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Versions: (draft-jennings-core-senml) 00 01 02 03 04 05 06 07 08 09 10

Network Working Group                                        C. Jennings
Internet-Draft                                                     Cisco
Intended status: Standards Track                               Z. Shelby
Expires: November 5, 2017                                            ARM
                                                                J. Arkko
                                                              A. Keranen
                                                                Ericsson
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                             May 4, 2017


            Media Types for Sensor Measurement Lists (SenML)
                        draft-ietf-core-senml-07

Abstract

   This specification defines media types for representing simple sensor
   measurements and device parameters in the Sensor Measurement Lists
   (SenML).  Representations are defined in JavaScript Object Notation
   (JSON), Concise Binary Object Representation (CBOR), eXtensible
   Markup Language (XML), and Efficient XML Interchange (EXI), which
   share the common SenML data model.  A simple sensor, such as a
   temperature sensor, could use this media type in protocols such as
   HTTP or CoAP to transport the measurements of the sensor or to be
   configured.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   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 November 5, 2017.








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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements and Design Goals . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  SenML Structure and Semantics . . . . . . . . . . . . . . . .   5
     4.1.  Base attributes . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Regular attributes  . . . . . . . . . . . . . . . . . . .   6
     4.3.  Considerations  . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Resolved Records  . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Associating Meta-data . . . . . . . . . . . . . . . . . .   8
   5.  JSON Representation (application/senml+json)  . . . . . . . .   9
     5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.1.  Single Datapoint  . . . . . . . . . . . . . . . . . .  10
       5.1.2.  Multiple Datapoints . . . . . . . . . . . . . . . . .  10
       5.1.3.  Multiple Measurements . . . . . . . . . . . . . . . .  11
       5.1.4.  Resolved Data . . . . . . . . . . . . . . . . . . . .  12
       5.1.5.  Multiple Data Types . . . . . . . . . . . . . . . . .  13
       5.1.6.  Collection of Resources . . . . . . . . . . . . . . .  13
       5.1.7.  Setting an Actuator . . . . . . . . . . . . . . . . .  14
   6.  CBOR Representation (application/senml+cbor)  . . . . . . . .  15
   7.  XML Representation (application/senml+xml)  . . . . . . . . .  17
   8.  EXI Representation (application/senml+exi)  . . . . . . . . .  19
   9.  Fragment Identification Methods . . . . . . . . . . . . . . .  22
     9.1.  Fragment Identification Examples  . . . . . . . . . . . .  22
   10. Usage Considerations  . . . . . . . . . . . . . . . . . . . .  23
   11. CDDL  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Units Registry . . . . . . . . . . . . . . . . . . . . .  26
     12.2.  SenML Label Registry . . . . . . . . . . . . . . . . . .  29
     12.3.  Media Type Registration  . . . . . . . . . . . . . . . .  30
       12.3.1.  senml+json Media Type Registration . . . . . . . . .  30
       12.3.2.  sensml+json Media Type Registration  . . . . . . . .  32



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       12.3.3.  senml+cbor Media Type Registration . . . . . . . . .  33
       12.3.4.  sensml+cbor Media Type Registration  . . . . . . . .  34
       12.3.5.  senml+xml Media Type Registration  . . . . . . . . .  36
       12.3.6.  sensml+xml Media Type Registration . . . . . . . . .  37
       12.3.7.  senml+exi Media Type Registration  . . . . . . . . .  38
       12.3.8.  sensml+exi Media Type Registration . . . . . . . . .  40
     12.4.  XML Namespace Registration . . . . . . . . . . . . . . .  41
     12.5.  CoAP Content-Format Registration . . . . . . . . . . . .  41
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  42
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  42
   15. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  42
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     16.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Links Extension  . . . . . . . . . . . . . . . . . .  45
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

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.  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.  SenML can be used for a variety of data flow
   models, most notably data feeds pushed from a sensor to a collector,
   and the web resource model where the sensor is requested as a
   resource representation (e.g., "GET /sensor/temperature").

   There are many types of more complex measurements and measurements
   that this media type would not be suitable for.  SenML strikes a
   balance between having some information about the sensor carried with
   the sensor data so that the data is self describing but it also tries
   to make that a fairly minimal set of auxiliary information for
   efficiency reason.  Other information about the sensor can be
   discovered by other methods such as using the CoRE Link Format
   [RFC6690].

   SenML is defined by a data model for measurements and simple meta-
   data about measurements and devices.  The data is structured as a
   single array that contains a series of SenML Records which can each
   contain attributes such as an unique identifier for the sensor, the
   time the measurement was made, the unit the measurement is in, and
   the current value of the sensor.  Serializations for this data model
   are defined for JSON [RFC7159], CBOR [RFC7049], XML, and Efficient
   XML Interchange (EXI) [W3C.REC-exi-20140211].



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   For example, the following shows a measurement from a temperature
   gauge encoded in the JSON syntax.

   [
     {"n":"urn:dev:ow:10e2073a01080063","u":"Cel","v":23.1}
   ]

   In the example above, the array has a single SenML Record with a
   measurement for a sensor named "urn:dev:ow:10e2073a01080063" with a
   current value of 23.1 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
   implement this in roughly 1 KB of flash on a 8 bit microprocessor.
   Experience with Google power meter and large scale deployments has
   indicated that the solution needs to support allowing multiple
   measurements to be batched into a single HTTP or CoAP request.  This
   "batch" upload capability allows the server side to efficiently
   support a large number of devices.  It also conveniently supports
   batch transfers from proxies and storage devices, even in situations
   where the sensor itself sends just a single data item at a time.  The
   multiple measurements could be from multiple related sensors or from
   the same sensor but at different times.

   The basic design is an array with a series of measurements.  The
   following example shows two measurements made at different times.
   The value of a measurement is in the "v" tag, the time of a
   measurement is in the "t" tag, the "n" tag has a unique sensor name,
   and the unit of the measurement is carried in the "u" tag.

   [
     {"n":"urn:dev:ow:10e2073a01080063","u":"Cel","t":1.276020076e+09,
      "v":23.5},
     {"n":"urn:dev:ow:10e2073a01080063","u":"Cel","t":1.276020091e+09,
      "v":23.6}
   ]

   To keep the messages small, it does not make sense to repeat the "n"
   tag in each SenML Record so there is a concept of a Base Name which
   is simply a string that is prepended to the Name field of all
   elements in that record and any records that follow it.  So a more
   compact form of the example above is the following.




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   [
     {"bn":"urn:dev:ow:10e2073a01080063","u":"Cel","t":1.276020076e+09,
      "v":23.5},
     {"u":"Cel","t":1.276020091e+09,
      "v":23.6}
   ]

   In the above example the Base Name is in the "bn" tag and the "n"
   tags in each Record are the empty string so they are omitted.

   Some devices have accurate time while others do not so SenML supports
   absolute and relative times.  Time is represented in floating point
   as seconds and values greater than zero represent an absolute time
   relative to the Unix epoch while values of 0 or less represent a
   relative time in the past from the current time.  A simple sensor
   with no absolute wall clock time might take a measurement every
   second, batch up 60 of them, and then send the batch to a server.  It
   would include the relative time each measurement was made compared to
   the time the batch was sent in each SenML Record.  The server might
   have accurate NTP time and use the time it received the data, and the
   relative offset, to replace the times in the SenML with absolute
   times before saving the SenML Pack in a document database.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   This document also uses the following terms:

   SenML Record:  One measurement or configuration instance in time
      presented using the SenML data model.

   SenML Pack:  One or more SenML Records in an array structure.

4.  SenML Structure and Semantics

   Each SenML Pack carries a single array that represents a set of
   measurements and/or parameters.  This array contains a series of
   SenML Records with several attributes described below.  There are two
   kind of attributes: base and regular.  The base attributes can only
   be included in the first SenML Record and they apply to the entries
   in all Records.  All base attributes are optional.  Regular
   attributes can be included in any SenML Record and apply only to that
   Record.




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4.1.  Base attributes

   Base Name:  This is a string that is prepended to the names found in
      the entries.

   Base Time:  A base time that is added to the time found in an entry.

   Base Unit:  A base unit that is assumed for all entries, unless
      otherwise indicated.  If a record does not contain a Unit value,
      then the Base Unit is used.  Otherwise the value found in the Unit
      (if any) is used.

   Base Value:  A base value is added to the value found in an entry,
      similar to Base Time.

   Base Sum:  A base sum is added to the sum found in an entry, similar
      to Base Time.

   Version:  Version number of media type format.  This attribute is an
      optional positive integer and defaults to 5 if not present.  [RFC
      Editor: change the default value to 10 when this specification is
      published as an RFC and remove this note]

4.2.  Regular attributes

   Name:  Name of the sensor or parameter.  When appended to the Base
      Name attribute, this must result in a globally unique identifier
      for the resource.  The name is optional, if the Base Name is
      present.  If the name is missing, Base Name must uniquely identify
      the resource.  This can be used to represent a large array of
      measurements from the same sensor without having to repeat its
      identifier on every measurement.

   Unit:  Units for a measurement value.  Optional.  If the Record has
      no Unit, the Base Unit is used as the Unit.  Having no Unit and no
      Base Unit is allowed.

   Value  Value of the entry.  Optional if a Sum value is present,
      otherwise required.  Values are represented using basic data
      types.  This specification defines floating point numbers ("v"
      field for "Value"), booleans ("vb" for "Boolean Value"), strings
      ("vs" for "String Value") and binary data ("vd" for "Data Value").
      Exactly one value field MUST appear unless there is Sum field in
      which case it is allowed to have no Value field.

   Sum:  Integrated sum of the values over time.  Optional.  This
      attribute is in the units specified in the Unit value multiplied
      by seconds.



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   Time:  Time when value was recorded.  Optional.

   Update Time:  An optional time in seconds that represents the maximum
      time before this sensor will provide an updated reading for a
      measurement.  This can be used to detect the failure of sensors or
      communications path from the sensor.

4.3.  Considerations

   The SenML format can be extended with further custom attributes.
   Both new base and regular attributes are allowed.  See Section 12.2
   for details.  Implementations MUST ignore attributes they don't
   recognize unless that attribute has a label name that ends with the
   '_' character in which case an error MUST be generated.

   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
   name is to include some bit string that has guaranteed uniqueness
   (such as a 1-wire address) that is assigned to the device.  Some of
   the examples in this draft use the device URN type as specified in
   [I-D.arkko-core-dev-urn].  UUIDs [RFC4122] are another way to
   generate a unique name.  Note that long-term stable unique
   identifiers are problematic for privacy reasons [RFC7721] and should
   be used with care or avoided.

   The resulting concatenated name MUST consist only of characters out
   of the set "A" to "Z", "a" to "z", "0" to "9", "-", ":", ".", or "_"
   and it MUST start with a character out of the set "A" to "Z", "a" to
   "z", or "0" to "9".  This restricted character set was chosen so that
   these names can be directly used as in other types of URI including
   segments of an HTTP path with no special encoding and can be directly



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   used in many databases and analytic systems.  [RFC5952] contains
   advice on encoding an IPv6 address in a name.

   If either the Base Time or Time value is missing, the missing
   attribute is considered to have a value of zero.  The Base Time and
   Time values are added together to get the time of measurement.  A
   time of zero indicates that the sensor does not know the absolute
   time and the measurement was made roughly "now".  A negative value is
   used to indicate seconds in the past from roughly "now".  A positive
   value is used to indicate the number of seconds, excluding leap
   seconds, since the start of the year 1970 in UTC.

   If only one of the Base Sum or Sum value is present, the missing
   attribute is considered to have a value of zero.  The Base Sum and
   Sum values are added together to get the sum of measurement.  If
   neither the Base Sum or Sum are present, then the measurement does
   not have a sum value.

   Representing the statistical characteristics of measurements, such as
   accuracy, can be very complex.  Future specification may add new
   attributes to provide better information about the statistical
   properties of the measurement.

4.4.  Resolved Records

   Sometimes it is useful to be able to refer to a defined normalized
   format for SenML records.  This normalized format tends to get used
   for big data applications and intermediate forms when converting to
   other formats.

   A SenML Record is referred to as "resolved" if it does not contain
   any base values and has no relative times, but the base values of the
   SenML Pack (if any) are applied to the Record.  That is, name and
   base name are concatenated, base time is added to the time of the
   Record, if the Record did not contain Unit the Base Unit is applied
   to the record, etc.  In addition the records need to be in
   chronological order.  An example of this is show in Section 5.1.4.

   Future specification that defines new base attributes need to specify
   how the attribute is resolved.

4.5.  Associating Meta-data

   SenML is designed to carry the minimum dynamic information about
   measurements, and for efficiency reasons does not carry significant
   static meta-data about the device, object or sensors.  Instead, it is
   assumed that this meta-data is carried out of band.  For web
   resources using SenML Packs, this meta-data can be made available



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   using the CoRE Link Format [RFC6690].  The most obvious use of this
   link format is to describe that a resource is available in a SenML
   format in the first place.  The relevant media type indicator is
   included in the Content-Type (ct=) attribute.

5.  JSON Representation (application/senml+json)

   The SenML labels (JSON object member names) shown in Table 1 are used
   in JSON SenML Record attributes.

                    +---------------+-------+---------+
                    |          Name | label | Type    |
                    +---------------+-------+---------+
                    |     Base Name | bn    | String  |
                    |     Base Time | bt    | Number  |
                    |     Base Unit | bu    | String  |
                    |    Base Value | bv    | Number  |
                    |      Base Sum | bs    | Number  |
                    |       Version | bver  | Number  |
                    |          Name | n     | String  |
                    |          Unit | u     | String  |
                    |         Value | v     | Number  |
                    |  String Value | vs    | String  |
                    | Boolean Value | vb    | Boolean |
                    |    Data Value | vd    | String  |
                    |     Value Sum | s     | Number  |
                    |          Time | t     | Number  |
                    |   Update Time | ut    | Number  |
                    |          Link | l     | String  |
                    +---------------+-------+---------+

                        Table 1: JSON SenML Labels

   The root content consists of an array with one JSON object for each
   SenML Record.  All the fields in the above table MAY occur in the
   records with the type specified in the table.

   Only the UTF-8 form of JSON is allowed.  Characters in the String
   Value are encoded using the escape sequences defined in [RFC7159].
   Octets in the Data Value are base64 encoded with URL safe alphabet as
   defined in Section 5 of [RFC4648].

   Systems receiving measurements MUST be able to process the range of
   floating point numbers that are representable as an IEEE double
   precision floating point numbers [IEEE.754.1985].  The number of
   significant digits in any measurement is not relevant, so a reading
   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



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   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.

5.1.  Examples

5.1.1.  Single Datapoint

   The following shows a temperature reading taken approximately "now"
   by a 1-wire sensor device that was assigned the unique 1-wire address
   of 10e2073a01080063:

   [
     {"n":"urn:dev:ow:10e2073a01080063","u":"Cel","v":23.1}
   ]

5.1.2.  Multiple Datapoints

   The following example shows voltage and current now, i.e., at an
   unspecified time.

[
  {"bn":"urn:dev:ow:10e2073a01080063:","n":"voltage","u":"V","v":120.1},
  {"n":"current","u":"A","v":1.2}
]

   The next example is similar to the above one, but shows current at
   Tue Jun 8 18:01:16.001 UTC 2010 and at each second for the previous 5
   seconds.

   [
     {"bn":"urn:dev:ow:10e2073a0108006:","bt":1.276020076001e+09,
      "bu":"A","bver":5,
      "n":"voltage","u":"V","v":120.1},
     {"n":"current","t":-5,"v":1.2},
     {"n":"current","t":-4,"v":1.3},
     {"n":"current","t":-3,"v":1.4},
     {"n":"current","t":-2,"v":1.5},
     {"n":"current","t":-1,"v":1.6},
     {"n":"current","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.  SenML defines a
   separate media type to indicate Sensor Streaming Measurement Lists



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   (SensML) for this usage (see Section 12.3.1).  In this situation the
   SensML stream can be sent and received in a partial fashion, i.e., a
   measurement entry can be read as soon as the SenML Record is received
   and not have to wait for the full SensML Stream to be complete.

   For instance, the following stream of measurements may be sent via a
   long lived HTTP POST from the producer of a SensML to the consumer of
   that, and each measurement object may be reported at the time it was
   measured:

   [
     {"bn":"urn:dev:ow:10e2073a01080063","bt":1.320067464e+09,
      "bu":"%RH","v":21.2},
     {"t":10,"v":21.3},
     {"t":20,"v":21.4},
     {"t":30,"v":21.4},
     {"t":40,"v":21.5},
     {"t":50,"v":21.5},
     {"t":60,"v":21.5},
     {"t":70,"v":21.6},
     {"t":80,"v":21.7},
   ...

5.1.3.  Multiple Measurements

   The following example shows humidity measurements from a mobile
   device with a 1-wire address 10e2073a01080063, starting at Mon Oct 31
   13:24:24 UTC 2011.  The device also provides position data, which is
   provided in the same measurement or parameter array as separate
   entries.  Note time is used to for correlating data that belongs
   together, e.g., a measurement and a parameter associated with it.
   Finally, the device also reports extra data about its battery status
   at a separate time.


















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   [
     {"bn":"urn:dev:ow:10e2073a01080063","bt":1.320067464e+09,
      "bu":"%RH","v":20},
     {"u":"lon","v":24.30621},
     {"u":"lat","v":60.07965},
     {"t":60,"v":20.3},
     {"u":"lon","t":60,"v":24.30622},
     {"u":"lat","t":60,"v":60.07965},
     {"t":120,"v":20.7},
     {"u":"lon","t":120,"v":24.30623},
     {"u":"lat","t":120,"v":60.07966},
     {"u":"%EL","t":150,"v":98},
     {"t":180,"v":21.2},
     {"u":"lon","t":180,"v":24.30628},
     {"u":"lat","t":180,"v":60.07967}
   ]

   The size of this example represented in various forms, as well as
   that form compressed with gzip is given in the following table.

                   +----------+------+-----------------+
                   | Encoding | Size | Compressed Size |
                   +----------+------+-----------------+
                   | JSON     |  573 |             206 |
                   | XML      |  649 |             235 |
                   | CBOR     |  254 |             196 |
                   | EXI      |  162 |             185 |
                   +----------+------+-----------------+

                         Table 2: Size Comparisons

   Note the EXI sizes are not using the schema guidance so the EXI
   representation could be a bit smaller.

5.1.4.  Resolved Data

   The following shows the example from the previous section show in
   resolved format.













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   [
     {"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067464e+09,
      "v":20},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067464e+09,
      "v":24.30621},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067464e+09,
      "v":60.07965},
     {"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067524e+09,
      "v":20.3},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067524e+09,
      "v":24.30622},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067524e+09,
      "v":60.07965},
     {"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067584e+09,
      "v":20.7},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067584e+09,
      "v":24.30623},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067584e+09,
      "v":60.07966},
     {"n":"urn:dev:ow:10e2073a01080063","u":"%EL","t":1.320067614e+09,
      "v":98},
     {"n":"urn:dev:ow:10e2073a01080063","u":"%RH","t":1.320067644e+09,
      "v":21.2},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lon","t":1.320067644e+09,
      "v":24.30628},
     {"n":"urn:dev:ow:10e2073a01080063","u":"lat","t":1.320067644e+09,
      "v":60.07967}
   ]

5.1.5.  Multiple Data Types

   The following example shows a sensor that returns different data
   types.

  [
    {"bn":"urn:dev:ow:10e2073a01080063:","n":"temp","u":"Cel","v":23.1},
    {"n":"label","vs":"Machine Room"},
    {"n":"open","vb":false},
    {"n":"nfv-reader","vd":"aGkgCg=="}
  ]

5.1.6.  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,




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   and has gotten two separate values as a result, a temperature and
   humidity measurement.

   [
     {"bn":"2001:db8::2","bt":1.320078429e+09,
      "n":"temperature","u":"Cel","v":27.2},
     {"n":"humidity","u":"%RH","v":80}
   ]

5.1.7.  Setting an Actuator

   The following example show the SenML that could be used to set the
   current set point of a typical residential thermostat which has a
   temperature set point, a switch to turn on and off the heat, and a
   switch to turn on the fan override.

   [
     {"bn":"urn:dev:ow:10e2073a01080063:"},
     {"n":"temp","u":"Cel","v":23.1},
     {"n":"heat","u":"/","v":1},
     {"n":"fan","u":"/","v":0}
   ]

   In the following example two different lights are turned on.  It is
   assumed that the lights are on a 802.1BA network that can guarantee
   delivery of the messages to the two lights within 15 ms and uses
   802.1AS for time synchronization.  The controller has set the time of
   the lights coming on to 20 ms in the future from the current time.
   This allows both lights to receive the message, wait till that time,
   then apply the switch command so that both lights come on at the same
   time.

   [
     {"bt":1.320078429e+09,"bu":"/","n":"2001:db8::3","v":1},
     {"n":"2001:db8::4","v":1}
   ]

   The following shows two lights being turned off using a non
   deterministic network that has a high odds of delivering a message in
   less than 100 ms and uses NTP for time synchronization.  The current
   time is 1320078429.  The user has just turned off a light switch
   which is turning off two lights.  Both lights are dimmed to 50%
   brightness immediately to give the user instant feedback that
   something is changing.  However given the network, the lights will
   probably dim at somewhat different times.  Then 100 ms in the future,
   both lights will go off at the same time.  The instant but not
   synchronized dimming gives the user the sensation of quick responses




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   and the timed off 100 ms in the future gives the perception of both
   lights going off at the same time.

   [
     {"bt":1.320078429e+09,"bu":"/","n":"2001:db8::3","v":0.5},
     {"n":"2001:db8::4","v":0.5},
     {"n":"2001:db8::3","t":0.1,"v":0},
     {"n":"2001:db8::4","t":0.1,"v":0}
   ]

6.  CBOR Representation (application/senml+cbor)

   The CBOR [RFC7049] representation is equivalent to the JSON
   representation, with the following changes:

   o  For JSON Numbers, the CBOR representation can use integers,
      floating point numbers, or decimal fractions (CBOR Tag 4); however
      a representation SHOULD be chosen such that when the CBOR value is
      converted back to an IEEE double precision floating point value,
      it has exactly the same value as the original Number.  For the
      version number, only an unsigned integer is allowed.

   o  Characters in the String Value are encoded using a definite length
      text string (type 3).  Octets in the Data Value are encoded using
      a definite length byte string (type 2) .

   o  For compactness, the CBOR representation uses integers for the map
      keys defined in Table 3.  This table is conclusive, i.e., there is
      no intention to define any additional integer map keys; any
      extensions will use string map keys.  This allows translators
      converting between CBOR and JSON representations to convert also
      all future labels without needing to update implementations.



















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                  +---------------+-------+------------+
                  |          Name | Label | CBOR Label |
                  +---------------+-------+------------+
                  |       Version | bver  |         -1 |
                  |     Base Name | bn    |         -2 |
                  |     Base Time | bt    |         -3 |
                  |    Base Units | bu    |         -4 |
                  |    Base Value | bv    |         -5 |
                  |      Base Sum | bs    |         -6 |
                  |          Name | n     |          0 |
                  |         Units | u     |          1 |
                  |         Value | v     |          2 |
                  |  String Value | vs    |          3 |
                  | Boolean Value | vb    |          4 |
                  |     Value Sum | s     |          5 |
                  |          Time | t     |          6 |
                  |   Update Time | ut    |          7 |
                  |    Data Value | vd    |          8 |
                  |          Link | l     |          9 |
                  +---------------+-------+------------+

            Table 3: CBOR representation: integers for map keys

   o  For streaming SensML in CBOR representation, the array containing
      the records SHOULD be an CBOR indefinite length array while for
      non streaming SenML, a definite length array MUST be used.

   The following example shows a dump of the CBOR example for the same
   sensor measurement as in Section 5.1.2.

 0000 87 a7 21 78 1b 75 72 6e 3a 64 65 76 3a 6f 77 3a |..!x.urn:dev:ow:|
 0010 31 30 65 32 30 37 33 61 30 31 30 38 30 30 36 3a |10e2073a0108006:|
 0020 22 fb 41 d3 03 a1 5b 00 10 62 23 61 41 20 05 00 |".A...[..b#aA ..|
 0030 67 76 6f 6c 74 61 67 65 01 61 56 02 fb 40 5e 06 |gvoltage.aV..@^.|
 0040 66 66 66 66 66 a3 00 67 63 75 72 72 65 6e 74 06 |fffff..gcurrent.|
 0050 24 02 fb 3f f3 33 33 33 33 33 33 a3 00 67 63 75 |$..?.333333..gcu|
 0060 72 72 65 6e 74 06 23 02 fb 3f f4 cc cc cc cc cc |rrent.#..?......|
 0070 cd a3 00 67 63 75 72 72 65 6e 74 06 22 02 fb 3f |...gcurrent."..?|
 0080 f6 66 66 66 66 66 66 a3 00 67 63 75 72 72 65 6e |.ffffff..gcurren|
 0090 74 06 21 02 f9 3e 00 a3 00 67 63 75 72 72 65 6e |t.!..>...gcurren|
 00a0 74 06 20 02 fb 3f f9 99 99 99 99 99 9a a3 00 67 |t. ..?.........g|
 00b0 63 75 72 72 65 6e 74 06 00 02 fb 3f fb 33 33 33 |current....?.333|
 00c0 33 33 33                                        |333|
 00c3







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7.  XML Representation (application/senml+xml)

   A SenML Pack or Stream can also be represented in XML format as
   defined in this section.

   Only the UTF-8 form of XML is allowed.  Characters in the String
   Value are encoded using the escape sequences defined in [RFC7159].
   Octets in the Data Value are base64 encoded with URL safe alphabet as
   defined in Section 5 of [RFC4648].

   The following example shows an XML example for the same sensor
   measurement as in Section 5.1.2.

   <sensml xmlns="urn:ietf:params:xml:ns:senml">
     <senml bn="urn:dev:ow:10e2073a0108006:" bt="1.276020076001e+09"
     bu="A" bver="5" n="voltage" u="V" v="120.1"></senml>
     <senml n="current" t="-5" v="1.2"></senml>
     <senml n="current" t="-4" v="1.3"></senml>
     <senml n="current" t="-3" v="1.4"></senml>
     <senml n="current" t="-2" v="1.5"></senml>
     <senml n="current" t="-1" v="1.6"></senml>
     <senml n="current" v="1.7"></senml>
   </sensml>

   The SenML Stream is represented as a sensml tag that contains a
   series of senml tags for each SenML Record.  The SenML Fields are
   represents as XML attributes.  The following table shows the mapping
   of the SenML labels, which are used for the attribute name, to the
   attribute types used in the XML senml tags.






















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                    +---------------+-------+---------+
                    |          Name | Label | Type    |
                    +---------------+-------+---------+
                    |     Base Name | bn    | string  |
                    |     Base Time | bt    | double  |
                    |     Base Unit | bu    | string  |
                    |    Base Value | bv    | double  |
                    |      Base Sum | bs    | double  |
                    |  Base Version | bver  | int     |
                    |          Name | n     | string  |
                    |          Unit | u     | string  |
                    |         Value | v     | double  |
                    |  String Value | vs    | string  |
                    |    Data Value | vd    | string  |
                    | Boolean Value | vb    | boolean |
                    |     Value Sum | s     | double  |
                    |          Time | t     | double  |
                    |   Update Time | ut    | double  |
                    |          Link | l     | string  |
                    +---------------+-------+---------+

                         Table 4: XML SenML Labels

   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"


   senml = element senml {
     attribute bn { xsd:string }?,
     attribute bt { xsd:double }?,
     attribute bv { xsd:double }?,
     attribute bs { xsd:double }?,
     attribute bu { xsd:string }?,
     attribute bver { xsd:int }?,

     attribute l { xsd:string }?,

     attribute n { xsd:string }?,
     attribute s { xsd:double }?,
     attribute t { xsd:double }?,
     attribute u { xsd:string }?,
     attribute ut { xsd:double }?,

     attribute v { xsd:double }?,
     attribute vb { xsd:boolean }?,
     attribute vs { xsd:string }?,
     attribute vd { xsd:string }?
   }

   sensml =
     element sensml {
       senml+
   }

   start = sensml

8.  EXI Representation (application/senml+exi)

   For efficient transmission of SenML over e.g. a constrained network,
   Efficient XML Interchange (EXI) can be used.  This encodes the XML
   Schema structure of SenML into binary tags and values rather than
   ASCII text.  An EXI representation of SenML SHOULD be made using the
   strict schema-mode of EXI.  This mode however does not allow tag
   extensions to the schema, and therefore any extensions will be lost
   in the encoding.  For uses where extensions need to be preserved in
   EXI, the non-strict schema mode of EXI MAY be used.

   The EXI header MUST include an "EXI Options", as defined in
   [W3C.REC-exi-20140211], with an schemaId set to the value of "a"
   indicating the schema provided in this specification.  Future
   revisions to the schema can change the value of the schemaId to allow



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   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 is the XSD Schema to be used for strict schema guided
   EXI processing.  It is generated from the RelaxNG.

   <?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="senml">
       <xs:complexType>
         <xs:attribute name="bn" type="xs:string" />
         <xs:attribute name="bt" type="xs:double" />
         <xs:attribute name="bv" type="xs:double" />
         <xs:attribute name="bs" type="xs:double" />
         <xs:attribute name="bu" type="xs:string" />
         <xs:attribute name="bver" type="xs:int" />
         <xs:attribute name="l" type="xs:string" />
         <xs:attribute name="n" type="xs:string" />
         <xs:attribute name="s" type="xs:double" />
         <xs:attribute name="t" type="xs:double" />
         <xs:attribute name="u" type="xs:string" />
         <xs:attribute name="ut" type="xs:double" />
         <xs:attribute name="v" type="xs:double" />
         <xs:attribute name="vb" type="xs:boolean" />
         <xs:attribute name="vs" type="xs:string" />
         <xs:attribute name="vd" type="xs:string" />
       </xs:complexType>
     </xs:element>
     <xs:element name="sensml">
       <xs:complexType>
         <xs:sequence>
           <xs:element maxOccurs="unbounded" ref="ns1:senml" />
         </xs:sequence>
       </xs:complexType>
     </xs:element>
   </xs:schema>

   The following shows a hexdump of the EXI produced from encoding the
   following XML example.  Note this example is the same information as
   the first example in Section 5.1.2 in JSON format.






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   <sensml xmlns="urn:ietf:params:xml:ns:senml">
     <senml bn="urn:dev:ow:10e2073a01080063:" n="voltage" u="V"
     v="120.1"></senml>
     <senml n="current" u="A" v="1.2"></senml>
   </sensml>

   Which compresses with EXI to the following displayed in hexdump:

 0000 a0 30 0d 84 80 79 d5 c9 b8 e9 91 95 d8 e9 bd dc |.0...y..........|
 0010 e8 c4 c1 94 c8 c0 dc cd 84 c0 c4 c0 e0 c0 c0 d8 |................|
 0020 cc e9 82 5d 9b db 1d 18 59 d9 48 0d 58 ac 42 60 |...]....Y.H.X.B`|
 0030 18 e1 2c 6e ae 4e 4c ad ce 84 06 82 41 90 0e    |..,n.NL.....A..|
 003f

   The above example used the bit packed form of EXI but it is also
   possible to use a byte packed form of EXI which can makes it easier
   for a simple sensor to produce valid EXI without really implementing
   EXI.  Consider the example of a temperature sensor that produces a
   value in tenths of degrees Celsius over a range of 0.0 to 55.0.  It
   would produce an XML SenML file such as:

   <sensml xmlns="urn:ietf:params:xml:ns:senml">
     <senml n="urn:dev:ow:10e2073a01080063" u="Cel" v="23.1"></senml>
   </sensml>

   The compressed form, using the byte alignment option of EXI, for the
   above XML is the following:

 0000 a0 00 48 80 6c 20 01 07 1d 75 72 6e 3a 64 65 76 |..H.l ...urn:dev|
 0010 3a 6f 77 3a 31 30 65 32 30 37 33 61 30 31 30 38 |:ow:10e2073a0108|
 0020 30 30 36 33 02 05 43 65 6c 01 00 e7 01 01 00 03 |0063..Cel.......|
 0030 01                                              |.|
 0031

   A small temperature sensor devices that only generates this one EXI
   file does not really need an full EXI implementation.  It can simply
   hard code the output replacing the 1-wire device ID starting at byte
   0x20 and going to byte 0x2F with it's device ID, and replacing the
   value "0xe7 0x01" at location 0x37 and 0x38 with the current
   temperature.  The EXI Specification [W3C.REC-exi-20140211] contains
   the full information on how floating point numbers are represented,
   but for the purpose of this sensor, the temperature can be converted
   to an integer in tenths of degrees (231 in this example).  EXI stores
   7 bits of the integer in each byte with the top bit set to one if
   there are further bytes.  So the first bytes at is set to low 7 bits
   of the integer temperature in tenths of degrees plus 0x80.  In this
   example 231 & 0x7F + 0x80 = 0xE7.  The second byte is set to the




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   integer temperature in tenths of degrees right shifted 7 bits.  In
   this example 231 >> 7 = 0x01.

9.  Fragment Identification Methods

   A SenML Pack typically consists of multiple SenML Records and for
   some applications it may be useful to be able to refer with a
   Fragment Identifier to a single record, or a set of records, in a
   Pack.  The fragment identifier is only interpreted by a client and
   does not impact retrieval of a representation.  The SenML Fragment
   Identification is modeled after CSV Fragment Identifiers [RFC7111].

   To select a single SenML Record, the "rec" scheme followed by a
   single number is used.  For the purpose of numbering records, the
   first record is at position 1.  A range of records can be selected by
   giving the first and the last record number separated by a '-'
   character.  Instead of the second number, the '*' character can be
   used to indicate the last Senml Record in the Pack.  A set of records
   can also be selected using a comma separated list of record positions
   or ranges.

   (We use the term "selecting a record" for identifying it as part of
   the fragment, not in the sense of isolating it from the Pack -- the
   record still needs to be interpreted as part of the Pack, e.g., using
   the base values defined in record 1.)

9.1.  Fragment Identification Examples

   The 3rd SenML Record from "coap://example.com/temp" resource can be
   selected with:

   coap://example.com/temp#rec=3

   Records from 3rd to 6th can be selected with:

   coap://example.com/temp#rec=3-6

   Records from 19th to the last can be selected with:

   coap://example.com/temp#rec=19-*

   The 3rd and 5th record can be selected with:

   coap://example.com/temp#rec=3,5

   To select the Records from third to fifth, the 10th record, and all
   from 19th to the last:




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   coap://example.com/temp#rec=3-5,10,19-*

10.  Usage Considerations

   The measurements support sending both the current value of a sensor
   as well as the an integrated sum.  For many types of measurements,
   the sum is more useful than the current value.  For example, an
   electrical meter that measures the energy a given computer uses will
   typically want to measure the cumulative amount of energy used.  This
   is less prone to error than reporting the power each second and
   trying to have something on the server side sum together all the
   power measurements.  If the network between the sensor and the meter
   goes down over some period of time, when it comes back up, the
   cumulative sum helps reflect what happened while the network was
   down.  A meter like this would typically report a measurement with
   the units set to watts, but it would put the sum of energy used in
   the "s" attribute of the measurement.  It might optionally include
   the current power in the "v" attribute.

   While the benefit of using the integrated sum is fairly clear for
   measurements like power and energy, it is less obvious for something
   like temperature.  Reporting the sum of the temperature makes it easy
   to compute averages even when the individual temperature values are
   not reported frequently enough to compute accurate averages.
   Implementors are encouraged to report the cumulative sum as well as
   the raw value of a given sensor.

   Applications that use the cumulative sum values need to understand
   they are very loosely defined by this specification, and depending on
   the particular sensor implementation may behave in unexpected ways.
   Applications should be able to deal with the following issues:

   1.  Many sensors will allow the cumulative sums to "wrap" back to
       zero after the value gets sufficiently large.

   2.  Some sensors will reset the cumulative sum back to zero when the
       device is reset, loses power, or is replaced with a different
       sensor.

   3.  Applications cannot make assumptions about when the device
       started accumulating values into the sum.

   Typically applications can make some assumptions about specific
   sensors that will allow them to deal with these problems.  A common
   assumption is that for sensors whose measurement values are always
   positive, the sum should never get smaller; so if the sum does get
   smaller, the application will know that one of the situations listed
   above has happened.



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11.  CDDL

   For reference, the JSON and CBOR representations can be described
   with the common CDDL [I-D.greevenbosch-appsawg-cbor-cddl]
   specification in Figure 1.

   SenML-Pack = [initial-record, * follow-on-record]

   initial-record = initial-defined .and initial-generic
   follow-on-record = follow-on-defined .and follow-on-generic

   ; first do a specification of the labels as defined:

   initial-defined = {
     ? bn => tstr,        ; Base Name
     ? bt => numeric,     ; Base Time
     ? bu => tstr,        ; Base Units
     ? bv => numeric,     ; Base value
     ? bs => numeric,     ; Base sum
     ? bver => uint,      ; Base Version
     follow-on-defined-group,
     + base-key-value-pair
   }

   follow-on-defined-group = (
      ? n => tstr,        ; Name
      ? u => tstr,        ; Units
      ? s => numeric,     ; Value Sum
      ? t => numeric,     ; Time
      ? ut => numeric,    ; Update Time
      ? l => tstr,        ; Link
      * key-value-pair,
      ? ( v => numeric // ; Numeric Value
          vs => tstr //   ; String Value
          vb => bool //   ; Boolean Value
          vd => binary-value ) ; Data Value
   )
   follow-on-defined = { follow-on-defined-group }

   ; now define the generic versions

   initial-generic = {
     follow-on-generic-group,
     * base-key-value-pair,
   }

   follow-on-generic-group = (
     + key-value-pair,



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   )
   follow-on-generic = { follow-on-generic-group }

   key-value-pair = ( non-b-label => value )

   base-key-value-pair = ( b-label => value )

   non-b-label = tstr .regexp  "[A-Zac-z0-9][-_:.A-Za-z0-9]*" / uint
   b-label = tstr .regexp  "b[-_:.A-Za-z0-9]+" / nint

   value = tstr / binary-value / numeric / bool
   numeric = number / decfrac

        Figure 1: Common CDDL specification for CBOR and JSON SenML

   For JSON, we use text labels and base64url-encoded binary data
   (Figure 2).

   bver = "bver" n  = "n"   s  = "s"
   bn  = "bn"    u  = "u"   t  = "t"
   bt  = "bt"    v  = "v"   ut = "ut"
   bu  = "bu"    vs = "vs"  vd = "vd"
   bv  = "bv"    vb = "vb"  l  = "l"
   bs  = "bs"

   binary-value = tstr             ; base64url encoded

           Figure 2: JSON-specific CDDL specification for SenML

   For CBOR, we use integer labels and native binary data (Figure 3).

   bver = -1  n  = 0   s  = 5
   bn  = -2   u  = 1   t  = 6
   bt  = -3   v  = 2   ut = 7
   bu  = -4   vs = 3   vd = 8
   bv  = -5   vb = 4   l  = 9
   bs  = -6

   binary-value = bstr

           Figure 3: CBOR-specific CDDL specification for SenML

12.  IANA Considerations

   Note to RFC Editor: Please replace all occurrences of "RFC-AAAA" with
   the RFC number of this specification.





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12.1.  Units Registry

   IANA will create a registry of SenML unit symbols.  The primary
   purpose of this registry is to make sure that symbols uniquely map to
   give type of measurement.  Definitions for many of these units can be
   found in location such as [NIST811] and [BIPM].  Units marked with an
   asterisk are NOT RECOMMENDED to be produced by new implementations,
   but are in active use and SHOULD be implemented by consumers that can
   use the related base units.

   +----------+------------------------------------+-------+-----------+
   |   Symbol | Description                        | Type  | Reference |
   +----------+------------------------------------+-------+-----------+
   |        m | meter                              | float | RFC-AAAA  |
   |       kg | kilogram                           | float | RFC-AAAA  |
   |        g | gram*                              | float | RFC-AAAA  |
   |        s | second                             | float | RFC-AAAA  |
   |        A | ampere                             | float | RFC-AAAA  |
   |        K | kelvin                             | float | RFC-AAAA  |
   |       cd | candela                            | float | RFC-AAAA  |
   |      mol | mole                               | float | RFC-AAAA  |
   |       Hz | hertz                              | float | RFC-AAAA  |
   |      rad | radian                             | float | RFC-AAAA  |
   |       sr | steradian                          | float | RFC-AAAA  |
   |        N | newton                             | float | RFC-AAAA  |
   |       Pa | pascal                             | float | RFC-AAAA  |
   |        J | joule                              | float | RFC-AAAA  |
   |        W | watt                               | float | RFC-AAAA  |
   |        C | coulomb                            | float | RFC-AAAA  |
   |        V | volt                               | float | RFC-AAAA  |
   |        F | farad                              | float | RFC-AAAA  |
   |      Ohm | ohm                                | float | RFC-AAAA  |
   |        S | siemens                            | float | RFC-AAAA  |
   |       Wb | weber                              | float | RFC-AAAA  |
   |        T | tesla                              | float | RFC-AAAA  |
   |        H | henry                              | float | RFC-AAAA  |
   |      Cel | degrees Celsius                    | float | RFC-AAAA  |
   |       lm | lumen                              | float | RFC-AAAA  |
   |       lx | lux                                | float | RFC-AAAA  |
   |       Bq | becquerel                          | float | RFC-AAAA  |
   |       Gy | gray                               | float | RFC-AAAA  |
   |       Sv | sievert                            | float | RFC-AAAA  |
   |      kat | katal                              | float | RFC-AAAA  |
   |       m2 | square meter (area)                | float | RFC-AAAA  |
   |       m3 | cubic meter (volume)               | float | RFC-AAAA  |
   |        l | liter (volume)*                    | float | RFC-AAAA  |
   |      m/s | meter per second (velocity)        | float | RFC-AAAA  |
   |     m/s2 | meter per square second            | float | RFC-AAAA  |



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   |          | (acceleration)                     |       |           |
   |     m3/s | cubic meter per second (flow rate) | float | RFC-AAAA  |
   |      l/s | liter per second (flow rate)*      | float | RFC-AAAA  |
   |     W/m2 | watt per square meter (irradiance) | float | RFC-AAAA  |
   |    cd/m2 | candela per square meter           | float | RFC-AAAA  |
   |          | (luminance)                        |       |           |
   |      bit | bit (information content)          | float | RFC-AAAA  |
   |    bit/s | bit per second (data rate)         | float | RFC-AAAA  |
   |      lat | degrees latitude (note 2)          | float | RFC-AAAA  |
   |      lon | degrees longitude (note 2)         | float | RFC-AAAA  |
   |       pH | pH value (acidity; logarithmic     | float | RFC-AAAA  |
   |          | quantity)                          |       |           |
   |       dB | decibel (logarithmic quantity)     | float | RFC-AAAA  |
   |     Bspl | bel (sound pressure level;         | float | RFC-AAAA  |
   |          | logarithmic quantity)*             |       |           |
   |    count | 1 (counter value)                  | float | RFC-AAAA  |
   |        / | 1 (Ratio e.g., value of a switch,  | float | RFC-AAAA  |
   |          | note 1)                            |       |           |
   |        % | 1 (Ratio e.g., value of a switch,  | float | RFC-AAAA  |
   |          | note 1)*                           |       |           |
   |      %RH | Percentage (Relative Humidity)     | float | RFC-AAAA  |
   |      %EL | Percentage (remaining battery      | float | RFC-AAAA  |
   |          | energy level)                      |       |           |
   |       EL | seconds (remaining battery energy  | float | RFC-AAAA  |
   |          | level)                             |       |           |
   |      1/s | 1 per second (event rate)          | float | RFC-AAAA  |
   |    1/min | 1 per minute (event rate, "rpm")*  | float | RFC-AAAA  |
   | beat/min | 1 per minute (Heart rate in beats  | float | RFC-AAAA  |
   |          | per minute)*                       |       |           |
   |    beats | 1 (Cumulative number of heart      | float | RFC-AAAA  |
   |          | beats)*                            |       |           |
   |      S/m | Siemens per meter (conductivity)   | float | RFC-AAAA  |
   +----------+------------------------------------+-------+-----------+

                                  Table 5

   o  Note 1: A value of 0.0 indicates the switch is off while 1.0
      indicates on and 0.5 would be half on.  The preferred name of this
      unit is "/".  For historical reasons, the name "%" is also
      provided for the same unit - but note that while that name
      strongly suggests a percentage (0..100) -- it is however NOT a
      percentage, but the absolute ratio!

   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



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   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
        recommended.  Instead one can represent the value using
        scientific notation such a 1.2e3.  The "kg" unit is exception to
        this rule since it is an SI base unit; the "g" unit is provided
        for legacy compatibility.

   5.   For a given type of measurement, there will only be one unit
        type defined.  So for length, meters are defined and other
        lengths such as mile, foot, light year are not allowed.  For
        most cases, the SI unit is preferred.

   6.   Symbol names that could be easily confused with existing common
        units or units combined with prefixes should be avoided.  For
        example, selecting a unit name of "mph" to indicate something
        that had nothing to do with velocity would be a bad choice, as
        "mph" is commonly used to mean miles per hour.

   7.   The following should not be used because the are common SI
        prefixes: Y, Z, E, P, T, G, M, k, h, da, d, c, n, u, p, f, a, z,
        y, Ki, Mi, Gi, Ti, Pi, Ei, Zi, Yi.

   8.   The following units should not be used as they are commonly used
        to represent other measurements Ky, Gal, dyn, etg, P, St, Mx, G,
        Oe, Gb, sb, Lmb, mph, 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.




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   10.  A number after a unit typically indicates the previous unit
        raised to that power, and the / indicates that the units that
        follow are the reciprocal.  A unit should have only one / in the
        name.

   11.  A good list of common units can be found in the Unified Code for
        Units of Measure [UCUM].

12.2.  SenML Label Registry

   IANA will create a new registry for SenML labels.  The initial
   content of the registry is:

        +---------------+-------+------+----------+----+---------+
        |          Name | Label | CBOR | XML Type | ID | Note    |
        +---------------+-------+------+----------+----+---------+
        |     Base Name | bn    | -2   | string   | a  | RFCXXXX |
        |      Base Sum | bs    | -6   | double   | a  | RFCXXXX |
        |     Base Time | bt    | -3   | double   | a  | RFCXXXX |
        |     Base Unit | bu    | -4   | string   | a  | RFCXXXX |
        |    Base Value | bv    | -5   | double   | a  | RFCXXXX |
        |  Base Version | bver  | -1   | int      | a  | RFCXXXX |
        | Boolean Value | vb    | 4    | boolean  | a  | RFCXXXX |
        |    Data Value | vd    | 8    | string   | a  | RFCXXXX |
        |          Name | n     | 0    | string   | a  | RFCXXXX |
        |  String Value | vs    | 3    | string   | a  | RFCXXXX |
        |          Time | t     | 6    | double   | a  | RFCXXXX |
        |          Unit | u     | 1    | string   | a  | RFCXXXX |
        |   Update Time | ut    | 7    | double   | a  | RFCXXXX |
        |         Value | v     | 2    | double   | a  | RFCXXXX |
        |     Value Sum | s     | 5    | double   | a  | RFCXXXX |
        |          Link | l     | 9    | string   | a  | RFCXXXX |
        +---------------+-------+------+----------+----+---------+

                           Table 6: SenML Labels

   Note to RFC Editor.  Please replace RFCXXXX with the number for this
   RFC.

   All new entries must define the Label Name, Label, and XML Type but
   the CBOR labels SHOULD be left empty as CBOR will use the string
   encoding for any new labels.  The ID fields contains the EXI schemaId
   value of the first Schema which includes this label or is empty if
   this label was not intended for use with EXI.  The Note field SHOULD
   contain information about where to find out more information about
   this label.





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   The JSON, CBOR, and EXI types are derived from the XML type.  All XML
   numeric types such as double, float, integer and int become a JSON
   Number.  XML boolean and string become a JSON Boolean and String
   respectively.  CBOR represents numeric values with a CBOR type that
   does not loose any information from the JSON value.  EXI uses the XML
   types.

   New entries can be added to the registration by either Expert Review
   or IESG Approval as defined in [RFC5226].  Experts should exercise
   their own good judgment but need to consider that shorter labels
   should have more strict review.

   All new SenML labels that have "base" semantics (see Section 4.1)
   MUST start with character 'b'.  Regular labels MUST NOT start with
   that character.

   Extensions that add a label that is intended for use with XML need to
   create a new RelaxNG scheme that includes all the labels in the IANA
   registry.

   Extensions that add a label that is intended for use with EXI need to
   create a new XSD Schema that includes all the labels in the IANA
   registry then allocate a new EXI schemaId value.  Moving to the next
   letter in the alphabet is the suggested way to create the new value
   for the EXI schemaId.  Any labels with previously blank ID values
   SHOULD be updated in the IANA table to have their ID set to this new
   schemaId value.

   Extensions that are mandatory to understand to correctly process the
   Pack MUST have a label name that ends with the '_' character.

12.3.  Media Type Registration

   The following registrations are done following the procedure
   specified in [RFC6838] and [RFC7303].  Clipboard formats are defined
   for the JSON and XML form of lists but do not make sense for streams
   or other formats.

   Note to RFC Editor - please remove this paragraph.  Note that a
   request for media type review for senml+json was sent to the media-
   types@iana.org on Sept 21, 2010.  A second request for all the types
   was sent on October 31, 2016.

12.3.1.  senml+json Media Type Registration

   Type name: application

   Subtype name: senml+json



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   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using a subset of the
   encoding allowed in [RFC7159].  See RFC-AAAA for details.  This
   simplifies implementation of very simple system and does not impose
   any significant limitations as all this data is meant for machine to
   machine communications and is not meant to be human readable.

   Security considerations: Sensor data can contain a wide range of
   information ranging from information that is very public, such the
   outside temperature in a given city, to very private information that
   requires integrity and confidentiality protection, such as patient
   health information.  This format does not provide any security and
   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 "bver" 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 e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+json is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): senml

   Windows Clipboard Name: "JSON Sensor Measurement List"

   Macintosh file type code(s): none





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   Macintosh Universal Type Identifier code: org.ietf.senml-json
   conforms to public.text

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.2.  sensml+json Media Type Registration

   Type name: application

   Subtype name: sensml+json

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using a subset of the
   encoding allowed in [RFC7159].  See RFC-AAAA for details.  This
   simplifies implementation of very simple system and does not impose
   any significant limitations as all this data is meant for machine to
   machine communications and is not meant to be human readable.

   Security considerations: Sensor data can contain a wide range of
   information ranging from information that is very public, such the
   outside temperature in a given city, to very private information that
   requires integrity and confidentiality protection, such as patient
   health information.  This format does not provide any security and
   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 "bver" 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




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   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+json is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): sensml

   Macintosh file type code(s): none

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.3.  senml+cbor Media Type Registration

   Type name: application

   Subtype name: senml+cbor

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using [RFC7049].  See
   RFC-AAAA for details.

   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



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   how this media type will be used to decide if the security is
   adequate.

   Interoperability considerations: Applications should ignore any key
   value pairs that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" field can
   be used to ensure the receiver supports a minimal level of
   functionality needed by the creator of the CBOR object.

   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+cbor is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): senmlc

   Macintosh file type code(s): none

   Macintosh Universal Type Identifier code: org.ietf.senml-cbor
   conforms to public.data

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.4.  sensml+cbor Media Type Registration

   Type name: application

   Subtype name: sensml+cbor




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   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using [RFC7049].  See
   RFC-AAAA for details.

   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 key
   value pairs that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" field can
   be used to ensure the receiver supports a minimal level of
   functionality needed by the creator of the CBOR object.

   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+cbor is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): sensmlc

   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



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   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.5.  senml+xml Media Type Registration

   Type name: application

   Subtype name: senml+xml

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using
   [W3C.REC-xml-20081126].  See RFC-AAAA for details.

   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 tags
   or attributes that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" attribute
   in the senml tag can be used to ensure the receiver supports a
   minimal level of functionality needed by the creator of the XML.

   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+xml is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none




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   File extension(s): senmlx

   Windows Clipboard Name: "XML Sensor Measurement List"

   Macintosh file type code(s): none

   Macintosh Universal Type Identifier code: org.ietf.senml-xml conforms
   to public.xml

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.6.  sensml+xml Media Type Registration

   Type name: application

   Subtype name: sensml+xml

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using
   [W3C.REC-xml-20081126].  See RFC-AAAA for details.

   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 tags
   or attributes that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" attribute
   in the senml tag can be used to ensure the receiver supports a
   minimal level of functionality needed by the creator of the XML.



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   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+xml is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): sensmlx

   Macintosh file type code(s): none

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.3.7.  senml+exi Media Type Registration

   Type name: application

   Subtype name: senml+exi

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using
   [W3C.REC-exi-20140211].  See RFC-AAAA for details.

   Security considerations: Sensor data can contain a wide range of
   information ranging from information that is very public, such the
   outside temperature in a given city, to very private information that
   requires integrity and confidentiality protection, such as patient
   health information.  This format does not provide any security and



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   instead relies on the transport protocol that carries it to provide
   security.  Given applications need to look at the overall context of
   how this media type will be used to decide if the security is
   adequate.

   Interoperability considerations: Applications should ignore any tags
   or attributes that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" attribute
   in the senml tag can be used to ensure the receiver supports a
   minimal level of functionality needed by the creator of the XML.
   Further information on using schemas to guide the EXI can be found in
   RFC-AAAA.

   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+exi is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): senmle

   Macintosh file type code(s): none

   Macintosh Universal Type Identifier code: org.ietf.senml-exi conforms
   to public.data

   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






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12.3.8.  sensml+exi Media Type Registration

   Type name: application

   Subtype name: sensml+exi

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using
   [W3C.REC-exi-20140211].  See RFC-AAAA for details.

   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 tags
   or attributes that they do not understand.  This allows backwards
   compatibility extensions to this specification.  The "bver" attribute
   in the senml tag can be used to ensure the receiver supports a
   minimal level of functionality needed by the creator of the XML.
   Further information on using schemas to guide the EXI can be found in
   RFC-AAAA.

   Published specification: RFC-AAAA

   Applications that use this media type: The type is used by systems
   that report e.g., electrical power usage and environmental
   information such as temperature and humidity.  It can be used for a
   wide range of sensor reporting systems.

   Fragment identifier considerations: Fragment identification for
   application/senml+exi is supported by using fragment identifiers as
   specified by RFC-AAAA.

   Additional information:

   Magic number(s): none

   File extension(s): sensmle




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   Macintosh file type code(s): none

   Person & email address to contact for further information: Cullen
   Jennings <fluffy@iii.ca>

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

12.4.  XML Namespace Registration

   This document registers the following XML namespaces in the IETF XML
   registry defined in [RFC3688].

   URI: urn:ietf:params:xml:ns:senml

   Registrant Contact: The IESG.

   XML: N/A, the requested URIs are XML namespaces

12.5.  CoAP Content-Format Registration

   IANA is requested to assign CoAP Content-Format IDs for the SenML
   media types in the "CoAP Content-Formats" sub-registry, within the
   "CoRE Parameters" registry [RFC7252].  All IDs are assigned from the
   "Expert Review" (0-255) range.  The assigned IDs are show in Table 7.

                     +-------------------------+-----+
                     | Media type              | ID  |
                     +-------------------------+-----+
                     | application/senml+json  | TBD |
                     | application/sensml+json | TBD |
                     | application/senml+cbor  | TBD |
                     | application/sensml+cbor | TBD |
                     | application/senml+xml   | TBD |
                     | application/sensml+xml  | TBD |
                     | application/senml+exi   | TBD |
                     | application/sensml+exi  | TBD |
                     +-------------------------+-----+

                     Table 7: CoAP Content-Format IDs






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13.  Security Considerations

   See Section 14.  Further discussion of security properties can be
   found in Section 12.3.

14.  Privacy Considerations

   Sensor data can range from information with almost no security
   considerations, such as the current temperature in a given city, to
   highly sensitive medical or location data.  This specification
   provides no security protection for the data but is meant to be used
   inside another container or transport protocol such as S/MIME or HTTP
   with TLS that can provide integrity, confidentiality, and
   authentication information about the source of the data.

15.  Acknowledgement

   We would like to thank Alexander Pelov, Andrew McClure, Andrew
   Mcgregor, Bjoern Hoehrmann, Christian Amsuess, Christian Groves,
   Daniel Peintner, Jan-Piet Mens, Joe Hildebrand, John Klensin, Karl
   Palsson, Lennart Duhrsen, Lisa Dusseault, Lyndsay Campbell, Martin
   Thomson, Michael Koster, and Stephen Farrell, for their review
   comments.

16.  References

16.1.  Normative References

   [BIPM]     Bureau International des Poids et Mesures, "The
              International System of Units (SI)", 8th edition, 2006.

   [IEEE.754.1985]
              Institute of Electrical and Electronics Engineers,
              "Standard for Binary Floating-Point Arithmetic",
              IEEE Standard 754, August 1985.

   [NIST811]  Thompson, A. and B. Taylor, "Guide for the Use of the
              International System of Units (SI)", NIST Special
              Publication 811, 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <http://www.rfc-editor.org/info/rfc3688>.



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   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <http://www.rfc-editor.org/info/rfc7303>.

   [W3C.REC-exi-20140211]
              Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
              "Efficient XML Interchange (EXI) Format 1.0 (Second
              Edition)", World Wide Web Consortium Recommendation REC-
              exi-20140211, February 2014,
              <http://www.w3.org/TR/2014/REC-exi-20140211>.

   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126>.







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16.2.  Informative References

   [I-D.arkko-core-dev-urn]
              Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", draft-arkko-core-dev-urn-03
              (work in progress), July 2012.

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-greevenbosch-appsawg-
              cbor-cddl-10 (work in progress), March 2017.

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing
              Constrained RESTful Environments (CoRE) Link Format in
              JSON and CBOR", draft-ietf-core-links-json-08 (work in
              progress), April 2017.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, DOI 10.17487/RFC2141,
              May 1997, <http://www.rfc-editor.org/info/rfc2141>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <http://www.rfc-editor.org/info/rfc4122>.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,
              <http://www.rfc-editor.org/info/rfc5952>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

   [RFC7111]  Hausenblas, M., Wilde, E., and J. Tennison, "URI Fragment
              Identifiers for the text/csv Media Type", RFC 7111,
              DOI 10.17487/RFC7111, January 2014,
              <http://www.rfc-editor.org/info/rfc7111>.






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   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <http://www.rfc-editor.org/info/rfc7721>.

   [UCUM]     Schadow, G. and C. McDonald, "The Unified Code for Units
              of Measure (UCUM)", Regenstrief Institute and Indiana
              University School of Informatics, 2013,
              <http://unitsofmeasure.org/ucum.html>.

Appendix A.  Links Extension

   An attribute to support a link extension for SenML is defined as a
   string attribute by this specification.  The link extension can be
   used for additional information about a SenML Record.  The definition
   and usage of the contents of this value are specified in
   [I-D.ietf-core-links-json].

   For JSON and XML the attribute has a label of "l" and a value that is
   a string.

   The following shows an example of the links extension.

   [
     {"bn":"urn:dev:ow:10e2073a01080063:","bt":1.320078429e+09,
      "l":"[{\"href\":\"humidity\",\"foo\":\"bar1\"}",
      "n":"temperature","u":"Cel","v":27.2},
     {"n":"humidity","u":"%RH","v":80}
   ]

Authors' Addresses

   Cullen Jennings
   Cisco
   400 3rd Avenue SW
   Calgary, AB  T2P 4H2
   Canada

   Email: fluffy@iii.ca












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   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


   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

















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