draft-ietf-core-senml-versions-05.txt   rfc9100.txt 
CoRE C. Bormann Internet Engineering Task Force (IETF) C. Bormann
Internet-Draft Universitaet Bremen TZI Request for Comments: 9100 Universität Bremen TZI
Updates: 8428 (if approved) 4 June 2021 Updates: 8428 August 2021
Intended status: Standards Track Category: Standards Track
Expires: 6 December 2021 ISSN: 2070-1721
SenML Features and Versions Sensor Measurement Lists (SenML) Features and Versions
draft-ietf-core-senml-versions-05
Abstract Abstract
This short document updates RFC 8428, Sensor Measurement Lists This short document updates RFC 8428, "Sensor Measurement Lists
(SenML), by specifying the use of independently selectable "SenML (SenML)", by specifying the use of independently selectable "SenML
Features" and mapping them to SenML version numbers. 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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 6 December 2021. 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.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
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described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology
2. Feature Codes and the Version number . . . . . . . . . . . . 3 2. Feature Codes and the Version Number
2.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Discussion
2.2. Updating RFC8428 . . . . . . . . . . . . . . . . . . . . 4 2.2. Updating Section 4.4 of RFC 8428
3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 5 3. Features: Reserved0, Reserved1, Reserved2, Reserved3
4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 5 4. Feature: Secondary Units
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. References
7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.1. Normative References
7.2. Informative References . . . . . . . . . . . . . . . . . 7 7.2. Informative References
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8 Acknowledgements
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address
1. Introduction 1. Introduction
The Sensor Measurement Lists (SenML) specification [RFC8428] provides The Sensor Measurement Lists (SenML) specification [RFC8428] provides
a version number that is initially set to 10, without further a version number that is initially set to 10, without further
specification on the way to make use of different version numbers. specification on the way to make use of different version numbers.
The traditional idea of using a version number to indicate the The common idea of using a version number to indicate the evolution
evolution of an interchange format generally assumes an incremental of an interchange format generally assumes an incremental progression
progression of the version number as the format accretes additional of the version number as the format accretes additional features over
features over time. However, in the case of SenML, it is expected time. However, in the case of SenML, it is expected that the likely
that the likely evolution will be for independently selectable evolution will be for independently selectable capability _features_
capability _features_ to be added to the basic specification that is to be added to the basic specification that is indicated by version
indicated by version number 10. To support this model, this document number 10. To support this model, this document repurposes the
repurposes the single version number accompanying a pack of SenML single version number accompanying a pack of SenML records so that it
records so that it is interpreted as a bitmap that indicates the set is interpreted as a bitmap that indicates the set of features a
of features a recipient would need to have implemented to be able to recipient would need to have implemented to be able to process the
process the pack. pack.
This short document specifies the use of SenML Features and maps them This short document specifies the use of SenML Features and maps them
to SenML version number space, updating [RFC8428]. to SenML version number space, updating [RFC8428].
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Where bit arithmetic is explained, this document uses the notation Where bit arithmetic is explained, this document uses the notation
familiar from the programming language C [C], including the "0b" familiar from the programming language C [C], including the "0b"
prefix for binary numbers defined in Section 5.13.2 of the C++ prefix for binary numbers defined in Section 5.13.2 of the C++
language standard [Cplusplus], except that superscript notation language standard [CPLUSPLUS], except that superscript notation
(example for two to the power of 64: 2^64) denotes exponentiation; in (example for two to the power of 64: 2^64) denotes exponentiation; in
the plain text version of this draft, superscript notation is the plain text version of this document, superscript notation is
rendered in paragraph text by C-incompatible surrogate notation as rendered in paragraph text by C-incompatible surrogate notation as
seen in this example, and in display math by a crude plaintext seen in this example, and in display math by a crude plain text
representation, as is the sum (Sigma) sign. representation, as is the sum (Sigma) sign.
2. Feature Codes and the Version number 2. Feature Codes and the Version Number
The present specification defines "SenML Features", each identified The present specification defines "SenML Features", each identified
by a "feature name" (a text string) and a "feature code" (an unsigned by a "feature name" (a text string) and a "feature code" (an unsigned
integer less than 53). integer less than 53).
The specific version of a SenML pack is composed of a set of The specific version of a SenML pack is composed of a set of
features. The SenML version number ("bver" field) is then a bitmap features. The SenML version number ("bver" field) is then a bitmap
of these features represented as an unsigned integer, specifically of these features represented as an unsigned integer, specifically
the sum of, for each feature present, two taken to the power of the the sum of, for each feature present, two taken to the power of the
feature code of that feature (Figure 1). feature code of that feature (Figure 1).
__ 52 fc __ 52 fc
version = \ present(fc) ⋅ 2 version = \ present(fc) ⋅ 2
/__ fc = 0 /__ fc = 0
Figure 1: Feature bitmap as a sum of feature bits 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 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 present, 0 otherwise. (The expression 2^fc can be implemented as "1
<< fc" in C and related languages.) << fc" in C and related languages.)
RFC editor: Please check that, in the TXT version, no "&nbsp;" 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.
2.1. Discussion 2.1. Discussion
Representing features as a bitmap within a number is quite efficient Representing features as a bitmap within a number is quite efficient
as long as feature codes are sparingly allocated (see also as long as feature codes are sparingly allocated (see also
Section 6). Section 6).
Compatibility with the existing SenML version number, 10 decimal Compatibility with the existing SenML version number, 10 decimal
(0b1010), requires reserving four of the least significant bit (0b1010), requires reserving four of the least significant bit
positions for the base version as described in Section 3. There is 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 an upper limit to the range of the integer numbers that can be
represented in all SenML representations: practical JSON limits this represented in all SenML representations: practical JSON limits this
to 2^53-1 [RFC7493]. This means the feature codes 4 to 52 are 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, available, one of which is taken by the feature defined in Section 4,
leaving 48 for allocation. (The current version 10 (with all other leaving 48 for allocation. (The current version 10 (with all other
feature codes unset) can be visualized as feature codes unset) can be visualized as
"0b00000000000000000000000000000000000000000000000001010".) For a "0b00000000000000000000000000000000000000000000000001010".) For a
lifetime of this scheme of several decades, approximately two feature lifetime of this scheme of several decades, approximately two feature
codes per year or fewer should be allocated. Note that less codes per year or fewer should be allocated. Note that less
generally applicable features can always be communicated via fields generally applicable features can always be communicated via fields
labeled with names that end with the "_" character ("must-understand labeled with names that end with the "_" character ("must-understand
fields"), see Section 4.4 of [RFC8428].) fields"). See Section 4.4 of [RFC8428] for details.
Most representations visible to engineers working with SenML will use Most representations visible to engineers working with SenML will use
decimal numbers, e.g., 26 (0b11010, 0x1a) for a version that adds the decimal numbers. For instance, 26 (0b11010, 0x1a) denotes a version
"Secondary Units" feature (Section 4). This is slightly unwieldy, that adds the "Secondary Units" feature (Section 4). This is
but will be quickly memorized in practice. slightly unwieldy but will be quickly memorized in practice.
As a general observation, ending up over time with dozens of As a general observation, ending up over time with dozens of
individually selectable optional extensions may lead to too many individually selectable optional extensions may lead to too many
variants of what is supported by different implementations, reducing variants of what is supported by different implementations, reducing
interoperability. So, in practice, it is still desirable to batch up interoperability. So, in practice, it is still desirable to batch up
extensions that are expected to be supported together into a single extensions that are expected to be supported together into a single
feature bit, leading to a sort of hybrid between completely feature bit, leading to a sort of hybrid between completely
independent extensions and a linear version scheme. This is also independent extensions and a linear version scheme. This is also
another reason why a space of 48 remaining feature codes should another reason why a space of 48 remaining feature codes should
suffice for a while. suffice for a while.
2.2. Updating Section 4.4 of [RFC8428] 2.2. Updating Section 4.4 of RFC 8428
The last paragraph of Section 4.4 of [RFC8428] may be read to give 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., the impression that SenML version numbers are totally ordered, i.e.,
that an implementation that understands version n also always that an implementation that understands version n also always
understands all versions k < n. If this ever was true for SenML understands all versions k < n. If this ever was true for SenML
versions before 10, it certainly is no longer true with this versions before 10, it certainly is no longer true with this
specification. specification.
Any SenML pack that sets feature bits beyond the first four will lead 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 to a version number that actually is greater than 10, so the
requirement in Section 4.4 of [RFC8428] will prevent false requirement in Section 4.4 of [RFC8428] will prevent false
interoperability with version 10 implementations. interoperability with version 10 implementations.
Implementations that do implement feature bits beyond the first four, Implementations that do implement feature bits beyond the first four,
i.e., versions greater than 10, will instead need to perform a i.e., versions greater than 10, will instead need to perform a
bitwise comparison of the feature bitmap as described in this bitwise comparison of the feature bitmap as described in this
specification and ensure that all features indicated are understood specification and ensure that all features indicated are understood
before using the pack. E.g., an implementation that implements basic before using the pack. For example, an implementation that
SenML (version number 10) plus only a future feature code 5, will implements basic SenML (version number 10) plus only a future feature
accept version number 42, but would not be able to work with a pack code 5 will accept version number 42, but it would not be able to
indicating version number 26 (base specification plus feature code work with a pack indicating version number 26 (base specification
4). (If the implementation _requires_ feature code 5 without being plus feature code 4). (If the implementation _requires_ feature code
backwards compatible, it will accept 42, but not 10.) 5 without being backwards compatible, it will accept 42, but not 10.)
3. Features: Reserved0, Reserved1, Reserved2, Reserved3 3. Features: Reserved0, Reserved1, Reserved2, Reserved3
For SenML Version 10 as described in [RFC8428], the feature codes 0 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 to 3 are already in use. Reserved1 (1) and Reserved3 (3) are always
present and the features Reserved0 (0) and Reserved2 (2) 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 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 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 reserved feature codes are not to be used with any more specific
semantics except in a specification that updates the present semantics except in a specification that updates the present
specification. (Note that Reserved0 and Reserved2 could be used in specification. (Note that Reserved0 and Reserved2 could be used in
such a specification in a similar way to the way the feature codes 4 such a specification in a way similar to that of feature codes 4 to
to 52 are in the present specification.) 52 in the present specification.)
4. Feature: Secondary Units 4. Feature: Secondary Units
The feature "Secondary Units" (code number 4) indicates that The feature "Secondary Units" (code number 4) indicates that
secondary unit names [RFC8798] MAY be used in the "u" field of SenML secondary unit names [RFC8798] MAY be used in the "u" field of SenML
Records, in addition to the primary unit names already allowed by records in addition to the primary unit names already allowed by
[RFC8428]. [RFC8428].
Note that the most basic use of this feature simply sets the SenML Note that the most basic use of this feature simply sets the SenML
version number to 26 (10 + 2^4). version number to 26 (10 + 2^4).
5. Security Considerations 5. Security Considerations
The security considerations of [RFC8428] apply. This specification The security considerations of [RFC8428] apply. This specification
provides structure to the interpretation of the SenML version number, provides structure to the interpretation of the SenML version number,
which poses no additional security considerations except for some which poses no additional security considerations except for some
potential for surprise that version numbers do not simply increase potential for surprise that version numbers do not simply increase
linearly. linearly.
6. IANA Considerations 6. IANA Considerations
IANA is requested to create a new subregistry "SenML features" within IANA has created a new "SenML Features" subregistry within the
the SenML registry [IANA.senml], with the registration policy "Sensor Measurement Lists (SenML)" registry [IANA.SENML] with the
"specification required" [RFC8126] and the columns: registration policy "Specification Required" [RFC8126] and the
columns:
* Feature code (an unsigned integer less than 53) * Feature Code (an unsigned integer less than 53)
* Feature name (text) * Feature Name (text)
* Specification * Reference
To facilitate the use of feature names in programs, the designated To facilitate the use of feature names in programs, the designated
expert is requested to ensure that feature names are usable as expert is requested to ensure that feature names are usable as
identifiers in most programming languages, after lower-casing the identifiers in most programming languages, after lowercasing the
feature name in the registry entry and replacing whitespace with feature name in the registry entry and replacing blank space with
underscores or hyphens, and that they also are distinct in this form. underscores or hyphens, and that they also are distinct in this form.
The initial content of this registry is as follows: The initial content of this registry is as follows:
+==============+=================+====================+ +==============+=================+=====================+
| Feature code | Feature name | Specification | | Feature Code | Feature Name | Reference |
+==============+=================+====================+ +==============+=================+=====================+
| 0 | Reserved0 | RFCthis | | 0 | Reserved0 | [RFC9100] |
+--------------+-----------------+--------------------+ +--------------+-----------------+---------------------+
| 1 | Reserved1 | RFCthis | | 1 | Reserved1 | [RFC9100] |
+--------------+-----------------+--------------------+ +--------------+-----------------+---------------------+
| 2 | Reserved2 | RFCthis | | 2 | Reserved2 | [RFC9100] |
+--------------+-----------------+--------------------+ +--------------+-----------------+---------------------+
| 3 | Reserved3 | RFCthis | | 3 | Reserved3 | [RFC9100] |
+--------------+-----------------+--------------------+ +--------------+-----------------+---------------------+
| 4 | Secondary Units | RFCthis, [RFC8798] | | 4 | Secondary Units | [RFC9100] [RFC8798] |
+--------------+-----------------+--------------------+ +--------------+-----------------+---------------------+
Table 1: Features defined for SenML at the time of Table 1: Features Defined for SenML at the Time of
writing Writing
As the number of features that can be registered has a hard limit (48 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 codes left at the time of writing), the designated expert is
specifically instructed to maintain a frugal regime of code point specifically instructed to maintain a frugal regime of code point
allocation, keeping code points available for SenML Features that are allocation, keeping code points available for SenML Features that are
likely to be useful for non-trivial subsets of the SenML ecosystem. likely to be useful for non-trivial subsets of the SenML ecosystem.
Quantitatively, the expert could for instance steer the allocation to Quantitatively, the expert could, for instance, steer the allocation
a target of not allocating more than 10 % of the remaining set per to a target of not allocating more than 10% of the remaining set per
year. year.
Where the specification of the feature code is provided in a document Where the specification of the feature code is provided in a document
that is separate from the specification of the feature itself (as that is separate from the specification of the feature itself (as
with feature code 4 above), both specifications should be listed. with feature code 4 above), both specifications should be listed.
7. References 7. References
7.1. Normative References 7.1. Normative References
[C] International Organization for Standardization, [C] International Organization for Standardization,
"Information technology — Programming languages — C", ISO/ "Information technology - Programming languages - C", ISO/
IEC 9899:2018, Fourth Edition, June 2018, IEC 9899:2018, Fourth Edition, June 2018,
<https://www.iso.org/standard/74528.html>. <https://www.iso.org/standard/74528.html>.
[Cplusplus] [CPLUSPLUS]
International Organization for Standardization, International Organization for Standardization,
"Programming languages C++", ISO/IEC 14882:2020, Sixth "Programming languages - C++", ISO/IEC 14882:2020, Sixth
Edition, December 2020, Edition, December 2020,
<https://www.iso.org/standard/79358.html>. <https://www.iso.org/standard/79358.html>.
[IANA.senml] [IANA.SENML]
IANA, "Sensor Measurement Lists (SenML)", IANA, "Sensor Measurement Lists (SenML)",
<http://www.iana.org/assignments/senml>. <https://www.iana.org/assignments/senml>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
skipping to change at page 8, line 13 skipping to change at line 319
7.2. Informative References 7.2. Informative References
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015, DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>. <https://www.rfc-editor.org/info/rfc7493>.
Acknowledgements Acknowledgements
Ari Keränen proposed to use the version number as a bitmap and Ari Keränen proposed to use the version number as a bitmap and
provided further input on this specification. Jaime Jiménez helped provided further input on this specification. Jaime Jiménez helped
clarify the document by providing a review. Elwyn Davies provided a clarify the document by providing a review and acted as Document
detailed GENART review, with directly implementable text suggestions Shepherd. Elwyn Davies provided a detailed GENART review with
that now form part of this specification. Rob Wilton supplied directly implementable text suggestions that now form part of this
comments one of which became the last paragraph of Section 2.1; Éric specification. Rob Wilton supplied comments, one of which became the
Vyncke helped with Section 2. Additional thanks go to the other IESG last paragraph of Section 2.1; Éric Vyncke helped with Section 2.
reviewers. Additional thanks go to the other IESG reviewers.
Author's Address Author's Address
Carsten Bormann Carsten Bormann
Universitaet Bremen TZI Universität Bremen TZI
Postfach 330440 Postfach 330440
D-28359 Bremen D-28359 Bremen
Germany Germany
Phone: +49-421-218-63921 Phone: +49-421-218-63921
Email: cabo@tzi.org Email: cabo@tzi.org
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