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Table of Contents

1.Introduction

The Sensor Measurement Lists (SenML) specification[RFC8428] provides a versionnumber that is initially set to 10, without furtherspecification on the way to make use of different version numbers.¶

The traditional idea of using a version number for evolving aninterchange format presupposes a linear progression of that format.A more likely form of evolution of SenML is the addition ofindependently selectable "features"that can be added to the base version (version 10) in a fashion thatthese are mostly independent of each other. A recipient of a SenML pack can check thefeatures it implements against those required by the pack, processing thepack only if all required features are provided in the implementation.¶

This short document specifies the use of SenML Features and mapsthem to SenML version number space, updating[RFC8428].¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED","MAY", and "OPTIONAL" in this document are to be interpreted asdescribed in BCP 14[RFC2119][RFC8174] when, and only when, theyappear in all capitals, as shown here.¶

Where bit arithmetic is explained, this document usesthe notation familiar from the programming language C, except that"**" denotes exponentiation.¶

2.Feature Codes and the Version number

The present specification defines "SenML Features", each identified by a "featurename" (a text string) and a "feature code", an unsigned integer lessthan 53.¶

The specific version of a SenML pack is composed of a set offeatures.The SenML version number (bver field) is then a bitmap of thesefeatures, specifically the sum of, for each feature present, two takento the power of the feature code of that feature.¶

            \~~   52                   fcversion  =   >          present(fc) x 2            /__ fc=0

where present(fc) is 1 if the feature with the feature codefc ispresent, 0 otherwise.¶

3.Features: Reserved0, Reserved1, Reserved2, Reserved3

For SenML Version 10 as described in[RFC8428], the feature codes 0 to 3 are already in use.Reserved1 (1) and Reserved3 (3) are always presentand the features Reserved0 (0) and Reserved2 (2) are always absent,yielding a version number of 10 if no other feature is in use.These four reserved feature codes are not to be used with any more specificsemantics except in a specification that updates the present specification.¶

4.Feature: Secondary Units

The feature "Secondary Units" (code number 4) indicates that secondaryunit names[I-D.ietf-core-senml-more-units] MAY be be used in the "u" field of SenML Records, in addition to theprimary unit names already allowed by[RFC8428].¶

Note that the most basic use of this feature simply sets the SenMLversion number to 26 (10 + 2**4).¶

5.Security Considerations

The security considerations of[RFC8428] apply.This specification provides structure to the interpretation of theSenML version number, which poses no additional securityconsiderations except for some potential for surprise that versionnumbers do not simply increase linearly.¶

6.IANA Considerations

IANA is requested to create a new subregistry "SenML features" within the SenMLregistry[IANA.senml], with the registration policy "specification required"[RFC8126]and the columns:¶

• Feature code (an unsigned integer less than 53)¶
• Feature name (text)¶
• Specification¶

The initial content of this registry is as follows:¶

As the number of features that can be registered has a hard limit (48codes left at the time of writing), the designated expert isspecifically instructed to maintain a frugal regime of code pointallocation, keeping code points available for SenML Features that arelikely to be useful for non-trivial subsets of the SenML ecosystem.Quantitatively, the expert could for instance steer the allocation tonot allocate more than 10 % of the remaining set per year.¶

Acknowledgements

Ari Keranen proposed to use the version number as a bitmap andprovided further input on this specification.¶

Author's Address