Multipath: add Tempo, an arrival-time scheduler - #522
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July 19, 2026 19:16
A path can blackhole while remaining schedulable: with sparse traffic the congestion window never fills and smoothed_rtt stays frozen at its last-known-good value, so RTT-based schedulers keep picking a dead path forever (measured: 10s link blackout = 100% loss with live alternatives available). A path can also be 'zombie': up and delivering, but parking packets in oversized carrier queues for seconds (measured on a real cellular link: srtt up to 37s at only 7% loss) — worse than dead for interactive traffic, and invisible to PTO-based health because ACKs do eventually arrive. - recovery: expose consecutive_pto_count() (resets on any ACK — a self-reviving health signal) - path: Path::unhealthy() (>= 2 consecutive PTOs) and unhealthy_with() adding an optional smoothed-RTT ceiling; PathMap::max_srtt holds the ceiling - schedulers (minrtt, round-robin): two-pass selection — prefer healthy paths, fall back to blackhole suspects only when no healthy path can send, so a fully-degraded path set still transmits - connection: set_multipath_algorithm() switches the scheduler live, set_scheduler_max_rtt() sets/clears the RTT ceiling, path_health() returns per-path (addrs, srtt, consecutive PTOs, active, unhealthy) for external control planes With the keepalive PING API an application can probe all paths periodically; a dead path accumulates PTOs and is avoided within ~1s, and the first ACK after link recovery restores it automatically. (cherry picked from commit 1f9f12349dd62160d076f10e6f1f478f8a9dee85)
Tempo selects the path where the next packet would ARRIVE earliest: bytes_in_flight / pacing_rate + srtt/2. Compared to MinRtt it distributes load by available capacity rather than piling onto the lowest-RTT path until its cwnd fills, which reduces the reordering that per-packet striping inflicts on heterogeneous paths while still using every path's capacity. Falls back to cwnd/srtt when the congestion controller does not expose a pacing rate. Honors the same path-health rules as the other schedulers (healthy-first, blackhole suspects only as a last resort). (cherry picked from commit 905210667a3f3c4cf6cbdfe2416abdf51cfe66eb)
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Tempo selects the path where the next packet would ARRIVE earliest:
bytes_in_flight / pacing_rate + srtt/2. Compared to MinRtt it
distributes load by available capacity rather than piling onto the
lowest-RTT path until its cwnd fills, which reduces the reordering that
per-packet striping inflicts on heterogeneous paths while still using
every path's capacity.
Falls back to cwnd/srtt when the congestion controller does not expose
a pacing rate. Honors the same path-health rules as the other
schedulers (healthy-first, blackhole suspects only as a last resort).
Note: includes the path-health commit as its base — if the path-health PR merges first this reduces to the Tempo commit alone.
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https://claude.ai/code/session_01LqZK49rKWmBNsihzCrsbgD