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stand_alone: true ipr: trust200902 docname: draft-ietf-core-senml-data-ct-latest keyword: Internet-Draft cat: std consensus: true pi: toc: 'yes' tocompact: 'yes' tocdepth: '3' tocindent: 'yes' symrefs: 'yes' sortrefs: 'yes' comments: 'yes' inline: 'yes' strict: 'no' compact: 'no' subcompact: 'no' title: SenML Data Value Content-Format Indication abbrev: SenML Data Content-Format Indication date: 2021 author:

  • ins: A. Keränen name: Ari Keränen org: Ericsson street: '' city: Jorvas code: '02420' country: Finland email: ari.keranen@ericsson.com
  • ins: C. Bormann name: Carsten Bormann org: Universität Bremen TZI street: Postfach 330440 city: Bremen code: D-28359 country: Germany phone: +49-421-218-63921 email: cabo@tzi.org

normative: IANA.senml: RFC7252: coap RFC5234: abnf RFC7230: http0 RFC7231: http IANA.media-types: media-types IANA.core-parameters: core-parameters IANA.http-parameters: http-parameters informative: RFC4648: base RFC8949: cbor RFC6838: mediatype-reg

--- 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.

--- middle

Introduction {#intro}

The Sensor Measurement Lists (SenML) media types {{!RFC8428}} can be used to send various kinds of data. In the example given in {{ex-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"}
]

{: #ex-1 title="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 -coap}}) provides just this information; enclosing a Content-Format number (in this case number 60 as defined for content-type application/cbor in {{-cbor}}) in the Record is illustrated in {{ex-2}}. All registered CoAP Content-Formats are listed in the {{content-formats (COAP Content-Formats registry)<IANA.core-parameters}} {{-core-parameters}} as specified by {{Section 12.3 of -coap}}.

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

{: #ex-2 title="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 {{-cbor}} 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 -base}}). The data value after base64 decoding is shown with CBOR diagnostic notation in {{ex-2-cbor}}.

82           # array(2)
   63        # text(3)
      666F6F # "foo"
   18 2A     # unsigned(42)

{: #ex-2-cbor title="Example Data Value in CBOR diagnostic notation"}

Terminology

{::boilerplate bcp14-tagged}

Media Type: : A registered label for representations (byte strings) prepared for interchange, identified by a Media-Type-Name {{?RFC1590}}, {{-mediatype-reg}}.

Media-Type-Name: : A combination of a type-name and a subtype-name registered in {{-media-types}} as per {{-mediatype-reg}}, 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 name registered in the {{content-coding (HTTP Content Coding registry)<IANA.http-parameters}} {{-http-parameters}} as specified by {{Section 8.5 of RFC7230}}, indicating an encoding transformation with semantics further specified in {{Section 3.1.2.1 of RFC7231}}. Confusingly, in HTTP the Content-Coding is found in a header field called "Content-Encoding", however "Content-Coding" is the correct term.

Content-Format: : the combination of a Content-Type and a Content-Coding, identified by (1) a numeric identifier defined in the {{content-formats (COAP Content-Formats registry)<IANA.core-parameters}} {{-core-parameters}} as per {{Section 12.3 of -coap}}, or (2) a Content-Format-String.

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

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}}.

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 {{abnf}}.

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.

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.

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)

ABNF

This specification provides a formal definition of the syntax of Content-Format-Spec strings using ABNF notation {{-abnf}}, which contains three new rules and a number of rules collected and adapted from various RFCs {{-http}} {{-mediatype-reg}} {{-abnf}} {{?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
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

{: #content-format-spec title="ABNF syntax of Content-Format-Spec"}

Security Considerations {#seccons}

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.

IANA Considerations {#iana}

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

IANA is requested to assign new labels in the "SenML Labels" {{subregistry<IANA.senml}}{: relative="#senml-labels"} of the SenML registry {{IANA.senml}} (as defined in {{RFC8428}}) for the Content-Format indication as per {{tbl-senml-reg}}:

| Name | Label | JSON Type | XML Type | Reference | | Base Content-Format | bct | String | string | RFC-AAAA | | Content-Format | ct | String | string | RFC-AAAA | {: #tbl-senml-reg cols='r l l' title="IANA Registration for new SenML Labels"}

--- back

Acknowledgements {#acks}

{: numbered="no"}

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.