29.2443GPPInterface between the Control Plane and the User Plane nodesRelease 17TS
Clause 5.31.6 of 3GPP TS 23.501 , clause 4.3.2 of 3GPP TS 23.502  and clause 22.214.171.124 of 3GPP TS 23.682 , specify that The Reliable Data Service (RDS) may be used between the UE and UP function when using a PDU Session of PDU Type ‘Unstructured’ in 5GS or using PDN Connection of PDN Type ‘Non-IP’ in EPS. The service is enabled or disabled based on DNN and NSSAI Configuration per SLA in 5GS or APN Configuration per SLA in EPS.
If the UE indicates its capability of supporting RDS in the Protocol Configuration Options (PCO) and if the UP function indicated support of the RDS feature, the CP function may request the UP fuction to apply the RDS functionality for the PDU session by sending "Provide RDS Configuration Information" IE within PFCP Session Establishment Request message (see clause 126.96.36.199).
If the UP function supports and accepts RDS, it should respond to CP function by setting RDS flag in "RDS Configuration Information" IE within PFCP Session Establishment Response message (see clause 188.8.131.52), and the UP function should place unstructed DL data from N6/SGi in the payload of RDS protocol and then insert it as GTP-U payload, and vice versa for UL data.
Then the CP function should indicate to the UE, in the PCO, that the RDS shall be used if enabled in the DNN and NSSAI configuration in 5GS or APN configuration in EPS. The Reliable Data Service is enabled afterwards.
Reliable Data Service protocol is defined in 3GPP TS 24.250 .