Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Updates: 8428 (if approved) October 12, 2019
Intended status: Standards Track
Expires: April 14, 2020

Additional Units for SenML


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; RFC 8428 is updated to also accept these units.

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Table of Contents

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. [RFC8428] is updated to also accept these secondary Unit names in place of the (primary) Unit names defined in [RFC8428].

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 Units

IANA is requested to assign new units in the “SenML Units” subregistry of the SenML registry [IANA.senml] (as defined in [RFC8428]):

New units registered for SenML
Symbol Description Type Reference
B Byte (information content) float RFCthis
VA volt-ampere (Apparent Power) float RFCthis
VAs volt-ampere seconds (Apparent Energy) float RFCthis
var volt-ampere reactive (Reactive Power) float RFCthis
vars volt-ampere reactive seconds (Reactive Energy) float RFCthis
J/m joule per meter (Energy per distance) float RFCthis
kg/m3 kilograms per cubic meter (mass concentration) float RFCthis
deg degrees (angle)* float RFCthis

3. 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 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”.

4. New Registry

IANA is requested to create a “secondary units” subregistry in the 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 registry is provided in Table 2:

Secondary units registered for SenML
secondary unit description SenML unit scale off set refer- ence
ms millisecond s 1/1000 0 RFCthis
min minute s 60 0 RFCthis
h hour s 3600 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 watts-hour per kilometer J/m 3.6 0 RFCthis
KiB kibibyte B 1024 0 RFCthis
mV millivolt V 1/1000 0 RFCthis
mA milliampere A 1/1000 0 RFCthis
dBm decibel (milliwatt) dBW 1 -30 RFCthis
ug/m3 micrograms per cubic meter kg/m3 1e-9 0 RFCthis
mm/h millimeter per hour m/s 1/3600000 0 RFCthis
ppm parts per million / 1e-6 0 RFCthis
hPa hectopascal Pa 100 0 RFCthis
mm millimeter m 1/1000 0 RFCthis
km kilometer km 1000 0 RFCthis
km/h kilometer per hour m/s 1/3.6 0 RFCthis

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 and 5. Guidelines to the difference between units (which can go into the registry) and quantities are widely available, see for instance [RS] and [BIPM].

SenML packs MAY, but SHOULD NOT, use secondary units in place of SenML units, where the exception of the “SHOULD NOT” lies in the context of specific existing data models that are based on these secondary units.

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.

6. IANA Considerations

See Section 2 and Section 4.


Ari Keranen pointed out the need for additional units in SenML. Comments provided by him as well as by Thomas Fossati, Joaquin Prado, and Jaime Jimenez helped improve the document.

8. References

8.1. Normative References

[IANA.senml] IANA, "Sensor Measurement Lists (SenML)"
[IEC-80000-13] "Quantities and units – Part 13: Information science and technology", IEC 80000-13, Edition 1.0, March 2008.
[IEC-80000-6] "Quantities and units – Part 6: Electromagnetism", IEC 80000-6, Edition 1.0, 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, 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.
[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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A. and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, August 2018.

8.2. Informative References

[BIPM] Bureau International des Poids es Mesures, "The International System of Units (SI), 9th edition", 2019.
[RS] Rohde&Schwarz, "Standard-compliant usage of quantities, units and equations", version 4.0, November 2016.

Author's Address

Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen, D-28359 Germany Phone: +49-421-218-63921 EMail: