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Versions: (draft-groves-core-dynlink) 00 01 02 03

CoRE Working Group                                             Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                   Z. Vial
Expires: September 14, 2017                           Schneider-Electric
                                                               M. Koster
                                                             SmartThings
                                                               C. Groves
                                                                  Huawei
                                                          March 13, 2017


     Dynamic Resource Linking for Constrained RESTful Environments
                       draft-ietf-core-dynlink-03

Abstract

   For CoAP [RFC7252] Dynamic linking of state updates between
   resources, either on an endpoint or between endpoints, is defined
   with the concept of Link Bindings.  This specification defines
   conditional observation attributes that work with Link Bindings or
   with CoAP Observe [RFC7641].

   Editor's note:

   o The git repository for the draft is found at https://github.com/
   core-wg/dynlink

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 September 14, 2017.








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Copyright Notice

   Copyright (c) 2017 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Link Bindings . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Binding Methods . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Polling . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Observe . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Push  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Link Relation . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Binding Attributes  . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Bind Method (bind)  . . . . . . . . . . . . . . . . .   6
       3.3.2.  Minimum Period (pmin) . . . . . . . . . . . . . . . .   6
       3.3.3.  Maximum Period (pmax) . . . . . . . . . . . . . . . .   7
       3.3.4.  Change Step (st)  . . . . . . . . . . . . . . . . . .   7
       3.3.5.  Greater Than (gth)  . . . . . . . . . . . . . . . . .   7
       3.3.6.  Less Than (lth) . . . . . . . . . . . . . . . . . . .   7
       3.3.7.  Attribute Interactions  . . . . . . . . . . . . . . .   8
   4.  Binding Table . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Binding Interface Description . . . . . . . . . . . . . .   8
     4.2.  Resource Observation Attributes . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Interface Description . . . . . . . . . . . . . . . . . .  11
     6.2.  Link Relations Type . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  14
     A.1.  Greater Than (gth) example  . . . . . . . . . . . . . . .  14
     A.2.  Greater Than (gth) and Period Max (pmax) example  . . . .  14



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   IETF Standards for machine to machine communication in constrained
   environments describe a REST protocol and a set of related
   information standards that may be used to represent machine data and
   machine metadata in REST interfaces.  CoRE Link-format is a standard
   for doing Web Linking [RFC5988] in constrained environments.

   This specification introduces the concept of a Link Binding, which
   defines a new link relation type to create a dynamic link between
   resources over which to exchange state updates.  Specifically, a Link
   Binding is a link for binding the state of 2 resources together such
   that updates to one are sent over the link to the other.  CoRE Link
   Format representations are used to configure, inspect, and maintain
   Link Bindings.  This specification additionally defines a set of
   conditional Observe Attributes for use with Link Bindings and with
   the standalone CoRE Observe [RFC7641] method.

2.  Terminology

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

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC5988] and [RFC6690].  This
   specification makes use of the following additional terminology:

   Link Binding:  A unidirectional logical link between a source
      resource and a destination resource, over which state information
      is synchronized.

   State Synchronization:  Depending on the binding method (Polling,
      Observe, Push) different REST methods may be used to synchronize
      the resource values between a source and a destination.  The
      process of using a REST method to achieve this is defined as
      "State Synchronization".  The endpoint triggering the state
      synchronization is the synchronization initiator.

3.  Link Bindings

   In a M2M RESTful environment, endpoints may directly exchange the
   content of their resources to operate the distributed system.  For
   example, a light switch may supply on-off control information that
   may be sent directly to a light resource for on-off control.



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   Beforehand, a configuration phase is necessary to determine how the
   resources of the different endpoints are related to each other.  This
   can be done either automatically using discovery mechanisms or by
   means of human intervention and a so-called commissioning tool.  In
   this specification the abstract relationship between two resources is
   called a link Binding.  The configuration phase necessitates the
   exchange of binding information so a format recognized by all CoRE
   endpoints is essential.  This specification defines a format based on
   the CoRE Link-Format to represent binding information along with the
   rules to define a binding method which is a specialized relationship
   between two resources.  The purpose of a binding is to synchronize
   the content between a source resource and a destination resource.
   The destination resource MAY be a group resource if the authority
   component of the destination URI contains a group address (either a
   multicast address or a name that resolves to a multicast address).
   Since a binding is unidirectional, the binding entry defining a
   relationship is present only on one endpoint.  The binding entry may
   be located either on the source or the destination endpoint depending
   on the binding method.

3.1.  Binding Methods

   A binding method defines the rules to generate the web-transfer
   exchanges that synchronize state between source and destination
   resources.  By using REST methods content is sent from the source
   resource to the destination resource.

   The following table gives a summary of the binding methods defined in
   this specification.

          +---------+------------+-------------+---------------+
          | Name    | Identifier | Location    | Method        |
          +---------+------------+-------------+---------------+
          | Polling | poll       | Destination | GET           |
          |         |            |             |               |
          | Observe | obs        | Destination | GET + Observe |
          |         |            |             |               |
          | Push    | push       | Source      | PUT           |
          +---------+------------+-------------+---------------+

                      Table 1: Binding Method Summary

   The description of a binding method must define the following
   aspects:

   Identifier:  This is the value of the "bind" attribute used to
      identify the method.




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   Location:  This information indicates whether the binding entry is
      stored on the source or on the destination endpoint.

   REST Method:  This is the REST method used in the Request/Response
      exchanges.

   Conditions:  A binding method definition must state how the condition
      attributes of the abstract binding definition are actually used in
      this specialized binding.

   The binding methods are described in more detail below.

3.1.1.  Polling

   The Polling method consists of sending periodic GET requests from the
   destination endpoint to the source resource and copying the content
   to the destination resource.  The binding entry for this method MUST
   be stored on the destination endpoint.  The destination endpoint MUST
   ensure that the polling frequency does not exceed the limits defined
   by the pmin and pmax attributes of the binding entry.  The copying
   process MAY filter out content from the GET requests using value-
   based conditions (e.g based on the Change Step, Less Than, Greater
   Than attributes).

3.1.2.  Observe

   The Observe method creates an observation relationship between the
   destination endpoint and the source resource.  On each notification
   the content from the source resource is copied to the destination
   resource.  The creation of the observation relationship requires the
   CoAP Observation mechanism [RFC7641] hence this method is only
   permitted when the resources are made available over CoAP.  The
   binding entry for this method MUST be stored on the destination
   endpoint.  The binding conditions are mapped as query string
   parameters (see Section 4.2).

3.1.3.  Push

   When the Push method is assigned to a binding, the source endpoint
   sends PUT requests to the destination resource when the binding
   condition attributes are satisfied for the source resource.  The
   source endpoint MUST only send a notification request if the binding
   conditions are met.  The binding entry for this method MUST be stored
   on the source endpoint.







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3.2.  Link Relation

   Since Binding involves the creation of a link between two resources,
   Web Linking and the CoRE Link-Format are a natural way to represent
   binding information.  This involves the creation of a new relation
   type, named "boundto".  In a Web link with this relation type, the
   target URI contains the location of the source resource and the
   context URI points to the destination resource.

3.3.  Binding Attributes

   Web link attributes allow a fine-grained control of the type of state
   synchronization along with the conditions that trigger an update.
   This specification defines the attributes below:

           +--------------------+-----------+------------------+
           | Attribute          | Parameter | Value            |
           +--------------------+-----------+------------------+
           | Binding method     | bind      | xsd:string       |
           |                    |           |                  |
           | Minimum Period (s) | pmin      | xsd:integer (>0) |
           |                    |           |                  |
           | Maximum Period (s) | pmax      | xsd:integer (>0) |
           |                    |           |                  |
           | Change Step        | st        | xsd:decimal (>0) |
           |                    |           |                  |
           | Greater Than       | gth       | xsd:decimal      |
           |                    |           |                  |
           | Less Than          | lth       | xsd:decimal      |
           +--------------------+-----------+------------------+

                    Table 2: Binding Attributes Summary

   **Editor's note: Naming of lth and gth to be confirmed at IETF98.

3.3.1.  Bind Method (bind)

   This is the identifier of a binding method which defines the rules to
   synchronize the destination resource.  This attribute is mandatory.

3.3.2.  Minimum Period (pmin)

   When present, the minimum period indicates the minimum time to wait
   (in seconds) before triggering a new state synchronization (even if
   it has changed).  In the absence of this parameter, the minimum
   period is up to the synchronization initiator.  The minimum period
   MUST be greater than zero otherwise the receiver MUST return a CoAP
   error code 4.00 "Bad Request" (or equivalent).



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3.3.3.  Maximum Period (pmax)

   When present, the maximum period indicates the maximum time in
   seconds between two consecutive state synchronizations (regardless if
   it has changed).  In the absence of this parameter, the maximum
   period is up to the synchronization initiator.  The maximum period
   MUST be greater than zero and MUST be greater than the minimum period
   parameter (if present) otherwise the receiver MUST return a CoAP
   error code 4.00 "Bad Request" (or equivalent).

3.3.4.  Change Step (st)

   When present, the change step indicates how much the value of a
   resource SHOULD change before triggering a new state synchronization
   (compared to the value of the previous synchronization).  Upon
   reception of a query including the st attribute the current value
   (CurrVal) of the resource is set as the initial value (STinit).  Once
   the resource value differs from the STinit value (i.e.  CurrVal >=
   STinit + ST or CurrVal <= STint - ST) then a new state
   synchronization occurs.  STinit is then set to the state
   synchronization value and new state synchronizations are based on a
   change step against this value.  The change step MUST be greater than
   zero otherwise the receiver MUST return a CoAP error code 4.00 "Bad
   Request" (or equivalent).

   Note: Due to the state synchronization based update of STint it may
   result in that resource value received in two sequential state
   synchronizations differs by more than st.

3.3.5.  Greater Than (gth)

   When present, Greater Than indicates the upper limit value the
   resource value SHOULD cross before triggering a new state
   synchronization.  State synchronization only occurs when the resource
   value exceeds the specified upper limit value.  The actual resource
   value is used for the synchronization rather than the gth value.  If
   the value continues to rise, no new state synchronizations are
   generated as a result of gth.  If the value drops below the upper
   limit value and then exceeds the upper limit then a new state
   synchronization is generated.

3.3.6.  Less Than (lth)

   When present, Less Than indicates the lower limit value the resource
   value SHOULD cross before triggering a new state synchronization.
   State synchronization only occurs when the resource value is less
   than the specified lower limit value.  The actual resource value is
   used for the synchronization rather than the lth value.  If the value



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   continues to fall no new state synchronizations are generated as a
   result of lth.  If the value rises above the lower limit value and
   then drops below the lower limit then a new state synchronization is
   generated.

3.3.7.  Attribute Interactions

   Pmin, pmax, st, gth and lth may be present in the same query.

   If pmin and pmax are present in a query then they take precedence
   over the other parameters.  Thus even if st, gth or lth are met, if
   pmin has not been exceeded then no state synchronization occurs.
   Likewise if st, gth or lth have not been met and pmax time has
   expired then state synchronization occurs.  The current value of the
   resource is used for the synchronization.  If pmin time is exceeded
   and st, gth or lth are met then the current value of the resource is
   synchronized.  If st is also included, a state synchronization
   resulting from pmin or pmax updates STinit with the synchronized
   value.

   If gth and lth are included gth MUST be greater than lth otherwise an
   error CoAP error code 4.00 "Bad Request" (or equivalent) MUST be
   returned.

   If st is included in a query with a gth or lth attribute then state
   synchronizations occur only when the conditions described by st AND
   gth or st AND gl are met.

4.  Binding Table

   The binding table is a special resource that gives access to the
   bindings on a endpoint.  A binding table resource MUST support the
   Binding interface defined below.  A profile SHOULD allow only one
   resource table per endpoint.

4.1.  Binding Interface Description

   This section defines a REST interface for Binding table resources.
   The interface supports the link-format type.

   The if= column defines the Interface Description (if=) attribute
   value to be used in the CoRE Link Format for a resource conforming to
   that interface.  When this value appears in the if= attribute of a
   link, the resource MUST support the corresponding REST interface
   described in this section.  The resource MAY support additional
   functionality, which is out of scope for this specification.
   Although this interface description is intended to be used with the




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   CoRE Link Format, it is applicable for use in any REST interface
   definition.

   The Methods column defines the REST methods supported by the
   interface, which are described in more detail below.

      +-----------+----------+-------------------+-----------------+
      | Interface | if=      | Methods           | Content-Formats |
      +-----------+----------+-------------------+-----------------+
      | Binding   | core.bnd | GET, POST, DELETE | link-format     |
      +-----------+----------+-------------------+-----------------+

                  Table 3: Binding Interface Description

   The Binding interface is used to manipulate a binding table.  A
   request with a POST method and a content format of application/link-
   format simply appends new bindings to the table.  All links in the
   payload MUST have a relation type "boundTo".  A GET request simply
   returns the current state of a binding table whereas a DELETE request
   empties the table.  Individual entries may be dreeleted from the
   table by specifying the resource path in a DELETE request.

   The following example shows requests for adding, retrieving and
   deleting bindings in a binding table.

   Req: POST /bnd/ (Content-Format: application/link-format)
   <coap://sensor.example.com/s/switch>;
     rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"
   Res: 2.04 Changed

   Req: GET /bnd/
   Res: 2.05 Content (application/link-format)
   <coap://sensor.example.com/s/switch>;
     rel="boundto";anchor="/a/light";bind="obs";pmin="10";pmax="60"

   Req: DELETE /bnd/a/switch
   Res: 2.04 Changed

   Req: DELETE /bnd/
   Res: 2.04 Changed

                    Figure 1: Binding Interface Example

4.2.  Resource Observation Attributes

   When resource interfaces following this specification are made
   available over CoAP, the CoAP Observation mechanism [RFC7641] MAY be
   used to observe any changes in a resource, and receive asynchronous



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   notifications as a result.  In addition, a set of query string
   parameters are defined here to allow a client to control how often a
   client is interested in receiving notifications and how much a
   resource value should change for the new representation to be
   interesting.  These query parameters are described in the following
   table.  A resource using an interface description defined in this
   specification and marked as Observable in its link description SHOULD
   support these observation parameters.  The Change Step parameter can
   only be supported on resources with an atomic numeric value.

   *Editor's note: There is a proposal to use the query parameters on
   the GET Observe as the default pattern.  This allows multiple
   observations of the same resource.  The PUT behaviour below would be
   treated as a legacy option.  This will be discussed at IETF98.*

   These query parameters MUST be treated as resources that are read
   using GET and updated using PUT, and MUST NOT be included in the
   Observe request.  Multiple parameters MAY be updated at the same time
   by including the values in the query string of a PUT.  Before being
   updated, these parameters have no default value.

         +----------------+------------------+------------------+
         | Resource       | Parameter        | Data Format      |
         +----------------+------------------+------------------+
         | Minimum Period | /{resource}?pmin | xsd:integer (>0) |
         |                |                  |                  |
         | Maximum Period | /{resource}?pmax | xsd:integer (>0) |
         |                |                  |                  |
         | Change Step    | /{resource}?st   | xsd:decimal (>0) |
         |                |                  |                  |
         | Less Than      | /{resource}?lth  | xsd:decimal      |
         |                |                  |                  |
         | Greater Than   | /{resource}?gth  | xsd:decimal      |
         +----------------+------------------+------------------+

              Table 4: Resource Observation Attribute Summary

   Minimum Period:  As per Section 3.3.2

   Maximum Period:  As per Section 3.3.3

   Change Step:  As per Section 3.3.4

   Greater Than:  As per Section 3.3.5

   Less Than:  As per Section 3.3.6





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5.  Security Considerations

   An implementation of a client needs to be prepared to deal with
   responses to a request that differ from what is specified in this
   specification.  A server implementing what the client thinks is a
   resource with one of these interface descriptions could return
   malformed representations and response codes either by accident or
   maliciously.  A server sending maliciously malformed responses could
   attempt to take advantage of a poorly implemented client for example
   to crash the node or perform denial of service.

6.  IANA Considerations

6.1.  Interface Description

   The specification registers the "binding" CoRE interface description
   link target attribute value as per [RFC6690].

   Attribute Value:  core.binding

   Description: The binding interface is used to manipulate a binding
   table which describes the link bindings between source and
   destination resources for the purposes of synchronizing their
   content.

   Reference: This specification.  Note to RFC editor: please insert the
   RFC of this specification.

   Notes: None

6.2.  Link Relations Type

   This specification registers the new "bounto" link relation type as
   per [RFC5988].

   Relation Name:  boundto

   Description:  The purpose of a boundto relation type is to indicate
      that there is a binding between a source resource and a
      destination resource for the purposes of synchronizing their
      content.

   Reference:  This specification.  Note to RFC editor: please insert
      the RFC of this specification.

   Notes:  None

   Application Data:  None



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7.  Acknowledgements

   Acknowledgement is given to colleagues from the SENSEI project who
   were critical in the initial development of the well-known REST
   interface concept, to members of the IPSO Alliance where further
   requirements for interface types have been discussed, and to Szymon
   Sasin, Cedric Chauvenet, Daniel Gavelle and Carsten Bormann who have
   provided useful discussion and input to the concepts in this
   specification.

8.  Changelog

   draft-ietf-core-dynlink-03

   o  Section 4.2: Update the Href to use "switch" instead of "light".

   o  General: Added editor's notes for issues to be resolved at IETF98.

   draft-ietf-core-dynlink-02

   o  General: Changed the name of the greater than attribute "gt" to
      "gth" and the name of the less than attribute "lt" to "lth" due to
      conlict with the core resource directory draft lifetime "lt"
      attribute.

   o  Clause 6.1: Addressed the editor's note by changing the link
      target attribute to "core.binding".

   o  Added Appendix A for examples.

   draft-ietf-core-dynlink-01

   o  General: The term state synchronization has been introduced to
      describe the process of synchronization between destination and
      source resources.

   o  General: The document has been restructured the make the
      information flow better.

   o  Clause 3.1: The descriptions of the binding attributes have been
      updated to clarify their usage.

   o  Clause 3.1: A new clause has been added to discuss the
      interactions between the resources.

   o  Clause 3.4: Has been simplified to refer to the descriptions in
      3.1.  As the text was largely duplicated.




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   o  Clause 4.1: Added a clarification that individual resources may be
      removed from the binding table.

   o  Clause 6: Formailised the IANA considerations.

   draft-ietf-core-dynlink Initial Version 00:

   o  This is a copy of draft-groves-core-dynlink-00

   draft-groves-core-dynlink Draft Initial Version 00:

   o  This initial version is based on the text regarding the dynamic
      linking functionality in I.D.ietf-core-interfaces-05.

   o  The WADL description has been dropped in favour of a thorough
      textual description of the REST API.

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

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

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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7641>.






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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.  Greater Than (gth) 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: gth="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 2: Client Registers and Receives one Notification of the
      Current State and One of a New State when it passes through the
                      greather than threshold of 25.

A.2.  Greater Than (gth) 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



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    5                 |      |                Uri-Path: temperature
    6                 |      |         Uri-Query: pmax="20";gth="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                 |      |

      Figure 3: 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 greather than threshold of 25.

Authors' Addresses







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Internet-Draft      Dynamic Resource Linking for CoRE         March 2017


   Zach Shelby
   ARM
   150 Rose Orchard
   San Jose  95134
   FINLAND

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


   Matthieu Vial
   Schneider-Electric
   Grenoble
   FRANCE

   Phone: +33 (0)47657 6522
   Email: matthieu.vial@schneider-electric.com


   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

   Email: michael.koster@smartthings.com


   Christian Groves
   Huawei
   Australia

   Email: cngroves.std@gmail.com


















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