Host rules

Box contents, card placement, and interface assignments live in state.md. This file carries the rules that stay true regardless of what's plugged in.

Interfaces

Test pair: the two ports whose modules hold the cable under test. Find by driver name, never by ethN — names shift with kernel link order.
LAN uplink: carries the default route. Never repurpose or down it; identify by default route, not name.
Test-path ports live in NetworkManager's unmanaged list (/etc/NetworkManager/conf.d/99-unmanaged-10g.conf), up with no IPv4.

Module identities, EEPROM honesty (and why the X520 needs allow_unsupported_sfp=1), and the physical-layer model live in modules/.

Required for zero baseline loss (without them baseline loss is 0.02–0.3% and masks cable faults) and for the power budget.

CPU governor → performance on all cores. Biggest single contributor: under powersave, cores idle at 400 MHz, ramp too slowly, and drop frames at startup.
CPU max frequency → 2.0 GHz (scaling_max_freq, all cores). Under the performance governor this is a pinned clock, not a ceiling, and it is the power budget: ~30 W package at line rate against ~80 W unbounded (measurement.md, "Power"). The value is host-silicon-bound — 1.6 GHz still holds line rate with thinner margin, 1.2 GHz cannot fill the wire — so re-derive it on new host silicon and re-qualify zero-loss.
Disable adaptive coalescing: ethtool -C <dev> adaptive-rx off adaptive-tx off rx-usecs 25 tx-usecs 25. Syntax/values differ per driver — re-derive per NIC.
Rings deep, raised toward the max via ethtool -G (the tool asks RX 8160 / TX 4096; ixgbe takes it as-is). Effect was not isolated in testing; don't claim it as a proven win.
Socket buffers: no sysctl change needed — the tool forces SO_RCVBUFFORCE/SO_SNDBUFFORCE (needs root) past the 4 MB rmem_max clamp.

Verify a clean run shows tx frames == rx frames exactly before a run counts.

Through ./harness: sudo go run ./harness -for 8s -at 4s -- -a <portA> -b <portB>.

The harness starts go run . in its own process group and kills the whole group on exit, confirming with kill(-pgid,0)==ESRCH. Do not wrap in timeout (kills go run but orphans the compiled binary to PID 1); do not follow runs with pgrep orphan checks — the group kill is the guarantee.
It dumps the panel to shots/<offset>.png for remote assessment.
The tell for two competing instances (an orphan, or the user's own hand-run): RX pps > TX pps from cross-received frames on shared ethertypes/MACs. Ask rather than killing something that might be theirs.
Default config only — no -sizes/-streams overrides, no editing the hardcoded frameSizes/numStreams/batchSize (same violation). The default is the config that must work and the only one comparable across runs; small-frame runs measure the host, not the cable. Ask before running a specific non-default case.
The first ~5 s of any run is a settling transient (flow rules, rings, workers coming up) — not residual error.