<?xmlversion='1.0' encoding='utf-8'?> <?xml-stylesheet type="text/xsl" href="rfc2629.xslt" ?> <!-- generated by https://github.com/cabo/kramdown-rfc2629 version 1.4.14 -->version="1.0" encoding="UTF-8"?> <!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent"><?rfc toc="yes"?> <?rfc tocompact="yes"?> <?rfc tocdepth="3"?> <?rfc tocindent="yes"?> <?rfc symrefs="yes"?> <?rfc sortrefs="yes"?> <?rfc comments="yes"?> <?rfc inline="yes"?> <?rfc strict="no"?> <?rfc compact="yes"?> <?rfc subcompact="no"?><rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-core-senml-versions-05" number="9100" submissionType="IETF" category="std" consensus="true" updates="8428" obsoletes=""submissionType="IETF"xml:lang="en" tocInclude="true" tocDepth="3" symRefs="true" sortRefs="true" version="3"><!-- xml2rfc v2v3 conversion 3.8.0 --><front> <title abbrev="SenML Features and Versions">SenML">Sensor Measurement Lists (SenML) Features and Versions</title> <seriesInfoname="Internet-Draft" value="draft-ietf-core-senml-versions-05"/>name="RFC" value="9100"/> <author initials="C." surname="Bormann" fullname="Carsten Bormann"><organization>Universitaet<organization>Universität Bremen TZI</organization> <address> <postal> <street>Postfach 330440</street> <city>Bremen</city> <code>D-28359</code> <country>Germany</country> </postal> <phone>+49-421-218-63921</phone> <email>cabo@tzi.org</email> </address> </author> <date year="2021"month="June" day="04"/>month="August"/> <workgroup>CoRE</workgroup><keyword>Internet-Draft</keyword><keyword>Internet of Things (IoT)</keyword> <keyword>Internet of Things</keyword> <keyword>IOT</keyword> <keyword>data model</keyword> <abstract> <t>This short document updates RFC 8428,Sensor"Sensor Measurement Lists(SenML),(SenML)", by specifying the use of independently selectable "SenML Features" and mapping them to SenML version numbers.</t> </abstract><note removeInRFC="true"> <name>Discussion Venues</name> <t>Discussion of this document takes place on the CORE Working Group mailing list (core@ietf.org), which is archived at <eref target="https://mailarchive.ietf.org/arch/browse/core/">https://mailarchive.ietf.org/arch/browse/core/</eref>.</t> <t>Source for this draft and an issue tracker can be found at <eref target="https://github.com/core-wg/senml-versions">https://github.com/core-wg/senml-versions</eref>.</t> </note></front> <middle> <section anchor="intro" numbered="true" toc="default"> <name>Introduction</name> <t>The Sensor Measurement Lists (SenML) specification <xref target="RFC8428" format="default"/> provides a version number that is initially set to 10, without further specification on the way to make use of different version numbers.</t> <t>Thetraditionalcommon 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 <em>features</em> 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.</t> <t>This short document specifies the use of SenML Features and maps them to SenML version number space, updating <xref target="RFC8428" format="default"/>.</t> <section anchor="terminology" numbered="true" toc="default"> <name>Terminology</name> <t>The key words"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY","<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>", "<bcp14>MAY</bcp14>", and"OPTIONAL""<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as described inBCP 14BCP 14 <xref target="RFC2119" format="default"/> <xref target="RFC8174" format="default"/> when, and only when, they appear in all capitals, as shown here.</t> <t>Where bit arithmetic is explained, this document uses the notation familiar from the programming language C <xref target="C" format="default"/>, including the <tt>0b</tt> prefix for binary numbers defined in Section 5.13.2 of the C++ language standard <xreftarget="Cplusplus"target="CPLUSPLUS" format="default"/>, except that superscript notation (example for two to the power of 64: 2<sup>64</sup>) denotes exponentiation; in the plain text version of thisdraft,document, superscript notation is rendered in paragraph text by C-incompatible surrogate notation as seen in this example, and in display math by a crudeplaintextplain text representation, as is the sum (Sigma) sign.</t> </section> </section> <section anchor="codes" numbered="true" toc="default"> <name>Feature Codes and the Versionnumber</name>Number</name> <t>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).</t> <t>The specific version of a SenML pack is composed of a set of features. 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(The expression 2<sup>fc</sup> can be implemented as <tt>1 << fc</tt> in C and related languages.)</t><t>RFC editor: Please check that, in the TXT version, no <tt>&nbsp;</tt> 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.</t><section anchor="discussion" numbered="true" toc="default"> <name>Discussion</name> <t>Representing features as a bitmap within a number is quite efficient as long as feature codes are sparingly allocated (see also <xref target="iana" format="default"/>).</t> <t>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 <xref target="resv" format="default"/>. 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<sup>53</sup>-1 <xref target="RFC7493" format="default"/>. This means the feature codes 4 to 52 are available, one of which is taken by the feature defined in <xref target="secondary-units" format="default"/>, leaving 48 for allocation. (The current version 10 (with all other feature codes unset) can be visualized as <tt>0b00000000000000000000000000000000000000000000000001010</tt>.) 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-understandfields"), seefields"). See <xref section="4.4" sectionFormat="of" target="RFC8428"format="default"/>.)</t>format="default"/> for details.</t> <t>Most representations visible to engineers working with SenML will use decimalnumbers, e.g.,numbers. For instance, 26 (0b11010, 0x1a)fordenotes a version that adds the "Secondary Units" feature (<xref target="secondary-units" format="default"/>). This is slightlyunwieldy,unwieldy but will be quickly memorized in practice.</t> <t>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.</t> </section> <section anchor="updating-section-44-of-rfc8428" numbered="true" toc="default"> <name>Updating<xref section="4.4" sectionFormat="of" target="RFC8428" format="default"/></name>Section 4.4 of RFC 8428</name> <t>The last paragraph of <xref section="4.4" sectionFormat="of" target="RFC8428" format="default"/> 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.</t> <t>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 <xref section="4.4" sectionFormat="of" target="RFC8428" format="default"/> will prevent false interoperability with version 10 implementations.</t> <t>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.,For example, an implementation that implements basic SenML (version number 10) plus only a future feature code5,5 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 <em>requires</em> feature code 5 without being backwards compatible, it will accept 42, but not 10.)</t> </section> </section> <section anchor="resv" numbered="true" toc="default"> <name>Features: Reserved0, Reserved1, Reserved2, Reserved3</name> <t>For SenMLVersionversion 10 as described in <xref target="RFC8428" format="default"/>, the feature codes 0 to 3 are already in use. Reserved1 (1) and Reserved3 (3) are alwayspresentpresent, 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 asimilarway similar tothe way thethat of feature codes 4 to 52arein the present specification.)</t> </section> <section anchor="secondary-units" numbered="true" toc="default"> <name>Feature: Secondary Units</name> <t>The feature "Secondary Units" (code number 4) indicates that secondary unit names <xref target="RFC8798" format="default"/>MAY<bcp14>MAY</bcp14> be used in the "u" field of SenMLRecords,records in addition to the primary unit names already allowed by <xref target="RFC8428" format="default"/>.</t> <t>Note that the most basic use of this feature simply sets the SenML version number to 26 (10 + 2<sup>4</sup>).</t> </section> <section anchor="seccons" numbered="true" toc="default"> <name>Security Considerations</name> <t>The security considerations of <xref target="RFC8428" format="default"/> 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.</t> </section> <section anchor="iana" numbered="true" toc="default"> <name>IANA Considerations</name> <t>IANAis requested to createhas created a new "<xref section="SenML Features" sectionFormat="bare" target="IANA.SENML" relative="#senml-features"/>" subregistry"SenML features"within theSenML"Sensor Measurement Lists (SenML)" registry <xreftarget="IANA.senml" format="default"/>,target="IANA.SENML" format="default"/> with the registration policy"specification required""Specification Required" <xref target="RFC8126" format="default"/> and the columns:</t> <ul spacing="normal"> <li>FeaturecodeCode (an unsigned integer less than 53)</li> <li>FeaturenameName (text)</li><li>Specification</li><li>Reference</li> </ul> <t>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, afterlower-casinglowercasing the feature name in the registry entry and replacingwhitespaceblank space with underscores or hyphens, and that they also are distinct in this form.</t> <t>The initial content of this registry is as follows:</t> <table anchor="feat" align="center"> <name>FeaturesdefinedDefined for SenML at thetimeTime ofwriting</name>Writing</name> <thead> <tr> <th align="left">Featurecode</th>Code</th> <th align="left">Featurename</th>Name</th> <thalign="left">Specification</th>align="left">Reference</th> </tr> </thead> <tbody> <tr> <td align="left">0</td> <td align="left">Reserved0</td> <tdalign="left">RFCthis</td>align="left">[RFC9100]</td> </tr> <tr> <td align="left">1</td> <td align="left">Reserved1</td> <tdalign="left">RFCthis</td>align="left">[RFC9100]</td> </tr> <tr> <td align="left">2</td> <td align="left">Reserved2</td> <tdalign="left">RFCthis</td>align="left">[RFC9100]</td> </tr> <tr> <td align="left">3</td> <td align="left">Reserved3</td> <tdalign="left">RFCthis</td>align="left">[RFC9100]</td> </tr> <tr> <td align="left">4</td> <td align="left">Secondary Units</td> <tdalign="left">RFCthis,align="left">[RFC9100] <xref target="RFC8798" format="default"/></td> </tr> </tbody> </table> <t>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 expertcouldcould, forinstanceinstance, steer the allocation to a target of not allocating more than10 %10% of the remaining set per year.</t> <t>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.</t> </section> </middle> <back> <references> <name>References</name> <references> <name>Normative References</name> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8428.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/> <referenceanchor="RFC8428" target="https://www.rfc-editor.org/info/rfc8428"> <front> <title>Sensor Measurement Lists (SenML)</title> <author fullname="C. Jennings" initials="C." surname="Jennings"> <organization/> </author> <author fullname="Z. Shelby" initials="Z." surname="Shelby"> <organization/> </author> <author fullname="J. Arkko" initials="J." surname="Arkko"> <organization/> </author> <author fullname="A. Keranen" initials="A." surname="Keranen"> <organization/> </author> <author fullname="C. Bormann" initials="C." surname="Bormann"> <organization/> </author> <date month="August" year="2018"/> <abstract> <t>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.</t> </abstract> </front> <seriesInfo name="RFC" value="8428"/> <seriesInfo name="DOI" value="10.17487/RFC8428"/> </reference> <reference anchor="RFC8126" target="https://www.rfc-editor.org/info/rfc8126"> <front> <title>Guidelines for Writing an IANA Considerations Section in RFCs</title> <author fullname="M. Cotton" initials="M." surname="Cotton"> <organization/> </author> <author fullname="B. Leiba" initials="B." surname="Leiba"> <organization/> </author> <author fullname="T. Narten" initials="T." surname="Narten"> <organization/> </author> <date month="June" year="2017"/> <abstract> <t>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).</t> <t>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.</t> <t>This is the third edition of this document; it obsoletes RFC 5226.</t> </abstract> </front> <seriesInfo name="BCP" value="26"/> <seriesInfo name="RFC" value="8126"/> <seriesInfo name="DOI" value="10.17487/RFC8126"/> </reference> <reference anchor="IANA.senml" target="http://www.iana.org/assignments/senml">anchor="IANA.SENML" target="https://www.iana.org/assignments/senml"> <front> <title>Sensor Measurement Lists (SenML)</title> <author> <organization>IANA</organization> </author> <date/> </front> </reference><reference anchor="RFC8798" target="https://www.rfc-editor.org/info/rfc8798"> <front> <title>Additional Units for Sensor Measurement Lists (SenML)</title> <author fullname="C. Bormann" initials="C." surname="Bormann"> <organization/> </author> <date month="June" year="2020"/> <abstract> <t>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.</t> </abstract> </front> <seriesInfo name="RFC" value="8798"/> <seriesInfo name="DOI" value="10.17487/RFC8798"/> </reference><xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8798.xml"/> <reference anchor="C" target="https://www.iso.org/standard/74528.html"> <front> <title>Information technology—- Programming languages—- C</title> <author> <organization>International Organization for Standardization</organization> </author> <date year="2018" month="June"/> </front> <seriesInfo name="ISO/IEC" value="9899:2018, Fourth Edition"/> </reference> <referenceanchor="Cplusplus"anchor="CPLUSPLUS" target="https://www.iso.org/standard/79358.html"> <front> <title>Programming languages—- C++</title> <author> <organization>International Organization for Standardization</organization> </author> <date year="2020" month="December"/> </front> <seriesInfo name="ISO/IEC" value="14882:2020, Sixth Edition"/> </reference><reference anchor="RFC2119" target="https://www.rfc-editor.org/info/rfc2119"> <front> <title>Key words for use in RFCs to Indicate Requirement Levels</title> <author fullname="S. Bradner" initials="S." surname="Bradner"> <organization/> </author> <date month="March" year="1997"/> <abstract> <t>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.</t> </abstract> </front> <seriesInfo name="BCP" value="14"/> <seriesInfo name="RFC" value="2119"/> <seriesInfo name="DOI" value="10.17487/RFC2119"/> </reference> <reference anchor="RFC8174" target="https://www.rfc-editor.org/info/rfc8174"> <front> <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title> <author fullname="B. Leiba" initials="B." surname="Leiba"> <organization/> </author> <date month="May" year="2017"/> <abstract> <t>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.</t> </abstract> </front> <seriesInfo name="BCP" value="14"/> <seriesInfo name="RFC" value="8174"/> <seriesInfo name="DOI" value="10.17487/RFC8174"/> </reference><xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/> </references> <references> <name>Informative References</name><reference anchor="RFC7493" target="https://www.rfc-editor.org/info/rfc7493"> <front> <title>The I-JSON Message Format</title> <author fullname="T. Bray" initials="T." role="editor" surname="Bray"> <organization/> </author> <date month="March" year="2015"/> <abstract> <t>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.</t> </abstract> </front> <seriesInfo name="RFC" value="7493"/> <seriesInfo name="DOI" value="10.17487/RFC7493"/> </reference><xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7493.xml"/> </references> </references> <section numbered="false" anchor="acks" toc="default"> <name>Acknowledgements</name> <t><contact fullname="Ari Keränen"/> proposed to use the version number as a bitmap and provided further input on this specification. <contact fullname="Jaime Jiménez"/> helped clarify the document by providing areview.review and acted as Document Shepherd. <contact fullname="Elwyn Davies"/> provided a detailed GENARTreview,review with directly implementable text suggestions that now form part of this specification. <contact fullname="Rob Wilton"/> suppliedcommentscomments, one of which became the last paragraph of <xref target="discussion" format="default"/>; <contact fullname="Éric Vyncke"/> helped with <xref target="codes" format="default"/>. 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