Section: 5.31.6 Reliable Data Service
Content: The Reliable Data Service (RDS) may be used between the UE and NEF or UPF when using a PDU Session of PDU Type 'Unstructured'. The service provides a mechanism for the NEF or UPF to determine if the data was successfully delivered to the UE and for the UE to determine if the data was successfully delivered to the NEF or UPF. When a requested acknowledgement is not received, the Reliable Data Service retransmits the packet. The service is enabled or disabled based on DNN and NSSAI Configuration per SLA. When the service is enabled, a protocol is used between the end-points of the unstructured PDU Session. The protocol uses a packet header to identify if the packet requires no acknowledgement, requires an acknowledgement, or is an acknowledgment and to allow detection and elimination of duplicate PDUs at the receiving endpoint. RDS supports both single and multiple applications within the UE. Port Numbers in the header are used to identify the application on the originator and to identify the application on the receiver. The UE, NEF and the UPF may support reservation of the source and destination port numbers for their use and subsequent release of the reserved port numbers. Reliable Data Service protocol (as defined in TS 24.250[ Protocol for Reliable Data Service; Stage 3 ]  [80]) also enables applications to query their peer entities to determine which port numbers are reserved and which are available for use at any given time. The header is configured based on Reliable Data Service Configuration information which is obtained in the NIDD configuration, MT NIDD, and MO NIDD procedures with the AF as specified in TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. During NIDD Configuration, the AF may indicate which serialization formats it supports for mobile originated and mobile terminated traffic in the Reliable Data Server Configuration. When port numbers are reserved by the UE, the serialization format that will be used by the application may be indicated to the NEF. When port numbers are reserved by the NEF, the serialization format that will be used by the application may be indicated to the UE. If the receiver does not support the indicated serialization format, it rejects the port number reservation request and the sender may re-attempt to reserve the port number with a different serialization format. If, during NIDD Configuration, the AF indicated that it supports multiple serialization formats, the NEF determines the serialization format that it will indicate to the UE based on local policies and previous negotiations with the UE (e.g. the NEF may indicate the same serialization format that was indicated by the UE or avoid indicating a serialization format that was previously rejected by the UE). When serialization formats are configured for reserved port numbers, the NEF stores the serialization formats as part of the Reliable Data Service Configuration and provides the updated Reliable Data Service Configuration to the AF. NOTE: Whether the UE Application or AF supports a given serialization format is outside the scope of 3GPP specifications. The UE indicates its capability of supporting RDS in the Protocol Configuration Options (PCO) and the SMF negotiates RDS support with the NEF or UPF. If the NEF or UPF supports and accepts RDS then the SMF indicates to the UE, in the PCO, that the RDS shall be used if enabled in the DNN and NSSAI configuration. In order to prevent situations where an RDS instance needs to interface to both the user and control plane, RDS may only be used with PDU Sessions for which the "Control Plane CIoT 5GS Optimisation" indication is set or with PDU sessions using the Control Plane CIoT 5GS Optimisation when the AMF does not move the PDU session to the user plane. Reliable Data Service protocol is defined in TS 24.250[ Protocol for Reliable Data Service; Stage 3 ]  [80].