Media Types for Sensor Markup Language (SenML)
draft-ietf-core-senml-00

Abstract

This specification defines media types for representing simple sensor measurements and device parameters in the Sensor Markup Language (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 this media type in protocols such as HTTP or CoAP to transport the measurements of the sensor or to be configured.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on October 21, 2016.

Copyright Notice

Copyright (c) 2016 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 (http://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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Overview
• 2. Requirements and Design Goals
• 3. Terminology
• 4. Semantics
• 5. Associating Meta-data
• 6. JSON Representation (application/senml+json)
• 6.1. Examples
• 6.1.1. Single Datapoint
• 6.1.2. Multiple Datapoints
• 6.1.3. Multiple Measurements
• 6.1.4. Collection of Resources
• 7. CBOR Representation (application/senml+cbor)
• 8. XML Representation (application/senml+xml)
• 9. EXI Representation (application/senml-exi)
• 10. Usage Considerations
• 11. CDDL
• 12. IANA Considerations
• 12.1. Units Registry
• 12.2. SenML label registry
• 12.3. Media Type Registration
• 12.3.1. senml+json Media Type Registration
• 12.3.2. senml+cbor Media Type Registration
• 12.3.3. senml+xml Media Type Registration
• 12.3.4. senml-exi Media Type Registration
• 12.4. XML Namespace Registration
• 12.5. CoAP Content-Format Registration
• 13. Security Considerations
• 14. Privacy Considerations
• 15. Acknowledgement
• 16. References
• 16.1. Normative References
• 16.2. Informative References
• Appendix A. Links extension
• Authors' Addresses

1. Overview

Connecting sensors to the internet is not new, and there have been many protocols designed to facilitate it. This specification defines new media types for carrying simple sensor information in a protocol such as HTTP or CoAP called the Sensor Markup Language (SenML). This format was designed so that processors with very limited capabilities could easily encode a sensor measurement into the media type, while at the same time a server parsing the data could relatively efficiently collect a large number of sensor measurements. The markup language can be used for a variety of data flow models, most notably data feeds pushed from a sensor to a collector, and the web resource model where the sensor is requested as a resource representation (e.g., “GET /sensor/temperature”).

There are many types of more complex measurements and measurements that this media type would not be suitable for. SenML strikes a balance between having some information about the sensor carried with the sensor data so that the data is self describing but it also tries to make that a fairly minimal set of auxiliary information for efficiency reason. Other information about the sensor can be discovered by other methods such as using the CoRE Link Format [RFC6690].

SenML is defined by a data model for measurements and simple meta-data about measurements and devices. The data is structured as a single array that contains a series of SenML Records which can each contain attributes such as an unique identifier for the sensor, the time the measurement was made, the unit the measurement is in, and the current value of the sensor. Serializations for this data model are defined for JSON [RFC7159], CBOR [RFC7049], XML, and Efficient XML Interchange (EXI) [W3C.REC-exi-20110310].

For example, the following shows a measurement from a temperature gauge encoded in the JSON syntax.

[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.1, "u":"Cel" }]

In the example above, the array has a single SenML Record with a measurement for a sensor named “urn:dev:ow:10e2073a01080063” with a current value of 23.5 degrees Celsius.

2. Requirements and Design Goals

The design goal is to be able to send simple sensor measurements in small packets on mesh networks from large numbers of constrained devices. Keeping the total size of payload under 80 bytes makes this easy to use on a wireless mesh network. It is always difficult to define what small code is, but there is a desire to be able to implement this in roughly 1 KB of flash on a 8 bit microprocessor. Experience with Google power meter and large scale deployments has indicated that the solution needs to support allowing multiple measurements to be batched into a single HTTP or CoAP request. This “batch” upload capability allows the server side to efficiently support a large number of devices. It also conveniently supports batch transfers from proxies and storage devices, even in situations where the sensor itself sends just a single data item at a time. The multiple measurements could be from multiple related sensors or from the same sensor but at different times.

The basic design is an array with a series of measurements. The following example shows two measurements made at different times. The value of a measurement is in the “v” tag, the time of a measurement is in the “t” tag, the “n” tag has a unique sensor name, and the unit of the measurement is carried in the “u” tag.

[
    { "n": "urn:dev:ow:10e2073a01080063",
      "t": 1276020076, "v":23.5, "u":"Cel" },
    { "n": "urn:dev:ow:10e2073a01080063",
      "t": 1276020091, "v":23.6, "u":"Cel" }
]

To keep the messages small, it does not make sense to repeat the “n” tag in each SenML Record so there is a concept of a Base Name which is simply a string that is prepended to the Name field of all elements in that record and any records that follow it. So a more compact form of the example above is the following.

[
    { "bn": "urn:dev:ow:10e2073a01080063",
      "t": 1276020076, "v":23.5, "u":"Cel" },
    {  "t": 1276020091, "v":23.6, "u":"Cel" }
]

In the above example the Base Name is in the “bn” tag and the “n” tags in each Record are the empty string so they are omitted. The Base Name also could be put in a separate Record such as in the following example.

[
    { "bn": "urn:dev:ow:10e2073a01080063" },
    { "t": 1276020076, "v":23.5, "u":"Cel" },
    { "t": 1276020091, "v":23.6, "u":"Cel" }
]

Some devices have accurate time while others do not so SenML supports absolute and relative times. Time is represented in floating point as seconds and values greater than zero represent an absolute time relative to the unix epoch while values of 0 or less represent a relative time in the past from the current time. A simple sensor with no absolute wall clock time might take a measurement every second and batch up 60 of them then send it to a server. It would include the relative time the measurement was made to the time the batch was send in the SenML Pack. The server might have accurate NTP time and use the time it received the data, and the relative offset, to replace the times in the SenML with absolute times before saving the SenML Pack in a document database.

3. 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 [RFC2119].

This document also uses the following terms:

SenML Record:

One measurement or configuration instance in time presented using the SenML data model.

SenML Pack:

One or more SenML Records in an array structure.

4. Semantics

Each SenML Pack carries a single array that represents a set of measurements and/or parameters. This array contains a series of objects with several optional attributes described below:

Base Name:

This is a string that is prepended to the names found in the entries. This attribute is optional. This applies to the entries in all Records. A Base Name can only be included in the first Record of the array.

Base Time:

A base time that is added to the time found in an entry. This attribute is optional. This applies to the entries in all Records. A Base Time can only be included in the first Record of the array.

Base Unit:

A base unit that is assumed for all entries, unless otherwise indicated. This attribute is optional. If a record does not contain a unit value, then the base unit is used otherwise the value of found in the Unit is used. This applies to the entries in all Records. A Base Unit can only be included in the first object of the array.

Base Value:

A base value is added to the value found in an entry, similar to Base Time. This attribute is optional. This applies to the entries in all Records. A Base Value can only be included in the first Record of the array.

Version:

Version number of media type format. This attribute is optional positive integer and defaults to 5 if not present. A Version can only be included in the first object of the array.

Name:

Name of the sensor or parameter. When appended to the Base Name attribute, this must result in a globally unique identifier for the resource. The name is optional, if the Base Name is present. If the name is missing, Base Name must uniquely identify the resource. This can be used to represent a large array of measurements from the same sensor without having to repeat its identifier on every measurement.

Unit:

Units for a measurement value. Optional. If the Record has not Unit, the Base Unit is used as the Unit. Having no Unit and no Base Unit is allowed.

Value

Value of the entry. Optional if a Sum value is present, otherwise required. Values are represented using three basic data types, Floating point numbers (“v” field for “Value”), Booleans (“vb” for “Boolean Value”), Strings (“vs” for “String Value”) and Data (“vd” for “Binary Data Value”) . Exactly one of these three fields MUST appear unless there is Sum field in which case it is allowed to have no Value field or to have “v” field.

Sum:

Integrated sum of the values over time. Optional. This attribute is in the units specified in the Unit value multiplied by seconds.

Time:

Time when value was recorded. Optional.

Update Time:

An optional time in seconds that represents the maximum time before this sensor will provide an updated reading for a measurement. This can be used to detect the failure of sensors or communications path from the sensor.

The SenML format can be extended with further custom attributes. TODO - describe what extensions are possible and how to do them.

Systems reading one of the objects MUST check for the Version attribute. If this value is a version number larger than the version which the system understands, the system SHOULD NOT use this object. This allows the version number to indicate that the object contains mandatory to understand attributes. New version numbers can only be defined in an RFC that updates this specification or it successors.

The Name value is concatenated to the Base Name value to get the name of the sensor. The resulting name needs to uniquely identify and differentiate the sensor from all others. If the object is a representation resulting from the request of a URI [RFC3986], then in the absence of the Base Name attribute, this URI is used as the default value of Base Name. Thus in this case the Name field needs to be unique for that URI, for example an index or subresource name of sensors handled by the URI.

Alternatively, for objects not related to a URI, a unique name is required. In any case, it is RECOMMENDED that the full names are represented as URIs or URNs [RFC2141]. One way to create a unique name is to include some bit string that has guaranteed uniqueness (such as a 1-wire address) that is assigned to the device. Some of the examples in this draft use the device URN type as specified in [I-D.arkko-core-dev-urn]. UUIDs [RFC4122] are another way to generate a unique name. Note that long-term stable unique identifiers are problematic for privacy reasons [RFC7721] and should be used with care or avoided.

The resulting concatenated name MUST consist only of characters out of the set “A” to “Z”, “a” to “z”, “0” to “9”, “-“, “:”, “.”, or “_” and it MUST start with a character out of the set “A” to “Z”, “a” to “z”, or “0” to “9”. This restricted character set was chosen so that these names can be directly used as in other types of URI including segments of an HTTP path with no special encoding and can be directly used in many databases and analytic systems. [RFC5952] contains advice on encoding an IPv6 address in a name.

If either the Base Time or Time value is missing, the missing attribute is considered to have a value of zero. The Base Time and Time values are added together to get the time of measurement. A time of zero indicates that the sensor does not know the absolute time and the measurement was made roughly “now”. A negative value is used to indicate seconds in the past from roughly “now”. A positive value is used to indicate the number of seconds, excluding leap seconds, since the start of the year 1970 in UTC.

Representing the statistical characteristics of measurements, such as accuracy, can be very complex. Future specification may add new attributes to provide better information about the statistical properties of the measurement.

5. Associating Meta-data

SenML is designed to carry the minimum dynamic information about measurements, and for efficiency reasons does not carry significant static meta-data about the device, object or sensors. Instead, it is assumed that this meta-data is carried out of band. For web resources using SenML Packs, this meta-data can be made available using the CoRE Link Format [RFC6690]. The most obvious use of this link format is to describe that a resource is available in a SenML format in the first place. The relevant media type indicator is included in the Content-Type (ct=) attribute.

6. JSON Representation (application/senml+json)

Record atributes:

The root content consists of an array with JSON objects for each SenML Record. All the fields in the above table MAY occur in the records with the type specified in the table.

Only the UTF-8 form of JSON is allowed. Characters in the String Value are encoded using the escape sequences defined in [RFC4627]. Characters in the Data Value are base64 encoded with URL safe alphabet as defined in Section 5 of [RFC4648].

Systems receiving measurements MUST be able to process the range of floating point numbers that are representable as an IEEE double-precision floating-point numbers [IEEE.754.1985]. The number of significant digits in any measurement is not relevant, so a reading of 1.1 has exactly the same semantic meaning as 1.10. If the value has an exponent, the “e” MUST be in lower case. The mantissa SHOULD be less than 19 characters long and the exponent SHOULD be less than 5 characters long. This allows time values to have better than micro second precision over the next 100 years.

6.1. Examples

TODO - Add example with string, data, boolean, and base value

6.1.1. Single Datapoint

The following shows a temperature reading taken approximately “now” by a 1-wire sensor device that was assigned the unique 1-wire address of 10e2073a01080063:

[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.1, "u":"Cel" }]

6.1.2. Multiple Datapoints

The following example shows voltage and current now, i.e., at an unspecified time.

[{"bn": "urn:dev:ow:10e2073a01080063/"},
 { "n": "voltage", "t": 0, "u": "V", "v": 120.1 },
 { "n": "current", "t": 0, "u": "A", "v": 1.2 } 
]

The next example is similar to the above one, but shows current at Tue Jun 8 18:01:16 UTC 2010 and at each second for the previous 5 seconds.

[{"bn": "urn:dev:ow:10e2073a01080063/",
  "bt": 1276020076.001,
  "bu": "A",
  "bver": 5},
   { "n": "voltage", "u": "V", "v": 120.1 },
   { "n": "current", "t": -5, "v": 1.2 },
   { "n": "current", "t": -4, "v": 1.30 },
   { "n": "current", "t": -3, "v": 0.14e1 },
   { "n": "current", "t": -2, "v": 1.5 },
   { "n": "current", "t": -1, "v": 1.6 },
   { "n": "current", "t": 0,  "v": 1.7 } 
]

Note that in some usage scenarios of SenML the implementations MAY store or transmit SenML in a stream-like fashion, where data is collected over time and continuously added to the object. This mode of operation is optional, but systems or protocols using SenML in this fashion MUST specify that they are doing this. SenML defines a separate mime type (TODO) to indicate Sensor Streaming Markup Language (SensML) for this usage. In this situation the SensML stream can be sent and received in a partial fashion, i.e., a measurement entry can be read as soon as the SenML Record is received and not have to wait for the full SensML Stream to be complete.

For instance, the following stream of measurements may be sent via a long lived HTTP POST from the producer of a SensML to the consumer of that, and each measurement object may be reported at the time it measured:

[ {"bn": "urn:dev:ow:10e2073a01080063",
  "bt": 1320067464,
  "bu": "%RH"},
   { "v": 21.2, "t": 0 },
   { "v": 21.3, "t": 10 },
   { "v": 21.4, "t": 20 },
   { "v": 21.4, "t": 30 },
   { "v": 21.5, "t": 40 },
   { "v": 21.5, "t": 50 },
   { "v": 21.5, "t": 60 },
   { "v": 21.6, "t": 70 },
   { "v": 21.7, "t": 80 },
   { "v": 21.5, "t": 90 },
...

6.1.3. Multiple Measurements

The following example shows humidity measurements from a mobile device with an IPv6 address 2001:db8::1, starting at Mon Oct 31 13:24:24 UTC 2011. The device also provides position data, which is provided in the same measurement or parameter array as separate entries. Note time is used to for correlating data that belongs together, e.g., a measurement and a parameter associated with it. Finally, the device also reports extra data about its battery status at a separate time.

[{"bn": "urn:dev:ow:10e2073a01080063",
  "bt": 1320067464,
  "bu": "%RH"},
   { "v": 20.0, "t": 0 },
   { "v": 24.30621, "u": "lon", "t": 0 },
   { "v": 60.07965, "u": "lat", "t": 0 },  
   { "v": 20.3, "t": 60 },
   { "v": 24.30622, "u": "lon", "t": 60 },
   { "v": 60.07965, "u": "lat", "t": 60 },
   { "v": 20.7, "t": 120 },
   { "v": 24.30623, "u": "lon", "t": 120 },
   { "v": 60.07966, "u": "lat", "t": 120 },
   { "v": 98.0, "u": "%EL", "t": 150 },
   { "v": 21.2, "t": 180 },
   { "v": 24.30628, "u": "lon", "t": 180 },
   { "v": 60.07967, "u": "lat", "t": 180 } 
]

The size of this example represented in various forms, as well as that form compressed with gzip is given in the following table.

[{"bn": "urn:dev:ow:10e2073a01080063/",
  "bt": 1320078429,
  "bver": 5
  },
  { "n": "temperature", "v": 27.2, "u": "Cel" },
  { "n": "humidity", "v": 80, "u": "%RH" } 
]

0000 88 a4 62 62 6e 78 1c 75 72 6e 3a 64 65 76 3a 6f |..bbnx.urn:dev:o|
0010 77 3a 31 30 65 32 30 37 33 61 30 31 30 38 30 30 |w:10e2073a010800|
0020 36 33 2f 62 62 74 fb 41 d3 03 a1 5b 00 10 62 62 |63/bbt.A...[..bb|
0030 62 75 61 41 64 62 76 65 72 05 a3 61 6e 67 76 6f |buaAdbver..angvo|
0040 6c 74 61 67 65 61 75 61 56 61 76 fb 40 5e 06 66 |ltageauaVav.@^.f|
0050 66 66 66 66 a3 61 6e 67 63 75 72 72 65 6e 74 61 |ffff.angcurrenta|
0060 74 fb c0 14 00 00 00 00 00 00 61 76 fb 3f f3 33 |t.........av.?.3|
0070 33 33 33 33 33 a3 61 6e 67 63 75 72 72 65 6e 74 |33333.angcurrent|
0080 61 74 fb c0 10 00 00 00 00 00 00 61 76 fb 3f f4 |at.........av.?.|
0090 cc cc cc cc cc cd a3 61 6e 67 63 75 72 72 65 6e |.......angcurren|
00a0 74 61 74 fb c0 08 00 00 00 00 00 00 61 76 fb 3f |tat.........av.?|
00b0 f6 66 66 66 66 66 66 a3 61 6e 67 63 75 72 72 65 |.ffffff.angcurre|
00c0 6e 74 61 74 fb c0 00 00 00 00 00 00 00 61 76 fb |ntat.........av.|
00d0 3f f8 00 00 00 00 00 00 a3 61 6e 67 63 75 72 72 |?........angcurr|
00e0 65 6e 74 61 74 fb bf f0 00 00 00 00 00 00 61 76 |entat.........av|
00f0 fb 3f f9 99 99 99 99 99 9a a2 61 6e 67 63 75 72 |.?........angcur|
0100 72 65 6e 74 61 76 fb 3f fb 33 33 33 33 33 33    |rentav.?.333333|
010f

<sensml xmlns="urn:ietf:params:xml:ns:senml">
  <senml bn="urn:dev:ow:10e2073a01080063/" bt="1.276020076001e+09"
  bu="A" bver="5"></senml>
  <senml n="voltage" u="V" v="120.1"></senml>
  <senml n="current" t="-5" v="1.2"></senml>
  <senml n="current" t="-4" v="1.3"></senml>
  <senml n="current" t="-3" v="1.4"></senml>
  <senml n="current" t="-2" v="1.5"></senml>
  <senml n="current" t="-1" v="1.6"></senml>
  <senml n="current" v="1.7"></senml>
</sensml>

default namespace = "urn:ietf:params:xml:ns:senml"
namespace rng = "http://relaxng.org/ns/structure/1.0"

link = element l {
      attribute * { xsd:string }*
}


senml = element senml {
  attribute bn { xsd:string }?,
  attribute bt { xsd:double }?,
  attribute bv { xsd:double }?,
  attribute bu { xsd:string }?,
  attribute bver { xsd:int }?,

  attribute n { xsd:string }?,
  attribute s { xsd:double }?,
  attribute t { xsd:double }?,
  attribute u { xsd:string }?,
  attribute ut { xsd:double }?,

  attribute v { xsd:double }?,
  attribute vb { xsd:boolean }?,
  attribute vs { xsd:string }?,
  attribute vd { xsd:string }?,
  
  link*
}

sensml =
  element sensml {
    senml+
}

start = sensml

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
targetNamespace="urn:ietf:params:xml:ns:senml"
xmlns:ns1="urn:ietf:params:xml:ns:senml">
  <xs:element name="l">
    <xs:complexType>
      <xs:anyAttribute processContents="skip" />
    </xs:complexType>
  </xs:element>
  <xs:element name="senml">
    <xs:complexType>
      <xs:sequence>
        <xs:element minOccurs="0" maxOccurs="unbounded"
        ref="ns1:l" />
      </xs:sequence>
      <xs:attribute name="bn" type="xs:string" />
      <xs:attribute name="bt" type="xs:double" />
      <xs:attribute name="bv" type="xs:double" />
      <xs:attribute name="bu" type="xs:string" />
      <xs:attribute name="bver" type="xs:int" />
      <xs:attribute name="n" type="xs:string" />
      <xs:attribute name="s" type="xs:double" />
      <xs:attribute name="t" type="xs:double" />
      <xs:attribute name="u" type="xs:string" />
      <xs:attribute name="ut" type="xs:double" />
      <xs:attribute name="v" type="xs:double" />
      <xs:attribute name="vb" type="xs:boolean" />
      <xs:attribute name="vs" type="xs:string" />
      <xs:attribute name="vd" type="xs:string" />
    </xs:complexType>
  </xs:element>
  <xs:element name="sensml">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" ref="ns1:senml" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

<sensml xmlns="urn:ietf:params:xml:ns:senml">
  <senml bn="urn:dev:ow:10e2073a01080063/"></senml>
  <senml n="voltage" u="V" v="120.1"></senml>
  <senml n="current" u="A" v="1.2"></senml>
</sensml>

0000 a0 30 3d cd 95 b9 b5 b0 b9 9d 95 b8 b9 e1 cd 91 |.0=.............|
0010 00 f3 ab 93 71 d3 23 2b b1 d3 7b b9 d1 89 83 29 |....q.#+..{....)|
0020 91 81 b9 9b 09 81 89 81 c1 81 81 b1 99 7f 14 25 |...............%|
0030 d9 bd b1 d1 85 9d 94 80 d5 8a c4 26 01 0a 12 c6 |...........&....|
0040 ea e4 e4 ca dc e8 40 68 24 19 00 90             |......@h$...|
004c

<sensml xmlns="urn:ietf:params:xml:ns:senml">
  <senml n="urn:dev:ow:10e2073a01080063" u="Cel" v="23.1"></senml>
</sensml>

0000 a0 00 48 81 ee 6c ad cd ad 85 cc ec ad c5 cf 0e |..H..l..........|
0010 6c 80 02 05 1d 75 72 6e 3a 64 65 76 3a 6f 77 3a |l....urn:dev:ow:|
0020 31 30 65 32 30 37 33 61 30 31 30 38 30 30 36 33 |10e2073a01080063|
0030 02 05 43 65 6c 01 00 e7 01 01 00 04 01          |..Cel........|
003d

SenML-Pack = [initial-record, * follow-on-record]

initial-record = initial-defined .and initial-generic
follow-on-record = follow-on-defined .and follow-on-generic

; first do a specification of the labels as defined:

initial-defined = {
  ? bn => tstr,        ; Base Name
  ? bt => numeric,     ; Base Time
  ? bu => tstr,        ; Base Units
  ? bv => numeric,     ; Base value
  ? bver => uint,      ; Base Version
  follow-on-defined-group,
  + base-key-value-pair
}

follow-on-defined-group = (
   ? n => tstr,        ; Name
   ? u => tstr,        ; Units
   ? ( v => numeric // ; Numeric Value
       vs => tstr //   ; String Value
       vb => bool //   ; Boolean Value
       vd => bstr )    ; Data Value
   ? s => numeric,     ; Value Sum
   ? t => numeric,     ; Time
   ? ut => numeric,    ; Update Time
   * key-value-pair
)
follow-on-defined = { follow-on-defined-group }

; CBOR version (use the labels)
bver = -1  n  = 0   s  = 5
bn  = -2   u  = 1   t  = 6
bt  = -3   v  = 2   ut = 7
bu  = -4   vs = 3   vd = 8
bv  = -5   vb = 4
; use the label *names* for JSON

; now define the generic versions

initial-generic = {
  follow-on-generic-group,
  * base-key-value-pair,
}

follow-on-generic-group = (
  + key-value-pair,
)
follow-on-generic = { follow-on-generic-group }

key-value-pair = ( non-b-label => value )

base-key-value-pair = ( b-label => value )

non-b-label = tstr .regexp  "[A-Zac-z0-9][-_:.A-Za-z0-9]*" / uint
b-label = tstr .regexp  "b[-_:.A-Za-z0-9]+" / nint

value = tstr / bstr / numeric / bool
numeric = number / decfrac
 

[{"bn": "urn:dev:ow:10e2073a01080063/",
  "bt": 1320078429,
  "l": "[{\"href\":\"humidity\",\"foo\":\"bar1\"}\"
  },
  { "n": "temperature", "v": 27.2, "u": "Cel" },
  { "n": "humidity", "v": 80, "u": "%RH" } 
]