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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                       C. Jennings
Request for Comments: 8428                                         Cisco
Category: Standards Track                                      Z. Shelby
ISSN: 2070-1721                                                      ARM
                                                                J. Arkko
                                                              A. Keranen
                                                                Ericsson
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                             August 2018


                    Sensor Measurement Lists (SenML)

Abstract

   This specification defines a format for representing simple sensor
   measurements and device parameters in 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 one of these media types in protocols
   such as HTTP or the Constrained Application Protocol (CoAP) to
   transport the measurements of the sensor or to be configured.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8428.












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Copyright Notice

   Copyright (c) 2018 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
   (https://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 . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  SenML Structure and Semantics . . . . . . . . . . . . . . . .   6
     4.1.  Base Fields . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Regular Fields  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  SenML Labels  . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   9
     4.5.  Records and Their Fields  . . . . . . . . . . . . . . . .   9
       4.5.1.  Names . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.5.2.  Units . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.5.3.  Time  . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.5.4.  Values  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.6.  Resolved Records  . . . . . . . . . . . . . . . . . . . .  12
     4.7.  Associating Metadata  . . . . . . . . . . . . . . . . . .  12
     4.8.  Sensor Streaming Measurement Lists (SenSML) . . . . . . .  12
     4.9.  Configuration and Actuation Usage . . . . . . . . . . . .  13
   5.  JSON Representation (application/senml+json)  . . . . . . . .  13
     5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  14
       5.1.1.  Single Data Point . . . . . . . . . . . . . . . . . .  14
       5.1.2.  Multiple Data Points  . . . . . . . . . . . . . . . .  14
       5.1.3.  Multiple Measurements . . . . . . . . . . . . . . . .  15
       5.1.4.  Resolved Data . . . . . . . . . . . . . . . . . . . .  17
       5.1.5.  Multiple Data Types . . . . . . . . . . . . . . . . .  17
       5.1.6.  Collection of Resources . . . . . . . . . . . . . . .  18
       5.1.7.  Setting an Actuator . . . . . . . . . . . . . . . . .  18
   6.  CBOR Representation (application/senml+cbor)  . . . . . . . .  19
   7.  XML Representation (application/senml+xml)  . . . . . . . . .  21
   8.  EXI Representation (application/senml-exi)  . . . . . . . . .  23





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   9.  Fragment Identification Methods . . . . . . . . . . . . . . .  26
     9.1.  Fragment Identification Examples  . . . . . . . . . . . .  26
     9.2.  Fragment Identification for XML and EXI Formats . . . . .  27
   10. Usage Considerations  . . . . . . . . . . . . . . . . . . . .  27
   11. CDDL  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  SenML Units Registry . . . . . . . . . . . . . . . . . .  30
     12.2.  SenML Labels Registry  . . . . . . . . . . . . . . . . .  35
     12.3.  Media Type Registrations . . . . . . . . . . . . . . . .  36
       12.3.1.  senml+json Media Type Registration . . . . . . . . .  37
       12.3.2.  sensml+json Media Type Registration  . . . . . . . .  38
       12.3.3.  senml+cbor Media Type Registration . . . . . . . . .  39
       12.3.4.  sensml+cbor Media Type Registration  . . . . . . . .  41
       12.3.5.  senml+xml Media Type Registration  . . . . . . . . .  42
       12.3.6.  sensml+xml Media Type Registration . . . . . . . . .  43
       12.3.7.  senml-exi Media Type Registration  . . . . . . . . .  44
       12.3.8.  sensml-exi Media Type Registration . . . . . . . . .  45
     12.4.  XML Namespace Registration . . . . . . . . . . . . . . .  47
     12.5.  CoAP Content-Format Registration . . . . . . . . . . . .  47
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  47
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  48
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     15.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Overview

   Connecting sensors to the Internet is not new, and there have been
   many protocols designed to facilitate it.  This specification defines
   a format and media types for carrying simple sensor information in
   protocols such as HTTP [RFC7230] or CoAP [RFC7252].  The SenML format
   is 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 collect a large number of
   sensor measurements in a relatively efficient manner.  SenML can be
   used for a variety of data flow models, most notably data feeds
   pushed from a sensor to a collector, and for the web resource model
   where the sensor data 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




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   efficiency reasons.  Other information about the sensor can be
   discovered by other methods such as using the Constrained RESTful
   Environments (CoRE) Link Format [RFC6690].

   SenML is defined by a data model for measurements and simple metadata
   about measurements and devices.  The data is structured as a single
   array that contains a series of SenML Records that can each contain
   fields such as a 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 [RFC8259], CBOR [RFC7049], XML [W3C.REC-xml-20081126], and
   Efficient XML Interchange (EXI) [W3C.REC-exi-20140211].

   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 from large numbers of constrained devices.  Keeping the
   total size of the payload small makes it easy to also use SenML in
   constrained networks, e.g., in an IPv6 over Low-Power Wireless
   Personal Area Network (6LoWPAN) [RFC4944].  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 an 8-bit microprocessor.
   Experience with power meters and other 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.









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   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 given by the "v" field, the time of a
   measurement is in the "t" field, the "n" field has a unique sensor
   name, and the unit of the measurement is carried in the "u" field.

   [
     {"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"
   field 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.

   [
     {"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" field, and the "n"
   fields in each Record are empty strings, 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.  Values greater than or equal to 2**28
   represent an absolute time relative to the Unix epoch.  Values less
   than 2**28 represent time relative to 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.  If the server has accurate time based on, e.g., the
   Network Time Protocol (NTP), it may 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 information in a document
   database.







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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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   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.

   SenML Label:  A short name used in SenML Records to denote different
      SenML fields (e.g., "v" for "value").

   SenML Field:  A component of a record that associates a value to a
      SenML Label for this record.

   SenSML:  Sensor Streaming Measurement List (see Section 4.8).

   SenSML Stream:  One or more SenML Records to be processed as a
      stream.

   This document uses the terms "attribute" and "tag" where they occur
   with the underlying technologies (XML, CBOR [RFC7049], and the CoRE
   Link Format [RFC6690]); they are not used for SenML concepts, per se.
   However, note that "attribute" has been widely used in the past as a
   synonym for the SenML "field".

   All comparisons of text strings are performed byte by byte, which
   results in the comparisons being case sensitive.

   Where arithmetic is used, this specification uses the familiar
   notation of the programming language C, except that the operator "**"
   stands for exponentiation.

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 fields described below.  There are two
   kinds of fields: base and regular.  Both the base and regular fields
   can be included in any SenML Record.  The base fields apply to the
   entries in the Record and also to all Records after it up to, but not




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   including, the next Record that has that same base field.  All base
   fields are optional.  Regular fields can be included in any SenML
   Record and apply only to that Record.

4.1.  Base Fields

   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.

   Base Version:  Version number of the media type format.  This field
      is an optional positive integer and defaults to 10 if not present.

4.2.  Regular Fields

   Name:  Name of the sensor or parameter.  When appended to the Base
      Name field, 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, the 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:  Unit for a measurement value.  Optional.

   Value:  Value of the entry.  Optional if a Sum value is present;
      otherwise, it's 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 a
      Sum field, in which case it is allowed to have no Value field.

   Sum:  Integrated sum of the values over time.  Optional.  This field
      is in the unit specified in the Unit value multiplied by seconds.
      For historical reasons, it is named "sum" instead of "integral".



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   Time:  Time when the value was recorded.  Optional.

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

4.3.  SenML Labels

   Table 1 provides an overview of all SenML fields defined by this
   document with their respective labels and data types.

     +---------------+-------+------------+------------+------------+
     |          Name | Label | CBOR Label | JSON Type  | XML Type   |
     +---------------+-------+------------+------------+------------+
     |     Base Name | bn    |         -2 | String     | string     |
     |     Base Time | bt    |         -3 | Number     | double     |
     |     Base Unit | bu    |         -4 | String     | string     |
     |    Base Value | bv    |         -5 | Number     | double     |
     |      Base Sum | bs    |         -6 | Number     | double     |
     |  Base Version | bver  |         -1 | Number     | int        |
     |          Name | n     |          0 | String     | string     |
     |          Unit | u     |          1 | String     | string     |
     |         Value | v     |          2 | Number     | double     |
     |  String Value | vs    |          3 | String     | string     |
     | Boolean Value | vb    |          4 | Boolean    | boolean    |
     |    Data Value | vd    |          8 | String (*) | string (*) |
     |           Sum | s     |          5 | Number     | double     |
     |          Time | t     |          6 | Number     | double     |
     |   Update Time | ut    |          7 | Number     | double     |
     +---------------+-------+------------+------------+------------+

                           Table 1: SenML Labels

   (*) Data Value is a base64-encoded string with the URL-safe alphabet
   as defined in Section 5 of [RFC4648], with padding omitted.  (In
   CBOR, the octets in the Data Value are encoded using a definite-
   length byte string, major type 2.)

   For details of the JSON representation, see Section 5; for CBOR, see
   Section 6; and for XML, see Section 7.










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4.4.  Extensibility

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

   All SenML Records in a Pack MUST have the same version number.  This
   is typically done by adding a Base Version field to only the first
   Record in the Pack or by using the default value.

   Systems reading one of the objects MUST check for the Base Version
   field.  If this value is a version number larger than the version
   that the system understands, the system MUST NOT use this object.
   This allows the version number to indicate that the object contains
   structure or semantics that is different from what is defined in the
   present document beyond just making use of the extension points
   provided here.  New version numbers can only be defined in an RFC
   that updates this specification or its successors.

4.5.  Records and Their Fields

4.5.1.  Names

   The Name value is concatenated to the Base Name value to yield the
   name of the sensor.  The resulting concatenated name needs to
   uniquely identify and differentiate the sensor from all others.  The
   concatenated name MUST consist only of characters out of the set "A"
   to "Z", "a" to "z", and "0" to "9", as well as "-", ":", ".", "/",
   and "_"; furthermore, 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 concatenated names can be used directly within
   various URI schemes (including segments of an HTTP path with no
   special encoding; note that a name that contains "/" characters maps
   into multiple URI path segments) and can be used directly in many
   databases and analytic systems.  [RFC5952] contains advice on
   encoding an IPv6 address in a name.  See Section 14 for privacy
   considerations that apply to the use of long-term stable unique
   identifiers.

   Although it is RECOMMENDED that concatenated names be represented as
   URIs [RFC3986] or URNs [RFC8141], the restricted character set
   specified above puts strict limits on the URI schemes and URN
   namespaces that can be used.  As a result, implementers need to take
   care in choosing the naming scheme for concatenated names, because
   such names both need to be unique and need to conform to the
   restricted character set.  One approach is to include a bit string



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   that has guaranteed uniqueness (such as a 1-wire address [AN1796]).
   Some of the examples within this document use the device URN
   namespace as specified in [DEVICE-URN].  Universally Unique
   Identifiers (UUIDs) [RFC4122] are another way to generate a unique
   name.  However, the restricted character set does not allow the use
   of many URI schemes, such as the "tag" scheme [RFC4151] and the "ni"
   scheme [RFC6920], in names as such.  The use of URIs with characters
   incompatible with this set and possible mapping rules between the two
   are outside the scope of the present document.

4.5.2.  Units

   If the Record has no Unit, the Base Unit is used as the Unit.  Having
   no Unit and no Base Unit is allowed; any information that may be
   required about units applicable to the value then needs to be
   provided by the application context.

4.5.3.  Time

   If either the Base Time or Time value is missing, the missing field
   is considered to have a value of zero.  The Base Time and Time values
   are added together to get a value representing the time of
   measurement.

   Values less than 268,435,456 (2**28) represent time relative to the
   current time.  That is, a time of zero indicates that the sensor does
   not know the absolute time and the measurement was made roughly
   "now".  A negative value indicates seconds in the past from roughly
   "now".  Positive values up to 2**28 indicate seconds in the future
   from "now".  An example for employing positive values would be
   actuation use, when the desired change should happen in the future,
   but the sender or the receiver does not have accurate time available.

   Values greater than or equal to 2**28 represent an absolute time
   relative to the Unix epoch (1970-01-01T00:00Z in UTC time), and the
   time is counted the same way as the Portable Operating System
   Interface (POSIX) "seconds since the epoch" [TIME_T].  Therefore, the
   smallest absolute Time value that can be expressed (2**28) is
   1978-07-04 21:24:16 UTC.

   Because Time values up to 2**28 are used for representing time
   relative to "now" and Time and Base Time are added together, care
   must be taken to ensure that the sum does not inadvertently reach
   2**28 (i.e., absolute time) when relative time was intended to be
   used.






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   Obviously, SenML Records referenced to "now" are only useful within a
   specific communication context (e.g., based on information on when
   the SenML Pack, or a specific Record in a SenSML Stream, was sent) or
   together with some other context information that can be used for
   deriving a meaning of "now"; the expectation for any archival use is
   that they will be processed into UTC-referenced records before that
   context would cease to be available.  This specification deliberately
   leaves the accuracy of "now" very vague as it is determined by the
   overall systems that use SenML.  In a system where a sensor without
   wall-clock time sends a SenML Record with a time referenced to "now"
   over a high-speed RS-485 link to an embedded system with accurate
   time that resolves "now" based on the time of reception, the
   resulting time uncertainty could be within 1 ms.  At the other
   extreme, a deployment that sends SenML wind-speed readings over a
   Low-Earth Orbit (LEO) satellite link from a mountain valley might
   have resulting reception Time values that are easily a dozen minutes
   off the actual time of the sensor reading, with the time uncertainty
   depending on satellite locations and conditions.

4.5.4.  Values

   If only one of the Base Sum or Sum value is present, the missing
   field 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 nor the Sum is present, then the measurement does not
   have a Sum value.

   If the Base Value or Value is not present, the missing field(s) is
   considered to have a value of zero.  The Base Value and Value are
   added together to get the value of the measurement.

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

   In summary, the structure of a SenML Record is laid out to support a
   single measurement per Record.  If multiple data values are measured
   at the same time (e.g., air pressure and altitude), they are best
   kept as separate Records linked through their Time value; this is
   even true when one of the data values is more "meta" than others
   (e.g., describes a condition that influences other measurements at
   the same time).








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4.6.  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.  Also, if SenML Records are used outside of a SenML
   Pack, they need to be resolved first to ensure applicable base values
   are applied.

   A SenML Record is referred to as "resolved" if it does not contain
   any base values, i.e., labels starting with the character "b", except
   for Base Version fields (see below), and has no relative times.  To
   resolve the Records, the applicable base values of the SenML Pack (if
   any) are applied to the Record.  That is, for the base values in the
   Record or before the Record in the Pack, Name and Base Name are
   concatenated, the Base Time is added to the time of the Record, the
   Base Unit is applied to the Record if it did not contain a Unit, etc.
   In addition, the Records need to be in chronological order in the
   Pack.  An example of this is shown in Section 5.1.4.

   The Base Version field MUST NOT be present in resolved Records if the
   SenML version defined in this document is used; otherwise, it MUST be
   present in all the resolved SenML Records.

   A future specification that defines new base fields needs to specify
   how the field is resolved.

4.7.  Associating Metadata

   SenML is designed to carry the minimum dynamic information about
   measurements and, for efficiency reasons, does not carry significant
   static metadata about the device, object, or sensors.  Instead, it is
   assumed that this metadata is carried out of band.  For web resources
   using SenML Packs, this metadata can be made available 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=) link attribute (which is defined for the link
   format in Section 7.2.1 of [RFC7252]).

4.8.  Sensor Streaming Measurement Lists (SenSML)

   In some usage scenarios of SenML, the implementations 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 separate media types
   to indicate Sensor Streaming Measurement Lists (SenSML) for this



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RFC 8428                          SenML                      August 2018


   usage (see Section 12.3.2).  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 does not have
   to wait for the full SenSML Stream to be complete.

   If times relative to "now" (see Section 4.5.3) are used in SenML
   Records of a SenSML Stream, their interpretation of "now" is based on
   the time when the specific Record is sent in the stream.

4.9.  Configuration and Actuation Usage

   SenML can also be used for configuring parameters and controlling
   actuators.  When a SenML Pack is sent (e.g., using an HTTP/CoAP POST
   or PUT method) and the semantics of the target are such that SenML is
   interpreted as configuration/actuation, SenML Records are interpreted
   as a request to change the values of given (sub)resources (given as
   names) to given values at the given time(s).  The semantics of the
   target resource supporting this usage can be described, e.g., using
   [RID-CoRE].  Examples of actuation usage are shown in Section 5.1.7.

5.  JSON Representation (application/senml+json)

   For the SenML fields shown in Table 2, the SenML Labels are used as
   the JSON object member names within JSON objects representing the
   JSON SenML Records.

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

                        Table 2: JSON SenML Labels




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RFC 8428                          SenML                      August 2018


   The root JSON value 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 member values of the type specified in the table.

   Only the UTF-8 [RFC3629] form of JSON is allowed.  Characters in the
   String Value are encoded using the escape sequences defined in
   [RFC8259].  Octets in the Data Value are base64 encoded with the URL-
   safe alphabet as defined in Section 5 of [RFC4648], with padding
   omitted.

   Systems receiving measurements MUST be able to process the range of
   floating-point numbers that are representable as IEEE double-
   precision, floating-point numbers [IEEE.754].  This allows Time
   values to have better than microsecond precision over the next 100
   years.  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.  In the interest of avoiding unnecessary verbosity and speeding
   up processing, the mantissa SHOULD be less than 19 characters long,
   and the exponent SHOULD be less than 5 characters long.

5.1.  Examples

5.1.1.  Single Data Point

   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 Data Points

   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}
]









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RFC 8428                          SenML                      August 2018


   The next example is similar to the above one, but it 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}
   ]

   As an example of SenSML, the following stream of measurements may be
   sent via a long-lived HTTP POST from the producer of the stream to
   its consumer, 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 that time is used to correlate 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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RFC 8428                          SenML                      August 2018


   [
     {"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 following table shows the size of this example in various forms,
   as well as the size of each of these forms compressed with gzip.

                   +----------+------+-----------------+
                   | Encoding | Size | Compressed Size |
                   +----------+------+-----------------+
                   | JSON     | 573  |       206       |
                   | XML      | 649  |       235       |
                   | CBOR     | 254  |       196       |
                   | EXI      | 161  |       184       |
                   +----------+------+-----------------+

                         Table 3: Size Comparisons





















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RFC 8428                          SenML                      August 2018


5.1.4.  Resolved Data

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

   [
     {"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":"nfc-reader","vd":"aGkgCg"}
  ]






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RFC 8428                          SenML                      August 2018


5.1.6.  Collection of Resources

   The following example shows the results from a query to one device
   that aggregates multiple measurements from other devices.  The
   example assumes that a client has fetched information from a device
   at 2001:db8::2 by performing a GET operation on http://[2001:db8::2]
   at Mon Oct 31 16:27:09 UTC 2011 and has gotten two separate values as
   a result: a temperature and humidity measurement as well as the
   results from another device at http://[2001:db8::1] that also had a
   temperature and humidity measurement.  Note that the last record
   would use the Base Name from the 3rd record but the Base Time from
   the first record.

   [
     {"bn":"2001:db8::2/","bt":1.320078429e+09,
      "n":"temperature","u":"Cel","v":25.2},
     {"n":"humidity","u":"%RH","v":30},
     {"bn":"2001:db8::1/","n":"temperature","u":"Cel","v":12.3},
     {"n":"humidity","u":"%RH","v":67}
   ]

5.1.7.  Setting an Actuator

   The following example shows the SenML that could be used to set the
   current set point of a typical residential thermostat that 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 network that can guarantee delivery
   of the messages to the two lights within 15 ms (e.g., a network using
   802.1BA [IEEE802.1BA] and 802.1AS [IEEE802.1AS] for time
   synchronization).  The controller has set the time of the lights to
   come on at 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}
   ]



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RFC 8428                          SenML                      August 2018


   The following shows two lights being turned off using a
   non-deterministic network that has 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 that is turning off two lights.  Both lights are immediately
   dimmed to 50% brightness 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, 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 to an IEEE double-precision, floating-point
      value, it has exactly the same value as the original JSON Number
      converted to that form.  For the version number, only an unsigned
      integer is allowed.

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

   o  For compactness, the CBOR representation uses integers for the
      labels, as defined in Table 4.  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 also convert
      all future labels without needing to update implementations.  Base
      values are given negative CBOR labels, and others are given
      non-negative labels.





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RFC 8428                          SenML                      August 2018


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

            Table 4: CBOR Representation: Integers for Map Keys

   o  For streaming SenSML in CBOR representation, the array containing
      the records SHOULD be a CBOR array with an indefinite length; for
      non-streaming SenML, an array with a definite length 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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RFC 8428                          SenML                      August 2018


   In CBOR diagnostic notation (Section 6 of [RFC7049]), this is:

  [{-2: "urn:dev:ow:10e2073a0108006:",
    -3: 1276020076.001, -4: "A", -1: 5, 0: "voltage", 1: "V", 2: 120.1},
   {0: "current", 6: -5, 2: 1.2}, {0: "current", 6: -4, 2: 1.3},
   {0: "current", 6: -3, 2: 1.4}, {0: "current", 6: -2, 2: 1.5},
   {0: "current", 6: -1, 2: 1.6}, {0: "current", 6: 0, 2: 1.7}]

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.  Octets in the Data Value are
   base64 encoded with the URL-safe alphabet as defined in Section 5 of
   [RFC4648], with padding omitted.

   The following 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 element that contains a
   series of senml elements for each SenML Record.  The SenML fields are
   represented as XML attributes.  For each field defined in this
   document, the following table shows the SenML Labels, which are used
   for the XML attribute name, as well as the according restrictions on
   the XML attribute values ("type") as used in the XML senml elements.














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RFC 8428                          SenML                      August 2018


                   +---------------+-------+----------+
                   |          Name | Label | XML 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  |
                   |           Sum | s     | double   |
                   |          Time | t     | double   |
                   |   Update Time | ut    | double   |
                   +---------------+-------+----------+

                         Table 5: XML SenML Labels

   The RelaxNG [RNC] Schema for the XML is:

   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 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 }?
   }





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RFC 8428                          SenML                      August 2018


   sensml =
     element sensml {
       senml+
   }

   start = sensml

8.  EXI Representation (application/senml-exi)

   For efficient transmission of SenML over, e.g., a constrained
   network, EXI can be used.  This encodes the XML Schema
   [W3C.REC-xmlschema-1-20041028] 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.  However, this
   mode does not allow tag extensions to the schema; 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 "EXI Options", as defined in
   [W3C.REC-exi-20140211], with a 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
   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.
























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RFC 8428                          SenML                      August 2018


   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="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 that this example is the same
   information as the first example in Section 5.1.2 but in JSON format.

   <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>






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RFC 8428                          SenML                      August 2018


   Which compresses with EXI to the following displayed in hexdump:

 0000 a0 30 0d 84 80 f3 ab 93 71 d3 23 2b b1 d3 7b b9 |.0......q.#+..{.|
 0010 d1 89 83 29 91 81 b9 9b 09 81 89 81 c1 81 81 b1 |...)............|
 0020 99 d2 84 bb 37 b6 3a 30 b3 b2 90 1a b1 58 84 c0 |....7.:0.....X..|
 0030 33 04 b1 ba b9 39 32 b7 3a 10 1a 09 06 40 38    |3....92.:....@8|
 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 make 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 06 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 device that only generates this one EXI
   file does not really need a full EXI implementation.  It can simply
   hard code the output, replacing the 1-wire device ID starting at byte
   0x14 and going to byte 0x23 with its device ID and replacing the
   value "0xe7 0x01" at location 0x2b and 0x2c 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 byte is set to the 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
   integer temperature in tenths of degrees right-shifted 7 bits.  In
   this example, 231 >> 7 = 0x01.








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RFC 8428                          SenML                      August 2018


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 to a single
   Record, or a set of Records, in a Pack with a fragment identifier.
   The fragment identifier is only interpreted by a client and does not
   impact retrieval of a representation.  The SenML fragment
   identification is modeled after Comma-Separated Value (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 earlier Records.)

9.1.  Fragment Identification Examples

   The 3rd SenML Record from the "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 Records can be selected with:

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

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

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




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RFC 8428                          SenML                      August 2018


9.2.  Fragment Identification for XML and EXI Formats

   In addition to the SenML fragment identifiers described above, with
   the XML and EXI SenML formats, the syntax defined in the XPointer
   element() Scheme [XPointerElement] of the XPointer Framework
   [XPointerFramework] can be used.  (This is required by [RFC7303] for
   media types using the syntax suffix structured with "+xml".  For
   consistency, SenML allows this for the EXI formats as well.)

   Note that fragment identifiers are available to the client side only;
   they are not provided in transfer protocols such as CoAP or HTTP.
   Thus, they cannot be used by the server in deciding which media type
   to send.  Where a server has multiple representations available for a
   resource identified by a URI, it might send a JSON or CBOR
   representation when the client was directed to use an XML/EXI
   fragment identifier with it.  Clients can prevent running into this
   problem by explicitly requesting an XML or EXI media type (e.g.,
   using the CoAP Accept option) when XML-/EXI-only fragment identifier
   syntax is in use in the URI.

10.  Usage Considerations

   The measurements support sending both the current value of a sensor
   as well as an integrated sum.  For many types of measurements, the
   sum is more useful than the current value.  For historical reasons,
   this field is called "Sum" instead of "integral", which would more
   accurately describe its function.  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 unit set to watts, but
   it would put the sum of energy used in the "s" field of the
   measurement.  It might optionally include the current power in the
   "v" field.

   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.
   Implementers are encouraged to report the cumulative sum as well as
   the raw value of a given sensor.





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RFC 8428                          SenML                      August 2018


   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, they 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 non-
   negative, the sum should never get smaller; if the sum does get
   smaller, the application will know that one of the situations listed
   above has happened.

   Despite the name "Sum", the Sum field is not useful for applications
   that maintain a running count of the number of times an event
   happened or that keep track of a counter such as the total number of
   bytes sent on an interface.  Data like that can be sent directly in
   the Value field.
























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RFC 8428                          SenML                      August 2018


11.  CDDL

   As a convenient reference, the JSON and CBOR representations can be
   described with the common Concise Data Definition Language (CDDL)
   specification [CDDL-CBOR] in Figure 1 (informative).

   SenML-Pack = [1* record]

   record = {
     ? bn => tstr,        ; Base Name
     ? bt => numeric,     ; Base Time
     ? bu => tstr,        ; Base Units
     ? bv => numeric,     ; Base Value
     ? bs => numeric,     ; Base Sum
     ? bver => uint,      ; Base Version
     ? n => tstr,        ; Name
     ? u => tstr,        ; Units
     ? s => numeric,     ; Sum
     ? t => numeric,     ; Time
     ? ut => numeric,    ; Update Time
     ? ( v => numeric // ; Numeric Value
         vs => tstr //   ; String Value
         vb => bool //   ; Boolean Value
         vd => binary-value ) ; Data Value
     * key-value-pair
   }

   ; now define the generic versions
   key-value-pair = ( label => value )

   label = non-b-label / b-label
   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













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RFC 8428                          SenML                      August 2018


   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"
   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
   bs  = -6

   binary-value = bstr

           Figure 3: CBOR-Specific CDDL Specification for SenML

12.  IANA Considerations

   IANA has created a new "Sensor Measurement Lists (SenML)" registry
   that contains the subregistries defined in Sections 12.1 and 12.2.

12.1.  SenML Units Registry

   IANA has created a registry of SenML unit symbols called the "SenML
   Units" registry.  The primary purpose of this registry is to make
   sure that symbols uniquely map to indicate a type of measurement.
   Definitions for many of these units can be found in other
   publications 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 corresponding SenML units that are closer to the unscaled SI
   units.








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RFC 8428                          SenML                      August 2018


   +----------+------------------------------------+-------+-----------+
   |   Symbol | Description                        | Type  | Reference |
   +----------+------------------------------------+-------+-----------+
   |        m | meter                              | float | RFC 8428  |
   |       kg | kilogram                           | float | RFC 8428  |
   |        g | gram*                              | float | RFC 8428  |
   |        s | second                             | float | RFC 8428  |
   |        A | ampere                             | float | RFC 8428  |
   |        K | kelvin                             | float | RFC 8428  |
   |       cd | candela                            | float | RFC 8428  |
   |      mol | mole                               | float | RFC 8428  |
   |       Hz | hertz                              | float | RFC 8428  |
   |      rad | radian                             | float | RFC 8428  |
   |       sr | steradian                          | float | RFC 8428  |
   |        N | newton                             | float | RFC 8428  |
   |       Pa | pascal                             | float | RFC 8428  |
   |        J | joule                              | float | RFC 8428  |
   |        W | watt                               | float | RFC 8428  |
   |        C | coulomb                            | float | RFC 8428  |
   |        V | volt                               | float | RFC 8428  |
   |        F | farad                              | float | RFC 8428  |
   |      Ohm | ohm                                | float | RFC 8428  |
   |        S | siemens                            | float | RFC 8428  |
   |       Wb | weber                              | float | RFC 8428  |
   |        T | tesla                              | float | RFC 8428  |
   |        H | henry                              | float | RFC 8428  |
   |      Cel | degrees Celsius                    | float | RFC 8428  |
   |       lm | lumen                              | float | RFC 8428  |
   |       lx | lux                                | float | RFC 8428  |
   |       Bq | becquerel                          | float | RFC 8428  |
   |       Gy | gray                               | float | RFC 8428  |
   |       Sv | sievert                            | float | RFC 8428  |
   |      kat | katal                              | float | RFC 8428  |
   |       m2 | square meter (area)                | float | RFC 8428  |
   |       m3 | cubic meter (volume)               | float | RFC 8428  |
   |        l | liter (volume)*                    | float | RFC 8428  |
   |      m/s | meter per second (velocity)        | float | RFC 8428  |
   |     m/s2 | meter per square second            | float | RFC 8428  |
   |          | (acceleration)                     |       |           |
   |     m3/s | cubic meter per second (flow rate) | float | RFC 8428  |
   |      l/s | liter per second (flow rate)*      | float | RFC 8428  |
   |     W/m2 | watt per square meter (irradiance) | float | RFC 8428  |
   |    cd/m2 | candela per square meter           | float | RFC 8428  |
   |          | (luminance)                        |       |           |
   |      bit | bit (information content)          | float | RFC 8428  |
   |    bit/s | bit per second (data rate)         | float | RFC 8428  |
   |      lat | degrees latitude (Note 1)          | float | RFC 8428  |
   |      lon | degrees longitude (Note 1)         | float | RFC 8428  |



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RFC 8428                          SenML                      August 2018


   |       pH | pH value (acidity; logarithmic     | float | RFC 8428  |
   |          | quantity)                          |       |           |
   |       dB | decibel (logarithmic quantity)     | float | RFC 8428  |
   |      dBW | decibel relative to 1 W (power     | float | RFC 8428  |
   |          | level)                             |       |           |
   |     Bspl | bel (sound pressure level;         | float | RFC 8428  |
   |          | logarithmic quantity)*             |       |           |
   |    count | 1 (counter value)                  | float | RFC 8428  |
   |        / | 1 (ratio, e.g., value of a switch; | float | RFC 8428  |
   |          | Note 2)                            |       |           |
   |        % | 1 (ratio, e.g., value of a switch; | float | RFC 8428  |
   |          | Note 2)*                           |       |           |
   |      %RH | percentage (relative humidity)     | float | RFC 8428  |
   |      %EL | percentage (remaining battery      | float | RFC 8428  |
   |          | energy level)                      |       |           |
   |       EL | seconds (remaining battery energy  | float | RFC 8428  |
   |          | level)                             |       |           |
   |      1/s | 1 per second (event rate)          | float | RFC 8428  |
   |    1/min | 1 per minute (event rate, "rpm")*  | float | RFC 8428  |
   | beat/min | 1 per minute (heart rate in beats  | float | RFC 8428  |
   |          | per minute)*                       |       |           |
   |    beats | 1 (cumulative number of heart      | float | RFC 8428  |
   |          | beats)*                            |       |           |
   |      S/m | siemens per meter (conductivity)   | float | RFC 8428  |
   +----------+------------------------------------+-------+-----------+

                  Table 6: IANA Registry for SenML Units

   o  Note 1: Assumed to be in World Geodetic System 1984 (WGS84),
      unless another reference frame is known for the sensor.

   o  Note 2: A value of 0.0 indicates the switch is off, 1.0 indicates
      on, and 0.5 indicates 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 NOT a percentage but the absolute
      ratio!














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RFC 8428                          SenML                      August 2018


   New entries can be added to the registration by Expert Review as
   defined in [RFC8126].  Experts should exercise their own good
   judgment but need to consider the following guidelines:

   1.   There needs to be a real and compelling use for any new unit to
        be added.

   2.   Each unit should define the semantic information and be chosen
        carefully.  Implementers 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
        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 System of Units (SI) prefixes such as "k" before the unit
        is not recommended.  Instead, one can represent the value using
        scientific notation such as 1.2e3.  The "kg" unit is an
        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, meter is defined, and other
        lengths such as mile, foot, and light year are not allowed.  For
        most cases, the SI unit is preferred.

        (Note that some amount of judgment will be required here, as
        even SI itself is not entirely consistent in this respect.  For
        instance, for temperature, [ISO-80000-5] defines a quantity,
        item 5-1 (thermodynamic temperature), and a corresponding unit
        of 5-1.a (Kelvin); [ISO-80000-5] goes on to define another
        quantity, item 5-2 ("Celsius temperature"), and the
        corresponding unit of 5-2.a (degree Celsius).  The latter
        quantity is defined such that it gives the thermodynamic
        temperature as a delta from T0 = 275.15 K.  ISO 80000-5 is
        defining both units side by side and not really expressing a
        preference.  This level of recognition of the alternative unit
        degree Celsius is the reason why Celsius temperatures seem
        exceptionally acceptable in the SenML units list alongside
        Kelvin.)






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RFC 8428                          SenML                      August 2018


   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 they are common SI
        prefixes: Y, Z, E, P, T, G, M, k, h, da, d, c, u, n, p, f, a, z,
        y, Ki, Mi, Gi, Ti, Pi, Ei, Zi, and 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, and kWh.

   9.   The unit names are case sensitive, and the correct case needs to
        be used; however, symbols that differ only in case should not be
        allocated.

   10.  A number after a unit typically indicates the previous unit
        raised to that power, and "/" indicates that the units that
        follow are the reciprocals.  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].

























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RFC 8428                          SenML                      August 2018


12.2.  SenML Labels Registry

   IANA has created a new registry for SenML Labels called the "SenML
   Labels" registry.  The initial contents of the registry are as
   follows:

   +--------------+-------+----+-----------+----------+----+-----------+
   |         Name | Label | CL | JSON Type | XML Type | EI | Reference |
   +--------------+-------+----+-----------+----------+----+-----------+
   |    Base Name | bn    | -2 | String    | string   | a  | RFC 8428  |
   |    Base Time | bt    | -3 | Number    | double   | a  | RFC 8428  |
   |    Base Unit | bu    | -4 | String    | string   | a  | RFC 8428  |
   |   Base Value | bv    | -5 | Number    | double   | a  | RFC 8428  |
   |     Base Sum | bs    | -6 | Number    | double   | a  | RFC 8428  |
   | Base Version | bver  | -1 | Number    | int      | a  | RFC 8428  |
   |         Name | n     | 0  | String    | string   | a  | RFC 8428  |
   |         Unit | u     | 1  | String    | string   | a  | RFC 8428  |
   |        Value | v     | 2  | Number    | double   | a  | RFC 8428  |
   | String Value | vs    | 3  | String    | string   | a  | RFC 8428  |
   |      Boolean | vb    | 4  | Boolean   | boolean  | a  | RFC 8428  |
   |        Value |       |    |           |          |    |           |
   |   Data Value | vd    | 8  | String    | string   | a  | RFC 8428  |
   |          Sum | s     | 5  | Number    | double   | a  | RFC 8428  |
   |         Time | t     | 6  | Number    | double   | a  | RFC 8428  |
   |  Update Time | ut    | 7  | Number    | double   | a  | RFC 8428  |
   +--------------+-------+----+-----------+----------+----+-----------+

                Note that CL = CBOR Label and EI = EXI ID.

                  Table 7: IANA Registry for SenML Labels

   This is the same table as Table 1, with notes removed and columns
   added for the information that is all the same for this initial set
   of registrations, but it will need to be supplied with different
   values for new registrations.

   All new entries must define the 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 EI column contains the EXI schemaId value of
   the first schema that includes this label, or it is empty if this
   label was not intended for use with EXI.  The Reference column SHOULD
   contain information about where to find out more information about
   this label.

   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,




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RFC 8428                          SenML                      August 2018


   respectively.  CBOR represents numeric values with a CBOR type that
   does not lose any information from the JSON value.  EXI uses the XML
   types.

   New entries can be added to the registration by Expert Review as
   defined in [RFC8126].  Experts should exercise their own good
   judgment but need to consider that shorter labels should have more
   strict review.  New entries should not be made that counteract the
   advice at the end of Section 4.5.4.

   All new SenML Labels that have "base" semantics (see Section 4.1)
   MUST start with the character "b".  Regular labels MUST NOT start
   with that character.  All new SenML Labels with Value semantics (see
   Section 4.2) MUST have "Value" in their (long-form) name.

   Extensions that add a label intended for use with XML need to create
   a new RelaxNG Schema that includes all the labels in the "SenML
   Labels" 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 "SenML
   Labels" registry and 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 "SenML Labels" registry 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 Registrations

   The registrations in the subsections below follow the procedures
   specified in [RFC6838] and [RFC7303].  This document registers media
   types for each serialization format of SenML (JSON, CBOR, XML, and
   EXI) and also a corresponding set of media types for streaming use
   (SenSML; see Section 4.8).  Clipboard formats are defined for the
   JSON and XML forms of SenML but not for streams or non-textual
   formats.

   The reason there are both SenML and the streaming SenSML formats is
   that they are not the same data formats, and they require separate
   negotiation to understand if they are supported and which one is
   being used.  The non-streaming format is required to have some sort
   of end-of-pack syntax that indicates there will be no more records.
   Many implementations that receive SenML wait for this end-of-pack
   marker before processing any of the records.  On the other hand, with
   the streaming formats, it is explicitly not required to wait for this



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RFC 8428                          SenML                      August 2018


   end-of-pack marker.  Many implementations that produce streaming
   SenSML will never send this end-of-pack marker, so implementations
   that receive streaming SenSML cannot wait for the end-of-pack marker
   before they start processing the records.  Given that SenML and
   streaming SenML are different data formats, and considering the
   requirement for separate negotiation, a media type for each one is
   needed.

12.3.1.  senml+json Media Type Registration

   Type name: application

   Subtype name: senml+json

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using a subset of the
   encoding allowed in [RFC8259].  See RFC 8428 for details.  This
   simplifies implementation of a 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: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any JSON
   key-value pairs that they do not understand unless the key ends with
   the "_" character, in which case an error MUST be generated.  This
   allows backwards-compatible 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 8428

   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 8428.








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RFC 8428                          SenML                      August 2018


   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      File extension(s): senml

      Windows Clipboard Name: "JSON Sensor Measurement List"

      Macintosh file type code(s): none

      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 [RFC8259].  See RFC 8428 for details.  This
   simplifies implementation of a 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: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any JSON
   key-value pairs that they do not understand unless the key ends with
   the "_" character, in which case an error MUST be generated.  This




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RFC 8428                          SenML                      August 2018


   allows backwards-compatible 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 8428

   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/sensml+json is supported by using fragment identifiers as
   specified by RFC 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      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
   8428 for details.



Jennings, et al.             Standards Track                   [Page 39]


RFC 8428                          SenML                      August 2018


   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any key-
   value pairs that they do not understand unless the key ends with the
   "_" character, in which case an error MUST be generated.  This allows
   backwards-compatible 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 8428

   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 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      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








Jennings, et al.             Standards Track                   [Page 40]


RFC 8428                          SenML                      August 2018


12.3.4.  sensml+cbor Media Type Registration

   Type name: application

   Subtype name: sensml+cbor

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using [RFC7049].  See RFC
   8428 for details.

   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any key-
   value pairs that they do not understand unless the key ends with the
   "_" character, in which case an error MUST be generated.  This allows
   backwards-compatible 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 8428

   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/sensml+cbor is supported by using fragment identifiers as
   specified by RFC 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      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




Jennings, et al.             Standards Track                   [Page 41]


RFC 8428                          SenML                      August 2018


   Restrictions on usage: None

   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 8428 for details.

   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any XML
   tags or attributes that they do not understand unless the attribute
   name ends with the "_" character, in which case an error MUST be
   generated.  This allows backwards-compatible extensions to this
   specification.  The "bver" attribute in the senml XML tag can be used
   to ensure the receiver supports a minimal level of functionality
   needed by the creator of the XML SenML Pack.

   Published specification: RFC 8428

   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 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      File extension(s): senmlx




Jennings, et al.             Standards Track                   [Page 42]


RFC 8428                          SenML                      August 2018


      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 8428 for details.

   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any XML
   tags or attributes that they do not understand unless the attribute
   name ends with the "_" character, in which case an error MUST be
   generated.  This allows backwards-compatible extensions to this
   specification.  The "bver" attribute in the senml XML tag can be used
   to ensure the receiver supports a minimal level of functionality
   needed by the creator of the XML SenML Pack.

   Published specification: RFC 8428

   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.




Jennings, et al.             Standards Track                   [Page 43]


RFC 8428                          SenML                      August 2018


   Fragment identifier considerations: Fragment identification for
   application/sensml+xml is supported by using fragment identifiers as
   specified by RFC 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      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 8428 for details.

   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any XML
   tags or attributes that they do not understand unless the attribute
   name ends with the "_" character, in which case an error MUST be
   generated.  This allows backwards-compatible extensions to this
   specification.  The "bver" attribute in the senml XML tag can be used
   to ensure the receiver supports a minimal level of functionality
   needed by the creator of the XML SenML Pack.  Further information on
   using schemas to guide the EXI can be found in RFC 8428.



Jennings, et al.             Standards Track                   [Page 44]


RFC 8428                          SenML                      August 2018


   Published specification: RFC 8428

   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 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      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

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 8428 for details.




Jennings, et al.             Standards Track                   [Page 45]


RFC 8428                          SenML                      August 2018


   Security considerations: See Section 13 of RFC 8428.

   Interoperability considerations: Applications MUST ignore any XML
   tags or attributes that they do not understand unless the attribute
   name ends with the "_" character, in which case an error MUST be
   generated.  This allows backwards-compatible extensions to this
   specification.  The "bver" attribute in the senml XML tag can be used
   to ensure the receiver supports a minimal level of functionality
   needed by the creator of the XML SenML Pack.  Further information on
   using schemas to guide the EXI can be found in RFC 8428.

   Published specification: RFC 8428

   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/sensml-exi is supported by using fragment identifiers as
   specified by RFC 8428.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      File extension(s): sensmle

      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









Jennings, et al.             Standards Track                   [Page 46]


RFC 8428                          SenML                      August 2018


12.4.  XML Namespace Registration

   This document registers the following XML namespace 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 has assigned CoAP Content-Format IDs for the SenML media types
   in the "CoAP Content-Formats" subregistry within the "Constrained
   RESTful Environments (CoRE) Parameters" registry [RFC7252].  IDs for
   the JSON, CBOR, and EXI Content-Formats have been assigned in the
   0-255 range (Expert Review), and IDs for the XML Content-Formats have
   been assigned in the 256-9999 range (IETF Review or IESG Approval).
   The assigned IDs are shown in the table below:

         +-------------------------+----------+-----+-----------+
         | Media Type              | Encoding | ID  | Reference |
         +-------------------------+----------+-----+-----------+
         | application/senml+json  | -        | 110 | RFC 8428  |
         | application/sensml+json | -        | 111 | RFC 8428  |
         | application/senml+cbor  | -        | 112 | RFC 8428  |
         | application/sensml+cbor | -        | 113 | RFC 8428  |
         | application/senml-exi   | -        | 114 | RFC 8428  |
         | application/sensml-exi  | -        | 115 | RFC 8428  |
         | application/senml+xml   | -        | 310 | RFC 8428  |
         | application/sensml+xml  | -        | 311 | RFC 8428  |
         +-------------------------+----------+-----+-----------+

                     Table 8: CoAP Content-Format IDs

13.  Security Considerations

   Sensor data presented with SenML can contain a wide array of
   information that ranges from very public (such as the outside
   temperature in a given city) to very private (such as patient health
   information that requires integrity and confidentiality protection).
   When SenML is used for configuration or actuation, it can be used to
   change the state of systems and also impact the physical world, e.g.,
   by turning off a heater or opening a lock.  Malicious use of SenML to
   change system state could have severe consequences, potentially
   including violation of physical security, property damage, and even
   loss of life.



Jennings, et al.             Standards Track                   [Page 47]


RFC 8428                          SenML                      August 2018


   SenML formats alone do not provide any security and instead rely on
   the protocol that carries them to provide security.  Applications
   using SenML need to look at the overall context of how these formats
   will be used to decide if the security is adequate.  In particular,
   for sensitive sensor data and actuation use, it is important to
   ensure that proper security mechanisms are used to provide, e.g.,
   confidentiality, data integrity, and authentication as appropriate
   for the usage.

   SenML formats defined by this specification do not contain any
   executable content.  However, future extensions could potentially
   embed application-specific executable content in the data.

   SenML Records are intended to be interpreted in the context of any
   applicable base values.  If Records become separated from the Record
   that establishes the base values, the data will be useless or, worse,
   wrong.  Care needs to be taken in keeping the integrity of a Pack
   that contains unresolved SenML Records (see Section 4.6).

   See also Section 14.

14.  Privacy Considerations

   Sensor data can range from information with almost no privacy
   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 transfer protocol such as S/MIME
   [RFC5751] or HTTP with TLS [RFC2818] that can provide integrity,
   confidentiality, and authentication information about the source of
   the data.

   The Name fields need to uniquely identify the sources or destinations
   of the values in a SenML Pack.  However, the use of long-term stable
   and unique identifiers can be problematic for privacy reasons
   [RFC6973], depending on the application and the potential of these
   identifiers to be used in correlation with other information.  They
   should be used with care or avoided, for example, as described for
   IPv6 addresses in [RFC7721].












Jennings, et al.             Standards Track                   [Page 48]


RFC 8428                          SenML                      August 2018


15.  References

15.1.  Normative References

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

   [IEEE.754] IEEE, "Standard for Binary Floating-Point Arithmetic",
              IEEE Standard 754.

   [NIST811]  Thompson, A. and B. Taylor, "Guide for the Use of the
              International System of Units (SI)", NIST Special
              Publication 811, DOI 10.6028/NIST.SP.811e2008, March 2008.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

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

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

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://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, <https://www.rfc-editor.org/info/rfc7049>.

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

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



Jennings, et al.             Standards Track                   [Page 49]


RFC 8428                          SenML                      August 2018


   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RNC]      ISO/IEC, "Information technology -- Document Schema
              Definition Language (DSDL) -- Part 2: Regular-grammar-
              based validation -- RELAX NG", ISO/IEC 19757-2, Annex
              C: RELAX NG Compact syntax, December 2008.

   [TIME_T]   The Open Group Base Specifications, "Open Group Standard -
              Vol. 1: Base Definitions, Issue 7", Section 4.16, "Seconds
              Since the Epoch", IEEE Standard 1003.1, 2018,
              <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
              V1_chap04.html#tag_04_16>.

   [W3C.REC-exi-20140211]
              Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
              "Efficient XML Interchange (EXI) Format 1.0 (Second
              Edition)", W3C 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)", W3C Recommendation REC-xml-20081126, November
              2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.

   [W3C.REC-xmlschema-1-20041028]
              Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn,
              "XML Schema Part 1: Structures Second Edition", W3C
              Recommendation REC-xmlschema-1-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.

   [XPointerElement]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              element() Scheme", W3C Recommendation REC-xptr-element,
              March 2003,
              <https://www.w3.org/TR/2003/REC-xptr-element-20030325/>.



Jennings, et al.             Standards Track                   [Page 50]


RFC 8428                          SenML                      August 2018


   [XPointerFramework]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", W3C Recommendation REC-XPointer-Framework,
              March 2003,
              <http://www.w3.org/TR/2003/REC-xptr-framework-20030325/>.

15.2.  Informative References

   [AN1796]   Linke, B., "Overview of 1-Wire Technology and Its Use",
              Maxim Integrated, Tutorial 1796, June 2008,
              <http://pdfserv.maximintegrated.com/en/an/AN1796.pdf>.

   [CDDL-CBOR]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR and JSON data structures", Work in Progress,
              draft-ietf-cbor-cddl-05, August 2018.

   [DEVICE-URN]
              Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", Work in Progress,
              draft-ietf-core-dev-urn-02, July 2018.

   [IEEE802.1AS]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks - Timing and Synchronization for Time-Sensitive
              Applications in Bridged Local Area Networks", IEEE
              Standard 802.1AS.

   [IEEE802.1BA]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Audio Video Bridging (AVB) Systems", IEEE
              Standard 802.1BA.

   [ISO-80000-5]
              ISO, "Quantities and units - Part 5: Thermodynamics",
              ISO 80000-5, Edition 1.0, May 2007.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [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,
              <https://www.rfc-editor.org/info/rfc3986>.





Jennings, et al.             Standards Track                   [Page 51]


RFC 8428                          SenML                      August 2018


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

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,
              <https://www.rfc-editor.org/info/rfc4151>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <https://www.rfc-editor.org/info/rfc5751>.

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

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

   [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
              <https://www.rfc-editor.org/info/rfc6920>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [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,
              <https://www.rfc-editor.org/info/rfc7111>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.



Jennings, et al.             Standards Track                   [Page 52]


RFC 8428                          SenML                      August 2018


   [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,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
              <https://www.rfc-editor.org/info/rfc8141>.

   [RID-CoRE]
              Shelby, Z., Vial, M., Groves, C., Zhu, J., and B.
              Silverajan, Ed., "Reusable Interface Definitions for
              Constrained RESTful Environments", Work in Progress,
              draft-ietf-core-interfaces-12, June 2018.

   [UCUM]     Schadow, G. and C. McDonald, "The Unified Code for Units
              of Measure", Version 2.1, Regenstrief Institute and
              the UCUM Organization, November 2017,
              <http://unitsofmeasure.org/ucum.html>.

Acknowledgements

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






















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RFC 8428                          SenML                      August 2018


Authors' Addresses

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

   Email: fluffy@iii.ca


   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   United States of America

   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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