Building Power-Efficient CoAP Devices for Cellular Networks
Ericsson
Jorvas 02420
Finland
jari.arkko@piuha.net
Ericsson
Stockholm 164 83
Sweden
anders.e.eriksson@ericsson.com
Ericsson
Jorvas 02420
Finland
ari.keranen@ericsson.com
CoAP
cellular networks
This memo discusses the use of the Constrained Application Protocol
(CoAP) protocol in building sensors and other devices that employ
cellular networks as a communications medium. Building communicating
devices that employ these networks is obviously well known, but this
memo focuses specifically on techniques necessary to minimize power
consumption.
This memo discusses the use of the Constrained Application Protocol
(CoAP) protocol in building
sensors and other devices that employ cellular networks as a
communications medium. Building communicating devices that employ
these networks is obviously well known, but this memo focuses
specifically on techniques necessary to minimize power consumption.
CoAP has many advantages, including being simple to implement; a
thousand lines for the entire software above IP layer is plenty for a
CoAP-based sensor, for instance. However, while many of these
advantages are obvious and easily obtained, optimizing power
consumption remains challenging and requires careful design .
The memo targets primarily 3GPP cellular networks in their 2G, 3G,
and LTE variants and their future enhancements, including possible
power efficiency improvements at the radio and link layers. The exact
standards or details of the link layer or radios are not relevant for
our purposes, however. To be more precise, the material in this memo
is suitable for any large-scale, public network that employs
point-to-point communications model and radio technology for the
devices in the network.
Our focus is devices that need to be optimized for power usage, and
on devices that employ CoAP. As a general technology, CoAP is similar
to HTTP. It can be used in various ways and network entities may take
on different roles. This freedom allows the technology to be used in
efficient and less efficient ways. Some guidance is needed to
understand what communication models over CoAP are recommended when
low power usage is a critical goal.
The recommendations in this memo should be taken as complementary
to device hardware optimization, microelectronics improvements, and
further evolution of the underlying link and radio layers. Further
gains in power efficiency can certainly be gained on several fronts;
the approach that we take in this memo is to do what can be done at
the IP, transport, and application layers to provide the best possible
power efficiency. Application implementors generally have to use the
current generation microelectronics, currently available radio
networks and standards, and so on. This focus in our memo should by no
means be taken as an indication that further evolution in these other
areas is unnecessary. Such evolution is useful, is ongoing, and is
generally complementary to the techniques presented in this memo. The
evolution of underlying technologies may change what techniques
described here are useful for a particular application, however.
The rest of this memo is structured as follows. discusses the need and goals for low-power
devices. outlines our expectations for the low
layer communications model. describes the two
scenarios that we address, and , , and give
guidelines for use of CoAP in these scenarios.
There are many situations where power usage optimization is
unnecessary. Optimization may not be necessary on devices that can run
on power feed over wired communications media, such as in
Power-over-Ethernet (PoE) solutions. These devices may require a
rudimentary level of power optimization techniques just to keep
overall energy costs and aggregate power feed sizes at a reasonable
level, but more extreme techniques necessary for battery powered
devices are not required. The situation is similar with devices that
can easily be connected to mains power. Other types of devices may
get an occasional charge of power from energy harvesting techniques.
For instance, some environmental sensors can run on solar
cells. Typically, these devices still have to regulate their power
usage in a strict manner, for instance to be able to use as small and
inexpensive solar cells as possible.
In battery operated devices the power usage is even more
important. For instance, one of the authors employs over a hundred different
sensor devices in his home network. A majority of these devices are
wired and run on PoE, but in most environments this would be
impractical because the necessary wires do not exist. The future is in
wireless solutions that can cover buildings and other environments
without assuming a pre-existing wired infrastructure. In addition, in
many cases it is impractical to provide a mains power source. Often
there are no power sockets easily available in the locations that the
devices need to be in, and even if there were, setting up the wires
and power adapters would be more complicated than installing a
standalone device without any wires.
Yet, with a large number of devices the battery lifetimes become
critical. Cost and practical limits dictate that devices can be
largely just bought and left on their own. For instance, with hundred
devices, even a ten-year battery lifetime results in a monthly battery
change for one device within the network. This may be impractical in
many environments. In addition, some devices may be physically
difficult to reach for a battery change. Or, a large group of devices
-- such as utility meters or environmental sensors -- cannot be
economically serviced too often, even if in theory the batteries could
be changed.
Many of these situations lead to a requirement for minimizing power
usage and/or maximizing battery lifetimes. Using the power usage
strategies described in , mains-powered
sensor-type devices can use the Always-on strategy whereas battery or
energy harvesting devices need to adjust behavior based on the
communication interval. For intervals in the order of seconds,
Low-power strategy is appropriate. For intervals ranging from minutes
to hours either Low-power or Normally-off strategies are
suitable. Finally, for intervals lasting days and longer, Normally-off
is usually the best choice. Unfortunately, much of our current
technology has been built with different objectives in mind. Networked
devices that are "always on", gadgets that require humans to recharge
them every couple of days, and protocols that have been optimized to
maximize throughput rather than conserve resources.
Long battery lifetimes are required for many applications,
however. In some cases these lifetimes should be in the order of years
or even a decade or longer. Some communication devices already reach
multi-year lifetimes, and continuous improvement in low-power
electronics and advances in radio technology keep pushing these
lifetimes longer. However, it is perhaps fair to say that battery
lifetimes are generally too short at present time.
Power usage can not be evaluated solely based on lower layer
communications. The entire system, including upper layer protocols and
applications is responsible for the power consumption as a whole. The
lower communication layers have already adopted many techniques that
can be used to reduce power usage, such as scheduling device wake-up
times. Further reductions will likely need some co-operation from the
upper layers so that unnecessary communications, denial-of-service
attacks on power consumption, and other power drains are
eliminated.
Of course, application requirements ultimately determine what kinds
of communications are necessary. For instance, some applications
require more data to be sent than others. The purpose of the
guidelines in this memo is not to prefer one or the other application,
but to provide guidance on how to minimize the amount of
communications overhead that is not directly required by the
application. While such optimization is generally useful, it is
relatively speaking most noticeable in applications that transfer only
a small amount of data, or operate only infrequently.
We assume that the underlying communications network can be any
large-scale, public network that employs point-to-point communications
model and radio technology. 2G, 3G, and LTE networks are examples of such
networks, but not the only possible networks with these characteristics.
In the following we look at some of these characteristics and their
implications. Note that in most cases these characteristics are not
properties of the specific networks but rather inherent in the concept
of public networks.
Using a public network service implies that applications can be
deployed without having to build a network to go with them. For
economic reasons, only the largest users (such as utility companies)
could afford to build their own network, and even they would not be
able to provide a world-wide coverage. This means that applications
where coverage is important can be built. For instance, most transport
sector applications require national or even world-wide coverage to
work.
But there are other implications, as well. By definition, the network
is not tailored for this application and with some exceptions, the
traffic passes through the Internet. One implication of this is that
there are generally no application-specific network configurations or
discovery support. For instance, the public network helps devices to
get on the Internet, set up default routers, configure DNS servers,
and so on, but does nothing for configuring possible higher-layer
functions, such as servers the device might need to contact to perform
its application functions.
Public networks often provide web proxies and other functionality that
can in some cases make a significant improvement for delays and cost
of communication over the wireless link. For instance, resolving
server DNS names in a proxy instead of the user's device may cut down
on the general chattiness of the communications, therefore reducing
overall delay in completing the entire transaction. Likewise, a CoAP
proxy or pub/sub broker
can assist a CoAP device in communication. However, unlike HTTP web
proxies, CoAP proxies and brokers are not yet widely deployed in
public networks.
Similarly, given the lack of available IPv4 addresses, the chances are
that many devices are behind a network address translation (NAT)
device. This means that they are not easily reachable as servers.
Alternatively, the devices may be directly on the global Internet
(either on IPv4 or IPv6) and easily reachable as
servers. Unfortunately, this may mean that they also receive unwanted
traffic, which may have implications for both power consumption and
service costs.
This is a common link model in cellular networks. One implication of
this model is that there will be no other nodes on the same link,
except maybe for the service provider's router. As a result, multicast
discovery can not be reasonably used for any local discovery purposes.
While the configuration of the service provider's router for specific
users is theoretically possible, in practice this is difficult to
achieve, at least for any small user that can not afford a
network-wide contract for a private APN (Access Point Name). The
public network access service has little per-user tailoring.
The use of radio technology means that power is needed to operate the
radios. Transmission generally requires more power
than reception. However, radio protocols have generally been designed
so that a device checks periodically whether it has messages. In a
situation where messages arrive seldom or not at all, this checking
consumes energy. Research has shown that these periodic checks (such
as LTE paging message reception) are often a far bigger contributor to
energy consumption than message transmission.
Note that for situations where there are several applications on the
same device wishing to communicate with the Internet in some manner,
bundling those applications together so that they can communicate at
the same time can be very useful. Some guidance for these techniques
in the smartphone context can be found in .
Naturally, each device has a freedom to decide when it sends
messages. In addition, we assume that there is some way for the
devices to control when or how often it wants to receive messages.
Specific methods for doing this depend on the specific network being
used and also tend to change as improvements in the design of these
networks are incorporated. The reception control methods generally
come in two variants, fine grained mechanisms that deal with how often
the device needs to wake-up for paging messages, and more crude
mechanisms where the device simply disconnects from the network for a
period of time. There are associated costs and benefits to each
method, but those are not relevant for this memo, as long as some
control method exists. Furthermore, devices could use Delay-Tolerant
Networking (DTN) mechanisms to relax the
requirements for timeliness of connectivity and message delivery.
Not all applications or situations are equal. They may require
different solutions or communication models. This memo focuses on two
common scenarios at cellular networks:
This scenario involves all communication that requires real-time or
near real-time communications with a device. That is, a network entity
must be able to reach the device with a small time lag at any time,
and no pre-agreed wake-up schedule can be arranged. By "real-time" we
mean any reasonable end-to-end communications latency, be it measured
in milliseconds or seconds. However, unpredictable sleep states are
not expected.
Examples of devices in this category include sensors that must be measurable
from a remote source at any instant in time, such as process automation sensors
and actuators that require immediate action, such as light bulbs or door locks.
This scenario involves freedom to choose when device communicates. The
device is often expected to be able to be in a sleep state for much of
its time. The device itself can choose when it communicates, or it lets
the network assist in this task.
Examples of devices in this category include sensors that track slowly
changing values, such as temperature sensors and actuators that
control a relatively slow process, such as heating systems.
Note that there may be hard real-time requirements, but they are
expressed in terms of how fast the device can communicate, not in
terms of how fast it can respond to a network stimuli. For instance,
a fire detector can be classified as a sleepy device as long as it
can internally quickly wake up on detecting fire and initiate the necessary
communications without delay.
In both scenarios the device will be attached to a public network.
Without special arrangements, the device will also get a dynamically
assigned IP address or an IPv6 prefix. At least one but typically
several router hops separate the device from its communicating peers
such as application servers. As a result, the address or even the
existence of the device is typically not immediately obvious to the
other nodes participating in the application. As discussed earlier,
multicast discovery has limited value in public networks; network
nodes cannot practically discover individual devices in a large public
network. And the devices can not discover who they need to talk, as
the public network offers just basic Internet connectivity.
Our recommendation is to initiate a discovery and registration
process. This allows each device to inform its peers that it has
connected to the network and that it is reachable at a given IP
address. Registration also facilitates low-power operation since a
device can delegate part of the discovery signaling and reachability
requirements to another node.
The registration part is easy e.g., with a resource directory. The
device should perform the necessary registration with these devices,
for instance, as specified in . In order to do this
registration, the device needs to know its CoRE Link Format
description, as specified in . In essence, the
registration process involves performing a GET on
.well-known/core/?rt=core-rd at the address of the resource directory,
and then doing a POST on the path of the discovered resource.
Other mechanisms enabling device discovery and delegation of
functionality to a non-sleepy node include and .
However, current CoAP specifications provide only limited support
for discovering the resource directory or other registration
services. Local multicast discovery only works in LAN-type networks,
but not in these public cellular networks. Our recommended alternate
methods for discovery are the following:
The DNS name of the resource directory is manually configured. This
approach is suitable in situations where the owner of the devices has
the resources and capabilities to do the configuration. For instance,
a utility company can typically program its metering devices to point
to the company servers.
The DNS name of the directory or proxy is hardwired to the software by
the manufacturer, and the directory or proxy is actually run by the
manufacturer. This approach is suitable in many consumer usage
scenarios, where it would be unreasonable to assume that the consumer
runs any specific network services. The manufacturer's web interface
and the directory/proxy servers can co-operate to provide the desired
functionality to the end user. For instance, the end user can register
a device identity in the manufacturer's web interface and ask specific
actions to be taken when the device does something.
The DNS name of the directory or proxy is hardwired to the software by
the manufacturer, but this directory or proxy merely redirects the
request to a directory or proxy run by the whoever bought the
device. This approach is suitable in many enterprise environments, as
it allows the enterprise to be in charge of actual data collection and
device registries; only the initial bootstrap goes through the
manufacturer. In many cases there are even legal requirements (such as
EU privacy laws) that prevent providing unnecessary information to
third parties.
The delegating manufacturer server model could be generalized into a
reverse-DNS -like discovery infrastructure that could answer the question
"this is device with identity ID, where is my home registration server?".
However, at present no such resolution system exists.
(Note: The EPCGlobal system for RFID resolution is reminiscent
of this approach.)
Besides manual configuration, these alternate mechanisms are mostly
suitable for large manufacturers and deployments. Good automated
mechanism for discovery of devices that are manufactured and deployed
in small quantities are still needed.
A variety of data formats exist for passing around data. These data
formats include XML, JavaScript Object Notation (JSON) , Efficient XML Interchange (EXI) , and text formats. Message lengths can
have a significant effect on the amount of energy required for the
communications, and such it is highly desirable to keep message
lengths minimal. At the same time, extreme optimization can affect
flexibility and ease of programming. The authors recommend as a compact, yet easily processed
and extendable textual format.
These devices are often best modeled as CoAP servers. The device
will have limited control on when it receives messages, and it will
have to listen actively for messages, up to the limits of the
underlying link layer. If the device acts also in client role in some
phase of its operation, it can control how many transmissions it makes
on its own behalf.
The packet reception checks should be tailored according to the
requirements of the application. If sub-second response time is not
needed, a more infrequent checking process may save some power.
For sensor-type devices, the CoAP Observe extension may be supported. This allows the sensor to track
changes to the sensed value, and make an immediate observation
response upon a change. This may reduce the amount of polling needed
to be done by the client. Unfortunately, it does not reduce the time
that the device needs to be listening for requests. Subscription
requests from other clients than the currently registered one may come
at any time, the current client may change its request, and the device
still needs to respond to normal queries as a server. As a result, the
sensor can not rely having to communicate only on its own choice of
observation interval.
In order to act as a server, the device needs to be placed in a
public IPv4 address, be reachable over IPv6, or hosted in a private
network. If the the device is hosted on a private network, then all
other nodes need to access this device also need to reside in the same
private network. There are multiple ways to provide private networks
over public cellular networks. One approach is to dedicate a special
APN for the private network. Corporate access via cellular networks
has often been arranged in this manner, for instance. Another approach
is to use Virtual Private Networking (VPN) technology, for instance
IPsec-based VPNs.
Power consumption from unwanted traffic is problematic in these
devices, unless placed in a private network or protected by a
operator-provided firewall service. Devices on an IPv6 network will
have some protection through the nature of the 2^64 address allocation
for a single terminal in a 3GPP cellular network; the attackers will
be unable to guess the full IP address of the device. However, this
protects only the device from processing a packet, but since the
network will still deliver the packet to any of the addresses within
the assigned 64-bit prefix, packet reception costs are still
incurred.
Note that the the VPN approach can not prevent unwanted traffic
received at the tunnel endpoint address, and may require keep-alive
traffic. Special APNs can solve this issue, but require explicit
arrangement with the service provider.
These devices are best modeled as devices that can delegate queries
to some other node. For instance, as mirror proxy or CoAP Publish-Subscribe clients. When the device initializes
itself, it makes a registration of itself in a proxy as described
above in and then continues to send periodic
updates of sensor values.
As a result, the device acts only as a client, not a server, and
can shut down all communication channels while it is during its
sleeping period. The length of the sleeping period depends on power
and application requirements. Some environmental sensors might use a
day or a week as the period, while other devices may use a smaller
values ranging from minutes to hours.
Other approaches for delegation include CoAP-options described in
. In this memo we
use mirror proxies as an example, because of their ability to work
with both HTTP and CoAP implementations; but the concepts are similar
and the IETF work is still in progress so the final protocol details
are yet to be decided.
The ability to shut down communications and act as only a client
has four impacts:
Radio transmission and reception can be turned off during the
sleeping period, reducing power consumption significantly.
However, some power and time is consumed by having to re-attach to
the network after the end of a sleep period.
The window of opportunity for unwanted traffic to arrive is much
smaller, as the device is listening for traffic only part of the
time. Note that networks may cache packets for some time though. On
the other hand, stateful firewalls can effectively remove much of
unwanted traffic for client type devices.
The device may exist behind a NAT or a firewall without being
impacted. Note that "Simple Security" basic IPv6 firewall capability
blocks inbound UDP traffic by default, so just
moving to IPv6 is not direct solution to this problem.
For sleepy devices that represent actuators, it is also possible to
use the mirror proxy model. The device can make periodic polls to the
proxy to determine if a variable has changed.
There are several challenges in implementing sleepy devices. They
need hardware that can be put to an appropriate sleep mode but yet
awakened when it is time to do something again. This is not always
easy in all hardware platforms. It is important to be able to shut
down as much of the hardware as possible, preferably down to
everything else except a clock circuit. The platform also needs to support
re-awakening at suitable time scales, as otherwise the device needs to be
powered up too frequently.
Most commercial cellular modem platforms do not allow applications
to suspend the state of the communications stack. Hence, after a
power-off period they need to re-establish communications, which takes
some amount of time and extra energy.
Implementations should have a coordinated understanding of the
state and sleeping schedule. For instance, it makes no sense to keep a
CPU powered up, waiting for a message when the lower layer has been
told that the next possible paging opportunity is some time away.
The cellular networks have a number of adjustable configuration
parameters, such as the maximum used paging interval. Proper setting
of these values has an impact on the power consumption of the device,
but with the current business practices, such settings are rarely
negotiated when the user's subscription is provisioned.
There are no particular security aspects with what has been
discussed in this memo, except for the ability to delegate queries for
a resource to another node. Depending on how this is done, there are
obvious security issues which have largely NOT yet been addressed in
the relevant Internet Drafts
. However,
we point out that in general, security issues in delegation can be
solved either through reliance on your local network support nodes
(which may be quite reasonable in many environments) or explicit
end-to-end security. Explicit end-to-end security through nodes that
are awake at different times means in practice end-to-end data object
security. We have implemented one such mechanism for sleepy nodes as
described in .
The security considerations relating to CoAP and the relevant link layers should
apply. Note that cellular networks universally employ per-device
authentication, integrity protection, and for most of the world,
encryption of all their communications. Additional protection of
transport sessions is possible through mechanisms described in or data objects.
There are no IANA impacts in this memo.
Efficient XML Interchange (EXI) Format 1.0
Optimizing Downloads for Efficient Network Access
The authors would like to thank Zach Shelby, Jan Holler, Salvatore
Loreto, Matthew Vial, Thomas Fossati, Mohit Sethi, Jan Melen, Joachim
Sachs, Heidi-Maria Rissanen, Sebastien Pierrel, Kumar Balachandran,
Muhammad Waqas Mir, Cullen Jennings, Markus Isomaki, Hannes
Tschofenig, and Anna Larmo for interesting discussions in this problem
space.