Internet DRAFT - draft-ietf-lwig-7228bis
draft-ietf-lwig-7228bis
LWIG Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational M. Ersue
Expires: 31 December 2022
A. Keranen
Ericsson
C. Gomez
Universitat Politecnica de Catalunya
29 June 2022
Terminology for Constrained-Node Networks
draft-ietf-lwig-7228bis-00
Abstract
The Internet Protocol Suite is increasingly used on small devices
with severe constraints on power, memory, and processing resources,
creating constrained-node networks. This document provides a number
of basic terms that have been useful in the standardization work for
constrained-node networks.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-lwig-7228bis/.
Discussion of this document takes place on the Light-Weight
Implementation Guidance (lwig) Working Group mailing list
(mailto:lwip@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/lwip/.
Source for this draft and an issue tracker can be found at
https://github.com/lwig-wg/terminology.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Core Terminology . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Constrained Nodes . . . . . . . . . . . . . . . . . . . . 4
2.2. Constrained Networks . . . . . . . . . . . . . . . . . . 5
2.2.1. Challenged Networks . . . . . . . . . . . . . . . . . 6
2.3. Constrained-Node Networks . . . . . . . . . . . . . . . . 7
2.3.1. LLN . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.2. LoWPAN, 6LoWPAN . . . . . . . . . . . . . . . . . . . 8
2.3.3. LPWAN . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Classes of Constrained Devices . . . . . . . . . . . . . . . 8
3.1. Firmware/Software upgradability . . . . . . . . . . . . . 12
3.2. Isolation functionality . . . . . . . . . . . . . . . . . 13
3.3. Shielded secrets . . . . . . . . . . . . . . . . . . . . 13
4. Power Terminology . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Scaling Properties . . . . . . . . . . . . . . . . . . . 14
4.2. Classes of Energy Limitation . . . . . . . . . . . . . . 15
4.3. Strategies for Using Power for Communication . . . . . . 16
4.4. Strategies of Keeping Time over Power Events . . . . . . 17
5. Classes of Networks . . . . . . . . . . . . . . . . . . . . . 20
5.1. Classes of link layer MTU size . . . . . . . . . . . . . 20
5.2. Class of Internet Integration . . . . . . . . . . . . . . 21
5.3. Classes of physical layer bit rate . . . . . . . . . . . 21
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. Informative References . . . . . . . . . . . . . . . . . . . 23
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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
Small devices with limited CPU, memory, and power resources, so-
called "constrained devices" (often used as sensors/actuators, smart
objects, or smart devices) can form a network, becoming "constrained
nodes" in that network. Such a network may itself exhibit
constraints, e.g., with unreliable or lossy channels, limited and
unpredictable bandwidth, and a highly dynamic topology.
Constrained devices might be in charge of gathering information in
diverse settings, including natural ecosystems, buildings, and
factories, and sending the information to one or more server
stations. They might also act on information, by performing some
physical action, including displaying it. Constrained devices may
work under severe resource constraints such as limited battery and
computing power, little memory, and insufficient wireless bandwidth
and ability to communicate; these constraints often exacerbate each
other. Other entities on the network, e.g., a base station or
controlling server, might have more computational and communication
resources and could support the interaction between the constrained
devices and applications in more traditional networks.
Today, diverse sizes of constrained devices with different resources
and capabilities are becoming connected. Mobile personal gadgets,
building-automation devices, cellular phones, machine-to-machine
(M2M) devices, and other devices benefit from interacting with other
"things" nearby or somewhere in the Internet. With this, the
Internet of Things (IoT) becomes a reality, built up out of uniquely
identifiable and addressable objects (things). Over the next decade,
this could grow to large numbers of Internet-connected constrained
devices ([IoT-2025] predicts that by, 2025, more than 2500 devices
will be connected to the Internet per second), greatly increasing the
Internet's size and scope.
The present document provides a number of basic terms that have been
useful in the standardization work for constrained environments. The
intention is not to exhaustively cover the field but to make sure a
few core terms are used consistently between different groups
cooperating in this space.
The present document is a revision of [RFC7228].
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In this document, the term "byte" is used in its now customary sense
as a synonym for "octet". Where sizes of semiconductor memory are
given, the prefix "kibi" (1024) is combined with "byte" to
"kibibyte", abbreviated "KiB", for 1024 bytes [ISQ-13]. Powers of 10
are given as 10^100 where 100 is the exponent.
In computing, the term "power" is often used for the concept of
"computing power" or "processing power", as in CPU performance. In
this document, the term stands for electrical power unless explicitly
stated otherwise. "Mains-powered" is used as a shorthand for being
permanently connected to a stable electrical power grid.
2. Core Terminology
There are two important aspects to _scaling_ within the Internet of
Things:
* scaling up Internet technologies to a large number [IoT-2025] of
inexpensive nodes, while
* scaling down the characteristics of each of these nodes and of the
networks being built out of them, to make this scaling up
economically and physically viable.
The need for scaling down the characteristics of nodes leads to
"constrained nodes".
2.1. Constrained Nodes
The term "constrained node" is best defined by contrasting the
characteristics of a constrained node with certain widely held
expectations on more familiar Internet nodes:
Constrained Node: A node where some of the characteristics that are
otherwise pretty much taken for granted for Internet nodes at the
time of writing are not attainable, often due to cost constraints
and/or physical constraints on characteristics such as size,
weight, and available power and energy. The tight limits on
power, memory, and processing resources lead to hard upper bounds
on state, code space, and processing cycles, making optimization
of energy and network bandwidth usage a dominating consideration
in all design requirements. Also, some layer-2 services such as
full connectivity and broadcast/multicast may be lacking.
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While this is not a rigorous definition, it is grounded in the state
of the art and clearly sets apart constrained nodes from server
systems, desktop or laptop computers, powerful mobile devices such as
smartphones, etc. There may be many design considerations that lead
to these constraints, including cost, size, weight, and other scaling
factors.
(An alternative term, when the properties as a network node are not
in focus, is "constrained device".)
There are multiple facets to the constraints on nodes, often applying
in combination, for example:
* constraints on the maximum code complexity (ROM/Flash),
* constraints on the size of state and buffers (RAM),
* constraints on the amount of computation feasible in a period of
time ("processing power"),
* constraints on the available power, and
* constraints on user interface and accessibility in deployment
(ability to set keys, update software, etc.).
Section 3 defines a number of interesting classes ("class-N") of
constrained nodes focusing on relevant combinations of the first two
constraints. With respect to available power, [RFC6606]
distinguishes "power-affluent" nodes (mains-powered or regularly
recharged) from "power-constrained nodes" that draw their power from
primary batteries or by using energy harvesting; more detailed power
terminology is given in Section 4.
The use of constrained nodes in networks often also leads to
constraints on the networks themselves. However, there may also be
constraints on networks that are largely independent of those of the
nodes. We therefore distinguish "constrained networks" from
"constrained-node networks".
2.2. Constrained Networks
We define "constrained network" in a similar way:
Constrained Network: A network where some of the characteristics
pretty much taken for granted with link layers in common use in
the Internet at the time of writing are not attainable.
Constraints may include:
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* low achievable bitrate/throughput (including limits on duty
cycle),
* high packet loss and high variability of packet loss (delivery
rate),
* highly asymmetric link characteristics,
* severe penalties for using larger packets (e.g., high packet loss
due to link-layer fragmentation),
* limits on reachability over time (a substantial number of devices
may power off at any point in time but periodically "wake up" and
can communicate for brief periods of time), and
* lack of (or severe constraints on) advanced services such as IP
multicast.
More generally, we speak of constrained networks whenever at least
some of the nodes involved in the network exhibit these
characteristics.
Again, there may be several reasons for this:
* cost constraints on the network,
* constraints posed by the nodes (for constrained-node networks),
* physical constraints (e.g., power constraints, environmental
constraints, media constraints such as underwater operation,
limited spectrum for very high density, electromagnetic
compatibility),
* regulatory constraints, such as very limited spectrum availability
(including limits on effective radiated power and duty cycle) or
explosion safety, and
* technology constraints, such as older and lower-speed technologies
that are still operational and may need to stay in use for some
more time.
2.2.1. Challenged Networks
A constrained network is not necessarily a "challenged network"
[FALL]:
Challenged Network: A network that has serious trouble maintaining
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what an application would today expect of the end-to-end IP model,
e.g., by:
* not being able to offer end-to-end IP connectivity at all,
* exhibiting serious interruptions in end-to-end IP connectivity,
or
* exhibiting delay well beyond the Maximum Segment Lifetime (MSL)
defined by TCP [RFC0793].
All challenged networks are constrained networks in some sense, but
not all constrained networks are challenged networks. There is no
well-defined boundary between the two, though. Delay-Tolerant
Networking (DTN) has been designed to cope with challenged networks
[RFC4838].
2.3. Constrained-Node Networks
Constrained-Node Network: A network whose characteristics are
influenced by being composed of a significant portion of
constrained nodes.
A constrained-node network always is a constrained network because of
the network constraints stemming from the node constraints, but it
may also have other constraints that already make it a constrained
network.
The rest of this subsection introduces two additional terms that are
in active use in the area of constrained-node networks, without an
intent to define them: LLN and (6)LoWPAN.
2.3.1. LLN
A related term that has been used to describe the focus of the IETF
ROLL working group is "Low-Power and Lossy Network (LLN)". The ROLL
(Routing Over Low-Power and Lossy) terminology document [RFC7102]
defines LLNs as follows:
LLN: Low-Power and Lossy Network. Typically composed of many
embedded devices with limited power, memory, and processing
resources interconnected by a variety of links, such as IEEE
802.15.4 or low-power Wi-Fi. There is a wide scope of application
areas for LLNs, including industrial monitoring, building
automation (heating, ventilation, and air conditioning (HVAC),
lighting, access control, fire), connected home, health care,
environmental monitoring, urban sensor networks, energy
management, assets tracking, and refrigeration.
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Beyond that, LLNs often exhibit considerable loss at the physical
layer, with significant variability of the delivery rate, and some
short-term unreliability, coupled with some medium-term stability
that makes it worthwhile to both (1) construct directed acyclic
graphs that are medium-term stable for routing and (2) do
measurements on the edges such as Expected Transmission Count (ETX)
[RFC6551]. Not all LLNs comprise low-power nodes
[I-D.hui-vasseur-roll-rpl-deployment].
LLNs typically are composed of constrained nodes; this leads to the
design of operation modes such as the "non-storing mode" defined by
RPL (the IPv6 Routing Protocol for Low-Power and Lossy Networks
[RFC6550]). So, in the terminology of the present document, an LLN
is a constrained-node network with certain network characteristics,
which include constraints on the network as well.
2.3.2. LoWPAN, 6LoWPAN
One interesting class of a constrained network often used as a
constrained-node network is "LoWPAN" [RFC4919], a term inspired from
the name of an IEEE 802.15.4 working group (low-rate wireless
personal area networks (LR-WPANs)). The expansion of the LoWPAN
acronym, "Low-Power Wireless Personal Area Network", contains a hard-
to-justify "Personal" that is due to the history of task group naming
in IEEE 802 more than due to an orientation of LoWPANs around a
single person. Actually, LoWPANs have been suggested for urban
monitoring, control of large buildings, and industrial control
applications, so the "Personal" can only be considered a vestige.
Occasionally, the term is read as "Low-Power Wireless Area Networks"
[WEI]. Originally focused on IEEE 802.15.4, "LoWPAN" (or when used
for IPv6, "6LoWPAN") also refers to networks built from similarly
constrained link-layer technologies [RFC7668] [RFC8105] [RFC7428]
[RFC9159].
2.3.3. LPWAN
An overview over Low-Power Wide Area Network (LPWAN) technologies is
provided by [RFC8376].
3. Classes of Constrained Devices
Despite the overwhelming variety of Internet-connected devices that
can be envisioned, it may be worthwhile to have some succinct
terminology for different classes of constrained devices.
Before we get to that, let's first distinguish two big rough groups
of devices based on their CPU capabilities:
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* Microcontroller-class devices (sometimes called "M-class"). These
often (but not always) include RAM and code storage on chip and
would struggle to support more powerful general-purpose operating
systems, e.g., they do not have an MMU (memory management unit).
They use most of their pins for interfaces to application hardware
such as digital in/out (the latter often Pulse Width Modulation
(PWM)-controllable), ADC/DACs (analog-to-digital and digital-to-
analog converters), etc. Where this hardware is specialized for
an application, we may talk about "Systems on a Chip" (SOC).
These devices often implement elaborate sleep modes to achieve
microwatt- or at least milliwatt-level sustained power usage (Ps,
see below).
* General-purpose-class devices (sometimes called "A-class"). These
usually have RAM and Flash storage on separate chips (not always
separate packages), and offer support for general-purpose
operating systems such as Linux, e.g. an MMU. Many of the pins on
the CPU chip are dedicated to interfacing with RAM and other
memory. Some general-purpose-class devices integrate some
application hardware such as video controllers, these are often
also called "Systems on a Chip" (SOC). While these chips also
include sleep modes, they are usually more on the watt side of
sustained power usage (Ps).
If the distinction between these groups needs to be made in this
document, we distinguish group "M" (microcontroller) from group "J"
(general purpose).
In this document, the class designations in Table 1 may be used as
rough indications of device capabilities. Note that the classes from
10 upwards are not really constrained devices in the sense of the
previous section; they may still be useful to discuss constraints in
larger devices:
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+=======+=========+===================+===============+=============+
| Group | Name | data size (e.g., | code size | Examples |
| | | RAM) | (e.g., Flash) | |
+=======+=========+===================+===============+=============+
| M | Class | << 10 KiB | << 100 KiB | ATtiny |
| | 0, C0 | | | |
+-------+---------+-------------------+---------------+-------------+
| M | Class | ~ 10 KiB | ~ 100 KiB | STM32F103CB |
| | 1, C1 | | | |
+-------+---------+-------------------+---------------+-------------+
| M | Class | ~ 50 KiB | ~ 250 KiB | STM32F103RC |
| | 2, C2 | | | |
+-------+---------+-------------------+---------------+-------------+
| M | Class | ~ 100 KiB | ~ 500..1000 | STM32F103RG |
| | 3, C3 | | KiB | |
+-------+---------+-------------------+---------------+-------------+
| M | Class | ~ 300..1000 KiB | ~ 1000..2000 | "Luxury" |
| | 4, C4 | | KiB | |
+-------+---------+-------------------+---------------+-------------+
| J | Class | (16..)32..64..128 | 4..8..16 MiB | OpenWRT |
| | 10, | MiB | | routers |
| | C10 | | | |
+-------+---------+-------------------+---------------+-------------+
| J | Class | 0.5..1 GiB | (lots) | Raspberry |
| | 15, | | | PI |
| | C15 | | | |
+-------+---------+-------------------+---------------+-------------+
| J | Class | 1..4 GiB | (lots) | Smartphones |
| | 16, | | | |
| | C16 | | | |
+-------+---------+-------------------+---------------+-------------+
| J | Class | 4..32 GiB | (lots) | Laptops |
| | 17, | | | |
| | C17 | | | |
+-------+---------+-------------------+---------------+-------------+
| J | Class | (lots) | (lots) | Servers |
| | 19, | | | |
| | C19 | | | |
+-------+---------+-------------------+---------------+-------------+
Table 1: Classes of Constrained Devices (KiB = 1024 bytes)
As of the writing of this document, these characteristics correspond
to distinguishable clusters of commercially available chips and
design cores for constrained devices. While it is expected that the
boundaries of these classes will move over time, Moore's law tends to
be less effective in the embedded space than in personal computing
devices: gains made available by increases in transistor count and
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density are more likely to be invested in reductions of cost and
power requirements than into continual increases in computing power.
(This effect is less pronounced in the multi-chip J-group
architectures; e.g., class 10 usage for OpenWRT has started at 4/16
MiB Flash/RAM, with an early lasting minimum at 4/32, to now
requiring 8/64 and preferring 16/128 for modern software releases
[W432].)
Class 0 devices are very constrained sensor-like motes. They are so
severely constrained in memory and processing capabilities that most
likely they will not have the resources required to communicate
directly with the Internet in a secure manner (rare heroic, narrowly
targeted implementation efforts notwithstanding). Class 0 devices
will participate in Internet communications with the help of larger
devices acting as proxies, gateways, or servers. Class 0 devices
generally cannot be secured or managed comprehensively in the
traditional sense. They will most likely be preconfigured (and will
be reconfigured rarely, if at all) with a very small data set. For
management purposes, they could answer keepalive signals and send on/
off or basic health indications.
Class 1 devices are quite constrained in code space and processing
capabilities, such that they cannot easily talk to other Internet
nodes employing a full protocol stack such as using HTTP, Transport
Layer Security (TLS), and related security protocols and XML-based
data representations. However, they are capable enough to use a
protocol stack specifically designed for constrained nodes (such as
the Constrained Application Protocol (CoAP) over UDP [RFC7252]) and
participate in meaningful conversations without the help of a gateway
node. In particular, they can provide support for the security
functions required on a large network. Therefore, they can be
integrated as fully developed peers into an IP network, but they need
to be parsimonious with state memory, code space, and often power
expenditure for protocol and application usage.
Class 2 devices are less constrained and fundamentally capable of
supporting most of the same protocol stacks as used on notebooks or
servers. However, even these devices can benefit from lightweight
and energy-efficient protocols and from consuming less bandwidth.
Furthermore, using fewer resources for networking leaves more
resources available to applications. Thus, using the protocol stacks
defined for more constrained devices on Class 2 devices might reduce
development costs and increase the interoperability.
Constrained devices with capabilities significantly beyond Class 2
devices exist. They are less demanding from a standards development
point of view as they can largely use existing protocols unchanged.
The previous version of the present document therefore did not make
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any attempt to define constrained classes beyond Class 2. These
devices, and to a certain extent even J-group devices, can still be
constrained by a limited energy supply. Class 3 and 4 devices are
less clearly defined than the lower classes; they are even less
constrained. In particular Class 4 devices are powerful enough to
quite comfortably run, say, JavaScript interpreters, together with
elaborate network stacks. Additional classes may need to be defined
based on protection capabilities, e.g., an MPU (memory protection
unit; true MMUs are typically only found in J-group devices).
With respect to examining the capabilities of constrained nodes,
particularly for Class 1 devices, it is important to understand what
type of applications they are able to run and which protocol
mechanisms would be most suitable. Because of memory and other
limitations, each specific Class 1 device might be able to support
only a few selected functions needed for its intended operation. In
other words, the set of functions that can actually be supported is
not static per device type: devices with similar constraints might
choose to support different functions. Even though Class 2 devices
have some more functionality available and may be able to provide a
more complete set of functions, they still need to be assessed for
the type of applications they will be running and the protocol
functions they would need. To be able to derive any requirements,
the use cases and the involvement of the devices in the application
and the operational scenario need to be analyzed. Use cases may
combine constrained devices of multiple classes as well as more
traditional Internet nodes.
3.1. Firmware/Software upgradability
Platforms may differ in their firmware or software upgradability.
The below is a first attempt at classifying this.
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+======+============================================================+
| Name | Firmware/Software upgradability |
+======+============================================================+
| F0 | no (discard for upgrade) |
+------+------------------------------------------------------------+
| F1 | replaceable, out of service during replacement, reboot |
+------+------------------------------------------------------------+
| F2 | patchable during operation, reboot required |
+------+------------------------------------------------------------+
| F3 | patchable during operation, restart not visible |
| | externally |
+------+------------------------------------------------------------+
| F9 | app-level upgradability, no reboot required |
| | ("hitless") |
+------+------------------------------------------------------------+
Table 2: Levels of software update capabilities
3.2. Isolation functionality
TBD. This section could discuss the ability of the platform to
isolate different components. The categories below are not mutually
exclusive; we need to build relevant clusters.
+======+===========================================================+
| Name | Isolation functionality |
+======+===========================================================+
| Is0 | no isolation |
+------+-----------------------------------------------------------+
| Is2 | MPU (memory protection unit), at least boundary registers |
+------+-----------------------------------------------------------+
| Is5 | MMU with Linux-style kernel/user |
+------+-----------------------------------------------------------+
| Is7 | Virtualization-style isolation |
+------+-----------------------------------------------------------+
| Is8 | Secure enclave isolation |
+------+-----------------------------------------------------------+
Table 3: Levels of isolation capabilities
3.3. Shielded secrets
[Need to identify clusters]
Some platforms can keep shielded secrets (usually in conjunction with
secure enclave functionality).
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+======+================================+
| Name | Secret shielding functionality |
+======+================================+
| Sh0 | no secret shielding |
+------+--------------------------------+
| Sh1 | some secret shielding |
+------+--------------------------------+
| Sh9 | perfect secret shielding |
+------+--------------------------------+
Table 4: Levels of secret shielding
capabilities
4. Power Terminology
Devices not only differ in their computing capabilities but also in
available power and/or energy. While it is harder to find
recognizable clusters in this space, it is still useful to introduce
some common terminology.
4.1. Scaling Properties
The power and/or energy available to a device may vastly differ, from
kilowatts to microwatts, from essentially unlimited to hundreds of
microjoules.
Instead of defining classes or clusters, we simply state, using the
International System of Units (SI units), an approximate value for
one or both of the quantities listed in Table 5:
+======+============================================+=========+
| Name | Definition | SI Unit |
+======+============================================+=========+
| Ps | Sustainable average power available for | W |
| | the device over the time it is functioning | (Watt) |
+------+--------------------------------------------+---------+
| Et | Total electrical energy available before | J |
| | the energy source is exhausted | (Joule) |
+------+--------------------------------------------+---------+
Table 5: Quantities Relevant to Power and Energy
The value of Et may need to be interpreted in conjunction with an
indication over which period of time the value is given; see
Section 4.2.
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Some devices enter a "low-power" mode before the energy available in
a period is exhausted or even have multiple such steps on the way to
exhaustion. For these devices, Ps would need to be given for each of
the modes/steps.
4.2. Classes of Energy Limitation
As discussed above, some devices are limited in available energy as
opposed to (or in addition to) being limited in available power.
Where no relevant limitations exist with respect to energy, the
device is classified as E9. The energy limitation may be in total
energy available in the usable lifetime of the device (e.g., a device
that is discarded when its non-replaceable primary battery is
exhausted), classified as E2. Where the relevant limitation is for a
specific period, the device is classified as E1, e.g., a solar-
powered device with a limited amount of energy available for the
night, a device that is manually connected to a charger and has a
period of time between recharges, or a device with a periodic
(primary) battery replacement interval. Finally, there may be a
limited amount of energy available for a specific event, e.g., for a
button press in an energy-harvesting light switch; such devices are
classified as E0. Note that, in a sense, many E1 devices are also
E2, as the rechargeable battery has a limited number of useful
recharging cycles.
Table 6 provides a summary of the classifications described above.
+======+========================+==============================+
| Name | Type of energy | Example Power Source |
| | limitation | |
+======+========================+==============================+
| E0 | Event energy-limited | Event-based harvesting |
+------+------------------------+------------------------------+
| E1 | Period energy-limited | Battery that is periodically |
| | | recharged or replaced |
+------+------------------------+------------------------------+
| E2 | Lifetime energy- | Non-replaceable primary |
| | limited | battery |
+------+------------------------+------------------------------+
| E9 | No direct quantitative | Mains-powered |
| | limitations to | |
| | available energy | |
+------+------------------------+------------------------------+
Table 6: Classes of Energy Limitation
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4.3. Strategies for Using Power for Communication
Especially when wireless transmission is used, the radio often
consumes a big portion of the total energy consumed by the device.
Design parameters, such as the available spectrum, the desired range,
and the bitrate aimed for, influence the power consumed during
transmission and reception; the duration of transmission and
reception (including potential reception) influence the total energy
consumption.
Different strategies for power usage and network attachment may be
used, based on the type of the energy source (e.g., battery or mains-
powered) and the frequency with which a device needs to communicate.
The general strategies for power usage can be described as follows:
Always-on: This strategy is most applicable if there is no reason
for extreme measures for power saving. The device can stay on in
the usual manner all the time. It may be useful to employ power-
friendly hardware or limit the number of wireless transmissions,
CPU speeds, and other aspects for general power-saving and cooling
needs, but the device can be connected to the network all the
time.
Normally-off: Under this strategy, the device sleeps such long
periods at a time that once it wakes up, it makes sense for it to
not pretend that it has been connected to the network during
sleep: the device reattaches to the network as it is woken up.
The main optimization goal is to minimize the effort during the
reattachment process and any resulting application communications.
If the device sleeps for long periods of time and needs to
communicate infrequently, the relative increase in energy
expenditure during reattachment may be acceptable.
Low-power: This strategy is most applicable to devices that need to
operate on a very small amount of power but still need to be able
to communicate on a relatively frequent basis. This implies that
extremely low-power solutions need to be used for the hardware,
chosen link-layer mechanisms, and so on. Typically, given the
small amount of time between transmissions, despite their sleep
state, these devices retain some form of attachment to the
network. Techniques used for minimizing power usage for the
network communications include minimizing any work from re-
establishing communications after waking up and tuning the
frequency of communications (including "duty cycling", where
components are switched on and off in a regular cycle) and other
parameters appropriately.
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Table 7 provides a summary of the strategies described above.
+======+==============+===========================+
| Name | Strategy | Ability to communicate |
+======+==============+===========================+
| P0 | Normally-off | Reattach when required |
+------+--------------+---------------------------+
| P1 | Low-power | Appears connected, |
| | | perhaps with high latency |
+------+--------------+---------------------------+
| P9 | Always-on | Always connected |
+------+--------------+---------------------------+
Table 7: Strategies of Using Power for
Communication
Note that the discussion above is at the device level; similar
considerations can apply at the communications-interface level. This
document does not define terminology for the latter.
A term often used to describe power-saving approaches is "duty-
cycling". This describes all forms of periodically switching off
some function, leaving it on only for a certain percentage of time
(the "duty cycle").
[RFC7102] only distinguishes two levels, defining a Non-Sleepy Node
as a node that always remains in a fully powered-on state (always
awake) where it has the capability to perform communication (P9) and
a Sleepy Node as a node that may sometimes go into a sleep mode (a
low-power state to conserve power) and temporarily suspend protocol
communication (P0); there is no explicit mention of P1.
4.4. Strategies of Keeping Time over Power Events
Many applications for a device require it to keep some concept of
time.
Time-keeping can be relative to a previous event (last packet
received), absolute on a device-specific scale (e.g., last reboot),
or absolute on a world-wide scale ("wall-clock time").
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Some devices lose the concept of time when going to sleep: after
wakeup, they don't know how long they slept. Some others do keep
some concept of time during sleep, but not precise enough to use as a
basis for keeping absolute time. Some devices have a continuously
running source of a reasonably accurate time (often a 32,768 Hz watch
crystal). Finally, some devices can keep their concept of time even
during a battery change, e.g., by using a backup battery or a
supercapacitor to power the real-time clock (RTC).
The actual accuracy of time may vary, with errors ranging from tens
of percent from on-chip RC oscillators (not useful for keeping
absolute time, but still useful for, e.g., timing out some state) to
approximately 10^-4 to 10^-5 ("watch crystal") of error. More
precise timing is available with temperature compensated crystal
oscillators (TCXO). Further improvement requires significantly
higher power usage, bulk, fragility, and device cost, e.g. oven-
controlled crystal oscillators (OCXO) can reach 10^-8 accuracy, and
Rubidium frequency sources can reach 10^-11 over the short term and
10^-9 over the long term.
A device may need to fire up a more accurate frequency source during
wireless communication, this may also allow it to keep more precise
time during the period.
The various time sources available on the device can be assisted by
external time input, e.g. via the network using the NTP protocol
[RFC5905]. Information from measuring the deviation between external
input and local time source can be used to increase the accuracy of
maintaining time even during periods of no network use.
Errors of the frequency source can be compensated if known
(calibrated against a known better source, or even predicted, e.g.,
in a software TCXO). Even with errors partially compensated, an
uncertainty remains, which is the more fundamental characteristic to
discuss.
Battery solutions may allow the device to keep a wall-clock time
during its entire life, or the wall-clock time may need to be reset
after a battery change. Even devices that have a battery lasting for
their lifetime may not be set to wall-clock time at manufacture time,
possibly because the battery is only activated at installation time
where time sources may be questionable or because setting the clock
during manufacture is deemed too much effort.
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Devices that keep a good approximation of wall-clock time during
their life may be in a better position to securely validate external
time inputs than devices that need to be reset episodically, which
can possibly be tricked by their environment into accepting a long-
past time, for instance with the intent of exploiting expired
security assertions such as certificates.
From a practical point of view, devices can be divided at least on
the two dimensions proposed in Table 8 and Table 9. Corrections to
the local time of a device performed over the network can be used to
improve the uncertainty exhibited by these basic device classes.
+======+===========================+=============================+
| Name | Type | Uncertainty (roughly) |
+======+===========================+=============================+
| T0 | no concept of time | infinite |
+------+---------------------------+-----------------------------+
| T1 | relative time while awake | (usually high) |
+------+---------------------------+-----------------------------+
| T2 | relative time | (usually high during sleep) |
+------+---------------------------+-----------------------------+
| T3 | relative time | 10^-4 or better |
+------+---------------------------+-----------------------------+
| T5 | absolute time (e.g., | 10^-4 or better |
| | since boot) | |
+------+---------------------------+-----------------------------+
| T7 | wall-clock time | 10^-4 or better |
+------+---------------------------+-----------------------------+
| T8 | wall-clock time | 10^-5 or better |
+------+---------------------------+-----------------------------+
| T9 | wall-clock time | 10^-6 or better (TCXO) |
+------+---------------------------+-----------------------------+
| T10 | wall-clock time | 10^-7 or better (OCXO or |
| | | Rb) |
+------+---------------------------+-----------------------------+
Table 8: Strategies of Keeping Time over Power Events
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+======+====================================+=================+
| Name | Permanency (from type T5 upwards): | Uncertainty |
+======+====================================+=================+
| TP0 | time needs to be reset on certain | |
| | occasions | |
+------+------------------------------------+-----------------+
| TP1 | time needs to be set during | (possibly |
| | installation | reduced... |
+------+------------------------------------+-----------------+
| TP9 | reliable time is maintained during | ...by using |
| | lifetime | external input) |
+------+------------------------------------+-----------------+
Table 9: Permanency of Keeping Time
Further parameters that can be used to discuss clock quality can be
found in Section 3.5 of [I-D.ietf-cbor-time-tag].
5. Classes of Networks
5.1. Classes of link layer MTU size
Link layer technologies used by constrained devices can be
categorized on the basis of link layer MTU size. Depending on this
parameter, the fragmentation techniques needed (if any) to support
the IPv6 MTU requirement may vary.
We define the following classes of link layer MTU size:
+======+=====================+====================================+
| Name | L2 MTU size (bytes) | 6LoWPAN Fragmentation applicable*? |
+======+=====================+====================================+
| S0 | 3 - 12 | need new kind of fragmentation |
+------+---------------------+------------------------------------+
| S1 | 13 - 127 | yes |
+------+---------------------+------------------------------------+
| S2 | 128 - 1279 | yes |
+------+---------------------+------------------------------------+
| S3 | >= 1280 | no fragmentation needed |
+------+---------------------+------------------------------------+
Table 10
* if no link layer fragmentation is available (note: 'Sx' stands for
'Size x')
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S0 technologies require fragmentation to support the IPv6 MTU
requirement. If no link layer fragmentation is available,
fragmentation is needed at the adaptation layer below IPv6. However,
6LoWPAN fragmentation [RFC4944] cannot be used for these
technologies, given the extremely reduced link layer MTU. In this
case, lightweight fragmentation formats must be used (e.g.
[RFC8724]).
S1 and S2 technologies require fragmentation at the subnetwork level
to support the IPv6 MTU requirement. If link layer fragmentation is
unavailable or insufficient, fragmentation is needed at the
adaptation layer below IPv6. 6LoWPAN fragmentation [RFC4944] can be
used to carry 1280-byte IPv6 packets over these technologies.
S3 technologies do not require fragmentation to support the IPv6 MTU
requirement.
5.2. Class of Internet Integration
The term "Internet of Things" is sometimes confusingly used for
connected devices that are not actually employing Internet
technology. Some devices do use Internet technology, but only use it
to exchange packets with a fixed communication partner ("device-to-
cloud" scenarios, see also Section 2.2 of [RFC7452]). More general
devices are prepared to communicate with other nodes in the Internet
as well.
We define the following classes of Internet technology level:
+======+======================================+
| Name | Internet technology |
+======+======================================+
| I0 | none (local interconnect only) |
+------+--------------------------------------+
| I1 | device-to-cloud only |
+------+--------------------------------------+
| I9 | full Internet connectivity supported |
+------+--------------------------------------+
Table 11
5.3. Classes of physical layer bit rate
[This section is a trial balloon. We could also talk about burst
rate, sustained rate; bits/s, messages/s, ...]
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Physical layer technologies used by constrained devices can be
categorized on the basis of physical layer (PHY) bit rate. The PHY
bit rate class of a technology has important implications with regard
to compatibility with existing protocols and mechanisms on the
Internet, responsiveness to frame transmissions and need for header
compression techniques.
We define the following classes of PHY bit rate:
+======+==============+============================================+
| Name | PHY bit rate | Comment |
| | (bit/s) | |
+======+==============+============================================+
| B0 | < 10 | Transmission time of 150-byte frame > MSL |
+------+--------------+--------------------------------------------+
| B1 | 10 -- 10^3 | Unresponsiveness if human expects reaction |
| | | to sent frame (frame size > 62.5 byte) |
+------+--------------+--------------------------------------------+
| B2 | 10^3 -- 10^6 | Responsiveness if human expects reaction |
| | | to sent frame, but header compression |
| | | still needed |
+------+--------------+--------------------------------------------+
| B3 | > 10^6 | Header compression yields relatively low |
| | | performance benefits |
+------+--------------+--------------------------------------------+
Table 12
(note: 'Bx' stands for 'Bit rate x')
B0 technologies lead to very high transmission times, which may be
close to or even greater than the Maximum Segment Lifetime (MSL)
assumed on the Internet [RFC0793]. Many Internet protocols and
mechanisms will fail when transmit times are greater than the MSL.
B0 technologies lead to a frame transmission time greater than the
MSL for a frame size greater than 150 bytes.
B1 technologies offer transmission times which are lower than the MSL
(for a frame size greater than 150 bytes). However, transmission
times for B1 technologies are still significant if a human expects a
reaction to the transmission of a frame. With B1 technologies, the
transmission time of a frame greater than 62.5 bytes exceeds 0.5
seconds, i.e. a threshold time beyond which any response or reaction
to a frame transmission will appear not to be immediate [RFC5826].
B2 technologies do not incur responsiveness problems, but still
benefit from using header compression techniques (e.g. [RFC6282]) to
achieve performance improvements.
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Over B3 technologies, the relative performance benefits of header
compression are low. For example, in a duty-cycled technology
offering B3 PHY bit rates, energy consumption decrease due to header
compression may be comparable with the energy consumed while in a
sleep interval. On the other hand, for B3 PHY bit rates, a human
user will not be able to perceive whether header compression has been
used or not in a frame transmission.
6. IANA Considerations
This document makes no requests to IANA.
7. Security Considerations
This document introduces common terminology that does not raise any
new security issues. Security considerations arising from the
constraints discussed in this document need to be discussed in the
context of specific protocols. For instance, Section 11.6 of
[RFC7252], "Constrained node considerations", discusses implications
of specific constraints on the security mechanisms employed.
[RFC7416] provides a security threat analysis for the RPL routing
protocol. Implementation considerations for security protocols on
constrained nodes are discussed in [RFC7815] and
[I-D.ietf-lwig-tls-minimal]. A wider view of security in
constrained-node networks is provided in [RFC8576].
8. Informative References
[FALL] Fall, K., "A Delay-Tolerant Network Architecture for
Challenged Internets", SIGCOMM 2003,
DOI 10.1145/863955.863960, 2003,
<https://doi.org/10.1145/863955.863960>.
[I-D.hui-vasseur-roll-rpl-deployment]
Vasseur, J., Hui, J., Dasgupta, S., and G. Yoon, "RPL
deployment experience in large scale networks", Work in
Progress, Internet-Draft, draft-hui-vasseur-roll-rpl-
deployment-01, 5 July 2012,
<https://www.ietf.org/archive/id/draft-hui-vasseur-roll-
rpl-deployment-01.txt>.
[I-D.ietf-cbor-time-tag]
Bormann, C., Gamari, B., and H. Birkholz, "Concise Binary
Object Representation (CBOR) Tags for Time, Duration, and
Period", Work in Progress, Internet-Draft, draft-ietf-
cbor-time-tag-00, 19 May 2021,
<https://www.ietf.org/archive/id/draft-ietf-cbor-time-tag-
00.txt>.
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[I-D.ietf-lwig-tls-minimal]
Kumar, S. S., Keoh, S. L., and H. Tschofenig, "A
Hitchhiker's Guide to the (Datagram) Transport Layer
Security Protocol for Smart Objects and Constrained Node
Networks", Work in Progress, Internet-Draft, draft-ietf-
lwig-tls-minimal-01, 7 March 2014,
<https://www.ietf.org/archive/id/draft-ietf-lwig-tls-
minimal-01.txt>.
[IoT-2025] Rosen, M. and IDC, "Driving the Digital Agenda Requires
Strategic Architecture", 16 November 2016, <https://idc-
cema.com/dwn/SF_177701/driving_the_digital_agenda_requires
_strategic_architecture_rosen_idc.pdf>. Slide 11
[ISQ-13] International Electrotechnical Commission, "International
Standard -- Quantities and units -- Part 13: Information
science and technology", IEC 80000-13, March 2008.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, DOI 10.17487/RFC4919, August 2007,
<https://www.rfc-editor.org/info/rfc4919>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, DOI 10.17487/RFC5826, April 2010,
<https://www.rfc-editor.org/info/rfc5826>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
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[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<https://www.rfc-editor.org/info/rfc6551>.
[RFC6606] Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Routing",
RFC 6606, DOI 10.17487/RFC6606, May 2012,
<https://www.rfc-editor.org/info/rfc6606>.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <https://www.rfc-editor.org/info/rfc7102>.
[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>.
[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>.
[RFC7416] Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
and M. Richardson, Ed., "A Security Threat Analysis for
the Routing Protocol for Low-Power and Lossy Networks
(RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
<https://www.rfc-editor.org/info/rfc7416>.
[RFC7428] Brandt, A. and J. Buron, "Transmission of IPv6 Packets
over ITU-T G.9959 Networks", RFC 7428,
DOI 10.17487/RFC7428, February 2015,
<https://www.rfc-editor.org/info/rfc7428>.
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[RFC7452] Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
"Architectural Considerations in Smart Object Networking",
RFC 7452, DOI 10.17487/RFC7452, March 2015,
<https://www.rfc-editor.org/info/rfc7452>.
[RFC7668] Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
<https://www.rfc-editor.org/info/rfc7668>.
[RFC7815] Kivinen, T., "Minimal Internet Key Exchange Version 2
(IKEv2) Initiator Implementation", RFC 7815,
DOI 10.17487/RFC7815, March 2016,
<https://www.rfc-editor.org/info/rfc7815>.
[RFC8105] Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
M., and D. Barthel, "Transmission of IPv6 Packets over
Digital Enhanced Cordless Telecommunications (DECT) Ultra
Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
2017, <https://www.rfc-editor.org/info/rfc8105>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
[RFC8576] Garcia-Morchon, O., Kumar, S., and M. Sethi, "Internet of
Things (IoT) Security: State of the Art and Challenges",
RFC 8576, DOI 10.17487/RFC8576, April 2019,
<https://www.rfc-editor.org/info/rfc8576>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
[RFC9159] Gomez, C., Darroudi, S.M., Savolainen, T., and M. Spoerk,
"IPv6 Mesh over BLUETOOTH(R) Low Energy Using the Internet
Protocol Support Profile (IPSP)", RFC 9159,
DOI 10.17487/RFC9159, December 2021,
<https://www.rfc-editor.org/info/rfc9159>.
[W432] "Warning about 4/32 devices", OpenWRT wiki, last accessed
2021-12-01,
<https://openwrt.org/supported_devices/432_warning>.
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[WEI] Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
Internet", Wiley-Blackwell monograph,
DOI 10.1002/9780470686218, ISBN 9780470747995, 2009,
<https://doi.org/10.1002/9780470686218>.
Acknowledgements
TBD
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Mehmet Ersue
Munich
Germany
Email: mersue@gmail.com
Ari Keranen
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Phone: +34-93-413-7206
Email: carlesgo@entel.upc.edu
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