The startup speed-test probe deliberately overloads the link (2 Gb/s for
800 ms) to measure capacity. The report tick is suppressed while it runs
and the window anchors are rebased when it ends — but two residues leak
past the rebase: probe frames still pending in the reassembler age out as
frames_dropped for another ~120 ms (the loss-window fuse), and the burst
can latch flush_in_window (a jump-to-live it caused itself). Either one
reads as a SEVERE window: the controller backs off ×0.7 and — worse —
slow start ends at the first congestion signal, permanently.
That is precisely the 2026-07 field report's Automatic session: 20→14
Mb/s exactly one report tick after the probe, then a purely additive
+6 %/4.5 s crawl that took the whole 4.5-minute match to reach 308 Mb/s
on a link the probe had just measured at ~1.2 Gb/s. Slow start (doubling
per clean window to the probed ceiling, 'seconds instead of minutes')
never got to run.
The first post-probe report window is now discarded outright: no
LossReport (probe residue would also spike the host's adaptive FEC), the
standing-latency detector closes it as not-loss-free (its clean-run just
resets), and the ABR is fed nothing.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehlerandClaude Fable 5
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