Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-00
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| Authors | Zach Shelby , Matthieu Vial , Michael Koster , Christian Groves | ||
| Last updated | 2016-10-19 | ||
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draft-ietf-core-dynlink-00
CoRE Working Group Z. Shelby
Internet-Draft ARM
Intended status: Informational Z. Vial
Expires: April 22, 2017 Schneider-Electric
M. Koster
SmartThings
C. Groves
Huawei
October 19, 2016
Dynamic Resource Linking for Constrained RESTful Environments
draft-ietf-core-dynlink-00
Abstract
For CoAP [RFC7252] Dynamic linking of state updates between
resources, either on an endpoint or between endpoints, is defined
with the concept of Link Bindings. This document defines conditional
observation attributes that work with Link Bindings or with simple
CoAP Observe [RFC7641].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Link Bindings and Observe Attributes . . . . . . . . . . . . 3
3.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Binding Methods . . . . . . . . . . . . . . . . . . . . . 5
3.3. Binding Table . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Resource Observation Attributes . . . . . . . . . . . . . 6
4. Interface Descriptions . . . . . . . . . . . . . . . . . . . 8
4.1. Binding . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
IETF Standards for machine to machine communication in constrained
environments describe a REST protocol and a set of related
information standards that may be used to represent machine data and
machine metadata in REST interfaces. CoRE Link-format is a standard
for doing Web Linking [RFC5988] in constrained environments.
This document introduces the concept of a Link Binding, which defines
a new link relation type to create a dynamic link between resources
over which to exchange state updates. Specifically, a Link Binding
is a link for binding the state of 2 resources together such that
updates to one are sent over the link to the other. CoRE Link Format
representations are used to configure, inspect, and maintain Link
Bindings. This document additionally defines a set of conditional
Observe Attributes for use with Link Bindings and with the standalone
CoRE Observe [RFC7641] method.
Editor's note: This initial version is based on the text of I.D.ietf-
core-interfaces-04. Further work is needed around link bindings and
extending the obeserve attributes with another use case that requires
3 new optional attributes.
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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
[RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC5988] and [RFC6690]. This
specification makes use of the following additional terminology:
Link Binding: A unidirectional logical link between a source
resource and a destination resource, over which state information
is synchronized.
3. Link Bindings and Observe Attributes
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 document the abstract relationship between two resources is
called a link Binding. The configuration phase necessitates the
exchange of binding information so a format recognized by all CoRE
endpoints is essential. This document defines a format based on the
CoRE Link-Format to represent binding information along with the
rules to define a binding method which is a specialized relationship
between two resources. The purpose of a binding is to synchronize
the content between a source resource and a destination resource.
The destination resource MAY be a group resource if the authority
component of the destination URI contains a group address (either a
multicast address or a name that resolves to a multicast address).
Since a binding is unidirectional, the binding entry defining a
relationship is present only on one endpoint. The binding entry may
be located either on the source or the destination endpoint depending
on the binding method. The following table gives a summary of the
binding methods described in more detail in Section 3.2
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+---------+------------+-------------+---------------+
| Name | Identifier | Location | Method |
+---------+------------+-------------+---------------+
| Polling | poll | Destination | GET |
| | | | |
| Observe | obs | Destination | GET + Observe |
| | | | |
| Push | push | Source | PUT |
+---------+------------+-------------+---------------+
Table 1: Binding Method Summary
3.1. Format
Since Binding involves the creation of a link between two resources,
Web Linking and the CoRE Link-Format are a natural way to represent
binding information. This involves the creation of a new relation
type, purposely named "boundto". In a Web link with this relation
type, the target URI contains the location of the source resource and
the context URI points to the destination resource. The Web link
attributes allow a fine-grained control of the type of
synchronization exchange along with the conditions that trigger an
update. This specification defines the attributes below:
+--------------------+-----------+------------------+
| Attribute | Parameter | Value |
+--------------------+-----------+------------------+
| Binding method | bind | xsd:string |
| | | |
| Minimum Period (s) | pmin | xsd:integer (>0) |
| | | |
| Maximum Period (s) | pmax | xsd:integer (>0) |
| | | |
| Change Step | st | xsd:decimal (>0) |
| | | |
| Greater Than | gt | xsd:decimal |
| | | |
| Less Than | lt | xsd:decimal |
+--------------------+-----------+------------------+
Table 2: Binding Attributes Summary
Bind Method: This is the identifier of a binding method which
defines the rules to synchronize the destination resource. This
attribute is mandatory.
Minimum Period: When present, the minimum period indicates the
minimum time to wait (in seconds) before sending a new
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synchronization message (even if it has changed). In the absence
of this parameter, the minimum period is up to the notifier.
Maximum Period: When present, the maximum period indicates the
maximum time in seconds between two consecutive state
synchronization messages (regardless if it has changed). In the
absence of this parameter, the maximum period is up to the
notifier. The maximum period MUST be greater than the minimum
period parameter (if present).
Change Step: When present, the change step indicates how much the
value of a resource SHOULD change before sending a new
notification (compared to the value of the last notification).
This parameter has lower priority than the period parameters, thus
even if the change step has been fulfilled, the time since the
last notification SHOULD be between pmin and pmax.
Greater Than: When present, Greater Than indicates the upper limit
value the resource value SHOULD cross before sending a new
notification. This parameter has lower priority than the period
parameters, thus even if the Greater Than limit has been crossed,
the time since the last notification SHOULD be between pmin and
pmax.
Less Than: When present, Less Than indicates the lower limit value
the resource value SHOULD cross before sending a new notification.
This parameter has lower priority than the period parameters, thus
even if the Less Than limit has been crossed, the time since the
last notification SHOULD be between pmin and pmax.
3.2. Binding Methods
A binding method defines the rules to generate the web-transfer
exchanges that will effectively send content from the source resource
to the destination resource. The description of a binding method
must define the following aspects:
Identifier: This is 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.
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Conditions: A binding method definition must state how the condition
attributes of the abstract binding definition are actually used in
this specialized binding.
This specification supports 3 binding methods described below:
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 Change
Step, Less Than, Greater Than).
Observe: The Observe method creates an observation relationship
between the destination endpoint and the source resource. On each
notification the content from the source resource is copied to the
destination resource. The creation of the observation
relationship requires the CoAP Observation mechanism [RFC7641]
hence this method is only permitted when the resources are made
available over CoAP. The binding entry for this method MUST be
stored on the destination endpoint. The binding conditions are
mapped as query string parameters (see Section 3.4).
Push: When the Push method is assigned to a binding, the source
endpoint sends PUT requests to the destination resource when the
binding condition attributes are satisfied for the source
resource. The source endpoint MUST only send a notification
request if the binding conditions are met. The binding entry for
this method MUST be stored on the source endpoint.
3.3. Binding Table
The binding table is a special resource that gives access to the
bindings on a endpoint. A binding table resource MUST support the
Binding interface defined in Section 4.1. A profile SHOULD allow
only one resource table per endpoint.
3.4. Resource Observation Attributes
When resource interfaces following this specification are made
available over CoAP, the CoAP Observation mechanism [RFC7641] MAY be
used to observe any changes in a resource, and receive asynchronous
notifications as a result. In addition, a set of query string
parameters are defined here to allow a client to control how often a
client is interested in receiving notifications and how much a
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resource value should change for the new representation to be
interesting. These query parameters are described in the following
table. A resource using an interface description defined in this
specification and marked as Observable in its link description SHOULD
support these observation parameters. The Change Step parameter can
only be supported on resources with an atomic numeric value.
These query parameters MUST be treated as resources that are read
using GET and updated using PUT, and MUST NOT be included in the
Observe request. Multiple parameters MAY be updated at the same time
by including the values in the query string of a PUT. Before being
updated, these parameters have no default value.
+----------------+------------------+------------------+
| Resource | Parameter | Data Format |
+----------------+------------------+------------------+
| Minimum Period | /{resource}?pmin | xsd:integer (>0) |
| | | |
| Maximum Period | /{resource}?pmax | xsd:integer (>0) |
| | | |
| Change Step | /{resource}?st | xsd:decimal (>0) |
| | | |
| Less Than | /{resource}?lt | xsd:decimal |
| | | |
| Greater Than | /{resource}?gt | xsd:decimal |
+----------------+------------------+------------------+
Table 3: Resource Observation Attribute Summary
Minimum Period: When present, the minimum period indicates the
minimum time to wait (in seconds) before sending a new
synchronization message (even if it has changed). In the absence
of this parameter, the minimum period is up to the notifier.
Maximum Period: When present, the maximum period indicates the
maximum time in seconds between two consecutive state
synchronization messages (regardless if it has changed). In the
absence of this parameter, the maximum period is up to the
notifier. The maximum period MUST be greater than the minimum
period parameter (if present).
Change Step: When present, the change step indicates how much the
value of a resource SHOULD change before sending a new
notification (compared to the value of the last notification).
This parameter has lower priority than the period parameters, thus
even if the change step has been fulfilled, the time since the
last notification SHOULD be between pmin and pmax.
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Greater Than: When present, Greater Than indicates the upper limit
value the resource value SHOULD cross before sending a new
notification. This parameter has lower priority than the period
parameters, thus even if the Greater Than limit has been crossed,
the time since the last notification SHOULD be between pmin and
pmax.
Less Than: When present, Less Than indicates the lower limit value
the resource value SHOULD cross before sending a new notification.
This parameter has lower priority than the period parameters, thus
even if the Less Than limit has been crossed, the time since the
last notification SHOULD be between pmin and pmax.
4. Interface Descriptions
This section defines REST interfaces for Binding table resources.
The interface supports the link-format type.
The if= column defines the Interface Description (if=) attribute
value to be used in the CoRE Link Format for a resource conforming to
that interface. When this value appears in the if= attribute of a
link, the resource MUST support the corresponding REST interface
described in this section. The resource MAY support additional
functionality, which is out of scope for this specification.
Although this interface descriptions is intended to be used with the
CoRE Link Format, it is applicable for use in any REST interface
definition.
The Methods column defines the methods supported by that interface,
which are described in more detail below.
+-----------+----------+-------------------+-----------------+
| Interface | if= | Methods | Content-Formats |
+-----------+----------+-------------------+-----------------+
| Binding | core.bnd | GET, POST, DELETE | link-format |
+-----------+----------+-------------------+-----------------+
Table 4: Inteface Description
4.1. Binding
The Binding interface is used to manipulate a binding table. A
request with a POST method and a content format of application/link-
format simply appends new bindings to the table. All links in the
payload MUST have a relation type "boundTo". A GET request simply
returns the current state of a binding table whereas a DELETE request
empties the table.
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The following example shows requests for adding, retrieving and
deleting bindings in a binding table.
Req: POST /bnd/ (Content-Format: application/link-format)
<coap://sensor.example.com/s/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/
Res: 2.04 Changed
Figure 1: Binding Interface Example
5. Security Considerations
An implementation of a client needs to be prepared to deal with
responses to a request that differ from what is specified in this
document. A server implementing what the client thinks is a resource
with one of these interface descriptions could return malformed
representations and response codes either by accident or maliciously.
A server sending maliciously malformed responses could attempt to
take advantage of a poorly implemented client for example to crash
the node or perform denial of service.
6. IANA Considerations
The "binding" interface description types requires registration
The new link relations type "boundto" requires registration.
7. Acknowledgements
Acknowledgement is given to colleagues from the SENSEI project who
were critical in the initial development of the well-known REST
interface concept, to members of the IPSO Alliance where further
requirements for interface types have been discussed, and to Szymon
Sasin, Cedric Chauvenet, Daniel Gavelle and Carsten Bormann who have
provided useful discussion and input to the concepts in this
document.
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8. Changelog
draft-ietf-core-dynlink Initial Version 00:
o This is a copy of draft-groves-core-dynlink-00
draft-groves-core-dynlink Draft Initial Version 00:
o This initial version is based on the text regarding the dynamic
linking functionality in I.D.ietf-core-interfaces-05.
o The WADL description has been dropped in favour of a thorough
textual description of the REST API.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<http://www.rfc-editor.org/info/rfc5988>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
9.2. Informative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
Authors' Addresses
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Zach Shelby
ARM
150 Rose Orchard
San Jose 95134
FINLAND
Phone: +1-408-203-9434
Email: zach.shelby@arm.com
Matthieu Vial
Schneider-Electric
Grenoble
FRANCE
Phone: +33 (0)47657 6522
Email: matthieu.vial@schneider-electric.com
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View 94043
USA
Email: michael.koster@smartthings.com
Christian Groves
Huawei
Australia
Email: Christian.Groves@nteczone.com
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