Internet Engineering Task Force (IETF)                        A. Keränen
Request for Comments: 9193                                      Ericsson
Category: Standards Track                                     C. Bormann
ISSN: 2070-1721                                   Universität Bremen TZI
                                                              June 2022


  Sensor Measurement Lists (SenML) Fields for Indicating Data Value
                            Content-Format

Abstract

  The Sensor Measurement Lists (SenML) media types support multiple
  types of values, from numbers to text strings and arbitrary binary
  Data Values.  In order to facilitate processing of binary Data
  Values, this document specifies a pair of new SenML fields for
  indicating the content format of those binary Data Values, i.e.,
  their Internet media type, including parameters as well as any
  content codings applied.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9193.

Copyright Notice

  Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

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

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 a Near Field
  Communication (NFC) reader is sent in a single SenML Pack.  The
  example is given in SenML JSON representation, so the "vd" (Data
  Value) field is encoded as a base64url string (without padding), as
  per Section 5 of [RFC8428].

  [
    {"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., the 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, especially if the Pack is propagated over time and via multiple
  entities.  To facilitate automatic interpretation, it is useful to be
  able to indicate an Internet media type and, optionally, content
  codings right in the SenML Record.

  The Constrained Application Protocol (CoAP) Content-Format
  (Section 12.3 of [RFC7252]) provides this information in the form of
  a single unsigned integer.  For instance, [RFC8949] defines the
  Content-Format number 60 for Content-Type application/cbor.
  Enclosing this Content-Format number in the Record is illustrated in
  Figure 2.  All registered CoAP Content-Format numbers are listed in
  the "CoAP Content-Formats" registry [IANA.core-parameters], as
  specified by Section 12.3 of [RFC7252].  Note that, at the time of
  writing, the structure of this registry only provides for zero or one
  content coding; nothing in the present document needs to change if
  the registry is extended to allow sequences of content codings.

  {"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 Concise Binary Object Representation
  (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

1.1.  Evolution

  As with SenML in general, there is no expectation that the creator of
  a SenML Pack knows (or has negotiated with) each consumer of that
  Pack, which may be very remote in space and particularly in time.
  This means that the SenML creator in general has no way to know
  whether the consumer knows:

  *  each specific Media-Type-Name used,

  *  each parameter and each parameter value used,

  *  each content coding in use, and

  *  each Content-Format number in use for a combination of these.

  What SenML, as well as the new fields defined here, guarantees is
  that a recipient implementation _knows_ when it needs to be updated
  to understand these field values and the values controlled by them;
  registries are used to evolve these name spaces in a controlled way.
  SenML Packs can be processed by a consumer while not understanding
  all the information in them, and information can generally be
  preserved in this processing such that it is useful for further
  consumers.

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 [RFC1590] [RFC6838], identified by a
     Media-Type-Name.

  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 (Section 5 of [RFC2045], separated from the Media-Type-
     Name and from each other by a semicolon).  In HTTP and many other
     protocols, it is used in a Content-Type header field.

  Content coding:  A name registered in the "HTTP Content Coding
     Registry" [IANA.http-parameters], as specified by Sections 16.6.1
     and 18.6 of [RFC9110], indicating an encoding transformation with
     semantics further specified in Section 8.4.1 of [RFC9110].
     Confusingly, in HTTP, content coding values are found in a header
     field called "Content-Encoding"; however, "content coding" is the
     correct term for the process and the registered values.

  Content format:  The combination of a Content-Type and zero or more
     content codings, identified by (1) a numeric identifier defined in
     the "CoAP Content-Formats" registry [IANA.core-parameters], as per
     Section 12.3 of [RFC7252] (referred to as Content-Format number),
     or (2) a Content-Format-String.

  Content-Format-String:  The string representation of the combination
     of a Content-Type and zero or more content codings.

  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 the
  following:

  *  a CoAP Content-Format number 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 "ct"
     attribute defined in Section 7.2.1 of [RFC7252] for CoRE Link
     Format [RFC6690].

  *  a Content-Format-String containing a Content-Type and zero or more
     content codings (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 zero or more
  content codings 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 number.

  To indicate that one or more content codings are used with a Content-
  Type, each of the content coding values is appended to the Content-
  Type value (media type and parameters, if any), separated by an "@"
  sign, in the order of when the content codings were applied (the same
  order as in Section 8.4 of [RFC9110]).  For example (using a content
  coding value of "deflate", as defined in Section 8.4.1.2 of
  [RFC9110]):

  text/plain; charset=utf-8@deflate

  If no "@" sign is present after the media type and parameters, then
  no content coding has been 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.  The process of resolving
  (Section 4.6 of [RFC8428]) 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.

  Figure 4 shows a variation of Figure 2 with multiple records, with
  the "nfc-reader" records resolving to the base field value "60" and
  the "iris-photo" record overriding this with the "image/png" media
  type (actual data left out for brevity).

  [
    {"n":"nfc-reader", "vd":"gmNmb28YKg",
     "bct":"60", "bt":1627430700},
    {"n":"nfc-reader", "vd":"gmNiYXIYKw", "t":10},
    {"n":"iris-photo", "vd":".....", "ct":"image/png", "t":10},
    {"n":"nfc-reader", "vd":"gmNiYXoYLA", "t":20}
  ]

                 Figure 4: SenML Pack with the bct Field

5.  Examples

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

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

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

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

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

  *  "application/json@deflate@aes128gcm" (JSON Content-Type with
     "deflate" followed by "aes128gcm" as content codings)

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

  *  "text/csv;header=present@gzip" (CSV with header row, using "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 [RFC9110] [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 9110,
  ; leaving only SP as blank space,
  ; removing legacy 8-bit characters, and
  ; leaving the parameter as mandatory with each semicolon:

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

  token          = 1*tchar
  tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                 / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                 / DIGIT / ALPHA
  quoted-string  = %x22 *( qdtext / quoted-pair ) %x22
  qdtext         = SP / %x21 / %x23-5B / %x5D-7E
  quoted-pair    = "\" ( SP / VCHAR )

  ; Adapted from Section 8.4.1 of RFC 9110

  Content-Coding   = token

  ; Adapted from various specs

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

  ; From 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
  VCHAR     =  %x21-7E           ; printable ASCII (no SP)

               Figure 5: 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 specifies
  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

  IANA has assigned the following new labels in the "SenML Labels"
  subregistry of the "Sensor Measurement Lists (SenML)" registry
  [IANA.senml] (as defined in Section 12.2 of [RFC8428]) for the
  Content-Format indication, as per Table 1:

   +=====================+=======+===========+==========+===========+
   |                Name | Label | JSON Type | XML Type | Reference |
   +=====================+=======+===========+==========+===========+
   | Base Content-Format | bct   | String    | string   | RFC 9193  |
   +---------------------+-------+-----------+----------+-----------+
   |      Content-Format | ct    | String    | string   | RFC 9193  |
   +---------------------+-------+-----------+----------+-----------+

            Table 1: IANA Registration for New SenML Labels

  Note that, per Section 12.2 of [RFC8428], no CBOR labels nor
  Efficient XML Interchange (EXI) schemaId values (EXI ID column) are
  supplied.

9.  References

9.1.  Normative References

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

  [IANA.http-parameters]
             IANA, "Hypertext Transfer Protocol (HTTP) Parameters",
             <https://www.iana.org/assignments/http-parameters>.

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

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

  [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
             <https://www.rfc-editor.org/info/rfc2045>.

  [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>.

  [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>.

  [RFC9110]  Fielding, R., Nottingham, M., and J. Reschke, "HTTP
             Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110,
             February 2022, <https://www.rfc-editor.org/rfc/rfc9110>.

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>.

  [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>.

  [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>.

Acknowledgments

  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 document
  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: [email protected]


  Carsten Bormann
  Universität Bremen TZI
  Postfach 330440
  D-28359 Bremen
  Germany
  Phone: +49-421-218-63921
  Email: [email protected]

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