wdiff draft-ietf-core-dynlink-07.txt draft-ietf-core-dynlink-08.txt

CoRE Working Group Z. Shelby
Internet-Draft ARM
Intended status: Informational M. Koster
Expires: April 25, September 9, 2019 SmartThings
C. Groves

J. Zhu
Huawei
B. Silverajan, Ed.
Tampere University of Technology
October 22, 2018
March 08, 2019

Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-07
draft-ietf-core-dynlink-08

Abstract

For CoAP (RFC7252), Dynamic

This specification defines Link Bindings, which provide dynamic
linking of state updates between resources, either on an endpoint or
between endpoints, is defined
with the concept of Link Bindings. for systems using CoAP (RFC7252). This
specification also defines
conditional observation attributes Conditional Notification 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 April 25, September 9, 2019.

This document is subject to BCP 78 and the IETF Trust's Legal
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(https://trustee.ietf.org/license-info) in effect on the date of
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carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Link Bindings . Conditional Notification Attributes . . . . . . . . . . . . . 4
3.1. Attribute Definitions . . . . . . . . . . 4
3.1. The "bind" attribute and Binding Methods . . . . . . . . 4
3.1.1. Polling . . Minimum Period (pmin) . . . . . . . . . . . . . . . . 5
3.1.2. Maximum Period (pmax) . . . . . 5
3.1.2. Observe . . . . . . . . . . . 5
3.1.3. Change Step (st) . . . . . . . . . . . . 5
3.1.3. Push . . . . . . 5
3.1.4. Greater Than (gt) . . . . . . . . . . . . . . . . . . 6
3.2. Link Relation . . .
3.1.5. Less Than (lt) . . . . . . . . . . . . . . . . . . . 6
4. Binding and Resource Observation Attributes .
3.1.6. Notification Band (band) . . . . . . . . 6
4.1. Minimum Period (pmin) . . . . . . 7
3.2. Server processing of Conditional Notification Attributes 8
4. Link Bindings . . . . . . . . . . . . 7
4.2. Maximum Period (pmax) . . . . . . . . . . . . 8
4.1. The "bind" attribute and Binding Methods . . . . . . 7
4.3. Change Step (st) . . 9
4.1.1. Polling . . . . . . . . . . . . . . . . . . 7
4.4. Greater Than (gt) . . . . . 10
4.1.2. Observe . . . . . . . . . . . . . . . 8
4.5. Less Than (lt) . . . . . . . . 10
4.1.3. Push . . . . . . . . . . . . . 8
4.6. Notification Band (band) . . . . . . . . . . . 11
4.2. Link Relation . . . . . 8
4.7. Attribute Interactions . . . . . . . . . . . . . . . . . 9 11
5. Binding Table . . . . . . . . . . . . . . . . . . . . . . . . 10 11
6. Implementation Considerations . . . . . . . . . . . . . . . . 11 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 13
8.1. Interface Description . . . . . Resource Type value 'core.bnd' . . . . . . . . . . . . . 12 13
8.2. Link Relation Type . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 14
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13 14
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 15 16
12.2. Informative References . . . . . . . . . . . . . . . . . 16 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 16 17
A.1. Greater Than (gt) example . . . . . . . . . . . . . . . . 16 17
A.2. Greater Than (gt) and Period Max (pmax) example . . . . . 17 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 19

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 Conditional
Notification 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.

3. Conditional Notification Attributes

3.1. Attribute Definitions

This specification defines Conditional Notification Attributes, which
provide for fine-grained control of notification and state
synchronization when using CoRE Observe [RFC7641] or Link Bindings

In a M2M RESTful environment, endpoints may directly exchange the
content of their resources to operate
(see Section 4). Conditional Notification Attributes define the distributed system. For
example, a light switch may supply on-off control information
conditions that
may be sent directly to trigger a light notification.

When 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 interfaces following 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 are made
available over CoAP, the rules CoAP Observation mechanism [RFC7641] MAY
also be used to define observe any changes in a binding method which is resource, and receive
asynchronous notifications as a specialized relationship between two resources. result. A resource marked as
Observable in its link description SHOULD support these Conditional
Notification Attributes.

The purpose set of
such parameters defined here allow a binding is client to synchronize the content between control how
often a source
resource client is interested in receiving notifications and how much
a destination resource. The destination resource MAY be
a group resource if the authority component of value should change for the destination URI
contains a group address (either a multicast address or a name that
resolves new representation 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
interesting.

One 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.

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

These attributes are defined below:

Table 1: The bind attribute

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

Table 2: Binding Method Summary

The description of 1: Conditional Notification Attributes

Conditional Notification Attributes SHOULD be evaluated on all
potential notifications from a binding method must define resource, whether resulting from an
internal server-driven sampling process or from external update
requests to the following
aspects:

Identifier: This is server.

Note: In this draft, we assume that there are finite quantization
effects in the internal or external updates to the value of a
resource; specifically, that a resource may be updated at any time
with any valid value. We therefore avoid any continuous-time
assumptions in the "bind" attribute used to
identify the method.

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

REST Method: This is Conditional Notification
Attributes and instead use the REST method used in phrase "sampled value" to refer to a
member of a sequence of values that may be internally observed from
the Request/Response
exchanges.

Conditions: A binding method definition must resource state how over time.

3.1.1. Minimum Period (pmin)

When present, the condition
attributes of minimum period indicates the abstract binding definition are actually used in
this specialized binding.

The binding methods are described minimum time, in more detail below.

3.1.1. Polling

The Polling method consists of sending periodic GET requests from the
destination endpoint to
seconds, between two consecutive notifications (whether or not the source
resource and copying value has changed). In the content absence of this parameter, the
minimum period is up to the destination resource. server. The binding entry for this method minimum period MUST be stored on
greater than zero otherwise the destination endpoint. The destination endpoint receiver MUST
ensure return a CoAP error
code 4.00 "Bad Request" (or equivalent).

A server MAY report the last sampled value that occured during the polling frequency does not exceed
pmin interval, after the limits defined
by pmin interval expires.

Note: Due to finite quantization effects, the time between
notifications may be greater than pmin and pmax attributes of even when the binding entry. The copying
process MAY filter out content from sampled value
changes within the GET requests using value-
based conditions (e.g based on pmin interval. Pmin may or may not be used to
drive the Change Step, Less Than, Greater
Than attributes). internal sampling process.

3.1.2. Observe

The Observe method creates an observation relationship between the
destination endpoint and Maximum Period (pmax)

When present, the source resource. On each notification maximum period indicates the content from maximum time, in
seconds, between two consecutive notifications (whether or not the source
resource is copied to the destination
resource. The creation of the observation relationship requires value has changed). In the
CoAP Observation mechanism [RFC7641] hence absence of this method parameter, the
maximum period is only
permitted when up to the resources are made available over CoAP. server. The
binding entry for this method maximum period 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
greater than zero and MUST be greater than the binding
condition attributes are satisfied for minimum period
parameter (if present) otherwise the source resource. The
source endpoint receiver MUST only send return a notification request if the binding
conditions are met. The binding entry for this method MUST be stored
on CoAP
error code 4.00 "Bad Request" (or equivalent).

3.1.3. Change Step (st)

When present, the source endpoint.

3.2. Link Relation

Since Binding involves change step indicates how much the creation value of a link between two resources,
Web Linking and the CoRE Link-Format are
resource SHOULD change before triggering a natural way notification, compared to represent
binding information. This involves
the creation value of the previous notification. Upon reception of a new relation
type, named "boundto". In a Web link with this relation type, query
including the
target URI contains st attribute, the location most recently sampled value of the source
resource is reported, and then set as 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 last reported value
(last_rep_v). When a fine-grained control subsequent sample or update of the type of state
synchronization along with resource
value differs from the conditions that trigger last reported value by an update.

When resource interfaces following this specification are made
available over CoAP, the CoAP Observation mechanism [RFC7641] MAY
also be used amount, positive or
negative, greater than or equal to observe any changes in a resource, and receive
asynchronous notifications as a result. A resource using an
interface description defined in this specification st, 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 time for pmin has
elapsed since the last notification, a client notification is interested in receiving notifications sent and how much a resource value should change for the new
representation
last reported value is updated to the value sent in the notification.
The change step MUST be interesting, as query parameters.

These query parameters 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 treated as supported on resources that are read
using GET and updated using PUT, with a
scalar numeric value.

Note: Due to sampling and MUST NOT be included other constraints, e.g. pmin, the resource
value received in two sequential notifications may differ by more
than st.

3.1.4. Greater Than (gt)

When present, Greater Than indicates the
Observe request. Multiple parameters MAY be updated at 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 same time
by including
fpr pmin has elapsed since the values last notification. The sampled value
is sent in the query string of a PUT. Before being
updated, these parameters have notification. If the value continues to rise, no default value.

These attributes
notifications are defined below:

Table 3: Binding Attributes Summary

4.1. Minimum Period (pmin) (lt)

When present, the minimum period Less Than indicates the minimum time to wait
(in seconds) lower limit value the resource
value SHOULD cross before triggering a new state synchronization (even if
it has changed). In the absence of this parameter, the minimum
period notification. A notification
is up to sent when the synchronization initiator. The minimum period
MUST be greater than zero otherwise samples value crosses the receiver MUST return a CoAP
error code 4.00 "Bad Request" (or equivalent).

4.2. Maximum Period (pmax)

When present, specified lower limit
value, relative to the maximum period indicates last reported value, and the maximum time in
seconds between two consecutive state synchronizations (regardless if
it fpr pmin has changed). In
elapsed since the absence last notification. The sampled value is sent in
the notification. If the value continues to fall no notifications
are generated as a result of this parameter, lt. If the maximum
period value rises above the lower
limit value then a new notification is up sent, subject to the synchronization initiator. pmin
time..

The maximum period
MUST be greater than zero and MUST Less Than parameter can only be greater than the supported on resources with a
scalar numeric value.

3.1.6. Notification Band (band)

The notification band attribute allows a bounded or unbounded (based
on a minimum period
parameter (if present) otherwise or maximum) value range that may trigger multiple
notifications. This enables use cases where different ranges results
in differing behaviour. For example: monitoring the receiver MUST return a CoAP
error code 4.00 "Bad Request" (or equivalent).

4.3. Change Step (st)

When present, temperature of
machinery. Whilst the change step indicates how much temperature is in the value of normal operating range
only periodic observations are needed. However as the temperature
moves to more abnormal ranges more frequent synchronization/reporting
may be needed.

Without a
resource SHOULD change before triggering notification band, a new state synchronization
(compared transition across a less than (lt), or
greater than (gt) limit only generates one notification. This means
that it is not possible to the value of the previous synchronization). Upon
reception of describe a query including case where multiple
notifications are sent so long as the st limit is exceeded.

The band attribute works as a modifier to the current value
(CurrVal) behaviour of the resource gt and lt.
Therefore, if band is set as present in a query, gt, lt or both, MUST be
included.

When band is present with the initial value (STinit). Once lt attribute, it defines the resource value differs from lower
bound for the notification band (notification band minimum).
Notifications occur when the STinit resource value (i.e. CurrVal >=
STinit + ST or CurrVal <= STint - ST) then a new state
synchronization occurs. STinit is then set equal to or above the state
synchronization
notification band minimum. If lt is not present there is no minimum
value and new state synchronizations are based on a
change step against this value. The change step MUST be greater than
zero otherwise for the receiver MUST return a CoAP error code 4.00 "Bad
Request" (or equivalent).

The Change Step parameter can only be supported on resources band.

When band is present with an
atomic numeric value.

Note: Due to the state synchronization based update of STint gt attribute, 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 defines the upper limit value
bound for the
resource value SHOULD cross before triggering a new state
synchronization. State synchronization only occurs notification band (notification band maximum).
Notifications occur 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 equal to rise, no new state synchronizations are
generated as a result of gt. If the value drops or below the upper
limit
notification band maximum. If gt is not present there is no maximum
value and then exceeds for the upper limit then a new state
synchronization band.

If band is generated.

4.5. Less Than (lt)

When present, Less Than indicates the lower limit value present with both the resource
value SHOULD cross before triggering a new state synchronization.
State synchronization only gt and lt attributes, notification
occurs when the resource value is less greater than or equal to gt or when
the specified lower limit value. The actual resource value is
used for the synchronization rather less than the lt value. or equal to lt.

If a band is specified in which the value
continues to fall no new state synchronizations are generated as a
result of lt. If gt is less than that of
lt, in-band notification occurs. That is, notification occurs
whenever the resource value rises above is between the lower limit value gt and
then drops below lt values,
including equal to gt or lt.

If 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 is specified in differing behaviour. For example: monitoring which the
temperature value of machinery. Whilst the temperature gt 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.

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 of lt, out-of-band notification occurs. That is, notification
occurs when the resource value not possible to describe a case where multiple
notifications are sent so long as between the limit is exceeded.

The band attribute works as a modifier gt and lt values,
excluding equal to the behaviour of gt and lt.
Therefore, if band is present in

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

3.2. Server processing of Conditional Notification Attributes

Pmin, pmax, st, gt, lt or both, MUST be
included.

When and band is may be present with in the lt attribute, it defines same query.
Howver, they are not defined at multiple prioritization levels. The
server sends a notification whenever any of the lower
bound parameter conditions
are met, upon which it updates its last notification value and time
to prepare for the next notification. Only one 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 there are multiple conditions being met at the same time. The
reference code below illustrates how
notifications are generated. the logic to determine when a
notification is to be sent.

bool notifiable( Resource * r ) {

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

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

Figure 1: Code logic for conditional notification attribute
interactions for observe
notification

5. Binding Table

4. 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 content updates
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.

Conditional Notification Attributes defined in Section 3 can be used
with Link Bindings in order to customize the notification behavior
and timing.

4.1. The "bind" attribute and Binding Methods

A binding method defines the rules to generate the network-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.

This specification defines a new CoRE link attribute "bind". This is
the identifier for a binding method which defines the rules to
synchronize the destination resource. This attribute is mandatory.

Table 2: The bind attribute

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

Table 3: Binding Method Summary

The description of a binding method defines 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.

Conditional Notification: How Conditional Notification Attributes
are used in the binding.

The binding methods are described in more detail below.

4.1.1. 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).

4.1.2. Observe

The Binding table is a special Observe method creates an observation relationship between the
destination endpoint and the source resource. On each notification
the content from the source resource that gives access is copied to the
bindings 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 a the destination
endpoint. This section defines The binding conditions are mapped as query parameters in
the Observe request (see Section 3).

4.1.3. Push

When the Push method is assigned to a REST interface binding, the source endpoint
sends PUT requests to the destination resource when the Conditional
Notification Attributes are satisfied for
Binding table resources. the source resource. The Binding table resource
source endpoint SHOULD only send a notification request if any
included Conditional Notification Attributes are met. The binding
entry for this method MUST support be stored on the source endpoint.

4.2. Link Relation

Since Binding interface defined below. The interface supports involves the link-
format type.

The if= column defines creation of a link between two resources,
Web Linking and the Interface Description (if=) attribute
value to be CoRE Link-Format used in to represent binding
information. This involves the CoRE Link Format for creation of a new relation type,
"boundto". In a Web link with this relation type, the target URI
contains the location of the source resource conforming and the context URI
points to the destination resource.

5. Binding Table

The Binding Table is a special resource that interface. When this value appears in describes the if= attribute of bindings
on an endpoint. An endpoint offering a
link, representation of the Binding
Table resource MUST support SHOULD indicate its presence and enable its discovery
by advertising a link at "/.well-known/core" [RFC6690]. If so, the corresponding REST interface
described in this section. The
Binding Table resource MAY support additional
functionality, which is out of scope for this specification.
Although this interface description is intended to MUST be used with discoverable by using the
CoRE Link Format, it is applicable for use in any REST interface
definition. Resource
Type (rt) 'core.bnd'.

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

+-----------+----------+-------------------+-----------------+

Table 4: Binding Interface Table Description

The Binding interface is REST methods GET and PUT are used to manipulate a binding table. Binding Table.
A GET request simply returns the current state of a Binding Table. A
request with a POST PUT method and a content format of application/link-
format simply appends new is used to clear the bindings to the table. table or replaces its
entire contents. All links in the payload of a PUT rquest MUST have
a relation type "boundto". A GET request simply
returns the current state

(Editor's Note: Usage 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. PATCH method for fine-grained addition
and removal of individual bindings is under study.)

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

Req: POST GET /.well-known/core?rt=core.bnd (application/link-format)
Res: 2.05 Content (application/link-format)
</bnd/>;rt=core.bnd;ct=40

Req: GET /bnd/
Res: 2.05 Content (application/link-format)
<coap://sensor.example.com/a/switch1/>;
rel=boundto;bind=obs;anchor=/a/fan,;bind="obs",
<coap://sensor.example.com/a/switch2/>;
rel=boundto;bind=obs;anchor=/a/light;bind="obs"

Req: PUT /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 2: Binding Interface Table 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.

The initiation of a link binding Link Binding can be delegated from a client to a
link state machine implementation, which can be an embedded client or
a configuration tool. Implementation considerations have to be given
to how to monitor transactions made by the configuration tool with
regards to link bindings, Link Bindings, as well as any errors that may arise with
establishing link bindings as well as with Link Bindings in addition to established link bindings. Link Bindings.

7. Security Considerations

Consideration has to be given to what kinds of security credentials
the state machine of a configuration tool or an embedded client needs
to be configured with, and what kinds of access control lists client
implementations should possess, so that transactions on creating link
bindings Link
Bindings and handling error conditions can be processed by the state
machine.

8. IANA Considerations

8.1. Interface Description

The Resource Type value 'core.bnd'

This specification registers a new Resource Type Link Target
Attribute 'core.bnd' in the "binding" CoRE interface description
link target attribute value Resource Type (rt=) registry established
as per [RFC6690].

Attribute Value: core.bnd

Description: The binding interface See Section 5. This attribute value is used to manipulate discover
the resource representing a binding
table 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

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. Christian Amsuss supplied a comprehensive review of
draft -06.

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-08

o Reorganize the draft to introduce Conditional Notification
Attributes at the beginning

o Made pmin and pmax type xsd:decimal to accommodate fractional
second timing

o updated the attribute descriptions. lt and gt notify on all
crossings, both directions

o updated Binding Table description, removed interface description
but introduced core.bnd rt attribute value

draft-ietf-core-dynlink-07

o Added reference code to illustrate attribute interactions for
observations

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

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.

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.

Figure 3: 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 | |
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 4: 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
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
Kalevantie 4
Tampere FI-33720 FI-33100
Finland

Email: bilhanan.silverajan@tut.fi bilhanan.silverajan@tuni.fi