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Eclipse Tahu addresses the existence of legacy SCADA/DCS/ICS protocols and infrastructures and provides a much-needed definition of how best to apply MQTT into these existing industrial operational environments.

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Must NBIRTH sequence number always be '0'?

SeppPenner opened this issue · comments

commented

A question from my community under SeppPenner/SparkplugNet#57:

For me it is not quite clear, if the NBIRTH sequence number must always be '0' as it is currently implemented. The SparkplugB 3.0.0 documentation is ambiguous. In chapter 6.4.19 the sequence number "MUST have a value between 0 and 255". But in chapter 12.13. it is written "The NBIRTH MUST include a sequence number in the payload and it MUST have a value of 0."

The following theoretical example shows wrong data if the NBIRTH sequence number is always '0':
Let's assume, the 'bdseq' remains the same. The following messages are sent:
-> NBIRTH (seq=0)
-> NDATA (seq=1)
<- receiving NCMD "Node Control/Rebirth"
-> NBIRTH (seq=0)
Now let's assume the host application receives the 2nd NBIRTH message before the NDATA message. As a result, the host application would process the NDATA message as the sequence number is higher than the sequence number of the 2nd NBIRTH. This may result in wrong data. Unless the timestamp is taken into account as well.

Hence my question: what is the correct behavior of NBIRTH sequence number from your point of view? Thank you very much for any thoughts.

So: How should the sequence number behave?

commented

NBIRTH sequence number must always be 0