Constrained RESTful Environments
Constrained RESTful Environments WG
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CoRE provides a framework for resource-oriented applications intended to
run on constrained IP networks. A constrained IP network has limited
packet sizes, may exhibit a high degree of packet loss, and may have a
substantial number of devices that may be powered off at any point in
time but periodically "wake up" for brief periods of time. These
networks and the nodes within them are characterized by severe limits on
throughput, available power, and particularly on the complexity that can
be supported with limited code size and limited RAM per node [RFC 7228].
More generally, we speak of constrained node networks whenever at least
some of the nodes and networks involved exhibit these characteristics.
Low-Power Wireless Personal Area Networks (LoWPANs) are an example of
this type of network. Constrained networks can occur as part of home and
building automation, energy management, and the Internet of Things.
The CoRE working group will define a framework for a limited class of
applications: those that deal with the manipulation of simple resources
on constrained networks. This includes applications to monitor simple
sensors (e.g. temperature sensors, light switches, and power meters), to
control actuators (e.g. light switches, heating controllers, and door
locks), and to manage devices.
The general architecture consists of nodes on the constrained network,
called Devices, that are responsible for one or more Resources that may
represent sensors, actuators, combinations of values, and/or other
information. Devices send messages to change and query resources on
other Devices. Devices can send notifications about changed resource
values to Devices that have expressed their interest to receive
notification about changes. A Device can also publish or be queried
about its resources. (Typically a single physical host on the network
would have just one Device but a host might represent multiple logical
Devices. The specific terminology to be used here is to be decided by
the working group.) As part of the framework for building these
applications, the working group has defined a Constrained Application
Protocol (CoAP) for the manipulation of Resources on a Device.
CoAP is designed for use between Devices on the same constrained
network, between Devices and general nodes on the Internet, and between
Devices on different constrained networks both joined by an internet.
(CoAP is also being used via other mechanisms, such as SMS on mobile
communication networks.) CoAP targets the type of operating
environments defined in the ROLL and 6Lo working groups which have
additional constraints compared to normal IP networks, but the CoAP
protocol also operates over traditional IP networks.
There also may be proxies that interconnect between other Internet
protocols and the Devices using the CoAP protocol. It is worth noting
that proxy does not have to occur at the boundary between the
constrained network and the more general network, but can be deployed at
various locations in the less-constrained network.
CoAP supports various forms of "caching". Beyond the benefits of
caching already well known from REST, caching can be used to increase
energy savings of low-power nodes by allowing them to be normally-off
[RFC 7228]. For example, a temperature sensor might wake up every five
minutes and send the current temperature to a proxy that has expressed
interest in notifications; when the proxy receives a request over CoAP
or HTTP for that temperature resource, it can respond with the last
notified value (instead of trying to query the Device which may not be
reachable at this time). The working group will continue to evolve this
model to increase its practical applicability.
The working group will perform maintenance on its first four
- RFC 6690
- RFC 7252
- RFC 7641
and will continue to evolve the experimental group communications
support (RFC 7390). The working group will not develop a reliable
CoAP today works over UDP and DTLS. The working group will define
transport mappings for alternative transports as required, both IP
(starting with TCP and a secure version over TLS) and non-IP (e.g., SMS,
working with the security area on potentially addressing the security gap);
includes defining appropriate URI schemes. Continued compatibility with
CoAP over SMS as defined in OMA LWM2M will be considered.
CoRE will continue and complete its work on
draft-ietf-core-resource-directory, as already partially adopted by OMA
LWM2M. Interoperability with DNS-SD (and the work of the dnssd working
group) will be a primary consideration. The working group will also
work on a specification enabling broker-based publish-subscribe-style
communication over CoAP.
CoRE will work on related data formats, such as alternative
representations of RFC 6690 link format and RFC 7390 group communication
information. The working group will complete the SenML specification,
again with consideration to its adoption in OMA LWM2M.
RFC 7252 defines a basic HTTP mapping for CoAP, with further discussion
in draft-ietf-core-http-mapping. This mapping will be evolved and
supported by further documents.
Besides continuing to examine operational and manageability aspects of
the CoAP protocol itself, CoRE will also develop a way to make
RESTCONF-style management functions available via CoAP that is
appropriate for constrained node networks. This will require very close
coordination with NETCONF and other operations and management working
groups. YANG data models will be used for manageability. Note that
the YANG modeling language is not a target for change in
this process, though additional mechanisms that support YANG
modules may be employed in specific cases where significant
performance gains are both attainable and required. The working
group will continue to consider the OMA LWM2M management functions
as a well-accepted alternative form of management and provide
support at the CoAP protocol level where required.
The working group has selected DTLS as the basis for the communications
security in CoAP. CoRE will work with the TLS working group on the
efficiency of this solution. The preferred cipher suites will evolve in
cooperation with the TLS working group and CFRG. The ACE working group
is expected to provide solutions to authorization that may need
complementary elements on the CoRE side. Object security as defined in
JOSE and being adapted to the constrained node network requirements in
COSE also may need additions on the CoRE side.
The working group will coordinate on requirements from many
organizations and SDO. The working group will closely coordinate with
other IETF working groups, particularly of the constrained node networks
cluster (6Lo, 6TiSCH, LWIG, ROLL, ACE, COSE), and appropriate
groups in the IETF OPS and Security areas. Work on these subjects, as
well as on interaction models and design patterns (including follow-up
work around the CoRE Interfaces draft) may benefit from close
cooperation with the proposed Thing-to-Thing Research Group.