Network Working Group C. Jennings
Internet-Draft Cisco
Intended status: Standards Track Z. Shelby
Expires: July 16, 2016 ARM
J. Arkko
A. Keranen
Ericsson
January 13, 2016
Media Types for Sensor Markup Language (SENML)
draft-jennings-core-senml-03
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 beq
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 July 16, 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
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publication of this document. Please review these documents
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements and Design Goals . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Associating Meta-data . . . . . . . . . . . . . . . . . . . . 6
6. JSON Representation (application/senml+json) . . . . . . . . 7
6.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1. Single Datapoint . . . . . . . . . . . . . . . . . . 8
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) . . . . . . . . 11
8. XML Representation (application/senml+xml) . . . . . . . . . 12
9. EXI Representation (application/senml-exi) . . . . . . . . . 13
10. Usage Considerations . . . . . . . . . . . . . . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11.1. Units Registry . . . . . . . . . . . . . . . . . . . . . 16
11.2. Media Type Registration . . . . . . . . . . . . . . . . 19
11.2.1. senml+json Media Type Registration . . . . . . . . . 19
11.2.2. senml+cbor Media Type Registration . . . . . . . . . 21
11.2.3. senml+xml Media Type Registration . . . . . . . . . 21
11.2.4. senml-exi Media Type Registration . . . . . . . . . 22
11.3. XML Namespace Registration . . . . . . . . . . . . . . . 23
11.4. CoAP Content-Format Registration . . . . . . . . . . . . 23
12. Security Considerations . . . . . . . . . . . . . . . . . . . 24
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24
14. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 24
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
15.1. Normative References . . . . . . . . . . . . . . . . . . 24
15.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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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. 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 [RFC6690]. 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").
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 base value object(s) and array(s) 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 [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":23.5, "u":"Cel" }]]
In the example above, the first element of the root array is empty
object since there are no base values. The second array inside the
root array 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 of payload under 80 bytes makes this
easy to use on a wireless mesh network. It is always difficult to
define what small code is, but there is a desire to be able to
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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.
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].
4. Semantics
Each SenML representation carries a single array that represents a
set of measurements and/or parameters. This array contains a base
object with 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.
Version: Version number of media type format. This attribute is
optional positive integer and defaults to 2 if not present.
The measurement or parameter entries array contains values for sensor
measurements or other generic parameters, such as configuration
parameters. There must be at least one entry in the array. This
array is called simply "measurement array" in the following text.
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
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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.
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: 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
communications path from the sensor. Optional.
The SenML format can be extended with further custom attributes
placed in a base object, or in an entry. Extensions in a base object
pertain to all entries following the base object, whereas extensions
in an entry object only pertain to that entry.
Systems reading one of the objects MUST check for the Version
attribute. If this value is a version number larger than the version
which the system understands, the system SHOULD NOT use this object.
This allows the version number to indicate that the object contains
mandatory to understand attributes. New version numbers can only be
defined in an RFC that updates this specification or it successors.
The Name value is concatenated to the Base Name value to get the name
of the sensor. The resulting name needs to uniquely identify and
differentiate the sensor from all others. If the object is a
representation resulting from the request of a URI [RFC3986], then in
the absence of the Base Name attribute, this URI is used as the
default value of Base Name. Thus in this case the Name field needs
to be unique for that URI, for example an index or subresource name
of sensors handled by the URI.
Alternatively, for objects not related to a URI, a unique name is
required. In any case, it is RECOMMENDED that the full names are
represented as URIs or URNs [RFC2141]. One way to create a unique
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name is to include a EUI-48 or EUI-64 identifier (a MAC address) or
some other 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.
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
value is used to indicate the number of seconds, excluding leap
seconds, since the start of the year 1970 in UTC.
A measurement array MAY be followed by another base object and
measurement array. The new base object can add, change, and/or
remove base values from the previous base object(s). The new base
values are applied to the following measurement arrays. Every base
object MUST be followed by a measurement array, and hence base
objects are found in the root array at even indexes and measurement
arrays at odd indexes.
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 [RFC6690].
The CoRE Link Format provides a simple way to describe Web Links, and
in particular allows a web server to describe resources it is
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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="outdoor-temperature" 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)
Base object variables:
+------------+------+--------+
| SenML | JSON | Type |
+------------+------+--------+
| Base Name | bn | String |
| Base Time | bt | Number |
| Base Units | bu | Number |
| Version | ver | Number |
+------------+------+--------+
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 |
+---------------+------+----------------+
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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 content consists of an array with even amount of JSON
objects where the first (and then every odd) element is a base object
and the second (and then every even) element is a measurements array.
The base object MAY contain a "bn" attribute with a value of type
string. The 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.
The base object MUST be followed by an array. The array MUST have
one or more measurement or parameter objects.
If the root array has more than one base object, each following base
object modifies the base values using the JSON merge patch format
[RFC7396]. That is, base values can be added or modified by defining
their new values and existing base values can removed by defining the
value as "null".
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.
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:
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[{}, [{ "n": "urn:dev:ow:10e2073a01080063", "v":23.5, "u":"Cel" }]]
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.
[{"bn": "urn:dev:mac:0024befffe804ff1/"},
[ { "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:mac:0024befffe804ff1/",
"bt": 1276020076,
"bu": "A",
"ver": 2},
[ { "n": "voltage", "u": "V", "v": 120.1 },
{ "n": "current", "t": -5, "v": 1.2 },
{ "n": "current", "t": -4, "v": 1.30 },
{ "n": "current", "t": -3, "v": 0.14e1 },
{ "n": "current", "t": -2, "v": 1.5 },
{ "n": "current", "t": -1, "v": 1.6 },
{ "n": "current", "t": 0, "v": 1.7 } ]
]
Note that in some usage scenarios of SenML the implementations MAY
store or transmit SenML in a stream-like fashion, where data is
collected over time and continuously added to the object. This mode
of operation is optional, but systems or protocols using SenML in
this fashion MUST specify that they are doing this. In this
situation the SenML stream can be sent and received in a partial
fashion, i.e., a measurement entry can be read as soon as it is
received and only not when the entire SenML object is complete.
For instance, the following stream of measurements may be sent from
the producer of a SenML object to the consumer of that SenML object,
and each measurement object may be reported at the time it arrives:
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[{"bn": "http://[2001:db8::1]",
"bt": 1320067464,
"bu": "%RH"},
[ { "v": 21.2, "t": 0 },
{ "v": 21.3, "t": 10 },
{ "v": 21.4, "t": 20 },
{ "v": 21.4, "t": 30 },
{ "v": 21.5, "t": 40 },
{ "v": 21.5, "t": 50 },
{ "v": 21.5, "t": 60 },
{ "v": 21.6, "t": 70 },
{ "v": 21.7, "t": 80 },
{ "v": 21.5, "t": 90 },
...
6.1.3. Multiple Measurements
The following example shows humidity measurements from a mobile
device with an IPv6 address 2001:db8::1, starting at Mon Oct 31
13:24:24 UTC 2011. The device also provides position data, which is
provided in the same measurement or parameter array as separate
entries. Note time is used to for correlating data that belongs
together, e.g., a measurement and a parameter associated with it.
Finally, the device also reports extra data about its battery status
at a separate time.
[{"bn": "http://[2001:db8::1]",
"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 } ]
]
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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.
[{"bn": "http://[2001:db8::2]/",
"bt": 1320078429,
"ver": 2},
[ { "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 1. 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.
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+---------------+------------+------------+
| Name | JSON label | CBOR label |
+---------------+------------+------------+
| Version | ver | -1 |
| Base Name | bn | -2 |
| Base Time | bt | -3 |
| Base Units | bu | -4 |
| Name | n | 0 |
| Units | u | 1 |
| Value | v | 2 |
| String Value | sv | 3 |
| Boolean Value | bv | 4 |
| Value Sum | s | 5 |
| Time | t | 6 |
| Update Time | ut | 7 |
+---------------+------------+------------+
Table 1: CBOR representation: integers for map keys
8. 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.
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"
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:int }?,
attribute ut { xsd:int }?
}
senml =
element senml {
attribute bn { xsd:string }?,
attribute bt { xsd:int }?,
attribute bu { xsd:string }?,
attribute ver { xsd:int }?,
e*
}
start = senml
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.
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.
The following XSD Schema is generated from the RelaxNG and used for
strict schema guided EXI processing.
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The following shows a hexdump of the EXI produced from encoding the
following XML example. Note that while this example is similar to
the first example in Section 6.1.2 in JSON format.
Which compresses to the following displayed in hexdump:
0000000 a0 30 49 cd 95 b9 b5 b0 b5 d8 c8 b5 9d 95 b8 b9
0000010 e1 cd 90 81 d7 57 26 e3 a6 46 57 63 a6 f7 73 a3
0000020 13 06 53 23 03 73 36 13 03 13 03 83 03 03 63 36
0000030 21 2e cd ed 8e 8c 2c ec a8 00 01 ab 2b 10 98 00
0000040 42 58 dd 5c 9c 99 5b 9d 10 00 03 41 41 90 08
000004f
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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:
The compressed form, using the byte alignment option of EXI, for the
above XML is the following:
00000000 a0 00 48 82 4e 6c ad cd ad 85 ae c6 45 ac ec ad |..H.Nl......E...|
00000010 c5 cf 0e 6c 80 01 00 1d 75 72 6e 3a 64 65 76 3a |...l....urn:dev:|
00000020 6f 77 3a 31 30 65 32 30 37 33 61 30 31 30 38 30 |ow:10e2073a01080|
00000030 30 36 33 03 01 06 74 65 6d 70 03 05 43 65 6c 01 |063...temp..Cel.|
00000040 00 e7 01 01 00 01 |......|
00000046
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 0x24 and going to byte 0x33 with it's device ID, and replacing
the value "0xe7 0x01" at location 0x41 to 0x42 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 location 0x41 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 at
location 0x42 is set to the 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
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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.
11. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with
the RFC number of this specification.
11.1. 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
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of measurement. Definitions for many of these units can be found in
[NIST811] and [BIPM].
All the registry entries in Table 2 use "v" (numeric) values. New
entries allocated in the registry must define what kind of values
they use.
+--------+--------------------------------------------+-----------+
| 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 |
| Cel | 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 (note 1) | RFC-AAAA |
| count | counter value | RFC-AAAA |
| %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 |
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| 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 (note 2) | RFC-AAAA |
| lon | degrees longitude (note 2) | RFC-AAAA |
| %EL | remaining battery energy level in percents | RFC-AAAA |
| EL | remaining battery energy level in seconds | RFC-AAAA |
| beat/m | Heart rate in beats per minute | RFC-AAAA |
| beats | Cumulative number of heart beats | RFC-AAAA |
+--------+--------------------------------------------+-----------+
Table 2
o Note 1: A value of 0.0 indicates the switch is off while 100.0
indicates on.
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.
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.
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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.
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].
11.2. Media Type Registration
The following registrations are done following the procedure
specified in [RFC6838] and [RFC7303].
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with
the RFC number of this specification.
11.2.1. 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 [RFC7159]. Specifically, only the ASCII
[RFC0020] subset of the UTF-8 characters are allowed. This
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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
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
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings
Change controller: IESG
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11.2.2. senml+cbor Media Type Registration
Type name: application
Subtype name: senml+cbor
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: 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
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings
Change controller: IESG
11.2.3. senml+xml Media Type Registration
Type name: application
Subtype name: senml+xml
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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
Person & email address to contact for further information: Cullen
Jennings
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings
Change controller: IESG
11.2.4. 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
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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
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings
Change controller: IESG
11.3. 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
11.4. 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 3.
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+------------------------+-----+
| Media type | ID |
+------------------------+-----+
| application/senml+json | TBD |
| application/senml+cbor | TBD |
| application/senml+xml | TBD |
| application/senml-exi | TBD |
+------------------------+-----+
Table 3: CoAP Content-Format IDs
12. Security Considerations
See Section 13. Further discussion of security properties can be
found in Section 11.2.
13. 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.
14. 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 was contributed by Carsten Bormann.
15. References
15.1. 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.
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[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,
.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
.
[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,
.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, DOI 10.17487/RFC6838, January 2013,
.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, .
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, .
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
DOI 10.17487/RFC7303, July 2014,
.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
.
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[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,
.
15.2. 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.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
May 1997, .
[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,
.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI
10.17487/RFC4122, July 2005,
.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, DOI 10.17487/
RFC5952, August 2010,
.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
.
[UCUM] Schadow, G. and C. McDonald, "The Unified Code for Units
of Measure (UCUM)", Regenstrief Institute and Indiana
University School of Informatics, 2013,
.
[WADL] Hadley, M., "Web Application Description Language (WADL)",
2009,
.
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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
Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
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