wdiff rfc9178v1.txt rfc9178.txt

Internet Engineering Task Force (IETF) J. Arkko
Request for Comments: 9178 A. Eriksson Ericsson
Category: Informational A. Keränen Eriksson
ISSN: 2070-1721 Independent
A. Keränen
Ericsson
February
May 2022

Building Power-Efficient Constrained Application Protocol (CoAP) Devices
for Cellular Networks

Abstract

This memo discusses the use of the Constrained Application Protocol
(CoAP) 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.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9178.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Goals for Low-Power Operation
3. Link-Layer Assumptions
4. Scenarios
5. Discovery and Registration
6. Data Formats
7. Real-Time Reachable Devices
8. Sleepy Devices
8.1. Implementation Considerations
9. Security Considerations
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

This memo discusses the use of the Constrained Application Protocol
(CoAP) [RFC7252] 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 [Tiny-CoAP].

This memo 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.

Our focus is on devices that need to be optimized for power usage and
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 types of 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,
ongoing, and generally complementary to the techniques presented in
this memo. However, the list of techniques described in this
document as useful for a particular application may change with the
evolution of these underlying technologies.

The rest of this memo is structured as follows. Section 2 discusses
the need and goals for low-power devices. Section 3 outlines our
expectations for the low-layer communications model. Section 4
describes the two scenarios that we address. Sections 5, 6, 7, and 8
give guidelines for the use of CoAP in these scenarios.

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.

2. Goals for Low-Power Operation

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
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 solar cells that
are as small and inexpensive as possible.

In 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,
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 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.

Many of these situations lead to a requirement for minimizing power
usage and/or maximizing battery lifetimes. Using the power usage
strategies described in [RFC7228], mains-powered sensor-type devices
can use the Always-on strategy, whereas battery-operated or energy-
harvesting devices need to adjust behavior based on the communication
interval. For intervals on the order of seconds, the Low-power
strategy is appropriate. For intervals ranging from minutes to
hours, either the Low-power or Normally-off strategy is suitable.
Finally, for intervals lasting days 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.

Long battery lifetimes are required for many applications, however.
In some cases, these lifetimes should be on the order of years or
even a decade or longer. Some communication devices already reach
multi-year lifetimes, and continuous 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.

Power usage cannot be evaluated based solely 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
cooperation 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.

3. Link-Layer Assumptions

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, 3G, LTE, and LTE 5G
networks are examples of such networks but are 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 are inherent in the
concept of public networks.

* 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 worldwide coverage. This means that
applications where coverage is important can be built. For
instance, most transport-sector applications require national or
even worldwide 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 that a 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 significant improvements related to
delays and 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 Publish-Subscribe
(pub/sub) Broker [CoAP-PubSub] 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, 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
(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.

* Point-to-Point Link Model

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 cannot be reasonably used for any
local discovery purposes. While the configuration of the service
provider's router for specific users is theoretically possible,
this is difficult to achieve in practice, at least for any small
user that cannot afford a network-wide contract for a private APN
(Access Point Name). The public network access service has little
per-user tailoring.

* Radio Technology

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.

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
[Android-Bundle].

Naturally, each device has 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 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: (1) fine-grained mechanisms that deal with how
often the device needs to wake up for paging messages and (2) cruder
mechanisms where the device simply disconnects from the network for a
period of time. There are costs and benefits 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) mechanisms [RFC4838] to relax the requirements for
timeliness of connectivity and message delivery.

4. Scenarios

Not all applications or situations are equal. They may require
different solutions or communication models. This memo focuses on
two common scenarios in cellular networks:

* Real-Time Reachable Devices

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 previously agreed-upon 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.

* Sleepy Devices

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.

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 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.

5. Discovery and Registration

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 cannot discover who they need to
talk,
talk to, 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 peer
devices such a resource
directory -- for instance, as specified in [RFC9176]. In order to do
this registration, the device needs to know its Constrained RESTful
Environments (CoRE) Link Format description, as specified in
[RFC6690]. 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 those discussed in
[CoRE-Mirror] and [CoAP-PubSub].

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. networks discussed in this document.
We recommend the following alternate methods for discovery:

* Manual Configuration

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.

* Manufacturer Server

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
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 be
taken when the device does something.

* Delegating Manufacturer Server

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 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.

* Common Global Resolution Infrastructure

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

Besides manual configuration, these alternate mechanisms are mostly
suitable for large manufacturers and deployments. Good automated
mechanisms for discovery of devices that are manufactured and
deployed in small quantities are still needed.

6. Data Formats

A variety of data formats exist for passing around data. These data
formats include XML, JavaScript Object Notation (JSON) [RFC8259],
Efficient XML Interchange (EXI) [W3C.REC-exi-20110310], [W3C.REC-exi-20140211], Concise
Binary Object Representation (CBOR) [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 [RFC8428] for a compact but
easily processed and extendable textual format.

7. Real-Time Reachable Devices

These devices are often best modeled as CoAP servers. The device
will have limited control 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
also acts in the role of a client, 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 (Observe option)
[RFC7641] 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 clients other than the currently registered 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 cannot rely on 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 be hosted in a
private network. If 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 Network (VPN)
technology -- for instance, IPsec-based VPNs.

Power consumption from unwanted traffic is problematic in these
devices, unless they are placed in a private network or protected by
an operator-provided firewall service. Devices on an IPv6 network
will be afforded some protection due to 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 VPN approach cannot prevent unwanted traffic received
at the tunnel endpoint address and may require keep-alive traffic.
Special APNs can solve this issue but require an explicit arrangement
with the service provider.

8. Sleepy Devices

These devices are best modeled as devices that can delegate queries
to some other node -- for instance, as mirror proxies servers [CoRE-Mirror]
or CoAP pub/sub Clients [CoAP-PubSub]. When the device initializes
itself, it makes a registration of itself in a proxy server or broker as
described above in Section 5 and then continues to send periodic
updates of sensor values.

As a result, the device acts only as a client and not as a server,
and can shut down all communication channels 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 smaller values
ranging from minutes to hours.

Other approaches for delegation include CoAP options such as those
described in [CoAP-Alive] and [CoAP-Publ-Monitor]. In this memo, we
use mirror proxies as an example, because of their ability to work
with both HTTP and CoAP implementations; however, the concepts are
similar, and work in the IETF is still in progress, so the final
protocol details have 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 are consumed by having to reattach 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, however, that networks may cache packets for some
time. On the other hand, stateful firewalls can effectively
remove much of the unwanted traffic for client-type devices.

* The device may exist behind a NAT or a firewall without being
impacted. Note that the "simple security" basic IPv6 firewall
capability [RFC6092] blocks inbound UDP traffic by default, so
just moving to IPv6 is not a direct solution to this problem.

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 periodically poll receive
information from the
proxy to determine if a server or broker about variable has changed. changes via
either polling or notifications.

8.1. Implementation Considerations

There are several challenges related to implementing sleepy devices.
They need hardware that can be placed in an appropriate sleep mode
but 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 reawakening at suitable timescales, 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
settings of these values have an impact on the power consumption of
the device, but with current business practices, such settings are
rarely negotiated when the user's subscription is provisioned.

9. Security Considerations

There are no particular security aspects 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 that have largely NOT yet been addressed
in the relevant Internet-Drafts [CoRE-Mirror] [CoAP-Alive]
[CoAP-Publ-Monitor]. However, we point out that, in general,
security issues in delegation can be solved 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,
in practice, end-to-end data object security. We have implemented
one such mechanism for sleepy nodes as described in [RFC8387].

The security considerations relating to CoAP [RFC7252] 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 [RFC7252] or data objects.

10. IANA Considerations

This document has no IANA actions.

11. References

11.1. Normative References

[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.

[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://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,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.

[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.

[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176,
February April
2022, <https://www.rfc-editor.org/info/rfc9176>.

[W3C.REC-exi-20110310]

[W3C.REC-exi-20140211]
Schneider, J., Kamiya, T. T., Peintner, D., and J. Schneider, R. Kyusakov,
"Efficient XML Interchange (EXI) Format 1.0", 1.0 (Second
Edition)", World Wide Web Consortium Recommendation REC-exi-20110310, March 2011,
<https://www.w3.org/TR/2011/REC-exi-20110310/>. REC-
exi-20140211, February 2014, <https://www.w3.org/TR/exi/>.

[RFC8428] Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Sensor Measurement Lists (SenML)", RFC 8428,
DOI 10.17487/RFC8428, August 2018,
<https://www.rfc-editor.org/info/rfc8428>.

[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.

11.2. Informative References

[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
April 2007, <https://www.rfc-editor.org/info/rfc4838>.

[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security
Capabilities in Customer Premises Equipment (CPE) for
Providing Residential IPv6 Internet Service", RFC 6092,
DOI 10.17487/RFC6092, January 2011,
<https://www.rfc-editor.org/info/rfc6092>.

[Tiny-CoAP]
Arkko, J., Rissanen, H., Loreto, S., Turanyi, Z., and O.
Novo, "Implementing Tiny COAP Sensors", Work in Progress,
Internet-Draft, draft-arkko-core-sleepy-sensors-01, 5 July
2011, <https://datatracker.ietf.org/doc/html/draft-arkko-
core-sleepy-sensors-01>.

[RFC8387] Sethi, M., Arkko, J., Keranen, A., and H. Back, "Practical
Considerations and Implementation Experiences in Securing
Smart Object Networks", RFC 8387, DOI 10.17487/RFC8387,
May 2018, <https://www.rfc-editor.org/info/rfc8387>.

[CoAP-Alive]
Castellani, A. P. and S. Loreto, "CoAP Alive Message", Work
in Progress, Internet-Draft, draft-castellani-core-
alive-00, draft-castellani-core-alive-
00, 29 March 2012,
<https://datatracker.ietf.org/doc/html/draft-castellani-
core-alive-00>. <https://datatracker.ietf.org/doc/html/
draft-castellani-core-alive-00>.

[CoAP-Publ-Monitor]
Fossati, T., Giacomin, P., and S. Loreto, "Publish and
Monitor Options for CoAP", Work in Progress, Internet-
Draft, draft-fossati-core-publish-monitor-options-01, 10
March 2012, <https://datatracker.ietf.org/doc/html/draft-
fossati-core-publish-monitor-options-01>.

[CoRE-Mirror]
Vial, M., "CoRE Mirror Server", Work in Progress,
Internet-Draft, draft-vial-core-mirror-proxy-01, 13 July
2012, <https://datatracker.ietf.org/doc/html/draft-vial-
core-mirror-proxy-01>.

[CoAP-PubSub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", Work in Progress, Internet-Draft, draft-ietf-
core-coap-pubsub-09, 30 September 2019,
core-coap-pubsub-10, 4 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-09>.
coap-pubsub-10>.

[Android-Bundle]
"Optimizing Downloads for Efficient Network Access",
"Optimize network access", Android developer note, August 2020, May
2022, <https://developer.android.com/training/efficient-
downloads/efficient-network-access.html>.

Acknowledgments

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.

Authors' Addresses

Jari Arkko
Ericsson
FI-02420 Jorvas
Finland
Email: jari.arkko@piuha.net

Anders Eriksson
Ericsson
Independent
SE-164 83 Stockholm
Sweden
Email: anders.e.eriksson@ericsson.com anders.e.eriksson@posthem.se

Ari Keränen
Ericsson
FI-02420 Jorvas
Finland
Email: ari.keranen@ericsson.com