LWIG Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational M. Ersue
Expires: October 25, 2013 Nokia Siemens Networks
A. Keranen
Ericsson
April 23, 2013
Terminology for Constrained Node Networks
draft-ietf-lwig-terminology-04
Abstract
The Internet Protocol Suite is increasingly used on small devices
with severe constraints, creating constrained node networks. This
document provides a number of basic terms that have turned out to be
useful in the standardization work for constrained environments.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Constrained Nodes . . . . . . . . . . . . . . . . . . . . 3
2.2. Constrained Networks . . . . . . . . . . . . . . . . . . 4
2.2.1. Challenged Networks . . . . . . . . . . . . . . . . . 5
2.3. Constrained Node Networks . . . . . . . . . . . . . . . . 5
2.3.1. LLN ("low-power lossy network") . . . . . . . . . . . 5
2.3.2. LoWPAN, 6LoWPAN . . . . . . . . . . . . . . . . . . . 6
3. Classes of Constrained Devices . . . . . . . . . . . . . . . 7
4. Power Terminology . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Scaling Properties . . . . . . . . . . . . . . . . . . . 9
4.2. Classes of Energy Limitation . . . . . . . . . . . . . . 9
4.3. Strategies of Using Power for Communication . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Small devices with limited CPU, memory, and power resources, so
called constrained devices (also known as sensor, smart object, or
smart device) can constitute 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.
Constrained devices may work under severe resource constraints such
as limited battery and computing power, little memory and
insufficient wireless bandwidth, and communication capabilities.
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, etc. benefit from interacting with other "things"
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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). And over the next decade, this
could grow to large numbers [fifty-billion] of Internet-connected
constrained devices, greatly increasing the Internet's size and
scope.
The present document provides a number of basic terms that have
turned out to be 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.
2. Terminology
The main focus of this field of work appears to be _scaling_:
o Scaling up Internet technologies to a large number [fifty-billion]
of inexpensive nodes, while
o 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 in 2013
are not attainable, often due to cost constraints and/or physical
constraints on characteristics such as size, weight, and available
power and energy.
While this is less than satisfying as 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 name, when the properties as a network node are not
in focus, is "constrained device".)
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There are multiple facets to the constraints on nodes, often applying
in combination, e.g.:
o constraints on the maximum code complexity (ROM/Flash);
o constraints on the size of state and buffers (RAM);
o constraints on the available power.
Section 3 defines a small number of interesting classes ("class-N"
for N=0,1,2) 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 from those of
the nodes. We therefore distinguish _constrained networks_ and
_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 for Internet link layers in 2013 are
not attainable.
Again, there may be several reasons for this:
o cost constraints on the network,
o constraints of the nodes (for constrained node networks),
o physical constraints (e.g., power constraints, media constraints
such as underwater operation, limited spectrum for very high
density, electromagnetic compatibility),
o regulatory constraints, such as very limited spectrum availability
(including limits on effective radiated power and duty cycle), or
explosion safety.
Constraints may include:
o low achievable bit rate (including limits on duty cycle),
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o high packet loss, packet loss (delivery rate) variability,
o severe penalties for using larger packets (e.g., high packet loss
due to link layer fragmentation),
o lack of (or severe constraints on) advanced services such as IP
multicast.
2.2.1. Challenged Networks
A constrained network is not necessarily a _challenged_ network
[FALL]:
Challenged Network: A network that has serious trouble maintaining
what an application would today expect of the end-to-end IP model,
e.g., by:
o not being able to offer end-to-end IP connectivity at all;
o exhibiting serious interruptions in end-to-end IP connectivity;
o 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 may
also have other constraints that already make it a constrained
network.
2.3.1. LLN ("low-power lossy network")
A related term that has been used recently is "low-power lossy
network" (LLN). In its terminology document, the ROLL working group
is saying [I-D.ietf-roll-terminology]:
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LLN: Low power and Lossy networks (LLNs) are 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 WiFi. There is a wide scope of application
areas for LLNs, including industrial monitoring, building
automation (HVAC, lighting, access control, fire), connected home,
healthcare, environmental monitoring, urban sensor networks,
energy management, assets tracking and refrigeration.. [sic]
In common usage, LLN often stands for "the network characteristics
that RPL has been designed for". Beyond what is said in the ROLL
terminology document, LLNs do appear to have significant 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 construct medium-term stable
directed acyclic graphs for routing and do measurements on the edges
such as ETX [RFC6551]. Actual "low power" does not seem to be
required for an LLN [I-D.hui-vasseur-roll-rpl-deployment], and the
positions on scaling of LLNs appear to vary widely
[I-D.clausen-lln-rpl-experiences].
The ROLL terminology document states that LLNs typically are composed
of constrained nodes; this is also supported by the design of
operation modes such as RPL's "non-storing mode". So, in the
terminology of the present document, an LLN seems to be 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 the "LoWPAN" [RFC4919], a term inspired
from the name of the IEEE 802.15.4 working group (low-rate wireless
personal area networks (LR-WPANs)). The expansion of that acronym,
"Low-Power Wireless Personal Area Network" contains a hard to justify
"Personal" that is due to IEEE politics 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. Maybe the term is best read as "Low-Power
Wireless Area Networks" (LoWPANs) [WEI]. Originally focused on IEEE
802.15.4, "LoWPAN" (or when used for IPv6, "6LoWPAN") is now also
being used for networks built from similarly constrained link layer
technologies [I-D.ietf-6lowpan-btle]
[I-D.mariager-6lowpan-v6over-dect-ule] [I-D.brandt-6man-lowpanz].
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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. In this
document, the class designations in Table 1 may be used as rough
indications of device capabilities:
+-------------+-----------------------+-------------------------+
| Name | data size (e.g., RAM) | code size (e.g., Flash) |
+-------------+-----------------------+-------------------------+
| Class 0, C0 | << 10 KiB | << 100 KiB |
| | | |
| Class 1, C1 | ~ 10 KiB | ~ 100 KiB |
| | | |
| Class 2, C2 | ~ 50 KiB | ~ 250 KiB |
+-------------+-----------------------+-------------------------+
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
density are more likely to be invested in reductions of cost and
power requirements than into continual increases in computing power.
Class 0 devices are very constrained sensor-like motes. Most likely
they will not be able to communicate directly with the Internet in a
secure manner. 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 cannot easily talk to other Internet nodes employing
a full protocol stack such as using HTTP, TLS and related security
protocols and XML-based data representations. However, they have
enough power to use a protocol stack specifically designed for
constrained nodes (e.g., CoAP over UDP) 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
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memory, code space, and often power expenditure for protocol and
application usage.
Class 2 can already support mostly 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 very constrained devices also 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 present document therefore does not make any attempt to define
classes beyond Class 2. These devices can still be constrained by a
limited energy supply.
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.
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4. Power Terminology
Devices not only differ in their computing capabilities, but also in
available electrical 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 propose simply stating,
in SI units, an approximate value for one or both of the quantities
listed in Table 2:
+--------+---------------------------------------------+------------+
| Name | Definition | SI Unit |
+--------+---------------------------------------------+------------+
| Ps | Sustainable average power available for the | W (Watt) |
| | device over the time it is functioning | |
| | | |
| Et | Total electrical energy available before | J (Joule) |
| | the energy source is exhausted | |
+--------+---------------------------------------------+------------+
Table 2: 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 the next
subsection.
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 E3. The energy limitation may be in total
energy available in the usable lifetime of the device (e.g. a device
with a non-replaceable primary battery, which is discarded when this
battery is exhausted), classified as E2. Where the relevant
limitation is for a specific period, this is classified as E1, e.g.
a limited amount of energy available for the night with a solar-
powered device, or for the period between recharges with a device
that is manually connected to a charger, or by 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
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an energy harvesting light switch; this is classified as E0. Note
that many E1 devices in a sense also are E2, as the rechargeable
battery has a limited number of useful recharging cycles.
In summary, we distinguish (Table 3):
+------+------------------------------+-----------------------------+
| Name | Type of energy limitation | Example Power Source |
+------+------------------------------+-----------------------------+
| E0 | Event energy-limited | Event-based harvesting |
| | | |
| E1 | Period energy-limited | Battery that is |
| | | periodically recharged or |
| | | replaced |
| | | |
| E2 | Lifetime energy-limited | Non-replaceable primary |
| | | battery |
| | | |
| E3 | No direct quantitative | Mains powered |
| | limitations to available | |
| | energy | |
+------+------------------------------+-----------------------------+
Table 3: Classes of Energy Limitation
4.3. Strategies of 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.
Based on the type of the energy source (e.g., battery or mains power)
and how often device needs to communicate, it may use different kinds
of strategies for power usage and network attachment.
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.
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Always-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 re-attaches to the network as it is woken up. The main
optimization goal is to minimize the effort during such re-
attachment 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 needs 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 network 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, tuning the frequency
of communications, and other parameters appropriately.
In summary, we distinguish (Table 4):
+------+------------+----------------------------------------------+
| Name | Strategy | Ability to communicate |
+------+------------+----------------------------------------------+
| S0 | Always-off | Re-attach when required |
| | | |
| S1 | Low-power | Appears connected, perhaps with high latency |
| | | |
| S2 | Always-on | Always connected |
+------+------------+----------------------------------------------+
Table 4: Strategies of Using Power for Communication
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5. Security Considerations
This draft introduces common terminology that does not raise any new
security issue.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
Dominique Barthel and Peter van der Stok provided useful comments;
Charles Palmer provided a full editorial review.
Peter van der Stok insisted that we should have power terminology,
hence Section 4. The text for Section 4.3 is mostly lifted from
[I-D.arkko-lwig-cellular] and has been adapted for this document.
8. Informative References
[FALL] Fall, K., "A Delay-Tolerant Network Architecture for
Challenged Internets", SIGCOMM 2003, 2003.
[I-D.arkko-lwig-cellular]
Arkko, J., Eriksson, A., and A. Keraenen, "Building Power-
Efficient CoAP Devices for Cellular Networks", draft-
arkko-lwig-cellular-00 (work in progress), February 2013.
[I-D.brandt-6man-lowpanz]
Brandt, A. and J. Buron, "Transmission of IPv6 packets
over ITU-T G.9959 Networks", draft-brandt-6man-lowpanz-00
(work in progress), February 2013.
[I-D.clausen-lln-rpl-experiences]
Clausen, T., Verdiere, A., Yi, J., Herberg, U., and Y.
Igarashi, "Observations of RPL: IPv6 Routing Protocol for
Low power and Lossy Networks", draft-clausen-lln-rpl-
experiences-06 (work in progress), February 2013.
[I-D.hui-vasseur-roll-rpl-deployment]
Vasseur, J., Hui, J., Dasgupta, S., and G. Yoon, "RPL
deployment experience in large scale networks", draft-hui-
vasseur-roll-rpl-deployment-01 (work in progress), July
2012.
[I-D.ietf-6lowpan-btle]
Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
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over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
(work in progress), February 2013.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-12 (work in
progress), March 2013.
[I-D.mariager-6lowpan-v6over-dect-ule]
Mariager, P. and J. Petersen, "Transmission of IPv6
Packets over DECT Ultra Low Energy", draft-mariager-
6lowpan-v6over-dect-ule-02 (work in progress), May 2012.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC4838] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
Networking Architecture", RFC 4838, April 2007.
[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, August 2007.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[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, May 2012.
[WEI] Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
Internet", ISBN 9780470747995, 2009.
[fifty-billion]
Ericsson, "More Than 50 Billion Connected Devices",
Ericsson White Paper 284 23-3149 Uen, February 2011,
<http://www.ericsson.com/res/docs/whitepapers/
wp-50-billions.pdf>.
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Mehmet Ersue
Nokia Siemens Networks
St.-Martinstrasse 76
81541 Munich
Germany
Phone: +49 172 8432301
Email: mehmet.ersue@nsn.com
Ari Keranen
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
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