Internet DRAFT - draft-zotti-core-sleepy-nodes
draft-zotti-core-sleepy-nodes
CoRE Working Group T. Zotti
Internet-Draft Philips Research
Intended status: Informational P. van der Stok
Expires: March 31, 2016 Consultant
E. Dijk
Philips Research
September 28, 2015
Sleepy CoAP Nodes
draft-zotti-core-sleepy-nodes-04
Abstract
Control networks rely on application protocols like CoAP to enable
RESTful communications in constrained environments. Many of these
networks make use of "Sleepy Nodes": battery powered devices that
switch off their (radio) interface during most of the time to
conserve battery energy. As a result of this, Sleepy Nodes cannot be
reached most of the time. This fact prevents using normal
communication patterns as specified in the CoRE group, since the
server-model is not applicable to these devices. This document
discusses and specifies an architecture to support Sleepy Nodes such
as battery-powered sensors in mesh networks with the goal of
proposing a standardisation solution for Sleepy Node proxies.
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
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This Internet-Draft will expire on March 31, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problem statement . . . . . . . . . . . . . . . . . . . . 3
1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.4. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Use cases and architecture . . . . . . . . . . . . . . . . . 5
2.1. Node interactions and use cases . . . . . . . . . . . . . 6
2.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Example contents . . . . . . . . . . . . . . . . . . . . 10
3. Design motivation . . . . . . . . . . . . . . . . . . . . . . 10
4. Interactions involving Resource Directory . . . . . . . . . . 10
5. Synchronize interface . . . . . . . . . . . . . . . . . . . . 12
5.1. Sleepy Node discovers proxy . . . . . . . . . . . . . . . 12
5.2. Registration at a Proxy . . . . . . . . . . . . . . . . . 12
5.3. De-registration at a Proxy . . . . . . . . . . . . . . . 15
5.4. Initialization of delegated resource . . . . . . . . . . 16
5.5. Sleepy Node updates delegated resource at Proxy . . . . . 17
5.6. Sleepy Node READs resource updates from Proxy . . . . . . 18
6. Delegate Interface . . . . . . . . . . . . . . . . . . . . . 18
6.1. Discovering Endpoint discovers Sleepy Node at Proxy . . . 19
6.2. Proxy REPORTs events to Endpoint . . . . . . . . . . . . 20
6.3. A Node WRITEs to Sleepy Node via Proxy . . . . . . . . . 21
6.4. A Node READs information from Sleepy Node via Proxy . . . 22
7. Direct Interface . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Sleepy Node REPORTs events directly to Destination Node . 22
7.2. A Sleepy Node READs information from a Server Node . . . 23
8. Realization with PubSub broker . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 24
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
13.1. Normative References . . . . . . . . . . . . . . . . . . 25
13.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
Control networks rely on application protocols such as CoAP to enable
RESTful communications in constrained environments. Many of these
networks feature "Sleepy Nodes": battery-powered nodes which switch
on/off their communication interface to conserve battery energy. As
a result of this, Sleepy Nodes cannot be reached most of the time.
This fact prevents using normal communication patterns as specified
by the CoRE group, since the server model is clearly not applicable
to the most energy constrained devices.
This document discusses and specifies an architecture to support
Sleepy Nodes such as battery-powered sensors in wireless networks.
The proposed solution makes use of a Proxy Node to which a Sleepy
Node delegates part of its communication tasks while it is not
accessible in the wireless network. Direct interactions between
Sleepy Nodes and non-Sleepy Nodes are only possible, when the Sleepy
Node initiates the communication.
Earlier related documents treating the Sleepy Node subject are the
CoRE mirror server [I-D.vial-core-mirror-server] and the Publish-
Subscribe in the Constrained Application Protocol (CoAP)
[I-D.koster-core-coap-pubsub]. Both documents describe the
interfaces to the proxy accompanying the Sleepy Node. Both make use
of the observe option discussed in [I-D.ietf-core-observe]. This
document describes the roles of the nodes communicating with the
Sleepy Node and/or its proxy. The draft describes the differences
between the concepts supporting the Sleepy Node, and the concepts
underlying the PubSub paradigm.
The draft relies heavily on the concepts introduced by the Resource
Directory [I-D.ietf-core-resource-directory], and describes how the
Sleepy Node profits of the introduction of a Resource Directory into
the network.
The issues that need to be addressed to provide support for Sleepy
Nodes in Control networks are summarized in Section 1.1. Section 2
provides a set of use case descriptions that introduce communication
patterns to be used in home and building control scenarios.
Section 4, Section 5,Section 6, and Section 7 specify interfaces to
support each of these scenarios. Many interface specifications and
examples are taken over from [I-D.vial-core-mirror-server].
1.1. Problem statement
During typical operation, a Sleepy Node has its radio disabled and
the CPU may be in a sleeping state. If an external event occurs
(e.g. person walks into the room activating a presence sensor), the
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CPU and radio are powered back on and they send out a message to
another node, or to a group of nodes. After sending this message,
the radio and CPU are powered off again, and the Sleepy Node sleeps
until the next external event or until a predefined time period has
passed. The main problems when introducing Sleepy Nodes into a
wireless network are as follows:
Problem 1: How to contact a Sleepy Node that has its radio turned off
most of the time for:
- Writing configuration settings.
- Reading out sensor data, settings or log data.
- Configuring additional event destination nodes or node groups.
Problem 2: How to discover a Sleepy Node and its services, while the
node is asleep:
- Direct node discovery (CoAP GET /.well-known/core as defined in
[RFC7252]) does not find the node with high probability.
- Mechanisms may be needed to provide, as the result of node
discovery, the IP address of a Proxy instead of the IP address of
the node directly.
Problem 3: How a Sleepy Node can convey data to a node or groups of
nodes, with good reliability and minimal energy consumption.
1.2. Assumptions
The solution architecture specified here assumes that a Sleepy Node
has enough energy to perform bidirectional communication during its
normal operational state. This solution may be applicable also to
extreme low-power devices such as solar powered sensors as long as
they have enough energy to perform commissioning and the initial
registration steps. These installation operations may require, in
some cases, an additional source of power. Since a Sleepy Node is
unreachable for relatively long periods of times, the data exchanges
in the interaction model are always initiated by a Sleepy Node when
its sleep period ends.
1.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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This document assumes readers are familiar with the terms and
concepts discussed in [RFC7252],[RFC5988],
[I-D.ietf-core-resource-directory],
[I-D.ietf-core-interfaces],[I-D.ietf-core-observe] and
[I-D.vial-core-mirror-server].
In addition, this document makes use of the following additional
terminology:
Sleepy Node: a battery-powered node which does the on/off switching
of its communication interface with the purpose of conserving battery
energy
Sleeping/Asleep: A Sleepy Node being in a "sleeping state" i.e. its
network interface is switched off and a Sleepy Node is not able to
send or receive messages.
Awake/Not Sleeping: A Sleepy Node being in an "awake state" i.e. its
network interface is switched on and the Sleepy Node is able to send
or receive messages.
Wake up reporting duration: the duration between a wake up from a
Sleepy Node and the next wake up and report of the same Node.
Proxy: any node that is configured to, or selected to, perform
communication tasks on behalf of one or more Sleepy Nodes.
Regular Node: any node in the network which is not a Proxy or a
Sleepy Node.
1.4. Acronyms
This Internet-Draft contains the following acronyms:
DTLS: Datagram Transport Layer Security
EP: Endpoint
MC: Multicast
RD: Resource Directory
2. Use cases and architecture
To describe the application viewpoint of the solution, we introduce
some example scenarios for the various interactions shown in
Figure 1. The figure assigns the following roles taken up by a
regular node:
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o Reading Node: any regular node that reads information from the
Sleepy Node.
o Configuring Node: any regular node that writes information/
configuration into Sleepy Node(s). Examples of configuration are
new thresholds for a sensor or a new value for the wake-up cycle
time.
o Discovering Node: any regular node that performs discovery of the
nodes in a network, including Sleepy Nodes.
o Destination Node: any regular node or node in a group that
receives a message that is generated by the Sleepy Node.
o Server Node: an optional server that the Sleepy Node knows about,
or is told about, which is used to fetch
information/configuration/firmware updates/etc.
o Discovery Server: an optional server that enables nodes to
discover all the devices in the network, including Sleepy Nodes,
and query their capabilities. For example, a Resource Directory
server as defined in [I-D.ietf-core-resource-directory] or a DNS-
SD server as defined in [RFC6763]. For the rest of this document
the discovery server is a Resource Directory. Specifically, the
functionalities of the Resource Directory related to the
architecture presented in this Internet-Draft are described in
more details in Section 4.
o Delegated resource is the copy at the Proxy of a resource present
in the Sleepy Node.
2.1. Node interactions and use cases
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+------------+ +-------------+
| Discovery | <-DISCOVERY-| Discovering |
| server | | Node |
| (Optional) | +-------------+
+------------+ |
|
.--DISCOVERY--' +---------+
| | Reading |
| .---| Node |
v | +---------+
+---------+ +-----------+ |
| Sleepy |---REPORT(A)-->| |<--READ--' +-------------+
| Node |---READ------->| Proxy |<--WRITE---| Configuring |
| |---WRITE------>| | | Node |
+---------+ +-----------+ +-------------+
| | | +-------------+
| | '---REPORT(B)->| Destination |
| '-----DIRECT REPORT---------------------->| Node |
| +-------------+
| +-----------+
'------------READ--------------------------->| Server |
| Node |
+-----------+
Figure 1: Interaction model for Sleepy Nodes in IP-based networks
The interactions visualized in Figure 1 are discussed and motivated
with their use cases. The arrows in the figure indicate that the
initiative for an interaction is taken by the source of the arrow.
DISCOVERY Interaction: a Discovering Node discovers Sleepy Node(s)via
Proxy or Discovery Server; for example:
- A Discovering Node wants to discover given services related to a
group of deployed sensors by sending a multicast to /.well-known/
core. It gets responses for the sleeping sensors from the Proxy
nodes.
- During commissioning phase, a discovering node queries a
Discovery Server to find all the proxies providing a given
service.
REPORT Interaction: On request of a Destination Node or because of
configuration settings which have instructed the Node to do so, a
Node sends a sequence of event notifications to destination Node(s),
(A) directly or (B) via Proxy; for example:
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- A battery-powered sensor sends a notification with "battery low"
event directly to a designated Destination Node (REPORT(A)).
- A battery-powered occupancy sensor detects an event "people
present", switches on the radio and multicasts an "ON" command to
a group of lights (REPORT(A)).
- A battery-powered temperature sensor reports periodically the
room temperature to a proxy Node (REPORT(A)). The proxy node
reports to all associated HVAC destination nodes when the
temperature change deviates from a predefined range (REPORT(B)).
WRITE Interaction: A node sends a request to a proxy to set a value.
o A Sleepy Node WRITES to the proxy; for example:
- A battery-powered sensor wants to extend the registration
lifetime of its delegated resource at the Proxy.
o A configuring Node WRITEs information to a Proxy; for example:
- A configuring Node changes the reporting frequency of a
deployed sensor by contacting the Proxy node to which the
sensor is registered.
- Sensor firmware is upgraded. A configuring Node pushes
firmware data blocks to the Proxy, which pushes the blocks to
the Sleepy Node.
- A configuring Node adds a new subscription to an operational
sensor via the Proxy. From that moment on, the new Node
receives also the sensor events and status updates from the
sensor.
READ Interaction: A node sends a read request to a node that returns
a value.
o Sleepy Node sends a read request to a server Node; for example:
- A sensor (periodically) updates internal data tables by
fetching it from a predetermined remote node.
- A sensor (periodically) checks for new firmware with a remote
node. If new firmware is found, the sensor switches to a non-
sleepy operation mode, and fetches the data.
o A Sleepy Node sends a read request to its proxy; for example:
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- A sensor (periodically) checks with his Proxy availability of
configuration updates or changes of its delegated resources
(e.g. a sensor may detect in this way that a configuring Node
has changed its name or modified its reporting frequency).
o A reading Node sends a read request to a proxy; for example:
- A Node (e.g. in the backend) requests the status of a
deployed sensor, e.g. asking the sensor state and/or firmware
version and/or battery status and/or its error log. The Proxy
returns this information.
- A Node requests a Proxy when a Sleepy sensor was 'last
active' (i.e. identified as being awake) in the network.
2.2. Architecture
The architecture associated with the support of Sleepy Nodes is
illustrated in Figure 2. Three High level interfaces are shown.
direct synchronize delegate
| | |
+----+ | +--------+ | +-------+ | +----+
| EP |---|---| sleepy |---|---| proxy |---|---| EP |
+----+ | +--------+ | +-------+ | +----+
| | |
Figure 2: Architecture of Sleepy Node support
o Direct interface: it allows the Sleepy Node to communicate
directly to endpoints (i.e. for sending or reading information).
The operations performed via this interface are always initiated
by the Sleepy Node when its sleep period ends.
o Delegate interface: via this interface the Proxy exposes the
values of delegated resources to interested endpoints on behalf of
the Sleepy Node. The same interface is used by endpoints which
want to communicate with the Sleepy Node (e.g. for reading or
writing information).
o Synchronize interface: used by Sleepy Node and Proxy to
synchronize values of delegated resources. Through this interface
operations as discovery of the Proxy, registration, initialization
and update of resources at the Proxy are performed, along with a
de-registration operation to explicitly remove resources already
registered to the Proxy.
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The interfaces consist of a set of functions which together realize
the interactions described in Section 2.1.
Endpoints and the proxy communicate with a Resource Directory (RD) to
discover resources of the Sleepy Node and delegated resources on the
proxy (not shown in the Figure 2).
2.3. Example contents
The examples presented in this specification make use of a smart
temperature sensor the resources of which are defined below using
Link Format [RFC6690]. Three resources are dedicated to the Device
Description (manufacturer, model, name) and one contains the current
temperature in degree Celsius.
</dev/mfg >;rt="ipso.dev.mfg";if="core.rp",
</dev/mdl>;rt="ipso.dev.mdl";if="core.rp",
</dev/n>;rt="ipso.dev.n";if="core.p",
</sen/temp>;rt="ucum.Cel";if="core.s"
3. Design motivation
The Sleepy Node stack features a CoAP interface, to make the Sleepy
Node part of the IP-based network. Adding CoAP with a transport
protocol increases the possibilities to configure the Sleepy Node
within the network. The increased energy consumption coming from the
overhead of the CoAP and IP headers can be acceptable in many cases.
The proxy and Sleepy Node make use of the /.well-known/core resource
to handle discovery during network initialization. Using the
Resource Directory during operation of the Sleepy Node reduces its
participation in the discovery traffic.
A Sleepy Node delegates its resources to a proxy. The proxy
functionality extends the functionality of the RD, because the proxy
handles the value of the resource, and the RD does not. A proxy may
support multiple Sleepy Nodes. A Sleepy Node may also delegate its
resources to multiple proxies. A node can select a proxy that
handles the resource of the Sleepy Node of choice.
The complexity of the discovery and delegation interfaces is
minimized by reusing the RD interface as much as possible.
4. Interactions involving Resource Directory
It is assumed that the Proxy has a resource type rt="core.sp", where
sp stands for sleepy proxy.
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In order to become fully operational in a network and to communicate
over the functional interfaces shown in Figure 2, a Sleepy Node and
the Proxy need to perform operations via the Registration interface
of the RD:
- Discovery of Proxy via RD. The Sleepy Node MAY discover the
Proxy by sending a request to the RD to return all EP with
rt=core.sp.
- Register existence of Proxy. When a RD is present and a Sleepy
Node has registered itself to a Proxy (see Section 5.2), the Proxy
MUST register the Sleepy Node at the RD and MUST keep this
registration up-to-date.
- Register delegated resources. When a RD is present, the Proxy
MUST register the delegated resources at the RD and keep them up-
to date.
A Configuring Endpoint (often part of a so-called Commissioning Tool)
registers the services that are reported directly by the Sleepy Node
in the resource directory, by registering the resource type and the
multicast address. The multicast address can be associated with a
group as described in [I-D.ietf-core-resource-directory].
A discovering Endpoint can discover one or more Sleepy Node resources
via the Resource Directory.
+-------------+ +-----------------+
| Configuring | | Discovering |---.
| Endpoint | | Endpoint | |
+-------------+ +-----------------+ |
| |
| +------------+ |
.-Register MC------>| |<--Discover resources -.
| Resource |
| Directory |<--Register Proxy -----.
.-Proxy Discovery-->| |<--Register resources -.
| +------------+ |
| |
+---------+ +-----------+ |
| Sleepy | | Proxy |---------'
| Node | | |
+---------+ +-----------+
Figure 3: Interactions involving Resource Directory
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5. Synchronize interface
The functions of the synchronize interface implemented by the Proxy
are described in this section.
5.1. Sleepy Node discovers proxy
A Sleepy Node can discover the proxy in two ways:
- via the CoAP interface [RFC7390] by sending a multicast message
to discover an endpoint with rt=core.sp.
- via RD as already described in Section 4.
The following example shows a sleeping endpoint discovering a proxy
using this interface, thus learning that the base Proxy resource,
where the Sleepy Node resources are registered, is at /sp.
Sleepy Proxy
| |
| ----- GET /.well-known/core?rt=core.sp ------> |
| |
| |
| <---- 2.05 Content "</sp>; rt="core.sp" ------ |
| |
Req: GET coap://[ff02::1]/.well-known/core?rt=core.sp
Res: 2.05 Content
</sp>;rt="core.sp"
The use of /sp is recommended and not compulsory.
5.2. Registration at a Proxy
Once a Sleepy Node has discovered a Proxy by means of one of the
procedures described in Section 5.1, the registration step can be
performed. To perform registration, a Sleepy Node sends to the Proxy
Node a CoAP POST request containing a description of the resources to
be delegated to the Proxy as the message payload in the CoRE Link
Format [RFC6690]. The description of the resource includes the
Sleepy Node identifier, its domain and the lifetime of the
registration.
Upon successful registration a Proxy creates a new delegated resource
or updates an existing delegated resource and returns its location.
The resources specified by the Sleepy Node during registration are
created with path that has as prefix the base Proxy resource path
(e.g. /sp). The registration interface MUST be implemented to be
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idempotent, so that registering twice with the same endpoint
parameter does not create multiple delegated resources. The
delegated resource SHOULD implement the Interface Type CoRE Link List
defined in [I-D.ietf-core-interfaces]. A GET request on this
resource MUST return the list of delegated resources for the
corresponding Sleepy Node.
After successful registration, a Proxy SHOULD enable resource
discovery for the new resources by updating its "/.well-known/core"
resource. A Proxy MUST wait for the initial representation of a
resource before it can be visible during resource discovery. The top
level delegated resource MUST be published in "/.well-known/core" to
enable the discovery of the resources via RD as described in
Section 4. Resources of a delegated container SHOULD be discoverable
either directly in "/.well-known/core" or indirectly through gradual
reveal from the delegated resource. The Web Link of a delegated
resource MUST contain an "ep" attribute with the value of the End-
Point parameter received during registration.
A Proxy MAY be configured to register the Sleepy Node's resources in
a RD. In this case, a Sleepy Node MUST NOT register the resources in
a RD by itself since it is the responsibility of the Proxy to perform
the registration in the RD on behalf of the Sleepy Node. Since each
Sleepy Node may register resources with different lifetimes, a Proxy
MUST register the resources of a given Sleepy Node in a dedicated
path of the RD.
In case a Sleepy Node delegates its own resources to more than one
Proxy and each Proxy registers the Sleepy Node's resource in a RD,
the RD entries from the different Proxies for the same Sleepy Node
risk to overlap.
To avoid this problem, a Proxy MUST create its own resource path to
register the resources of a Sleepy Node on the RD.
The new path name is typically formed by concatenating the Proxy's
endpoint identifier with the path in use. This precaution ensures
that the ep identifier of a Sleepy Node is unique for each resource
path in the RD.
Implementation note: It is not recommended to reuse the value of the
ep parameter in the URI of the delegated resource. This parameter
may be a relatively long identifier to guarantee global uniqueness
(e.g. EUI64) and would generate inefficient URIs on the Proxy where
only a local handler is necessary.
The following example shows a Sleepy Node registering with a Proxy.
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Sleepy Proxy
| |
| --- POST /sp?ep=0224e8fffe925dcf;rt=sensor "</dev..."---> |
| |
| |
| <-- 2.01 Created Location: /sp/0 ----------------------- |
| |
Req: POST coap://sp.example.org/sp?ep=0224e8fffe925dcf;rt=sensor
Etag: 0x3f
Payload:
</dev/mfg >;rt="ipso.dev.mfg";if="core.rp",
</dev/mdl>;rt="ipso.dev.mdl";if="core.rp",
</dev/n>;rt="ipso.dev.n";if="core.p",
</sen/temp>;rt="ucum.Cel";if="core.s"
Res: 2.01 Created
Location: /sp/0
The delegated resource has been created with path /sp/0 on the Proxy
in the example above. The path to the ep can be discovered as shown
below:
Req: GET coap://sp.example.org/.well-known/core
Res: 2.05 Content
</sp>;rt="core.sp",
</sp/0>;ep="0224e8fffe925dcf";rt="sensor"
A node can discover the delegated resources of the ep as shown below:
Req: GET coap://sp.example.org/sp/0
Res: 2.05 Content
Payload:
</sp/0/dev/mfg >;rt="ipso.dev.mfg";if="core.rp",
</sp/0/dev/mdl>;rt="ipso.dev.mdl";if="core.rp",
</sp/0/dev/n>;rt="ipso.dev.n";if="core.p",
</sp/0/sen/temp>;rt="ucum.Cel";if="core.s"
Once the resources are registered in the Proxy, the Proxy registers
the delegated resources in the RD.
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Proxy RD
| |
| --- POST /rd?ep=0224e8fffe925dcf "</sp/0..." --------> |
| |
| |
| <-- 2.01 Created Location: /rd/6534 ------------------- |
| |
Req: POST coap://rd.example.org/rd?ep=0224e8fffe925dcf
Etag: 0x6a
Payload:
</sp/0/dev/mfg >;rt="ipso.dev.mfg";if="core.rp",
</sp/0/dev/mdl>;rt="ipso.dev.mdl";if="core.rp",
</sp/0/dev/n>;rt="ipso.dev.n";if="core.p",
</sp/0/sen/temp>;rt="ucum.Cel";if="core.s"
Res: 2.01 Created
Location: /rd/6534
5.3. De-registration at a Proxy
Sleepy Node resources in the Proxy are kept active for the period
indicated by the lifetime parameter. The Sleepy Node is responsible
for refreshing the delegated resource within this period using either
the registration or update function (see Section 5.5 of the
Synchronize interface). Once a delegated resource has expired, the
Proxy deletes all resources associated to that resource and updates
its "/.well-known/core" resource. When the Proxy resources are also
registered in a RD, the RD and delegated resources are supposed to
have the same lifetime. Consequently, when the delegated resource
expires, a Proxy MAY let the RD resource expire too instead of
explicitly deleting it. When the delegated resource is deleted by
means of explicit de-registration operation then also the RD resource
MUST be explicitly removed.
A Proxy could lose or delete the delegated resource associated to a
Sleepy Node without sending an explicit notification (e.g. after
reboot). A Sleepy Node SHOULD be able to detect this situation by
processing the response code while using the Sleepy Node Operation or
Update interface. Especially an error code "4.04 Not Found" SHOULD
cause the Sleepy Node to register again. A Sleepy Node MAY also
register with multiple proxies to alleviate the risk of interruption
of service.
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5.4. Initialization of delegated resource
Once registration has been successfully performed, the Sleepy Node
must initialize the delegated resource. To send the initial contents
(e.g. values, device name, manufacturer name) of the delegated
resources to the Proxy, the Sleepy Node uses CoAP PUT repeatedly.
The basic interface is specified as follows:
Interaction: Sleepy -> Proxy
Method: PUT
URI Template: /{+location}{+resource}{?lt}
URI Template Variables:
location := This is the Location path returned by the Proxy as a
result of a successful registration.
resource := This is the relative path to a delegated resource
managed by the registered Sleepy Node.
lt := Lifetime (optional). The number of seconds by which the
lifetime of the whole delegated resource is extended. Range of
1-4294967295. If no lifetime is included, the current
remaining lifetime stays unchanged.
Request Content-Type: Defined at registration
Response Content-Type: Defined at registration for GET method.
application/link-format for PUT method if at least one of the
mutable resources has been updated since the last PUT request.
Etag: The Etag option MAY be included to allow clients to validate a
resource on multiple Proxies.
Success: 2.01 "Created", the request MUST include the initial
representation of the delegated resource.
Success: 2.04 "Changed", the request MUST include the new
representation of the delegated resource.
Success: 2.05 "Content", the response MUST include the current
representation of the delegated resource.
Failure: 4.00 "Bad Request". Malformed request.
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Failure: 5.03 "Service Unavailable". Service could not perform the
operation.
The following example describes how a Sleepy Node can initialize the
resource containing its manufacturer name just after registration.
Sleepy Proxy
| |
| --- PUT /sp/0/dev/mfg "acme" ---------------> |
| |
| |
| <-- 2.01 Created ----------------------------- |
| |
Req: PUT /sp/0/dev/mfg
Payload: acme
Res: 2.01 Created
The example below shows how a Sleepy Node can indicate that it is
supposed to send a temperature value at least every hour to keep its
delegated resource active.
Sleepy Proxy
| |
| --- PUT /sp/0/sen/temp?lt=3600 "22" --------> |
| |
| |
| <-- 2.04 Changed ----------------------------- |
| |
Req: PUT /sp/0/sen/temp?lt=3600
Payload: 22
Res: 2.04 Changed
The use of repeated CoAP PUT can be avoided by writing all relevant
resources into the Proxy in one operation by means of the Batch
interface described in [I-D.ietf-core-interfaces]. After successful
initialization, a Proxy SHOULD enable resource discovery for the new
delegated resources by updating its /.well-known/core resource.
5.5. Sleepy Node updates delegated resource at Proxy
A Sleepy Node can update a delegated resource at the Proxy (REPORT A)
using standard CoAP PUT requests on the delegated resource as shown
in Section 5.4.
When a Sleepy Node sends a PUT request to update its resources, the
response MAY contain a link-format payload. The payload does not
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directly relate to the target resource of the PUT request. Instead,
it is a list of web links to resources that have been modified by
clients since either the last PUT request or the last call to the
modification check interface (see Section 5.6).
5.6. Sleepy Node READs resource updates from Proxy
This function allows a Sleepy Node to retrieve a list of delegated
resources that have been modified at the Proxy by other nodes. The
interface format for GET is the same as the one specified for PUT in
Section 5.4.
A configuring Node (EP) can update a resource in the Proxy. The
Sleepy Node receives an indication of the changed resources as
specified in Section 5.5.
The Sleepy Node can send GET requests to its Proxy on each delegated
resource in order to receive their updated representation. The
example in Figure 4 shows a configuration node which changes the name
of a Sleepy Node at the Proxy. The Sleepy Node can then check and
read the modification in its resource.
Sleepy Proxy EP
| | <---PUT /sp/0/dev/n----|
| | Payload: Sensor1 |
Wake-up |---2.04 Changed-------->|
event | |
| | |
|--POST /sp/0/dev/.. -->| |
|<----2.04 Changed------| |
| Payload: <sp/0/dev/n> | |
| | |
|---GET /sp/0/dev/n---->| |
|<-----2.05 Content-----| |
| Payload: Sensor1 | |
| | |
Figure 4: Example: A Sleepy Node READs resource updates from his
Proxy
6. Delegate Interface
This section details the functions belonging to the delegate
interface.
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6.1. Discovering Endpoint discovers Sleepy Node at Proxy
Through this function, a Discovering Endpoint can discover one or
more Sleepy Node(s) at a Proxy. In case a Resource Directory is not
present, this is the only way to discover Sleepy Nodes. A CoAP
client discovers resources owned by the Sleepy Node but hosted on the
Proxy using typical mechanisms such as one or more GETs on the
resource /.well-known/core [RFC6690].
Resource discovery between an Endpoint and a proxy or an Endpoint and
a RD needs special care to take into account the fact that resources
from a Sleepy Node might appear duplicated. EPs SHOULD employ 2-step
resource discovery by looking up Sleepy Nodes AND resource types to
detect duplicate resources. EPs MAY use single-step resource
discovery only if the Sleepy Node can register with no more than one
Proxy. An EP can use the "ep" link attribute as a filter on the
"/.well-known/core" resource to retrieve a list of endpoints and
detect duplicate Sleepy Nodes registered on multiple proxies. An EP
can use the "ep" type of lookup to do the same on a RD. The result
of endpoint discovery is then used to filter out duplicate resources
returned from simple resource discovery.
The following example shows a client discovering the Sleepy Nodes and
learning that the Sleepy Node 0224e8fffe925dcf is registered on two
Proxies.
EP proxy1 proxy2
| | |
| ----- GET /.well-known/core?ep=* ------->|------>|
| | |
| | |
| <---- 2.05 Content "</sp/0>..." --------| |
| | |
| <---- 2.05 Content "</sp/0>..." --------|-------|
Req: GET coap://[ff02::1]/.well-known/core?ep=*
Res: 2.05 Content
</sp/0>;ep="0224e8fffe925dcf"
Res: 2.05 Content
</sp/0>;ep="02004cfffe4f4f50"
</sp/1>;ep="0224e8fffe925dcf"
From the previous exchange and the next resource discovery request,
the EP can infer that the resources coap://sp1/sp/0/sen/temp and
coap://sp2/sp/1/sen/temp actually come from the same Sleepy Node with
ep=0224e8fffe925dcf.
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EP proxy1 proxy2
| | |
| - GET /.well-known/core?rt=ipso:ucum.Cel ->|------>|
| | |
| | |
| <---- 2.05 Content "</sp/0>..." ----------| |
| | |
| <---- 2.05 Content "</sp/1>..." ----------|-------|
Req: GET coap://[ff02::1]/.well-known/core?rt=ucum.Cel
&ep=0224e8fffe925dcf
Res: 2.05 Content
</sp/0/sen/temp;rt="ucum.Cel"
Res: 2.05 Content
</sp/1/sen/temp>;rt="ucum.Cel"
6.2. Proxy REPORTs events to Endpoint
This interface can be used by the Endpoint to receive event report
message to Proxy (REPORT A) which further notifies it to interested
Destination Endpoint(s)(REPORT B). This indirect reporting is useful
for a scalable solution, e.g. there may be many interested
subscribers but the Sleepy Node itself can only support a limited
number of subscribers given its limits on battery energy. A client
interested in the events related with a specific resource may send a
CoAP GET to the Proxy, to obtain the last published state. If a
Reading node is interested in receiving updates whenever the Sleepy
Node reports new event to its Proxy, it can use observe
[I-D.ietf-core-observe] at the Proxy for that specific resource.
A proxy using the CoAP protocol [RFC7252] SHOULD accept to establish
a CoAP observation relationship between the delegated resource and a
client as defined in [I-D.ietf-core-observe].
A Sleepy Node may stop updating its delegated resources without
explicitly removing its delegated resource (e.g. transition to
another proxy after network unreachability detection). An Endpoint
can detect this situation when the corresponding delegated resource
has expired. Upon receipt of a response with error code 4.04 "Not
Found", an Endpoint SHOULD restart resource discovery to determine if
the resources are now delegated to another proxy.
The interface function is specified as follows:
Interaction: EP -> Proxy
Method: Defined at registration
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URI Template: /{+location}{+resource}
URI Template Variables:
location := This is the Location path returned by the Proxy as a
result of a successful registration.
resource := This is the relative path to a delegated resource
managed by a Sleepy Node.
Content-Type: Defined at registration
In the example below an EP observes the changes of temperature
through the Proxy.
Sleepy Proxy EP
| | |
| | <- GET /sp/0/sen/temp - |
| | (observe) |
| | |
| | -- 2.05 Content "22" -> |
| | |
| - PUT /sp/0/sen/temp "23" -> | |
| | |
| <- 2.04 Changed ------------ | |
| | |
| | -- 2.05 Content "23" -> |
6.3. A Node WRITEs to Sleepy Node via Proxy
A Configuring Node uses CoAP PUT to write information (such as
configuration data) to the Proxy, where the information is destined
for a Sleepy Node. Upon change of a delegated resource, an internal
flag is set in the Proxy that the specific resource has changed.
Next time the Sleepy Node wakes up, the Sleepy Node checks the Proxy
for any modification of its delegated resources and reads those
changed resources using CoAP GET requests, as shown in Figure 4. The
allowed resources that a Configuring Node can write to, and the CoAP
Content-Format of those CoAP resources, is determined in the initial
registration phase.
The following example shows a commissioning tool (EP) changing the
name of a Sleepy Node through a Proxy. The Sleepy Node detects this
change right after updating its current temperature.
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Sleepy Proxy EP
| | |
| | <-- PUT /sp/0/dev/n --- |
| | |
| | -- 2.04 Changed ------> |
| | |
| - PUT /sp/0/sen/temp ---> | |
| <- 2.04 Changed --------- | |
| Payload: <sp/0/dev/n> --- | |
| | |
| - GET /sp/0/dev/n ------> | |
| | |
| <- 2.05 Content --------- | |
| | |
Req: PUT /sp/0/dev/n
Payload: "sensor-1"
Res: 2.04 Changed
Req: PUT /sp/0/sen/temp
Payload: "24"
Res: 2.04 Changed, Content-Type: application/link-format
Payload: "</sp/0/dev/n>"
Req: GET /sp/0/dev/n
Res: 2.05 Content
Payload: "sensor-1"
6.4. A Node READs information from Sleepy Node via Proxy
A Reading Node uses standard CoAP GET to read information of a Sleepy
Node via a Proxy. However, not all information/resources from the
Sleepy Node may be copied to the Proxy. In that case, the Reading
Node cannot get direct access to resources that are not delegated to
the Proxy. The strategy to follow in that case is to first WRITE to
the Sleepy Node (via the Proxy, Section 6.3) a request for reporting
this missing information; where the request can be fulfilled by the
Sleepy Node the next time the Sleepy Node wakes up.
7. Direct Interface
This section details the functions belonging to the direct interface.
7.1. Sleepy Node REPORTs events directly to Destination Node
When the Sleepy Node needs to report an event to Destination nodes or
groups of Destination nodes present in the subscribers list, it
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becomes Awake and then it can use standard CoAP POST unicast or
multicast requests to report the event.
TODO: MC example
7.2. A Sleepy Node READs information from a Server Node
A Sleepy Node while Awake uses standard CoAP GET to read any
information from a Server Node. While the Sleepy Node awaits a CoAP
response containing the requested information, it remains awake. To
increase battery life of Sleepy Nodes, such an operation should not
be performed frequently.
8. Realization with PubSub broker
The PubSub broker [I-D.koster-core-coap-pubsub] can be used to
implement the REPORT function of the Sleepy Node proxy specified in
this document. However, there are some differences to be taken into
account:
- The PubSub broker handles topics. In the case of the proxy the
topics must be equated to resources.
- Clients publish anonymously updates to a topic. In the case of
the proxy, a delegated resource is bound to one given node that is
allowed to update it. For the same functionality, the PubSub
broker must restrict topic updates to one client only. The client
linked to the topic must be visible to the clients which subscribe
to the topic.
In addition, some other functionality needs to be added to the PubSub
broker to satisfy the interaction model shown in Figure 1:
- the READ function from Sleepy Node to proxy is not covered by
the PubSub broker. The PubSub broker needs to piggy-back a "check
topic" on the confirmation of a publication by the proxy. The
proxy can then perform a Read on the signalled topic.
- The interaction "register resources" from proxy to Resource
Directory, shown in Figure 3, is not part of the PubSub broker.
9. IANA Considerations
The new Resource Type (rt=) Link Target Attribute, 'core.sp' needs to
be registered in the "Resource Type (rt=) Link Target Attribute
Values" sub registry under the "Constrained RESTful Environments
(CoRE) Parameters" registry.
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10. Security Considerations
For the communication between Sleepy Node and Proxy it MAY be
sufficient to use Layer 2 (MAC) security without the recommended use
of DTLS. However, it must be ascertained that the Sleepy Node can
communicate only with a given secured Proxy. A Sleepy Node may
obtain the Layer 2 network key using the bootstrapping mechanism
described in [I-D.kumar-6lo-selective-bootstrap]. DTLS MUST be used
over link-layer security for further transport-layer protection of
messages between Regular Nodes and Proxies in the network. There are
no special adaptations needed of the DTLS handshake to support Sleepy
Nodes. During the whole handshake, Sleepy Nodes are required to
remain awake to avoid that, in case of small retransmission timers,
the other node may think the handshake message was lost and starts
retransmitting. In view of this, the only key point, therefore, is
that DTLS handshakes are not performed frequently to save on battery
power. Based on the DTLS authentication, also an authorization
method could be implemented so that only authorized nodes can e.g.
- Act as a Proxy for a Sleepy Node. (The Proxy shall be a trusted
device given its important role of storing values of parameters
for the delegated resources);
- READ data from Sleepy Nodes;
- WRITE data to Sleepy Nodes (via the Proxy);
- Receive REPORTs from Sleepy Nodes (direct or via Proxy).
11. Acknowledgements
Much of the text and examples in this document are copied from
[I-D.vial-core-mirror-server]. Matthieu Vial has generously
authorized us to use his text. Rahman Akbar has pointed out the CoAP
dependency of earlier versions.
12. Changelog
RFC editor, please delete this section before publication.
From version 2 to version 3:
Introduced interfaces and copied examples and text from mirror
server draft.
From version 3 to version 4:
Comparison with PubSub Broker completed.
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Mistakes in examples removed.
Less dependence on 6LowPAN networks.
Added Design motivation section.
13. References
13.1. Normative References
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-16 (work in progress), December 2014.
[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>.
[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>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<http://www.rfc-editor.org/info/rfc7390>.
13.2. Informative References
[I-D.ietf-core-interfaces]
Shelby, Z., Vial, M., and M. Koster, "CoRE Interfaces",
draft-ietf-core-interfaces-03 (work in progress), July
2015.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-04
(work in progress), July 2015.
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[I-D.koster-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-koster-core-coap-pubsub-02 (work in
progress), July 2015.
[I-D.kumar-6lo-selective-bootstrap]
Kumar, S. and P. Stok, "Security Bootstrapping over IEEE
802.15.4 in selective order", draft-kumar-6lo-selective-
bootstrap-00 (work in progress), March 2015.
[I-D.vial-core-mirror-server]
Vial, M., "CoRE Mirror Server", draft-vial-core-mirror-
server-01 (work in progress), April 2013.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
Authors' Addresses
Teresa Zotti
Philips Research
High Tech Campus 34
Eindhoven 5656 AE
The Netherlands
Phone: +31 6 21175346
Email: teresa.zotti@philips.com
Peter van der Stok
Consultant
Phone: +31 492474673
Email: consultancy@vanderstok.org
Esko Dijk
Philips Research
High Tech Campus 34
Eindhoven 5656 AE
The Netherlands
Phone: +31 6 55408986
Email: esko.dijk@philips.com
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