Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-13
CoRE Working Group M. Koster
Internet-Draft SmartThings
Intended status: Informational B. Silverajan, Ed.
Expires: August 26, 2021 Tampere University
February 22, 2021
Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-13
Abstract
This specification defines Link Bindings, which provide dynamic
linking of state updates between resources, either on an endpoint or
between endpoints, for systems using CoAP (RFC7252). This
specification also defines Conditional Notification and Control
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
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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 August 26, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conditional Attributes . . . . . . . . . . . . . . . . . . . 4
3.1. Conditional Notification Attributes . . . . . . . . . . . 4
3.1.1. Greater Than (gt) . . . . . . . . . . . . . . . . . . 5
3.1.2. Less Than (lt) . . . . . . . . . . . . . . . . . . . 5
3.1.3. Change Step (st) . . . . . . . . . . . . . . . . . . 6
3.1.4. Notification Band (band) . . . . . . . . . . . . . . 6
3.1.5. Edge (edge) . . . . . . . . . . . . . . . . . . . . . 7
3.2. Conditional Control Attributes . . . . . . . . . . . . . 7
3.2.1. Minimum Period (pmin) . . . . . . . . . . . . . . . . 8
3.2.2. Maximum Period (pmax) . . . . . . . . . . . . . . . . 8
3.2.3. Minimum Evaluation Period (epmin) . . . . . . . . . . 9
3.2.4. Maximum Evaluation Period (epmax) . . . . . . . . . . 9
3.2.5. Confirmable Notification (con) . . . . . . . . . . . 9
3.3. Server processing of Conditional Attributes . . . . . . . 9
4. Link Bindings . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. The "bind" attribute and Binding Methods . . . . . . . . 11
4.1.1. Polling . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.2. Observe . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.3. Push . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.4. Execute . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. Link Relation . . . . . . . . . . . . . . . . . . . . . . 13
5. Binding Table . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Implementation Considerations . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8.1. Resource Type value 'core.bnd' . . . . . . . . . . . . . 16
8.2. Link Relation Type . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 22
A.1. Minimum Period (pmin) example . . . . . . . . . . . . . . 22
A.2. Maximum Period (pmax) example . . . . . . . . . . . . . . 22
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A.3. Greater Than (gt) example . . . . . . . . . . . . . . . . 24
A.4. Greater Than (gt) and Period Max (pmax) example . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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 Conditional
Notification and Control Attributes for use with Link Bindings and
with CoRE Observe [RFC7641].
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], [RFC6690] and
[RFC7641]. 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.
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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 Attributes
This specification defines conditional attributes, which provide for
fine-grained control of notification and state synchronization when
using CoRE Observe [RFC7641] or Link Bindings (see Section 4). When
resource interfaces following this specification are made available
over CoAP, the CoAP Observation mechanism [RFC7641] MAY also be used
to observe any changes in a resource, and receive asynchronous
notifications as a result. A resource marked as Observable in its
link description SHOULD support these conditional attributes.
Note: In this draft, we assume that there are finite quantization
effects in the internal or external updates to the value representing
the state of a resource; specifically, that a resource state may be
updated at any time with any valid value. We therefore avoid any
continuous-time assumptions in the description of the conditional
attributes and instead use the phrase "sampled value" to refer to a
member of a sequence of values that may be internally observed from
the resource state over time.
3.1. Conditional Notification Attributes
Conditional Notification Attributes define the conditions that
trigger a notification. Conditional Notification Attributes SHOULD
be evaluated on all potential notifications from a resource, whether
resulting from an internal server-driven sampling process or from
external update requests to the server.
The set of Conditional Notification Attributes defined here allow a
client to control how often a client is interested in receiving
notifications and how much a value should change for the new
representation state to be interesting. One or more Conditional
Notification Attributes MAY be included as query parameters in an
Observe request.
Conditional Notification Attributes are defined below:
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+-------------------+-----------+-----------------+
| Attribute | Parameter | Value |
+-------------------+-----------+-----------------+
| Greater Than | gt | xs:decimal |
| | | |
| Less Than | lt | xs:decimal |
| | | |
| Change Step | st | xs:decimal (>0) |
| | | |
| Notification Band | band | xs:boolean |
| | | |
| Edge | edge | xs:boolean |
+-------------------+-----------+-----------------+
Table 1: Conditional Notification Attributes
3.1.1. Greater Than (gt)
When present, Greater Than indicates the upper limit value the
sampled value SHOULD cross before triggering a notification. A
notification is sent whenever the sampled value crosses the specified
upper limit value, relative to the last reported value, and the time
fpr pmin has elapsed since the last notification. The sampled value
is sent in the notification. If the value continues to rise, no
notifications are generated as a result of gt. If the value drops
below the upper limit value then a notification is sent, subject
again to the pmin time.
The Greater Than parameter can only be supported on resources with a
scalar numeric value.
3.1.2. Less Than (lt)
When present, Less Than indicates the lower limit value the resource
value SHOULD cross before triggering a notification. A notification
is sent when the samples value crosses the specified lower limit
value, relative to the last reported value, and the time fpr pmin has
elapsed since the last notification. The sampled value is sent in
the notification. If the value continues to fall no notifications
are generated as a result of lt. If the value rises above the lower
limit value then a new notification is sent, subject to the pmin
time.
The Less Than parameter can only be supported on resources with a
scalar numeric value.
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3.1.3. Change Step (st)
When present, the change step indicates how much the value
representing a resource state SHOULD change before triggering a
notification, compared to the old state. Upon reception of a query
including the st attribute, the current resource state representing
the most recently sampled value is reported, and then set as the last
reported value (last_rep_v). When a subsequent sampled value or
update of the resource state differs from the last reported state by
an amount, positive or negative, greater than or equal to st, and the
time for pmin has elapsed since the last notification, a notification
is sent and the last reported value is updated to the new resource
state sent in the notification. The change step MUST be greater than
zero otherwise the receiver MUST return a CoAP error code 4.00 "Bad
Request" (or equivalent).
The Change Step parameter can only be supported on resource states
represented with a scalar numeric value.
Note: Due to sampling and other constraints, e.g. pmin, the change in
resource states received in two sequential notifications may differ
by more than st.
3.1.4. Notification Band (band)
The notification band attribute allows a bounded or unbounded (based
on a minimum or maximum) value range that may trigger multiple
notifications. This enables use cases where different ranges results
in differing behaviour. For example, in monitoring the temperature
of machinery, whilst the temperature is in the normal operating
range, only periodic updates are needed. However as the temperature
moves to more abnormal ranges more frequent state updates may be sent
to clients.
Without a notification band, a transition across a less than (lt), or
greater than (gt) limit only generates one notification. This means
that it is not possible to describe a case where multiple
notifications are sent so long as the limit is exceeded.
The band attribute works as a modifier to the behaviour of gt and lt.
Therefore, if band is present in a query, gt, lt or both, MUST be
included.
When band is present with the lt attribute, it defines the lower
bound for the notification band (notification band minimum).
Notifications occur when the resource value is equal to or above the
notification band minimum. If lt is not present there is no minimum
value for the band.
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When band is present with the gt attribute, it defines the upper
bound for the notification band (notification band maximum).
Notifications occur when the resource value is equal to or below the
notification band maximum. If gt is not present there is no maximum
value for the band.
If band is present with both the gt and lt attributes, notification
occurs when the resource value is greater than or equal to gt or when
the resource value is less than or equal to lt.
If a band is specified in which the value of gt is less than that of
lt, in-band notification occurs. That is, notification occurs
whenever the resource value is between the gt and lt values,
including equal to gt or lt.
If the band is specified in which the value of gt is greater than
that of lt, out-of-band notification occurs. That is, notification
occurs when the resource value not between the gt and lt values,
excluding equal to gt and lt.
The Notification Band parameter can only be supported on resources
with a scalar numeric value.
3.1.5. Edge (edge)
When present, the Edge attribute indicates interest for receiving
notifications of either the falling edge or the rising edge
transition of a boolean resource state. When the value of the Edge
attribute is 0, the server notifies the client each time a resource
state changes from True to False. When the value of the Edge
attribute is 1, the server notifies the client each time a resource
state changes from False to True.
The Edge attribute can only be supported on resources with a boolean
value.
3.2. Conditional Control Attributes
Conditional Control Attributes define the time intervals between
consecutive notifications as well as the cadence of the measurement
of the conditions that trigger a notification. Conditional Control
Attributes can be used to configure the internal server-driven
sampling process for performing measurements of the conditions of a
resource. One or more Conditional Control Attributes MAY be included
as query parameters in an Observe request.
Conditional Control Attributes are defined below:
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+-------------------------------+-----------+-----------------+
| Attribute | Parameter | Value |
+-------------------------------+-----------+-----------------+
| Minimum Period (s) | pmin | xs:decimal (>0) |
| | | |
| Maximum Period (s) | pmax | xs:decimal (>0) |
| | | |
| Minimum Evaluation Period (s) | epmin | xs:decimal (>0) |
| | | |
| Maximum Evaluation Period (s) | epmax | xs:decimal (>0) |
| | | |
| Confirmable Notification | con | xs:boolean |
+-------------------------------+-----------+-----------------+
Table 2: Conditional Control Attributes
3.2.1. Minimum Period (pmin)
When present, the minimum period indicates the minimum time, in
seconds, between two consecutive notifications (whether or not the
resource state has changed). In the absence of this parameter, the
minimum period is up to the server. The minimum period MUST be
greater than zero otherwise the receiver MUST return a CoAP error
code 4.00 "Bad Request" (or equivalent).
A server MAY update the resource state with the last sampled value
that occured during the pmin interval, after the pmin interval
expires.
Note: Due to finite quantization effects, the time between
notifications may be greater than pmin even when the sampled value
changes within the pmin interval. Pmin may or may not be used to
drive the internal sampling process.
3.2.2. Maximum Period (pmax)
When present, the maximum period indicates the maximum time, in
seconds, between two consecutive notifications (whether or not the
resource state has changed). In the absence of this parameter, the
maximum period is up to the server. The maximum period MUST be
greater than zero and MUST be greater than, or equal to, the minimum
period parameter (if present) otherwise the receiver MUST return a
CoAP error code 4.00 "Bad Request" (or equivalent).
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3.2.3. Minimum Evaluation Period (epmin)
When present, the minimum evaluation period indicates the minimum
time, in seconds, the client recommends to the server to wait between
two consecutive measurements of the conditions of a resource since
the client has no interest in the server doing more frequent
measurements. When the minimum evaluation period expires after the
previous measurement, the server MAY immediately perform a new
measurement. In the absence of this parameter, the minimum
evaluation period is not defined and thus not used by the server.
The server MAY use pmin, if defined, as a guidance on the desired
measurement cadence. The minimum evaluation period MUST be greater
than zero otherwise the receiver MUST return a CoAP error code 4.00
"Bad Request" (or equivalent).
3.2.4. Maximum Evaluation Period (epmax)
When present, the maximum evaluation period indicates the maximum
time, in seconds, the server MAY wait between two consecutive
measurements of the conditions of a resource. When the maximum
evaluation period expires after the previous measurement, the server
MUST immediately perform a new measurement. In the absence of this
parameter, the maximum evaluation period is not defined and thus not
used by the server. The maximum evaluation period MUST be greater
than zero and MUST be greater than the minimum evaluation period
parameter (if present) otherwise the receiver MUST return a CoAP
error code 4.00 "Bad Request" (or equivalent).
3.2.5. Confirmable Notification (con)
When present with a value of 1 in a query, the con attribute
indicates a notification MUST be confirmable, i.e., the server MUST
send the notification in a confirmable CoAP message, to request an
acknowledgement from the client. When present with a value of 0 in a
query, the con attribute indicates a notification can be confirmable
or non-confirmable, i.e., it can be sent in a confirmable or a non-
confirmable CoAP message.
3.3. Server processing of Conditional Attributes
Conditional Notification Attributes and Conditional Control
Attributes may be present in the same query. However, they are not
defined at multiple prioritization levels. The server sends a
notification whenever any of the parameter conditions are met, upon
which it updates its last notification value and time to prepare for
the next notification. Only one notification occurs when there are
multiple conditions being met at the same time. The reference code
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below illustrates the logic to determine when a notification is to be
sent.
bool notifiable( Resource * r ) {
#define BAND r->band
#define SCALAR_TYPE ( num_type == r->type )
#define STRING_TYPE ( str_type == r->type )
#define BOOLEAN_TYPE ( bool_type == r->type )
#define PMIN_EX ( r->last_sample_time - r->last_rep_time >= r->pmin )
#define PMAX_EX ( r->last_sample_time - r->last_rep_time > r->pmax )
#define LT_EX ( r->v < r->lt ^ r->last_rep_v < r->lt )
#define GT_EX ( r->v > r->gt ^ r->last_rep_v > r->gt )
#define ST_EX ( abs( r->v - r->last_rep_v ) >= r->st )
#define IN_BAND ( ( r->gt <= r->v && r->v <= r->lt ) || ( r->lt <= r->gt && r->gt <= r->v ) || ( r->v <= r->lt && r->lt <= r->gt ) )
#define VB_CHANGE ( r->vb != r->last_rep_vb )
#define VS_CHANGE ( r->vs != r->last_rep_vs )
return (
PMIN_EX &&
( SCALAR_TYPE ?
( ( !BAND && ( GT_EX || LT_EX || ST_EX || PMAX_EX ) ) ||
( BAND && IN_BAND && ( ST_EX || PMAX_EX) ) )
: STRING_TYPE ?
( VS_CHANGE || PMAX_EX )
: BOOLEAN_TYPE ?
( VB_CHANGE || PMAX_EX )
: false )
);
}
Figure 1: Code logic for conditional notification attribute
interactions
4. 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
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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.
+----------------+-----------+-----------+
| Attribute | Parameter | Value |
+----------------+-----------+-----------+
| Binding method | bind | xs:string |
+----------------+-----------+-----------+
Table 3: 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 |
| | | | |
| Execute | exec | Source | POST |
+---------+------------+-------------+---------------+
Table 4: 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 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 parameters in
the Observe request (see Section 3).
4.1.3. Push
The Push method can be used to allow a source endpoint to replace an
outdated resource state at the destination with a newer
representation. When the Push method is assigned to a binding, the
source endpoint sends PUT requests to the destination resource when
the Conditional Notification Attributes are satisfied for the source
resource. The source endpoint SHOULD only send a notification
request if any included Conditional Notification Attributes are met.
The binding entry for this method MUST be stored on the source
endpoint.
4.1.4. Execute
An alternative means for a source endpoint to deliver change-of-state
notifications to a destination resource is to use the Execute Method.
While the Push method simply updates the state of the destination
resource with the representation of the source resource, Execute can
be used when the destination endpoint wishes to receive all state
changes from a source. This allows, for example, the existence of a
resource collection consisting of all the state changes at the
destination endpoint. When the Execute method is assigned to a
binding, the source endpoint sends POST requests to the destination
resource when the Conditional Notification Attributes are satisfied
for the source resource. The source endpoint SHOULD only send a
notification request if any included Conditional Notification
Attributes are met. The binding entry for this method MUST be stored
on the source endpoint.
Note: Both the Push and the Execute methods are examples of Server
Push mechanisms that are being researched in the Thing-to-Thing
Research Group (T2TRG) [I-D.irtf-t2trg-rest-iot].
4.2. Link Relation
Since Binding involves the creation of a link between two resources,
Web Linking and the CoRE Link-Format used to represent binding
information. This involves the 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 and the context URI
points to the destination resource.
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5. Binding Table
The Binding Table is a special resource that describes the bindings
on an endpoint. An endpoint offering a representation of the Binding
Table resource SHOULD indicate its presence and enable its discovery
by advertising a link at "/.well-known/core" [RFC6690]. If so, the
Binding Table resource MUST be discoverable by using the Resource
Type (rt) 'core.bnd'.
The Methods column defines the REST methods supported by the Binding
Table, which are described in more detail below.
+---------------+----------+----------+----------------+
| Resource | rt= | Methods | Content-Format |
+---------------+----------+----------+----------------+
| Binding Table | core.bnd | GET, PUT | link-format |
+---------------+----------+----------+----------------+
Table 5: Binding Table Description
The REST methods GET and PUT are used to manipulate a Binding Table.
A GET request simply returns the current state of a Binding Table. A
request with a PUT method and a content format of application/link-
format is used to clear the bindings to the table or replaces its
entire contents. All links in the payload of a PUT rquest MUST have
a relation type "boundto".
The following example shows requests for discovering, retrieving and
replacing bindings in a binding table.
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Req: 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;anchor=/a/fan,;bind="obs",
<coap://sensor.example.com/a/switch2/>;
rel=boundto;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
Figure 2: Binding Table Example
Additional operations on the Binding Table can be specified in future
documents. Such operations can include, for example, the usage of
the iPATCH or PATCH methods [RFC8132] for fine-grained addition and
removal of individual bindings or binding subsets.
6. Implementation Considerations
When pmax and pmin are equal, the expected behaviour is that
notifications will be sent every (pmin == pmax) seconds. However,
these notifications can only be fulfilled by the server on a best
effort basis. Because pmin and pmax are designed as acceptable
tolerance bounds for sending state updates, a query from an
interested client containing equal pmin and pmax values must not be
seen as a hard real-time scheduling contract between the client and
the server.
When using multiple resource bindings (e.g. multiple Observations of
resource) with different bands, consideration should be given to the
resolution of the resource value when setting sequential bands. For
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.
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The use of the notification band minimum and maximum allow for a
synchronization whenever a change in the resource value occurs.
Theoretically this could occur in-line with the server internal
sample period or the configuration of epmin and epmax values for
determining the resource value. Implementors SHOULD consider the
resolution needed before updating the resource, e.g. updating the
resource when a temperature sensor value changes by 0.001 degree
versus 1 degree.
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. Implementation considerations have to be given
to how to monitor transactions made by the configuration tool with
regards to Link Bindings, as well as any errors that may arise with
establishing Link Bindings in addition to established Link Bindings.
When a server has multiple observations with different measurement
cadences as defined by the epmin and epmax values, the server MAY
evaluate all observations when performing the measurement of any one
observation.
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 and handling error conditions can be processed by the state
machine.
8. IANA Considerations
8.1. Resource Type value 'core.bnd'
This specification registers a new Resource Type Link Target
Attribute 'core.bnd' in the Resource Type (rt=) registry established
as per [RFC6690].
Attribute Value: core.bnd
Description: See Section 5. This attribute value is used to discover
the resource representing 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.
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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. Hannes Tschofenig and Mert Ocak highlighted syntactical
corrections in the usage of pmax and pmin in a query. Discussions
with Ari Keraenen led to the addition of an extra binding method
supporting POST operations. Alan Soloway contributed text leading to
the inclusion of epmin and epmax. David Navarro proposed allowing
for pmax to be equal to pmin.
10. Contributors
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Christian Groves
Australia
email: cngroves.std@gmail.com
Zach Shelby
ARM
Vuokatti
FINLAND
phone: +358 40 7796297
email: zach.shelby@arm.com
Matthieu Vial
Schneider-Electric
Grenoble
France
phone: +33 (0)47657 6522
eMail: matthieu.vial@schneider-electric.com
Jintao Zhu
Huawei
Xi'an, Shaanxi Province
China
email: jintao.zhu@huawei.com
11. Changelog
draft-ietf-core-dynlink-13
o Conditional Atttributes section restructured
o "edge" and "con" attributes added
o Implementation considerations, clarifications added when pmax ==
pmin
o rewritten to remove talk of server reporting values to clients
draft-ietf-core-dynlink-12
o Attributes epmin and epmax included
o pmax now can be equal to pmin
draft-ietf-core-dynlink-11
o Updates to author list
draft-ietf-core-dynlink-10
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o Binding methods now support both POST and PUT operations for
server push.
draft-ietf-core-dynlink-09
o Corrections in Table 1, Table 2, Figure 2.
o Clarifications for additional operations to binding table added in
section 5
o Additional examples in Appendix A
draft-ietf-core-dynlink-08
o Reorganize the draft to introduce Conditional Notification
Attributes at the beginning
o Made pmin and pmax type xs: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
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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:
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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
[I-D.irtf-t2trg-rest-iot]
Keranen, A., Kovatsch, M., and K. Hartke, "RESTful Design
for Internet of Things Systems", draft-irtf-t2trg-rest-
iot-06 (work in progress), May 2020.
[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>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
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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. Minimum Period (pmin) 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: pmin="10"
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
13 | | ____________
14 | |
15 | | 23 Cel
16 | |
17 | |
18 | |
19 | | ____________
20 ____________ |<-----+ Header: 2.05
21 | 2.05 | 26 Cel Token: 0x4a
22 26 Cel | | Observe: 20
23 | | Payload: "26 Cel"
24 | |
25 | |
Figure 3: Client registers and receives one notification of the
current state and one of a new state state when pmin time expires.
A.2. Maximum Period (pmax) example
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
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2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: pmax="20"
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
13 | |
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05
17 | 2.05 | 23 Cel Token: 0x4a
18 23 Cel | | Observe: 16
19 | | Payload: "23 Cel"
20 | |
21 | |
22 | |
23 | |
24 | |
25 | |
26 | |
27 | |
28 | |
29 | |
30 | |
31 | |
32 | |
33 | |
34 | |
35 | |
36 | | ____________
37 ____________ |<-----+ Header: 2.05
38 | 2.05 | 23 Cel Token: 0x4a
39 23 Cel | | Observe: 37
40 | | Payload: "23 Cel"
41 | |
42 | |
Figure 4: Client registers and receives one notification of the
current state, one of a new state and one of an unchanged state when
pmax time expires.
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A.3. Greater Than (gt) example
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: 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 5: Client registers and receives one notification of the
current state and one of a new state when it passes through the
greater than threshold of 25.
A.4. Greater Than (gt) and Period Max (pmax) example
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: pmax=20;gt=25
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
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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 6: Client registers and receives one notification of the
current state, one when pmax time expires and one of a new state when
it passes through the greater than threshold of 25.
Authors' Addresses
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View 94043
USA
Email: michael.koster@smartthings.com
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Bilhanan Silverajan (editor)
Tampere University
Kalevantie 4
Tampere FI-33100
Finland
Email: bilhanan.silverajan@tuni.fi
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