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fix(srtla-core): keep the rtt floor and stall rejoin honest under load - #26
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the fast and slow min-rtt windows held 10 and 100 samples, sized for one keepalive sample every 300 ms. srtla acks now feed a sample per acked packet, so on a loaded link the "30 s" floor spanned well under a second and tracked a standing queue instead of the propagation floor. the bdp in-flight cap then grew with the queue it should bound, and the queue-building detector could not see a standing queue at all. track both minima over 3 s and 30 s of wall time with a constant-space windowed min (the filter bbr uses), and drop the per-sample pre-filter the time windows make redundant. refs #25
a latched link carries no payload, so the keepalive echo is its only steady delivery proof. three things kept that proof from ever holding fresh for the rejoin dwell on a low-rtt link: - an echo only counted as proof when the rtt probe was armed, which waits for 3 s without any rtt sample - keepalives slipped a whole housekeeping tick whenever one tick was serviced earlier than the last, leaving 2 s gaps - the freshness window floor (1000 ms) equalled the keepalive period, so each echo landed just after the previous one went stale count every timestamped echo as proof, give the keepalive check the same half-tick slack as reconnect pacing, and judge rejoin freshness on a window of at least one keepalive period plus half a tick. engaging the latch still uses the bare staleness window. refs #25
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The title clearly describes the main changes: preserving the RTT floor and improving stall rejoin behavior under load. |
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Review comments at @crates/srtla-core/src/connection/rtt.rs:
- Around line 177-186: Refresh or invalidate the cached RTT minima before
`get_rtt_min_ms()` supplies them to `in_flight_cap_packets`, so enhanced
admission never uses a sample older than its configured window. Update the
`RttTracker` flow to evaluate expiry using the current time, while preserving
the existing dual-window minimum behavior for valid samples.
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crates/srtla-core/src/config_snapshot.rscrates/srtla-core/src/connection/mod.rscrates/srtla-core/src/connection/reconnection.rscrates/srtla-core/src/connection/rtt.rscrates/srtla-core/src/lib.rscrates/srtla-core/src/windowed_min.rssrc/sender/uplink_recv.rssrc/tests/connection_tests.rssrc/tests/stall_deselect_tests.rs
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an srt ack carries last received + 1, but the sender took the packet it names as delivered. that packet is usually still in flight, so it was pruned from the in-flight count and its few-millisecond age became an rtt sample far below the real round trip (6-8 ms on 70 ms links in the issue #25 rerun). with the rtt floor now held for 30 s, one such sample shrinks every link's bdp cap to a packet and the cap stops gating. step the field back one sequence before the ownership lookup and the cumulative prune, matching moblin's sn < ack. refs #25
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Refs #25.
In #25 a bufferbloated leg carries most of the stream while the clean leg sits idle. The run 3 debug log shows two causes in our code.
The RTT floor follows the queue. The fast and slow min RTT windows held 10 and 100 samples. The sizes assumed one keepalive sample every 300 ms. SRTLA ACKs now feed one sample per acked packet (since a094863), so on a loaded link the "30 s" floor covered well under a second. On a leg with a standing queue,
rtt_minbecame the queue delay. The BDP in-flight cap then grew with the queue: about 80 packets on an 800 ms leg against 2 on the clean leg. The queue-building detector compares the fast and slow minima, so it could not see a standing queue at all.A stall-latched link could not rejoin. In the log the clean leg stays latched from 38.7 s to 54.6 s. A latched link carries no payload, so its steady delivery proof is the keepalive echo. Three things kept that proof from staying fresh for the rejoin dwell:
The SRT ACK field was read off by one. An SRT ACK carries last received + 1, the first sequence still owed. The sender treated that packet as delivered, so it pruned a packet still in flight and sampled its age as an RTT. In the #25 rerun
rtt_minread 6 to 8 ms on legs with 30 to 35 ms of netem delay. With the floor held for 30 s, one such sample shrinks every cap to 1 packet, every leg is then over cap, and the cap stops gating.Changes
windowed_min: a constant-space windowed minimum (the Nichols filter that BBR uses).RttTrackeruses it for 3 s and 30 s windows measured in wall time. The per-sample pre-filter is gone because the time windows make it redundant.SrtlaConnection::handle_keepalive_echo: every timestamped echo is delivery proof. It is still an RTT sample only when armed.needs_keepalivegets the same half-tick slack as reconnect pacing.packet_handlerconverts the ACK field withseq::srt_ack_last_acked(one sequence back, as Moblin'ssn < ack) before the ownership lookup and the cumulative prune.handle_srt_ackkeeps its "at or before" contract on the converted value.max(staleness window, STALL_REJOIN_PROOF_FLOOR_MS), which is one keepalive period plus half a tick. Engaging the latch still uses the bare staleness window, so the gate drops a link just as fast.Review notes
crates/srtla-core/src/windowed_min.rs. Its estimate is approximate by design: when the best sample expires, the next best can sit slightly above the true windowed minimum. It never reports a sample older than the window.keepalive_echoes_alone_release_a_low_rtt_linkfails without the proof floor. The echoes in it jitter by a few ms on purpose, because perfectly periodic echoes pass either way.Risks and follow-ups
--mode classicdoes not use the cap or the classifier, so the classic-mode results in Enhanced mode: a bufferbloated leg ends up carrying most of the stream while the clean leg sits nearly idle #25 change only through the stall rejoin fix.Summary by CodeRabbit