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Bormann 3 Internet-Draft Universitaet Bremen TZI 4 Updates: 8428 (if approved) 15 November 2020 5 Intended status: Standards Track 6 Expires: 19 May 2021 8 SenML Features and Versions 9 draft-ietf-core-senml-versions-01 11 Abstract 13 This short document updates RFC 8428, Sensor Measurement Lists 14 (SenML), by specifying the use of independently selectable "SenML 15 Features" and mapping them to SenML version numbers. 17 Discussion Venues 19 This note is to be removed before publishing as an RFC. 21 Discussion of this document takes place on the CORE Working Group 22 mailing list (core@ietf.org), which is archived at 23 https://mailarchive.ietf.org/arch/browse/core/ 24 (https://mailarchive.ietf.org/arch/browse/core/). 26 Source for this draft and an issue tracker can be found at 27 https://github.com/core-wg/senml-versions (https://github.com/core- 28 wg/senml-versions). 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at https://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on 19 May 2021. 47 Copyright Notice 49 Copyright (c) 2020 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 54 license-info) in effect on the date of publication of this document. 55 Please review these documents carefully, as they describe your rights 56 and restrictions with respect to this document. Code Components 57 extracted from this document must include Simplified BSD License text 58 as described in Section 4.e of the Trust Legal Provisions and are 59 provided without warranty as described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 64 2. Feature Codes and the Version number . . . . . . . . . . . . 3 65 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 3 66 4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 3 67 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 69 7. Normative References . . . . . . . . . . . . . . . . . . . . 5 70 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5 71 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 73 1. Introduction 75 The Sensor Measurement Lists (SenML) specification [RFC8428] provides 76 a version number that is initially set to 10, without further 77 specification on the way to make use of different version numbers. 79 The traditional idea of using a version number for evolving an 80 interchange format presupposes a linear progression of that format. 81 A more likely form of evolution of SenML is the addition of 82 independently selectable "features" that can be added to the base 83 version (version 10) in a fashion that these are mostly independent 84 of each other. A recipient of a SenML pack can check the features it 85 implements against those required by the pack, processing the pack 86 only if all required features are provided in the implementation. 88 This short document specifies the use of SenML Features and maps them 89 to SenML version number space, updating [RFC8428]. 91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 93 "OPTIONAL" in this document are to be interpreted as described in 94 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 95 capitals, as shown here. 97 Where bit arithmetic is explained, this document uses the notation 98 familiar from the programming language C [C], except that superscript 99 notation (example for two to the power of 64: 2^(64)) denotes 100 exponentiation; in the plain text version of this draft, superscript 101 notation is rendered by C-incompatible surrogate notation as seen in 102 this example. 104 2. Feature Codes and the Version number 106 The present specification defines "SenML Features", each identified 107 by a "feature name" (a text string) and a "feature code", an unsigned 108 integer less than 53. 110 The specific version of a SenML pack is composed of a set of 111 features. The SenML version number ("bver" field) is then a bitmap 112 of these features, specifically the sum of, for each feature present, 113 two taken to the power of the feature code of that feature. 115 \~~ 52 fc 116 version = > present(fc) x 2 117 /__ fc=0 119 where present(fc) is 1 if the feature with the feature code "fc" is 120 present, 0 otherwise. 122 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 124 For SenML Version 10 as described in [RFC8428], the feature codes 0 125 to 3 are already in use. Reserved1 (1) and Reserved3 (3) are always 126 present and the features Reserved0 (0) and Reserved2 (2) are always 127 absent, yielding a version number of 10 if no other feature is in 128 use. These four reserved feature codes are not to be used with any 129 more specific semantics except in a specification that updates the 130 present specification. 132 4. Feature: Secondary Units 134 The feature "Secondary Units" (code number 4) indicates that 135 secondary unit names [RFC8798] MAY be be used in the "u" field of 136 SenML Records, in addition to the primary unit names already allowed 137 by [RFC8428]. 139 Note that the most basic use of this feature simply sets the SenML 140 version number to 26 (10 + 2^(4)). 142 5. Security Considerations 144 The security considerations of [RFC8428] apply. This specification 145 provides structure to the interpretation of the SenML version number, 146 which poses no additional security considerations except for some 147 potential for surprise that version numbers do not simply increase 148 linearly. 150 6. IANA Considerations 152 IANA is requested to create a new subregistry "SenML features" within 153 the SenML registry [IANA.senml], with the registration policy 154 "specification required" [RFC8126] and the columns: 156 * Feature code (an unsigned integer less than 53) 158 * Feature name (text) 160 * Specification 162 The initial content of this registry is as follows: 164 +==============+=================+===============+ 165 | Feature code | Feature name | Specification | 166 +==============+=================+===============+ 167 | 0 | Reserved0 | RFCthis | 168 +--------------+-----------------+---------------+ 169 | 1 | Reserved1 | RFCthis | 170 +--------------+-----------------+---------------+ 171 | 2 | Reserved2 | RFCthis | 172 +--------------+-----------------+---------------+ 173 | 3 | Reserved3 | RFCthis | 174 +--------------+-----------------+---------------+ 175 | 4 | Secondary Units | RFCthis | 176 +--------------+-----------------+---------------+ 178 Table 1: Features defined for SenML at the 179 time of writing 181 As the number of features that can be registered has a hard limit (48 182 codes left at the time of writing), the designated expert is 183 specifically instructed to maintain a frugal regime of code point 184 allocation, keeping code points available for SenML Features that are 185 likely to be useful for non-trivial subsets of the SenML ecosystem. 186 Quantitatively, the expert could for instance steer the allocation to 187 not allocate more than 10 % of the remaining set per year. 189 7. Normative References 191 [C] International Organization for Standardization, 192 "Information technology — Programming languages — C", ISO/ 193 IEC 9899:2018, Fourth Edition, June 2018. 195 [IANA.senml] 196 IANA, "Sensor Measurement Lists (SenML)", 197 . 199 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 200 Requirement Levels", BCP 14, RFC 2119, 201 DOI 10.17487/RFC2119, March 1997, 202 . 204 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 205 Writing an IANA Considerations Section in RFCs", BCP 26, 206 RFC 8126, DOI 10.17487/RFC8126, June 2017, 207 . 209 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 210 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 211 May 2017, . 213 [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. 214 Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, 215 DOI 10.17487/RFC8428, August 2018, 216 . 218 [RFC8798] Bormann, C., "Additional Units for Sensor Measurement 219 Lists (SenML)", RFC 8798, DOI 10.17487/RFC8798, June 2020, 220 . 222 Acknowledgements 224 Ari Keranen proposed to use the version number as a bitmap and 225 provided further input on this specification. 227 Author's Address 228 Carsten Bormann 229 Universitaet Bremen TZI 230 Postfach 330440 231 D-28359 Bremen 232 Germany 234 Phone: +49-421-218-63921 235 Email: cabo@tzi.org