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fix(abcip): CompactLogix native default connection size matches libplctag's Large Forward Open (found by T21 latency harness)

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Joseph Doherty
2026-10-07 19:34:30 -04:00
parent 2dc55ecb8f
commit c306c6f674
9 changed files with 101 additions and 22 deletions
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docs/plans/2026-10-07-native-ab-transport-gate-results.md
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@@ -199,9 +199,31 @@ ab_server has 7 atomic scalars, so the 100-tag set was **repeated cyclically** (
| CompactLogix (504, the press default) | 7.7 / 10.9 | 22.1 / 25.3 | 6 | **FAIL** |
| ControlLogix (4002) | 7.3 / 9.1 | 7.7 / 9.2 | 2 | pass |
The CompactLogix result is a real finding, not a harness fault: native opens a 504-byte connection there, while libplctag
opens a Large one (see the T18 capture above), so native needs 3× the round trips per pass. The T21 press result is
recorded under `## Latency (T21)` and is not filled in here.
The CompactLogix result was a real finding, not a harness fault. Native opened a 504-byte connection there, while libplctag
opened a Large one (see the T18 capture above), so native needed 3× the round trips per pass.
**Fixed:** the CompactLogix profile now defaults to 4002, so native opens Large like libplctag. A refusal still falls back to
a regular 504 open. Re-run 2026-10-07 23:30–23:32Z against the `compactlogix` compose profile on :44818 (3 distinct atomic
tags, repeated to 100), with a capture in the container's network namespace:
| Run | libplctag p50 / p95 ms | native p50 / p95 ms | native req/pass | Criterion |
|---|---|---|---|---|
| 1 | — | — | — | aborted in the latency libplctag phase (see below) |
| 2 | 7.6 / 9.6 | 7.5 / 10.8 | 2 | FAIL (limit 10.6) |
| 3 | 7.8 / 9.3 | 8.1 / 9.4 | 2 | PASS (limit 10.2) |
| 4 | 7.7 / 10.0 | 7.8 / 11.7 | 2 | FAIL (limit 11.1) |
| 5 | — | — | — | aborted in the latency libplctag phase |
The structural gap is gone: p50s are equal, and both transports now send 2 requests per pass. At ~10 ms the p95 margin is about
1 ms, which is within this network path's jitter, so the ab_server verdict varies from run to run.
Capture: every session opened with **0x5B (Large Forward Open)** and was accepted (0xDB, status 0). That covers 15 sessions:
four per complete invocation and three in the aborted run 5. Each session had one Forward Close (0x4E to Connection Manager 6/1).
There were **zero** 0x4D/0x53/0x4B. Both aborts (runs 1 and 5) were the simulator: libplctag's ~2 KB packed request was split
across TCP segments (1448 + 606), and ab_server answered with FIN. The harness aborted on the first connection-level failure,
as designed. A real controller reassembles TCP; ab_server does not.
The T21 press result is recorded under `## Latency (T21)` and is not filled in here.
## AdminUI /run (T19)