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
Abstract
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
https://www.rfc-editor.org/info/rfc9100.
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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
Acknowledgements
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
pack.
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: 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
specification.
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
[RFC8428].
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
linearly.
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
columns:
* 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
Writing
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
year.
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,
<https://www.iso.org/standard/74528.html>.
[CPLUSPLUS]
International Organization for Standardization,
"Programming languages - C++", ISO/IEC 14882:2020, Sixth
Edition, December 2020,
<https://www.iso.org/standard/79358.html>.
[IANA.SENML]
IANA, "Sensor Measurement Lists (SenML)",
<https://www.iana.org/assignments/senml>.
[RFC2119] Bradner, S., "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] 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,
<https://www.rfc-editor.org/info/rfc8126>.
[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,
<https://www.rfc-editor.org/info/rfc8428>.
[RFC8798] Bormann, C., "Additional Units for Sensor Measurement
Lists (SenML)", RFC 8798, DOI 10.17487/RFC8798, June 2020,
<https://www.rfc-editor.org/info/rfc8798>.
7.2. Informative References
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
Acknowledgements
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
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org