Media Types for Sensor Markup Language (SENML)

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 . 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 , CBOR , XML, and Efficient XML Interchange (EXI) . For example, the following shows a measurement from a temperature gauge encoded in the JSON syntax.

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.

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 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”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in .

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. This is a string that is prepended to the names found in the entries. This attribute is optional. A base time that is added to the time found in an entry. This attribute is optional. A base unit that is assumed for all entries, unless otherwise indicated. This attribute is optional. 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 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 for a measurement value. Optional. 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. Integrated sum of the values over time. Optional. This attribute is in the units specified in the Unit value multiplied by seconds. Time when value was recorded. Optional. 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 , 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 . 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 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 . UUIDs 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. 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.

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 . 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=”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 .

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 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 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 . 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 , 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 . 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:

The following example shows voltage and current now, i.e., at an unspecified time. The device has an EUI-64 MAC address of 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.

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:

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.

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.

The CBOR representation is equivalent to the JSON representation, with the following changes: For compactness, the CBOR representation uses integers for the map keys defined in . This table is conclusive, i.e., there is no intention to define any additional integer map keys; any extensions will use string map keys. For JSON Numbers, the CBOR representation can use integers, floating point numbers, or decimal fractions (CBOR Tag 4); the common limitations of JSON implementations are not relevant for these. For the version number, however, only an unsigned integer is allowed. Name JSON label CBOR label Version 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

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 .

]]> The RelaxNG schema for the XML is:

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.

]]> 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 in JSON format.

]]> Which compresses to the following displayed in hexdump:

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:

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 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.

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: Many sensors will allow the cumulative sums to “wrap” back to zero after the value gets sufficiently large. Some sensors will reset the cumulative sum back to zero when the device is reset, loses power, or is replaced with a different sensor. 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.

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 and . All the registry entries in 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 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 Note 1: A value of 0.0 indicates the switch is off while 100.0 indicates on. 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 . Experts should exercise their own good judgment but need to consider the following guidelines: There needs to be a real and compelling use for any new unit to be added. 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. 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. 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. 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. 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. 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. 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. 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. 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. A good list of common units can be found in the Unified Code for Units of Measure .

The following registrations are done following the procedure specified in and . 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 . Specifically, only the ASCII 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 <fluffy@iii.ca> Intended usage: COMMON Restrictions on usage: None Author: Cullen Jennings <fluffy@iii.ca> Change controller: IESG

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 <fluffy@iii.ca> Intended usage: COMMON Restrictions on usage: None Author: Cullen Jennings <fluffy@iii.ca> 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 <fluffy@iii.ca> Intended usage: COMMON Restrictions on usage: None Author: Cullen Jennings <fluffy@iii.ca> 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 <fluffy@iii.ca> Intended usage: COMMON Restrictions on usage: None Author: Cullen Jennings <fluffy@iii.ca> Change controller: IESG

This document registers the following XML namespaces in the IETF XML registry defined in . URI: urn:ietf:params:xml:ns:senml Registrant Contact: The IESG. XML: N/A, the requested URIs are XML namespaces

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 . All IDs are assigned from the “Expert Review” (0-255) range. The assigned IDs are show in . Media type ID application/senml+json TBD application/senml+cbor TBD application/senml+xml TBD application/senml-exi TBD

See . Further discussion of security properties can be found in .

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, Carsten Bormann, and Christian Amsuess for their review comments. The CBOR Representation text was contributed by Carsten Bormann.