Feature Description

3GPP specifies provisions for UEs capable of supporting both 5G and 4G NAS to connect to E-UTRAN and 5G core network.

To forward data (G-PDUs and End Marker packets) during an EPS to 5GS handover, the SMF:

  • Provisions one PDR per E-RAB (that supports data forwarding for at least one QoS flow).

  • Creates and associate one QER with each PDR, including the QFI IE set to the QFI value of one of the QoS flows mapped to the E-RAB, to request the UPF to insert a GTP-U PDU Session Container extension header including the QFI.

Data forwarding during handovers between 5GS and EPS is supported as follows (see, 3GPP TS 38.300):

  • For 5G to 4G handover, the source NG-RAN node sends one or several end-markers including one QFI of those QoS flows mapped to the same E-RAB and sends the end-marker packets to the UPF over the PDU session tunnel. UPF removes the QFI and maps to an appropriate E-RAB tunnel toward SGW.

  • For 4G to 5G handover, the source eNB forwards the received end markers in the EPS bearer tunnel to the SGW, which forwards them to the UPF. The UPF adds one QFI among the QoS flows mapped to that E-RAB to the end-markers and sends those end-markers to the target NG-RAN node in the per PDU session tunnel.

Error Indication and GTP-U Path Failure

The UPF notifies an Error Indication message for a GTP-U peer to the sender when a GTP-PDU is received with a TEID that does not exist. This ensures that there are no stale sessions or bearers, and maintains consistency in the network.

Error Indication and GTP-U Path Failure between SMF and UPF nodes are supported over N4 interface. For the neighbor nodes, it is supported over the S1u/S5u interfaces.

Behavior variations of local-purge or signal-peer for Error Indication and GTP-U Path Failure are considered in this implementation.

  • When Error Indication is received, the UPF communicates the TEID and GTPU-peer information with the SMF to ensure deletion or modification of the GTPU-peer.

  • On receiving GTP-U packet with non-existing TEID, the UPF generates and sends Error Indication with TEID and GTP-U peer entries.

  • The deletion of a session or a bearer is decided based on the Path Failure detection at SMF or UPF.

  • GTP-U Path Failure is detected using GTP-U echo messages between UPF nodes, and between the UPF and SMF nodes.