idnits 2.17.1 draft-ietf-core-senml-versions-05.txt: -(292): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- == There are 6 instances of lines with non-ascii characters in the document. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (4 June 2021) is 1057 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'C' -- Possible downref: Non-RFC (?) normative reference: ref. 'Cplusplus' Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 CoRE C. Bormann 3 Internet-Draft Universitaet Bremen TZI 4 Updates: 8428 (if approved) 4 June 2021 5 Intended status: Standards Track 6 Expires: 6 December 2021 8 SenML Features and Versions 9 draft-ietf-core-senml-versions-05 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 6 December 2021. 47 Copyright Notice 49 Copyright (c) 2021 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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 65 2. Feature Codes and the Version number . . . . . . . . . . . . 3 66 2.1. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4 67 2.2. Updating RFC8428 . . . . . . . . . . . . . . . . . . . . 4 68 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 . . . . 5 69 4. Feature: Secondary Units . . . . . . . . . . . . . . . . . . 5 70 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 72 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 73 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 74 7.2. Informative References . . . . . . . . . . . . . . . . . 7 75 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8 76 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 78 1. Introduction 80 The Sensor Measurement Lists (SenML) specification [RFC8428] provides 81 a version number that is initially set to 10, without further 82 specification on the way to make use of different version numbers. 84 The traditional idea of using a version number to indicate the 85 evolution of an interchange format generally assumes an incremental 86 progression of the version number as the format accretes additional 87 features over time. However, in the case of SenML, it is expected 88 that the likely evolution will be for independently selectable 89 capability _features_ to be added to the basic specification that is 90 indicated by version number 10. To support this model, this document 91 repurposes the single version number accompanying a pack of SenML 92 records so that it is interpreted as a bitmap that indicates the set 93 of features a recipient would need to have implemented to be able to 94 process the pack. 96 This short document specifies the use of SenML Features and maps them 97 to SenML version number space, updating [RFC8428]. 99 1.1. Terminology 101 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 102 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 103 "OPTIONAL" in this document are to be interpreted as described in 104 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 105 capitals, as shown here. 107 Where bit arithmetic is explained, this document uses the notation 108 familiar from the programming language C [C], including the "0b" 109 prefix for binary numbers defined in Section 5.13.2 of the C++ 110 language standard [Cplusplus], except that superscript notation 111 (example for two to the power of 64: 2^64) denotes exponentiation; in 112 the plain text version of this draft, superscript notation is 113 rendered in paragraph text by C-incompatible surrogate notation as 114 seen in this example, and in display math by a crude plaintext 115 representation, as is the sum (Sigma) sign. 117 2. Feature Codes and the Version number 119 The present specification defines "SenML Features", each identified 120 by a "feature name" (a text string) and a "feature code" (an unsigned 121 integer less than 53). 123 The specific version of a SenML pack is composed of a set of 124 features. The SenML version number ("bver" field) is then a bitmap 125 of these features represented as an unsigned integer, specifically 126 the sum of, for each feature present, two taken to the power of the 127 feature code of that feature (Figure 1). 129 __ 52 fc 130 version = \ present(fc) ⋅ 2 131 /__ fc = 0 133 Figure 1: Feature bitmap as a sum of feature bits 135 where present(fc) is 1 if the feature with the feature code "fc" is 136 present, 0 otherwise. (The expression 2^fc can be implemented as "1 137 << fc" in C and related languages.) 139 RFC editor: Please check that, in the TXT version, no " " crept 140 into the above due to xml2rfc bug 641, and remove this paragraph. If 141 possible with today's RFCXML, add the Sigma character as a 142 parenthesis after "sum" in the caption. 144 2.1. Discussion 146 Representing features as a bitmap within a number is quite efficient 147 as long as feature codes are sparingly allocated (see also 148 Section 6). 150 Compatibility with the existing SenML version number, 10 decimal 151 (0b1010), requires reserving four of the least significant bit 152 positions for the base version as described in Section 3. There is 153 an upper limit to the range of the integer numbers that can be 154 represented in all SenML representations: practical JSON limits this 155 to 2^53-1 [RFC7493]. This means the feature codes 4 to 52 are 156 available, one of which is taken by the feature defined in Section 4, 157 leaving 48 for allocation. (The current version 10 (with all other 158 feature codes unset) can be visualized as 159 "0b00000000000000000000000000000000000000000000000001010".) For a 160 lifetime of this scheme of several decades, approximately two feature 161 codes per year or fewer should be allocated. Note that less 162 generally applicable features can always be communicated via fields 163 labeled with names that end with the "_" character ("must-understand 164 fields"), see Section 4.4 of [RFC8428].) 166 Most representations visible to engineers working with SenML will use 167 decimal numbers, e.g., 26 (0b11010, 0x1a) for a version that adds the 168 "Secondary Units" feature (Section 4). This is slightly unwieldy, 169 but will be quickly memorized in practice. 171 As a general observation, ending up over time with dozens of 172 individually selectable optional extensions may lead to too many 173 variants of what is supported by different implementations, reducing 174 interoperability. So, in practice, it is still desirable to batch up 175 extensions that are expected to be supported together into a single 176 feature bit, leading to a sort of hybrid between completely 177 independent extensions and a linear version scheme. This is also 178 another reason why a space of 48 remaining feature codes should 179 suffice for a while. 181 2.2. Updating Section 4.4 of [RFC8428] 183 The last paragraph of Section 4.4 of [RFC8428] may be read to give 184 the impression that SenML version numbers are totally ordered, i.e., 185 that an implementation that understands version n also always 186 understands all versions k < n. If this ever was true for SenML 187 versions before 10, it certainly is no longer true with this 188 specification. 190 Any SenML pack that sets feature bits beyond the first four will lead 191 to a version number that actually is greater than 10, so the 192 requirement in Section 4.4 of [RFC8428] will prevent false 193 interoperability with version 10 implementations. 195 Implementations that do implement feature bits beyond the first four, 196 i.e., versions greater than 10, will instead need to perform a 197 bitwise comparison of the feature bitmap as described in this 198 specification and ensure that all features indicated are understood 199 before using the pack. E.g., an implementation that implements basic 200 SenML (version number 10) plus only a future feature code 5, will 201 accept version number 42, but would not be able to work with a pack 202 indicating version number 26 (base specification plus feature code 203 4). (If the implementation _requires_ feature code 5 without being 204 backwards compatible, it will accept 42, but not 10.) 206 3. Features: Reserved0, Reserved1, Reserved2, Reserved3 208 For SenML Version 10 as described in [RFC8428], the feature codes 0 209 to 3 are already in use. Reserved1 (1) and Reserved3 (3) are always 210 present and the features Reserved0 (0) and Reserved2 (2) are always 211 absent, i.e., the four least significant bits set to 0b1010 indicate 212 a version number of 10 if no other feature is in use. These four 213 reserved feature codes are not to be used with any more specific 214 semantics except in a specification that updates the present 215 specification. (Note that Reserved0 and Reserved2 could be used in 216 such a specification in a similar way to the way the feature codes 4 217 to 52 are in the present specification.) 219 4. Feature: Secondary Units 221 The feature "Secondary Units" (code number 4) indicates that 222 secondary unit names [RFC8798] MAY be used in the "u" field of SenML 223 Records, in addition to the primary unit names already allowed by 224 [RFC8428]. 226 Note that the most basic use of this feature simply sets the SenML 227 version number to 26 (10 + 2^4). 229 5. Security Considerations 231 The security considerations of [RFC8428] apply. This specification 232 provides structure to the interpretation of the SenML version number, 233 which poses no additional security considerations except for some 234 potential for surprise that version numbers do not simply increase 235 linearly. 237 6. IANA Considerations 239 IANA is requested to create a new subregistry "SenML features" within 240 the SenML registry [IANA.senml], with the registration policy 241 "specification required" [RFC8126] and the columns: 243 * Feature code (an unsigned integer less than 53) 245 * Feature name (text) 247 * Specification 249 To facilitate the use of feature names in programs, the designated 250 expert is requested to ensure that feature names are usable as 251 identifiers in most programming languages, after lower-casing the 252 feature name in the registry entry and replacing whitespace with 253 underscores or hyphens, and that they also are distinct in this form. 255 The initial content of this registry is as follows: 257 +==============+=================+====================+ 258 | Feature code | Feature name | Specification | 259 +==============+=================+====================+ 260 | 0 | Reserved0 | RFCthis | 261 +--------------+-----------------+--------------------+ 262 | 1 | Reserved1 | RFCthis | 263 +--------------+-----------------+--------------------+ 264 | 2 | Reserved2 | RFCthis | 265 +--------------+-----------------+--------------------+ 266 | 3 | Reserved3 | RFCthis | 267 +--------------+-----------------+--------------------+ 268 | 4 | Secondary Units | RFCthis, [RFC8798] | 269 +--------------+-----------------+--------------------+ 271 Table 1: Features defined for SenML at the time of 272 writing 274 As the number of features that can be registered has a hard limit (48 275 codes left at the time of writing), the designated expert is 276 specifically instructed to maintain a frugal regime of code point 277 allocation, keeping code points available for SenML Features that are 278 likely to be useful for non-trivial subsets of the SenML ecosystem. 279 Quantitatively, the expert could for instance steer the allocation to 280 a target of not allocating more than 10 % of the remaining set per 281 year. 283 Where the specification of the feature code is provided in a document 284 that is separate from the specification of the feature itself (as 285 with feature code 4 above), both specifications should be listed. 287 7. References 289 7.1. Normative References 291 [C] International Organization for Standardization, 292 "Information technology — Programming languages — C", ISO/ 293 IEC 9899:2018, Fourth Edition, June 2018, 294 . 296 [Cplusplus] 297 International Organization for Standardization, 298 "Programming languages — C++", ISO/IEC 14882:2020, Sixth 299 Edition, December 2020, 300 . 302 [IANA.senml] 303 IANA, "Sensor Measurement Lists (SenML)", 304 . 306 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 307 Requirement Levels", BCP 14, RFC 2119, 308 DOI 10.17487/RFC2119, March 1997, 309 . 311 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 312 Writing an IANA Considerations Section in RFCs", BCP 26, 313 RFC 8126, DOI 10.17487/RFC8126, June 2017, 314 . 316 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 317 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 318 May 2017, . 320 [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. 321 Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, 322 DOI 10.17487/RFC8428, August 2018, 323 . 325 [RFC8798] Bormann, C., "Additional Units for Sensor Measurement 326 Lists (SenML)", RFC 8798, DOI 10.17487/RFC8798, June 2020, 327 . 329 7.2. Informative References 331 [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, 332 DOI 10.17487/RFC7493, March 2015, 333 . 335 Acknowledgements 337 Ari Keränen proposed to use the version number as a bitmap and 338 provided further input on this specification. Jaime Jiménez helped 339 clarify the document by providing a review. Elwyn Davies provided a 340 detailed GENART review, with directly implementable text suggestions 341 that now form part of this specification. Rob Wilton supplied 342 comments one of which became the last paragraph of Section 2.1; Éric 343 Vyncke helped with Section 2. Additional thanks go to the other IESG 344 reviewers. 346 Author's Address 348 Carsten Bormann 349 Universitaet Bremen TZI 350 Postfach 330440 351 D-28359 Bremen 352 Germany 354 Phone: +49-421-218-63921 355 Email: cabo@tzi.org