Status of This Memo

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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 thatcannot be in SenML's main registry, as they are derived by lineartransformation from units already in that registry.Although SenML version 10[RFC8428] does not provide for the direct useof these secondary units, future support is intended via the useof 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.¶

2.New Primary Units

IANA has assigned new units in the "SenML Units"subregistry of the "Sensor Measurement Lists (SenML)" registry[IANA.SENML] (as defined in[RFC8428]):¶

3.New Registry for Secondary Units

IANA has created a "Secondary Units" subregistry in the "Sensor Measurement Lists (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: 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 inTable 2:¶

Note 1: This registration does not use the obvious name "%" becausethis name has been taken in[RFC8428] already, where it is aNOT RECOMMENDED synonym for "/" (unity) for legacy reasons. Note that thepresence of both "%" and "/100" with different meanings is likely tocreate confusion, so the present document adds some weight to therecommendation against using the counterintuitive unit name "%".¶

Example: The value of a quantity given as 100 ms is first multiplied by1/1000, yielding the number 0.1, and the offset 0 is then added, yielding thenumber 0.1 again, leading to a quantity of 0.1 s. The value of a quantitygiven as 10 dBm is first multiplied by 1, yielding the number 10, and theoffset -30 is then added, yielding the number -20, leading to a quantity of -20dBW.¶

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 tousing primary units in "u" fields.The use of primary units enables direct comparison of measurementsfrom different sources. Also, it facilitates implementations thattrigger on the presence of a quantity in a certain unit, without theneed to track any additional secondary units that may be registeredfor this quantity.¶

Where the benefits of directly using a secondary unit in a SenML packoutweigh the above considerations,the use of secondary units in "u" fieldsMAY be enabled by indicating a new SenMLversion 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 ofthe present specification; one such definition is proposed in[SENML-VERSIONS].¶

4.Operational Considerations

The "Secondary Units" subregistry is expected to grow at a faster pace thanthe subregistry of primary unit names. It also is amenable to automaticinterpretation, by making use of the scale and offset columns.¶

Implementers may be tempted to equip each instance of their systemswith code to download new versions of the registry from IANAfrequently in order to be able to make use of newly defined secondaryunit names. This can create high load at IANA and a potential singlepoint of failure. Instead of pulling the registry in each individualinstance of the code, the software update mechanism (or a similarmechanism that leads to less frequent IANA visits)SHOULD be used to disseminateupdated units registries obtained from IANA towards the instances viacommon repositories.¶

5.Security Considerations

The security considerations of[RFC8428] apply.¶

The introduction of new measurement units poses no additional securityconsiderations except a potential for additionalconfusion about the proper unit to use and the risk that animplementation based on the assumption described in the penultimateparagraph ofSection 3 no longer works properly.However, an implementation processing a pack while making use ofsecondary units is guaranteed to have been developed with an awarenessof the risks of having multiple units available for the same logicaltype.In any case, the existing risk of an existing SenML implementation notunderstanding a unit that was not in the initial registry contentprovided in[RFC8428] is unchanged, and implementations are warnedproperly about the potential use of secondary units by the need for amust-understand field or an updated version field.¶

6.IANA Considerations

SeeSection 2 andSection 3.¶

Acknowledgements

Ari Keränen pointed out the need for additional units in SenML.Comments provided by him as well as byThomas Fossati,Joaquin Prado,Jaime Jiménez,Benjamin Kaduk, andRob Wilton helped improve the document.¶

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