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Versions: (draft-groves-core-dynlink) 00 01 02 03 04 05 06

CoRE Working Group                                             Z. Shelby
Internet-Draft                                                       ARM
Intended status: Informational                                 M. Koster
Expires: January 4, 2019                                     SmartThings
                                                               C. Groves

                                                                  J. Zhu
                                                                  Huawei
                                                      B. Silverajan, Ed.
                                        Tampere University of Technology
                                                           July 03, 2018


     Dynamic Resource Linking for Constrained RESTful Environments
                       draft-ietf-core-dynlink-06

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 note

   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 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 4, 2019.






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Copyright Notice

   Copyright (c) 2018 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
   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 . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The "bind" attribute and Binding Methods  . . . . . . . .   4
       3.1.1.  Polling . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Observe . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Push  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Link Relation . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Binding and Resource Observation Attributes . . . . . . . . .   6
     4.1.  Minimum Period (pmin) . . . . . . . . . . . . . . . . . .   7
     4.2.  Maximum Period (pmax) . . . . . . . . . . . . . . . . . .   7
     4.3.  Change Step (st)  . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Greater Than (gt) . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Less Than (lt)  . . . . . . . . . . . . . . . . . . . . .   8
     4.6.  Notification Band (band)  . . . . . . . . . . . . . . . .   8
     4.7.  Attribute Interactions  . . . . . . . . . . . . . . . . .   9
   5.  Binding Table . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Interface Description . . . . . . . . . . . . . . . . . .  12
     8.2.  Link Relation Type  . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Greater Than (gt) example . . . . . . . . . . . . . . . .  15
     A.2.  Greater Than (gt) and Period Max (pmax) example . . . . .  16



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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.  CoRE Link-format [RFC6690] is a
   standard for doing Web Linking [RFC8288] 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 state updates are conveyed.  Specifically, a
   Link Binding is a unidirectional link for binding the states of
   source and destination 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 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 [RFC8288] 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.

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



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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.
   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 such an abstract relationship between two
   resources is defined, 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
   such 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.  The "bind" attribute and 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.

   In order to use binding methods, this specification defines a special
   CoRE link attribute "bind".  This is the identifier of a binding
   method which defines the rules to synchronize the destination
   resource.  This attribute is mandatory.

                +----------------+-----------+------------+
                | Attribute      | Parameter | Value      |
                +----------------+-----------+------------+
                | Binding method | bind      | xsd:string |
                +----------------+-----------+------------+

                        Table 1: The bind attribute

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



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

                      Table 2: 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.

   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



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   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).

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.

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.

4.  Binding and Resource Observation Attributes

   In addition to "bind", this specification further defines Web link
   attributes allowing a fine-grained control of the type of state
   synchronization along with the conditions that trigger an update.

   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 using an
   interface description defined in this specification and marked as
   Observable in its link description SHOULD support these observation
   parameters.

   In addition, the set of parameters are defined here 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, as query parameters.

   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.



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   These attributes are defined below:

           +--------------------+-----------+------------------+
           | Attribute          | Parameter | Value            |
           +--------------------+-----------+------------------+
           | Minimum Period (s) | pmin      | xsd:integer (>0) |
           |                    |           |                  |
           | Maximum Period (s) | pmax      | xsd:integer (>0) |
           |                    |           |                  |
           | Change Step        | st        | xsd:decimal (>0) |
           |                    |           |                  |
           | Greater Than       | gt        | xsd:decimal      |
           |                    |           |                  |
           | Less Than          | lt        | xsd:decimal      |
           |                    |           |                  |
           | Notification Band  | band      | xsd:boolean      |
           +--------------------+-----------+------------------+

                    Table 3: Binding Attributes Summary

4.1.  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).

4.2.  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).

4.3.  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



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   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).

   The Change Step parameter can only be supported on resources with an
   atomic numeric value.

   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.

4.4.  Greater Than (gt)

   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 gt value.  If
   the value continues to rise, no new state synchronizations are
   generated as a result of gt.  If the value drops below the upper
   limit value and then exceeds the upper limit then a new state
   synchronization is generated.

4.5.  Less Than (lt)

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

4.6.  Notification Band (band)

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




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   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).  State
   synchronization occurs 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).  State
   synchronization occurs 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, two kinds of
   signaling bands are specified.

   If a band is specified in which the value of gt is less than that of
   lt, in-band signaling occurs.  State synchronization occurs whenever
   the resource value is between the notification band minimum and
   maximum or is equal to the notification band minimum or maximum.

   On the other hand if the band is specified in which the value of gt
   is greater than that of lt, out-of-band signaling occurs.  State
   synchronization occurs whenever the resource value is outside the
   notification band minimum and maximum or is equal to the notification
   band minimum or maximum.

4.7.  Attribute Interactions

   Pmin, pmax, st, gt and lt may be present in the same query.
   Parameters are not defined at multiple prioritization levels.
   Instead, the server state machine generates a notification whenever
   any of the parameter conditions are met, after which it performs a
   reset on all the requested conditions.  State synchronization also
   occurs only once even if there are multiple conditions being met at
   the same time.







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5.  Binding Table

   The Binding table is a special resource that gives access to the
   bindings on a endpoint.  This section defines a REST interface for
   Binding table resources.  The Binding table resource MUST support the
   Binding interface defined below.  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
   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 4: 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 deleted 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.












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

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

   Req: DELETE /bnd/
   Res: 2.04 Changed

                    Figure 1: Binding Interface Example

6.  Implementation Considerations

   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
   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 for the 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.

7.  Security Considerations

   The initiation of a link binding can be delegated from a client to a
   link state machine implementation, which can be an embedded client or
   a configuration tool.  Consequently, consideration has to be given to
   what kinds of security credentials the the state machine needs to be
   configured with, and what kinds of access control lists client
   implementations should possess, so that transactions on creating link
   bindings and handling error conditions can be processed by the state
   machine.




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   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.

8.  IANA Considerations

8.1.  Interface Description

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

   Attribute Value:  core.bnd

   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

8.2.  Link Relation Type

   This specification registers the new "boundto" link relation type as
   per [RFC8288].

   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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9.  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.

10.  Contributors

   Matthieu Vial
   Schneider-Electric
   Grenoble
   France

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

11.  Changelog

   draft-ietf-core-dynlink-06

   o  Document restructure and refactoring into three main sections

   o  Clarifications on band usage

   o  Implementation considerations introduced

   o  Additional text on security considerations

   draft-ietf-core-dynlink-05

   o  Addition of a band modifier for gt and lt, adapted from draft-
      groves-core-obsattr

   o  Removed statement prescribing gt MUST be greater than lt

   draft-ietf-core-dynlink-03

   o  General: Reverted to using "gt" and "lt" from "gth" and "lth" for
      this draft owing to concerns raised that the attributes are
      already used in LwM2M with the original names "gt" and "lt".

   o  New author and editor added.

   draft-ietf-core-dynlink-02



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   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.

   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.







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12.  References

12.1.  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>.

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

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

12.2.  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.  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 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 (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 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

   Zach Shelby
   ARM
   Kidekuja 2
   Vuokatti  88600
   FINLAND

   Phone: +358407796297
   Email: zach.shelby@arm.com








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   Michael Koster
   SmartThings
   665 Clyde Avenue
   Mountain View  94043
   USA

   Email: michael.koster@smartthings.com


   Christian Groves
   Australia

   Email: cngroves.std@gmail.com


   Jintao Zhu
   Huawei
   No.127 Jinye Road, Huawei Base, High-Tech Development District
   Xi'an, Shaanxi Province
   China

   Email: jintao.zhu@huawei.com


   Bilhanan Silverajan (editor)
   Tampere University of Technology
   Korkeakoulunkatu 10
   Tampere  FI-33720
   Finland

   Email: bilhanan.silverajan@tut.fi




















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