Continuous in-network round-trip time monitoring

Round-trip time (RTT) is a central metric that influences end-user QoE and can expose traffic-interception attacks. Many popular RTT monitoring techniques either send active probes (that do not capture application-level RTTs) or passively monitor only the TCP handshake (which can be inaccurate, especially for long-lived flows). High-speed programmable switches present a unique opportunity to monitor the RTTs continuously and react in real time to improve performance and security. In this paper, we present Dart, an inline, real-time, and continuous RTT measurement system that can enable automated detection of network events and adapt (e.g., routing, scheduling, marking, or dropping traffic) inside the network. However, designing Dart is fraught with challenges, due to the idiosyncrasies of the TCP protocol and the resource constraints in high-speed switches. Dart overcomes these challenges by strategically limiting the tracking of packets to only those that can generate useful RTT samples, and by identifying the synergy between per-flow state and per-packet state for efficient memory use. We present a P4 prototype of Dart for the Tofino switch, as well our experiments on a campus testbed and simulations using anonymized campus traces. Dart, running in real time and with limited data-plane memory, is able to collect 99% of the RTT samples of an offline, software baseline---a variant of the popular tcptrace tool that has access to unlimited memory.