Internet DRAFT - draft-jennings-core-senml
draft-jennings-core-senml
Network Working Group C. Jennings
Internet-Draft Cisco
Intended status: Standards Track Z. Shelby
Expires: October 7, 2016 ARM
J. Arkko
A. Keranen
Ericsson
April 5, 2016
Media Types for Sensor Markup Language (SenML)
draft-jennings-core-senml-06
Abstract
This specification defines media types for representing simple sensor
measurements and device parameters in the Sensor Markup Language
(SenML). Representations are defined in JavaScript Object Notation
(JSON), Concise Binary Object Representation (CBOR), eXtensible
Markup Language (XML), and Efficient XML Interchange (EXI), which
share the common SenML data model. A simple sensor, such as a
temperature sensor, could use this media type in protocols such as
HTTP or CoAP to transport the measurements of the sensor or to be
configured.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 7, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements and Design Goals . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Associating Meta-data . . . . . . . . . . . . . . . . . . . . 8
6. JSON Representation (application/senml+json) . . . . . . . . 8
6.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1.1. Single Datapoint . . . . . . . . . . . . . . . . . . 9
6.1.2. Multiple Datapoints . . . . . . . . . . . . . . . . . 9
6.1.3. Multiple Measurements . . . . . . . . . . . . . . . . 10
6.1.4. Collection of Resources . . . . . . . . . . . . . . . 11
7. CBOR Representation (application/senml+cbor) . . . . . . . . 12
8. XML Representation (application/senml+xml) . . . . . . . . . 13
9. EXI Representation (application/senml-exi) . . . . . . . . . 15
10. Usage Considerations . . . . . . . . . . . . . . . . . . . . 19
11. CDDL . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12.1. Units Registry . . . . . . . . . . . . . . . . . . . . . 21
12.2. SenML label registry . . . . . . . . . . . . . . . . . . 24
12.3. Media Type Registration . . . . . . . . . . . . . . . . 24
12.3.1. senml+json Media Type Registration . . . . . . . . . 24
12.3.2. senml+cbor Media Type Registration . . . . . . . . . 25
12.3.3. senml+xml Media Type Registration . . . . . . . . . 26
12.3.4. senml-exi Media Type Registration . . . . . . . . . 27
12.4. XML Namespace Registration . . . . . . . . . . . . . . . 28
12.5. CoAP Content-Format Registration . . . . . . . . . . . . 28
13. Security Considerations . . . . . . . . . . . . . . . . . . . 28
14. Privacy Considerations . . . . . . . . . . . . . . . . . . . 29
15. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 29
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
16.1. Normative References . . . . . . . . . . . . . . . . . . 29
16.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Links extension . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Overview
Connecting sensors to the internet is not new, and there have been
many protocols designed to facilitate it. This specification defines
new media types for carrying simple sensor information in a protocol
such as HTTP or CoAP called the Sensor Markup Language (SenML). This
format was designed so that processors with very limited capabilities
could easily encode a sensor measurement into the media type, while
at the same time a server parsing the data could relatively
efficiently collect a large number of sensor measurements. The
markup language can be used for a variety of data flow models, most
notably data feeds pushed from a sensor to a collector, and the web
resource model where the sensor is requested as a resource
representation (e.g., "GET /sensor/temperature").
There are many types of more complex measurements and measurements
that this media type would not be suitable for. SenML strikes a
balance between having some information about the sensor carried with
the sensor data so that the data is self describing but it also tries
to make that a fairly minimal set of auxiliary information for
efficiency reason. Other information about the sensor can be
discovered by other methods such as using the CoRE Link Format
[RFC6690].
SenML is defined by a data model for measurements and simple meta-
data about measurements and devices. The data is structured as a
single array that contains a series of SenML Records which can each
contain attributes such as an unique identifier for the sensor, the
time the measurement was made, the unit the measurement is in, and
the current value of the sensor. Serializations for this data model
are defined for JSON [RFC7159], CBOR [RFC7049], XML, and Efficient
XML Interchange (EXI) [W3C.REC-exi-20110310].
For example, the following shows a measurement from a temperature
gauge encoded in the JSON syntax.
[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.1, "u":"Cel" }]
In the example above, the array has a single SenML Record with a
measurement for a sensor named "urn:dev:ow:10e2073a01080063" with a
current value of 23.5 degrees Celsius.
2. Requirements and Design Goals
The design goal is to be able to send simple sensor measurements in
small packets on mesh networks from large numbers of constrained
devices. Keeping the total size of payload under 80 bytes makes this
easy to use on a wireless mesh network. It is always difficult to
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define what small code is, but there is a desire to be able to
implement this in roughly 1 KB of flash on a 8 bit microprocessor.
Experience with Google power meter and large scale deployments has
indicated that the solution needs to support allowing multiple
measurements to be batched into a single HTTP or CoAP request. This
"batch" upload capability allows the server side to efficiently
support a large number of devices. It also conveniently supports
batch transfers from proxies and storage devices, even in situations
where the sensor itself sends just a single data item at a time. The
multiple measurements could be from multiple related sensors or from
the same sensor but at different times.
The basic design is an array with a series of measurements. The
following example shows two measurements made at different times.
The value of a measurement is in the "v" tag, the time of a
measurement is in the "t" tag, the "n" tag has a unique sensor name,
and the unit of the measurement is carried in the "u" tag.
[
{ "n": "urn:dev:ow:10e2073a01080063",
"t": 1276020076, "v":23.5, "u":"Cel" },
{ "n": "urn:dev:ow:10e2073a01080063",
"t": 1276020091, "v":23.6, "u":"Cel" }
]
To keep the messages small, it does not make sense to repeat the "n"
tag in each SenML Record so there is a concept of a Base Name which
is simply a string that is prepended to the Name field of all
elements in that record and any records that follow it. So a more
compact form of the example above is the following.
[
{ "bn": "urn:dev:ow:10e2073a01080063",
"t": 1276020076, "v":23.5, "u":"Cel" },
{ "t": 1276020091, "v":23.6, "u":"Cel" }
]
In the above example the Base Name is in the "bn" tag and the "n"
tags in each Record are the empty string so they are omitted. The
Base Name also could be put in a separate Record such as in the
following example.
[
{ "bn": "urn:dev:ow:10e2073a01080063" },
{ "t": 1276020076, "v":23.5, "u":"Cel" },
{ "t": 1276020091, "v":23.6, "u":"Cel" }
]
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Some devices have accurate time while others do not so SenML supports
absolute and relative times. Time is represented in floating point
as seconds and values greater than zero represent an absolute time
relative to the unix epoch while values of 0 or less represent a
relative time in the past from the current time. A simple sensor
with no absolute wall clock time might take a measurement every
second and batch up 60 of them then send it to a server. It would
include the relative time the measurement was made to the time the
batch was send in the SenML Pack. The server might have accurate NTP
time and use the time it received the data, and the relative offset,
to replace the times in the SenML with absolute times before saving
the SenML Pack in a document database.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
This document also uses the following terms:
SenML Record: One measurement or configuration instance in time
presented using the SenML data model.
SenML Pack: One or more SenML Records in an array structure.
4. Semantics
Each SenML Pack carries a single array that represents a set of
measurements and/or parameters. This array contains a series of
objects with several optional attributes described below:
Base Name: This is a string that is prepended to the names found in
the entries. This attribute is optional. This applies to the
entries in all Records. A Base Name can only be included in the
first Record of the array.
Base Time: A base time that is added to the time found in an entry.
This attribute is optional. This applies to the entries in all
Records. A Base Time can only be included in the first Record of
the array.
Base Unit: A base unit that is assumed for all entries, unless
otherwise indicated. This attribute is optional. If a record
does not contain a unit value, then the base unit is used
otherwise the value of found in the Unit is used. This applies to
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the entries in all Records. A Base Unit can only be included in
the first object of the array.
Base Value: A base value is added to the value found in an entry,
similar to Base Time. This attribute is optional. This applies
to the entries in all Records. A Base Value can only be included
in the first Record of the array.
Version: Version number of media type format. This attribute is
optional positive integer and defaults to 5 if not present. A
Version can only be included in the first object of the array.
Name: Name of the sensor or parameter. When appended to the Base
Name attribute, this must result in a globally unique identifier
for the resource. The name is optional, if the Base Name is
present. If the name is missing, Base Name must uniquely identify
the resource. This can be used to represent a large array of
measurements from the same sensor without having to repeat its
identifier on every measurement.
Unit: Units for a measurement value. Optional. If the Record has
not Unit, the Base Unit is used as the Unit. Having no Unit and
no Base Unit is allowed.
Value Value of the entry. Optional if a Sum value is present,
otherwise required. Values are represented using three basic data
types, Floating point numbers ("v" field for "Value"), Booleans
("vb" for "Boolean Value"), Strings ("vs" for "String Value") and
Data ("vd" for "Binary Data Value") . Exactly one of these three
fields MUST appear unless there is Sum field in which case it is
allowed to have no Value field or to have "v" field.
Sum: Integrated sum of the values over time. Optional. This
attribute is in the units specified in the Unit value multiplied
by seconds.
Time: Time when value was recorded. Optional.
Update Time: An optional time in seconds that represents the maximum
time before this sensor will provide an updated reading for a
measurement. This can be used to detect the failure of sensors or
communications path from the sensor.
The SenML format can be extended with further custom attributes.
TODO - describe what extensions are possible and how to do them.
Systems reading one of the objects MUST check for the Version
attribute. If this value is a version number larger than the version
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which the system understands, the system SHOULD NOT use this object.
This allows the version number to indicate that the object contains
mandatory to understand attributes. New version numbers can only be
defined in an RFC that updates this specification or it successors.
The Name value is concatenated to the Base Name value to get the name
of the sensor. The resulting name needs to uniquely identify and
differentiate the sensor from all others. If the object is a
representation resulting from the request of a URI [RFC3986], then in
the absence of the Base Name attribute, this URI is used as the
default value of Base Name. Thus in this case the Name field needs
to be unique for that URI, for example an index or subresource name
of sensors handled by the URI.
Alternatively, for objects not related to a URI, a unique name is
required. In any case, it is RECOMMENDED that the full names are
represented as URIs or URNs [RFC2141]. One way to create a unique
name is to include some bit string that has guaranteed uniqueness
(such as a 1-wire address) that is assigned to the device. Some of
the examples in this draft use the device URN type as specified in
[I-D.arkko-core-dev-urn]. UUIDs [RFC4122] are another way to
generate a unique name. Note that long-term stable unique
identifiers are problematic for privacy reasons [RFC7721] and should
be used with care or avoided.
The resulting concatenated name MUST consist only of characters out
of the set "A" to "Z", "a" to "z", "0" to "9", "-", ":", ".", or "_"
and it MUST start with a character out of the set "A" to "Z", "a" to
"z", or "0" to "9". This restricted character set was chosen so that
these names can be directly used as in other types of URI including
segments of an HTTP path with no special encoding and can be directly
used in many databases and analytic systems. [RFC5952] contains
advice on encoding an IPv6 address in a name.
If either the Base Time or Time value is missing, the missing
attribute is considered to have a value of zero. The Base Time and
Time values are added together to get the time of measurement. A
time of zero indicates that the sensor does not know the absolute
time and the measurement was made roughly "now". A negative value is
used to indicate seconds in the past from roughly "now". A positive
value is used to indicate the number of seconds, excluding leap
seconds, since the start of the year 1970 in UTC.
Representing the statistical characteristics of measurements, such as
accuracy, can be very complex. Future specification may add new
attributes to provide better information about the statistical
properties of the measurement.
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5. Associating Meta-data
SenML is designed to carry the minimum dynamic information about
measurements, and for efficiency reasons does not carry significant
static meta-data about the device, object or sensors. Instead, it is
assumed that this meta-data is carried out of band. For web
resources using SenML Packs, this meta-data can be made available
using the CoRE Link Format [RFC6690]. The most obvious use of this
link format is to describe that a resource is available in a SenML
format in the first place. The relevant media type indicator is
included in the Content-Type (ct=) attribute.
6. JSON Representation (application/senml+json)
Record atributes:
+---------------+------+---------+
| SenML | JSON | Type |
+---------------+------+---------+
| Base Name | bn | String |
| Base Time | bt | Number |
| Base Unit | bu | String |
| Base Value | bv | Number |
| Version | bver | Number |
| Name | n | String |
| Unit | u | String |
| Value | v | Number |
| String Value | vs | String |
| Boolean Value | vb | Boolean |
| Data Value | vd | String |
| Value Sum | s | Number |
| Time | t | Number |
| Update Time | ut | Number |
+---------------+------+---------+
The root content consists of an array with JSON objects for each
SenML Record. All the fields in the above table MAY occur in the
records with the type specified in the table.
Only the UTF-8 form of JSON is allowed. Characters in the String
Value are encoded using the escape sequences defined in [RFC4627].
Characters in the Data Value are base64 encoded with URL safe
alphabet as defined in Section 5 of [RFC4648].
Systems receiving measurements MUST be able to process the range of
floating point numbers that are representable as an IEEE double-
precision floating-point numbers [IEEE.754.1985]. The number of
significant digits in any measurement is not relevant, so a reading
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of 1.1 has exactly the same semantic meaning as 1.10. If the value
has an exponent, the "e" MUST be in lower case. The mantissa SHOULD
be less than 19 characters long and the exponent SHOULD be less than
5 characters long. This allows time values to have better than micro
second precision over the next 100 years.
6.1. Examples
TODO - simplify examples
TODO - Examples are messed up on if time is an integer or float
TODO - Add example with string, data, boolean, and base value
6.1.1. Single Datapoint
The following shows a temperature reading taken approximately "now"
by a 1-wire sensor device that was assigned the unique 1-wire address
of 10e2073a01080063:
[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.1, "u":"Cel" }]
6.1.2. Multiple Datapoints
The following example shows voltage and current now, i.e., at an
unspecified time.
[{"bn": "urn:dev:ow:10e2073a01080063/"},
{ "n": "voltage", "t": 0, "u": "V", "v": 120.1 },
{ "n": "current", "t": 0, "u": "A", "v": 1.2 }
]
The next example is similar to the above one, but shows current at
Tue Jun 8 18:01:16 UTC 2010 and at each second for the previous 5
seconds.
[{"bn": "urn:dev:ow:10e2073a01080063/",
"bt": 1276020076.001,
"bu": "A",
"bver": 5},
{ "n": "voltage", "u": "V", "v": 120.1 },
{ "n": "current", "t": -5, "v": 1.2 },
{ "n": "current", "t": -4, "v": 1.30 },
{ "n": "current", "t": -3, "v": 0.14e1 },
{ "n": "current", "t": -2, "v": 1.5 },
{ "n": "current", "t": -1, "v": 1.6 },
{ "n": "current", "t": 0, "v": 1.7 }
]
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Note that in some usage scenarios of SenML the implementations MAY
store or transmit SenML in a stream-like fashion, where data is
collected over time and continuously added to the object. This mode
of operation is optional, but systems or protocols using SenML in
this fashion MUST specify that they are doing this. SenML defines a
separate mime type (TODO) to indicate Sensor Streaming Markup
Language (SensML) for this usage. In this situation the SensML
stream can be sent and received in a partial fashion, i.e., a
measurement entry can be read as soon as the SenML Record is received
and not have to wait for the full SensML Stream to be complete.
For instance, the following stream of measurements may be sent via a
long lived HTTP POST from the producer of a SensML to the consumer of
that, and each measurement object may be reported at the time it
measured:
[ {"bn": "urn:dev:ow:10e2073a01080063",
"bt": 1320067464,
"bu": "%RH"},
{ "v": 21.2, "t": 0 },
{ "v": 21.3, "t": 10 },
{ "v": 21.4, "t": 20 },
{ "v": 21.4, "t": 30 },
{ "v": 21.5, "t": 40 },
{ "v": 21.5, "t": 50 },
{ "v": 21.5, "t": 60 },
{ "v": 21.6, "t": 70 },
{ "v": 21.7, "t": 80 },
{ "v": 21.5, "t": 90 },
...
6.1.3. Multiple Measurements
The following example shows humidity measurements from a mobile
device with an IPv6 address 2001:db8::1, starting at Mon Oct 31
13:24:24 UTC 2011. The device also provides position data, which is
provided in the same measurement or parameter array as separate
entries. Note time is used to for correlating data that belongs
together, e.g., a measurement and a parameter associated with it.
Finally, the device also reports extra data about its battery status
at a separate time.
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[{"bn": "urn:dev:ow:10e2073a01080063",
"bt": 1320067464,
"bu": "%RH"},
{ "v": 20.0, "t": 0 },
{ "v": 24.30621, "u": "lon", "t": 0 },
{ "v": 60.07965, "u": "lat", "t": 0 },
{ "v": 20.3, "t": 60 },
{ "v": 24.30622, "u": "lon", "t": 60 },
{ "v": 60.07965, "u": "lat", "t": 60 },
{ "v": 20.7, "t": 120 },
{ "v": 24.30623, "u": "lon", "t": 120 },
{ "v": 60.07966, "u": "lat", "t": 120 },
{ "v": 98.0, "u": "%EL", "t": 150 },
{ "v": 21.2, "t": 180 },
{ "v": 24.30628, "u": "lon", "t": 180 },
{ "v": 60.07967, "u": "lat", "t": 180 }
]
The size of this example represented in various forms, as well as
that form compressed with gzip is given in the following table.
+----------+------+-----------------+
| Encoding | Size | Compressed Size |
+----------+------+-----------------+
| JSON | 567 | 200 |
| XML | 656 | 232 |
| CBOR | 292 | 192 |
| EXI | 160 | 183 |
+----------+------+-----------------+
Table 1: Size Comparisons
Note the CBOR and EXI sizes are not using the schema guidance so the
could be a bit smaller.
6.1.4. Collection of Resources
The following example shows how to query one device that can provide
multiple measurements. 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.
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[{"bn": "urn:dev:ow:10e2073a01080063/",
"bt": 1320078429,
"bver": 5
},
{ "n": "temperature", "v": 27.2, "u": "Cel" },
{ "n": "humidity", "v": 80, "u": "%RH" }
]
7. CBOR Representation (application/senml+cbor)
The CBOR [RFC7049] representation is equivalent to the JSON
representation, with the following changes:
o For compactness, the CBOR representation uses integers for the map
keys defined in Table 2. This table is conclusive, i.e., there is
no intention to define any additional integer map keys; any
extensions will use string map keys.
o For JSON Numbers, the CBOR representation can use integers,
floating point numbers, or decimal fractions (CBOR Tag 4); the
common limitations of JSON implementations are not relevant for
these. For the version number, however, only an unsigned integer
is allowed.
+---------------+------------+------------+
| Name | JSON label | CBOR label |
+---------------+------------+------------+
| Version | bver | -1 |
| Base Name | bn | -2 |
| Base Time | bt | -3 |
| Base Units | bu | -4 |
| Base Value | bv | -5 |
| Name | n | 0 |
| Units | u | 1 |
| Value | v | 2 |
| String Value | vs | 3 |
| Boolean Value | vb | 4 |
| Value Sum | s | 5 |
| Time | t | 6 |
| Update Time | ut | 7 |
| Data Value | vd | 8 |
+---------------+------------+------------+
Table 2: CBOR representation: integers for map keys
The following example shows an hexdump of the CBOR example for the
same sensor measurement as in Section 6.1.2.
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0000 88 a4 62 62 6e 78 1c 75 72 6e 3a 64 65 76 3a 6f |..bbnx.urn:dev:o|
0010 77 3a 31 30 65 32 30 37 33 61 30 31 30 38 30 30 |w:10e2073a010800|
0020 36 33 2f 62 62 74 1a 4c 0e 85 6c 62 62 75 61 41 |63/bbt.L..lbbuaA|
0030 63 76 65 72 05 a3 61 6e 67 76 6f 6c 74 61 67 65 |cver..angvoltage|
0040 61 75 61 56 61 76 fb 40 5e 06 66 66 66 66 66 a3 |auaVav.@^.fffff.|
0050 61 6e 67 63 75 72 72 65 6e 74 61 74 24 61 76 fb |angcurrentat$av.|
0060 3f f3 33 33 33 33 33 33 a3 61 6e 67 63 75 72 72 |?.333333.angcurr|
0070 65 6e 74 61 74 23 61 76 fb 3f f4 cc cc cc cc cc |entat#av.?......|
0080 cd a3 61 6e 67 63 75 72 72 65 6e 74 61 74 22 61 |..angcurrentat"a|
0090 76 fb 3f f6 66 66 66 66 66 66 a3 61 6e 67 63 75 |v.?.ffffff.angcu|
00a0 72 72 65 6e 74 61 74 21 61 76 fb 3f f8 00 00 00 |rrentat!av.?....|
00b0 00 00 00 a3 61 6e 67 63 75 72 72 65 6e 74 61 74 |....angcurrentat|
00c0 20 61 76 fb 3f f9 99 99 99 99 99 9a a2 61 6e 67 | av.?........ang|
00d0 63 75 72 72 65 6e 74 61 76 fb 3f fb 33 33 33 33 |currentav.?.3333|
00e0 33 33 0a |33.|
00e3
8. XML Representation (application/senml+xml)
A SenML Stream can also be represented in XML format as defined in
this section. The following example shows an XML example for the
same sensor measurement as in Section 6.1.2.
<sensml xmlns="urn:ietf:params:xml:ns:senml">
<senml bn="urn:dev:ow:10e2073a01080063/" bt="1.276020076e+09"
bu="A" bver="5"></senml>
<senml n="voltage" u="V" v="120.1"></senml>
<senml n="current" t="-5" v="1.2"></senml>
<senml n="current" t="-4" v="1.3"></senml>
<senml n="current" t="-3" v="1.4"></senml>
<senml n="current" t="-2" v="1.5"></senml>
<senml n="current" t="-1" v="1.6"></senml>
<senml n="current" v="1.7"></senml>
</sensml>
The SenML Stream is represented as a sensml tag that contains a
series of senml tags for each SenML Record. The SenML Fields are
represents as XML attributes. The following table shows the mapping
the SenML Field names to the attribute used in the XML senml tag.
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+---------------+------+---------+
| SenML Field | XML | Type |
+---------------+------+---------+
| Base Name | bn | string |
| Base Time | bt | float |
| Base Unit | bu | string |
| Base Value | bv | float |
| Version | bver | int |
| Name | n | string |
| Unit | u | string |
| Value | v | float |
| String Value | vs | string |
| Data Value | vd | string |
| Boolean Value | vb | boolean |
| Value Sum | s | float |
| Time | t | float |
| Update Time | ut | float |
+---------------+------+---------+
The RelaxNG schema for the XML is:
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default namespace = "urn:ietf:params:xml:ns:senml"
namespace rng = "http://relaxng.org/ns/structure/1.0"
link = element l {
attribute * { xsd:string }*
}
senml = element senml {
attribute bn { xsd:string }?,
attribute bt { xsd:double }?,
attribute bv { xsd:double }?,
attribute bu { xsd:string }?,
attribute bver { xsd:int }?,
attribute n { xsd:string }?,
attribute s { xsd:double }?,
attribute t { xsd:double }?,
attribute u { xsd:string }?,
attribute ut { xsd:double }?,
attribute v { xsd:double }?,
attribute vb { xsd:boolean }?,
attribute vs { xsd:string }?,
attribute vd { xsd:string }?,
link*
}
sensml =
element sensml {
senml+
}
start = sensml
9. EXI Representation (application/senml-exi)
For efficient transmission of SenML over e.g. a constrained network,
Efficient XML Interchange (EXI) can be used. This encodes the XML
Schema 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. This mode however does not allow tag
extensions to the schema, and 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.
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The EXI header option MUST be included. An EXI schemaID options MUST
be set to the value of "a" indicating the scheme provided in this
specification. Future revisions to the schema can change this
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.
TODO - examples probably have the wrong setting the schemaID
The following is the XSD Schema to be used for strict schema guided
EXI processing. It is generated from the RelaxNG.
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<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified"
targetNamespace="urn:ietf:params:xml:ns:senml"
xmlns:ns1="urn:ietf:params:xml:ns:senml">
<xs:element name="l">
<xs:complexType>
<xs:anyAttribute processContents="skip" />
</xs:complexType>
</xs:element>
<xs:element name="senml">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="0" maxOccurs="unbounded"
ref="ns1:l" />
</xs:sequence>
<xs:attribute name="bn" type="xs:string" />
<xs:attribute name="bt" type="xs:double" />
<xs:attribute name="bv" type="xs:double" />
<xs:attribute name="bu" type="xs:string" />
<xs:attribute name="bver" type="xs:int" />
<xs:attribute name="n" type="xs:string" />
<xs:attribute name="s" type="xs:double" />
<xs:attribute name="t" type="xs:double" />
<xs:attribute name="u" type="xs:string" />
<xs:attribute name="ut" type="xs:double" />
<xs:attribute name="v" type="xs:double" />
<xs:attribute name="vb" type="xs:boolean" />
<xs:attribute name="vs" type="xs:string" />
<xs:attribute name="vd" type="xs:string" />
</xs:complexType>
</xs:element>
<xs:element name="sensml">
<xs:complexType>
<xs:sequence>
<xs:element maxOccurs="unbounded" ref="ns1:senml" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
The following shows a hexdump of the EXI produced from encoding the
following XML example. Note this example is the same information as
the first example in Section 6.1.2 in JSON format.
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<sensml xmlns="urn:ietf:params:xml:ns:senml">
<senml bn="urn:dev:ow:10e2073a01080063/"></senml>
<senml n="voltage" u="V" v="120.1"></senml>
<senml n="current" u="A" v="1.2"></senml>
</sensml>
Which compresses with EXI to the following displayed in hexdump:
0000 a0 30 41 cd 95 b9 b5 b0 d4 b9 9d 95 b8 b9 e1 cd |.0A.............|
0010 91 00 f3 ab 93 71 d3 23 2b b1 d3 7b b9 d1 89 83 |.....q.#+..{....|
0020 29 91 81 b9 9b 09 81 89 81 c1 81 81 b1 99 7f 14 |)...............|
0030 25 d9 bd b1 d1 85 9d 94 80 d5 8a c4 26 01 0a 12 |%...........&...|
0040 c6 ea e4 e4 ca dc e8 40 68 24 19 00 90 |.......@h$...|
004d
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 makes 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 82 0e 6c ad cd ad 86 a5 cc ec ad c5 cf |..H..l..........|
0010 0e 6c 80 02 05 1d 75 72 6e 3a 64 65 76 3a 6f 77 |.l....urn:dev:ow|
0020 3a 31 30 65 32 30 37 33 61 30 31 30 38 30 30 36 |:10e2073a0108006|
0030 33 02 05 43 65 6c 01 00 e7 01 01 00 04 01 |3..Cel........|
003e
A small temperature sensor devices that only generates this one EXI
file does not really need an full EXI implementation. It can simple
hard code the output replacing the one wire device ID starting at
byte 0x16 and going to byte 0x31 with it's device ID, and replacing
the value "0xe7 0x01" at location 0x38 to 0x39 with the current
temperature. The EXI Specification [W3C.REC-exi-20110310] 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 bytes at is set to 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
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integer temperature in tenths of degrees right shifted 7 bits. In
this example 231 >> 7 = 0x01.
10. Usage Considerations
The measurements support sending both the current value of a sensor
as well as the an integrated sum. For many types of measurements,
the sum is more useful than the current value. 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 units set to watts, but it would put the sum of energy used in
the "s" attribute of the measurement. It might optionally include
the current power in the "v" attribute.
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.
Implementors are encouraged to report the cumulative sum as well as
the raw value of a given sensor.
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 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 always
positive, the sum should never get smaller; so if the sum does get
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smaller, the application will know that one of the situations listed
above has happened.
11. CDDL
For reference, the CBOR representation can be described with the CDDL
[I-D.greevenbosch-appsawg-cbor-cddl] specification in Figure 1.
SenML-Pack = [initial-record, * follow-on-record]
initial-record = initial-defined .and initial-generic
follow-on-record = follow-on-defined .and follow-on-generic
; first do a specification of the labels as defined:
initial-defined = {
? bn => tstr, ; Base Name
? bt => numeric, ; Base Time
? bu => tstr, ; Base Units
? bv => numeric, ; Base value
? bver => uint, ; Base Version
follow-on-defined-group,
+ base-key-value-pair
}
follow-on-defined-group = (
? n => tstr, ; Name
? u => tstr, ; Units
? ( v => numeric // ; Numeric Value
vs => tstr // ; String Value
vb => bool // ; Boolean Value
vd => bstr ) ; Data Value
? s => numeric, ; Value Sum
? t => numeric, ; Time
? ut => numeric, ; Update Time
* key-value-pair
)
follow-on-defined = { follow-on-defined-group }
; CBOR version (use the labels)
bver = -1 n = 0 s = 5
bn = -2 u = 1 t = 6
bt = -3 v = 2 ut = 7
bu = -4 vs = 3 vd = 8
bv = -5 vb = 4
; use the label *names* for JSON
; now define the generic versions
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initial-generic = {
follow-on-generic-group,
* base-key-value-pair,
}
follow-on-generic-group = (
+ key-value-pair,
)
follow-on-generic = { follow-on-generic-group }
key-value-pair = ( non-b-label => value )
base-key-value-pair = ( b-label => value )
non-b-label = tstr .regexp "[A-Zac-z0-9][-_:.A-Za-z0-9]*" / uint
b-label = tstr .regexp "b[-_:.A-Za-z0-9]+" / nint
value = tstr / bstr / numeric / bool
numeric = number / decfrac
Figure 1: CDDL specification for CBOR SenML
12. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with
the RFC number of this specification.
12.1. Units Registry
IANA will create a registry of SenML unit symbols. The primary
purpose of this registry is to make sure that symbols uniquely map to
give type of measurement. Definitions for many of these units can be
found in location such as [NIST811] and [BIPM].
+--------+--------------------------------------+-------+-----------+
| Symbol | Description | Type | Reference |
+--------+--------------------------------------+-------+-----------+
| m | meter | float | RFC-AAAA |
| g | gram | float | RFC-AAAA |
| s | second | float | RFC-AAAA |
| A | ampere | float | RFC-AAAA |
| K | kelvin | float | RFC-AAAA |
| cd | candela | float | RFC-AAAA |
| mol | mole | float | RFC-AAAA |
| Hz | hertz | float | RFC-AAAA |
| rad | radian | float | RFC-AAAA |
| sr | steradian | float | RFC-AAAA |
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| N | newton | float | RFC-AAAA |
| Pa | pascal | float | RFC-AAAA |
| J | joule | float | RFC-AAAA |
| W | watt | float | RFC-AAAA |
| C | coulomb | float | RFC-AAAA |
| V | volt | float | RFC-AAAA |
| F | farad | float | RFC-AAAA |
| Ohm | ohm | float | RFC-AAAA |
| S | siemens | float | RFC-AAAA |
| Wb | weber | float | RFC-AAAA |
| T | tesla | float | RFC-AAAA |
| H | henry | float | RFC-AAAA |
| Cel | degrees Celsius | float | RFC-AAAA |
| lm | lumen | float | RFC-AAAA |
| lx | lux | float | RFC-AAAA |
| Bq | becquerel | float | RFC-AAAA |
| Gy | gray | float | RFC-AAAA |
| Sv | sievert | float | RFC-AAAA |
| kat | katal | float | RFC-AAAA |
| pH | pH acidity | float | RFC-AAAA |
| % | Value of a switch (note 1) | float | RFC-AAAA |
| count | counter value | float | RFC-AAAA |
| %RH | Relative Humidity | float | RFC-AAAA |
| m2 | area | float | RFC-AAAA |
| l | volume in liters | float | RFC-AAAA |
| m/s | velocity | float | RFC-AAAA |
| m/s2 | acceleration | float | RFC-AAAA |
| l/s | flow rate in liters per second | float | RFC-AAAA |
| W/m2 | irradiance | float | RFC-AAAA |
| cd/m2 | luminance | float | RFC-AAAA |
| Bspl | bel sound pressure level | float | RFC-AAAA |
| bit/s | bits per second | float | RFC-AAAA |
| lat | degrees latitude (note 2) | float | RFC-AAAA |
| lon | degrees longitude (note 2) | float | RFC-AAAA |
| %EL | remaining battery energy level in | float | RFC-AAAA |
| | percents | | |
| EL | remaining battery energy level in | float | RFC-AAAA |
| | seconds | | |
| beat/m | Heart rate in beats per minute | float | RFC-AAAA |
| beats | Cumulative number of heart beats | float | RFC-AAAA |
+--------+--------------------------------------+-------+-----------+
Table 3
o Note 1: A value of 0.0 indicates the switch is off while 1.0
indicates on and 0.5 would be half on.
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o Note 2: Assumed to be in WGS84 unless another reference frame is
known for the sensor.
New entries can be added to the registration by either Expert Review
or IESG Approval as defined in [RFC5226]. 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. Units should define the semantic information and be chosen
carefully. Implementors 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 be
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 SI prefixes such as "k" before the unit is not allowed.
Instead one can represent the value using scientific notation
such a 1.2e3. TODO - Open Issue. Some people would like to
have SI prefixes to improve human readability.
5. For a given type of measurement, there will only be one unit
type defined. So for length, meters are defined and other
lengths such as mile, foot, light year are not allowed. For
most cases, the SI unit is preferred.
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 the are common SI
prefixes: Y, Z, E, P, T, G, M, k, h, da, d, c, n, u, p, f, a, z,
y, Ki, Mi, Gi, Ti, Pi, Ei, Zi, 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, ph, Ci, R, RAD, REM, gal, bbl, qt, degF, Cal,
BTU, HP, pH, B/s, psi, Torr, atm, at, bar, kWh.
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9. The unit names are case sensitive and the correct case needs to
be used, but 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 the / indicates that the units that
follow are the reciprocal. 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].
12.2. SenML label registry
IANA will create a registry for SenML labels. The initial content of
the registry are shown in TODO.
New entries can be added to the registration by either Expert Review
or IESG Approval as defined in [RFC5226]. Experts should exercise
their own good judgment but need to consider that shorter labels
should have more strict review.
12.3. Media Type Registration
The following registrations are done following the procedure
specified in [RFC6838] and [RFC7303].
12.3.1. senml+json Media Type Registration
Type name: application
Subtype name: senml+json and sensml+json
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using a subset of the
encoding allowed in [RFC7159]. See RFC-AAAA for details. This
simplifies implementation of 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: Sensor data can contain a wide range of
information ranging from information that is very public, such the
outside temperature in a given city, to very private information that
requires integrity and confidentiality protection, such as patient
health information. This format does not provide any security and
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instead relies on the transport protocol that carries it to provide
security. Given applications need to look at the overall context of
how this media type will be used to decide if the security is
adequate.
Interoperability considerations: Applications should ignore any JSON
key value pairs that they do not understand. This allows backwards
compatibility extensions to this specification. The "ver" 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-AAAA
Applications that use this media type: The type is used by systems
that report electrical power usage and environmental information such
as temperature and humidity. It can be used for a wide range of
sensor reporting systems.
Additional information:
Magic number(s): none
File extension(s): senml
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
12.3.2. senml+cbor Media Type Registration
Type name: application
Subtype name: senml+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: TBD
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Security considerations: TBD
Interoperability considerations: TBD
Published specification: RFC-AAAA
Applications that use this media type: The type is used by systems
that report electrical power usage and environmental information such
as temperature and humidity. It can be used for a wide range of
sensor reporting systems.
Additional information:
Magic number(s): none
File extension(s): senml
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+xml Media Type Registration
Type name: application
Subtype name: senml+xml and sensml+xml
Required parameters: none
Optional parameters: none
Encoding considerations: TBD
Security considerations: TBD
Interoperability considerations: TBD
Published specification: RFC-AAAA
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Applications that use this media type: TBD
Additional information:
Magic number(s): none
File extension(s): senml
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
12.3.4. senml-exi Media Type Registration
Type name: application
Subtype name: senml-exi
Required parameters: none
Optional parameters: none
Encoding considerations: TBD
Security considerations: TBD
Interoperability considerations: TBD
Published specification: RFC-AAAA
Applications that use this media type: TBD
Additional information:
Magic number(s): none
File extension(s): senml
Macintosh file type code(s): none
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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
12.4. XML Namespace Registration
This document registers the following XML namespaces 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 is requested to assign CoAP Content-Format IDs for the SenML
media types in the "CoAP Content-Formats" sub-registry, within the
"CoRE Parameters" registry [RFC7252]. All IDs are assigned from the
"Expert Review" (0-255) range. The assigned IDs are show in Table 4.
+-------------------------+-----+
| Media type | ID |
+-------------------------+-----+
| application/senml+json | TBD |
| application/sensml+json | TBD |
| application/senml+cbor | TBD |
| application/senml+xml | TBD |
| application/sensml+xml | TBD |
| application/senml-exi | TBD |
+-------------------------+-----+
Table 4: CoAP Content-Format IDs
13. Security Considerations
See Section 14. Further discussion of security properties can be
found in Section 12.3.
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14. Privacy Considerations
Sensor data can range from information with almost no security
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 transport protocol such as S/MIME or HTTP
with TLS that can provide integrity, confidentiality, and
authentication information about the source of the data.
15. Acknowledgement
We would like to thank Lisa Dusseault, Joe Hildebrand, Lyndsay
Campbell, Martin Thomson, John Klensin, Bjoern Hoehrmann, Carsten
Bormann, and Christian Amsuess for their review comments.
The CBOR Representation text and CDDL was contributed by Carsten
Bormann.
16. References
16.1. Normative References
[BIPM] Bureau International des Poids et Mesures, "The
International System of Units (SI)", 8th edition, 2006.
[IEEE.754.1985]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic", IEEE
Standard 754, August 1985.
[NIST811] Thompson, A. and B. Taylor, "Guide for the Use of the
International System of Units (SI)", NIST Special
Publication 811, 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<http://www.rfc-editor.org/info/rfc3688>.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, DOI
10.17487/RFC4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>.
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[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, DOI 10.17487/RFC6838, January 2013,
<http://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, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
DOI 10.17487/RFC7303, July 2014,
<http://www.rfc-editor.org/info/rfc7303>.
[W3C.REC-exi-20110310]
Schneider, J. and T. Kamiya, "Efficient XML Interchange
(EXI) Format 1.0", World Wide Web Consortium
Recommendation REC-exi-20110310, March 2011,
<http://www.w3.org/TR/2011/REC-exi-20110310>.
16.2. Informative References
[I-D.arkko-core-dev-urn]
Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
Names for Device Identifiers", draft-arkko-core-dev-urn-03
(work in progress), July 2012.
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[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C. and H. Birkholz, "CBOR data definition language
(CDDL): a notational convention to express CBOR data
structures", draft-greevenbosch-appsawg-cbor-cddl-08 (work
in progress), March 2016.
[I-D.ietf-core-links-json]
Li, K., Rahman, A., and C. Bormann, "Representing CoRE
Formats in JSON and CBOR", draft-ietf-core-links-json-04
(work in progress), November 2015.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
May 1997, <http://www.rfc-editor.org/info/rfc2141>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI
10.17487/RFC4122, July 2005,
<http://www.rfc-editor.org/info/rfc4122>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, DOI 10.17487/
RFC5952, August 2010,
<http://www.rfc-editor.org/info/rfc5952>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[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,
<http://www.rfc-editor.org/info/rfc7721>.
[UCUM] Schadow, G. and C. McDonald, "The Unified Code for Units
of Measure (UCUM)", Regenstrief Institute and Indiana
University School of Informatics, 2013,
<http://unitsofmeasure.org/ucum.html>.
Appendix A. Links extension
An extension to SenML to support links is expected to be registered
and defined by [I-D.ietf-core-links-json].
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The link extension can be an array of objects that can be used for
additional information. Each object in the Link array is constrained
to being a map of strings to strings with unique keys.
The following shows an example of the links extension.
[{"bn": "urn:dev:ow:10e2073a01080063/",
"bt": 1320078429,
"l": "[{\"href\":\"humidity\",\"foo\":\"bar1\"}\"
},
{ "n": "temperature", "v": 27.2, "u": "Cel" },
{ "n": "humidity", "v": 80, "u": "%RH" }
]
Authors' Addresses
Cullen Jennings
Cisco
400 3rd Avenue SW
Calgary, AB T2P 4H2
Canada
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Zach Shelby
ARM
150 Rose Orchard
San Jose 95134
USA
Phone: +1-408-203-9434
Email: zach.shelby@arm.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
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Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
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