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CoRE Working Group                                             C. Groves
Internet-Draft                                                   W. Yang
Intended status: Standards Track                                  Huawei
Expires: April 20, 2017                                 October 17, 2016


                    SenML Base Time Offset Attribute
                     draft-groves-core-senml-bto-00

Abstract

   SenML [I-D.ietf-core-senml] defines a base time attribute and time
   value which is used to determine the time when a value is recorded.
   In some applications a SenML package will contain a series of records
   related to a constant sample time interval, e.g. once every 60
   seconds.  This means that the time attribute will be required for
   each record.  This document defines a new "time offset" base
   attribute that allows a sender to include the time for the sample
   interval between records.  If the "time offset" base attribute is
   used the sender will not send the time attribute for each record,
   minimising message and storage size.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 20, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  SenML Structure and Semantics . . . . . . . . . . . . . . . .   4
     3.1.  Base Attributes . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Regular Attributes  . . . . . . . . . . . . . . . . . . .   4
     3.3.  Considerations  . . . . . . . . . . . . . . . . . . . . .   4
   4.  JSON Representation (application/senml+json)  . . . . . . . .   5
   5.  CBOR Representation (application/senml+cbor)  . . . . . . . .   5
   6.  XML representation (application/senml+xml)  . . . . . . . . .   5
   7.  EXI Representation (application/senml-exi)  . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   SenML currently defines the base time "bt" and time "t" attributes to
   indicate the time that each value in a SenML record is recorded.
   This means that for each record (value "v") that a "t" attribute is
   required.  The example from section 5.1.2/[I-D.ietf-core-senml] is
   copied below to illustrate a SenML package with multiple records.




















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   [ {"bn": "urn:dev:ow:10e2073a01080063",
         "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 },
      ...


               Figure 1: SenML Package with multiple records

   As can be seen in the example above there is a fixed offset between
   the data points of 10 seconds.

   This document proposes a base time offset "bto" attribute that is
   used to indicate the offset between datapoints when a fixed offset is
   used.  This negates the need to include the "t" attribute for each
   data point.  The example below takes the above example and applies
   the base time offset to it.

   [ {"bn": "urn:dev:ow:10e2073a01080063",
         "bt": 1320067464,
         "bto": 10,
         "bu": "%RH",
         "v": 21.2},
         { "v": 21.3},
         { "v": 21.4},
         { "v": 21.4},
         { "v": 21.5},
         { "v": 21.5},
         { "v": 21.5},
         { "v": 21.6},
         { "v": 21.7},
         { "v": 21.5},
      ...

   Figure 2: SenML Package with multiple records using base time offset

   It can be seen that it results in a record and overall pack size
   reduction.  The receiver of the record would use the base time "bt"
   for the initial data point, e.g. "v": 21.2 would have a timestamp of



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   1320067464.  Each subsequent record would have a time stamp equal to
   the previous record plus the base time offset "bto", e.g. "v": 21.3
   would have a timestamp of 1320067474, "v": 21.4 would have a
   timestamp of 1320067484, and so on.

2.  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].

   See [I-D.ietf-core-senml] for further definitions.

3.  SenML Structure and Semantics

3.1.  Base Attributes

   This document adds an additional "base time offset" attribute.

   Base Time Offset: The base time offset represents a fixed time offset
   between each record in a SenML pack.  The first record represents
   time zero (or the base time if provided) and the offset is then
   applied to each subsequent record.  Either a positive or negative
   base time offset is allowed.

3.2.  Regular Attributes

   This document modifies the behaviour of the time attribute.

   Time: If the base time offset attribute is used in a SenML pack then
   the time attribute shall not be used in the individual records.

   _Editor's note: One theoretical use case may be to include the time
   attribute if the entry's time deviates from the regular offset.  It
   is not seen that such a corner case warrants the extra complexity._

3.3.  Considerations

   To simplify implementation, the use of base time offset is limited
   to:

   o  Fixed time increasing or decreasing records from the time stamp
      associated with the first record in the SenML pack.  This means
      that usages such as described by the 2nd example in
      5.1.2/[I-D.ietf-core-senml] where negative time offset from time
      zero being provided by the last record in a SenML pack are not
      possible.



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   _Editor's note: It could be possible to facilitate this use case by
   allowing a t=0 to be set on the last record with the use of a
   negative base time offset value.  However again it's not seen that
   such a corner case warrants the extra complexity in having to store
   values and calculate offsets at the end of transmission/reception._

   o  It is not possible to for two records in the SenML pack to have
      the same time.  For example the inclusion of a record for a
      voltage measurement followed by a current measurement would result
      in the records having times with a difference of the time offset.

4.  JSON Representation (application/senml+json)

   This document defines an additional SenML label (JSON object member
   name) as shown in Table 1 below.

                   +------------------+-------+-------+
                   | Name             | label | Type  |
                   +------------------+-------+-------+
                   | Base Time Offset | bto   | Numer |
                   +------------------+-------+-------+

                        Table 1: JSON SenML Labels

5.  CBOR Representation (application/senml+cbor)

   As per section 6/[I-D.ietf-core-senml], for CBOR the string map key
   "bto" shall be used to indicate the use of the base time offset.

6.  XML representation (application/senml+xml)

   This document defines an addition XML attribute as shown in Table 2
   below.

                    +------------------+-----+--------+
                    | Name             | XML | Type   |
                    +------------------+-----+--------+
                    | Base Time Offset | bto | double |
                    +------------------+-----+--------+

                         Table 2: XML SenML Labels

   The RelaxNG schema for the XML is:








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     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 bto { xsd:double }?,

        attribute l { xsd:string }?,

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

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

      sensml =
        element sensml {
          senml+
      }

      start = sensml

7.  EXI Representation (application/senml-exi)

   As per clause 8/[I-D.ietf-core-senml] extensions are indicated
   through the use of an EXI schemaID options.  The EXI schemaID options
   MUST be set to the value of "b" indicating the scheme provided in
   this specification.

   The following is the XSD Schema to be 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="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="bu" type="xs:string" />
            <xs:attribute name="bver" type="xs:int" />
            <xs:attribute name="bto" type="xs:double" />
            <xs:attribute name="l" type="xs:string" />
            <xs:attribute name="n" type="xs:string" />
            <xs:attribute name="s" type="xs:double" />
            <xs:attribute name="t" type="xs:double" />
            <xs:attribute name="u" type="xs:string" />
            <xs:attribute name="ut" type="xs:double" />
            <xs:attribute name="v" type="xs:double" />
            <xs:attribute name="vb" type="xs:boolean" />
            <xs:attribute name="vs" type="xs:string" />
            <xs:attribute name="vd" type="xs:string" />
          </xs:complexType>
        </xs:element>
        <xs:element name="sensml">
          <xs:complexType>
            <xs:sequence>
              <xs:element maxOccurs="unbounded" ref="ns1:senml" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:schema>

8.  Security Considerations

   No extra security issues are seen beyond those described in
   [I-D.ietf-core-senml].

9.  IANA Considerations

   This document proposes one new entry in the IANA SenML label
   reegistry.

   Label Name: Base Time Offset

   Label: bto




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   CBOR: -

   XML Type: Double

   ID: b

   Reference: This specification.

10.  Acknowledgements

   TBD

11.  Normative References

   [I-D.ietf-core-senml]
              Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
              Bormann, "Media Types for Sensor Markup Language (SenML)",
              draft-ietf-core-senml-03 (work in progress), October 2016.

   [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>.

Authors' Addresses

   Christian Groves
   Huawei
   Australia

   Email: Christian.Groves@nteczone.com


   Weiwei Yang
   Huawei
   P.R.China

   Email: tommy@huawei.com













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