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Versions: 00                                                            
CoRE Working Group                                             M. Koster
Internet-Draft                                               SmartThings
Intended status: Informational                        B. Silverajan, Ed.
Expires: January 13, 2022                             Tampere University
                                                           July 12, 2021


      Conditional Attributes for Constrained RESTful Environments
               draft-ietf-core-conditional-attributes-00

Abstract

   This specification defines Conditional Notification and Control
   Attributes that work with CoAP Observe (RFC7641).

Editor note

   The git repository for the draft is found at TBD

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 https://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 January 13, 2022.

Copyright Notice

   Copyright (c) 2021 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Conditional Attributes  . . . . . . . . . . . . . . . . . . .   3
     3.1.  Conditional Notification Attributes . . . . . . . . . . .   3
       3.1.1.  Greater Than (gt) . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Less Than (lt)  . . . . . . . . . . . . . . . . . . .   4
       3.1.3.  Change Step (st)  . . . . . . . . . . . . . . . . . .   5
       3.1.4.  Notification Band (band)  . . . . . . . . . . . . . .   5
       3.1.5.  Edge (edge) . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Conditional Control Attributes  . . . . . . . . . . . . .   6
       3.2.1.  Minimum Period (pmin) . . . . . . . . . . . . . . . .   7
       3.2.2.  Maximum Period (pmax) . . . . . . . . . . . . . . . .   7
       3.2.3.  Minimum Evaluation Period (epmin) . . . . . . . . . .   8
       3.2.4.  Maximum Evaluation Period (epmax) . . . . . . . . . .   8
       3.2.5.  Confirmable Notification (con)  . . . . . . . . . . .   8
     3.3.  Server processing of Conditional Attributes . . . . . . .   8
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  Minimum Period (pmin) example . . . . . . . . . . . . . .  12
     A.2.  Maximum Period (pmax) example . . . . . . . . . . . . . .  13
     A.3.  Greater Than (gt) example . . . . . . . . . . . . . . . .  14
     A.4.  Greater Than (gt) and Period Max (pmax) example . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   IETF Standards for machine to machine communication in constrained
   environments describe a REST protocol [RFC7252] and a set of related
   information standards that may be used to represent machine data and
   machine metadata in REST interfaces.

   This specification defines Conditional Notification and Control
   Attributes for use with CoRE Observe [RFC7641].






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

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC7641].  This specification
   makes use of the following additional terminology:

   Notification Band:  A resource value range that may be bounded by a
      minimum and maximum value or may be unbounded having either a
      minimum or maximum value.

3.  Conditional Attributes

   This specification defines conditional attributes, which provide for
   fine-grained control of notification and state synchronization when
   using CoRE Observe [RFC7641].  When resource interfaces following
   this specification are made available over CoAP, the CoAP Observation
   mechanism [RFC7641] MAY also be used to observe any changes in a
   resource, and receive asynchronous notifications as a result.  A
   resource marked as Observable in its link description SHOULD support
   these conditional attributes.

   Note: In this draft, we assume that there are finite quantization
   effects in the internal or external updates to the value representing
   the state of a resource; specifically, that a resource state may be
   updated at any time with any valid value.  We therefore avoid any
   continuous-time assumptions in the description of the conditional
   attributes and instead use the phrase "sampled value" to refer to a
   member of a sequence of values that may be internally observed from
   the resource state over time.

3.1.  Conditional Notification Attributes

   Conditional Notification Attributes define the conditions that
   trigger a notification.  Conditional Notification Attributes SHOULD
   be evaluated on all potential notifications from a resource, whether
   resulting from an internal server-driven sampling process or from
   external update requests to the server.

   The set of Conditional Notification Attributes defined here allow a
   client to control how often a client is interested in receiving
   notifications and how much a value should change for the new
   representation state to be interesting.  One or more Conditional



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   Notification Attributes MAY be included as query parameters in an
   Observe request.

   Conditional Notification Attributes are defined below:

            +-------------------+-----------+-----------------+
            | Attribute         | Parameter | Value           |
            +-------------------+-----------+-----------------+
            | Greater Than      | gt        | xs:decimal      |
            |                   |           |                 |
            | Less Than         | lt        | xs:decimal      |
            |                   |           |                 |
            | Change Step       | st        | xs:decimal (>0) |
            |                   |           |                 |
            | Notification Band | band      | xs:boolean      |
            |                   |           |                 |
            | Edge              | edge      | xs:boolean      |
            +-------------------+-----------+-----------------+

               Table 1: Conditional Notification Attributes

3.1.1.  Greater Than (gt)

   When present, Greater Than indicates the upper limit value the
   sampled value SHOULD cross before triggering a notification.  A
   notification is sent whenever the sampled value crosses the specified
   upper limit value, relative to the last reported value, and the time
   fpr pmin has elapsed since the last notification.  The sampled value
   is sent in the notification.  If the value continues to rise, no
   notifications are generated as a result of gt.  If the value drops
   below the upper limit value then a notification is sent, subject
   again to the pmin time.

   The Greater Than parameter can only be supported on resources with a
   scalar numeric value.

3.1.2.  Less Than (lt)

   When present, Less Than indicates the lower limit value the resource
   value SHOULD cross before triggering a notification.  A notification
   is sent when the samples value crosses the specified lower limit
   value, relative to the last reported value, and the time fpr pmin has
   elapsed since the last notification.  The sampled value is sent in
   the notification.  If the value continues to fall no notifications
   are generated as a result of lt.  If the value rises above the lower
   limit value then a new notification is sent, subject to the pmin
   time.




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   The Less Than parameter can only be supported on resources with a
   scalar numeric value.

3.1.3.  Change Step (st)

   When present, the change step indicates how much the value
   representing a resource state SHOULD change before triggering a
   notification, compared to the old state.  Upon reception of a query
   including the st attribute, the current resource state representing
   the most recently sampled value is reported, and then set as the last
   reported value (last_rep_v).  When a subsequent sampled value or
   update of the resource state differs from the last reported state by
   an amount, positive or negative, greater than or equal to st, and the
   time for pmin has elapsed since the last notification, a notification
   is sent and the last reported value is updated to the new resource
   state sent in the notification.  The change step MUST be greater than
   zero otherwise the receiver MUST return a CoAP error code 4.00 "Bad
   Request" (or equivalent).

   The Change Step parameter can only be supported on resource states
   represented with a scalar numeric value.

   Note: Due to sampling and other constraints, e.g. pmin, the change in
   resource states received in two sequential notifications may differ
   by more than st.

3.1.4.  Notification Band (band)

   The notification band attribute allows a bounded or unbounded (based
   on a minimum or maximum) value range that may trigger multiple
   notifications.  This enables use cases where different ranges results
   in differing behaviour.  For example, in monitoring the temperature
   of machinery, whilst the temperature is in the normal operating
   range, only periodic updates are needed.  However as the temperature
   moves to more abnormal ranges more frequent state updates may be sent
   to clients.

   Without a notification band, a transition across a less than (lt), or
   greater than (gt) limit only generates one notification.  This means
   that it is not possible to describe a case where multiple
   notifications are sent so long as the limit is exceeded.

   The band attribute works as a modifier to the behaviour of gt and lt.
   Therefore, if band is present in a query, gt, lt or both, MUST be
   included.

   When band is present with the lt attribute, it defines the lower
   bound for the notification band (notification band minimum).



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   Notifications occur when the resource value is equal to or above the
   notification band minimum.  If lt is not present there is no minimum
   value for the band.

   When band is present with the gt attribute, it defines the upper
   bound for the notification band (notification band maximum).
   Notifications occur when the resource value is equal to or below the
   notification band maximum.  If gt is not present there is no maximum
   value for the band.

   If band is present with both the gt and lt attributes, notification
   occurs when the resource value is greater than or equal to gt or when
   the resource value is less than or equal to lt.

   If a band is specified in which the value of gt is less than that of
   lt, in-band notification occurs.  That is, notification occurs
   whenever the resource value is between the gt and lt values,
   including equal to gt or lt.

   If the band is specified in which the value of gt is greater than
   that of lt, out-of-band notification occurs.  That is, notification
   occurs when the resource value not between the gt and lt values,
   excluding equal to gt and lt.

   The Notification Band parameter can only be supported on resources
   with a scalar numeric value.

3.1.5.  Edge (edge)

   When present, the Edge attribute indicates interest for receiving
   notifications of either the falling edge or the rising edge
   transition of a boolean resource state.  When the value of the Edge
   attribute is 0, the server notifies the client each time a resource
   state changes from True to False.  When the value of the Edge
   attribute is 1, the server notifies the client each time a resource
   state changes from False to True.

   The Edge attribute can only be supported on resources with a boolean
   value.

3.2.  Conditional Control Attributes

   Conditional Control Attributes define the time intervals between
   consecutive notifications as well as the cadence of the measurement
   of the conditions that trigger a notification.  Conditional Control
   Attributes can be used to configure the internal server-driven
   sampling process for performing measurements of the conditions of a




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   resource.  One or more Conditional Control Attributes MAY be included
   as query parameters in an Observe request.

   Conditional Control Attributes are defined below:

      +-------------------------------+-----------+-----------------+
      | Attribute                     | Parameter | Value           |
      +-------------------------------+-----------+-----------------+
      | Minimum Period (s)            | pmin      | xs:decimal (>0) |
      |                               |           |                 |
      | Maximum Period (s)            | pmax      | xs:decimal (>0) |
      |                               |           |                 |
      | Minimum Evaluation Period (s) | epmin     | xs:decimal (>0) |
      |                               |           |                 |
      | Maximum Evaluation Period (s) | epmax     | xs:decimal (>0) |
      |                               |           |                 |
      | Confirmable Notification      | con       | xs:boolean      |
      +-------------------------------+-----------+-----------------+

                  Table 2: Conditional Control Attributes

3.2.1.  Minimum Period (pmin)

   When present, the minimum period indicates the minimum time, in
   seconds, between two consecutive notifications (whether or not the
   resource state has changed).  In the absence of this parameter, the
   minimum period is up to the server.  The minimum period MUST be
   greater than zero otherwise the receiver MUST return a CoAP error
   code 4.00 "Bad Request" (or equivalent).

   A server MAY update the resource state with the last sampled value
   that occured during the pmin interval, after the pmin interval
   expires.

   Note: Due to finite quantization effects, the time between
   notifications may be greater than pmin even when the sampled value
   changes within the pmin interval.  Pmin may or may not be used to
   drive the internal sampling process.

3.2.2.  Maximum Period (pmax)

   When present, the maximum period indicates the maximum time, in
   seconds, between two consecutive notifications (whether or not the
   resource state has changed).  In the absence of this parameter, the
   maximum period is up to the server.  The maximum period MUST be
   greater than zero and MUST be greater than, or equal to, the minimum
   period parameter (if present) otherwise the receiver MUST return a
   CoAP error code 4.00 "Bad Request" (or equivalent).



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3.2.3.  Minimum Evaluation Period (epmin)

   When present, the minimum evaluation period indicates the minimum
   time, in seconds, the client recommends to the server to wait between
   two consecutive measurements of the conditions of a resource since
   the client has no interest in the server doing more frequent
   measurements.  When the minimum evaluation period expires after the
   previous measurement, the server MAY immediately perform a new
   measurement.  In the absence of this parameter, the minimum
   evaluation period is not defined and thus not used by the server.
   The server MAY use pmin, if defined, as a guidance on the desired
   measurement cadence.  The minimum evaluation period MUST be greater
   than zero otherwise the receiver MUST return a CoAP error code 4.00
   "Bad Request" (or equivalent).

3.2.4.  Maximum Evaluation Period (epmax)

   When present, the maximum evaluation period indicates the maximum
   time, in seconds, the server MAY wait between two consecutive
   measurements of the conditions of a resource.  When the maximum
   evaluation period expires after the previous measurement, the server
   MUST immediately perform a new measurement.  In the absence of this
   parameter, the maximum evaluation period is not defined and thus not
   used by the server.  The maximum evaluation period MUST be greater
   than zero and MUST be greater than the minimum evaluation period
   parameter (if present) otherwise the receiver MUST return a CoAP
   error code 4.00 "Bad Request" (or equivalent).

3.2.5.  Confirmable Notification (con)

   When present with a value of 1 in a query, the con attribute
   indicates a notification MUST be confirmable, i.e., the server MUST
   send the notification in a confirmable CoAP message, to request an
   acknowledgement from the client.  When present with a value of 0 in a
   query, the con attribute indicates a notification can be confirmable
   or non-confirmable, i.e., it can be sent in a confirmable or a non-
   confirmable CoAP message.

3.3.  Server processing of Conditional Attributes

   Conditional Notification Attributes and Conditional Control
   Attributes may be present in the same query.  However, they are not
   defined at multiple prioritization levels.  The server sends a
   notification whenever any of the parameter conditions are met, upon
   which it updates its last notification value and time to prepare for
   the next notification.  Only one notification occurs when there are
   multiple conditions being met at the same time.  The reference code




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   below illustrates the logic to determine when a notification is to be
   sent.

bool notifiable( Resource * r ) {

#define BAND r->band
#define SCALAR_TYPE ( num_type == r->type )
#define STRING_TYPE ( str_type == r->type )
#define BOOLEAN_TYPE ( bool_type == r->type )
#define PMIN_EX ( r->last_sample_time - r->last_rep_time >= r->pmin )
#define PMAX_EX ( r->last_sample_time - r->last_rep_time > r->pmax )
#define LT_EX ( r->v < r->lt ^ r->last_rep_v < r->lt )
#define GT_EX ( r->v > r->gt ^ r->last_rep_v > r->gt )
#define ST_EX ( abs( r->v - r->last_rep_v ) >= r->st )
#define IN_BAND ( ( r->gt <= r->v && r->v <= r->lt ) || ( r->lt <= r->gt && r->gt <= r->v ) || ( r->v <= r->lt && r->lt <= r->gt ) )
#define VB_CHANGE ( r->vb != r->last_rep_vb )
#define VS_CHANGE ( r->vs != r->last_rep_vs )

  return (
    PMIN_EX &&
    ( SCALAR_TYPE ?
      ( ( !BAND && ( GT_EX || LT_EX || ST_EX || PMAX_EX ) ) ||
        ( BAND && IN_BAND && ( ST_EX || PMAX_EX) ) )
    : STRING_TYPE ?
      ( VS_CHANGE || PMAX_EX )
    : BOOLEAN_TYPE ?
      ( VB_CHANGE || PMAX_EX )
    : false )
  );
}

        Figure 1: Code logic for conditional notification attribute
                               interactions

4.  Implementation Considerations

   When pmax and pmin are equal, the expected behaviour is that
   notifications will be sent every (pmin == pmax) seconds.  However,
   these notifications can only be fulfilled by the server on a best
   effort basis.  Because pmin and pmax are designed as acceptable
   tolerance bounds for sending state updates, a query from an
   interested client containing equal pmin and pmax values must not be
   seen as a hard real-time scheduling contract between the client and
   the server.

   When using multiple resource bindings (e.g. multiple Observations of
   resource) with different bands, consideration should be given to the
   resolution of the resource value when setting sequential bands.  For



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   example: Given BandA (Abmn=10, Bbmx=20) and BandB (Bbmn=21, Bbmx=30).
   If the resource value returns an integer then notifications for
   values between and inclusive of 10 and 30 will be triggered.  Whereas
   if the resolution is to one decimal point (0.1) then notifications
   for values 20.1 to 20.9 will not be triggered.

   The use of the notification band minimum and maximum allow for a
   synchronization whenever a change in the resource value occurs.
   Theoretically this could occur in-line with the server internal
   sample period or the configuration of epmin and epmax values for
   determining the resource value.  Implementors SHOULD consider the
   resolution needed before updating the resource, e.g. updating the
   resource when a temperature sensor value changes by 0.001 degree
   versus 1 degree.

   When a server has multiple observations with different measurement
   cadences as defined by the epmin and epmax values, the server MAY
   evaluate all observations when performing the measurement of any one
   observation.

5.  Security Considerations

   TBD IANA Considerations ===================

   TBD

6.  Acknowledgements

   Hannes Tschofenig and Mert Ocak highlighted syntactical corrections
   in the usage of pmax and pmin in a query.  Alan Soloway contributed
   text leading to the inclusion of epmin and epmax.  David Navarro
   proposed allowing for pmax to be equal to pmin.

7.  Contributors

















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   Christian Groves
   Australia
   email: cngroves.std@gmail.com

   Zach Shelby
   ARM
   Vuokatti
   FINLAND
   phone: +358 40 7796297
   email: zach.shelby@arm.com

   Matthieu Vial
   Schneider-Electric
   Grenoble
   France
   phone: +33 (0)47657 6522
   eMail: matthieu.vial@schneider-electric.com

   Jintao Zhu
   Huawei
   Xi'an, Shaanxi Province
   China
   email: jintao.zhu@huawei.com

8.  Changelog

   draft-ietf-core-conditional-attributes-00

   o  Conditional Atttributes section from draft-ietf-core-dynlink-13
      separated into own WG draft

9.  References

9.1.  Normative References

   [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>.

   [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>.








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9.2.  Informative References

   [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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

Appendix A.  Examples

   This appendix provides some examples of the use of binding attribute
   / observe attributes.

   Note: For brevity the only the method or response code is shown in
   the header field.

A.1.  Minimum Period (pmin) example






























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           Observed   CLIENT  SERVER     Actual
       t   State         |      |         State
           ____________  |      |  ____________
       1                 |      |
       2    unknown      |      |     18.5 Cel
       3                 +----->|                  Header: GET
       4                 | GET  |                   Token: 0x4a
       5                 |      |                Uri-Path: temperature
       6                 |      |               Uri-Query: pmin="10"
       7                 |      |                 Observe: 0 (register)
       8                 |      |
       9   ____________  |<-----+                  Header: 2.05
      10                 | 2.05 |                   Token: 0x4a
      11    18.5 Cel     |      |                 Observe: 9
      12                 |      |                 Payload: "18.5 Cel"
      13                 |      |  ____________
      14                 |      |
      15                 |      |     23 Cel
      16                 |      |
      17                 |      |
      18                 |      |
      19                 |      |  ____________
      20   ____________  |<-----+                  Header: 2.05
      21                 | 2.05 |     26 Cel        Token: 0x4a
      22    26 Cel       |      |                 Observe: 20
      23                 |      |                 Payload: "26 Cel"
      24                 |      |
      25                 |      |

      Figure 2: Client registers and receives one notification of the
    current state and one of a new state state when pmin time expires.

A.2.  Maximum Period (pmax) example

           Observed   CLIENT  SERVER     Actual
       t   State         |      |         State
           ____________  |      |  ____________
       1                 |      |
       2    unknown      |      |     18.5 Cel
       3                 +----->|                  Header: GET
       4                 | GET  |                   Token: 0x4a
       5                 |      |                Uri-Path: temperature
       6                 |      |               Uri-Query: pmax="20"
       7                 |      |                 Observe: 0 (register)
       8                 |      |
       9   ____________  |<-----+                  Header: 2.05
      10                 | 2.05 |                   Token: 0x4a
      11    18.5 Cel     |      |                 Observe: 9



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      12                 |      |                 Payload: "18.5 Cel"
      13                 |      |
      14                 |      |
      15                 |      |  ____________
      16   ____________  |<-----+                  Header: 2.05
      17                 | 2.05 |     23 Cel        Token: 0x4a
      18    23 Cel       |      |                 Observe: 16
      19                 |      |                 Payload: "23 Cel"
      20                 |      |
      21                 |      |
      22                 |      |
      23                 |      |
      24                 |      |
      25                 |      |
      26                 |      |
      27                 |      |
      28                 |      |
      29                 |      |
      30                 |      |
      31                 |      |
      32                 |      |
      33                 |      |
      34                 |      |
      35                 |      |
      36                 |      |  ____________
      37   ____________  |<-----+                  Header: 2.05
      38                 | 2.05 |     23 Cel        Token: 0x4a
      39    23 Cel       |      |                 Observe: 37
      40                 |      |                 Payload: "23 Cel"
      41                 |      |
      42                 |      |

      Figure 3: Client registers and receives one notification of the
   current state, one of a new state and one of an unchanged state when
                            pmax time expires.

A.3.  Greater Than (gt) example














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        Observed   CLIENT  SERVER     Actual
    t   State         |      |         State
        ____________  |      |  ____________
    1                 |      |
    2    unknown      |      |     18.5 Cel
    3                 +----->|                  Header: GET
    4                 | GET  |                   Token: 0x4a
    5                 |      |                Uri-Path: temperature
    6                 |      |               Uri-Query: gt=25
    7                 |      |                 Observe: 0 (register)
    8                 |      |
    9   ____________  |<-----+                  Header: 2.05
   10                 | 2.05 |                   Token: 0x4a
   11    18.5 Cel     |      |                 Observe: 9
   12                 |      |                 Payload: "18.5 Cel"
   13                 |      |
   14                 |      |
   15                 |      |  ____________
   16   ____________  |<-----+                  Header: 2.05
   17                 | 2.05 |     26 Cel        Token: 0x4a
   18    26 Cel       |      |                 Observe: 16
   29                 |      |                 Payload: "26 Cel"
   20                 |      |
   21                 |      |

      Figure 4: Client registers and receives one notification of the
      current state and one of a new state when it passes through the
                       greater than threshold of 25.

A.4.  Greater Than (gt) and Period Max (pmax) example

        Observed   CLIENT  SERVER     Actual
    t   State         |      |         State
        ____________  |      |  ____________
    1                 |      |
    2    unknown      |      |     18.5 Cel
    3                 +----->|                  Header: GET
    4                 | GET  |                   Token: 0x4a
    5                 |      |                Uri-Path: temperature
    6                 |      |         Uri-Query: pmax=20;gt=25
    7                 |      |                 Observe: 0 (register)
    8                 |      |
    9   ____________  |<-----+                  Header: 2.05
   10                 | 2.05 |                   Token: 0x4a
   11    18.5 Cel     |      |                 Observe: 9
   12                 |      |                 Payload: "18.5 Cel"
   13                 |      |
   14                 |      |



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   15                 |      |
   16                 |      |
   17                 |      |
   18                 |      |
   19                 |      |
   20                 |      |
   21                 |      |
   22                 |      |
   23                 |      |
   24                 |      |
   25                 |      |
   26                 |      |
   27                 |      |
   28                 |      |
   29                 |      |  ____________
   30   ____________  |<-----+                  Header: 2.05
   31                 | 2.05 |     23 Cel        Token: 0x4a
   32    23 Cel       |      |                 Observe: 30
   33                 |      |                 Payload: "23 Cel"
   34                 |      |
   35                 |      |
   36                 |      |  ____________
   37   ____________  |<-----+                  Header: 2.05
   38                 | 2.05 |     26 Cel        Token: 0x4a
   39    26 Cel       |      |                 Observe: 37
   40                 |      |                 Payload: "26 Cel"
   41                 |      |
   42                 |      |

      Figure 5: Client registers and receives one notification of the
   current state, one when pmax time expires and one of a new state when
            it passes through the greater than threshold of 25.

Authors' Addresses

   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

   Email: michael.koster@smartthings.com









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   Bilhanan Silverajan (editor)
   Tampere University
   Kalevantie 4
   Tampere  FI-33100
   Finland

   Email: bilhanan.silverajan@tuni.fi












































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