CoRE Working Group M. Koster
Internet-Draft SmartThings
Intended status: Informational B. Silverajan, Ed.
Expires: January 13, 2022 Tampere University
July 12, 2021
Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-14
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).
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 13, 2022.
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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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Link Bindings . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The "bind" attribute and Binding Methods . . . . . . . . 4
3.1.1. Polling . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Observe . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.3. Push . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.4. Execute . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Link Relation . . . . . . . . . . . . . . . . . . . . . . 6
4. Binding Table . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Implementation Considerations . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7.1. Resource Type value 'core.bnd' . . . . . . . . . . . . . 8
7.2. Link Relation Type . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [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.
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 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).
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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
[I-D.ietf-core-conditional-attributes] can be used with Link Bindings
in order to customize the notification behavior and timing.
3.1. 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 1: The bind attribute
The following table gives a summary of the binding methods defined in
this specification.
+---------+------------+-------------+---------------+
| Name | Identifier | Location | Method |
+---------+------------+-------------+---------------+
| Polling | poll | Destination | GET |
| | | | |
| Observe | obs | Destination | GET + Observe |
| | | | |
| Push | push | Source | PUT |
| | | | |
| Execute | exec | Source | POST |
+---------+------------+-------------+---------------+
Table 2: Binding Method Summary
The description of a binding method defines the following aspects:
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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
defined in [I-D.ietf-core-conditional-attributes] are used in the
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 defined in [I-D.ietf-core-conditional-attributes]).
3.1.2. Observe
The Observe method creates an observation relationship between the
destination endpoint and the source resource. On each notification
the content from the source resource is copied to the destination
resource. The creation of the observation relationship requires the
CoAP Observation mechanism [RFC7641] hence this method is only
permitted when the resources are made available over CoAP. The
binding entry for this method MUST be stored on the destination
endpoint. The binding conditions are mapped as query parameters in
the Observe request (see [I-D.ietf-core-conditional-attributes]).
3.1.3. 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.
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The binding entry for this method MUST be stored on the source
endpoint.
3.1.4. 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].
3.2. 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.
4. 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.
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+---------------+----------+----------+----------------+
| Resource | rt= | Methods | Content-Format |
+---------------+----------+----------+----------------+
| Binding Table | core.bnd | GET, PUT | link-format |
+---------------+----------+----------+----------------+
Table 3: 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.
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 1: 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.
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5. Implementation 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. 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.
6. 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.
7. IANA Considerations
7.1. 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 4. 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.
Notes: None
7.2. 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
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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
8. 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. Discussions with Ari Keraenen led to the addition of an
extra binding method supporting POST operations.
9. Contributors
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
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10. Changelog
draft-ietf-core-dynlink-14
o Conditional Atttributes section removed and submitted as draft-
ietf-core-conditional-attributes-00
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
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
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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
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".
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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.
11. References
11.1. 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>.
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[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>.
11.2. Informative References
[I-D.ietf-core-conditional-attributes]
Koster, M. and B. Silverajan, "Conditional Attributes for
Constrained RESTful Environments", draft-ietf-core-
conditional-attributes-00 (work in progress), July 2021.
[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-07 (work in progress), February 2021.
[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>.
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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