Network Working Group A. Keranen Internet-Draft Ericsson Intended status: Standards Track C. Bormann Expires: January 9, 2020 Universitaet Bremen TZI July 8, 2019 SenML Data Value Content-Format Indication draft-keranen-core-senml-data-ct-02 Abstract The Sensor Measurement Lists (SenML) media type supports multiple types of values, from numbers to text strings and arbitrary binary data values. In order to simplify processing of the data values this document proposes to specify a new SenML field for indicating the Content-Format of the data. 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/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Keranen & Bormann Expires January 9, 2020 [Page 1] Internet-Draft SenML Data Value Content-Format Indication July 2019 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. SenML Content-Format ("ct") Field . . . . . . . . . . . . . . 3 4. Using Content-Type and Content-Coding With the "ct" Field . . 3 5. SenML Base Content-Format ("bct") Field . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 4 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 9.2. Informative References . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction The Sensor Measurement Lists (SenML) media type [RFC8428] can be used to send various different kinds of data. In the example given in Figure 1, a temperature value, an indication whether a lock is open, and a data value (with SenML field "vd") read from an NFC reader is sent in a single SenML pack. [ {"bn":"urn:dev:ow:10e2073a01080063:","n":"temp","u":"Cel","v":7.1}, {"n":"open","vb":false}, {"n":"nfc-reader","vd":"aGkgCg"} ] Figure 1: SenML pack with unidentified binary data The receiver is expected to know how to decode the data in the "vd" field based on the context, e.g., name of the data source and out-of- band knowledge of the application. However, this context may not always be easily available to entities processing the SenML pack. To facilitate automatic decoding it is useful to be able to indicate an Internet media type and content-coding right in the SenML Record. The CoAP Content-Format (Section 12.3 in [RFC7252]) provides just this information; enclosing a Content-Format number (in this case number 60 as defined for media type application/cbor in [RFC7049]) in the Record is illustrated in Figure 2. All registered CoAP content formats are listed in the Content Formats subregistry of the CoRE Parameters registry [IANA.core-parameters]. Keranen & Bormann Expires January 9, 2020 [Page 2] Internet-Draft SenML Data Value Content-Format Indication July 2019 {"n":"nfc-reader", "vd":"gmNmb28YKg", "ct":60} Figure 2: SenML Record with binary data identified as CBOR In this example SenML Record the data value contains a string "foo" and a number 42 encoded in a CBOR [RFC7049] array. Since the example above uses the JSON format of SenML, the data value containing the binary CBOR value is base64-encoded. The data value after base64 decoding is shown with CBOR diagnostic notation in Figure 3. 82 # array(2) 63 # text(3) 666F6F # "foo" 18 2A # unsigned(42) Figure 3: Example Data Value in CBOR diagnostic notation 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Readers should also be familiar with the terms and concepts discussed in [RFC8428]. Awareness of terminology issues discussed in [I-D.bormann-core-media-content-type-format] can also be very helpful. 3. SenML Content-Format ("ct") Field When a SenML Record contains a Data Value field ("vd"), the Record MAY also include a Content-Format indication field. The Content- Format indication uses label "ct" and an unsigned integer value in the range of 0-65535 indicating the CoAP Content-Format of the data (similar to CoRE Link Format [RFC6690] "ct" attribute, except that the Link Format attribute happens to be encoded as a string). 4. Using Content-Type and Content-Coding With the "ct" Field Since some Internet media types and their content coding and parameter alternatives do not have assigned CoAP Content-Format identifiers, the "ct" field can alternatively be used to indicate the Content-Type and Content-Coding of the data. If Content-Coding is not specified with Content-Type, identity (i.e., no) transformation is used. Keranen & Bormann Expires January 9, 2020 [Page 3] Internet-Draft SenML Data Value Content-Format Indication July 2019 If Content-Coding is used, the content-coding value (e.g., "deflate") MUST be prefixed with "@" and concatenated with the media type and parameters (if any). For example: "application/json@deflate". 5. SenML Base Content-Format ("bct") Field The Base Content-Format Field, label "bct", provides a default value for the Content-Format Field (label "ct") within its range. The range of the base field includes the record containing it, up to (but not including) the next record containing a "bct" field, if any, or up to the end of the pack otherwise. Resolution (Section 4.6 of [RFC8428]) of this base field is performed by adding its value with the label "ct" to all records in this range that carry a "vd" field but do not already contain a Content-Format ("ct") field. 6. Security Considerations The indication of a media type in the data does not exempt a consuming application from properly checking its inputs. Also, the ability for an attacker to supply crafted SenML data that specify media types chosen by the attacker may expose vulnerabilities of handlers for these media types to the attacker. 7. IANA Considerations IANA is requested to assign new labels in the "SenML Labels" subregistry of the SenML registry [IANA.senml] (as defined in [RFC8428]) for the Content-Format indication as per Table 1: +-----------------+-------+---------------+------------+------------+ | Name | Label | JSON Type | XML Type | Reference | +-----------------+-------+---------------+------------+------------+ | Base Content- | bct | Number/String | int/string | this | | Format | | | | document | | | | | | | | Content-Format | ct | Number/String | int/string | this | | | | | | document | +-----------------+-------+---------------+------------+------------+ Table 1: IANA Registration for new SenML Labels Acknowledgements The authors would like to thank Sergio Abreu for the discussions leading to the design of this extension. Keranen & Bormann Expires January 9, 2020 [Page 4] Internet-Draft SenML Data Value Content-Format Indication July 2019 9. References 9.1. Normative References [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, . [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, . [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C. Bormann, "Sensor Measurement Lists (SenML)", RFC 8428, DOI 10.17487/RFC8428, August 2018, . 9.2. Informative References [I-D.bormann-core-media-content-type-format] Bormann, C., "On Media-Types, Content-Types, and related terminology", draft-bormann-core-media-content-type- format-00 (work in progress), March 2019. [IANA.core-parameters] IANA, "Constrained RESTful Environments (CoRE) Parameters", . [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, . Keranen & Bormann Expires January 9, 2020 [Page 5] Internet-Draft SenML Data Value Content-Format Indication July 2019 Authors' Addresses Ari Keranen Ericsson Jorvas 02420 Finland Email: ari.keranen@ericsson.com Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Keranen & Bormann Expires January 9, 2020 [Page 6]