Network Working Group C. Bormann Internet-Draft Universitaet Bremen TZI Updates: 8428 (if approved) March 06, 2020 Intended status: Standards Track Expires: September 7, 2020 SenML Features and Versions draft-bormann-core-senml-versions-01 Abstract 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 7, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Bormann Expires September 7, 2020 [Page 1] Internet-Draft SenML Features and Versions March 2020 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Feature Codes and the Version number . . . . . . . . . . . . 2 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 3 4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . . 3 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 7. Normative References . . . . . . . . . . . . . . . . . . . . 4 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction The Sensor Measurement Lists (SenML) specification [RFC8428] 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 [RFC8428]. 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 [RFC2119] [RFC8174] 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, except that "**" denotes exponentiation. 2. 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 Bormann Expires September 7, 2020 [Page 2] Internet-Draft SenML Features and Versions March 2020 of these features, specifically the sum of, for each feature present, two taken to the power of the feature code of that feature. \~~ 52 fc version = > present(fc) x 2 /__ fc=0 where present(fc) is 1 if the feature with the feature code "fc" is present, 0 otherwise. 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 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 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. 4. Feature: Secondary Units The feature "Secondary Units" (code number 4) indicates that secondary unit names [I-D.ietf-core-senml-more-units] MAY be be used in the "u" field of SenML Records, in addition to the primary unit names already allowed by [RFC8428]. Note that the most basic use of this feature simply sets the SenML version number to 26 (10 + 2**4). 5. Security Considerations The security considerations of [RFC8428] 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. 6. IANA Considerations IANA is requested to create a new subregistry "SenML features" within the SenML registry [IANA.senml], with the registration policy "specification required" [RFC8126] and the columns: o Feature code (an unsigned integer less than 53) o Feature name (text) Bormann Expires September 7, 2020 [Page 3] Internet-Draft SenML Features and Versions March 2020 o Specification The initial content of this registry is as follows: +--------------+-----------------+---------------+ | Feature code | Feature name | Specification | +--------------+-----------------+---------------+ | 0 | Reserved0 | RFCthis | | 1 | Reserved1 | RFCthis | | 2 | Reserved2 | RFCthis | | 3 | Reserved3 | RFCthis | | 4 | Secondary Units | RFCthis | +--------------+-----------------+---------------+ Table 1: Features defined for SenML at the time of writing 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. 7. Normative References [I-D.ietf-core-senml-more-units] Bormann, C., "Additional Units for SenML", draft-ietf- core-senml-more-units-05 (work in progress), February 2020. [IANA.senml] IANA, "Sensor Measurement Lists (SenML)", . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Bormann Expires September 7, 2020 [Page 4] Internet-Draft SenML Features and Versions March 2020 [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, August 2018, . Acknowledgements Ari Keranen proposed to use the version number as a bitmap and provided further input on this specification. Author's Address Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Bormann Expires September 7, 2020 [Page 5]