Section: 5.37.6 UL/DL policy control based on round-trip latency requirement
Content: For XR and other interactive media services that require very low Round-Trip (RT) latency, Uplink-Downlink policy control may be supported to meet the RT latency requirement. RT latency requirement is the upper bound for the sum of UL delay and DL delay of a single data flow or two different data flows between UE and N6 termination point at the UPF. PCF may support Uplink-Downlink policy control based on RT latency requirement based on an RT latency indication from AF (as defined in clause 6.1.3.27.2 of TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]) during the AF session with the required QoS procedure as defined in clause 4.15.6.6 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. The AF can provide an RT latency indication with a single direction delay requirement between the UE and the PSA UPF expressed as the QoS Reference parameter or individual QoS parameters (as defined in clause 6.1.3.22 of TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]). The RT latency indication indicates the need to meet the RT latency requirement for data flow, i.e. doubling of the single direction delay requirement between the UE and the PSA UPF expressed by the QoS Reference parameter or individual QoS parameter. PCF determines the data flow's UL PDB and DL PDB based on the RT latency requirement. The UL PDB and DL PDB can be unequal, but their sum shall not exceed the RT latency requirement. The PCF shall generate two PCC rules, one for UL QoS flow for UL traffic of the data flow and one for DL QoS flow for DL traffic of the data flow, respectively. PCF shall assign the 5QIs for each of these two PCC rules according to the derived UL PDB and DL PDB. To support UL and DL delay tracking, the QoS monitoring for UL packet delay and the DL packet delay (as defined in clause 6.1.3.21 of TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]) shall be triggered respectively to request tracking the UL packet delay of the QoS flow used in UL and DL packet delay of the QoS flow used in DL independently. Based on the QoS monitoring results, the PCF may readjust the UL PDB and/or DL PDB under the consideration of the RT latency requirement to better fit the new situation. The Uplink-Downlink policy control based on round-trip latency requirement for two unidirectional service data flows is described in clause 6.1.3.27.2 of TS 23.503[ Policy and charging control framework for the 5G System (5GS); Stage 2 ]  [45]. NOTE: How the PCF derives the round-trip latency and takes policy decisions is up to the implementation.