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Versions: (draft-jennings-senml) 00 01 02 04 05 06 draft-ietf-core-senml

Network Working Group                                        C. Jennings
Internet-Draft                                                     Cisco
Intended status: Standards Track                               Z. Shelby
Expires: April 21, 2016                                              ARM
                                                                J. Arkko
                                                              A. Keranen
                                                                Ericsson
                                                        October 19, 2015


             Media Types for Sensor Markup Language (SENML)
                      draft-jennings-core-senml-02

Abstract

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements and Design Goals . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Semantics . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Associating Meta-data . . . . . . . . . . . . . . . . . . . .   6
   6.  JSON Representation (application/senml+json)  . . . . . . . .   7
     6.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.1.  Single Datapoint  . . . . . . . . . . . . . . . . . .   8
       6.1.2.  Multiple Datapoints . . . . . . . . . . . . . . . . .   9
       6.1.3.  Multiple Measurements . . . . . . . . . . . . . . . .  10
       6.1.4.  Collection of Resources . . . . . . . . . . . . . . .  11
   7.  CBOR Representation (application/senml+cbor)  . . . . . . . .  11
   8.  XML Representation (application/senml+xml)  . . . . . . . . .  12
   9.  EXI Representation (application/senml-exi)  . . . . . . . . .  13
   10. Usage Considerations  . . . . . . . . . . . . . . . . . . . .  15
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Units Registry . . . . . . . . . . . . . . . . . . . . .  17
     11.2.  Media Type Registration  . . . . . . . . . . . . . . . .  19
       11.2.1.  senml+json Media Type Registration . . . . . . . . .  19
       11.2.2.  senml+cbor Media Type Registration . . . . . . . . .  21
       11.2.3.  senml+xml Media Type Registration  . . . . . . . . .  22
       11.2.4.  senml-exi Media Type Registration  . . . . . . . . .  22
     11.3.  XML Namespace Registration . . . . . . . . . . . . . . .  23
     11.4.  CoAP Content-Format Registration . . . . . . . . . . . .  23
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   13. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  24
   14. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  24
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     15.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27








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1.  Overview

   Connecting sensors to the internet is not new, and there have been
   many protocols designed to facilitate it.  This specification defines
   new media types for carrying simple sensor information in a protocol
   such as HTTP or CoAP called the Sensor Markup Language (SenML).  This
   format was designed so that processors with very limited capabilities
   could easily encode a sensor measurement into the media type, while
   at the same time a server parsing the data could relatively
   efficiently collect a large number of sensor measurements.  There are
   many types of more complex measurements and measurements that this
   media type would not be suitable for.  A decision was made not to
   carry most of the meta data about the sensor in this media type to
   help reduce the size of the data and improve efficiency in decoding.
   Instead meta-data about a sensor resource can be described out-of-
   band using the CoRE Link Format [RFC6690].  The markup language can
   be used for a variety of data flow models, most notably data feeds
   pushed from a sensor to a collector, and the web resource model where
   the sensor is requested as a resource representation (e.g., "GET
   /sensor/temperature").

   SenML is defined by a data model for measurements and simple meta-
   data about measurements and devices.  The data is structured as a
   single array that contains base value object(s) and array(s) of
   entries.  Each entry is an object that has attributes such as a
   unique identifier for the sensor, the time the measurement was made,
   and the current value.  Serializations for this data model are
   defined for JSON [RFC7159], CBOR [RFC7049], XML, and Efficient XML
   Interchange (EXI) [W3C.REC-exi-20110310].

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

   [{}, [{ "n": "urn:dev:ow:10e2073a01080063", "v":23.5, "u":"Cel" }]]

   In the example above, the first element of the root array is empty
   object since there are no base values.  The second array inside the
   root array has a single measurement for a sensor named
   "urn:dev:ow:10e2073a01080063" with a temperature of 23.5 degrees
   Celsius.

2.  Requirements and Design Goals

   The design goal is to be able to send simple sensor measurements in
   small packets on mesh networks from large numbers of constrained
   devices.  Keeping the total size of payload under 80 bytes makes this
   easy to use on a wireless mesh network.  It is always difficult to
   define what small code is, but there is a desire to be able to



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

3.  Terminology

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

4.  Semantics

   Each SenML representation carries a single array that represents a
   set of measurements and/or parameters.  This array contains a base
   object with several optional attributes described below and a
   mandatory array of one or more entries.

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

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

   Base Units:  A base unit that is assumed for all entries, unless
      otherwise indicated.  This attribute is optional.

   Version:  Version number of media type format.  This attribute is
      optional positive integer and defaults to 2 if not present.

   The measurement or parameter entries array contains values for sensor
   measurements or other generic parameters, such as configuration
   parameters.  There must be at least one entry in the array.  This
   array is called simply "measurement array" in the following text.

   Each array entry contains several attributes, some of which are
   optional and some of which are mandatory:

   Name:  Name of the sensor or parameter.  When appended to the Base
      Name attribute, this must result in a globally unique identifier
      for the resource.  The name is optional, if the Base Name is



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      present.  If the name is missing, Base Name must uniquely identify
      the resource.  This can be used to represent a large array of
      measurements from the same sensor without having to repeat its
      identifier on every measurement.

   Units:  Units for a measurement value.  Optional.

   Value  Value of the entry.  Optional if a Sum value is present,
      otherwise required.  Values are represented using three basic data
      types, Floating point numbers ("v" field for "Value"), Booleans
      ("bv" for "Boolean Value") and Strings ("sv" for "String Value").
      Exactly one of these three fields MUST appear.

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

   Time:  Time when value was recorded.  Optional.

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

   The SenML format can be extended with further custom attributes
   placed in a base object, or in an entry.  Extensions in a base object
   pertain to all entries following the base object, whereas extensions
   in an entry object only pertain to that entry.

   Systems reading one of the objects MUST check for the Version
   attribute.  If this value is a version number larger than the version
   which the system understands, the system SHOULD NOT use this object.
   This allows the version number to indicate that the object contains
   mandatory to understand attributes.  New version numbers can only be
   defined in an RFC that updates this specification or it successors.

   The Name value is concatenated to the Base Name value to get the name
   of the sensor.  The resulting name needs to uniquely identify and
   differentiate the sensor from all others.  If the object is a
   representation resulting from the request of a URI [RFC3986], then in
   the absence of the Base Name attribute, this URI is used as the
   default value of Base Name.  Thus in this case the Name field needs
   to be unique for that URI, for example an index or subresource name
   of sensors handled by the URI.

   Alternatively, for objects not related to a URI, a unique name is
   required.  In any case, it is RECOMMENDED that the full names are
   represented as URIs or URNs [RFC2141].  One way to create a unique



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   name is to include a EUI-48 or EUI-64 identifier (a MAC address) or
   some other bit string that has guaranteed uniqueness (such as a
   1-wire address) that is assigned to the device.  Some of the examples
   in this draft use the device URN type as specified in
   [I-D.arkko-core-dev-urn].  UUIDs [RFC4122] are another way to
   generate a unique name.

   The resulting concatenated name MUST consist only of characters out
   of the set "A" to "Z", "a" to "z", "0" to "9", "-", ":", ".", or "_"
   and it MUST start with a character out of the set "A" to "Z", "a" to
   "z", or "0" to "9".  This restricted character set was chosen so that
   these names can be directly used as in other types of URI including
   segments of an HTTP path with no special encoding.  [RFC5952]
   contains advice on encoding an IPv6 address in a name.

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

   A measurement array MAY be followed by another base object and
   measurement array.  The new base object can add, change, and/or
   remove base values from the previous base object(s).  The new base
   values are applied to the following measurement arrays.  Every base
   object MUST be followed by a measurement array, and hence base
   objects are found in the root array at even indexes and measurement
   arrays at odd indexes.

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

5.  Associating Meta-data

   SenML is designed to carry the minimum dynamic information about
   measurements, and for efficiency reasons does not carry more static
   meta-data about the device, object or sensors.  Instead, it is
   assumed that this meta-data is carried out of band.  For web
   resources using SenML representations, this meta-data can be made
   available using the CoRE Link Format [RFC6690].

   The CoRE Link Format provides a simple way to describe Web Links, and
   in particular allows a web server to describe resources it is



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   hosting.  The list of links that a web server has available, can be
   discovered by retrieving the /.well-known/core resource, which
   returns the list of links in the CoRE Link Format.  Each link may
   contain attributes, for example title, resource type, interface
   description, and content-type.

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

   Further semantics about a resource can be included in the Resource
   Type and Interface Description attributes.  The Resource Type (rt=)
   attribute is meant to give a semantic meaning to that resource.  For
   example rt="outdoor-temperature" would indicate static semantic
   meaning in addition to the unit information included in SenML.  The
   Interface Description (if=) attribute is used to describe the REST
   interface of a resource, and may include e.g. a reference to a WADL
   description [WADL].

6.  JSON Representation (application/senml+json)

   Base object variables:

                      +------------+------+--------+
                      |      SenML | JSON | Type   |
                      +------------+------+--------+
                      |  Base Name | bn   | String |
                      |  Base Time | bt   | Number |
                      | Base Units | bu   | Number |
                      |    Version | ver  | Number |
                      +------------+------+--------+

   Measurement or Parameter Entries:

                 +---------------+------+----------------+
                 |         SenML | JSON | Notes          |
                 +---------------+------+----------------+
                 |          Name | n    | String         |
                 |         Units | u    | String         |
                 |         Value | v    | Floating point |
                 |  String Value | sv   | String         |
                 | Boolean Value | bv   | Boolean        |
                 |     Value Sum | s    | Floating point |
                 |          Time | t    | Number         |
                 |   Update Time | ut   | Number         |
                 +---------------+------+----------------+




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   All of the data is UTF-8, but since this is for machine to machine
   communications on constrained systems, only characters with code
   points between U+0001 and U+007F are allowed which corresponds to the
   ASCII [RFC0020] subset of UTF-8.

   The root content consists of an array with even amount of JSON
   objects where the first (and then every odd) element is a base object
   and the second (and then every even) element is a measurements array.
   The base object MAY contain a "bn" attribute with a value of type
   string.  The object MAY contain a "bt" attribute with a value of type
   number.  The object MAY contain a "bu" attribute with a value of type
   string.  The object MAY contain a "ver" attribute with a value of
   type number.  The object MAY contain other attribute value pairs.
   The base object MUST be followed by an array.  The array MUST have
   one or more measurement or parameter objects.

   If the root array has more than one base object, each following base
   object modifies the base values using the JSON merge patch format
   [RFC7396].  That is, base values can be added or modified by defining
   their new values and existing base values can removed by defining the
   value as "null".

   Inside each measurement or parameter object the "n", "u", and "sv"
   attributes are of type string, the "t" and "ut" attributes are of
   type number, the "bv" attribute is of type boolean, and the "v" and
   "s" attributes are of type floating point.  All the attributes are
   optional, but as specified in Section 4, one of the "v", "sv", or
   "bv" attributes MUST appear unless the "s" attribute is also present.
   The "v", and "sv", and "bv" attributes MUST NOT appear together.

   Systems receiving measurements MUST be able to process the range of
   floating point numbers that are representable as an IEEE double-
   precision floating-point numbers [IEEE.754.1985].  The number of
   significant digits in any measurement is not relevant, so a reading
   of 1.1 has exactly the same semantic meaning as 1.10.  If the value
   has an exponent, the "e" MUST be in lower case.  The mantissa SHOULD
   be less than 19 characters long and the exponent SHOULD be less than
   5 characters long.  This allows time values to have better than micro
   second precision over the next 100 years.

6.1.  Examples

6.1.1.  Single Datapoint

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




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   [ {}, [{ "n": "urn:dev:ow:10e2073a01080063", "v":23.5 }] ]

6.1.2.  Multiple Datapoints

   The following example shows voltage and current now, i.e., at an
   unspecified time.  The device has an EUI-64 MAC address of
   0024befffe804ff1.

   [{"bn": "urn:dev:mac:0024befffe804ff1/"},
    [ { "n": "voltage", "t": 0, "u": "V", "v": 120.1 },
      { "n": "current", "t": 0, "u": "A", "v": 1.2 } ]
   ]

   The next example is similar to the above one, but shows current at
   Tue Jun 8 18:01:16 UTC 2010 and at each second for the previous 5
   seconds.

   [{"bn": "urn:dev:mac:0024befffe804ff1/",
     "bt": 1276020076,
     "bu": "A",
     "ver": 1},
    [ { "n": "voltage", "u": "V", "v": 120.1 },
      { "n": "current", "t": -5, "v": 1.2 },
      { "n": "current", "t": -4, "v": 1.30 },
      { "n": "current", "t": -3, "v": 0.14e1 },
      { "n": "current", "t": -2, "v": 1.5 },
      { "n": "current", "t": -1, "v": 1.6 },
      { "n": "current", "t": 0,  "v": 1.7 } ]
   ]

   Note that in some usage scenarios of SenML the implementations MAY
   store or transmit SenML in a stream-like fashion, where data is
   collected over time and continuously added to the object.  This mode
   of operation is optional, but systems or protocols using SenML in
   this fashion MUST specify that they are doing this.  In this
   situation the SenML stream can be sent and received in a partial
   fashion, i.e., a measurement entry can be read as soon as it is
   received and only not when the entire SenML object is complete.

   For instance, the following stream of measurements may be sent from
   the producer of a SenML object to the consumer of that SenML object,
   and each measurement object may be reported at the time it arrives:









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   [{"bn": "http://[2001:db8::1]",
     "bt": 1320067464,
     "bu": "%RH"},
    [ { "v": 21.2, "t": 0 },
      { "v": 21.3, "t": 10 },
      { "v": 21.4, "t": 20 },
      { "v": 21.4, "t": 30 },
      { "v": 21.5, "t": 40 },
      { "v": 21.5, "t": 50 },
      { "v": 21.5, "t": 60 },
      { "v": 21.6, "t": 70 },
      { "v": 21.7, "t": 80 },
      { "v": 21.5, "t": 90 },
   ...

6.1.3.  Multiple Measurements

   The following example shows humidity measurements from a mobile
   device with an IPv6 address 2001:db8::1, starting at Mon Oct 31
   13:24:24 UTC 2011.  The device also provides position data, which is
   provided in the same measurement or parameter array as separate
   entries.  Note time is used to for correlating data that belongs
   together, e.g., a measurement and a parameter associated with it.
   Finally, the device also reports extra data about its battery status
   at a separate time.

   [{"bn": "http://[2001:db8::1]",
     "bt": 1320067464,
     "bu": "%RH"},
    [ { "v": 20.0, "t": 0 },
      { "v": 24.30621, "u": "lon", "t": 0 },
      { "v": 60.07965, "u": "lat", "t": 0 },
      { "v": 20.3, "t": 60 },
      { "v": 24.30622, "u": "lon", "t": 60 },
      { "v": 60.07965, "u": "lat", "t": 60 },
      { "v": 20.7, "t": 120 },
      { "v": 24.30623, "u": "lon", "t": 120 },
      { "v": 60.07966, "u": "lat", "t": 120 },
      { "v": 98.0, "u": "%EL", "t": 150 },
      { "v": 21.2, "t": 180 },
      { "v": 24.30628, "u": "lon", "t": 180 },
      { "v": 60.07967, "u": "lat", "t": 180 } ]
   ]








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6.1.4.  Collection of Resources

   The following example shows how to query one device that can provide
   multiple measurements.  The example assumes that a client has fetched
   information from a device at 2001:db8::2 by performing a GET
   operation on http://[2001:db8::2] at Mon Oct 31 16:27:09 UTC 2011,
   and has gotten two separate values as a result, a temperature and
   humidity measurement.

   [{"bn": "http://[2001:db8::2]/",
     "bt": 1320078429,
     "ver": 1},
    [ { "n": "temperature", "v": 27.2, "u": "Cel" },
      { "n": "humidity", "v": 80, "u": "%RH" } ]
   ]

7.  CBOR Representation (application/senml+cbor)

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

   o  For compactness, the CBOR representation uses integers for the map
      keys defined in Table 1.  This table is conclusive, i.e., there is
      no intention to define any additional integer map keys; any
      extensions will use string map keys.

   o  For JSON Numbers, the CBOR representation can use integers,
      floating point numbers, or decimal fractions (CBOR Tag 4); the
      common limitations of JSON implementations are not relevant for
      these.  For the version number, however, only an unsigned integer
      is allowed.




















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                +---------------+------------+------------+
                |          Name | JSON label | CBOR label |
                +---------------+------------+------------+
                |       Version | ver        |         -1 |
                |     Base Name | bn         |         -2 |
                |     Base Time | bt         |         -3 |
                |    Base Units | bu         |         -4 |
                |          Name | n          |          0 |
                |         Units | u          |          1 |
                |         Value | v          |          2 |
                |  String Value | sv         |          3 |
                | Boolean Value | bv         |          4 |
                |     Value Sum | s          |          5 |
                |          Time | t          |          6 |
                |   Update Time | ut         |          7 |
                +---------------+------------+------------+

            Table 1: CBOR representation: integers for map keys

8.  XML Representation (application/senml+xml)

   A SenML object can also be represented in XML format as defined in
   this section.  The following example shows an XML example for the
   same sensor measurement as in Section 6.1.2.

   <?xml version="1.0" encoding="UTF-8"?>
   <senml xmlns="urn:ietf:params:xml:ns:senml"
          bn="urn:dev:mac:0024befffe804ff1/"
          bt="1276020076"
          ver="1" bu="A">
     <e n="voltage" u="V" v="120.1" />
     <e n="current" t="-5" v="1.2" />
     <e n="current" t="-4" v="1.30" />
     <e n="current" t="-3" v="0.14e1" />
     <e n="current" t="-2" v="1.5" />
     <e n="current" t="-1" v="1.6" />
     <e n="current" t="0" v="1.7" />
   </senml>

   The RelaxNG schema for the XML is:











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   default namespace = "urn:ietf:params:xml:ns:senml"
   namespace rng = "http://relaxng.org/ns/structure/1.0"

   e = element e {
     attribute n { xsd:string }?,
     attribute u { xsd:string }?,
     attribute v { xsd:float }?,
     attribute sv { xsd:string }?,
     attribute bv { xsd:boolean }?,
     attribute s { xsd:decimal }?,
     attribute t { xsd:int }?,
     attribute ut { xsd:int }?,
     p*
   }

   senml =
     element senml {
       attribute bn { xsd:string }?,
       attribute bt { xsd:int }?,
       attribute bu { xsd:string }?,
       attribute ver { xsd:int }?,
       e*
     }

   start = senml

9.  EXI Representation (application/senml-exi)

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

   The EXI header option MUST be included.  An EXI schemaID options MUST
   be set to the value of "a" indicating the scheme provided in this
   specification.  Future revisions to the schema can change this
   schemaID to allow for backwards compatibility.  When the data will be
   transported over CoAP or HTTP, an EXI Cookie SHOULD NOT be used as it
   simply makes things larger and is redundant to information provided
   in the Content-Type header.

   The following XSD Schema is generated from the RelaxNG and used for
   strict schema guided EXI processing.




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   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
              elementFormDefault="qualified"
              targetNamespace="urn:ietf:params:xml:ns:senml"
              xmlns:ns1="urn:ietf:params:xml:ns:senml">

     <xs:element name="e">
       <xs:complexType>
         <xs:attribute name="n" type="xs:string"/>
         <xs:attribute name="u" type="xs:string"/>
         <xs:attribute name="v" type="xs:float"/>
         <xs:attribute name="sv" type="xs:string"/>
         <xs:attribute name="bv" type="xs:boolean"/>
         <xs:attribute name="s" type="xs:decimal"/>
         <xs:attribute name="t" type="xs:int"/>
         <xs:attribute name="ut" type="xs:int"/>
       </xs:complexType>
     </xs:element>
     <xs:element name="senml">
       <xs:complexType>
         <xs:sequence>
           <xs:element minOccurs="0" maxOccurs="unbounded" ref="ns1:e"/>
         </xs:sequence>
         <xs:attribute name="bn" type="xs:string"/>
         <xs:attribute name="bt" type="xs:int"/>
         <xs:attribute name="bu" type="xs:string"/>
         <xs:attribute name="ver" type="xs:int"/>
       </xs:complexType>
     </xs:element>
   </xs:schema>

   The following shows a hexdump of the EXI produced from encoding the
   following XML example.  Note that while this example is similar to
   the first example in Section 6.1.2 in JSON format.

   <?xml version="1.0" encoding="UTF-8"?>
   <senml xmlns="urn:ietf:params:xml:ns:senml"
          bn="urn:dev:ow:10e2073a01080063" >
     <e n="voltage" t="0" v="120.1" u="V" />
     <e n="current" t="0" v="1.2" u="A" />
   </senml>

   Which compresses to the following displayed in hexdump:








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   00000000  a0 30 0d 85 01 d7 57 26  e3 a6 46 57 63 a6 f7 73
   00000010  a3 13 06 53 23 03 73 36  13 03 13 03 83 03 03 63
   00000020  36 21 2e cd ed 8e 8c 2c  ec a8 00 00 d5 95 88 4c
   00000030  02 08 4b 1b ab 93 93 2b  73 a2 00 00 34 14 19 00
   00000040  c0

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

   <?xml version="1.0" encoding="UTF-8"?>
   <senml xmlns="urn:ietf:params:xml:ns:senml"
          bn="urn:dev:ow:10e2073a01080063" >
     <e n="temp" v="23.1" u="Cel" />
   </senml>

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

   00000000  a054c9006c200000 1d75726e3a646576 |.T..l ...urn:dev|
   00000010  3a6f773a31306532 3037336130313038 |:ow:10e2073a0108|
   00000020  3030363304010674 656d70030543656c |0063...temp..Cel|
   00000030  0100e70101000102                  |........|

   A small temperature sensor devices that only generates this one EXI
   file does not really need an full EXI implementation.  It can simple
   hard code the output replacing the one wire device ID starting at
   byte 0x14 and going to byte 0x23 with it's device ID, and replacing
   the value "0xe7 0x01" at location 0x32 to 0x33 with the current
   temperature.  The EXI Specification [W3C.REC-exi-20110310] contains
   the full information on how floating point numbers are represented,
   but for the purpose of this sensor, the temperature can be converted
   to an integer in tenths of degrees (231 in this example).  EXI stores
   7 bits of the integer in each byte with the top bit set to one if
   there are further bytes.  So the first bytes at location 0x32 is set
   to low 7 bits of the integer temperature in tenths of degrees plus
   0x80.  In this example 231 & 0x7F + 0x80 = 0xE7.  The second byte at
   location 0x33 is set to the integer temperature in tenths of degrees
   right shifted 7 bits.  In this example 231 >> 7 = 0x01.

10.  Usage Considerations

   The measurements support sending both the current value of a sensor
   as well as the an integrated sum.  For many types of measurements,
   the sum is more useful than the current value.  For example, an



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

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

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

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

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

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

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

11.  IANA Considerations

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





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11.1.  Units Registry

   IANA will create a registry of unit symbols.  The primary purpose of
   this registry is to make sure that symbols uniquely map to give type
   of measurement.  Definitions for many of these units can be found in
   [NIST811] and [BIPM].

   All the registry entries in Table 2 use "v" (numeric) values.  New
   entries allocated in the registry must define what kind of values
   they use.

    +--------+--------------------------------------------+-----------+
    | Symbol | Description                                | Reference |
    +--------+--------------------------------------------+-----------+
    |      m | meter                                      | RFC-AAAA  |
    |     kg | kilogram                                   | RFC-AAAA  |
    |      s | second                                     | RFC-AAAA  |
    |      A | ampere                                     | RFC-AAAA  |
    |      K | kelvin                                     | RFC-AAAA  |
    |     cd | candela                                    | RFC-AAAA  |
    |    mol | mole                                       | RFC-AAAA  |
    |     Hz | hertz                                      | RFC-AAAA  |
    |    rad | radian                                     | RFC-AAAA  |
    |     sr | steradian                                  | RFC-AAAA  |
    |      N | newton                                     | RFC-AAAA  |
    |     Pa | pascal                                     | RFC-AAAA  |
    |      J | joule                                      | RFC-AAAA  |
    |      W | watt                                       | RFC-AAAA  |
    |      C | coulomb                                    | RFC-AAAA  |
    |      V | volt                                       | RFC-AAAA  |
    |      F | farad                                      | RFC-AAAA  |
    |    Ohm | ohm                                        | RFC-AAAA  |
    |      S | siemens                                    | RFC-AAAA  |
    |     Wb | weber                                      | RFC-AAAA  |
    |      T | tesla                                      | RFC-AAAA  |
    |      H | henry                                      | RFC-AAAA  |
    |    Cel | degrees Celsius                            | RFC-AAAA  |
    |     lm | lumen                                      | RFC-AAAA  |
    |     lx | lux                                        | RFC-AAAA  |
    |     Bq | becquerel                                  | RFC-AAAA  |
    |     Gy | gray                                       | RFC-AAAA  |
    |     Sv | sievert                                    | RFC-AAAA  |
    |    kat | katal                                      | RFC-AAAA  |
    |     pH | pH acidity                                 | RFC-AAAA  |
    |      % | Value of a switch (note 1)                 | RFC-AAAA  |
    |  count | counter value                              | RFC-AAAA  |
    |    %RH | Relative Humidity                          | RFC-AAAA  |
    |     m2 | area                                       | RFC-AAAA  |



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    |      l | volume in liters                           | RFC-AAAA  |
    |    m/s | velocity                                   | RFC-AAAA  |
    |   m/s2 | acceleration                               | RFC-AAAA  |
    |    l/s | flow rate in liters per second             | RFC-AAAA  |
    |   W/m2 | irradiance                                 | RFC-AAAA  |
    |  cd/m2 | luminance                                  | RFC-AAAA  |
    |   Bspl | bel sound pressure level                   | RFC-AAAA  |
    |  bit/s | bits per second                            | RFC-AAAA  |
    |    lat | degrees latitude (note 2)                  | RFC-AAAA  |
    |    lon | degrees longitude (note 2)                 | RFC-AAAA  |
    |    %EL | remaining battery energy level in percents | RFC-AAAA  |
    |     EL | remaining battery energy level in seconds  | RFC-AAAA  |
    | beat/m | Heart rate in beats per minute             | RFC-AAAA  |
    |  beats | Cumulative number of heart beats           | RFC-AAAA  |
    +--------+--------------------------------------------+-----------+

                                  Table 2

   o  Note 1: A value of 0.0 indicates the switch is off while 100.0
      indicates on.

   o  Note 2: Assumed to be in WGS84 unless another reference frame is
      known for the sensor.

   New entries can be added to the registration by either Expert Review
   or IESG Approval as defined in [RFC5226].  Experts should exercise
   their own good judgment but need to consider the following
   guidelines:

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

   2.   Units should define the semantic information and be chosen
        carefully.  Implementors need to remember that the same word may
        be used in different real-life contexts.  For example, degrees
        when measuring latitude have no semantic relation to degrees
        when measuring temperature; thus two different units are needed.

   3.   These measurements are produced by computers for consumption by
        computers.  The principle is that conversion has to be easily be
        done when both reading and writing the media type.  The value of
        a single canonical representation outweighs the convenience of
        easy human representations or loss of precision in a conversion.

   4.   Use of SI prefixes such as "k" before the unit is not allowed.
        Instead one can represent the value using scientific notation
        such a 1.2e3.




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

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

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

   8.   The following units should not be used as they are commonly used
        to represent other measurements Ky, Gal, dyn, etg, P, St, Mx, G,
        Oe, Gb, sb, Lmb, ph, Ci, R, RAD, REM, gal, bbl, qt, degF, Cal,
        BTU, HP, pH, B/s, psi, Torr, atm, at, bar, kWh.

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

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

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

11.2.  Media Type Registration

   The following registrations are done following the procedure
   specified in [RFC6838] and [RFC7303].

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

11.2.1.  senml+json Media Type Registration

   Type name: application

   Subtype name: senml+json

   Required parameters: none




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   Optional parameters: none

   Encoding considerations: Must be encoded as using a subset of the
   encoding allowed in [RFC7159].  Specifically, only the ASCII
   [RFC0020] subset of the UTF-8 characters are allowed.  This
   simplifies implementation of very simple system and does not impose
   any significant limitations as all this data is meant for machine to
   machine communications and is not meant to be human readable.

   Security considerations: Sensor data can contain a wide range of
   information ranging from information that is very public, such the
   outside temperature in a given city, to very private information that
   requires integrity and confidentiality protection, such as patient
   health information.  This format does not provide any security and
   instead relies on the transport protocol that carries it to provide
   security.  Given applications need to look at the overall context of
   how this media type will be used to decide if the security is
   adequate.

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

   Published specification: RFC-AAAA

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

   Additional information:

   Magic number(s): none

   File extension(s): senml

   Macintosh file type code(s): none

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

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>



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   Change controller: IESG

11.2.2.  senml+cbor Media Type Registration

   Type name: application

   Subtype name: senml+cbor

   Required parameters: none

   Optional parameters: none

   Encoding considerations: TBD

   Security considerations: TBD

   Interoperability considerations: TBD

   Published specification: RFC-AAAA

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

   Additional information:

   Magic number(s): none

   File extension(s): senml

   Macintosh file type code(s): none

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

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG








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11.2.3.  senml+xml Media Type Registration

   Type name: application

   Subtype name: senml+xml

   Required parameters: none

   Optional parameters: none

   Encoding considerations: TBD

   Security considerations: TBD

   Interoperability considerations: TBD

   Published specification: RFC-AAAA

   Applications that use this media type: TBD

   Additional information:

   Magic number(s): none

   File extension(s): senml

   Macintosh file type code(s): none

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

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

11.2.4.  senml-exi Media Type Registration

   Type name: application

   Subtype name: senml-exi

   Required parameters: none

   Optional parameters: none



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   Encoding considerations: TBD

   Security considerations: TBD

   Interoperability considerations: TBD

   Published specification: RFC-AAAA

   Applications that use this media type: TBD

   Additional information:

   Magic number(s): none

   File extension(s): senml

   Macintosh file type code(s): none

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

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Cullen Jennings <fluffy@iii.ca>

   Change controller: IESG

11.3.  XML Namespace Registration

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

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

   Registrant Contact: The IESG.

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

11.4.  CoAP Content-Format Registration

   IANA is requested to assign CoAP Content-Format IDs for the SenML
   media types in the "CoAP Content-Formats" sub-registry, within the
   "CoRE Parameters" registry [RFC7252].  All IDs are assigned from the
   "Expert Review" (0-255) range.  The assigned IDs are show in Table 3.





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

                     Table 3: CoAP Content-Format IDs

12.  Security Considerations

   See Section 13.  Further discussion of security properties can be
   found in Section 11.2.

13.  Privacy Considerations

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

14.  Acknowledgement

   We would like to thank Lisa Dusseault, Joe Hildebrand, Lyndsay
   Campbell, Martin Thomson, John Klensin, Bjoern Hoehrmann, Carsten
   Bormann, and Christian Amsuess for their review comments.

   The CBOR Representation text was contributed by Carsten Bormann.

15.  References

15.1.  Normative References

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

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






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

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

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

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

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

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

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

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

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

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <http://www.rfc-editor.org/info/rfc7396>.








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   [W3C.REC-exi-20110310]
              Schneider, J. and T. Kamiya, "Efficient XML Interchange
              (EXI) Format 1.0", World Wide Web Consortium
              Recommendation REC-exi-20110310, March 2011,
              <http://www.w3.org/TR/2011/REC-exi-20110310>.

15.2.  Informative References

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

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.

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

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

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

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

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

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

   [WADL]     Hadley, M., "Web Application Description Language (WADL)",
              2009,
              <http://java.net/projects/wadl/sources/svn/content/trunk/
              www/wadl20090202.pdf>.



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Authors' Addresses

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

   Phone: +1 408 421-9990
   Email: fluffy@cisco.com


   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   USA

   Phone: +1-408-203-9434
   Email: zach.shelby@arm.com


   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@piuha.net


   Ari Keranen
   Ericsson
   Jorvas  02420
   Finland

   Email: ari.keranen@ericsson.com















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