Root cause of "connection takes 20-30s, sometimes 1-2 minutes, to reach
stable throughput" (trickle of KB for a while, then a sudden jump to full
speed): on_loss during SlowStart unconditionally halved cwnd AND
permanently switched to ProbeBandwidth's linear (+1 MTU/RTT) growth on the
very FIRST loss. Real mobile/Wi-Fi links have a non-zero background loss
rate from ordinary wireless noise and handover blips that has nothing to
do with congestion; on such a link the first RTT or two of slow start would
hit a loss, get knocked into linear growth from a still-small window, and
take an enormous number of RTTs to claw back up to full speed - directly
contradicting the module's own stated BBR-inspired design intent, since
real BBR is deliberately loss-tolerant during startup instead of treating
any loss as a hard congestion signal.
Fix: track losses within a short (500ms) window and only pay the full
exit-slow-start-and-halve cost once SLOW_START_LOSS_TOLERANCE (3) losses
land within it - sustained loss is still treated as real congestion. A
single isolated loss now takes a mild, temporary haircut (cwnd *= 0.8) but
stays in slow start, so exponential growth continues instead of being
abandoned over a one-off dropped packet.
- Karn's algorithm: drop_acked_frames no longer samples RTT from frames
that were retransmitted (last_sent is bumped on each retransmit, so an
ACK for the original transmission would measure a spuriously small RTT
and drag SRTT/RTO down). Added CongestionController::on_ack_no_rtt for
the case where every acked frame was ambiguous, so the window still
advances without polluting the RTT estimator. Refactored the shared
window-growth into grow_window.
- Frame decode: header+payload+pad length now uses checked_add. payload_len
is a u32 from the header and on 32-bit targets (MIPS/ARMv7 routers are
supported) the sum could wrap usize and slip past the truncation check.
- Replay cache: a full cache used to reject ALL new handshakes globally
until the next tick, letting one flooding key-holder deny service to
everyone. Now it reclaims expired entries and, if still full, evicts the
single oldest — new handshakes always get in. Fixed the mislabelled
"100000" log (cap is 50000) and named it REPLAY_CACHE_MAX.
ospab
ospab