Network Working Group C. Jennings
Internet-Draft Cisco
Intended status: Standards Track Z. Shelby
Expires: May 4, 2012 Sensinode
J. Arkko
Ericsson
Nov 1, 2011
Media Types for Sensor Markup Language (SENML)
draft-jennings-senml-07
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), eXtensible Markup Language (XML) and Efficient XML
Interchange (EXI), which share the common SenML data model. A simple
sensor, such as a temperature sensor, could use this media type in
protocols such as HTTP or CoAP to transport the measurements of the
sensor or to be configured.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 4, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
Jennings, et al. Expires May 4, 2012 [Page 1]
Internet-Draft Sensor Markup Nov 2011
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements and Design Goals . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Associating Meta-data . . . . . . . . . . . . . . . . . . . . 7
6. JSON Representation (application/senml+json) . . . . . . . . . 7
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 . . . . . . . . . . . . . . . 10
7. XML Representation (application/senml+xml) . . . . . . . . . . 11
8. EXI Representation (application/senml+exi) . . . . . . . . . . 12
9. Usage Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10.1. Units Registry . . . . . . . . . . . . . . . . . . . . . . 15
10.2. Media Type Registration . . . . . . . . . . . . . . . . . 17
10.2.1. senml+json Media Type Registration . . . . . . . . . . 17
10.2.2. senml+xml Media Type Registration . . . . . . . . . . 18
10.2.3. senml+exi Media Type Registration . . . . . . . . . . 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 20
13. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 20
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
14.1. Normative References . . . . . . . . . . . . . . . . . . . 20
14.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Jennings, et al. Expires May 4, 2012 [Page 2]
Internet-Draft Sensor Markup Nov 2011
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[I-D.ietf-core-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. There are many types of more complex measurements and
measurements that this media type would not be suitable for. A
decision was made not to carry most of the meta data about the sensor
in this media type to help reduce the size of the data and improve
efficiency in decoding. Instead meta-data about a sensor resource
can be described out-of-band using the CoRE Link Format
[I-D.ietf-core-link-format]. 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 (GET /sensor/temperature).
SenML is defined by a data model for measurements and simple meta-
data about measurements and devices. The data is structured as a
single object (with attributes) that contains an array of entries.
Each entry is an object that has attributes such as a unique
identifier for the sensor, the time the measurement was made, and the
current value. Serializations for this data model are defined for
JSON [RFC4627], XML and Efficient XML Interchange (EXI).
For example, the following shows a measurement from a temperature
gauge encoded in the JSON syntax.
{"e":[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.5, "u":"degC" }]}
In the example above, the array in the object has a single
measurement for a sensor named "urn:dev:ow:10e2073a01080063" with a
temperature 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 under 80 bytes makes this easy to
use on a wireless mesh network. It is always difficult to define
what small code is, but there is a desire to be able to implement
this in roughly 1 KB of flash on a 8 bit microprocessor. Experience
with Google power meter and large scale deployments has indicated
that the solution needs to support allowing multiple measurements to
Jennings, et al. Expires May 4, 2012 [Page 3]
Internet-Draft Sensor Markup Nov 2011
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.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. Semantics
Each representation caries a single SenML object that represents a
set of measurements and/or parameters. This object contains several
optional attributes described below and a mandatory array of one or
more entries.
Base Name
This is a string that is prepended to the names found in the
entries. This attribute is optional.
Base Time
A base time that is added to the time found in an entry. This
attribute is optional.
Base Units
A base unit that is assumed for all entries, unless otherwise
indicated. This attribute is optional. Acceptable values are
specified in Section 10.1.
Version
Version number of media type format. This attribute is optional
positive integer and defaults to 1 if not present.
Jennings, et al. Expires May 4, 2012 [Page 4]
Internet-Draft Sensor Markup Nov 2011
Measurement or Parameter Entries
Array of values for sensor measurements or other generic
parameters (such as configuration parameters). If present there
must be at least one entry in the array.
Each array entry contains several attributes, some of which are
optional and some of which are mandatory.
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.
Units
Units for a measurement value. Optional, if Base Unit is present
or if not required for a parameter. Acceptable values are
specified in Section 10.1.
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 ("bv" for
"Boolean Value") and Strings ("sv" for "String Value"). Exactly
one of these three fields MUST appear.
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
Update time. A 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
Jennings, et al. Expires May 4, 2012 [Page 5]
Internet-Draft Sensor Markup Nov 2011
communications path from the sensor. Optional.
The SenML format can be extended with further custom attributes
placed in the base object, or in an entry. Extensions in the base
object pertain to all entries, whereas extensions in an entry object
only pertain to that.
Systems reading one of the objects MUST check for the Version
attribute. If this value is a version number larger than the version
which the system understands, the system SHOULD NOT use this object.
This allows the version number to indicate that the object contains
mandatory to understand attributes. New version numbers can only be
defined in RFC which 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 a EUI-48 or EUI-64 identifier (A MAC address) or
some other bit string that is 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.
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. [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
Jennings, et al. Expires May 4, 2012 [Page 6]
Internet-Draft Sensor Markup Nov 2011
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 can be
very complex. Future specification may add new attributes to provide
better information about the statistical properties of the
measurement.
5. Associating Meta-data
SenML is designed to carry the minimum dynamic information about
measurements, and for efficiency reasons does not carry more 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 representations, this meta-data can be made
available using the CoRE Link Format [I-D.ietf-core-link-format].
The CoRE Link Format provides a simple way to describe Web Links, and
in particular allows a web server to describe resources it is
hosting. The list of links that a web server has available, can be
discovered by retrieving the /.well-known/core resource, which
returns the list of links in the CoRE Link Format. Each link may
contain attributes, for example title, resource type, interface
description and content-type.
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.
Further semantics about a resource can be included in the Resource
Type and Interface Description attributes. The Resource Type (rt=)
attribute is meant to give a semantic meaning to that resource. For
example rt="OutdoorTemperature" would indicate static semantic
meaning in addition to the unit information included in SenML. The
Interface Description (if=) attribute is used to describe the REST
interface of a resource, and may include e.g. a reference to a WADL
description [WADL].
6. JSON Representation (application/senml+json)
Root variables:
Jennings, et al. Expires May 4, 2012 [Page 7]
Internet-Draft Sensor Markup Nov 2011
+---------------------------+------+--------+
| SenML | JSON | Type |
+---------------------------+------+--------+
| Base Name | bn | String |
| Base Time | bt | Number |
| Base Units | bu | Number |
| Version | ver | Number |
| Measurement or Parameters | e | Array |
+---------------------------+------+--------+
Measurement or Parameter Entries:
+---------------+------+----------------+
| SenML | JSON | Notes |
+---------------+------+----------------+
| Name | n | String |
| Units | u | String |
| Value | v | Floating point |
| String Value | sv | String |
| Boolean Value | bv | Boolean |
| Value Sum | s | Floating point |
| Time | t | Number |
| Update Time | ut | Number |
+---------------+------+----------------+
All of the data is UTF-8, but since this is for machine to machine
communications on constrained systems, only characters with code
points between U+0001 and U+007F are allowed which corresponds to the
ASCII[RFC0020] subset of UTF-8.
The root contents MUST consist of exactly one JSON object as
specified by [RFC4627]. This object MAY contain a "bn" attribute
with a value of type string. This object MAY contain a "bt"
attribute with a value of type number. The object MAY contain a "bu"
attribute with a value of type string. The object MAY contain a
"ver" attribute with a value of type number. The object MAY contain
other attribute value pairs, and the object MUST contain exactly one
"e" attribute with a value of type array. The array MUST have one or
more measurement or parameter objects.
Inside each measurement or parameter object the "n", "u", and "sv"
attributes are of type string, the "t" and "ut" attributes are of
type number, the "bv" attribute is of type boolean, and the "v" and
"s" attributes are of type floating point. All the attributes are
optional, but as specified in Section 4, one of the "v", "sv", or
"bv" attributes MUST appear unless the "s" attribute is also present.
The "v", and "sv", and "bv" attributes MUST NOT appear together.
Jennings, et al. Expires May 4, 2012 [Page 8]
Internet-Draft Sensor Markup Nov 2011
Systems receiving measurements MUST be able to process the range of
floating point numbers that are representable as an IEEE double-
precision floating-point numbers [IEEE.754.1985]. The number of
significant digits in any measurement is not relevant, so a reading
of 1.1 has exactly the same semantic meaning as 1.10. If the value
has an exponent, the "e" MUST be in lower case. The mantissa SHOULD
be less than 19 characters long and the exponent SHOULD be less than
5 characters long. This allows time values to have better than micro
second precision over the next 100 years.
6.1. Examples
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:
{"m":[{ "n": "urn:dev:ow:10e2073a01080063", "v":23.5 }]}
6.1.2. Multiple Datapoints
The following example shows voltage and current now, i.e., at an
unspecified time. The device has an EUI-64 MAC address of
0024befffe804ff1.
{"e":[
{ "n": "voltage", "t": 0, "u": "V", "v": 120.1 },
{ "n": "current", "t": 0, "u": "A", "v": 1.2 }],
"bn": "urn:dev:mac:0024befffe804ff1/"
}
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.
{"e":[
{ "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 }],
"bn": "urn:dev:mac:0024befffe804ff1/",
"bt": 1276020076,
"ver": 1,
"bu": "A"
Jennings, et al. Expires May 4, 2012 [Page 9]
Internet-Draft Sensor Markup Nov 2011
}
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 provide 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.
{"e":[
{ "v": 20.0, "t": 0 },
{ "sv": "E 24' 30.621", "u": "lon", "t": 0 },
{ "sv": "N 60' 7.965", "u": "lat", "t": 0 },
{ "v": 20.3, "t": 60 },
{ "sv": "E 24' 30.622", "u": "lon", "t": 60 },
{ "sv": "N 60' 7.965", "u": "lat", "t": 60 },
{ "v": 20.7, "t": 120 },
{ "sv": "E 24' 30.623", "u": "lon", "t": 120 },
{ "sv": "N 60' 7.966", "u": "lat", "t": 120 },
{ "v": 98.0, "u": "%EL", "t": 150 },
{ "v": 21.2, "t": 180 },
{ "sv": "E 24' 30.628", "u": "lon", "t": 180 },
{ "sv": "N 60' 7.967", "u": "lat", "t": 180 }],
"bn": "http://[2001:db8::1]",
"bt": 1320067464,
"bu": "%RH"
}
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.
{"e":[
{ "n": "temperature", "v": 27.2, "u": "degC" },
{ "n": "humidity", "v": 80, "u": "%RH" }],
"bn": "http://[2001:db8::2]/",
"bt": 1320078429,
"ver": 1
}
Jennings, et al. Expires May 4, 2012 [Page 10]
Internet-Draft Sensor Markup Nov 2011
7. XML Representation (application/senml+xml)
A SenML object 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.
<?xml version="1.0" encoding="UTF-8"?>
<senml xmlns="urn:ietf:params:xml:ns:senml"
bn="urn:dev:mac:0024befffe804ff1/"
bt="1276020076"
ver="1" bu="A">
<e n="voltage" u="V" v="120.1" />
<e n="current" t="-5" v="1.2" />
<e n="current" t="-4" v="1.30" />
<e n="current" t="-3" v="0.14e1" />
<e n="current" t="-2" v="1.5" />
<e n="current" t="-1" v="1.6" />
<e n="current" t="0" v="1.7" />
</senml>
The RelaxNG schema for the XML is:
Jennings, et al. Expires May 4, 2012 [Page 11]
Internet-Draft Sensor Markup Nov 2011
default namespace = "urn:ietf:params:xml:ns:senml"
namespace rng = "http://relaxng.org/ns/structure/1.0"
e = element e {
attribute n { xsd:string }?,
attribute u { xsd:string }?,
attribute v { xsd:float }?,
attribute sv { xsd:string }?,
attribute bv { xsd:boolean }?,
attribute s { xsd:decimal }?,
attribute t { xsd:integer }?,
attribute ut { xsd:integer }?,
p*
}
senml =
element senml {
attribute bn { xsd:string }?,
attribute bt { xsd:integer }?,
attribute bu { xsd:string }?,
attribute ver { xsd:integer }?,
e*
}
start = senml
8. 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.
The following XSD Schema is generated from the RelaxNG and used for
strict schema guided EXI processing. (TODO: define a better schema
that include the common strings. I'd like to have something where
the stream ended up defined in a way that it was trivial to encode
sensor measurement into EXI in a small amount of C code. )
Jennings, et al. Expires May 4, 2012 [Page 12]
Internet-Draft Sensor Markup Nov 2011
<?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="e">
<xs:complexType>
<xs:attribute name="n" type="xs:string"/>
<xs:attribute name="u" type="xs:string"/>
<xs:attribute name="v" type="xs:float"/>
<xs:attribute name="sv" type="xs:string"/>
<xs:attribute name="bv" type="xs:boolean"/>
<xs:attribute name="s" type="xs:decimal"/>
<xs:attribute name="t" type="xs:integer"/>
<xs:attribute name="ut" type="xs:integer"/>
</xs:complexType>
</xs:element>
<xs:element name="senml">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="0" maxOccurs="unbounded" ref="ns1:e"/>
</xs:sequence>
<xs:attribute name="bn" type="xs:string"/>
<xs:attribute name="bt" type="xs:integer"/>
<xs:attribute name="bu" type="xs:string"/>
<xs:attribute name="ver" type="xs:integer"/>
</xs:complexType>
</xs:element>
</xs:schema>
The following shows a hexdump of the EXI produced from encoding the
following XML example. Note that while this example is similar to
the the first example in Section 6.1.2 in JSON format, it has been
simplified by omitting the URNs and units.
<?xml version="1.0" encoding="UTF-8"?>
<senml xmlns="urn:ietf:params:xml:ns:senml">
<e n="voltage" t="0" v="120.1" />
<e n="current" t="0" v="1.2" />
</senml>
Result:
0000000 90 60 84 bb 37 b6 3a 30 b3 b2 a0 02 58 84 c0 00
0000020 84 b1 ba b9 39 32 b7 3a 20 02 06 40 08
Jennings, et al. Expires May 4, 2012 [Page 13]
Internet-Draft Sensor Markup Nov 2011
9. 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
smaller, the application will know that one of the situations listed
above has happened.
Jennings, et al. Expires May 4, 2012 [Page 14]
Internet-Draft Sensor Markup Nov 2011
10. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA"
with the RFC number of this specification.
10.1. Units Registry
IANA will create a registry of 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
[NIST822] and [BIPM].
+--------+----------------------------------------------+-----------+
| Symbol | Description | Reference |
+--------+----------------------------------------------+-----------+
| m | meter | RFC-AAAA |
| kg | kilogram | RFC-AAAA |
| s | second | RFC-AAAA |
| A | ampere | RFC-AAAA |
| K | kelvin | RFC-AAAA |
| cd | candela | RFC-AAAA |
| mol | mole | RFC-AAAA |
| Hz | hertz | RFC-AAAA |
| rad | radian | RFC-AAAA |
| sr | steradian | RFC-AAAA |
| N | newton | RFC-AAAA |
| Pa | pascal | RFC-AAAA |
| J | joule | RFC-AAAA |
| W | watt | RFC-AAAA |
| C | coulomb | RFC-AAAA |
| V | volt | RFC-AAAA |
| F | farad | RFC-AAAA |
| Ohm | ohm | RFC-AAAA |
| S | siemens | RFC-AAAA |
| Wb | weber | RFC-AAAA |
| T | tesla | RFC-AAAA |
| H | henry | RFC-AAAA |
| degC | degrees Celsius | RFC-AAAA |
| lm | lumen | RFC-AAAA |
| lx | lux | RFC-AAAA |
| Bq | becquerel | RFC-AAAA |
| Gy | gray | RFC-AAAA |
| Sv | sievert | RFC-AAAA |
| kat | katal | RFC-AAAA |
| pH | pH acidity | RFC-AAAA |
| % | Value of a switch. A value of 0.0 indicates | RFC-AAAA |
| | the switch is off while 100.0 indicates on. | |
| count | counter value | RFC-AAAA |
Jennings, et al. Expires May 4, 2012 [Page 15]
Internet-Draft Sensor Markup Nov 2011
| %RH | Relative Humidity | RFC-AAAA |
| m2 | area | RFC-AAAA |
| l | volume in liters | RFC-AAAA |
| m/s | velocity | RFC-AAAA |
| m/s2 | acceleration | RFC-AAAA |
| l/s | flow rate in liters per second | RFC-AAAA |
| W/m2 | irradiance | RFC-AAAA |
| cd/m2 | luminance | RFC-AAAA |
| Bspl | bel sound pressure level | RFC-AAAA |
| bit/s | bits per second | RFC-AAAA |
| lat | degrees latitude. Assumed to be in WGS84 | RFC-AAAA |
| | unless another reference frame is known for | |
| | the sensor. | |
| lon | degrees longitude. Assumed to be in WGS84 | RFC-AAAA |
| | unless another reference frame is known for | |
| | the sensor. | |
| %EL | remaining battery energy level in percents | RFC-AAAA |
+--------+----------------------------------------------+-----------+
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.
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.
Jennings, et al. Expires May 4, 2012 [Page 16]
Internet-Draft Sensor Markup Nov 2011
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.
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.
10.2. Media Type Registration
The following registrations are done following the procedure
specified in [RFC4288] and [RFC3023].
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA"
with the RFC number of this specification.
10.2.1. 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 [RFC4627]. Specifically, only the ASCII[RFC0020]
subset of the UTF-8 characters are allowed. 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
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
Jennings, et al. Expires May 4, 2012 [Page 17]
Internet-Draft Sensor Markup Nov 2011
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 <c.jennings@ieee.org>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <c.jennings@ieee.org>
Change controller: IESG
10.2.2. senml+xml Media Type Registration
Type name: application
Subtype name: senml+xml
Required parameters: none
Optional parameters: none
Encoding considerations: TBD
Security considerations: TBD
Jennings, et al. Expires May 4, 2012 [Page 18]
Internet-Draft Sensor Markup Nov 2011
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
Person & email address to contact for further information: Cullen
Jennings <c.jennings@ieee.org>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <c.jennings@ieee.org>
Change controller: IESG
10.2.3. 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
Jennings, et al. Expires May 4, 2012 [Page 19]
Internet-Draft Sensor Markup Nov 2011
File extension(s): senml
Macintosh file type code(s): none
Person & email address to contact for further information: Cullen
Jennings <c.jennings@ieee.org>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <c.jennings@ieee.org>
Change controller: IESG
11. Security Considerations
See Section 12.Further discussion of security proprieties can be
found in Section 10.2.
12. 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.
13. Acknowledgement
We would like to thank Lisa Dusseault, Joe Hildebrand, Lyndsay
Campbell, Martin Thomson, John Klensin, Bjoern Hoehrmann, and Carsten
Bormann for their review comments.
14. References
14.1. Normative References
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Jennings, et al. Expires May 4, 2012 [Page 20]
Internet-Draft Sensor Markup Nov 2011
Types", RFC 3023, January 2001.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IEEE.754.1985]
Institute of Electrical and Electronics Engineers,
"Standard for Binary Floating-Point Arithmetic",
IEEE Standard 754, August 1985.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, May 2008.
14.2. Informative References
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[I-D.ietf-core-coap]
Shelby, Z. and B. Frank, "Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-07 (work in progress),
January 2011.
[I-D.ietf-core-link-format]
Shelby, Z., "Constrained Application Protocol (CoAP)",
draft-ietf-core-link-format-07 (work in progress),
January 2011.
[BIPM] Bureau International des Poids et Mesures, "The
International System of Units (SI)", 8th edition, 2006 .
[NIST822] Thompson, A. and B. Taylor, "Guide for the Use of the
International System of Units (SI)", NIST Special
Publication 811, 2008 Edition .
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
Jennings, et al. Expires May 4, 2012 [Page 21]
Internet-Draft Sensor Markup Nov 2011
[RFC0020] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[I-D.arkko-core-dev-urn]
Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
Names for Device Identifiers", draft-arkko-core-dev-urn-00
(work in progress), October 2011.
[WADL] Hadley, M., "Web Application Description Language (WADL)",
2009, <http://java.net/projects/wadl/sources/svn/content/
trunk/www/wadl20090202.pdf>.
Authors' Addresses
Cullen Jennings
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Zach Shelby
Sensinode
Kidekuja 2
Vuokatti 88600
FINLAND
Phone: +358407796297
Email: zach@sensinode.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
Jennings, et al. Expires May 4, 2012 [Page 22]