| Network Working Group | C. Jennings |
| Internet-Draft | Cisco |
| Intended status: Standards Track | Z. Shelby |
| Sensinode | |
| J. Arkko | |
| Ericsson | |
| Nov 1, 2011 |
Media Types for Sensor Markup Language (SENML)
draft-jennings-senml-07
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.
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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.
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 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 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].
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.
Each array entry contains several attributes, some of which are optional and some of which are mandatory.
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 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.
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].
Root variables:
| 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.
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.
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 }]}
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"
}
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"
}
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
}
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:
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
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. )
<?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>
<?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>
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.
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
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:
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.
Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with the RFC number of this specification.
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 the switch is off while 100.0 indicates on. | 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 |
| 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 unless another reference frame is known for the sensor. | RFC-AAAA |
| lon | degrees longitude. Assumed to be in WGS84 unless another reference frame is known for the sensor. | RFC-AAAA |
| %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:
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.
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 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
Type name: application
Subtype name: senml+xml
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 <c.jennings@ieee.org>
Intended usage: COMMON
Restrictions on usage: None
Author: Cullen Jennings <c.jennings@ieee.org>
Change controller: IESG
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
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
See Section 12.Further discussion of security proprieties can be found in Section 10.2.
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.
We would like to thank Lisa Dusseault, Joe Hildebrand, Lyndsay Campbell, Martin Thomson, John Klensin, Bjoern Hoehrmann, and Carsten Bormann for their review comments.
| [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 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. |