RDS establishes a peer-to-peer logical link between the UE and the network. The logical link is identified by,
a pair of port numbers and EPS bearer ID in EPS; or
a pair of port numbers, PDU session identity and the QoS Flow Identifier in 5GS.
Each port number is used to identify an application on the UE side or at the network side and is carried in the address field of each frame. The source port number identifies the application on the originator and the destination port number identifies the application on the receiver. When a single application on the originator conducts data transfer with a single application on the receiver, the source port number and destination port number need not be used. Each RDS frame shall consist of a header and an information field of variable length. The header shall contain information about port numbers and the frame number that is used to identify the frame and provide reliable transmission. The information field contains the payload to be transferred between the UE and the network.
the UE establishes a PDN connection with the SCEF or P-GW either during Attach or through UE requested PDN connectivity. The UE shall use the EPS bearer ID to select the bearer to transfer RDS PDUs to the SCEF or P-GW. The EPS bearer ID identifies the destination (at the UE or at the SCEF or P-GW) and is not carried in the frame as it is already included in the NAS ESM message header.
the UE establishes a PDU session with the SMF through UE requested PDU session establishment procedure. The UE shall use the PDU session identity and the QoS Flow Identifier to select the flow to transfer RDS PDUs to the NEF or UPF. The PDU session identity identifies the destination (at the UE or at the NEF or UPF) and is not carried in the frame as it is already included in the NAS 5GSM message header.
Once the UE and network successfully negotiate to use RDS for a particular PDN connection or a PDU session, the PDN connection or PDU session shall transfer data only using RDS protocol.
RDS shall support both single and multiple applications within the UE. RDS enables applications to reserve source and destination port numbers for their use and also subsequently release the reserved port numbers. When reserving ports applications can indicate the serialization format that they will be using. RDS 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. Applications can also additionally query their peer entities to determine the serialization format that will be used. RDS shall provide fuctionality for flow control and sequence control to maintain the sequential order of frames across the logical link. The UE and the network may support reservation of the source and the destination port numbers for their use and subsequent release of the reserved port numbers.