Internet DRAFT - draft-bormann-core-senml-versions
draft-bormann-core-senml-versions
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Updates: 8428 (if approved) March 06, 2020
Intended status: Standards Track
Expires: September 7, 2020
SenML Features and Versions
draft-bormann-core-senml-versions-01
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Feature Codes and the Version number . . . . . . . . . . . . 2
3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 3
4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 3
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
7. Normative References . . . . . . . . . . . . . . . . . . . . 4
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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].
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, except that "**" denotes
exponentiation.
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
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of these features, specifically the sum of, for each feature present,
two taken to the power of the feature code of that feature.
\~~ 52 fc
version = > present(fc) x 2
/__ fc=0
where present(fc) is 1 if the feature with the feature code "fc" is
present, 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
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 [I-D.ietf-core-senml-more-units] 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 + 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 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:
o Feature code (an unsigned integer less than 53)
o Feature name (text)
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o Specification
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 |
+--------------+-----------------+---------------+
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
not allocate more than 10 % of the remaining set per year.
7. Normative References
[I-D.ietf-core-senml-more-units]
Bormann, C., "Additional Units for SenML", draft-ietf-
core-senml-more-units-05 (work in progress), February
2020.
[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>.
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[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>.
Acknowledgements
Ari Keranen proposed to use the version number as a bitmap and
provided further input on this specification.
Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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