]> Ericsson

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Universität Bremen TZI

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Internet of Things (IoT) Internet of Things IOT data model media type 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.

Introduction The Sensor Measurement Lists (SenML) media types can be used to send various kinds of data. In the example given in , 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 .

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 () provides this information in the form of a single unsigned integer. For instance, defines the Content-Format number 60 for Content-Type application/cbor. Enclosing this Content-Format number in the Record is illustrated in . All registered CoAP Content-Format numbers are listed in the "" registry , as specified by . 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.

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) 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 .

Example Data Value in CBOR Diagnostic Notation

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.

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 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.

Media-Type-Name:

A combination of a type-name and a subtype-name registered in , as per , 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, it is used in a Content-Type header field.

Content coding:

A name registered in the "" , as specified by Sections and of , indicating an encoding transformation with semantics further specified in . 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 "" registry , as per (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 .

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 for CoRE Link Format .
• 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 . 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 ). For example (using a content coding value of "deflate", as defined in ): 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.

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 () 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. shows a variation of 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).

SenML Pack with the bct Field

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) Content-Type)
• "text/csv;header=present@gzip" (CSV with header row, using "gzip" as content coding)

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

ABNF Syntax of Content-Format-Spec

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.

IANA Considerations IANA has assigned the following new labels in the "" subregistry of the "Sensor Measurement Lists (SenML)" registry (as defined in ) for the Content-Format indication, as per : IANA Registration for New SenML Labels

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

Note that, per , no CBOR labels nor Efficient XML Interchange (EXI) schemaId values (EXI ID column) are supplied.

References Normative References IANA IANA IANA IANA Informative References

Acknowledgments The authors would like to thank for the discussions leading to the design of this extension and for reviews and feedback. suggested not burdening this document with a separate mandatory-to-implement version of the fields. , , and provided helpful comments at Working Group Last Call. asked for clarifying and using the term Content-Format-Spec.