Internet DRAFT - draft-groves-core-senml-bto
draft-groves-core-senml-bto
CoRE Working Group C. Groves
Internet-Draft
Intended status: Standards Track W. Yang
Expires: October 21, 2017 Huawei
April 19, 2017
SenML Base Time Offset Attribute
draft-groves-core-senml-bto-01
Abstract
SenML [I-D.ietf-core-senml] defines a base time attribute and time
value which is used to determine the time when a value is recorded.
In some applications a SenML pack will contain a series of records
related to a constant sample time interval, e.g. once every 60
seconds. This means that the time attribute will be required for
each record. This document defines a new "time offset" base
attribute that allows a sender to include the time for the sample
interval between records. If the "time offset" base attribute is
used the sender will not send the time attribute for each record,
minimising message and storage size.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SenML Structure and Semantics . . . . . . . . . . . . . . . . 4
3.1. Base Attributes . . . . . . . . . . . . . . . . . . . . . 4
3.2. Regular Attributes . . . . . . . . . . . . . . . . . . . 4
3.3. Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. JSON Representation (application/senml+json) . . . . . . . . 5
5. CBOR Representation (application/senml+cbor) . . . . . . . . 5
6. XML representation (application/senml+xml) . . . . . . . . . 5
7. EXI Representation (application/senml-exi) . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 8
12. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
SenML currently defines the base time "bt" and time "t" attributes to
indicate the time that each value in a SenML record is recorded.
This means that for each record (value "v") that a "t" attribute is
required. The example from section 5.1.2/[I-D.ietf-core-senml] is
copied below to illustrate a SenML pack with multiple records.
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[ {"bn": "urn:dev:ow:10e2073a01080063",
"bt": 1320067464,
"bu": "%RH",
"v": 21.2, "t": 0 },
{ "v": 21.3, "t": 10 },
{ "v": 21.4, "t": 20 },
{ "v": 21.4, "t": 30 },
{ "v": 21.5, "t": 40 },
{ "v": 21.5, "t": 50 },
{ "v": 21.5, "t": 60 },
{ "v": 21.6, "t": 70 },
{ "v": 21.7, "t": 80 },
{ "v": 21.5, "t": 90 },
...
Figure 1: SenML pack with multiple records
As can be seen in the example above there is a fixed offset between
the data points of 10 seconds.
This document proposes a base time offset "bto" attribute that is
used to indicate the offset between datapoints when a fixed offset is
used. This negates the need to include the "t" attribute for each
data point. The example below takes the above example and applies
the base time offset to it.
[ {"bn": "urn:dev:ow:10e2073a01080063",
"bt": 1320067464,
"bto": 10,
"bu": "%RH",
"v": 21.2},
{ "v": 21.3},
{ "v": 21.4},
{ "v": 21.4},
{ "v": 21.5},
{ "v": 21.5},
{ "v": 21.5},
{ "v": 21.6},
{ "v": 21.7},
{ "v": 21.5},
...
Figure 2: SenML pack with multiple records using base time offset
It can be seen that it results in a record and overall pack size
reduction. The receiver of the record would use the base time "bt"
for the initial data point, e.g. "v": 21.2 would have a timestamp of
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1320067464. Each subsequent record would have a time stamp equal to
the previous record plus the base time offset "bto", e.g. "v": 21.3
would have a timestamp of 1320067474, "v": 21.4 would have a
timestamp of 1320067484, and so on.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
See [I-D.ietf-core-senml] for further definitions.
3. SenML Structure and Semantics
3.1. Base Attributes
This document adds an additional "base time offset" attribute.
Base Time Offset: The base time offset represents a fixed time offset
between each record in a SenML pack. The first record represents
time zero (or the base time if provided) and the offset is then
applied to each subsequent record. Either a positive or negative
base time offset is allowed.
3.2. Regular Attributes
This document modifies the behaviour of the time attribute.
Time: If the base time offset attribute is used in a SenML pack then
the time attribute shall not be used in the individual records.
_Editor's note: One theoretical use case may be to include the time
attribute if the entry's time deviates from the regular offset. It
is not seen that such a corner case warrants the extra complexity._
3.3. Considerations
To simplify implementation, the use of base time offset is limited
to:
o Fixed time increasing or decreasing records from the time stamp
associated with the first record in the SenML pack. This means
that usages such as described by the 2nd example in
5.1.2/[I-D.ietf-core-senml] where negative time offset from time
zero being provided by the last record in a SenML pack are not
possible.
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_Editor's note: It could be possible to facilitate this use case by
allowing a t=0 to be set on the last record with the use of a
negative base time offset value. However again it's not seen that
such a corner case warrants the extra complexity in having to store
values and calculate offsets at the end of transmission/reception._
o It is not possible to for two records in the SenML pack to have
the same time. For example the inclusion of a record for a
voltage measurement followed by a current measurement would result
in the records having times with a difference of the time offset.
4. JSON Representation (application/senml+json)
This document defines an additional SenML label (JSON object member
name) as shown in Table 1 below.
+------------------+-------+-------+
| Name | label | Type |
+------------------+-------+-------+
| Base Time Offset | bto | Numer |
+------------------+-------+-------+
Table 1: JSON SenML Labels
5. CBOR Representation (application/senml+cbor)
As per section 6/[I-D.ietf-core-senml], for CBOR the string map key
"bto" shall be used to indicate the use of the base time offset.
6. XML representation (application/senml+xml)
This document defines an addition XML attribute as shown in Table 2
below.
+------------------+-----+--------+
| Name | XML | Type |
+------------------+-----+--------+
| Base Time Offset | bto | double |
+------------------+-----+--------+
Table 2: XML SenML Labels
The RelaxNG schema for the XML is:
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senml = element senml {
attribute bn { xsd:string }?,
attribute bt { xsd:double }?,
attribute bv { xsd:double }?,
attribute bs { xsd:double }?,
attribute bu { xsd:string }?,
attribute bver { xsd:int }?,
attribute bto { xsd:double }?,
attribute l { xsd:string }?,
attribute n { xsd:string }?,
attribute s { xsd:double }?,
attribute t { xsd:double }?,
attribute u { xsd:string }?,
attribute ut { xsd:double }?,
attribute v { xsd:double }?,
attribute vb { xsd:boolean }?,
attribute vs { xsd:string }?,
attribute vd { xsd:string }?
}
sensml =
element sensml {
senml+
}
start = sensml
7. EXI Representation (application/senml-exi)
As per clause 8/[I-D.ietf-core-senml] extensions are indicated
through the use of an EXI schemaID options. The EXI schemaID options
MUST be set to the value of "b" indicating the scheme provided in
this specification.
The following is the XSD Schema to be used for strict schema guided
EXI processing.
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<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
targetNamespace="urn:ietf:params:xml:ns:senml"
xmlns:ns1="urn:ietf:params:xml:ns:senml">
<xs:element name="senml">
<xs:complexType>
<xs:attribute name="bn" type="xs:string" />
<xs:attribute name="bt" type="xs:double" />
<xs:attribute name="bv" type="xs:double" />
<xs:attribute name="bu" type="xs:string" />
<xs:attribute name="bver" type="xs:int" />
<xs:attribute name="bto" type="xs:double" />
<xs:attribute name="l" type="xs:string" />
<xs:attribute name="n" type="xs:string" />
<xs:attribute name="s" type="xs:double" />
<xs:attribute name="t" type="xs:double" />
<xs:attribute name="u" type="xs:string" />
<xs:attribute name="ut" type="xs:double" />
<xs:attribute name="v" type="xs:double" />
<xs:attribute name="vb" type="xs:boolean" />
<xs:attribute name="vs" type="xs:string" />
<xs:attribute name="vd" type="xs:string" />
</xs:complexType>
</xs:element>
<xs:element name="sensml">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" ref="ns1:senml" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
8. Security Considerations
No extra security issues are seen beyond those described in
[I-D.ietf-core-senml].
9. IANA Considerations
This document proposes one new entry in the IANA SenML label
reegistry.
Label Name: Base Time Offset
Label: bto
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CBOR: -
XML Type: Double
ID: b
Reference: This specification.
10. Acknowledgements
TBD
11. Changelog
draft-groves-core-senml-bto-01
o General: Changed "SenML Package" to "SenML Pack"
12. Normative References
[I-D.ietf-core-senml]
Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Media Types for Sensor Measurement Lists
(SenML)", draft-ietf-core-senml-05 (work in progress),
March 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Christian Groves
Australia
Email: cngroves.std@gmail.com
Weiwei Yang
Huawei
P.R.China
Email: tommy@huawei.com
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