Universität Bremen TZI

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CoRE Internet of Things (IoT) Internet of Things IOT data model 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 .

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Table of Contents

• .  Introduction
  • .  Terminology
• .  Feature Codes and the Version Number
  • .  Discussion
  • .  Updating Section 4.4 of RFC 8428
• .  Features: Reserved0, Reserved1, Reserved2, Reserved3
• .  Feature: Secondary Units
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction The 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 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 .

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 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 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.

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 ().

Feature Bitmap as a Sum (Sigma Symbol) of Feature Bits __ 52 fc version = \ present(fc) ⋅ 2 /__ fc = 0

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.)

Discussion Representing 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 least significant bit positions for the base version as described in . 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 the feature defined in , 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 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 (). 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.

Updating Section 4.4 of RFC 8428 The last paragraph of 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 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.)

Features: Reserved0, Reserved1, Reserved2, Reserved3 For 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, 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.)

Feature: Secondary Units The feature "Secondary Units" (code number 4) indicates that secondary unit names MAY 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 Considerations The 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 Considerations IANA has created a new "" subregistry within the "Sensor Measurement Lists (SenML)" registry with the registration policy "Specification Required" 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: Features Defined for SenML at the Time of Writing

Feature Code	Feature Name	Reference
0	Reserved0	[RFC9100]
1	Reserved1	[RFC9100]
2	Reserved2	[RFC9100]
3	Reserved3	[RFC9100]
4	Secondary Units	[RFC9100]

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.

References Normative References International Organization for Standardization International Organization for Standardization IANA In 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. Many 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. RFC 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. 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. 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. Informative References I-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. helped clarify the document by providing a review and acted as Document Shepherd. provided a detailed GENART review with directly implementable text suggestions that now form part of this specification. supplied comments, one of which became the last paragraph of ; helped with . Additional thanks go to the other IESG reviewers.

Author's Address Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org