1. RFC 9100
Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 9100                        Universität Bremen TZI
Updates: 8428                                                August 2021
Category: Standards Track                                               
ISSN: 2070-1721

         Sensor Measurement Lists (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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Feature Codes and the Version Number
     2.1.  Discussion
     2.2.  Updating Section 4.4 of RFC 8428
   3.  Features: Reserved0, Reserved1, Reserved2, Reserved3
   4.  Feature: Secondary Units
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Author's Address

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 common idea of using a version number to indicate the evolution
   of an interchange format generally assumes an incremental progression
   of the version number as the format accretes additional features over
   time.  However, in the case of SenML, it is expected that the likely
   evolution will be for independently selectable capability _features_
   to be added to the basic specification that is indicated by version
   number 10.  To support this model, this document repurposes the
   single version number accompanying a pack of SenML records so that it
   is interpreted as a bitmap that indicates the set of features a
   recipient would need to have implemented to be able to process the

   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",
   "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: 2^64) denotes exponentiation; in
   the plain text version of this document, superscript notation is
   rendered in paragraph text by C-incompatible surrogate notation as
   seen in this example, and in display math by a crude plain text
   representation, as is the sum (Sigma) sign.

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 represented as an unsigned integer, specifically
   the sum of, for each feature present, two taken to the power of the
   feature code of that feature (Figure 1).

              __ 52                     fc
   version = \           present(fc) ⋅ 2
             /__ fc = 0

      Figure 1: Feature Bitmap as a Sum (Sigma Symbol) of Feature Bits

   where present(fc) is 1 if the feature with the feature code "fc" is
   present, 0 otherwise.  (The expression 2^fc can be implemented as "1
   << fc" in C and related languages.)

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 least significant bit
   positions for the base version as described in 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 2^53-1 [RFC7493].  This means the feature codes 4 to 52 are
   available, one of which is taken by the feature defined in 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 fewer should be allocated.  Note that less
   generally applicable features can always be communicated via fields
   labeled with names that end with the "_" character ("must-understand
   fields").  See Section 4.4 of [RFC8428] for details.

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

   As a general observation, ending up over time with dozens of
   individually selectable optional extensions may lead to too many
   variants of what is supported by different implementations, reducing
   interoperability.  So, in practice, it is still desirable to batch up
   extensions that are expected to be supported together into a single
   feature bit, leading to a sort of hybrid between completely
   independent extensions and a linear version scheme.  This is also
   another reason why a space of 48 remaining feature codes should
   suffice for a while.

2.2.  Updating Section 4.4 of RFC 8428

   The last paragraph of Section 4.4 of [RFC8428] may be read to give
   the impression that SenML version numbers are totally ordered, i.e.,
   that an implementation that understands version n also always
   understands all versions k < n.  If this ever was true for SenML
   versions before 10, it certainly is no longer true with this

   Any SenML pack that sets feature bits beyond the first four will lead
   to a version number that actually is greater than 10, so the
   requirement in Section 4.4 of [RFC8428] will prevent false
   interoperability with version 10 implementations.

   Implementations that do implement feature bits beyond the first four,
   i.e., versions greater than 10, will instead need to perform a
   bitwise comparison of the feature bitmap as described in this
   specification and ensure that all features indicated are understood
   before using the pack.  For example, an implementation that
   implements basic SenML (version number 10) plus only a future feature
   code 5 will accept version number 42, but it would not be able to
   work with a pack indicating version number 26 (base specification
   plus feature code 4).  (If the implementation _requires_ feature code
   5 without being backwards compatible, it will accept 42, but not 10.)

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, i.e., the four least significant bits set to 0b1010 indicate
   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.  (Note that Reserved0 and Reserved2 could be used in
   such a specification in a way similar to that of feature codes 4 to
   52 in the present specification.)

4.  Feature: Secondary Units

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

   Note that the most basic use of this feature simply sets the SenML
   version number to 26 (10 + 2^4).

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

6.  IANA Considerations

   IANA has created a new "SenML Features" subregistry within the
   "Sensor Measurement Lists (SenML)" registry [IANA.SENML] with the
   registration policy "Specification Required" [RFC8126] and the

   *  Feature Code (an unsigned integer less than 53)

   *  Feature Name (text)

   *  Reference

   To facilitate the use of feature names in programs, the designated
   expert is requested to ensure that feature names are usable as
   identifiers in most programming languages, after lowercasing the
   feature name in the registry entry and replacing blank space with
   underscores or hyphens, and that they also are distinct in this form.

   The initial content of this registry is as follows:

         | Feature Code | Feature Name    | Reference           |
         | 0            | Reserved0       | [RFC9100]           |
         | 1            | Reserved1       | [RFC9100]           |
         | 2            | Reserved2       | [RFC9100]           |
         | 3            | Reserved3       | [RFC9100]           |
         | 4            | Secondary Units | [RFC9100] [RFC8798] |

            Table 1: Features Defined for SenML at the Time of

   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 a target of not allocating more than 10% of the remaining set per

   Where the specification of the feature code is provided in a document
   that is separate from the specification of the feature itself (as
   with feature code 4 above), both specifications should be listed.

7.  References

7.1.  Normative References

   [C]        International Organization for Standardization,
              "Information technology - Programming languages - C", ISO/
              IEC 9899:2018, Fourth Edition, June 2018,

              International Organization for Standardization,
              "Programming languages - C++", ISO/IEC 14882:2020, Sixth
              Edition, December 2020,

              IANA, "Sensor Measurement Lists (SenML)",

   [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, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8428]  Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
              Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
              DOI 10.17487/RFC8428, August 2018,

   [RFC8798]  Bormann, C., "Additional Units for Sensor Measurement
              Lists (SenML)", RFC 8798, DOI 10.17487/RFC8798, June 2020,

7.2.  Informative References

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,


   Ari Keränen proposed to use the version number as a bitmap and
   provided further input on this specification.  Jaime Jiménez helped
   clarify the document by providing a review and acted as Document
   Shepherd.  Elwyn Davies provided a detailed GENART review with
   directly implementable text suggestions that now form part of this
   specification.  Rob Wilton supplied comments, one of which became the
   last paragraph of Section 2.1; Éric Vyncke helped with Section 2.
   Additional thanks go to the other IESG reviewers.

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921
   Email: cabo@tzi.org
  1. RFC 9100