<- RFC Index (5801..5900)
RFC 5826
Internet Engineering Task Force (IETF) A. Brandt
Request for Comments: 5826 J. Buron
Category: Informational Sigma Designs, Inc.
ISSN: 2070-1721 G. Porcu
Telecom Italia
April 2010
Home Automation Routing Requirements in Low-Power and Lossy Networks
Abstract
This document presents requirements specific to home control and
automation applications for Routing Over Low power and Lossy (ROLL)
networks. In the near future, many homes will contain high numbers
of wireless devices for a wide set of purposes. Examples include
actuators (relay, light dimmer, heating valve), sensors (wall switch,
water leak, blood pressure), and advanced controllers (radio-
frequency-based AV remote control, central server for light and heat
control). Because such devices only cover a limited radio range,
routing is often required. The aim of this document is to specify
the routing requirements for networks comprising such constrained
devices in a home-control and automation environment.
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 a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5286.
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RFC 5826 Home Automation Routing Requirements in LLNs April 2010
Copyright Notice
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RFC 5826 Home Automation Routing Requirements in LLNs April 2010
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
1.2. Requirements Language ......................................6
2. Home Automation Applications ....................................6
2.1. Lighting Application in Action .............................6
2.2. Energy Conservation and Optimizing Energy Consumption ......6
2.3. Moving a Remote Control Around .............................7
2.4. Adding a New Module to the System ..........................7
2.5. Controlling Battery-Operated Window Shades .................8
2.6. Remote Video Surveillance ..................................8
2.7. Healthcare .................................................9
2.7.1. At-Home Health Reporting ...........................10
2.7.2. At-Home Health Monitoring ..........................10
2.8. Alarm Systems .............................................10
3. Unique Routing Requirements of Home Automation Applications ....11
3.1. Constraint-Based Routing ..................................12
3.2. Support of Mobility .......................................12
3.3. Scalability ...............................................13
3.4. Convergence Time ..........................................13
3.5. Manageability .............................................14
3.6. Stability .................................................14
4. Traffic Pattern ................................................14
5. Security Considerations ........................................15
6. Acknowledgments ................................................16
7. References .....................................................16
7.1. Normative References ......................................16
7.2. Informative References ....................................17
1. Introduction
This document presents requirements specific to home control and
automation applications for Routing Over Low power and Lossy (ROLL)
networks. In the near future, many homes will contain high numbers
of wireless devices for a wide set of purposes. Examples include
actuators (relay, light dimmer, heating valve), sensors (wall switch,
water leak, blood pressure), and advanced controllers. Basic home-
control modules such as wall switches and plug-in modules may be
turned into an advanced home automation solution via the use of an
IP-enabled application responding to events generated by wall
switches, motion sensors, light sensors, rain sensors, and so on.
Network nodes may be sensors and actuators at the same time. An
example is a wall switch for replacement in existing homes. The push
buttons may generate events for a controller node or for activating
other actuator nodes. At the same time, a built-in relay may act as
actuator for a controller or other remote sensors.
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Because ROLL nodes only cover a limited radio range, routing is often
required. These devices are usually highly constrained in terms of
resources such as battery and memory and operate in unstable
environments. Persons moving around in a house, opening or closing a
door, or starting a microwave oven affect the reception of weak radio
signals. Reflection and absorption may cause a reliable radio link
to turn unreliable for a period of time and then become reusable
again, thus the term "lossy". All traffic in a ROLL network is
carried as IPv6 packets.
The connected home area is very much consumer oriented. The
implication on network nodes is that devices are very cost sensitive,
which leads to resource-constrained environments having slow CPUs and
small memory footprints. At the same time, nodes have to be
physically small, which puts a limit to the physical size of the
battery, and thus, the battery capacity. As a result, it is common
for battery-operated, sensor-style nodes to shut down radio and CPU
resources for most of the time. The radio tends to use the same
power for listening as for transmitting.
Although this document focuses its text on radio-based wireless
networks, home-automation networks may also operate using a variety
of links, such as IEEE 802.15.4, Bluetooth, Low-Power WiFi, wired or
other low-power PLC (Power-Line Communication) links. Many such low-
power link technologies share similar characteristics with low-power
wireless and this document should be regarded as applying equally to
all such links.
Section 2 describes a few typical use cases for home automation
applications. Section 3 discusses the routing requirements for
networks comprising such constrained devices in a home network
environment. These requirements may be overlapping requirements
derived from other application-specific routing requirements
presented in [BUILDING-REQS], [RFC5673], and [RFC5548].
A full list of requirements documents may be found in Section 7.
1.1. Terminology
ROLL: Routing Over Low-power and Lossy networks. A ROLL
node may be classified as a sensor, actuator, or
controller.
Actuator: Network node that performs some physical action.
Dimmers and relays are examples of actuators. If
sufficiently powered, actuator nodes may participate
in routing network messages.
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Border router: Infrastructure device that connects a ROLL network to
the Internet or some backbone network.
Channel: Radio frequency band used to carry network packets.
Controller: Network node that controls actuators. Control
decisions may be based on sensor readings, sensor
events, scheduled actions, or incoming commands from
the Internet or other backbone networks. If
sufficiently powered, controller nodes may participate
in routing network messages.
Downstream: Data direction traveling from a Local Area Network
(LAN) to a Personal Area Network (PAN) device.
DR: Demand-Response. The mechanism of users adjusting
their power consumption in response to the actual
pricing of power.
DSM: Demand-Side Management. Process allowing power
utilities to enable and disable loads in consumer
premises. Where DR relies on voluntary action from
users, DSM may be based on enrollment in a formal
program.
LLNs: Low-Power and Lossy Networks.
LAN: Local Area Network.
PAN: Personal Area Network. A geographically limited
wireless network based on, e.g., 802.15.4 or Z-Wave
radio.
PDA Personal Digital Assistant. A small, handheld
computer.
PLC Power-Line Communication.
RAM Random Access Memory.
Sensor: Network node that measures some physical parameter
and/or detects an event. The sensor may generate a
trap message to notify a controller or directly
activate an actuator. If sufficiently powered, sensor
nodes may participate in routing network messages.
Upstream: Data direction traveling from a PAN to a LAN device.
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Refer to the ROLL terminology reference document [ROLL-TERM] for a
full list of terms used in the IETF ROLL WG.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Home Automation Applications
Home automation applications represent a special segment of networked
devices with its unique set of requirements. Historically, such
applications used wired networks or power-line communication (PLC)
but wireless solutions have emerged, allowing existing homes to be
upgraded more easily.
To facilitate the requirements discussion in Section 3, this section
lists a few typical use cases of home automation applications. New
applications are being developed at a high pace and this section does
not mean to be exhaustive. Most home automation applications tend to
be running some kind of command/response protocol. The command may
come from several places.
2.1. Lighting Application in Action
A lamp may be turned on, not only by a wall switch but also by a
movement sensor. The wall-switch module may itself be a push-button
sensor and an actuator at the same time. This will often be the case
when upgrading existing homes as existing wiring is not prepared for
automation.
One event may cause many actuators to be activated at the same time.
Using the direct analogy to an electronic car key, a house owner may
activate the "leaving home" function from an electronic house key,
mobile phone, etc. For the sake of visual impression, all lights
should turn off at the same time; at least, it should appear to
happen at the same time.
2.2. Energy Conservation and Optimizing Energy Consumption
In order to save energy, air conditioning, central heating, window
shades, etc., may be controlled by timers, motion sensors, or
remotely via Internet or cell. Central heating may also be set to a
reduced temperature during nighttime.
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The power grid may experience periods where more wind-generated power
is produced than is needed. Typically this may happen during night
hours.
In periods where electricity demands exceed available supply,
appliances such as air conditioning, climate-control systems, washing
machines, etc., can be turned off to avoid overloading the power
grid.
This is known as Demand-Side Management (DSM). Remote control of
household appliances is well-suited for this application.
The start/stop decision for the appliances can also be regulated by
dynamic power pricing information obtained from the electricity
utility companies. This method, called Demand-Response (DR), works
by motivation of users via pricing, bonus points, etc. For example,
the washing machine and dish washer may just as well work while power
is cheap. The electric car should also charge its batteries on cheap
power.
In order to achieve effective electricity savings, the energy
monitoring application must guarantee that the power consumption of
the ROLL devices is much lower than that of the appliance itself.
Most of these appliances are mains powered and are thus ideal for
providing reliable, always-on routing resources. Battery-powered
nodes, by comparison, are constrained routing resources and may only
provide reliable routing under some circumstances.
2.3. Moving a Remote Control Around
A remote control is a typical example of a mobile device in a home
automation network. An advanced remote control may be used for
dimming the light in the dining room while eating and later on,
turning up the music while doing the dishes in the kitchen. Reaction
must appear to be instant (within a few hundred milliseconds) even
when the remote control has moved to a new location. The remote
control may be communicating to either a central home automation
controller or directly to the lamps and the media center.
2.4. Adding a New Module to the System
Small-size, low-cost modules may have no user interface except for a
single button. Thus, an automated inclusion process is needed for
controllers to find new modules. Inclusion covers the detection of
neighbors and the assignment of a unique node ID. Inclusion should
be completed within a few seconds.
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For ease of use in a consumer application space such as home control,
nodes may be included without having to type in special codes before
inclusion. One way to achieve an acceptable balance between security
and convenience is to block inclusion during normal operation,
explicitly enable inclusion support just before adding a new module,
and disable it again just after adding a new module.
For security considerations, refer to Section 5.
If assignment of unique addresses is performed by a central
controller, it must be possible to route the inclusion request from
the joining node to the central controller before the joining node
has been included in the network.
2.5. Controlling Battery-Operated Window Shades
In consumer premises, window shades are often battery-powered as
there is no access to mains power over the windows. For battery
conservation purposes, such an actuator node is sleeping most of the
time. A controller sending commands to a sleeping actuator node via
ROLL devices will have no problems delivering the packet to the
nearest powered router, but that router may experience a delay until
the next wake-up time before the command can be delivered.
2.6. Remote Video Surveillance
Remote video surveillance is a fairly classic application for home
networking. It provides the ability for the end-user to get a video
stream from a web cam reached via the Internet. The video stream may
be triggered by the end-user after receiving an alarm from a sensor
(movement or smoke detector) or the user simply wants to check the
home status via video.
Note that in the former case, more than likely, there will be a form
of inter-device communication: upon detecting some movement in the
home, the movement sensor may send a request to the light controller
to turn on the lights, to the Web Cam to start a video stream that
would then be directed to the end-user's cell phone or Personal
Digital Assistant (PDA) via the Internet.
In contrast to other applications, e.g., industrial sensors, where
data would mainly be originated by a sensor to a sink and vice versa,
this scenario implicates a direct inter-device communication between
ROLL devices.
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2.7. Healthcare
By adding communication capability to devices, patients and elderly
citizens may be able to do simple measurements at home.
Thanks to online devices, a doctor can keep an eye on the patient's
health and receive warnings if a new trend is discovered by automated
filters.
Fine-grained, daily measurements presented in proper ways may allow
the doctor to establish a more precise diagnosis.
Such applications may be realized as wearable products that
frequently do a measurement and automatically deliver the result to a
data sink locally or over the Internet.
Applications falling in this category are referred to as at-home
health reporting. Whether measurements are done in a fixed interval
or they are manually activated, they leave all processing to the
receiving data sink.
A more active category of applications may send an alarm if some
alarm condition is triggered. This category of applications is
referred to as at-home health monitoring. Measurements are
interpreted in the device and may cause reporting of an event if an
alarm is triggered.
Many implementations may overlap both categories.
Since wireless and battery operated systems may never reach 100%
guaranteed operational time, healthcare and security systems will
need a management layer implementing alarm mechanisms for low
battery, report activity, etc.
For instance, if a blood pressure sensor did not report a new
measurement, say five minutes after the scheduled time, some
responsible person must be notified.
The structure and performance of such a management layer is outside
the scope of the routing requirements listed in this document.
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2.7.1. At-Home Health Reporting
Applications might include:
o Temperature
o Weight
o Blood pressure
o Insulin level
Measurements may be stored for long-term statistics. At the same
time, a critically high blood pressure may cause the generation of an
alarm report. Refer to Section 2.7.2.
To avoid a high number of request messages, nodes may be configured
to autonomously do a measurement and send a report in intervals.
2.7.2. At-Home Health Monitoring
An alarm event may become active, e.g., if the measured blood
pressure exceeds a threshold or if a person falls to the ground.
Alarm conditions must be reported with the highest priority and
timeliness.
Applications might include:
o Temperature
o Weight
o Blood pressure
o Insulin level
o Electrocardiogram (ECG)
o Position tracker
2.8. Alarm Systems
A home security alarm system is comprised of various sensors
(vibration, fire, carbon monoxide, door/window, glass-break,
presence, panic button, etc.).
Some smoke alarms are battery powered and at the same time mounted in
a high place. Battery-powered safety devices should only be used for
routing if no other alternatives exist to avoid draining the battery.
A smoke alarm with a drained battery does not provide a lot of
safety. Also, it may be inconvenient to change the batteries in a
smoke alarm.
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Alarm system applications may have both a synchronous and an
asynchronous behavior; i.e., they may be periodically queried by a
central control application (e.g., for a periodical refreshment of
the network state) or send a message to the control application on
their own initiative.
When a node (or a group of nodes) identifies a risk situation (e.g.,
intrusion, smoke, fire), it sends an alarm message to a central
controller that could autonomously forward it via the Internet or
interact with other network nodes (e.g., try to obtain more detailed
information or ask other nodes close to the alarm event).
Finally, routing via battery-powered nodes may be very slow if the
nodes are sleeping most of the time (they could appear unresponsive
to the alarm detection). To ensure fast message delivery and avoid
battery drain, routing should be avoided via sleeping devices.
3. Unique Routing Requirements of Home Automation Applications
Home automation applications have a number of specific routing
requirements related to the set of home networking applications and
the perceived operation of the system.
The relations of use cases to requirements are outlined in the table
below:
+------------------------------+-----------------------------+
| Use case | Requirement |
+------------------------------+-----------------------------+
|2.1. Lighting Application in |3.2. Support of Mobility |
|Action |3.3. Scalability |
+------------------------------+-----------------------------+
|2.2. Energy Conservation and |3.1. Constraint-Based Routing|
|Optimizing Energy Consumption | |
+------------------------------+-----------------------------+
|2.3. Moving a Remote Control |3.2. Support of Mobility |
|Around |3.4. Convergence Time |
+------------------------------+-----------------------------+
|2.4. Adding a New Module to |3.4. Convergence Time |
|the System |3.5. Manageability |
+------------------------------+-----------------------------+
|2.7. Healthcare |3.1. Constraint-Based Routing|
| |3.2. Support of Mobility |
| |3.4. Convergence Time |
+------------------------------+-----------------------------+
|2.8. Alarm Systems |3.3. Scalability |
| |3.4. Convergence Time |
+------------------------------+-----------------------------+
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3.1. Constraint-Based Routing
For convenience and low-operational costs, power consumption of
consumer products must be kept at a very low level to achieve a long
battery lifetime. One implication of this fact is that Random Access
Memory (RAM) is limited and it may even be powered down, leaving only
a few 100 bytes of RAM alive during the sleep phase.
The use of battery-powered devices reduces installation costs and
does enable installation of devices even where main power lines are
not available. On the other hand, in order to be cost effective and
efficient, the devices have to maximize the sleep phase with a duty
cycle lower than 1%.
Some devices only wake up in response to an event, e.g., a push
button.
Simple battery-powered nodes such as movement sensors on garage doors
and rain sensors may not be able to assist in routing. Depending on
the node type, the node never listens at all, listens rarely, or
makes contact on demand to a pre-configured target node. Attempting
to communicate with such nodes may at best require a long time before
getting a response.
Other battery-powered nodes may have the capability to participate in
routing. The routing protocol SHOULD route via mains-powered nodes
if possible.
The routing protocol MUST support constraint-based routing taking
into account node properties (CPU, memory, level of energy, sleep
intervals, safety/convenience of changing battery).
3.2. Support of Mobility
In a home environment, although the majority of devices are fixed
devices, there is still a variety of mobile devices, for example, a
remote control is likely to move. Another example of mobile devices
is wearable healthcare devices.
While healthcare devices delivering measurement results can tolerate
route discovery times measured in seconds, a remote control appears
unresponsive if using more than 0.5 seconds to, e.g., pause the
music.
On more rare occasions, receiving nodes may also have moved.
Examples include a safety-off switch in a clothes iron, a vacuum
cleaner robot, or the wireless chime of doorbell set.
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Refer to Section 3.4 for routing protocol convergence times.
A non-responsive node can either be caused by 1) a failure in the
node, 2) a failed link on the path to the node, or 3) a moved node.
In the first two cases, the node can be expected to reappear at
roughly the same location in the network, whereas it can return
anywhere in the network in the latter case.
3.3. Scalability
Looking at the number of wall switches, power outlets, sensors of
various natures, video equipment, and so on in a modern house, it
seems quite realistic that hundreds of devices may form a home-
automation network in a fully populated "smart" home, and a large
proportion of those may be low-power devices. Moving towards
professional-building automation, the number of such devices may be
in the order of several thousands.
The routing protocol needs to be able to support a basic home
deployment and so MUST be able to support at least 250 devices in the
network. Furthermore, the protocol SHOULD be extensible to support
more sophisticated and future deployments with a larger number of
devices.
3.4. Convergence Time
A wireless home automation network is subject to various
instabilities due to signal strength variation, moving persons, and
the like.
Measured from the transmission of a packet, the following convergence
time requirements apply.
The routing protocol MUST converge within 0.5 seconds if no nodes
have moved (see Section 3.2 for motivation).
The routing protocol MUST converge within four seconds if nodes have
moved to re-establish connectivity within a time that a human
operator would find tolerable as, for example, when moving a remote
control unit.
In both cases, "converge" means "the originator node has received a
response from the destination node". The above-mentioned convergence
time requirements apply to a home control network environment of up
to 250 nodes with up to four repeating nodes between source and
destination.
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3.5. Manageability
The ability of the home network to support auto-configuration is of
the utmost importance. Indeed, most end-users will not have the
expertise and the skills to perform advanced configuration and
troubleshooting. Thus, the routing protocol designed for home-
automation networks MUST provide a set of features including zero-
configuration of the routing protocol for a new node to be added to
the network. From a routing perspective, zero-configuration means
that a node can obtain an address and join the network on its own,
almost without human intervention.
3.6. Stability
If a node is found to fail often compared to the rest of the network,
this node SHOULD NOT be the first choice for routing of traffic.
4. Traffic Pattern
Depending on the design philosophy of the home network, wall switches
may be configured to directly control individual lamps or
alternatively, all wall switches send control commands to a central
lighting control computer, which again sends out control commands to
relevant devices.
In a distributed system, the traffic tends to be multipoint-to-
multipoint. In a centralized system, it is a mix of multipoint-to-
point and point-to-multipoint.
Wall switches only generate traffic when activated, which typically
happens from one to ten times per hour.
Remote controls have a similar transmit pattern to wall switches but
may be activated more frequently in some deployments.
Temperature/air and pressure/rain sensors send frames when queried by
the user or can be preconfigured to send measurements at fixed
intervals (typically minutes). Motion sensors typically send a frame
when motion is first detected and another frame when an idle period
with no movement has elapsed. The highest transmission frequency
depends on the idle period used in the sensor. Sometimes, a timer
will trigger a frame transmission when an extended period without
status change has elapsed.
All frames sent in the above examples are quite short, typically less
than five bytes of payload. Lost frames and interference from other
transmitters may lead to retransmissions. In all cases,
acknowledgment frames with a size of a few bytes are used.
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5. Security Considerations
As is the case with every network, LLNs are exposed to routing
security threats that need to be addressed. The wireless and
distributed nature of these networks increases the spectrum of
potential routing security threats. This is further amplified by the
resource constraints of the nodes, thereby preventing resource-
intensive routing security approaches from being deployed. A viable
routing security approach SHOULD be sufficiently lightweight that it
may be implemented across all nodes in a LLN. These issues require
special attention during the design process, so as to facilitate a
commercially attractive deployment.
An attacker can snoop, replay, or originate arbitrary messages to a
node in an attempt to manipulate or disable the routing function.
To mitigate this, the LLN MUST be able to authenticate a new node
prior to allowing it to participate in the routing decision process.
The routing protocol MUST support message integrity.
A further example of routing security issues that may arise is the
abnormal behavior of nodes that exhibit an egoistic conduct, such as
not obeying network rules or forwarding no or false packets.
Other important issues may arise in the context of denial-of-service
(DoS) attacks, malicious address space allocations, advertisement of
variable addresses, a wrong neighborhood, etc. The routing
protocol(s) SHOULD support defense against DoS attacks and other
attempts to maliciously or inadvertently cause the mechanisms of the
routing protocol(s) to over-consume the limited resources of LLN
nodes, e.g., by constructing forwarding loops or causing excessive
routing protocol overhead traffic, etc.
The properties of self-configuration and self-organization that are
desirable in a LLN introduce additional routing security
considerations. Mechanisms MUST be in place to deny any node that
attempts to take malicious advantage of self-configuration and self-
organization procedures. Such attacks may attempt, for example, to
cause DoS, drain the energy of power-constrained devices, or to
hijack the routing mechanism. A node MUST authenticate itself to a
trusted node that is already associated with the LLN before the
former can take part in self-configuration or self-organization. A
node that has already authenticated and associated with the LLN MUST
deny, to the maximum extent possible, the allocation of resources to
any unauthenticated peer. The routing protocol(s) MUST deny service
to any node that has not clearly established trust with the HC-LLN.
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In a home-control environment, it is considered unlikely that a
network is constantly being snooped and at the same time, ease of use
is important. As a consequence, the network key MAY be exposed for
short periods during inclusion of new nodes.
Electronic door locks and other critical applications SHOULD apply
end-to-end application security on top of the network transport
security.
If connected to a backbone network, the LLN SHOULD be capable of
limiting the resources utilized by nodes in said backbone network so
as not to be vulnerable to DoS. This should typically be handled by
border routers providing access from a backbone network to resources
in the LLN.
With low-computation power and scarce energy resources, LLNs' nodes
may not be able to resist any attack from high-power malicious nodes
(e.g., laptops and strong radios). However, the amount of damage
generated to the whole network SHOULD be commensurate with the number
of nodes physically compromised. For example, an intruder taking
control over a single node SHOULD NOT be able to completely deny
service to the whole network.
In general, the routing protocol(s) SHOULD support the implementation
of routing security best practices across the LLN. Such an
implementation ought to include defense against, for example,
eavesdropping, replay, message insertion, modification, and man-in-
the-middle attacks.
The choice of the routing security solutions will have an impact on
the routing protocol(s). To this end, routing protocol(s) proposed
in the context of LLNs MUST support authentication and integrity
measures and SHOULD support confidentiality (routing security)
measures.
6. Acknowledgments
J. P. Vasseur, Jonathan Hui, Eunsook "Eunah" Kim, Mischa Dohler, and
Massimo Maggiorotti are gratefully acknowledged for their
contributions to this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Brandt, et al. Informational [Page 16]
RFC 5826 Home Automation Routing Requirements in LLNs April 2010
7.2. Informative References
[BUILDING-REQS] Martocci, J., Ed., De Mil, P., Vermeylen, W., and N.
Riou, "Building Automation Routing Requirements in
Low Power and Lossy Networks", Work in Progress,
January 2010.
[RFC5548] Dohler, M., Ed., Watteyne, T., Ed., Winter, T., Ed.,
and D. Barthel, Ed., "Routing Requirements for Urban
Low-Power and Lossy Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
Phinney, "Industrial Routing Requirements in Low-
Power and Lossy Networks", RFC 5673, October 2009.
[ROLL-TERM] Vasseur, JP. "Terminology in Low power And Lossy
Networks", Work in Progress, October 2009.
Authors' Addresses
Anders Brandt
Sigma Designs, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
EMail: abr@sdesigns.dk
Jakob Buron
Sigma Designs, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
EMail: jbu@sdesigns.dk
Giorgio Porcu
Telecom Italia
Piazza degli Affari, 2
20123 Milan
Italy
EMail: gporcu@gmail.com
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