Udpstp and the packets that never left

udpstp is a Rust implementation of the UDP Speed Test Protocol, designed to measure network capacity, packet loss, and delay in either direction. Its rate-search test uses feedback about loss and delay while sending traffic in bursts. The central argument is that results depend not only on the network but also on what the sender actually manages to transmit: packets waiting inside a process must not be mistaken for network loss, and delayed work must not be dumped onto the path in a way that distorts the load.

When a send plan contains six packets but the operating system accepts only two, udpstp carries the four unsent packets into the next plan with their original sequence numbers and fresh timestamps. This preserves sequence continuity without counting time spent waiting in the sender as network delay. If the backlog reaches a burst’s size, new traffic is withheld rather than allowing the queue to grow. A separate catch-up rule handles late timers: bursts less than a set window behind remain eligible, older ones are skipped, and the most recent missed burst is retained. The window is one millisecond on Linux and macOS and two on Windows; tests verify partial sends, timing boundaries, and recovery after delays. Results distinguish intended sending rate from traffic actually supplied, while warnings and logs expose sender shortfalls. The tool runs on Linux, macOS, and Windows, with terminal or JSON output for comparing paths and configurations.