Status of This Memo

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Table of Contents

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"}
]

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"}

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)

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}
]

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)

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:¶

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

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