A packet capture is the only record of what actually crossed the wire. It is also the most over-claimed artefact in substation work: people treat “we have a capture” as if it were “we have a result”. This note separates the two.
Why the capture is the only witness
A configuration file records intent. A device screen records an interpretation. Only the wire records behaviour: the bytes, in order, with timestamps. That is why every serious claim about a substation network eventually lands on a capture — and why a tool that cannot read one is describing a system it has not seen.
The mistake starts one step later. A capture proves that something happened. It does not prove that what happened was correct, complete, or caused by what you think.
What a GOOSE capture can prove
Read field by field, a capture answers a specific set of questions with no interpretation required:
| Question | What in the frame answers it |
|---|---|
| Was the message actually published? | The frame exists, with a timestamp |
| Which stream is it? | Destination MAC, APPID, VLAN ID and priority |
| Which control block, and which version of it? | gocbRef, datSet, confRev |
| Is this a new state or a retransmission? | stNum and sqNum |
| Is the publisher keeping its declared rhythm? | Intervals against timeAllowedToLive |
| Does the frame match the configuration? | Field values compared with the SCD, not with memory |
None of those rows needs the standard’s blessing. They are observations, and they are checkable by a second person with the same file.
What a capture cannot prove
This list is the important one, because every item on it has been presented as proven by a capture somewhere:
- Conformance. A capture shows one exchange, not compliance with a standard. Conformance is a test against requirements, normally by an accredited laboratory.
- Why the publisher sent it. The frame is the output of internal state you cannot see; you can infer, you cannot know.
- That nothing else is wrong. The capture has a window. Behaviour outside it is unobserved, and “no errors in the file” is not “no errors”.
- That your own implementation is interoperable. A capture of a peer shows the peer. It says nothing about whether your encoder writes what that peer would accept.
- Who produced the capture. Unless the file states the product and version, it is anonymous bytes — useful, but not attributable.
- Anything about a different protocol. GOOSE timing says nothing about sampled values, and an MMS read says nothing about a trip circuit.
Five things to record with every capture
A capture without this header becomes an anecdote within a week. The header costs five minutes:
| Field | Why it matters |
|---|---|
| Who produced it | Product and version; “a relay” is not a source |
| When, and for how long | The window is part of the evidence |
| How it was taken | Port mirror, TAP, or a host capture — each has blind spots |
| Clock source | Timestamps are only comparable if the clock is known |
| What was supposed to happen | The test intent, so “pass” has a definition |
An anonymised capture is still evidence. A capture with no provenance is a rumour with a checksum.
The case that changed how we test
In the project behind this note, the protocol tests were symmetric: our encoder produced bytes, our decoder read them back, and the tests passed. Five wire-format defects survived that arrangement, because both sides shared the same mistake.
| Defect | What symmetric tests showed | What foreign bytes showed |
|---|---|---|
| Identifier encoded as an OCTET STRING | green | a conformant peer reads a different type |
| Conclude with wrong choice tags | green | the release silently fails |
| Name-list service tags one too high | green | the request is not recognised |
| Read-Request access read from the wrong field | green | an empty list, no error |
| Session/presentation/ACSE layers skipped | green | no association at all |
The lesson is not “write more tests”. It is that a test written from one implementation’s output cannot find a mistake that implementation makes. The fix was to pin assertions to bytes produced by another implementation.
Turning a capture into evidence
Four rules, in order. Skip one and the evidence weakens:
- The assertion must fail before the fix. Otherwise you have described the current behaviour, not found a defect.
- The bytes under assertion must come from a different implementation. Your own output is a regression test, not evidence.
- The file is hashed and kept. A result that cannot be re-run against the same bytes is a memory.
- The limits are written down. What the capture does not cover belongs next to what it does.
What this means in practice
For an engineer: when someone hands you a capture, ask for the header before the analysis. For a vendor: if a page claims interoperability, ask which peer, which version, and whether a peer ever accepted your request. The answer is usually shorter than the page.
In the tool described on this site, the position is deliberate and uncomfortable: a capture of a peer is not proof that our own requests are accepted. That is why the interoperability claim is still open, and why the evidence page says so in plain words.
A capture is evidence about a moment, not a verdict about a system. The difference is the whole discipline.
The canonical version, with the Persian translation: https://rezafarsi.com/en/articles/goose-capture-evidence/
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