Draft: draft-ietf-core-senml

Commentary: and give an example how now - referenced data might work for SensML .
Sensor Streaming Measurement List (see sec-sensml).
One or more SenML Records to be processed as a stream.
Obviously, "now"-referenced SenML records are only useful within a
specific communication context (e.g., based on information on when
the SenML pack was sent) or together with some other context
information that can be used for deriving a meaning of "now"; the
expectation for any archival use is that they will be processed into
UTC-referenced records before that context would cease to be
available.  This specification deliberately leaves the accuracy of
"now" very vague as it is determined by the overall systems that use
SenML.  In a system where a sensor without wall-clock time sends a
SenML record with a "now"-referenced time over a high speed RS 485
link to an embedded system with accurate time that resolves "now"
based on the time of reception, the resulting time uncertainty could
be within 1 ms.  At the other extreme, a deployment that sends SenML
wind speed readings over a LEO satellite link from a mountain valley
might have resulting reception time values that are easily a dozen
minutes off the actual time of the sensor reading, with the time
uncertainty depending on satellite locations and conditions.
Obviously, "now"-referenced SenML records are only useful within a
specific communication context (e.g., based on information on when
the SenML pack, or a specific record in a SensML stream, was sent) or
together with some other context information that can be used for
deriving a meaning of "now"; the expectation for any archival use is
that they will be processed into UTC-referenced records before that
context would cease to be available.  This specification deliberately
leaves the accuracy of "now" very vague as it is determined by the
overall systems that use SenML.  In a system where a sensor without
wall-clock time sends a SenML record with a "now"-referenced time
over a high speed RS 485 link to an embedded system with accurate
time that resolves "now" based on the time of reception, the
resulting time uncertainty could be within 1 ms.  At the other
extreme, a deployment that sends SenML wind speed readings over a LEO
satellite link from a mountain valley might have resulting reception
time values that are easily a dozen minutes off the actual time of
the sensor reading, with the time uncertainty depending on satellite
locations and conditions.
In some usage scenarios of SenML, the implementations MAY store or
transmit SenML in a stream-like fashion, where data is collected over
time and continuously added to the object.  This mode of operation is
optional, but systems or protocols using SenML in this fashion MUST
specify that they are doing this.  SenML defines separate media types
to indicate Sensor Streaming Measurement Lists (SensML) for this
usage (see sec-sensml-json).  In this situation, the SensML stream
can be sent and received in a partial fashion, i.e., a measurement
entry can be read as soon as the SenML Record is received and does
not have to wait for the full SensML Stream to be complete.
Note that in some usage scenarios of SenML the implementations MAY
store or transmit SenML in a stream-like fashion, where data is
collected over time and continuously added to the object.  This mode
of operation is optional, but systems or protocols using SenML in
this fashion MUST specify that they are doing this.  SenML defines
separate media types to indicate Sensor Streaming Measurement Lists
(SensML) for this usage (see sec-sensml-json).  In this situation the
SensML stream can be sent and received in a partial fashion, i.e., a
measurement entry can be read as soon as the SenML Record is received
and not have to wait for the full SensML Stream to be complete.