Draw the views: software, transaction, link/channel and physical/timing.
Record the transaction identity and its scope: port, ID/tag, address, endpoint or cache line.
Mark flow control: READY, grant, credit, FIFO, retry or link state.
State the applicable ordering rule, completion boundary and software ownership contract.
Choose a metric: latency, throughput, errors, retries or measured margin.
Collect the matching waveform, monitor, analyser, firmware or counter evidence.
Propose one reduced experiment and one product regression.
The best protocol answers are layered, measurable, and tied to a specific transaction.
Never answer with only a block diagram; carry the failing transaction through the diagram.