SenML Features and VersionsUniversitaet Bremen TZIPostfach 330440BremenD-28359Germany+49-421-218-63921cabo@tzi.orgCoREInternet-DraftThis 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 VenuesDiscussion 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/.Source for this draft and an issue tracker can be found at
https://github.com/core-wg/senml-versions.IntroductionThe Sensor Measurement Lists (SenML) specification 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 .TerminologyThe 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 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 , including the 0b
prefix for binary numbers defined in Section 5.13.2 of the C++
language standard , except that superscript notation
(example for two to the power of 64: 264) denotes
exponentiation; in the plain text version of this draft, superscript
notation is rendered by C-incompatible surrogate notation as seen in
this example.Feature Codes and the Version numberThe 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, specifically the sum of, for each feature present, two taken
to the power of the feature code of that feature.where present(fc) is 1 if the feature with the feature code fc is
present, 0 otherwise.DiscussionRepresenting features as a bitmap within a number is quite efficient as long as
feature codes are sparingly allocated (see also ).Compatibility with the existing SenML version number, 10 decimal
(0b1010), requires reserving four of the lower-most bit positions .
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 253-1 .
This means the feature codes 4 to 52 are available, one of which is
taken by , 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 less should be allocated.
(More boutique features can always be communicated by must-understand
fields, see .)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 (). This is sightly unwieldy, but
will be quickly memorized in practice.Features: Reserved0, Reserved1, Reserved2, Reserved3For SenML Version 10 as described in , 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.Feature: Secondary UnitsThe feature "Secondary Units" (code number 4) indicates that secondary
unit names MAY be 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 + 24).Security ConsiderationsThe security considerations of 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.IANA ConsiderationsIANA is requested to create a new subregistry "SenML features" within the SenML
registry , with the registration policy "specification required"
and the columns:
Feature code (an unsigned integer less than 53)
Feature name (text)
Specification
The initial content of this registry is as follows:
Features defined for SenML at the time of writing
Feature code
Feature name
Specification
0
Reserved0
RFCthis
1
Reserved1
RFCthis
2
Reserved2
RFCthis
3
Reserved3
RFCthis
4
Secondary Units
RFCthis
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.ReferencesNormative ReferencesSensor Measurement Lists (SenML)This specification defines a format for representing simple sensor measurements and device parameters in Sensor Measurement Lists (SenML). Representations are defined in JavaScript Object Notation (JSON), Concise Binary Object Representation (CBOR), Extensible Markup Language (XML), and Efficient XML Interchange (EXI), which share the common SenML data model. A simple sensor, such as a temperature sensor, could use one of these media types in protocols such as HTTP or the Constrained Application Protocol (CoAP) to transport the measurements of the sensor or to be configured.Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226.Sensor Measurement Lists (SenML)IANAAdditional Units for Sensor Measurement Lists (SenML)The Sensor Measurement Lists (SenML) media type supports the indication of units for a quantity represented. This short document registers a number of additional unit names in the IANA registry for units in SenML. It also defines a registry for secondary units that cannot be in SenML's main registry, as they are derived by linear transformation from units already in that registry.Information technology — Programming languages — CInternational Organization for StandardizationProgramming languages — C++International Organization for StandardizationKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Informative ReferencesThe I-JSON Message FormatI-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results.Acknowledgements proposed to use the version number as a bitmap and
provided further input on this specification.
help clarify the document by providing a review.