5.33.2.3 Support for redundant transmission at transport layer

Redundant transmission can be supported within the 5G System without making any assumption on support for protocols such as IEEE FRER in the application layer (DN only) at the same time it can be supported without requiring redundant GTP-U tunnel over N3. The backhaul provides two disjoint transport paths between UPF and NG-RAN. The redundancy functionality within NG-RAN and UPF make use of the independent paths at transport layer. Support of redundant transmission at transport layer requires no 3GPP protocol impact.

Following are the required steps:

• UE establishes the PDU session for URLLC services. Based on DNN, S-NSSAI, knowledge of supporting redundant transmission at transport layer and other factors as described in clause 6.3.3, SMF selects a UPF that supports redundant transmission at transport layer for the PDU session. One N3 GTP-U tunnel is established between UPF and NG-RAN.

The knowledge of supporting redundant transmission at transport layer can be configured in the SMF, or be configured in UPF and then obtained by the SMF via N4 capability negotiation during N4 Association setup procedure.

• For DL data transmission, UPF sends the DL packets on N3 GTP-U tunnel. Redundant functionality in the UPF duplicates the DL data on the transport layer. Redundant functionality in the NG-RAN eliminates the received duplicated DL data and sends to NG-RAN.
• For UL data transmission, NG-RAN sends the received UL packets on N3 GTP-U tunnel, the Redundant functionality in the NG-RAN performs the redundant handling on the backhaul transport layer. The Redundant functionality in the UPF eliminates the received duplicated UL data and sends to UPF.