Network Working Group                                         A. Keränen
Internet-Draft                                                  Ericsson
Intended status: Standards Track                              C. Bormann
Expires: 9 January 2022                           Universität Bremen TZI
                                                             8 July 2021


               SenML Data Value Content-Format Indication
                    draft-ietf-core-senml-data-ct-04

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.

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   This Internet-Draft will expire on 9 January 2022.

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   provided without warranty as described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SenML Content-Format ("ct") Field . . . . . . . . . . . . . .   4
   4.  SenML Base Content-Format ("bct") Field . . . . . . . . . . .   4
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Sensor Measurement Lists (SenML) media types [RFC8428] can be
   used to send various 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 interpret 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 interpretation 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 of [RFC7252]) provides
   just this information; enclosing a Content-Format number (in this
   case number 60 as defined for content-type application/cbor in
   [RFC8949]) 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].

   {"n":"nfc-reader", "vd":"gmNmb28YKg", "ct":"60"}

         Figure 2: SenML Record with binary data identified as CBOR



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   In this example SenML Record, the data value contains a string "foo"
   and a number 42 encoded in a CBOR [RFC8949] array.  Since the example
   above uses the JSON format of SenML, the data value containing the
   binary CBOR value is base64-encoded (Section 5 of [RFC4648]).  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.

   Media Type:  A registered label for representations (byte strings)
      prepared for interchange, identified by a Media-Type-Name
      [RFC1590], [RFC6838].

   Media-Type-Name:  A combination of a type-name and a subtype-name
      registered in [IANA.media-types] as per [RFC6838], conventionally
      identified by the two names separated by a slash.

   Content-Type:  A Media-Type-Name, optionally associated with
      parameters (separated from the media type name and from each other
      by a semicolon).  In HTTP and many other protocols, used in a
      "Content-Type" header field.

   Content-Coding:  a registered name for an encoding transformation
      that has been or can be applied to a representation.  Confusingly,
      in HTTP the Content-Coding is then given in a header field called
      "Content-Encoding"; we *never* use this term (except when we are
      in error).

   Content-Format:  the combination of a Content-Type and a Content-
      Coding, identified by (1) a numeric identifier defined by the
      "CoAP Content-Formats" subregistry of [IANA.core-parameters] as
      per [RFC7252] or (2) a Content-Format-String.

   Content-Format-String:  the string representation of the combination
      of a Content-Type and a Content-Coding.



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   Content-Format-Spec:  the string representation of a Content-Format;
      either a Content-Format-String or the (decimal) string
      representation of a Content-Format number.

   Readers should also be familiar with the terms and concepts discussed
   in [RFC8428].

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, using label "ct".
   The value of this field is a Content-Format-Spec, i.e., one of:

   *  a CoAP Content-Format identifier in decimal form with no leading
      zeros (except for the value "0" itself).  This value represents an
      unsigned integer in the range of 0-65535, similar to the CoRE Link
      Format [RFC6690] "ct" attribute).

   *  or a Content-Format-String containing a Content-Type and
      optionally a Content-Coding (see below).

   The syntax of this field is formally defined in Section 6.

   The CoAP Content-Format number provides a simple and efficient way to
   indicate the type of the data.  Since some Internet media types and
   their content coding and parameter alternatives do not have assigned
   CoAP Content-Format numbers, using Content-Type and Content-Coding is
   also allowed.  Both methods use a string value in the "ct" field to
   keep its data type consistent across uses.  When the "ct" field
   contains only digits, it is interpreted as a CoAP Content-Format
   identifier.

   To indicate that a Content-Coding is used with a Content-Type, the
   Content-Coding value (e.g., "deflate" [RFC7230]) is appended to the
   Content-Type value (media type and parameters, if any), separated by
   a "@" sign.  For example: "text/plain; charset=utf-8@deflate".  If no
   "@" sign is present outside the media type parameters, the Content-
   Coding is not specified and the "identity" Content-Coding is used --
   no encoding transformation is employed.

4.  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



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   the label "ct" to all Records in this range that carry a "vd" field
   but do not already contain a Content-Format ("ct") field.

5.  Examples

   The following examples are valid values for the "ct" and "bct" fields
   (explanation/comments in parenthesis):

   *  "60" (CoAP Content-Format for "application/cbor")

   *  "0" (CoAP Content-Format for "text/plain" with parameter
      "charset=utf-8")

   *  "application/json" (JSON Content-Type -- equivalent to "50" CoAP
      Content-Format identifier)

   *  "application/json@deflate" (JSON Content-Type with "deflate" as
      Content-Coding -- equivalent to "11050" CoAP Content-Format
      identifier)

   *  "text/csv" (Comma-Separated Values (CSV) [RFC4180] Content-Type)

   *  "text/csv@gzip" (CSV with "gzip" as Content-Coding)

6.  ABNF

   This specification provides a formal definition of the syntax of
   Content-Format-Spec strings using ABNF notation [RFC5234], which
   contains three new rules and a number of rules collected and adapted
   from various RFCs [RFC7231] [RFC6838] [RFC5234] [RFC8866].

   ; New in this document

   Content-Format-Spec = Content-Format-Number / Content-Format-String

   Content-Format-Number = "0" / (POS-DIGIT *DIGIT)
   Content-Format-String   = Content-Type ["@" Content-Coding]

   ; Cleaned up from RFC 7231:

   Content-Type   = Media-Type-Name *( *SP ";" *SP parameter )
   parameter      = token "=" ( token / quoted-string )

   token          = 1*tchar
   tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                  / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                  / DIGIT / ALPHA
   quoted-string  = %x22 *qdtext %x22



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   qdtext         = SP / %x21 / %x23-5B / %x5D-7E

   ; Adapted from section 3.1.2.1 of RFC 7231

   Content-Coding   = token

   ; Adapted from various specs

   Media-Type-Name = type-name "/" subtype-name

   ; RFC 6838

   type-name = restricted-name
   subtype-name = restricted-name

   restricted-name = restricted-name-first *126restricted-name-chars
   restricted-name-first  = ALPHA / DIGIT
   restricted-name-chars  = ALPHA / DIGIT / "!" / "#" /
                            "$" / "&" / "-" / "^" / "_"
   restricted-name-chars =/ "." ; Characters before first dot always
                                ; specify a facet name
   restricted-name-chars =/ "+" ; Characters after last plus always
                                ; specify a structured syntax suffix


   ; Boilerplate from RFC 5234 and RFC 8866

   DIGIT     =  %x30-39           ; 0 – 9
   POS-DIGIT =  %x31-39           ; 1 – 9
   ALPHA     =  %x41-5A / %x61-7A ; A – Z / a – z
   SP        =  %x20

                Figure 4: ABNF syntax of Content-Format-Spec

7.  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.  This includes
   "decompression bombs", compressed data that is crafted to decompress
   to extremely large data items.

8.  IANA Considerations

   (Note to RFC Editor: Please replace all occurrences of "RFC-AAAA"
   with the RFC number of this specification and remove this note.)



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   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-Format | bct   | String    | string   | RFC-AAAA  |
    +---------------------+-------+-----------+----------+-----------+
    |      Content-Format | ct    | String    | string   | RFC-AAAA  |
    +---------------------+-------+-----------+----------+-----------+

             Table 1: IANA Registration for new SenML Labels

9.  References

9.1.  Normative References

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <http://www.iana.org/assignments/core-parameters>.

   [IANA.media-types]
              IANA, "Media Types",
              <http://www.iana.org/assignments/media-types>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.






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   [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

   [RFC1590]  Postel, J., "Media Type Registration Procedure", RFC 1590,
              DOI 10.17487/RFC1590, March 1994,
              <https://www.rfc-editor.org/info/rfc1590>.

   [RFC4180]  Shafranovich, Y., "Common Format and MIME Type for Comma-
              Separated Values (CSV) Files", RFC 4180,
              DOI 10.17487/RFC4180, October 2005,
              <https://www.rfc-editor.org/info/rfc4180>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

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

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,
              <https://www.rfc-editor.org/info/rfc8866>.




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   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

Acknowledgements

   The authors would like to thank Sérgio Abreu for the discussions
   leading to the design of this extension and Isaac Rivera for reviews
   and feedback.  Klaus Hartke suggested not burdening this draft with a
   separate mandatory-to-implement version of the fields.  Alexey
   Melnikov, Jim Schaad, and Thomas Fossati provided helpful comments at
   Working-Group last call.  Marco Tiloca asked for clarifying and using
   the term Content-Format-Spec.

Authors' Addresses

   Ari Keränen
   Ericsson
   FI-02420 Jorvas
   Finland

   Email: ari.keranen@ericsson.com


   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org


















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