Internet DRAFT - draft-ietf-core-senml-more-units
draft-ietf-core-senml-more-units
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track 19 March 2020
Expires: 20 September 2020
Additional Units for SenML
draft-ietf-core-senml-more-units-06
Abstract
The Sensor Measurement Lists (SenML) media type supports the
indication of units for a quantity represented. This short document
registers a number of additional unit names in the IANA registry for
Units in SenML. It also defines a registry for secondary units that
cannot be in SenML's main registry as they are derived by linear
transformation from units already in that registry.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 20 September 2020.
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Copyright (c) 2020 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. New Primary Units . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 3
3. New Registry for Secondary Units . . . . . . . . . . . . . . 4
4. Operational Considerations . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Sensor Measurement Lists (SenML, [RFC8428]) media type supports
the indication of a unit, using the SenML field "u", for the quantity
given as a data value in a SenML record. For this purpose, SenML
defines an IANA registry of defined Unit names and their meanings; in
the present document, we call the Unit names registered there
"primary Unit names".
This short document registers a number of additional units in the
IANA registry for Units in SenML that appear to be necessary for
further adopting SenML in other Standards Development Organizations
(SDOs).
The document also defines a registry for secondary Unit names that
cannot be in SenML's main registry as they are derived by linear
transformation from units already in that registry. Although SenML
version 10 [RFC8428] does not provide for the direct use of these
secondary units, they are planned to be supported via the use of
SenML extension mechanisms, one of which is proposed in
[I-D.bormann-core-senml-versions].
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. New Primary Units
IANA is requested to assign new units in the "SenML Units"
subregistry of the SenML registry [IANA.senml] (as defined in
[RFC8428]):
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+--------+--------------------------------------+-------+-----------+
| Symbol | Description | Type | Reference |
+========+======================================+=======+===========+
| B | Byte (information content) | float | RFCthis |
+--------+--------------------------------------+-------+-----------+
| VA | volt-ampere (Apparent Power) | float | RFCthis |
+--------+--------------------------------------+-------+-----------+
| VAs | volt-ampere second (Apparent | float | RFCthis |
| | Energy) | | |
+--------+--------------------------------------+-------+-----------+
| var | volt-ampere reactive (Reactive | float | RFCthis |
| | Power) | | |
+--------+--------------------------------------+-------+-----------+
| vars | volt-ampere reactive second | float | RFCthis |
| | (Reactive Energy) | | |
+--------+--------------------------------------+-------+-----------+
| J/m | joule per meter (Energy per | float | RFCthis |
| | distance) | | |
+--------+--------------------------------------+-------+-----------+
| kg/m3 | kilogram per cubic meter (mass | float | RFCthis |
| | density, mass concentration) | | |
+--------+--------------------------------------+-------+-----------+
| deg | degree (angle)* | float | RFCthis |
+--------+--------------------------------------+-------+-----------+
Table 1: New units registered for SenML
2.1. Rationale
SenML [RFC8428] takes the position that unscaled SI units should
always be used. However, SenML makes one exception: The degree
Celsius (as Cel) is allowed as an alternative to the K (Kelvin).
This document takes the position that the same should apply to a
small number of alternative units in wide use:
* The Byte. [IEC-80000-13] defines both the bit (item 13-9.b) and
the byte (item 13-9.c, also called octet) as alternative names for
the coherent unit used for dimensionless quantities, for the
purpose of giving storage capacity and related quantities. While
the name octet is associated with the symbol o, this is in wide
use only in French-speaking countries. Globally more wide-spread
is the symbol B for byte, even though B is already taken in SI for
bel. [RFC8428] therefore registers dB as the SenML unit for
logarithmic relative power, leaving B free for the usage proposed
here. While this is potentially confusing, the situation is
widely understood in engineering circles and is unlikely to cause
actual problems.
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* The Volt-Ampere. [IEC-80000-6]} item 6-57.a defines the VA (volt
ampere) as a unit for apparent power; items 6-59.a, 6-60.a and
6-61.a also use the unit for complex, reactive, and non-active
power.
* The Volt-Ampere-reactive. [IEC-80000-6] item 6-60.b defines the
var (volt ampere reactive) as an alternative (and fully
equivalent) unit to VA specifically for reactive power (with the
primary unit VA); it has become clear that there is strong
interest in using this unit specifically for the imaginary content
of complex power, i.e., reactive power [IEEE-1459].
The unit "degrees" is in wide use in practice for plane angles (as in
heading, bearing, etc.). It is marked with an asterisk because the
preferred coherent SI unit is radian ("rad").
The Joule per meter is not a traditional electromagnetic unit. It
and its scaled derivatives (in particular Wh/km) are used to describe
the energy expended for achieving motion over a given distance, e.g.,
as an equivalent for electrical cars of the inverse of "mileage".
3. New Registry for Secondary Units
IANA is requested to create a "secondary units" subregistry in the
SenML registry [IANA.senml] defined in [RFC8428].
The registry has six columns:
* secondary unit: a newly registered name allocated within the same
namespace as SenML units
* description: short description (usually just expansion of
abbreviation)
* SenML unit: an existing SenML unit from the SenML units registry
* scale, offset: two rational numbers, expressed in decimal
(optionally, with a decimal exponent given) or as a fraction
represented using a "/" character to separate numerator and
denominator.
* Reference: where does the entry come from.
Quantities expressed in the secondary unit can be converted into the
SenML unit by first multiplying their value with the scale number and
then adding the offset, yielding the value in the given SenML unit.
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The initial content of the secondary units registry is provided in
Table 2:
+-----------+----------------------+------+-----------+---+---------+
| secondary | description |SenML | scale |off| refer- |
| unit | | unit | |set| ence |
+===========+======================+======+===========+===+=========+
| ms | millisecond | s | 1/1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| min | minute | s | 60 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| h | hour | s | 3600 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| MHz | megahertz | Hz | 1000000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kW | kilowatt | W | 1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kVA | kilovolt-ampere | VA | 1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kvar | kilovar | var | 1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| Ah | ampere-hour | C | 3600 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| Wh | watt-hour | J | 3600 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kWh | kilowatt-hour | J | 3600000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| varh | var-hour | vars | 3600 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kvarh | kilovar-hour | vars | 3600000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| kVAh | kilovolt-ampere-hour | VAs | 3600000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| Wh/km | watt-hour per | J/m | 3.6 | 0 | RFCthis |
| | kilometer | | | | |
+-----------+----------------------+------+-----------+---+---------+
| KiB | kibibyte | B | 1024 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| GB | gigabyte | B | 1e9 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| Mbit/s | megabit per second |bit/s | 1000000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| B/s | byte per second |bit/s | 8 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| MB/s | megabyte per second |bit/s | 8000000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| mV | millivolt | V | 1/1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
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| mA | milliampere | A | 1/1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| dBm | decibel (milliwatt) | dBW | 1 |-30| RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| ug/m3 | microgram per cubic |kg/m3 | 1e-9 | 0 | RFCthis |
| | meter | | | | |
+-----------+----------------------+------+-----------+---+---------+
| mm/h | millimeter per hour | m/s | 1/3600000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| m/h | meter per hour | m/s | 1/3600 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| ppm | parts per million | / | 1e-6 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| /100 | percent (Note 1) | / | 1/100 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| /1000 | permille | / | 1/1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| hPa | hectopascal | Pa | 100 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| mm | millimeter | m | 1/1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| cm | centimeter | m | 1/100 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| km | kilometer | m | 1000 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
| km/h | kilometer per hour | m/s | 1/3.6 | 0 | RFCthis |
+-----------+----------------------+------+-----------+---+---------+
Table 2: Secondary units registered for SenML
Note 1: This registration does not use the obvious name "%" because
this name has been taken in [RFC8428] already, where it is a NOT
RECOMMENDED synonym for "/" (unity) for legacy reasons. Note that
the presence of both "%" and "/100" with different meanings is likely
to create confusion, so the present document adds some weight to the
recommendation against using the counterintuitive unit name "%".
Example: the value of a quantity given as 100 ms is first multiplied
by 1/1000, yielding the number 0.1, and then the offset 0 is added,
yielding the number 0.1 again, leading to a quantity of 0.1 s. The
value of a quantity given as 10 dBm is first multiplied by 1,
yielding the number 10, and then the offset -30 is added, yielding
the number -20, leading to a quantity of -20 dBW.
New entries can be added to the registration by Expert Review as
defined in [RFC8126]. Experts should exercise their own good
judgment, with the same guidelines as used for SenML units
(Section 12.1 of [RFC8428]), but without applying the rules 4, 5, and
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8. Note that rule 7 limits the use of what could be understood as
prefixes on their own, not the use of prefixes inside secondary unit
names. Guidelines to the difference between units (which can go into
the registry) and quantities (which cannot) are widely available, see
for instance [RS] and [BIPM].
As of SenML version 10 [RFC8428], SenML packs are limited to using
primary units in "u" fields. The use of primary units enables direct
comparison of measurements from different sources. Also, it
facilitates implementations that trigger on the presence of a
quantity in a certain unit, without the need to track any additional
secondary units that may be registered for this quantity.
Where the benefits of directly using a secondary unit in a SenML pack
outweigh the above considerations, the use of secondary units in "u"
fields MAY be enabled by indicating a new SenML version that
specifically allows this and/or by using a field with a label name
that ends with the "_" character ("must-understand" field) whose
definition specifically allows this. The definition of these
versions and fields is outside the scope of the present
specification; one such definition is proposed in
[I-D.bormann-core-senml-versions].
4. Operational Considerations
The secondary unit registry is expected to grow at a faster pace than
the registry of primary unit names. It also is amenable to automatic
interpretation, by making use of the scale and offset columns.
Implementers may be tempted to equip each instance of their systems
with code to download new versions of the registry from IANA
frequently, in order to be able to make use of newly defined
secondary unit names. This can create high load at IANA and a
potential single point of failure. Instead of pulling the registry
in each individual instance of the code, the software update
mechanism (or a similar mechanism that leads to less frequent IANA
visits) SHOULD be used to disseminate updated units registries
obtained from IANA towards the instances via common repositories.
5. Security Considerations
The security considerations of [RFC8428] apply.
The introduction of new measurement units poses no additional
security considerations except from a possible potential for
additional confusion about the proper unit to use, and from the risk
that an implementation based on the assumption described in the
penultimate paragraph of Section 3 no longer works properly.
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However, an implementation processing a pack while making use of
secondary units is guaranteed to have been developed with an
awareness of the risks of having multiple units available for the
same logical type. In any case, the existing risk of an existing
SenML implementation not understanding a unit that was not in the
initial registry content provided in [RFC8428], is unchanged, and
implementations are warned properly about the potential use of
secondary units by the need for a must-understand field or an updated
version field.
6. IANA Considerations
See Section 2 and Section 3.
7. References
7.1. Normative References
[IANA.senml]
IANA, "Sensor Measurement Lists (SenML)",
<http://www.iana.org/assignments/senml>.
[IEC-80000-13]
"Quantities and units - Part 13: Information science and
technology", Edition 1.0, IEC 80000-13, 1 March 2008.
[IEC-80000-6]
"Quantities and units - Part 6: Electromagnetism",
Edition 1.0, IEC 80000-6, 1 March 2008.
[IEEE-1459]
"IEEE Standard Definitions for the Measurement of Electric
Power Quantities Under Sinusoidal, Nonsinusoidal,
Balanced, or Unbalanced Conditions", IEEE Std 1459-2010,
19 March 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
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2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
DOI 10.17487/RFC8428, August 2018,
<https://www.rfc-editor.org/info/rfc8428>.
7.2. Informative References
[BIPM] Bureau International des Poids es Mesures, "The
International System of Units (SI), 9th edition", 2019,
<https://www.bipm.org/utils/common/pdf/si-brochure/SI-
Brochure-9-EN.pdf>.
[I-D.bormann-core-senml-versions]
Bormann, C., "SenML Features and Versions", Work in
Progress, Internet-Draft, draft-bormann-core-senml-
versions-01, 6 March 2020, <http://www.ietf.org/internet-
drafts/draft-bormann-core-senml-versions-01.txt>.
[RS] Rohde&Schwarz, "Standard-compliant usage of quantities,
units and equations", version 4.0, November 2016,
<https://karriere.rohde-schwarz.de/fileadmin/user_upload/
Standard-compliant_usage_of_quantities_units_and_equations
_bro_en_5214-5061-62_v0400_96dpi.pdf>.
Acknowledgements
Ari Keranen pointed out the need for additional units in SenML.
Comments provided by him as well as by Thomas Fossati, Joaquin Prado,
Jaime Jimenez, Benjamin Kaduk, and Rob Wilton helped improve the
document.
Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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