF receives the trigger to modify the existing PDU Session.
2The SMF sends an N4 session modification request message to the UPF. This message contains the structured control information, which defines the UPF's behavior.
3The UPF identifies the PFCP session context for the Session ID to modify. It updates the parameters of this session context according to the list of parameters sent by the SMF. It then responds with a PFCP Session Modification Response message. The message contains the information, which the UPF must provide to the SMF (in response to the control information received).
4
  • If the SMF sends the QAURR flag set in PFCPSMReq-Flag IE or URR ID (s) with Query URR IE (e), then UPF sends the usage report IE for the corresponding URR with the PFCP Session Modification response.

  • The custom Information Element (IE) called Recalculate Measurement is added to the Update-URR IE (for URR-ID: Gz-Bearer) to support the PGWCDR generation due to the Max-LOSDV condition with the N4 Session Modify Request. After receiving the IE, UPF reconciles the Volume and Duration of the URR according to the URR-ID present inside the IE.

5The UPF provisions and acts based on the Create URR, Update URR or Remove URR IE sent by the SMF.
6The SMF interacts with the network function, which triggered this procedure. For instance, AMF or PCF.