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.

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The Sensor Measurement Lists (SenML) [RFC 8428] 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 [RFC 8428] 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 [RFC 2119] [RFC 8174] 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 [RFC 8428]): Table 1: New Units Registered for SenML

SenML [RFC 8428] 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 widespread is the symbol B for byte, even though B is already taken in SI for bel. [RFC 8428] 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.

The Volt-Ampere:

[IEC-80000-6] defines the volt ampere (VA) (item 6-57.a) 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 volt-ampere reactive (var) 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 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 [RFC 8428]. The registry has six columns:

• Secondary Unit: a newly registered name allocated within the same namespace as SenML units
• Description: a short description (usually just the expansion of an 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 the entry comes 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. The initial content of the "Secondary Units" subregistry is provided in Table 2: Table 2: Secondary Units Registered for SenML 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 [RFC 8126]. Experts should exercise their own good judgment, with the same guidelines as used for SenML units (Section 12.1 of RFC 8428), 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 [RFC 8428], 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 [RFC 8428] 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 [RFC 8428] 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.

See Section 2 and Section 3.

[IANA.SENML]

IANA, "Sensor Measurement Lists (SenML)",
<https://www.iana.org/assignments/senml>.

[IEC-80000-13]

IEC, "Quantities and units - Part 13: Information science and technology", IEC 80000-13, March 2008.

[IEC-80000-6]

IEC, "Quantities and units - Part 6: Electromagnetism", IEC 80000-6, March 2008.

[IEEE-1459]

IEEE, "IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions", IEEE Std 1459-2010, DOI 10.1109/IEEESTD.2010.5439063, March 2010,
<https://doi.org/10.1109/IEEESTD.2010.5439063>.

[RFC2119]

S. Bradner, "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]

M. Cotton, B. Leiba, 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]

B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

[RFC8428]

C. Jennings, Z. Shelby, J. Arkko, A. Keranen, and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, August 2018,
<https://www.rfc-editor.org/info/rfc8428>.

[BIPM]

Bureau International des Poids et Mesures, "The International System of Units (SI)", 2019,
<https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf>.

[RS]

Rohde & Schwarz, "Standard-compliant usage of quantities, units and equations", July 2017,
<https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/Standard-compliant_usage_of_quantities_units_and_equations_bro_en_5214-5061-62_v0500.pdf>.

[SENML-VERSIONS]

C Bormann, "SenML Features and Versions", Internet-Draft draft-ietf-core-senml-versions-00, May 2020,
<https://tools.ietf.org/html/draft-ietf-core-senml-versions-00>.

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.

Carsten Bormann