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  <front>
    <title abbrev="Power-Efficient Cellular CoAP devices">Building Devices">Building
    Power-Efficient CoAP Constrained Application Protocol (CoAP) Devices for Cellular Networks</title>
    <seriesInfo name="RFC" value="9178"/>
    <author initials="J" surname="Arkko" fullname="Jari Arkko">
      <organization>Ericsson</organization>
      <address>
        <postal>
          <street/>
          <city>Jorvas</city>
          <code>02420</code>
          <country>Finland</country>
        </postal>
        <email>jari.arkko@piuha.net</email>
      </address>
    </author>
    <author initials="A" surname="Eriksson" fullname="Anders Eriksson">
<organization>Ericsson</organization>
      <organization>Independent</organization>
      <address>
        <postal>
          <street/>
          <city>Stockholm</city>
          <code>164 83</code>
          <country>Sweden</country>
        </postal>
<email>anders.e.eriksson@ericsson.com</email>
        <email>anders.e.eriksson@posthem.se</email>
      </address>
    </author>
    <author initials="A" surname="Keranen" surname="Keränen" fullname="Ari Keranen"> Keränen">
      <organization>Ericsson</organization>
      <address>
        <postal>
          <street/>
          <city>Jorvas</city>
          <code>02420</code>
          <country>Finland</country>
        </postal>
        <email>ari.keranen@ericsson.com</email>
      </address>
    </author>
    <date year="2016" /> month="May" year="2022"/>
    <keyword>CoAP</keyword>
    <keyword>cellular networks</keyword>
    <abstract>
      <t>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.</t>
    </abstract>
  </front>
  <middle>
    <section anchor="intro" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>This memo discusses the use of the Constrained Application Protocol
(CoAP) protocol <xref target="RFC7252"/> target="RFC7252" format="default"/> 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 of code for the entire software application above the 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 <xref
target="I-D.arkko-core-sleepy-sensors"/>.</t>

<t>The target="I-D.arkko-core-sleepy-sensors" format="default"/>.</t>
      <t>This memo targets primarily targets 3GPP cellular networks in their 2G, 3G,
LTE, and LTE 5G 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 a
point-to-point communications model and radio technology for the
devices in the network.</t>
      <t>Our focus is on devices that need to be optimized for power usage, usage and
on
devices that employ CoAP. As a general technology, CoAP is similar
to HTTP. It can be used in various ways 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 types of communication models over CoAP are recommended when
low power usage is a critical goal.</t>
      <t>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
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
However, the list of underlying technologies may change what techniques described here are in this document as useful for a
particular application, however.</t> application may change with the evolution of these underlying
technologies.</t>
      <t>The rest of this memo is structured as follows. <xref
target="lowpow"/> target="lowpow" format="default"/> discusses the need and goals for low-power
devices. <xref target="model"/> target="model" format="default"/> outlines our expectations for the low
layer low-layer communications model. <xref target="scen"/> target="scen" format="default"/> describes the two
scenarios that we address, and <xref target="disc"/>, address. Sections&nbsp;<xref target="disc" format="counter"/>, <xref
target="data"/>, target="data" format="counter"/>, <xref target="rt"/> target="rt" format="counter"/>, and <xref target="sens"/> target="sens" format="counter"/> give
guidelines for the use of CoAP in these scenarios.</t>
<t>This document was originally finalized in 2016 but is published six years later due to waiting for key references to reach RFC status. Therefore, some of the latest advancements in cellular network, CoAP, and other technologies are not discussed here, and some of the references point to documents that were state of the art in 2016.</t>
    </section>
    <section anchor="lowpow" title="Goals numbered="true" toc="default">
      <name>Goals for Low-Power Operation"> Operation</name>
      <t>There are many situations where power usage optimization is
unnecessary. Optimization may not be necessary on devices that can run
on a 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 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 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, manner -- for instance instance, to be able to use solar cells that
are as small and inexpensive solar cells as possible.</t>
      <t>In battery operated devices the battery-operated devices, power usage is even more
important. For instance, one of the authors employs over a hundred different
sensor devices in his their 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 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.</t>
      <t>Yet, with a large number of devices 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 a 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. </t>
      <t>Many of these situations lead to a requirement for minimizing power
usage and/or maximizing battery lifetimes.  Using the power usage
strategies described in <xref target="RFC7228"/>, target="RFC7228" format="default"/>, mains-powered
sensor-type devices can use the Always-on strategy strategy, whereas battery battery-operated or
energy harvesting
energy-harvesting devices need to adjust behavior based on the
communication interval. For intervals in on the order of seconds, the
Low-power strategy is appropriate. For intervals ranging from minutes
to hours hours, either the Low-power or Normally-off strategies are strategy is
suitable. Finally, for intervals lasting days and or longer, Normally-off
is usually the best choice. Unfortunately, much of our current technology has been built with different objectives in mind. Networked mind -- for instance, 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.</t>
      <t>Long battery lifetimes are required for many applications,
however. In some cases cases, these lifetimes should be in on the order of years
or even a decade or longer. Some communication devices already reach
multi-year lifetimes, and continuous improvement improvements 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.</t> present.</t>
      <t>Power usage can not cannot be evaluated solely based solely on lower layer lower-layer
communications. The entire system, including upper layer upper-layer protocols and
applications
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 cooperation from the
upper layers so that unnecessary communications, denial-of-service
attacks on power consumption, and other power drains are
eliminated.</t>
      <t>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 is,
relatively speaking speaking, most noticeable in applications that transfer only
a small amount of data, data or operate only infrequently.</t>
    </section>
    <section anchor="model" title="Link-Layer Assumptions"> numbered="true" toc="default">
      <name>Link-Layer Assumptions</name>
      <t>We assume that the underlying communications network can be any
large-scale, public network that employs a point-to-point communications
model and radio technology. 2G, &nbsp;2G, 3G, LTE, and LTE 5G networks are examples of such
networks,
networks but are not the only possible networks with these characteristics.</t>
      <t>In the following 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 are inherent in the concept
of public networks.

<list style="hanging">

<t hangText="Public networks"><vspace blankLines="1"/>

Using
</t>
        <ul spacing="normal">
          <li>
        <t>Public Networks</t>
	    <t>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 worldwide coverage. This means that applications
where coverage is important can be built. For instance, most transport
sector
transport-sector applications require national or even world-wide worldwide coverage to
work.

<vspace blankLines="1"/>
But
work.</t>
          <t>But there are other implications, implications as well. By definition, the network
is not tailored for this application and 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 that a device might need to contact to perform
its application functions.

<vspace blankLines="1"/>
Public
	  </t>
          <t>Public networks often provide web proxies and other functionality that
can
can, in some cases cases, make a significant improvement for improvements related to delays and cost costs
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 Publish-Subscribe (pub&wj;/sub) Broker <xref target="I-D.koster-core-coap-pubsub"/> target="I-D.ietf-core-coap-pubsub" format="default"/>
can assist a CoAP device in communication. However, unlike HTTP web
proxies, CoAP proxies and brokers are not yet widely deployed in
public networks.

<vspace blankLines="1"/>
Similarly, networks.</t>
          <t>Similarly, given the lack of available IPv4 addresses, the chances are
that many devices are behind a network address translation 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
(on either 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.</t>

<t hangText="Point-to-point link model"><vspace blankLines="1"/>

This
      </li>

      <li><t>Point-to-Point Link Model</t>
      <t>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 cannot 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,
achieve in practice, at least for any small user that can not cannot afford a
network-wide contract for a private APN (Access Point Name). The
public network access service has little per-user tailoring.</t>

<t hangText="Radio technology"><vspace blankLines="1"/>

The
      </li>

      <li>
        <t>Radio Technology</t>

	<t>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 to see 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.

<vspace blankLines="1"/>
Note transmission.</t>
<t>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 <xref
target="Android-Bundle"/>.
</t>

</list>
</t> target="Android-Bundle" format="default"/>.</t>
      </li>
</ul>

      <t>Naturally, each device has a the 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 they want 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 variants: (1)&nbsp;fine-grained mechanisms that deal with how often
the device needs to wake-up wake up for paging messages, messages and more crude (2)&nbsp;cruder
mechanisms where the device simply disconnects from the network for a
period of time. There are associated costs and benefits to associated with 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) <xref target="RFC4838"/> mechanisms <xref target="RFC4838" format="default"/> to relax the
requirements for timeliness of connectivity and message delivery. </t>
    </section>
    <section anchor="scen" title="Scenarios"> numbered="true" toc="default">
      <name>Scenarios</name>
      <t>Not all applications or situations are equal. They may require
different solutions or communication models. This memo focuses on two
common scenarios at in cellular networks:

<list style="hanging">

<t hangText="Real-Time
</t>

         <ul spacing="normal">
           <li>
	             <t>Real-Time Reachable Devices"><vspace blankLines="1"/>

This Devices</t>
<t>This scenario involves all communication that requires real-time or
near real-time
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 previously agreed-upon wake-up schedule can be arranged. By "real-time" "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.

<vspace blankLines="1"/>
Examples expected.</t>
<t>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.
</t>

<t hangText="Sleepy Devices"><vspace blankLines="1"/>
This locks.</t>
	 </li>

         <li>
	   <t>Sleepy Devices</t>
<t>This scenario involves the freedom to choose when a 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.

<vspace blankLines="1"/>
Examples task.</t>
<t>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.

<vspace blankLines="1"/>
Note systems.</t>
<t>Note that there may be hard real-time requirements, but they are
expressed in terms of how fast the device can communicate, 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.</t>

</list>
</t>
	 </li>
	</ul>
    </section>
    <section anchor="disc" title="Discovery numbered="true" toc="default">
      <name>Discovery and Registration"> Registration</name>
      <t>In both scenarios 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 cannot discover who they need to talk, talk to, as
the public network offers just basic Internet connectivity.</t>
      <t>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 operation, since a
device can delegate part of the discovery signaling and reachability
requirements to another node. </t> node.</t>
      <t>The registration part is easy easy, e.g., with a resource directory. The
device should perform the necessary registration with these devices, such a resource directory --
for instance, as specified in <xref
target="I-D.ietf-core-resource-directory"/>. target="RFC9176" format="default"/>. In order to do this
registration, the device needs to know its CoRE Constrained RESTful Environments (CoRE) Link Format
description, as specified in <xref target="RFC6690"/>. target="RFC6690" format="default"/>. In essence, the
registration process involves performing a GET on
.well-known/core/?rt=core-rd at the address of the resource directory, directory
and then doing a POST on the path of the discovered resource.</t>
      <t>Other mechanisms enabling device discovery and delegation of
functionality to a non-sleepy node include those discussed in <xref
target="I-D.vial-core-mirror-proxy"/> target="I-D.vial-core-mirror-proxy" format="default"/> and <xref
target="I-D.koster-core-coap-pubsub"/>.</t> target="I-D.ietf-core-coap-pubsub" format="default"/>.</t>
      <t>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 networks; it does
not work in these the public cellular networks. Our recommended networks discussed in this document. We recommend the following alternate methods for discovery are the following:

<list style="hanging">

<t hangText="Manual Configuration"><vspace blankLines="1"/>

The discovery:</t>

        <ul spacing="normal">
          <li>
        <t>Manual Configuration</t>

	<t>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.</t>

<t hangText="Manufacturer Server"><vspace blankLines="1"/>

The
	</li>

          <li>
	            <t>Manufacturer Server</t>
<t>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 cooperate 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 that specific
actions to be taken when the device does something.

</t>

<t hangText="Delegating something.</t>
	</li>

          <li>
        <t>Delegating Manufacturer Server"><vspace blankLines="1"/>

The Server</t>
	<t>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 cases, there are even legal requirements (such as
EU privacy laws) that prevent providing unnecessary information to
third parties.
</t>

<t hangText="Common parties.</t>
	</li>

        <li>
        <t>Common Global Resolution Infrastructure"><vspace blankLines="1"/>

The Infrastructure</t>

	<t>The delegating manufacturer server model could be generalized into
a
reverse-DNS -like reverse-DNS-like discovery infrastructure that could could, for example, answer the question
"this "This is a device with identity ID, ID 2456; where is my home registration server?". server?" However, at present present, no such resolution system exists.
(Note: The EPCGlobal system for RFID Radio Frequency Identification (RFID) resolution is reminiscent
	of this approach.)
</t>

</list>
</t>

<t>
Besides approach.)</t>
	</li>
	</ul>
      <t>Besides manual configuration, these alternate mechanisms are mostly
suitable for large manufacturers and deployments. Good automated
mechanism
mechanisms for discovery of devices that are manufactured and deployed
in small quantities are still needed.
</t> needed.</t>
    </section>
    <section anchor="data" title="Data Formats"> numbered="true" toc="default">
      <name>Data Formats</name>
      <t>A variety of data formats exist for passing around data. These data
formats include XML, JavaScript Object Notation (JSON) <xref
target="RFC7159"/>, target="RFC8259"
format="default"/>, Efficient XML Interchange (EXI) <xref
target="W3C.REC-exi-20110310"/>,
target="W3C.REC-exi-20140211" format="default"/>, Concise Binary Object Representation (CBOR) <xref target="RFC8949"/>, and various text formats. Message lengths can
have a significant effect on the amount of energy required for the
communications, and as 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 that readers refer
to <xref
target="I-D.jennings-core-senml"/> as target="RFC8428" format="default"/> for a compact, yet compact but easily processed
and extendable textual format.</t>
    </section>
    <section anchor="rt" title="Real-Time numbered="true" toc="default">
      <name>Real-Time Reachable Devices"> Devices</name>
      <t>These devices are often best modeled as CoAP servers. The device
will have limited control on over when it receives messages, and it will
have to listen actively for messages, up to the limits of the
underlying link layer. If in some phase of its operation the device acts also acts in client the role in some
phase of its operation, a client, it can control how many transmissions it makes
on its own behalf.</t>
      <t>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.</t>
      <t>For sensor-type devices, the CoAP Observe extension (Observe option) <xref
target="RFC7641"/> target="RFC7641" format="default"/> may be supported. This allows the sensor to track
changes to the sensed value, 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 other than the currently registered one client may come
in 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 cannot rely on having to communicate only on its own choice of
observation interval.</t>
      <t>In order to act as a server, the device needs to be placed in a
public IPv4 address, be reachable over IPv6, or be hosted in a private
network.  If the the device is hosted on a private network, then all
other nodes that 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 Network (VPN) technology, technology -- for instance instance,
IPsec-based VPNs.</t>
      <t>Power consumption from unwanted traffic is problematic in these
devices, unless they are placed in a private network or protected by a an
operator-provided firewall service. Devices on an IPv6 network will
have
be afforded some protection through due to the nature of the 2^64 2<sup>64</sup> 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.</t>
      <t>Note that the the VPN approach can not cannot prevent unwanted traffic
received at the tunnel endpoint address, address and may require keep-alive
traffic. Special APNs can solve this issue, issue but require an explicit
arrangement with the service provider.</t>
    </section>
    <section anchor="sens" title="Sleepy Devices"> numbered="true" toc="default">
      <name>Sleepy Devices</name>
      <t>These devices are best modeled as devices that can delegate queries
to some other node.  For node -- for instance, as mirror proxy servers <xref
target="I-D.vial-core-mirror-proxy"/>
target="I-D.vial-core-mirror-proxy" format="default"/> or CoAP Publish-Subscribe
pub&wj;/sub Clients <xref
target="I-D.koster-core-coap-pubsub"/> clients. target="I-D.ietf-core-coap-pubsub"
format="default"/>. When the device initializes
itself, it makes a registration of itself in a proxy server or broker as described
above in <xref target="disc"/> target="disc" format="default"/> and then continues to send periodic
updates of sensor values.</t>
      <t>As a result, the device acts only as a client, client and not as 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.</t>

<t>Other approaches for delegation include CoAP-options described in
<xref target="I-D.castellani-core-alive"/> <xref
target="I-D.fossati-core-publish-monitor-options"/>. 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.</t>
      <t>The ability to shut down communications and act as only a client
has four impacts:

<list style="symbols">

<t>Radio impacts:</t>
      <ul spacing="normal">
        <li>Radio transmission and reception can be turned off during the
sleeping period, reducing power consumption significantly.</t>

<t>However, significantly.</li>
        <li>However, some power and time is are consumed by having to re-attach reattach to
the network after the end of a sleep period.</t>

<t>The period.</li>
        <li>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 Note, however, that networks may cache packets for some time though. time. On
the other hand, stateful firewalls can effectively remove much of the
unwanted traffic for client type devices.</t>

<t>The client-type devices.</li>
        <li>The device may exist behind a NAT or a firewall without being
impacted. Note that "Simple Security" the "simple security" basic IPv6 firewall capability
<xref target="RFC6092"/> target="RFC6092" format="default"/> blocks inbound UDP traffic by default, so just
moving to IPv6 is not a direct solution to this problem.</t>

</list>

</t> problem.</li>
      </ul>
      <t>For sleepy devices that represent actuators, it is also possible to
use the mirror proxy server or pub/sub broker model.  The A device can make periodic polls to receive information from the
proxy to determine if a server or broker about variable has changed.</t> changes via either polling or notifications.</t>
      <section title="Implementation Considerations"> numbered="true" toc="default">
        <name>Implementation Considerations</name>
        <t>There are several challenges in related to implementing sleepy devices. They
need hardware that can be put to placed in 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
reawakening at suitable time scales, timescales, as otherwise the device needs to be
powered up too frequently.</t>
        <t>Most commercial cellular modem platforms do not allow applications
to suspend the state of the communications stack. Hence, after a
power-off period period, they need to re-establish communications, which takes
some amount of time and extra energy.</t>
        <t>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.</t>
        <t>The cellular networks have a number of adjustable configuration
parameters, such as the maximum used paging interval. Proper setting settings
of these values has have 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.</t>
      </section>
    </section>
    <section title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>There are no particular security aspects with related to 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 that have largely NOT yet been addressed in
the relevant Internet Drafts Internet-Drafts <xref
target="I-D.vial-core-mirror-proxy"/> target="I-D.vial-core-mirror-proxy" format="default"/>
        <xref target="I-D.castellani-core-alive"/> target="I-D.castellani-core-alive" format="default"/>
        <xref target="I-D.fossati-core-publish-monitor-options"/>. target="I-D.fossati-core-publish-monitor-options" format="default"/>.  However,
we point out that that, in general, security issues in delegation can be
solved either through either 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 means, in practice practice, end-to-end data object
security. We have implemented one such mechanism for sleepy nodes as
described in <xref
target="I-D.aks-lwig-crypto-sensors"/>.</t> target="RFC8387" format="default"/>.</t>
      <t>The security considerations relating to CoAP <xref
target="RFC7252"/> target="RFC7252" format="default"/> and the relevant link layers should
apply. Note that cellular networks universally employ per-device
authentication, integrity protection, and and, for most of the world,
encryption of all their communications. Additional protection of
transport sessions is possible through mechanisms described in <xref
target="RFC7252"/> target="RFC7252" format="default"/> or data objects.</t>
    </section>
    <section anchor="iana" title="IANA Considerations">

<t>There are numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document has no IANA impacts in this memo.</t> actions.</t>
    </section>
  </middle>
  <back>

<references title="Normative References">
      <?rfc include="reference.RFC.7159.xml"?>
      <?rfc include="reference.RFC.6690.xml"?>
      <?rfc include="reference.RFC.7252.xml"?>
      <?rfc include="reference.RFC.7641.xml"?>
      <?rfc include="reference.I-D.ietf-core-resource-directory.xml"?>

<displayreference target="I-D.arkko-core-sleepy-sensors" to="Tiny-CoAP"/>
<displayreference target="I-D.castellani-core-alive" to="CoAP-Alive"/>
<displayreference target="I-D.fossati-core-publish-monitor-options" to="CoAP-Publ-Monitor"/>
<displayreference target="I-D.vial-core-mirror-proxy" to="CoRE-Mirror"/>
<displayreference target="I-D.ietf-core-coap-pubsub" to="CoAP-PubSub"/>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8259.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6690.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7252.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7641.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8949.xml"/>

<!-- draft-ietf-core-resource-directory (RFC 9176 - published 2022-04-25) -->
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9176.xml"/>

        <reference anchor="W3C.REC-exi-20110310"> anchor="W3C.REC-exi-20140211" target="https://www.w3.org/TR/exi/">
          <front>
            <title>Efficient XML Interchange (EXI) Format 1.0</title> 1.0 (Second Edition)</title>
            <author initials='T' surname='Kamiya'>
	    <organization /> initials="J" surname="Schneider">
              <organization/>
            </author>
            <author initials='J' surname='Schneider'>
	    <organization /> initials="T" surname="Kamiya">
              <organization/>
            </author>
            <author initials="D" surname="Peintner">
              <organization/>
            </author>
            <author initials="R" surname="Kyusakov">
              <organization/>
            </author>
            <date month='March' year='2011' /> month="February" year="2014"/>
          </front>
	<seriesInfo name='World
          <refcontent>World Wide Web Consortium Recommendation REC-exi-20110310' value='http://www.w3.org/TR/2011/REC-exi-20110310' />
	<format type='HTML' target='http://www.w3.org/TR/2011/REC-exi-20110310' /> REC-exi-20140211</refcontent>
        </reference>

      <?rfc include="reference.I-D.jennings-core-senml.xml"?>
      <?rfc include="reference.RFC.7228.xml"?>

<!-- draft-jennings-core-senml (replaced by draft-ietf-core-senml, then
     published as RFC 8428) -->
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8428.xml"/>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7228.xml"/>
      </references>

<references title="Informative References">
      <?rfc include="reference.RFC.4838.xml"?>
      <?rfc include="reference.RFC.6092.xml"?>
      <?rfc include="reference.I-D.arkko-core-sleepy-sensors.xml"?>
      <?rfc include="reference.I-D.aks-lwig-crypto-sensors.xml"?>
      <?rfc include="reference.I-D.castellani-core-alive.xml"?>
      <?rfc include="reference.I-D.fossati-core-publish-monitor-options.xml"?>
      <?rfc include="reference.I-D.vial-core-mirror-proxy.xml"?>
      <?rfc include="reference.I-D.koster-core-coap-pubsub.xml"?>
      <references>
        <name>Informative References</name>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4838.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6092.xml"/>

<!-- draft-arkko-core-sleepy-sensors (Expired) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-arkko-core-sleepy-sensors.xml"/>

<!-- draft-aks-lwig-crypto-sensors (replaced by draft-ietf-lwig-crypto-sensors,
     then published as RFC 8387) -->
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8387.xml"/>

<!-- draft-castellani-core-alive (Expired)
  Have to do "long way" to fix author initials -->
<reference anchor='Android-Bundle'> anchor="I-D.castellani-core-alive">
   <front>
	  <title>Optimizing Downloads for Efficient Network Access</title>
      <title>CoAP Alive Message</title>
      <author fullname='Developer.Android.Com'>
	    <organization /> fullname="Angelo P. Castellani" surname="Castellani" initials="A">
	 </author>
      <author fullname="Salvatore Loreto" surname="Loreto" initials="S">
	 </author>
      <date month='February' year='2013' month="March" day="29" year="2012" />
   </front>
   <seriesInfo name='Android developer note' value='http://developer.android.com/training/efficient-downloads/efficient-network-access.html' />
	<format type='HTML' target='http://developer.android.com/training/efficient-downloads/efficient-network-access.html' name="Internet-Draft" value="draft-castellani-core-alive-00" />
</reference>

<!-- draft-fossati-core-publish-monitor-options (Expired) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-fossati-core-publish-monitor-options.xml"/>

<!-- draft-vial-core-mirror-proxy (Expired) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-vial-core-mirror-proxy.xml"/>

<!-- draft-koster-core-coap-pubsub (replaced by draft-ietf-core-coap-pubsub;
     Expired.  Since it's a "replaced by," had to change
     "I-D.koster-core-coap-pubsub" to "I-D.ietf-core-coap-pubsub" to get
     <displayreference> to work) (I-D Exists) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/draft-ietf-core-coap-pubsub.xml"/>

        <reference anchor="Android-Bundle"
target="https://developer.android.com/training/efficient-downloads/efficient-network-access.html">
          <front>
            <title>Optimize network access</title>
            <author/>
            <date month="May" year="2022"/>
          </front>
          <refcontent>Android developer note</refcontent>
        </reference>
      </references>
    </references>
    <section title="Acknowledgments"> numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>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 <contact fullname="Zach Shelby"/>,
      <contact fullname="Jan Holler"/>, <contact fullname="Salvatore
      Loreto"/>, <contact fullname="Matthew Vial"/>, <contact fullname="Thomas
      Fossati"/>, <contact fullname="Mohit Sethi"/>, <contact fullname="Jan
      Melen"/>, <contact fullname="Joachim Sachs"/>, <contact
      fullname="Heidi-Maria Rissanen"/>, <contact fullname="Sebastien
      Pierrel"/>, <contact fullname="Kumar Balachandran"/>, <contact
      fullname="Muhammad Waqas Mir, Cullen Jennings, Markus Isomaki, Hannes
Tschofenig, and Anna Larmo Mir"/>, <contact fullname="Cullen Jennings"/>,
      <contact fullname="Markus Isomaki"/>, <contact fullname="Hannes Tschofenig"/>, and <contact fullname="Anna Larmo"/> for interesting discussions in this problem
space.</t>
    </section>
  </back>
</rfc>