Internet DRAFT - draft-ietf-core-senml-versions
draft-ietf-core-senml-versions
CoRE C. Bormann
Internet-Draft Universitaet Bremen TZI
Updates: 8428 (if approved) 4 June 2021
Intended status: Standards Track
Expires: 6 December 2021
SenML Features and Versions
draft-ietf-core-senml-versions-05
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.
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 (core@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/core/
(https://mailarchive.ietf.org/arch/browse/core/).
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/senml-versions (https://github.com/core-
wg/senml-versions).
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 6 December 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Feature Codes and the Version number . . . . . . . . . . . . 3
2.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Updating RFC8428 . . . . . . . . . . . . . . . . . . . . 4
3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 5
4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
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 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.
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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 draft, superscript notation is
rendered in paragraph text by C-incompatible surrogate notation as
seen in this example, and in display math by a crude plaintext
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 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.)
RFC editor: Please check that, in the TXT version, no " " crept
into the above due to xml2rfc bug 641, and remove this paragraph. If
possible with today's RFCXML, add the Sigma character as a
parenthesis after "sum" in the caption.
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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].)
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 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 [RFC8428]
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.
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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. E.g., an implementation that implements basic
SenML (version number 10) plus only a future feature code 5, will
accept version number 42, but 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 similar way to the way the feature codes 4
to 52 are 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.
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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:
* Feature code (an unsigned integer less than 53)
* Feature name (text)
* Specification
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 lower-casing the
feature name in the registry entry and replacing whitespace 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 | Specification |
+==============+=================+====================+
| 0 | Reserved0 | RFCthis |
+--------------+-----------------+--------------------+
| 1 | Reserved1 | RFCthis |
+--------------+-----------------+--------------------+
| 2 | Reserved2 | RFCthis |
+--------------+-----------------+--------------------+
| 3 | Reserved3 | RFCthis |
+--------------+-----------------+--------------------+
| 4 | Secondary Units | RFCthis, [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.
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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)",
<http://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
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[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. 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
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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