Section: 5.33.3.3 GTP-U Path Measurement
Content: The SMF can request to activate QoS monitoring for the GTP-U path(s) between all UPF(s) and the (R)AN based on locally configured policies. Alternatively, when a QoS monitoring policy is received in a PCC rule and the QoS monitoring is not yet active for the DSCP corresponding to the 5QI in the PCC rule, the SMF activates QoS Monitoring for all UPFs currently in use for this PDU Session and the (R)AN. In this case, the SMF performs the QoS Flow Binding without taking the QoS Monitoring Policy within the PCC rule into account. The SMF sends the QoS monitoring policy to each involved UPF and the (R)AN via N4 interface and via N2 interface respectively. NOTE 1: The PCC rule containing a QoS monitoring policy is just a trigger for the SMF to instruct the UPFs to initiate the GTP-U based QoS Monitoring. A GTP-U sender performs an estimation of RTT to a GTP-U receiver on a GTP-U path by sending Echo messages and measuring time that elapses between the transmission of Request message and the reception of Response message. A GTP-U sender computes an accumulated packet delay by adding RTT/2, the processing time and, if available, an accumulated packet delay from an upstream GTP-U sender (i.e. an immediately preceding GTP-U sender in user plane path) thus the measured accumulated packet delay represents an estimated elapsed time since a user plane packet entered 3GPP domain. It is expected that a GTP-U sender determines RTT periodically in order to detect changes in transport delays. QoS monitoring is performed by a GTP-U end-point (UP function) that receives, stores and executes the QoS monitoring policy as described below for a QoS Flow. QoS monitoring is performed by comparing a measured accumulated packet delay with the stored parameters. If the GTP-U end-point (the PSA UPF, in the case of accumulated packet delay reporting) determines that the packet delay exceeds the value of a stored parameter, then the node triggers QoS monitoring alert signalling to the relevant SMF or to the OA&M function as further described in TS 29.244[ Interface between the Control Plane and the User Plane nodes ]  [65]. NOTE 2: Echo Request message and Echo Response message are sent outside GTP-U tunnels (the messages are using TEID set to 0). If underlying transport is using QoS differentiation (e.g. IP DiffServ) then it is up to the implementation to ensure that the Echo messages are classified correctly and receive similar treatment by the underlaying transport as GTP-U GTP-PDUs carrying QoS Flows (user data). When QoS Monitoring is used to measure the packet delay for a QoS Flow, the following applies: - Packet delay measurement is performed by using GTP-U Echo Request/Response as defined in the TS 28.552[ Management and orchestration; 5G performance measurements ]  [108], in the corresponding user plane transport path(s), independent of the corresponding PDU Session and the 5QI for a given QoS Flow. - RAN measures and provides the RAN part of UL/DL packet delay towards UPF (in the GTP-U header of the respective QoS Flow via N3). - The UPF calculates the UL/DL packet delay by combining the received measurements of RAN part with the measurements of N3/N9 interface (N9 is applicable when I-UPF exists). - The UPF reports the QoS Monitoring results as described in clause 5.8.2.18. QoS Monitoring can also be used to measure the packet delay for transport paths to influence the mapping of QoS Flows to appropriate network instances, DSCP values as follows: - SMF activates QoS monitoring for the GTP-U path(s) between all UPF(s) and all (R)AN nodes based on locally configured policies. - UPF does measurement of network hop delay per transport resources that it will use towards a peer network node identified by an IP destination address (the hop between these two nodes) and port. The network hop measured delay is computed by sending an Echo Request over such transport resource (Ti) and measuring RTT/2 when Echo Response is received. - UPF maps {network instance, DSCP} into Transport Resource and measures delay per IP destination address and port. Thus, for each IP destination address, the measured delay per (network instance, DSCP) entry is determined. - The UPF performing the QoS monitoring can provide the corresponding {Network instance, DSCP} along with the measured accumulated packet delay for the corresponding transport path to the SMF. The UPF reports the measurement results to the SMF based on some specific conditions e.g. first time, periodic, event triggered, when thresholds for reporting towards SMF (via N4) are reached. - Based on this, SMF can determine QoS Flow mapping to the appropriate {Network instance, DSCP} considering {5QI, QoS characteristics, ARP} for the given QoS Flow.