Internet Engineering Task Force (IETF) C. Jennings
Request for Comments: 8428 Cisco
Category: Standards Track Z. Shelby
ISSN: 2070-1721 ARM
J. Arkko
A. Keranen
Ericsson
C. Bormann
Universitaet Bremen TZI
August 2018
Sensor Measurement Lists (SenML)
Abstract
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.
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
https://www.rfc-editor.org/info/rfc8428.
Jennings, et al. Standards Track [Page 1]RFC 8428 SenML August 2018
Copyright Notice
Copyright (c) 2018 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements and Design Goals . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. SenML Structure and Semantics . . . . . . . . . . . . . . . . 6
4.1. Base Fields . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Regular Fields . . . . . . . . . . . . . . . . . . . . . 7
4.3. SenML Labels . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Extensibility . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Records and Their Fields . . . . . . . . . . . . . . . . 9
4.5.1. Names . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5.2. Units . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5.3. Time . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5.4. Values . . . . . . . . . . . . . . . . . . . . . . . 11
4.6. Resolved Records . . . . . . . . . . . . . . . . . . . . 12
4.7. Associating Metadata . . . . . . . . . . . . . . . . . . 12
4.8. Sensor Streaming Measurement Lists (SenSML) . . . . . . . 12
4.9. Configuration and Actuation Usage . . . . . . . . . . . . 13
5. JSON Representation (application/senml+json) . . . . . . . . 13
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Single Data Point . . . . . . . . . . . . . . . . . . 14
5.1.2. Multiple Data Points . . . . . . . . . . . . . . . . 14
5.1.3. Multiple Measurements . . . . . . . . . . . . . . . . 15
5.1.4. Resolved Data . . . . . . . . . . . . . . . . . . . . 17
5.1.5. Multiple Data Types . . . . . . . . . . . . . . . . . 17
5.1.6. Collection of Resources . . . . . . . . . . . . . . . 18
5.1.7. Setting an Actuator . . . . . . . . . . . . . . . . . 18
6. CBOR Representation (application/senml+cbor) . . . . . . . . 19
7. XML Representation (application/senml+xml) . . . . . . . . . 21
8. EXI Representation (application/senml-exi) . . . . . . . . . 23
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9. Fragment Identification Methods . . . . . . . . . . . . . . . 26
9.1. Fragment Identification Examples . . . . . . . . . . . . 26
9.2. Fragment Identification for XML and EXI Formats . . . . . 27
10. Usage Considerations . . . . . . . . . . . . . . . . . . . . 27
11. CDDL . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
12.1. SenML Units Registry . . . . . . . . . . . . . . . . . . 30
12.2. SenML Labels Registry . . . . . . . . . . . . . . . . . 35
12.3. Media Type Registrations . . . . . . . . . . . . . . . . 36
12.3.1. senml+json Media Type Registration . . . . . . . . . 37
12.3.2. sensml+json Media Type Registration . . . . . . . . 38
12.3.3. senml+cbor Media Type Registration . . . . . . . . . 39
12.3.4. sensml+cbor Media Type Registration . . . . . . . . 41
12.3.5. senml+xml Media Type Registration . . . . . . . . . 42
12.3.6. sensml+xml Media Type Registration . . . . . . . . . 43
12.3.7. senml-exi Media Type Registration . . . . . . . . . 44
12.3.8. sensml-exi Media Type Registration . . . . . . . . . 45
12.4. XML Namespace Registration . . . . . . . . . . . . . . . 47
12.5. CoAP Content-Format Registration . . . . . . . . . . . . 47
13. Security Considerations . . . . . . . . . . . . . . . . . . . 47
14. Privacy Considerations . . . . . . . . . . . . . . . . . . . 48
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 49
15.1. Normative References . . . . . . . . . . . . . . . . . . 49
15.2. Informative References . . . . . . . . . . . . . . . . . 51
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
1. Overview
Connecting sensors to the Internet is not new, and there have been
many protocols designed to facilitate it. This specification defines
a format and media types for carrying simple sensor information in
protocols such as HTTP [RFC7230] or CoAP [RFC7252]. The SenML format
is 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 collect a large number of
sensor measurements in a relatively efficient manner. SenML can be
used for a variety of data flow models, most notably data feeds
pushed from a sensor to a collector, and for the web resource model
where the sensor data 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
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efficiency reasons. Other information about the sensor can be
discovered by other methods such as using the Constrained RESTful
Environments (CoRE) Link Format [RFC6690].
SenML is defined by a data model for measurements and simple metadata
about measurements and devices. The data is structured as a single
array that contains a series of SenML Records that can each contain
fields such as a 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 [RFC8259], CBOR [RFC7049], XML [W3C.REC-xml-20081126], and
Efficient XML Interchange (EXI) [W3C.REC-exi-20140211].
For example, the following shows a measurement from a temperature
gauge encoded in the JSON syntax.
[
{"n":"urn:dev:ow:10e2073a01080063","u":"Cel","v":23.1}
]
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 23.1 degrees Celsius.
2. Requirements and Design Goals
The design goal is to be able to send simple sensor measurements in
small packets from large numbers of constrained devices. Keeping the
total size of the payload small makes it easy to also use SenML in
constrained networks, e.g., in an IPv6 over Low-Power Wireless
Personal Area Network (6LoWPAN) [RFC4944]. 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 an 8-bit microprocessor.
Experience with power meters and other 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.
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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 given by the "v" field, the time of a
measurement is in the "t" field, the "n" field has a unique sensor
name, and the unit of the measurement is carried in the "u" field.
[
{"n":"urn:dev:ow:10e2073a01080063","u":"Cel","t":1.276020076e+09,
"v":23.5},
{"n":"urn:dev:ow:10e2073a01080063","u":"Cel","t":1.276020091e+09,
"v":23.6}
]
To keep the messages small, it does not make sense to repeat the "n"
field 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","u":"Cel","t":1.276020076e+09,
"v":23.5},
{"u":"Cel","t":1.276020091e+09,
"v":23.6}
]
In the above example, the Base Name is in the "bn" field, and the "n"
fields in each Record are empty strings, so they are omitted.
Some devices have accurate time while others do not, so SenML
supports absolute and relative times. Time is represented in
floating point as seconds. Values greater than or equal to 2**28
represent an absolute time relative to the Unix epoch. Values less
than 2**28 represent time relative to the current time.
A simple sensor with no absolute wall-clock time might take a
measurement every second, batch up 60 of them, and then send the
batch to a server. It would include the relative time each
measurement was made compared to the time the batch was sent in each
SenML Record. If the server has accurate time based on, e.g., the
Network Time Protocol (NTP), it may 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 information in a document
database.
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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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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.
SenML Label: A short name used in SenML Records to denote different
SenML fields (e.g., "v" for "value").
SenML Field: A component of a record that associates a value to a
SenML Label for this record.
SenSML: Sensor Streaming Measurement List (see Section 4.8).
SenSML Stream: One or more SenML Records to be processed as a
stream.
This document uses the terms "attribute" and "tag" where they occur
with the underlying technologies (XML, CBOR [RFC7049], and the CoRE
Link Format [RFC6690]); they are not used for SenML concepts, per se.
However, note that "attribute" has been widely used in the past as a
synonym for the SenML "field".
All comparisons of text strings are performed byte by byte, which
results in the comparisons being case sensitive.
Where arithmetic is used, this specification uses the familiar
notation of the programming language C, except that the operator "**"
stands for exponentiation.
4. SenML Structure and Semantics
Each SenML Pack carries a single array that represents a set of
measurements and/or parameters. This array contains a series of
SenML Records with several fields described below. There are two
kinds of fields: base and regular. Both the base and regular fields
can be included in any SenML Record. The base fields apply to the
entries in the Record and also to all Records after it up to, but not
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including, the next Record that has that same base field. All base
fields are optional. Regular fields can be included in any SenML
Record and apply only to that Record.
4.1. Base Fields
Base Name: This is a string that is prepended to the names found in
the entries.
Base Time: A base time that is added to the time found in an entry.
Base Unit: A base unit that is assumed for all entries, unless
otherwise indicated. If a record does not contain a Unit value,
then the Base Unit is used. Otherwise, the value found in the
Unit (if any) is used.
Base Value: A base value is added to the value found in an entry,
similar to Base Time.
Base Sum: A base sum is added to the sum found in an entry, similar
to Base Time.
Base Version: Version number of the media type format. This field
is an optional positive integer and defaults to 10 if not present.
4.2. Regular Fields
Name: Name of the sensor or parameter. When appended to the Base
Name field, 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, the 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: Unit for a measurement value. Optional.
Value: Value of the entry. Optional if a Sum value is present;
otherwise, it's required. Values are represented using basic data
types. This specification defines floating-point numbers ("v"
field for "Value"), booleans ("vb" for "Boolean Value"), strings
("vs" for "String Value"), and binary data ("vd" for "Data
Value"). Exactly one Value field MUST appear unless there is a
Sum field, in which case it is allowed to have no Value field.
Sum: Integrated sum of the values over time. Optional. This field
is in the unit specified in the Unit value multiplied by seconds.
For historical reasons, it is named "sum" instead of "integral".
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Time: Time when the value was recorded. Optional.
Update Time: Period of time in seconds that represents the maximum
time before this sensor will provide an updated reading for a
measurement. Optional. This can be used to detect the failure of
sensors or the communications path from the sensor.
4.3. SenML Labels
Table 1 provides an overview of all SenML fields defined by this
document with their respective labels and data types.
+---------------+-------+------------+------------+------------+
| Name | Label | CBOR Label | JSON Type | XML Type |
+---------------+-------+------------+------------+------------+
| Base Name | bn | -2 | String | string |
| Base Time | bt | -3 | Number | double |
| Base Unit | bu | -4 | String | string |
| Base Value | bv | -5 | Number | double |
| Base Sum | bs | -6 | Number | double |
| Base Version | bver | -1 | Number | int |
| Name | n | 0 | String | string |
| Unit | u | 1 | String | string |
| Value | v | 2 | Number | double |
| String Value | vs | 3 | String | string |
| Boolean Value | vb | 4 | Boolean | boolean |
| Data Value | vd | 8 | String (*) | string (*) |
| Sum | s | 5 | Number | double |
| Time | t | 6 | Number | double |
| Update Time | ut | 7 | Number | double |
+---------------+-------+------------+------------+------------+
Table 1: SenML Labels
(*) Data Value is a base64-encoded string with the URL-safe alphabet
as defined in Section 5 of [RFC4648], with padding omitted. (In
CBOR, the octets in the Data Value are encoded using a definite-
length byte string, major type 2.)
For details of the JSON representation, see Section 5; for CBOR, see
Section 6; and for XML, see Section 7.
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4.4. Extensibility
The SenML format can be extended with further custom fields. Both
new base and regular fields are allowed. See Section 12.2 for
details. Implementations MUST ignore fields they don't recognize
unless that field has a label name that ends with the "_" character,
in which case an error MUST be generated.
All SenML Records in a Pack MUST have the same version number. This
is typically done by adding a Base Version field to only the first
Record in the Pack or by using the default value.
Systems reading one of the objects MUST check for the Base Version
field. If this value is a version number larger than the version
that the system understands, the system MUST NOT use this object.
This allows the version number to indicate that the object contains
structure or semantics that is different from what is defined in the
present document beyond just making use of the extension points
provided here. New version numbers can only be defined in an RFC
that updates this specification or its successors.
4.5. Records and Their Fields
4.5.1. Names
The Name value is concatenated to the Base Name value to yield the
name of the sensor. The resulting concatenated name needs to
uniquely identify and differentiate the sensor from all others. The
concatenated name MUST consist only of characters out of the set "A"
to "Z", "a" to "z", and "0" to "9", as well as "-", ":", ".", "/",
and "_"; furthermore, 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 concatenated names can be used directly within
various URI schemes (including segments of an HTTP path with no
special encoding; note that a name that contains "/" characters maps
into multiple URI path segments) and can be used directly in many
databases and analytic systems. [RFC5952] contains advice on
encoding an IPv6 address in a name. See Section 14 for privacy
considerations that apply to the use of long-term stable unique
identifiers.
Although it is RECOMMENDED that concatenated names be represented as
URIs [RFC3986] or URNs [RFC8141], the restricted character set
specified above puts strict limits on the URI schemes and URN
namespaces that can be used. As a result, implementers need to take
care in choosing the naming scheme for concatenated names, because
such names both need to be unique and need to conform to the
restricted character set. One approach is to include a bit string
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that has guaranteed uniqueness (such as a 1-wire address [AN1796]).
Some of the examples within this document use the device URN
namespace as specified in [DEVICE-URN]. Universally Unique
Identifiers (UUIDs) [RFC4122] are another way to generate a unique
name. However, the restricted character set does not allow the use
of many URI schemes, such as the "tag" scheme [RFC4151] and the "ni"
scheme [RFC6920], in names as such. The use of URIs with characters
incompatible with this set and possible mapping rules between the two
are outside the scope of the present document.
4.5.2. Units
If the Record has no Unit, the Base Unit is used as the Unit. Having
no Unit and no Base Unit is allowed; any information that may be
required about units applicable to the value then needs to be
provided by the application context.
4.5.3. Time
If either the Base Time or Time value is missing, the missing field
is considered to have a value of zero. The Base Time and Time values
are added together to get a value representing the time of
measurement.
Values less than 268,435,456 (2**28) represent time relative to the
current time. That is, a time of zero indicates that the sensor does
not know the absolute time and the measurement was made roughly
"now". A negative value indicates seconds in the past from roughly
"now". Positive values up to 2**28 indicate seconds in the future
from "now". An example for employing positive values would be
actuation use, when the desired change should happen in the future,
but the sender or the receiver does not have accurate time available.
Values greater than or equal to 2**28 represent an absolute time
relative to the Unix epoch (1970-01-01T00:00Z in UTC time), and the
time is counted the same way as the Portable Operating System
Interface (POSIX) "seconds since the epoch" [TIME_T]. Therefore, the
smallest absolute Time value that can be expressed (2**28) is
1978-07-04 21:24:16 UTC.
Because Time values up to 2**28 are used for representing time
relative to "now" and Time and Base Time are added together, care
must be taken to ensure that the sum does not inadvertently reach
2**28 (i.e., absolute time) when relative time was intended to be
used.
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Obviously, SenML Records referenced to "now" are only useful within a
specific communication context (e.g., based on information on when
the SenML Pack, or a specific Record in a SenSML Stream, was sent) or
together with some other context information that can be used for
deriving a meaning of "now"; the expectation for any archival use is
that they will be processed into UTC-referenced records before that
context would cease to be available. This specification deliberately
leaves the accuracy of "now" very vague as it is determined by the
overall systems that use SenML. In a system where a sensor without
wall-clock time sends a SenML Record with a time referenced to "now"
over a high-speed RS-485 link to an embedded system with accurate
time that resolves "now" based on the time of reception, the
resulting time uncertainty could be within 1 ms. At the other
extreme, a deployment that sends SenML wind-speed readings over a
Low-Earth Orbit (LEO) satellite link from a mountain valley might
have resulting reception Time values that are easily a dozen minutes
off the actual time of the sensor reading, with the time uncertainty
depending on satellite locations and conditions.
4.5.4. Values
If only one of the Base Sum or Sum value is present, the missing
field is considered to have a value of zero. The Base Sum and Sum
values are added together to get the sum of measurement. If neither
the Base Sum nor the Sum is present, then the measurement does not
have a Sum value.
If the Base Value or Value is not present, the missing field(s) is
considered to have a value of zero. The Base Value and Value are
added together to get the value of the measurement.
Representing the statistical characteristics of measurements, such as
accuracy, can be very complex. Future specification may add new
fields to provide better information about the statistical properties
of the measurement.
In summary, the structure of a SenML Record is laid out to support a
single measurement per Record. If multiple data values are measured
at the same time (e.g., air pressure and altitude), they are best
kept as separate Records linked through their Time value; this is
even true when one of the data values is more "meta" than others
(e.g., describes a condition that influences other measurements at
the same time).
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4.6. Resolved Records
Sometimes it is useful to be able to refer to a defined normalized
format for SenML Records. This normalized format tends to get used
for big data applications and intermediate forms when converting to
other formats. Also, if SenML Records are used outside of a SenML
Pack, they need to be resolved first to ensure applicable base values
are applied.
A SenML Record is referred to as "resolved" if it does not contain
any base values, i.e., labels starting with the character "b", except
for Base Version fields (see below), and has no relative times. To
resolve the Records, the applicable base values of the SenML Pack (if
any) are applied to the Record. That is, for the base values in the
Record or before the Record in the Pack, Name and Base Name are
concatenated, the Base Time is added to the time of the Record, the
Base Unit is applied to the Record if it did not contain a Unit, etc.
In addition, the Records need to be in chronological order in the
Pack. An example of this is shown in Section 5.1.4.
The Base Version field MUST NOT be present in resolved Records if the
SenML version defined in this document is used; otherwise, it MUST be
present in all the resolved SenML Records.
A future specification that defines new base fields needs to specify
how the field is resolved.
4.7. Associating Metadata
SenML is designed to carry the minimum dynamic information about
measurements and, for efficiency reasons, does not carry significant
static metadata about the device, object, or sensors. Instead, it is
assumed that this metadata is carried out of band. For web resources
using SenML Packs, this metadata 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=) link attribute (which is defined for the link
format in Section 7.2.1 of [RFC7252]).
4.8. Sensor Streaming Measurement Lists (SenSML)
In some usage scenarios of SenML, the implementations 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 separate media types
to indicate Sensor Streaming Measurement Lists (SenSML) for this
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usage (see Section 12.3.2). 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 does not have
to wait for the full SenSML Stream to be complete.
If times relative to "now" (see Section 4.5.3) are used in SenML
Records of a SenSML Stream, their interpretation of "now" is based on
the time when the specific Record is sent in the stream.
4.9. Configuration and Actuation Usage
SenML can also be used for configuring parameters and controlling
actuators. When a SenML Pack is sent (e.g., using an HTTP/CoAP POST
or PUT method) and the semantics of the target are such that SenML is
interpreted as configuration/actuation, SenML Records are interpreted
as a request to change the values of given (sub)resources (given as
names) to given values at the given time(s). The semantics of the
target resource supporting this usage can be described, e.g., using
[RID-CoRE]. Examples of actuation usage are shown in Section 5.1.7.
5. JSON Representation (application/senml+json)
For the SenML fields shown in Table 2, the SenML Labels are used as
the JSON object member names within JSON objects representing the
JSON SenML Records.
+---------------+-------+-----------+
| Name | Label | JSON Type |
+---------------+-------+-----------+
| Base Name | bn | String |
| Base Time | bt | Number |
| Base Unit | bu | String |
| Base Value | bv | Number |
| Base Sum | bs | Number |
| Base Version | bver | Number |
| Name | n | String |
| Unit | u | String |
| Value | v | Number |
| String Value | vs | String |
| Boolean Value | vb | Boolean |
| Data Value | vd | String |
| Sum | s | Number |
| Time | t | Number |
| Update Time | ut | Number |
+---------------+-------+-----------+
Table 2: JSON SenML Labels
Jennings, et al. Standards Track [Page 13]RFC 8428 SenML August 2018
The root JSON value consists of an array with one JSON object for
each SenML Record. All the fields in the above table MAY occur in
the Records with member values of the type specified in the table.
Only the UTF-8 [RFC3629] form of JSON is allowed. Characters in the
String Value are encoded using the escape sequences defined in
[RFC8259]. Octets in the Data Value are base64 encoded with the URL-
safe alphabet as defined in Section 5 of [RFC4648], with padding
omitted.
Systems receiving measurements MUST be able to process the range of
floating-point numbers that are representable as IEEE double-
precision, floating-point numbers [IEEE.754]. This allows Time
values to have better than microsecond precision over the next 100
years. 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. In the interest of avoiding unnecessary verbosity and speeding
up processing, the mantissa SHOULD be less than 19 characters long,
and the exponent SHOULD be less than 5 characters long.
5.1. Examples
5.1.1. Single Data Point
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","u":"Cel","v":23.1}
]
5.1.2. Multiple Data Points
The following example shows voltage and current "now", i.e., at an
unspecified time.
[
{"bn":"urn:dev:ow:10e2073a01080063:","n":"voltage","u":"V","v":120.1},
{"n":"current","u":"A","v":1.2}
]
Jennings, et al. Standards Track [Page 14]RFC 8428 SenML August 2018
The next example is similar to the above one, but it shows current at
Tue Jun 8 18:01:16.001 UTC 2010 and at each second for the previous 5
seconds.
[
{"bn":"urn:dev:ow:10e2073a0108006:","bt":1.276020076001e+09,
"bu":"A","bver":5,
"n":"voltage","u":"V","v":120.1},
{"n":"current","t":-5,"v":1.2},
{"n":"current","t":-4,"v":1.3},
{"n":"current","t":-3,"v":1.4},
{"n":"current","t":-2,"v":1.5},
{"n":"current","t":-1,"v":1.6},
{"n":"current","v":1.7}
]
As an example of SenSML, the following stream of measurements may be
sent via a long-lived HTTP POST from the producer of the stream to
its consumer, and each measurement object may be reported at the time
it was measured:
[
{"bn":"urn:dev:ow:10e2073a01080063","bt":1.320067464e+09,
"bu":"%RH","v":21.2},
{"t":10,"v":21.3},
{"t":20,"v":21.4},
{"t":30,"v":21.4},
{"t":40,"v":21.5},
{"t":50,"v":21.5},
{"t":60,"v":21.5},
{"t":70,"v":21.6},
{"t":80,"v":21.7},
...
5.1.3. Multiple Measurements
The following example shows humidity measurements from a mobile
device with a 1-wire address 10e2073a01080063, 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 that time is used to correlate 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.
Jennings, et al. Standards Track [Page 15]RFC 8428 SenML August 2018
[
{"bn":"urn:dev:ow:10e2073a01080063","bt":1.320067464e+09,
"bu":"%RH","v":20},
{"u":"lon","v":24.30621},
{"u":"lat","v":60.07965},
{"t":60,"v":20.3},
{"u":"lon","t":60,"v":24.30622},
{"u":"lat","t":60,"v":60.07965},
{"t":120,"v":20.7},
{"u":"lon","t":120,"v":24.30623},
{"u":"lat","t":120,"v":60.07966},
{"u":"%EL","t":150,"v":98},
{"t":180,"v":21.2},
{"u":"lon","t":180,"v":24.30628},
{"u":"lat","t":180,"v":60.07967}
]
The following table shows the size of this example in various forms,
as well as the size of each of these forms compressed with gzip.
+----------+------+-----------------+
| Encoding | Size | Compressed Size |
+----------+------+-----------------+
| JSON | 573 | 206 |
| XML | 649 | 235 |
| CBOR | 254 | 196 |
| EXI | 161 | 184 |
+----------+------+-----------------+
Table 3: Size Comparisons
Jennings, et al. Standards Track [Page 16]RFC 8428 SenML August 2018
5.1.4. Resolved Data
The following shows the example from the previous section in resolved
format.
[
{"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067464e+09,
"v":20},
{"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067464e+09,
"v":24.30621},
{"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067464e+09,
"v":60.07965},
{"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067524e+09,
"v":20.3},
{"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067524e+09,
"v":24.30622},
{"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067524e+09,
"v":60.07965},
{"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067584e+09,
"v":20.7},
{"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067584e+09,
"v":24.30623},
{"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067584e+09,
"v":60.07966},
{"n":"urn:dev:ow:10e2073a01080063","u":"%EL","t":1.320067614e+09,
"v":98},
{"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067644e+09,
"v":21.2},
{"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067644e+09,
"v":24.30628},
{"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067644e+09,
"v":60.07967}
]
5.1.5. Multiple Data Types
The following example shows a sensor that returns different data
types.
[
{"bn":"urn:dev:ow:10e2073a01080063:","n":"temp","u":"Cel","v":23.1},
{"n":"label","vs":"Machine Room"},
{"n":"open","vb":false},
{"n":"nfc-reader","vd":"aGkgCg"}
]
Jennings, et al. Standards Track [Page 17]RFC 8428 SenML August 2018
5.1.6. Collection of Resources
The following example shows the results from a query to one device
that aggregates multiple measurements from other devices. The
example assumes that a client has fetched information from a device
at 2001:db8::2 by performing a GET operation on http://[2001:db8::2]
at Mon Oct 31 16:27:09 UTC 2011 and has gotten two separate values as
a result: a temperature and humidity measurement as well as the
results from another device at http://[2001:db8::1] that also had a
temperature and humidity measurement. Note that the last record
would use the Base Name from the 3rd record but the Base Time from
the first record.
[
{"bn":"2001:db8::2/","bt":1.320078429e+09,
"n":"temperature","u":"Cel","v":25.2},
{"n":"humidity","u":"%RH","v":30},
{"bn":"2001:db8::1/","n":"temperature","u":"Cel","v":12.3},
{"n":"humidity","u":"%RH","v":67}
]
5.1.7. Setting an Actuator
The following example shows the SenML that could be used to set the
current set point of a typical residential thermostat that has a
temperature set point, a switch to turn on and off the heat, and a
switch to turn on the fan override.
[
{"bn":"urn:dev:ow:10e2073a01080063:"},
{"n":"temp","u":"Cel","v":23.1},
{"n":"heat","u":"/","v":1},
{"n":"fan","u":"/","v":0}
]
In the following example, two different lights are turned on. It is
assumed that the lights are on a network that can guarantee delivery
of the messages to the two lights within 15 ms (e.g., a network using
802.1BA [IEEE802.1BA] and 802.1AS [IEEE802.1AS] for time
synchronization). The controller has set the time of the lights to
come on at 20 ms in the future from the current time. This allows
both lights to receive the message, wait till that time, then apply
the switch command so that both lights come on at the same time.
[
{"bt":1.320078429e+09,"bu":"/","n":"2001:db8::3","v":1},
{"n":"2001:db8::4","v":1}
]
Jennings, et al. Standards Track [Page 18]RFC 8428 SenML August 2018
The following shows two lights being turned off using a
non-deterministic network that has high odds of delivering a message
in less than 100 ms and uses NTP for time synchronization. The
current time is 1320078429. The user has just turned off a light
switch that is turning off two lights. Both lights are immediately
dimmed to 50% brightness to give the user instant feedback that
something is changing. However, given the network, the lights will
probably dim at somewhat different times. Then 100 ms in the future,
both lights will go off at the same time. The instant, but not
synchronized, dimming gives the user the sensation of quick
responses, and the timed-off 100 ms in the future gives the
perception of both lights going off at the same time.
[
{"bt":1.320078429e+09,"bu":"/","n":"2001:db8::3","v":0.5},
{"n":"2001:db8::4","v":0.5},
{"n":"2001:db8::3","t":0.1,"v":0},
{"n":"2001:db8::4","t":0.1,"v":0}
]
6. CBOR Representation (application/senml+cbor)
The CBOR [RFC7049] representation is equivalent to the JSON
representation, with the following changes:
o For JSON Numbers, the CBOR representation can use integers,
floating-point numbers, or decimal fractions (CBOR Tag 4);
however, a representation SHOULD be chosen such that when the CBOR
value is converted to an IEEE double-precision, floating-point
value, it has exactly the same value as the original JSON Number
converted to that form. For the version number, only an unsigned
integer is allowed.
o Characters in the String Value are encoded using a text string
with a definite length (major type 3). Octets in the Data Value
are encoded using a byte string with a definite length (major type
2).
o For compactness, the CBOR representation uses integers for the
labels, as defined in Table 4. This table is conclusive, i.e.,
there is no intention to define any additional integer map keys;
any extensions will use string map keys. This allows translators
converting between CBOR and JSON representations to also convert
all future labels without needing to update implementations. Base
values are given negative CBOR labels, and others are given
non-negative labels.
Jennings, et al. Standards Track [Page 19]RFC 8428 SenML August 2018
+---------------+-------+------------+
| Name | Label | CBOR Label |
+---------------+-------+------------+
| Base Version | bver | -1 |
| Base Name | bn | -2 |
| Base Time | bt | -3 |
| Base Unit | bu | -4 |
| Base Value | bv | -5 |
| Base Sum | bs | -6 |
| Name | n | 0 |
| Unit | u | 1 |
| Value | v | 2 |
| String Value | vs | 3 |
| Boolean Value | vb | 4 |
| Sum | s | 5 |
| Time | t | 6 |
| Update Time | ut | 7 |
| Data Value | vd | 8 |
+---------------+-------+------------+
Table 4: CBOR Representation: Integers for Map Keys
o For streaming SenSML in CBOR representation, the array containing
the records SHOULD be a CBOR array with an indefinite length; for
non-streaming SenML, an array with a definite length MUST be used.
The following example shows a dump of the CBOR example for the same
sensor measurement as in Section 5.1.2.
0000 87 a7 21 78 1b 75 72 6e 3a 64 65 76 3a 6f 77 3a |..!x.urn:dev:ow:|
0010 31 30 65 32 30 37 33 61 30 31 30 38 30 30 36 3a |10e2073a0108006:|
0020 22 fb 41 d3 03 a1 5b 00 10 62 23 61 41 20 05 00 |".A...[..b#aA ..|
0030 67 76 6f 6c 74 61 67 65 01 61 56 02 fb 40 5e 06 |gvoltage.aV..@^.|
0040 66 66 66 66 66 a3 00 67 63 75 72 72 65 6e 74 06 |fffff..gcurrent.|
0050 24 02 fb 3f f3 33 33 33 33 33 33 a3 00 67 63 75 |$..?.333333..gcu|
0060 72 72 65 6e 74 06 23 02 fb 3f f4 cc cc cc cc cc |rrent.#..?......|
0070 cd a3 00 67 63 75 72 72 65 6e 74 06 22 02 fb 3f |...gcurrent."..?|
0080 f6 66 66 66 66 66 66 a3 00 67 63 75 72 72 65 6e |.ffffff..gcurren|
0090 74 06 21 02 f9 3e 00 a3 00 67 63 75 72 72 65 6e |t.!..>...gcurren|
00a0 74 06 20 02 fb 3f f9 99 99 99 99 99 9a a3 00 67 |t. ..?.........g|
00b0 63 75 72 72 65 6e 74 06 00 02 fb 3f fb 33 33 33 |current....?.333|
00c0 33 33 33 |333|
00c3
Jennings, et al. Standards Track [Page 20]RFC 8428 SenML August 2018
In CBOR diagnostic notation (Section 6 of [RFC7049]), this is:
[{-2: "urn:dev:ow:10e2073a0108006:",
-3: 1276020076.001, -4: "A", -1: 5, 0: "voltage", 1: "V", 2: 120.1},
{0: "current", 6: -5, 2: 1.2}, {0: "current", 6: -4, 2: 1.3},
{0: "current", 6: -3, 2: 1.4}, {0: "current", 6: -2, 2: 1.5},
{0: "current", 6: -1, 2: 1.6}, {0: "current", 6: 0, 2: 1.7}]
7. XML Representation (application/senml+xml)
A SenML Pack or Stream can also be represented in XML format as
defined in this section.
Only the UTF-8 form of XML is allowed. Octets in the Data Value are
base64 encoded with the URL-safe alphabet as defined in Section 5 of
[RFC4648], with padding omitted.
The following shows an XML example for the same sensor measurement as
in Section 5.1.2.
<sensml xmlns="urn:ietf:params:xml:ns:senml">
<senml bn="urn:dev:ow:10e2073a0108006:" bt="1.276020076001e+09"
bu="A" bver="5" 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>
The SenML Stream is represented as a sensml element that contains a
series of senml elements for each SenML Record. The SenML fields are
represented as XML attributes. For each field defined in this
document, the following table shows the SenML Labels, which are used
for the XML attribute name, as well as the according restrictions on
the XML attribute values ("type") as used in the XML senml elements.
Jennings, et al. Standards Track [Page 21]RFC 8428 SenML August 2018
+---------------+-------+----------+
| Name | Label | XML Type |
+---------------+-------+----------+
| Base Name | bn | string |
| Base Time | bt | double |
| Base Unit | bu | string |
| Base Value | bv | double |
| Base Sum | bs | double |
| Base Version | bver | int |
| Name | n | string |
| Unit | u | string |
| Value | v | double |
| String Value | vs | string |
| Data Value | vd | string |
| Boolean Value | vb | boolean |
| Sum | s | double |
| Time | t | double |
| Update Time | ut | double |
+---------------+-------+----------+
Table 5: XML SenML Labels
The RelaxNG [RNC] Schema for the XML is:
default namespace = "urn:ietf:params:xml:ns:senml"
namespace rng = "http://relaxng.org/ns/structure/1.0"
senml = element senml {
attribute bn { xsd:string }?,
attribute bt { xsd:double }?,
attribute bv { xsd:double }?,
attribute bs { 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 }?
}
Jennings, et al. Standards Track [Page 22]RFC 8428 SenML August 2018
sensml =
element sensml {
senml+
}
start = sensml
8. EXI Representation (application/senml-exi)
For efficient transmission of SenML over, e.g., a constrained
network, EXI can be used. This encodes the XML Schema
[W3C.REC-xmlschema-1-20041028] structure of SenML into binary tags
and values rather than ASCII text. An EXI representation of SenML
SHOULD be made using the strict schema mode of EXI. However, this
mode does not allow tag extensions to the schema; therefore, any
extensions will be lost in the encoding. For uses where extensions
need to be preserved in EXI, the non-strict schema mode of EXI MAY be
used.
The EXI header MUST include "EXI Options", as defined in
[W3C.REC-exi-20140211], with a schemaId set to the value of "a",
indicating the schema provided in this specification. Future
revisions to the schema can change the value of the schemaId to allow
for backwards compatibility. When the data will be transported over
CoAP or HTTP, an EXI Cookie SHOULD NOT be used as it simply makes
things larger and is redundant to information provided in the
Content-Type header.
Jennings, et al. Standards Track [Page 23]RFC 8428 SenML August 2018
The following is the XSD Schema to be used for strict schema-guided
EXI processing. It is generated from the RelaxNG.
<?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="senml">
<xs:complexType>
<xs:attribute name="bn" type="xs:string" />
<xs:attribute name="bt" type="xs:double" />
<xs:attribute name="bv" type="xs:double" />
<xs:attribute name="bs" 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>
The following shows a hexdump of the EXI produced from encoding the
following XML example. Note that this example is the same
information as the first example in Section 5.1.2 but in JSON format.
<sensml xmlns="urn:ietf:params:xml:ns:senml">
<senml bn="urn:dev:ow:10e2073a01080063:" n="voltage" u="V"
v="120.1"></senml>
<senml n="current" u="A" v="1.2"></senml>
</sensml>
Jennings, et al. Standards Track [Page 24]RFC 8428 SenML August 2018
Which compresses with EXI to the following displayed in hexdump:
0000 a0 30 0d 84 80 f3 ab 93 71 d3 23 2b b1 d3 7b b9 |.0......q.#+..{.|
0010 d1 89 83 29 91 81 b9 9b 09 81 89 81 c1 81 81 b1 |...)............|
0020 99 d2 84 bb 37 b6 3a 30 b3 b2 90 1a b1 58 84 c0 |....7.:0.....X..|
0030 33 04 b1 ba b9 39 32 b7 3a 10 1a 09 06 40 38 |3....92.:....@8|
003f
The above example used the bit-packed form of EXI, but it is also
possible to use a byte-packed form of EXI, which can make it easier
for a simple sensor to produce valid EXI without really implementing
EXI. Consider the example of a temperature sensor that produces a
value in tenths of degrees Celsius over a range of 0.0 to 55.0. It
would produce an XML SenML file such as:
<sensml xmlns="urn:ietf:params:xml:ns:senml">
<senml n="urn:dev:ow:10e2073a01080063" u="Cel" v="23.1"></senml>
</sensml>
The compressed form, using the byte-alignment option of EXI, for the
above XML is the following:
0000 a0 00 48 80 6c 20 01 06 1d 75 72 6e 3a 64 65 76 |..H.l ...urn:dev|
0010 3a 6f 77 3a 31 30 65 32 30 37 33 61 30 31 30 38 |:ow:10e2073a0108|
0020 30 30 36 33 02 05 43 65 6c 01 00 e7 01 01 00 03 |0063..Cel.......|
0030 01 |.|
0031
A small temperature sensor device that only generates this one EXI
file does not really need a full EXI implementation. It can simply
hard code the output, replacing the 1-wire device ID starting at byte
0x14 and going to byte 0x23 with its device ID and replacing the
value "0xe7 0x01" at location 0x2b and 0x2c with the current
temperature. The EXI specification [W3C.REC-exi-20140211] contains
the full information on how floating-point numbers are represented,
but for the purpose of this sensor, the temperature can be converted
to an integer in tenths of degrees (231 in this example). EXI stores
7 bits of the integer in each byte with the top bit set to one if
there are further bytes. So, the first byte is set to the low 7 bits
of the integer temperature in tenths of degrees plus 0x80. In this
example, 231 & 0x7F + 0x80 = 0xE7. The second byte is set to the
integer temperature in tenths of degrees right-shifted 7 bits. In
this example, 231 >> 7 = 0x01.
Jennings, et al. Standards Track [Page 25]RFC 8428 SenML August 2018
9. Fragment Identification Methods
A SenML Pack typically consists of multiple SenML Records, and for
some applications, it may be useful to be able to refer to a single
Record, or a set of Records, in a Pack with a fragment identifier.
The fragment identifier is only interpreted by a client and does not
impact retrieval of a representation. The SenML fragment
identification is modeled after Comma-Separated Value (CSV) fragment
identifiers [RFC7111].
To select a single SenML Record, the "rec" scheme followed by a
single number is used. For the purpose of numbering Records, the
first Record is at position 1. A range of records can be selected by
giving the first and the last record number separated by a "-"
character. Instead of the second number, the "*" character can be
used to indicate the last SenML Record in the Pack. A set of Records
can also be selected using a comma-separated list of Record positions
or ranges.
(We use the term "selecting a Record" for identifying it as part of
the fragment, not in the sense of isolating it from the Pack -- the
Record still needs to be interpreted as part of the Pack, e.g., using
the base values defined in earlier Records.)
9.1. Fragment Identification Examples
The 3rd SenML Record from the "coap://example.com/temp" resource can
be selected with:
coap://example.com/temp#rec=3
Records from 3rd to 6th can be selected with:
coap://example.com/temp#rec=3-6
Records from 19th to the last can be selected with:
coap://example.com/temp#rec=19-*
The 3rd and 5th Records can be selected with:
coap://example.com/temp#rec=3,5
To select the Records from third to fifth, the 10th Record, and all
Records from 19th to the last:
coap://example.com/temp#rec=3-5,10,19-*
Jennings, et al. Standards Track [Page 26]RFC 8428 SenML August 2018
9.2. Fragment Identification for XML and EXI Formats
In addition to the SenML fragment identifiers described above, with
the XML and EXI SenML formats, the syntax defined in the XPointer
element() Scheme [XPointerElement] of the XPointer Framework
[XPointerFramework] can be used. (This is required by [RFC7303] for
media types using the syntax suffix structured with "+xml". For
consistency, SenML allows this for the EXI formats as well.)
Note that fragment identifiers are available to the client side only;
they are not provided in transfer protocols such as CoAP or HTTP.
Thus, they cannot be used by the server in deciding which media type
to send. Where a server has multiple representations available for a
resource identified by a URI, it might send a JSON or CBOR
representation when the client was directed to use an XML/EXI
fragment identifier with it. Clients can prevent running into this
problem by explicitly requesting an XML or EXI media type (e.g.,
using the CoAP Accept option) when XML-/EXI-only fragment identifier
syntax is in use in the URI.
10. Usage Considerations
The measurements support sending both the current value of a sensor
as well as an integrated sum. For many types of measurements, the
sum is more useful than the current value. For historical reasons,
this field is called "Sum" instead of "integral", which would more
accurately describe its function. For example, an electrical meter
that measures the energy a given computer uses will typically want to
measure the cumulative amount of energy used. This is less prone to
error than reporting the power each second and trying to have
something on the server side sum together all the power measurements.
If the network between the sensor and the meter goes down over some
period of time, when it comes back up, the cumulative sum helps
reflect what happened while the network was down. A meter like this
would typically report a measurement with the unit set to watts, but
it would put the sum of energy used in the "s" field of the
measurement. It might optionally include the current power in the
"v" field.
While the benefit of using the integrated sum is fairly clear for
measurements like power and energy, it is less obvious for something
like temperature. Reporting the sum of the temperature makes it easy
to compute averages even when the individual temperature values are
not reported frequently enough to compute accurate averages.
Implementers are encouraged to report the cumulative sum as well as
the raw value of a given sensor.
Jennings, et al. Standards Track [Page 27]
RFC 8428 SenML August 2018
Applications that use the cumulative Sum values need to understand
they are very loosely defined by this specification, and depending on
the particular sensor implementation, they may behave in unexpected
ways. Applications should be able to deal with the following issues:
1. Many sensors will allow the cumulative sums to "wrap" back to
zero after the value gets sufficiently large.
2. Some sensors will reset the cumulative sum back to zero when the
device is reset, loses power, or is replaced with a different
sensor.
3. Applications cannot make assumptions about when the device
started accumulating values into the sum.
Typically, applications can make some assumptions about specific
sensors that will allow them to deal with these problems. A common
assumption is that for sensors whose measurement values are non-
negative, the sum should never get smaller; if the sum does get
smaller, the application will know that one of the situations listed
above has happened.
Despite the name "Sum", the Sum field is not useful for applications
that maintain a running count of the number of times an event
happened or that keep track of a counter such as the total number of
bytes sent on an interface. Data like that can be sent directly in
the Value field.
Jennings, et al. Standards Track [Page 28]RFC 8428 SenML August 2018
11. CDDL
As a convenient reference, the JSON and CBOR representations can be
described with the common Concise Data Definition Language (CDDL)
specification [CDDL-CBOR] in Figure 1 (informative).
SenML-Pack = [1* record]
record = {
? bn => tstr, ; Base Name
? bt => numeric, ; Base Time
? bu => tstr, ; Base Units
? bv => numeric, ; Base Value
? bs => numeric, ; Base Sum
? bver => uint, ; Base Version
? n => tstr, ; Name
? u => tstr, ; Units
? s => numeric, ; Sum
? t => numeric, ; Time
? ut => numeric, ; Update Time
? ( v => numeric // ; Numeric Value
vs => tstr // ; String Value
vb => bool // ; Boolean Value
vd => binary-value ) ; Data Value
* key-value-pair
}
; now define the generic versions
key-value-pair = ( label => value )
label = non-b-label / b-label
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 / binary-value / numeric / bool
numeric = number / decfrac
Figure 1: Common CDDL Specification for CBOR and JSON SenML
Jennings, et al. Standards Track [Page 29]RFC 8428 SenML August 2018
For JSON, we use text labels and base64url-encoded binary data
(Figure 2).
bver = "bver" n = "n" s = "s"
bn = "bn" u = "u" t = "t"
bt = "bt" v = "v" ut = "ut"
bu = "bu" vs = "vs" vd = "vd"
bv = "bv" vb = "vb"
bs = "bs"
binary-value = tstr ; base64url encoded
Figure 2: JSON-Specific CDDL Specification for SenML
For CBOR, we use integer labels and native binary data (Figure 3).
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
bs = -6
binary-value = bstr
Figure 3: CBOR-Specific CDDL Specification for SenML
12. IANA Considerations
IANA has created a new "Sensor Measurement Lists (SenML)" registry
that contains the subregistries defined in Sections 12.1 and 12.2.
12.1. SenML Units Registry
IANA has created a registry of SenML unit symbols called the "SenML
Units" registry. The primary purpose of this registry is to make
sure that symbols uniquely map to indicate a type of measurement.
Definitions for many of these units can be found in other
publications such as [NIST811] and [BIPM]. Units marked with an
asterisk are NOT RECOMMENDED to be produced by new implementations
but are in active use and SHOULD be implemented by consumers that can
use the corresponding SenML units that are closer to the unscaled SI
units.
Jennings, et al. Standards Track [Page 30]RFC 8428 SenML August 2018
+----------+------------------------------------+-------+-----------+
| Symbol | Description | Type | Reference |
+----------+------------------------------------+-------+-----------+
| m | meter | float | RFC 8428 |
| kg | kilogram | float | RFC 8428 |
| g | gram* | float | RFC 8428 |
| s | second | float | RFC 8428 |
| A | ampere | float | RFC 8428 |
| K | kelvin | float | RFC 8428 |
| cd | candela | float | RFC 8428 |
| mol | mole | float | RFC 8428 |
| Hz | hertz | float | RFC 8428 |
| rad | radian | float | RFC 8428 |
| sr | steradian | float | RFC 8428 |
| N | newton | float | RFC 8428 |
| Pa | pascal | float | RFC 8428 |
| J | joule | float | RFC 8428 |
| W | watt | float | RFC 8428 |
| C | coulomb | float | RFC 8428 |
| V | volt | float | RFC 8428 |
| F | farad | float | RFC 8428 |
| Ohm | ohm | float | RFC 8428 |
| S | siemens | float | RFC 8428 |
| Wb | weber | float | RFC 8428 |
| T | tesla | float | RFC 8428 |
| H | henry | float | RFC 8428 |
| Cel | degrees Celsius | float | RFC 8428 |
| lm | lumen | float | RFC 8428 |
| lx | lux | float | RFC 8428 |
| Bq | becquerel | float | RFC 8428 |
| Gy | gray | float | RFC 8428 |
| Sv | sievert | float | RFC 8428 |
| kat | katal | float | RFC 8428 |
| m2 | square meter (area) | float | RFC 8428 |
| m3 | cubic meter (volume) | float | RFC 8428 |
| l | liter (volume)* | float | RFC 8428 |
| m/s | meter per second (velocity) | float | RFC 8428 |
| m/s2 | meter per square second | float | RFC 8428 |
| | (acceleration) | | |
| m3/s | cubic meter per second (flow rate) | float | RFC 8428 |
| l/s | liter per second (flow rate)* | float | RFC 8428 |
| W/m2 | watt per square meter (irradiance) | float | RFC 8428 |
| cd/m2 | candela per square meter | float | RFC 8428 |
| | (luminance) | | |
| bit | bit (information content) | float | RFC 8428 |
| bit/s | bit per second (data rate) | float | RFC 8428 |
| lat | degrees latitude (Note 1) | float | RFC 8428 |
| lon | degrees longitude (Note 1) | float | RFC 8428 |
Jennings, et al. Standards Track [Page 31]
RFC 8428 SenML August 2018
| pH | pH value (acidity; logarithmic | float | RFC 8428 |
| | quantity) | | |
| dB | decibel (logarithmic quantity) | float | RFC 8428 |
| dBW | decibel relative to 1 W (power | float | RFC 8428 |
| | level) | | |
| Bspl | bel (sound pressure level; | float | RFC 8428 |
| | logarithmic quantity)* | | |
| count | 1 (counter value) | float | RFC 8428 |
| / | 1 (ratio, e.g., value of a switch; | float | RFC 8428 |
| | Note 2) | | |
| % | 1 (ratio, e.g., value of a switch; | float | RFC 8428 |
| | Note 2)* | | |
| %RH | percentage (relative humidity) | float | RFC 8428 |
| %EL | percentage (remaining battery | float | RFC 8428 |
| | energy level) | | |
| EL | seconds (remaining battery energy | float | RFC 8428 |
| | level) | | |
| 1/s | 1 per second (event rate) | float | RFC 8428 |
| 1/min | 1 per minute (event rate, "rpm")* | float | RFC 8428 |
| beat/min | 1 per minute (heart rate in beats | float | RFC 8428 |
| | per minute)* | | |
| beats | 1 (cumulative number of heart | float | RFC 8428 |
| | beats)* | | |
| S/m | siemens per meter (conductivity) | float | RFC 8428 |
+----------+------------------------------------+-------+-----------+
Table 6: IANA Registry for SenML Units
o Note 1: Assumed to be in World Geodetic System 1984 (WGS84),
unless another reference frame is known for the sensor.
o Note 2: A value of 0.0 indicates the switch is off, 1.0 indicates
on, and 0.5 indicates half on. The preferred name of this unit is
"/". For historical reasons, the name "%" is also provided for
the same unit, but note that while that name strongly suggests a
percentage (0..100), it is NOT a percentage but the absolute
ratio!
Jennings, et al. Standards Track [Page 32]RFC 8428 SenML August 2018
New entries can be added to the registration by Expert Review as
defined in [RFC8126]. Experts should exercise their own good
judgment but need to consider the following guidelines:
1. There needs to be a real and compelling use for any new unit to
be added.
2. Each unit should define the semantic information and be chosen
carefully. Implementers need to remember that the same word may
be used in different real-life contexts. For example, degrees
when measuring latitude have no semantic relation to degrees
when measuring temperature; thus, two different units are
needed.
3. These measurements are produced by computers for consumption by
computers. The principle is that conversion has to be easily
done when both reading and writing the media type. The value of
a single canonical representation outweighs the convenience of
easy human representations or loss of precision in a conversion.
4. Use of System of Units (SI) prefixes such as "k" before the unit
is not recommended. Instead, one can represent the value using
scientific notation such as 1.2e3. The "kg" unit is an
exception to this rule since it is an SI base unit; the "g" unit
is provided for legacy compatibility.
5. For a given type of measurement, there will only be one unit
type defined. So for length, meter is defined, and other
lengths such as mile, foot, and light year are not allowed. For
most cases, the SI unit is preferred.
(Note that some amount of judgment will be required here, as
even SI itself is not entirely consistent in this respect. For
instance, for temperature, [ISO-80000-5] defines a quantity,
item 5-1 (thermodynamic temperature), and a corresponding unit
of 5-1.a (Kelvin); [ISO-80000-5] goes on to define another
quantity, item 5-2 ("Celsius temperature"), and the
corresponding unit of 5-2.a (degree Celsius). The latter
quantity is defined such that it gives the thermodynamic
temperature as a delta from T0 = 275.15 K. ISO 80000-5 is
defining both units side by side and not really expressing a
preference. This level of recognition of the alternative unit
degree Celsius is the reason why Celsius temperatures seem
exceptionally acceptable in the SenML units list alongside
Kelvin.)
Jennings, et al. Standards Track [Page 33]RFC 8428 SenML August 2018
6. Symbol names that could be easily confused with existing common
units or units combined with prefixes should be avoided. For
example, selecting a unit name of "mph" to indicate something
that had nothing to do with velocity would be a bad choice, as
"mph" is commonly used to mean "miles per hour".
7. The following should not be used because they are common SI
prefixes: Y, Z, E, P, T, G, M, k, h, da, d, c, u, n, p, f, a, z,
y, Ki, Mi, Gi, Ti, Pi, Ei, Zi, and Yi.
8. The following units should not be used as they are commonly used
to represent other measurements: Ky, Gal, dyn, etg, P, St, Mx,
G, Oe, Gb, sb, Lmb, mph, Ci, R, RAD, REM, gal, bbl, qt, degF,
Cal, BTU, HP, pH, B/s, psi, Torr, atm, at, bar, and kWh.
9. The unit names are case sensitive, and the correct case needs to
be used; however, symbols that differ only in case should not be
allocated.
10. A number after a unit typically indicates the previous unit
raised to that power, and "/" indicates that the units that
follow are the reciprocals. A unit should have only one "/" in
the name.
11. A good list of common units can be found in the Unified Code for
Units of Measure [UCUM].
Jennings, et al. Standards Track [Page 34]RFC 8428 SenML August 2018
12.2. SenML Labels Registry
IANA has created a new registry for SenML Labels called the "SenML
Labels" registry. The initial contents of the registry are as
follows:
+--------------+-------+----+-----------+----------+----+-----------+
| Name | Label | CL | JSON Type | XML Type | EI | Reference |
+--------------+-------+----+-----------+----------+----+-----------+
| Base Name | bn | -2 | String | string | a | RFC 8428 |
| Base Time | bt | -3 | Number | double | a | RFC 8428 |
| Base Unit | bu | -4 | String | string | a | RFC 8428 |
| Base Value | bv | -5 | Number | double | a | RFC 8428 |
| Base Sum | bs | -6 | Number | double | a | RFC 8428 |
| Base Version | bver | -1 | Number | int | a | RFC 8428 |
| Name | n | 0 | String | string | a | RFC 8428 |
| Unit | u | 1 | String | string | a | RFC 8428 |
| Value | v | 2 | Number | double | a | RFC 8428 |
| String Value | vs | 3 | String | string | a | RFC 8428 |
| Boolean | vb | 4 | Boolean | boolean | a | RFC 8428 |
| Value | | | | | | |
| Data Value | vd | 8 | String | string | a | RFC 8428 |
| Sum | s | 5 | Number | double | a | RFC 8428 |
| Time | t | 6 | Number | double | a | RFC 8428 |
| Update Time | ut | 7 | Number | double | a | RFC 8428 |
+--------------+-------+----+-----------+----------+----+-----------+
Note that CL = CBOR Label and EI = EXI ID.
Table 7: IANA Registry for SenML Labels
This is the same table as Table 1, with notes removed and columns
added for the information that is all the same for this initial set
of registrations, but it will need to be supplied with different
values for new registrations.
All new entries must define the Name, Label, and XML Type, but the
CBOR labels SHOULD be left empty as CBOR will use the string encoding
for any new labels. The EI column contains the EXI schemaId value of
the first schema that includes this label, or it is empty if this
label was not intended for use with EXI. The Reference column SHOULD
contain information about where to find out more information about
this label.
The JSON, CBOR, and EXI types are derived from the XML type. All XML
numeric types such as double, float, integer, and int become a JSON
Number. XML boolean and string become a JSON Boolean and String,
Jennings, et al. Standards Track [Page 35]RFC 8428 SenML August 2018
respectively. CBOR represents numeric values with a CBOR type that
does not lose any information from the JSON value. EXI uses the XML
types.
New entries can be added to the registration by Expert Review as
defined in [RFC8126]. Experts should exercise their own good
judgment but need to consider that shorter labels should have more
strict review. New entries should not be made that counteract the
advice at the end of Section 4.5.4.
All new SenML Labels that have "base" semantics (see Section 4.1)
MUST start with the character "b". Regular labels MUST NOT start
with that character. All new SenML Labels with Value semantics (see
Section 4.2) MUST have "Value" in their (long-form) name.
Extensions that add a label intended for use with XML need to create
a new RelaxNG Schema that includes all the labels in the "SenML
Labels" registry.
Extensions that add a label that is intended for use with EXI need to
create a new XSD Schema that includes all the labels in the "SenML
Labels" registry and then allocate a new EXI schemaId value. Moving
to the next letter in the alphabet is the suggested way to create the
new value for the EXI schemaId. Any labels with previously blank ID
values SHOULD be updated in the "SenML Labels" registry to have their
ID set to this new schemaId value.
Extensions that are mandatory to understand to correctly process the
Pack MUST have a label name that ends with the "_" character.
12.3. Media Type Registrations
The registrations in the subsections below follow the procedures
specified in [RFC6838] and [RFC7303]. This document registers media
types for each serialization format of SenML (JSON, CBOR, XML, and
EXI) and also a corresponding set of media types for streaming use
(SenSML; see Section 4.8). Clipboard formats are defined for the
JSON and XML forms of SenML but not for streams or non-textual
formats.
The reason there are both SenML and the streaming SenSML formats is
that they are not the same data formats, and they require separate
negotiation to understand if they are supported and which one is
being used. The non-streaming format is required to have some sort
of end-of-pack syntax that indicates there will be no more records.
Many implementations that receive SenML wait for this end-of-pack
marker before processing any of the records. On the other hand, with
the streaming formats, it is explicitly not required to wait for this
Jennings, et al. Standards Track [Page 36]
RFC 8428 SenML August 2018
end-of-pack marker. Many implementations that produce streaming
SenSML will never send this end-of-pack marker, so implementations
that receive streaming SenSML cannot wait for the end-of-pack marker
before they start processing the records. Given that SenML and
streaming SenML are different data formats, and considering the
requirement for separate negotiation, a media type for each one is
needed.
12.3.1. senml+json Media Type Registration
Type name: application
Subtype name: senml+json
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using a subset of the
encoding allowed in [RFC8259]. See RFC 8428 for details. This
simplifies implementation of a very simple system and does not impose
any significant limitations as all this data is meant for machine-to-
machine communications and is not meant to be human readable.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any JSON
key-value pairs that they do not understand unless the key ends with
the "_" character, in which case an error MUST be generated. This
allows backwards-compatible extensions to this specification. The
"bver" field can be used to ensure the receiver supports a minimal
level of functionality needed by the creator of the JSON object.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/senml+json is supported by using fragment identifiers as
specified by RFC 8428.
Jennings, et al. Standards Track [Page 37]
RFC 8428 SenML August 2018
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senml
Windows Clipboard Name: "JSON Sensor Measurement List"
Macintosh file type code(s): none
Macintosh Universal Type Identifier code: org.ietf.senml-json
conforms to public.text
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.2. sensml+json Media Type Registration
Type name: application
Subtype name: sensml+json
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using a subset of the
encoding allowed in [RFC8259]. See RFC 8428 for details. This
simplifies implementation of a very simple system and does not impose
any significant limitations as all this data is meant for machine-to-
machine communications and is not meant to be human readable.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any JSON
key-value pairs that they do not understand unless the key ends with
the "_" character, in which case an error MUST be generated. This
Jennings, et al. Standards Track [Page 38]RFC 8428 SenML August 2018
allows backwards-compatible extensions to this specification. The
"bver" field can be used to ensure the receiver supports a minimal
level of functionality needed by the creator of the JSON object.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/sensml+json is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): sensml
Macintosh file type code(s): none
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.3. senml+cbor Media Type Registration
Type name: application
Subtype name: senml+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using [RFC7049]. See RFC
8428 for details.
Jennings, et al. Standards Track [Page 39]RFC 8428 SenML August 2018
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any key-
value pairs that they do not understand unless the key ends with the
"_" character, in which case an error MUST be generated. This allows
backwards-compatible extensions to this specification. The "bver"
field can be used to ensure the receiver supports a minimal level of
functionality needed by the creator of the CBOR object.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/senml+cbor is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senmlc
Macintosh file type code(s): none
Macintosh Universal Type Identifier code: org.ietf.senml-cbor
conforms to public.data
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
Jennings, et al. Standards Track [Page 40]RFC 8428 SenML August 2018
12.3.4. sensml+cbor Media Type Registration
Type name: application
Subtype name: sensml+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using [RFC7049]. See RFC
8428 for details.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any key-
value pairs that they do not understand unless the key ends with the
"_" character, in which case an error MUST be generated. This allows
backwards-compatible extensions to this specification. The "bver"
field can be used to ensure the receiver supports a minimal level of
functionality needed by the creator of the CBOR object.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/sensml+cbor is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): sensmlc
Macintosh file type code(s): none
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Jennings, et al. Standards Track [Page 41]RFC 8428 SenML August 2018
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.5. senml+xml Media Type Registration
Type name: application
Subtype name: senml+xml
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using
[W3C.REC-xml-20081126]. See RFC 8428 for details.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any XML
tags or attributes that they do not understand unless the attribute
name ends with the "_" character, in which case an error MUST be
generated. This allows backwards-compatible extensions to this
specification. The "bver" attribute in the senml XML tag can be used
to ensure the receiver supports a minimal level of functionality
needed by the creator of the XML SenML Pack.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/senml+xml is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senmlx
Jennings, et al. Standards Track [Page 42]RFC 8428 SenML August 2018
Windows Clipboard Name: "XML Sensor Measurement List"
Macintosh file type code(s): none
Macintosh Universal Type Identifier code: org.ietf.senml-xml
conforms to public.xml
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.6. sensml+xml Media Type Registration
Type name: application
Subtype name: sensml+xml
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using
[W3C.REC-xml-20081126]. See RFC 8428 for details.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any XML
tags or attributes that they do not understand unless the attribute
name ends with the "_" character, in which case an error MUST be
generated. This allows backwards-compatible extensions to this
specification. The "bver" attribute in the senml XML tag can be used
to ensure the receiver supports a minimal level of functionality
needed by the creator of the XML SenML Pack.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Jennings, et al. Standards Track [Page 43]RFC 8428 SenML August 2018
Fragment identifier considerations: Fragment identification for
application/sensml+xml is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): sensmlx
Macintosh file type code(s): none
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.7. senml-exi Media Type Registration
Type name: application
Subtype name: senml-exi
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using
[W3C.REC-exi-20140211]. See RFC 8428 for details.
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any XML
tags or attributes that they do not understand unless the attribute
name ends with the "_" character, in which case an error MUST be
generated. This allows backwards-compatible extensions to this
specification. The "bver" attribute in the senml XML tag can be used
to ensure the receiver supports a minimal level of functionality
needed by the creator of the XML SenML Pack. Further information on
using schemas to guide the EXI can be found in RFC 8428.
Jennings, et al. Standards Track [Page 44]RFC 8428 SenML August 2018
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/senml-exi is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): senmle
Macintosh file type code(s): none
Macintosh Universal Type Identifier code: org.ietf.senml-exi
conforms to public.data
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
12.3.8. sensml-exi Media Type Registration
Type name: application
Subtype name: sensml-exi
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using
[W3C.REC-exi-20140211]. See RFC 8428 for details.
Jennings, et al. Standards Track [Page 45]RFC 8428 SenML August 2018
Security considerations: See Section 13 of RFC 8428.
Interoperability considerations: Applications MUST ignore any XML
tags or attributes that they do not understand unless the attribute
name ends with the "_" character, in which case an error MUST be
generated. This allows backwards-compatible extensions to this
specification. The "bver" attribute in the senml XML tag can be used
to ensure the receiver supports a minimal level of functionality
needed by the creator of the XML SenML Pack. Further information on
using schemas to guide the EXI can be found in RFC 8428.
Published specification: RFC 8428
Applications that use this media type: The type is used by systems
that report, e.g., electrical power usage and environmental
information such as temperature and humidity. It can be used for a
wide range of sensor reporting systems.
Fragment identifier considerations: Fragment identification for
application/sensml-exi is supported by using fragment identifiers as
specified by RFC 8428.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): sensmle
Macintosh file type code(s): none
Person & email address to contact for further information:
Cullen Jennings <fluffy@iii.ca>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <fluffy@iii.ca>
Change controller: IESG
Jennings, et al. Standards Track [Page 46]RFC 8428 SenML August 2018
12.4. XML Namespace Registration
This document registers the following XML namespace in the "IETF XML
Registry" defined in [RFC3688].
URI: urn:ietf:params:xml:ns:senml
Registrant Contact: The IESG.
XML: N/A, the requested URIs are XML namespaces
12.5. CoAP Content-Format Registration
IANA has assigned CoAP Content-Format IDs for the SenML media types
in the "CoAP Content-Formats" subregistry within the "Constrained
RESTful Environments (CoRE) Parameters" registry [RFC7252]. IDs for
the JSON, CBOR, and EXI Content-Formats have been assigned in the
0-255 range (Expert Review), and IDs for the XML Content-Formats have
been assigned in the 256-9999 range (IETF Review or IESG Approval).
The assigned IDs are shown in the table below:
+-------------------------+----------+-----+-----------+
| Media Type | Encoding | ID | Reference |
+-------------------------+----------+-----+-----------+
| application/senml+json | - | 110 | RFC 8428 |
| application/sensml+json | - | 111 | RFC 8428 |
| application/senml+cbor | - | 112 | RFC 8428 |
| application/sensml+cbor | - | 113 | RFC 8428 |
| application/senml-exi | - | 114 | RFC 8428 |
| application/sensml-exi | - | 115 | RFC 8428 |
| application/senml+xml | - | 310 | RFC 8428 |
| application/sensml+xml | - | 311 | RFC 8428 |
+-------------------------+----------+-----+-----------+
Table 8: CoAP Content-Format IDs
13. Security Considerations
Sensor data presented with SenML can contain a wide array of
information that ranges from very public (such as the outside
temperature in a given city) to very private (such as patient health
information that requires integrity and confidentiality protection).
When SenML is used for configuration or actuation, it can be used to
change the state of systems and also impact the physical world, e.g.,
by turning off a heater or opening a lock. Malicious use of SenML to
change system state could have severe consequences, potentially
including violation of physical security, property damage, and even
loss of life.
Jennings, et al. Standards Track [Page 47]RFC 8428 SenML August 2018
SenML formats alone do not provide any security and instead rely on
the protocol that carries them to provide security. Applications
using SenML need to look at the overall context of how these formats
will be used to decide if the security is adequate. In particular,
for sensitive sensor data and actuation use, it is important to
ensure that proper security mechanisms are used to provide, e.g.,
confidentiality, data integrity, and authentication as appropriate
for the usage.
SenML formats defined by this specification do not contain any
executable content. However, future extensions could potentially
embed application-specific executable content in the data.
SenML Records are intended to be interpreted in the context of any
applicable base values. If Records become separated from the Record
that establishes the base values, the data will be useless or, worse,
wrong. Care needs to be taken in keeping the integrity of a Pack
that contains unresolved SenML Records (see Section 4.6).
See also Section 14.
14. Privacy Considerations
Sensor data can range from information with almost no privacy
considerations, such as the current temperature in a given city, to
highly sensitive medical or location data. This specification
provides no security protection for the data but is meant to be used
inside another container or transfer protocol such as S/MIME
[RFC5751] or HTTP with TLS [RFC2818] that can provide integrity,
confidentiality, and authentication information about the source of
the data.
The Name fields need to uniquely identify the sources or destinations
of the values in a SenML Pack. However, the use of long-term stable
and unique identifiers can be problematic for privacy reasons
[RFC6973], depending on the application and the potential of these
identifiers to be used in correlation with other information. They
should be used with care or avoided, for example, as described for
IPv6 addresses in [RFC7721].
Jennings, et al. Standards Track [Page 48]
RFC 8428 SenML August 2018
15. References
15.1. Normative References
[BIPM] Bureau International des Poids et Mesures, "The
International System of Units (SI)", 8th Edition, 2006.
[IEEE.754] IEEE, "Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754.
[NIST811] Thompson, A. and B. Taylor, "Guide for the Use of the
International System of Units (SI)", NIST Special
Publication 811, DOI 10.6028/NIST.SP.811e2008, March 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
DOI 10.17487/RFC7303, July 2014,
<https://www.rfc-editor.org/info/rfc7303>.
Jennings, et al. Standards Track [Page 49]RFC 8428 SenML August 2018
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RNC] ISO/IEC, "Information technology -- Document Schema
Definition Language (DSDL) -- Part 2: Regular-grammar-
based validation -- RELAX NG", ISO/IEC 19757-2, Annex
C: RELAX NG Compact syntax, December 2008.
[TIME_T] The Open Group Base Specifications, "Open Group Standard -
Vol. 1: Base Definitions, Issue 7", Section 4.16, "Seconds
Since the Epoch", IEEE Standard 1003.1, 2018,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16>.
[W3C.REC-exi-20140211]
Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
"Efficient XML Interchange (EXI) Format 1.0 (Second
Edition)", W3C Recommendation REC-exi-20140211, February
2014, <http://www.w3.org/TR/2014/REC-exi-20140211>.
[W3C.REC-xml-20081126]
Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", W3C Recommendation REC-xml-20081126, November
2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.
[W3C.REC-xmlschema-1-20041028]
Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn,
"XML Schema Part 1: Structures Second Edition", W3C
Recommendation REC-xmlschema-1-20041028, October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.
[XPointerElement]
Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
element() Scheme", W3C Recommendation REC-xptr-element,
March 2003,
<https://www.w3.org/TR/2003/REC-xptr-element-20030325/>.
Jennings, et al. Standards Track [Page 50]RFC 8428 SenML August 2018
[XPointerFramework]
Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
Framework", W3C Recommendation REC-XPointer-Framework,
March 2003,
<http://www.w3.org/TR/2003/REC-xptr-framework-20030325/>.
15.2. Informative References
[AN1796] Linke, B., "Overview of 1-Wire Technology and Its Use",
Maxim Integrated, Tutorial 1796, June 2008,
<http://pdfserv.maximintegrated.com/en/an/AN1796.pdf>.
[CDDL-CBOR]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR and JSON data structures", Work in Progress,
draft-ietf-cbor-cddl-05, August 2018.
[DEVICE-URN]
Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
Names for Device Identifiers", Work in Progress,
draft-ietf-core-dev-urn-02, July 2018.
[IEEE802.1AS]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Timing and Synchronization for Time-Sensitive
Applications in Bridged Local Area Networks", IEEE
Standard 802.1AS.
[IEEE802.1BA]
IEEE, "IEEE Standard for Local and metropolitan area
networks--Audio Video Bridging (AVB) Systems", IEEE
Standard 802.1BA.
[ISO-80000-5]
ISO, "Quantities and units - Part 5: Thermodynamics",
ISO 80000-5, Edition 1.0, May 2007.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
Jennings, et al. Standards Track [Page 51]RFC 8428 SenML August 2018
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC4151] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, DOI 10.17487/RFC4151, October 2005,
<https://www.rfc-editor.org/info/rfc4151>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/info/rfc5751>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7111] Hausenblas, M., Wilde, E., and J. Tennison, "URI Fragment
Identifiers for the text/csv Media Type", RFC 7111,
DOI 10.17487/RFC7111, January 2014,
<https://www.rfc-editor.org/info/rfc7111>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
Jennings, et al. Standards Track [Page 52]RFC 8428 SenML August 2018
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
[RID-CoRE]
Shelby, Z., Vial, M., Groves, C., Zhu, J., and B.
Silverajan, Ed., "Reusable Interface Definitions for
Constrained RESTful Environments", Work in Progress,
draft-ietf-core-interfaces-12, June 2018.
[UCUM] Schadow, G. and C. McDonald, "The Unified Code for Units
of Measure", Version 2.1, Regenstrief Institute and
the UCUM Organization, November 2017,
<http://unitsofmeasure.org/ucum.html>.
Acknowledgements
We would like to thank Alexander Pelov, Alexey Melnikov, Andrew
McClure, Andrew McGregor, Bjoern Hoehrmann, Christian Amsuess,
Christian Groves, Daniel Peintner, Jan-Piet Mens, Jim Schaad, Joe
Hildebrand, John Klensin, Karl Palsson, Lennart Duhrsen, Lisa
Dusseault, Lyndsay Campbell, Martin Thomson, Michael Koster, Peter
Saint-Andre, Roni Even, and Stephen Farrell, for their review
comments.
Jennings, et al. Standards Track [Page 53]RFC 8428 SenML August 2018
Authors' Addresses
Cullen Jennings
Cisco
400 3rd Avenue SW
Calgary, AB T2P 4H2
Canada
Email: fluffy@iii.ca
Zach Shelby
ARM
150 Rose Orchard
San Jose 95134
United States of America
Phone: +1-408-203-9434
Email: zach.shelby@arm.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Jennings, et al. Standards Track [Page 54]
