Ericsson

Jorvas 02420 Finland ari.keranen@ericsson.com

Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

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.

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 .

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Table of Contents

• .  Introduction
  • .  Evolution
• .  Terminology
• .  SenML Content-Format ("ct") Field
• .  SenML Base Content-Format ("bct") Field
• .  Examples
• .  ABNF
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Authors' Addresses

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

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 ): 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.

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 [ {"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} ]

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 ; 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)

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 This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This specification defines a format for representing simple sensor measurements and device parameters in Sensor Measurement Lists (SenML). Representations are defined in JavaScript Object Notation (JSON), Concise Binary Object Representation (CBOR), Extensible Markup Language (XML), and Efficient XML Interchange (EXI), which share the common SenML data model. A simple sensor, such as a temperature sensor, could use one of these media types in protocols such as HTTP or the Constrained Application Protocol (CoAP) to transport the measurements of the sensor or to be configured. Informative References Several questions have been raised about the requirements and administrative procedure for registering MIME content-type and subtypes, and the use of these Media Types for other applications. This document addresses these issues and specifies a procedure for the registration of new Media Types (content-type/subtypes). It also generalizes the scope of use of these Media Types to make it appropriate to use the same registrations and specifications with other applications. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. This RFC documents the format used for Comma-Separated Values (CSV) files and registers the associated MIME type "text/csv". This memo provides information for the Internet community. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. This memo defines the Session Description Protocol (SDP). SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. This document obsoletes RFC 4566. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format.

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.

Authors' Addresses Ericsson

Jorvas 02420 Finland ari.keranen@ericsson.com

Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org