Section: 5.37.5.2 PDU Set Information and Identification
Content: To support PDU Set based QoS handling, the PSA UPF identifies PDUs that belong to PDU Sets and determines the below PDU Set Information which it sends to the NG-RAN in the GTP-U header. The PDU Set information is used by the NG-RAN for PDU Set based QoS handling as described above. The PDU Set Information comprises: - PDU Set Sequence Number. - Indication of End PDU of the PDU Set. - PDU Sequence Number within a PDU Set. - PDU Set Size in bytes. - PDU Set Importance, which identifies the relative importance of a PDU Set compared to other PDU Sets within a QoS Flow. The NG-RAN may use the Priority Level (see clause 5.7.3.3) across QoS Flows and PDU Set Importance within a QoS Flow for PDU Set level packet discarding in presence of congestion. NOTE 1: In addition to considering the PDU Set Importance within a QoS Flow, NG-RAN could also consider the relative PDU Set Importance across QoS Flows of the same Priority Level when determining which PDU Set needs to be discarded, which is up to implementation and configuration of operator. NOTE 2: The PDU Set Information can be different for different PDU Sets within a QoS Flow. The SMF instructs PSA UPF to perform PDU Set marking and may provide the PSA UPF the Protocol Description used by the service data flow. The Protocol Description may be received in the PCC rule, based on information provided by the AF or by PCF local policies as described in clause 5.37.5.1. PSA UPF can identify the PDU Set Information using the Protocol Description and the received transport protocol headers and payload or using implementation specific means. The details of the RTP/SRTP headers, header extensions and/or payloads used to identify PDU Set Information are defined in TS 26.522[ 5G Real-time Media Transport Protocol Configurations ]  [179]. For each DL PDU received on N6 for which PDU Set based QoS handling is indicated from the SMF, the PSA UPF applies the rules for PDU Set identification and provides the available PDU Set Information to the RAN in the GTP-U header. NOTE 3: The PSA UPF is expected to assign a unique PDU Set Sequence Number in the GTP-U header to each PDU Set of the QoS Flow.