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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

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   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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].





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   {"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.




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   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.






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9.  References

9.1.  Normative References

   [IANA.senml]
              IANA, "Sensor Measurement Lists (SenML)",
              <http://www.iana.org/assignments/senml>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8428]  Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
              Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
              DOI 10.17487/RFC8428, August 2018,
              <https://www.rfc-editor.org/info/rfc8428>.

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",
              <http://www.iana.org/assignments/core-parameters>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.





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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

































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