Conditional Attributes for Constrained RESTful Environments
draft-ietf-core-conditional-attributes-05

CoRE Working Group                                             M. Koster
Internet-Draft                                             Dogtiger Labs
Intended status: Informational                                A. Soloway
Expires: 27 April 2023                       Qualcomm Technologies, Inc.
                                                      B. Silverajan, Ed.
                                                      Tampere University
                                                         24 October 2022

      Conditional Attributes for Constrained RESTful Environments
               draft-ietf-core-conditional-attributes-05

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 https://github.com/core-
   wg/conditional-attributes/

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
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   This Internet-Draft will expire on 27 April 2023.

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   Copyright (c) 2022 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/
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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised 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 (c.gt) . . . . . . . . . . . . . . . . .   4
       3.1.2.  Less Than (c.lt)  . . . . . . . . . . . . . . . . . .   4
       3.1.3.  Change Step (c.st)  . . . . . . . . . . . . . . . . .   5
       3.1.4.  Notification Band (c.band)  . . . . . . . . . . . . .   5
       3.1.5.  Edge (c.edge) . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Conditional Control Attributes  . . . . . . . . . . . . .   7
       3.2.1.  Minimum Period (c.pmin) . . . . . . . . . . . . . . .   7
       3.2.2.  Maximum Period (c.pmax) . . . . . . . . . . . . . . .   8
       3.2.3.  Minimum Evaluation Period (c.epmin) . . . . . . . . .   8
       3.2.4.  Maximum Evaluation Period (c.epmax) . . . . . . . . .   8
       3.2.5.  Confirmable Notification (c.con)  . . . . . . . . . .   8
     3.3.  Server processing of Conditional Attributes . . . . . . .   9
   4.  Implementation Considerations . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Minimum Period (c.pmin) example . . . . . . . . . . . . .  13
     A.2.  Maximum Period (c.pmax) example . . . . . . . . . . . . .  14
     A.3.  Greater Than (c.gt) example . . . . . . . . . . . . . . .  16
     A.4.  Greater Than (c.gt) and Period Max (c.pmax) example . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   IETF Standards for machine-to-machine communication in constrained
   environments describe the Constrained Application Protocol (CoAP)
   [RFC7252], a RESTful application protocol, as well as 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 CoAP 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 [RFC7252] and [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 for use with CoRE
   Observe [RFC7641].  Conditional attributes provide fine-grained
   control of notification and synchronization of resource states.  When
   observing a resource, a CoAP client conveys conditional attributes as
   metadata using the query component of a CoAP URI.  A conditional
   attribute can be represented as a "name=value" query parameter or
   simply a "name" without a value.  Multiple conditional attributes in
   a query component are separated with an ampersand "&".  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.

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   The set of Conditional Notification Attributes defined here allows a
   client to control how often a notification is received and how much a
   representation state should change in order to trigger a
   notification.  One or more Conditional Notification Attributes MAY be
   included in an Observe request.

   Conditional Notification Attributes are defined below:

             +===================+========+=================+
             | Attribute         | Name   | Value           |
             +===================+========+=================+
             | Greater Than      | c.gt   | xs:decimal      |
             +-------------------+--------+-----------------+
             | Less Than         | c.lt   | xs:decimal      |
             +-------------------+--------+-----------------+
             | Change Step       | c.st   | xs:decimal (>0) |
             +-------------------+--------+-----------------+
             | Notification Band | c.band | (none)          |
             +-------------------+--------+-----------------+
             | Edge              | c.edge | xs:boolean      |
             +-------------------+--------+-----------------+

               Table 1: Conditional Notification Attributes

3.1.1.  Greater Than (c.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
   for "c.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 "c.gt".  If the value
   drops below the upper limit value then a notification is sent,
   subject again to the "c.pmin" time.

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

3.1.2.  Less Than (c.lt)

   When present, Less Than indicates the lower limit value the resource
   value SHOULD cross before triggering a notification.  A notification
   is sent whenever the sampled value crosses the specified lower limit
   value, relative to the last reported value, and the time for "c.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 "c.lt".  If the value rises above the

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   lower limit value then a new notification is sent, subject to the
   "c.pmin" time.

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

3.1.3.  Change Step (c.st)

   When present, Change step indicates how much the value representing a
   resource state SHOULD change before triggering a notification,
   compared to the previous resource state.  Upon reception of a query
   including the "c.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 "c.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 resources with a
   scalar numeric value.

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

3.1.4.  Notification Band (c.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 result
   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 (c.lt),
   or Greater Than (c.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.

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   The "c.band" attribute works as a modifier to the behaviour of "c.gt"
   and "c.lt".  Its use is determined only by its presence, and not its
   value.  Therefore, if "c.band" is present in a query, "c.gt", "c.lt"
   or both, MUST be included.

   When "c.band" is present with "c.lt" but without "c.gt", the lower
   bound for the notification band (notification band minimum) is
   defined.  Notifications occur when the resource value is equal to or
   above the notification band minimum.  No maximum values exist for the
   band.

   When "c.band" is present with "c.gt" but without "c.lt", the upper
   bound for the notification band (notification band maximum) is
   defined.  Notifications occur when the resource value is equal to or
   below the notification band maximum.  No minimum values exist for the
   band.

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

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

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

3.1.5.  Edge (c.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
   "c.edge" attribute is 0 (False), the server notifies the client each
   time a resource state changes from True to False.  When the value of
   the "c.edge" attribute is 1 (True), the server notifies the client
   each time a resource state changes from False to True.

   The "c.edge" attribute can only be supported on resources with a
   boolean value.

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3.2.  Conditional Control Attributes

   Conditional Control Attributes define the time intervals between
   consecutive notifications as well as the cadence of the evaluation of
   the conditions that trigger a notification.  Conditional Control
   Attributes can be used to configure the internal server-driven
   sampling process for performing evaluations of the conditions of a
   resource.  One or more Conditional Control Attributes MAY be included
   in an Observe request.

   Conditional Control Attributes are defined below:

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

                  Table 2: Conditional Control Attributes

3.2.1.  Minimum Period (c.pmin)

   When present, 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.  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 occurred during the "c.pmin" interval, after the "c.pmin"
   interval expires.

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

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3.2.2.  Maximum Period (c.pmax)

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

3.2.3.  Minimum Evaluation Period (c.epmin)

   When present, Minimum Evaluation Period indicates the minimum time,
   in seconds, the client recommends to the server to wait between two
   consecutive evaluations of the conditions of a resource, since the
   client has no interest in the server doing more frequent evaluations.
   When the value of Minimum Evaluation Period expires after the
   previous evaluation, the server MAY immediately perform a new
   evaluation.  In the absence of this parameter, the minimum evaluation
   period is not defined and thus not used by the server.  The server
   MAY use "c.pmin", if defined, as a guidance on the desired evaluation
   cadence.  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 (c.epmax)

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

3.2.5.  Confirmable Notification (c.con)

   When present with a value of 1 (True), Confirmable Notification
   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
   (False), Confirmable Notification indicates a notification can be
   confirmable or non-confirmable, i.e., it can be sent in a confirmable
   or a non-confirmable CoAP message.

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

 bool notifiable( Resource * r ) {

   #define EDGE EXISTS(r->edge)
   #define BAND EXISTS(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 VB_EDGE ( r->vb && r->edge || !r->vb && !r->edge )
   #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 ?
       ( ( !EDGE && VB_CHANGE ) ||
         ( EDGE && VB_CHANGE && VB_EDGE ) ||
         PMAX_EX )
     : false )
   );
 }

      Figure 1: Code logic for conditional notification attribute
                              interactions

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4.  Implementation Considerations

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

   The use of the notification band minimum and maximum allows for a
   synchronization whenever a change in the resource value occurs.
   Theoretically this could occur in-line with the server internal
   sample period or as defined by the "c.epmin" and "c.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 "c.epmin" and "c.epmax" values, the server
   MAY evaluate all observations when performing the measurement of any
   one observation.

   This specification defines conditional attributes that can be used
   with CoAP Observe relationships between CoAP clients and CoAP
   servers.  However, it is recognised that the presence of one or more
   proxies between a client and a server can interfere with clients
   receiving resource updates, if a proxy does not supply resource
   representations when the value remains unchanged (e.g. if "c.pmax" is
   set, and the server sends multiple updates when the resource state
   contains the same value).  A server SHOULD use the Max-Age option to
   mitigate this by setting Max-Age to be less than or equal to
   "c.pmax".

5.  Security Considerations

   The security considerations in Section 11 of [RFC7252] apply.
   Additionally, the security considerations in Section 7 of [RFC7641]
   also apply.

6.  IANA Considerations

   This specification requests a new Conditional Attributes registry to
   ensure attributes map uniquely to parameter names.

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   Note to IANA: Please replace "RFC XXXX" with the assigned RFC number
   in the table below.

     +====================+===========+=================+===========+
     | Attribute          | Parameter | Value           | Reference |
     +====================+===========+=================+===========+
     | Minimum Period (s) | c.pmin    | xs:decimal (>0) | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Maximum Period (s) | c.pmax    | xs:decimal (>0) | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Minimum Evaluation | c.epmin   | xs:decimal (>0) | RFC XXXX  |
     | Period (s)         |           |                 |           |
     +--------------------+-----------+-----------------+-----------+
     | Maximum Evaluation | c.epmax   | xs:decimal (>0) | RFC XXXX  |
     | Period (s)         |           |                 |           |
     +--------------------+-----------+-----------------+-----------+
     | Confirmable        | c.con     | xs:boolean      | RFC XXXX  |
     | Notification       |           |                 |           |
     +--------------------+-----------+-----------------+-----------+
     | Greater Than       | c.gt      | xs:decimal      | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Less Than          | c.lt      | xs:decimal      | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Change Step        | c.st      | xs:decimal (>0) | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Notification Band  | c.band    | (none)          | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+
     | Edge               | c.edge    | xs:boolean      | RFC XXXX  |
     +--------------------+-----------+-----------------+-----------+

                                 Table 3

7.  Acknowledgements

   Hannes Tschofenig and Mert Ocak highlighted syntactical corrections
   in the usage of pmax and pmin in a query.  David Navarro proposed
   allowing for pmax to be equal to pmin.  Marco Tiloca provided an
   extensive review.

8.  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

9.  Changelog

   draft-ietf-core-conditional-attributes-05

   *  Multiple (mostly editorial) clarifications and updates based on
      review comments on mailing list from Marco Tiloca.

   draft-ietf-core-conditional-attributes-04

   *  Reference code updated to include behaviour for edge attribute.

   draft-ietf-core-conditional-attributes-03

   *  Attribute names updated to create uniqueness for use as
      conditional observe attributes.

   draft-ietf-core-conditional-attributes-02

   *  Clarifications on usage and value of the band parameter

   *  Implementation considerations for proxies added

   *  Security considerations added

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   *  IANA considerations added

   draft-ietf-core-conditional-attributes-01

   *  Clarifications on True and False values for Edge and Con
      Attributes

   *  Alan Soloway added as author

   draft-ietf-core-conditional-attributes-00

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

10.  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>.

   [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>.

   [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>.

Appendix A.  Examples

   This appendix provides some examples of the use of Conditional
   Attributes.

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

A.1.  Minimum Period (c.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: c.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 c.pmin time
                                  expires.

A.2.  Maximum Period (c.pmax) 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: c.pmax="20"
       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 |     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                 |      |

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

A.3.  Greater Than (c.gt) 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: c.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 (c.gt) and Period Max (c.pmax) 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: c.pmax=20;c.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                 |      |
   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                 |      |

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

Authors' Addresses

   Michael Koster
   Dogtiger Labs
   524 H Street
   Antioch, CA,  94509
   United States of America
   Email: michaeljohnkoster@gmail.com

   Alan Soloway
   Qualcomm Technologies, Inc.
   5775 Morehouse Drive
   San Diego,  92121
   United States of America
   Email: asoloway@qti.qualcomm.com

   Bilhanan Silverajan (editor)
   Tampere University
   Kalevantie 4
   FI-33100 Tampere
   Finland
   Email: bilhanan.silverajan@tuni.fi

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