Internet-Draft SenML Features and Versions February 2021
Bormann Expires 25 August 2021 [Page]
8428 (if approved)
Intended Status:
Standards Track
C. Bormann
Universitaet Bremen TZI

SenML Features and Versions


This short document updates RFC 8428, Sensor Measurement Lists (SenML), by specifying the use of independently selectable "SenML Features" and mapping them to SenML version numbers.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the CORE Working Group mailing list (, which is archived at

Source for this draft and an issue tracker can be found at

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 25 August 2021.

Table of Contents

1. Introduction

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

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

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

1.1. Terminology

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.

Where bit arithmetic is explained, this document uses the notation familiar from the programming language C [C], including the 0b prefix for binary numbers defined in Section 5.13.2 of the C++ language standard [Cplusplus], except that superscript notation (example for two to the power of 64: 264) denotes exponentiation; in the plain text version of this draft, superscript notation is rendered by C-incompatible surrogate notation as seen in this example.

2. Feature Codes and the Version number

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

The specific version of a SenML pack is composed of a set of features. The SenML version number (bver field) is then a bitmap of these features, specifically the sum of, for each feature present, two taken to the power of the feature code of that feature.

where present(fc) is 1 if the feature with the feature code fc is present, 0 otherwise.

2.1. Discussion

Representing features as a bitmap within a number is quite efficient as long as feature codes are sparingly allocated (see also Section 6).

Compatibility with the existing SenML version number, 10 decimal (0b1010), requires reserving four of the lower-most bit positions Section 3. There is an upper limit to the range of the integer numbers that can be represented in all SenML representations: practical JSON limits this to 253-1 [RFC7493]. This means the feature codes 4 to 52 are available, one of which is taken by Section 4, leaving 48 for allocation. (The current version 10 (with all other feature codes unset) can be visualized as 0b00000000000000000000000000000000000000000000000001010.) For a lifetime of this scheme of several decades, approximately two feature codes per year or less should be allocated. (More boutique features can always be communicated by must-understand fields, see Section 4.4 of [RFC8428].)

Most representations visible to engineers working with SenML will use decimal numbers, e.g. 26 (0b11010, 0x1a) for a version that adds the "Secondary Units" feature (Section 4). This is sightly unwieldy, but will be quickly memorized in practice.

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 present and 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 specific semantics except in a specification that updates the present specification.

4. Feature: Secondary Units

The feature "Secondary Units" (code number 4) indicates that secondary unit names [RFC8798] MAY be be used in the "u" field of SenML Records, in addition to the primary unit names already allowed by [RFC8428].

Note that the most basic use of this feature simply sets the SenML version number to 26 (10 + 24).

5. Security Considerations

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

6. IANA Considerations

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

The initial content of this registry is as follows:

Table 1: Features defined for SenML at the time of writing
Feature code Feature name Specification
0 Reserved0 RFCthis
1 Reserved1 RFCthis
2 Reserved2 RFCthis
3 Reserved3 RFCthis
4 Secondary Units RFCthis

As the number of features that can be registered has a hard limit (48 codes left at the time of writing), the designated expert is specifically instructed to maintain a frugal regime of code point allocation, keeping code points available for SenML Features that are likely to be useful for non-trivial subsets of the SenML ecosystem. Quantitatively, the expert could for instance steer the allocation to not allocate more than 10 % of the remaining set per year.

7. References

7.1. Normative References

International Organization for Standardization, "Information technology — Programming languages — C", ISO/IEC 9899:2018, Fourth Edition, , <>.
International Organization for Standardization, "Programming languages — C++", ISO/IEC 14882:2020, Sixth Edition, , <>.
IANA, "Sensor Measurement Lists (SenML)", <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, , <>.
Bormann, C., "Additional Units for Sensor Measurement Lists (SenML)", RFC 8798, DOI 10.17487/RFC8798, , <>.

7.2. Informative References

Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, , <>.


Ari Keränen proposed to use the version number as a bitmap and provided further input on this specification. Jaime Jiménez help clarify the document by providing a review.

Author's Address

Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen