N4 Session Establishment and Modification Procedure for IPv6 Call Flow

The following call flow provides a high-level description of the N4 session establishment and modification procedure for IPv6.

StepDescription
1The SMF receives the trigger to establish a new PDU session or change the UPF for an established PDU session.
2The SMF sends N4 Session Establishment Request message to the UPF, in which Create PDR IE has IPv6 UE address, and SDF filter has IPv6 filters to select the PDRs.
3The UPF responds with an N4 Session Establishment Response message containing any information that the UPF must provide to the SMF in response to the control information received.
4The SMF interacts with the network function and triggers to modify the PDU session.
5The SMF sends an N4 Session Modification Request message to the UPF that contains the update for the structured control information which defines how the UPF needs to behave.
6The UPF identifies the N4 session context to be modified by the N4 Session ID. Then, the UPF updates the parameters of this N4 session context according to the list of parameters sent by the SMF. The UPF responds with an N4 session modification response message containing any information that the UPF must provide to the SMF in response to the control information received.
7An extra procedure is required for IPv6 sessions, which is Router Solicitation (RS) and Router Advertisement (RA). After the session is established, UE sends the RS message to network to get the link layer address. The UPF forwards this message to SMF. The SMF sends RA message with required parameters to configure the IPv6 address of UE.
8The RS and RA between UPF and SMF is GTP-u encapsulated and SMF sends an extra pair of PDRs during session establishment and modification procedure, in which GTP-u Tunnel IDs are exchanged for GTP-u tunneling.
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The additional pair of PDRs that are sent from the SMF are as follows:

  • One PDR has Source interface as Access, and Destination Interface as CP-Function, to forward IPv6 RS from UE to SMF. The SDF filter is present so that UPF can select this PDR for IPv6 RS from UE

  • Example: Permit in 58 from any tp ff01::2 any

  • Another PDR has source interface as CP-Function, and Destination Interface as Access, to forward the IPv6 RA from SMF to UE. The SDF filter is present so that UPF can select this PDR for all the IPv6 RAs from SMF.

  • Example: Permit out 58 from any to ff01::2 any

10After RS and RA procedure is completed, the UE sends IPv6 traffic to PDN.