Scout: User Plane Telemetry for NextG Cellular Networks

5G's shift to disaggregated O-RAN architectures and high-capacity radio introduces unprecedented transport-layer complexity precisely as the ecosystem demands predictable, consistent low latency transport and mission-critical usecases. To understand and ultimately control latency, we need an accurate view of a packet's journey through this stack. This raises a central question: can network-centric methods suffice, or does an application-centric view demand a broader lens? The user plane is where application performance is realized, yet existing telemetry remains largely control-plane and KPI focused, missing the compound effects of protocols, interfaces, and buffers that packets must traverse. We address this gap with Scout, a packet-centric observability approach to the 5G user plane that spans the transport stack. By following packet trajectories, Scout reconstructs how application traffic is shaped inside the RAN and reveals how application-visible latency and transport dynamics arise from protocol processing, internal buffering, radio scheduling, packet loss, as well as platform-level execution effects. This packet-centric view enables the first full-stack attribution of latency within the user plane. Through end-to-end experiments, we show that application behavior can be directly observed and explained from within the network itself, collapsing the boundary between application experience and internal network dynamics.