| RFC 8798 | Additional Units for SenML | June 2020 |
| Bormann | Standards Track | [Page] |
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.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8798.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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, future support is intended via the use of SenML extension mechanisms, one of which is proposed in [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.¶
IANA has assigned new units in the "SenML Units" subregistry of the "Sensor Measurement Lists (SenML)" registry [IANA.SENML] (as defined in [RFC8428]):¶
| Symbol | Description | Type | Reference |
|---|---|---|---|
| B | Byte (information content) | float | RFC 8798 |
| VA | volt-ampere (Apparent Power) | float | RFC 8798 |
| VAs | volt-ampere second (Apparent Energy) | float | RFC 8798 |
| var | volt-ampere reactive (Reactive Power) | float | RFC 8798 |
| vars | volt-ampere-reactive second (Reactive Energy) | float | RFC 8798 |
| J/m | joule per meter (Energy per distance) | float | RFC 8798 |
| kg/m3 | kilogram per cubic meter (mass density, mass concentration) | float | RFC 8798 |
| deg | degree (angle)* | float | RFC 8798 |
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 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".¶
The unit "degree" 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").¶
IANA has created a "Secondary Units" subregistry in the "Sensor Measurement Lists (SenML)" registry [IANA.SENML] defined in [RFC8428].¶
The registry has six columns:¶
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.¶
The initial content of the "Secondary Units" subregistry is provided in Table 2:¶
| Secondary Unit | Description | SenML Unit | Scale | Offset | Reference |
|---|---|---|---|---|---|
| ms | millisecond | s | 1/1000 | 0 | RFC 8798 |
| min | minute | s | 60 | 0 | RFC 8798 |
| h | hour | s | 3600 | 0 | RFC 8798 |
| MHz | megahertz | Hz | 1000000 | 0 | RFC 8798 |
| kW | kilowatt | W | 1000 | 0 | RFC 8798 |
| kVA | kilovolt-ampere | VA | 1000 | 0 | RFC 8798 |
| kvar | kilovar | var | 1000 | 0 | RFC 8798 |
| Ah | ampere-hour | C | 3600 | 0 | RFC 8798 |
| Wh | watt-hour | J | 3600 | 0 | RFC 8798 |
| kWh | kilowatt-hour | J | 3600000 | 0 | RFC 8798 |
| varh | var-hour | vars | 3600 | 0 | RFC 8798 |
| kvarh | kilovar-hour | vars | 3600000 | 0 | RFC 8798 |
| kVAh | kilovolt-ampere-hour | VAs | 3600000 | 0 | RFC 8798 |
| Wh/km | watt-hour per kilometer | J/m | 3.6 | 0 | RFC 8798 |
| KiB | kibibyte | B | 1024 | 0 | RFC 8798 |
| GB | gigabyte | B | 1e9 | 0 | RFC 8798 |
| Mbit/s | megabit per second | bit/s | 1000000 | 0 | RFC 8798 |
| B/s | byte per second | bit/s | 8 | 0 | RFC 8798 |
| MB/s | megabyte per second | bit/s | 8000000 | 0 | RFC 8798 |
| mV | millivolt | V | 1/1000 | 0 | RFC 8798 |
| mA | milliampere | A | 1/1000 | 0 | RFC 8798 |
| dBm | decibel (milliwatt) | dBW | 1 | -30 | RFC 8798 |
| ug/m3 | microgram per cubic meter | kg/m3 | 1e-9 | 0 | RFC 8798 |
| mm/h | millimeter per hour | m/s | 1/3600000 | 0 | RFC 8798 |
| m/h | meter per hour | m/s | 1/3600 | 0 | RFC 8798 |
| ppm | parts per million | / | 1e-6 | 0 | RFC 8798 |
| /100 | percent (Note 1) | / | 1/100 | 0 | RFC 8798 |
| /1000 | permille | / | 1/1000 | 0 | RFC 8798 |
| hPa | hectopascal | Pa | 100 | 0 | RFC 8798 |
| mm | millimeter | m | 1/1000 | 0 | RFC 8798 |
| cm | centimeter | m | 1/100 | 0 | RFC 8798 |
| km | kilometer | m | 1000 | 0 | RFC 8798 |
| km/h | kilometer per hour | m/s | 1/3.6 | 0 | RFC 8798 |
Example: The value of a quantity given as 100 ms is first multiplied by 1/1000, yielding the number 0.1, and the offset 0 is then 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 the offset -30 is then 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 rules 4, 5, and 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 [SENML-VERSIONS].¶
The "Secondary Units" subregistry is expected to grow at a faster pace than the subregistry 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.¶
The security considerations of [RFC8428] apply.¶
The introduction of new measurement units poses no additional security considerations except a potential for additional confusion about the proper unit to use and the risk that an implementation based on the assumption described in the penultimate paragraph of Section 3 no longer works properly. 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.¶
Ari Keränen pointed out the need for additional units in SenML. Comments provided by him as well as by Thomas Fossati, Joaquin Prado, Jaime Jiménez, Benjamin Kaduk, and Rob Wilton helped improve the document.¶