Failure And Ambiguity¶

A valid-looking decode can still be wrong.

Core Idea¶

Protocol inspection fails when field extraction is mistaken for truth. Checksums, parity, status bits, and parsers can prove limited structural facts. They do not prove that a value is fresh, semantically correct, physically true, or acted on by a controller.

Common Failure Modes¶

Failure mode	Looks like	Better interpretation
Wrong dictionary	Plausible speed, pressure, altitude, state	Plausible false value until authority is verified
Stale data	Repeated valid value	Freshness hypothesis, not proof of stable state
Replay or buffering	Coherent sequence	Gateway/logging behavior may hide event time
Timing mismatch	Missing or late message	Sleep, bus load, schedule change, loss, or capture gap
Transport-vs-meaning confusion	Decoded frame or packet	Structure only; semantic source still required
Version mismatch	Decoder works but values drift	Database/profile may not match firmware or installation

Visual Model¶

flowchart LR valid["Structurally valid artifact"] --> decode["Plausible decode"] decode --> wrong["Wrong authority / stale timing"] wrong --> false["Plausible false value"] decode --> checked["Authority + freshness checked"] checked --> hypothesis["Bounded state hypothesis"]

Observer Questions¶

What assumption made this decode possible?
Which authority assigns meaning?
Could the value be stale, cached, replayed, or logged after the event?
Does timing support the claimed state?
What evidence would contradict this hypothesis?

Protocol Examples¶

CAN can carry valid bytes decoded with the wrong DBC. J1939 can expose a PGN while proprietary SPNs remain opaque. ARINC 429 can carry a valid label with an SSM that warns against using the data. MIL-STD-1553 can show a valid transaction whose data words are meaningless without RT maps. Modbus RTU can return valid registers whose engineering units come only from the device map. MDF can preserve samples while metadata controls the physical-value conversion.

Binder Rule¶

Every Tier 1 page must include Failure And Ambiguity. A page that cannot explain how interpretation fails is not ready to teach inspection.