Networking Working Group M. Dohler, Ed.
Internet-Draft CTTC
Intended status: Informational T. Watteyne, Ed.
Expires: December 3, 2008 France Telecom R&D
T. Winter, Ed.
Eka Systems
June 30, 2008
Urban WSNs Routing Requirements in Low Power and Lossy Networks
draft-ietf-roll-urban-routing-reqs-01
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Abstract
The application-specific routing requirements for Urban Low Power and
Lossy Networks (U-LLNs) are presented in this document. In the near
future, sensing and actuating nodes will be placed outdoors in urban
environments so as to improve the people's living conditions as well
as to monitor compliance with increasingly strict environmental laws.
These field nodes are expected to measure and report a wide gamut of
data, such as required in smart metering, waste disposal,
meteorological, pollution and allergy reporting applications. The
majority of these nodes is expected to communicate wirelessly which -
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given the limited radio range and the large number of nodes -
requires the use of suitable routing protocols. The design of such
protocols will be mainly impacted by the limited resources of the
nodes (memory, processing power, battery, etc.) and the
particularities of the outdoors urban application scenarios. As
such, for a wireless ROLL solution to be useful, the protocol(s)
ought to be energy-efficient, scalable, and autonomous. This
documents aims to specify a set of requirements reflecting these and
further U-LLNs tailored characteristics.
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].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Urban Low Power Lossy Networks . . . . . . . . . . 5
3.1. Canonical Network Elements . . . . . . . . . . . . . . . . 5
3.1.1. Access Points . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Repeaters . . . . . . . . . . . . . . . . . . . . . . 6
3.1.3. Actuators . . . . . . . . . . . . . . . . . . . . . . 6
3.1.4. Sensors . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Resource Constraints . . . . . . . . . . . . . . . . . . . 7
3.4. Link Reliability . . . . . . . . . . . . . . . . . . . . . 8
4. Urban LLN Application Scenarios . . . . . . . . . . . . . . . 9
4.1. Deployment of Nodes . . . . . . . . . . . . . . . . . . . 9
4.2. Association and Disassociation/Disappearance of Nodes . . 10
4.3. Regular Measurement Reporting . . . . . . . . . . . . . . 11
4.4. Queried Measurement Reporting . . . . . . . . . . . . . . 11
4.5. Alert Reporting . . . . . . . . . . . . . . . . . . . . . 12
5. Traffic Pattern . . . . . . . . . . . . . . . . . . . . . . . 12
6. Requirements of Urban LLN Applications . . . . . . . . . . . . 14
6.1. Scalability . . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Parameter Constrained Routing . . . . . . . . . . . . . . 14
6.3. Support of Autonomous and Alien Configuration . . . . . . 15
6.4. Support of Highly Directed Information Flows . . . . . . . 15
6.5. Support of Heterogeneous Field Devices . . . . . . . . . . 15
6.6. Support of Multicast, Anycast, and Implementation of
Groupcast . . . . . . . . . . . . . . . . . . . . . . . . 16
6.7. Network Dynamicity . . . . . . . . . . . . . . . . . . . . 16
6.8. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
This document details application-specific routing requirements for
Urban Low Power and Lossy Networks (U-LLNs). U-LLN use cases and
associated routing protocol requirements will be described.
Section 2 defines terminology useful in describing U-LLNs.
Section 3 provides an overview of U-LLN applications.
Section 4 describes a few typical use cases for U-LLN applications
exemplifying deployment problems and related routing issues.
Section 5 describes traffic flows that will be typical for U-LLN
applications.
Section 6 discusses the routing requirements for networks comprising
such constrained devices in a U-LLN environment. These requirements
may be overlapping requirements derived from other application-
specific requirements documents or as listed in
[I-D.culler-rl2n-routing-reqs].
Section 7 provides an overview of security considerations of U-LLN
implementations.
2. Terminology
Access Point: The access point is an infrastructure device that
connects the low power and lossy network system to a backbone
network.
Actuator: a field device that moves or controls equipment
AMI: Advanced Metering Infrastructure, part of Smart Grid.
Encompasses smart-metering applications.
DA: Distribution Automation, part of Smart Grid. Encompasses
technologies for maintenance and management of electrical
distribution systems.
Field Device: physical device placed in the urban operating
environment. Field devices include sensors, actuators and
repeaters.
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LLN: Low power and Lossy Network
ROLL: Routing over Low power and Lossy networks
Smart Grid: a broad class of applications to network and automate
utility infrastructure.
Schedule: An agreed execution, wake-up, transmission, reception,
etc., time-table between two or more field devices.
U-LLN: Urban LLN
3. Overview of Urban Low Power Lossy Networks
3.1. Canonical Network Elements
A U-LLN is understood to be a network composed of four key elements,
i.e.
1. access points,
2. repeaters,
3. actuators, and
4. sensors
which communicate wirelessly.
3.1.1. Access Points
The access point can be used as:
1. router to a wider infrastructure (e.g. Internet),
2. data sink (e.g. data collection & processing from sensors), and
3. data source (e.g. instructions towards actuators)
There can be several access points connected to the same U-LLN;
however, the number of access points is well below the amount of
sensing nodes. The access points are mainly static, i.e. fixed to a
random or pre- planned location, but can be nomadic, i.e. in form of
a walking supervisor. Access points may but generally do not suffer
from any form of (long-term) resource constraint, except that they
need to be small and sufficiently cheap.
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3.1.2. Repeaters
Repeaters generally act as relays with the aim to close coverage and
routing gaps; examples of their use are:
1. prolong the U-LLN's lifetime,
2. balance nodes' energy depletion,
3. build advanced sensing infrastructures.
There can be several repeaters supporting the same U-LLN; however,
the number of repeaters is well below the amount of sensing nodes.
The repeaters are mainly static, i.e. fixed to a random or pre-
planned location. Repeaters may but generally do not suffer from any
form of (long-term) resource constraint, except that they need to be
small and sufficiently cheap. Repeaters differ from access points in
that they do not act as a data sink/source. They differ from
actuator and sensing nodes in that they neither control nor sense.
3.1.3. Actuators
Actuator nodes control urban devices upon being instructed by
signaling arriving from or being forwarded by the access point(s);
examples are street or traffic lights. The amount of actuator points
is well below the number of sensing nodes. Some sensing nodes may
include an actuator component, e.g. an electric meter node with
integrated support for remote service disconnect. Actuators are
capable to forward data. Actuators may generally be mobile but are
likely to be static in the majority of near-future roll-outs.
Similar to the access points, actuator nodes do not suffer from any
long-term resource constraints.
3.1.4. Sensors
Sensing nodes measure a wide gamut of physical data, including but
not limited to:
1. municipal consumption data, such as smart-metering of gas, water,
electricity, waste, etc;
2. meteorological data, such as temperature, pressure, humidity, sun
index, strength and direction of wind, etc;
3. pollution data, such as polluting gases (SO2, NOx, CO, Ozone),
heavy metals (e.g. Mercury), pH, radioactivity, etc;
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4. ambient data, such as allergic elements (pollen, dust),
electromagnetic pollution, noise levels, etc.
A prominent example is a Smart Grid application which consists of a
city-wide network of smart meters and distribution monitoring
sensors. Smart meters in an urban Smart Grid application will
include electric, gas, and/or water meters typically administered by
one or multiple utility companies. These meters will be capable of
advanced sensing functionalities such as measuring quality of
service, providing granular interval data, or automating the
detection of alarm conditions. In addition they may be capable of
advanced interactive functionalities such as remote service
disconnect or remote demand reset. More advanced scenarios include
demand response systems for managing peak load, and distribution
automation systems to monitor the infrastructure which delivers
energy throughout the urban environment. Sensor nodes capable of
providing this type of functionality may sometimes be referred to as
Advanced Metering Infrastructure (AMI).
3.2. Topology
Whilst millions of sensing nodes may very well be deployed in an
urban area, they are likely to be associated to more than one network
where these networks may or may not communicate between one other.
The number of sensing nodes deployed in the urban environment in
support of some applications is expected to be in the order of 10^2-
10^7; this is still very large and unprecedented in current roll-
outs. The network MUST be capable of supporting the organization of
a large number of sensing nodes into regions containing on the order
of 10^2 to 10^4 sensing nodes each.
Deployment of nodes is likely to happen in batches, e.g. boxes of
hundreds to thousands of nodes arrive and are deployed. The location
of the nodes is random within given topological constraints, e.g.
placement along a road, river, or at individual residences.
3.3. Resource Constraints
The nodes are highly resource constrained, i.e. cheap hardware, low
memory and no infinite energy source. Different node powering
mechanisms are available, such as:
1. non-rechargeable battery;
2. rechargeable battery with regular recharging (e.g. sunlight);
3. rechargeable battery with irregular recharging (e.g.
opportunistic energy scavenging);
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4. capacitive/inductive energy provision (e.g. active RFID);
5. always on (e.g. powered electricity meter).
In the case of a battery powered sensing node, the battery life-time
is usually in the order of 10-15 years, rendering network lifetime
maximization with battery-powered nodes beyond this lifespan useless.
The physical and electromagnetic distances between the four key
elements, i.e. sensors, actuators, repeaters and access points, can
generally be very large, i.e. from several hundreds of meters to one
kilometer. Not every field node is likely to reach the access point
in a single hop, thereby requiring suitable routing protocols which
manage the information flow in an energy-efficient manner. Sensor
nodes are capable of forwarding data.
3.4. Link Reliability
The links between the network elements are volatile due to the
following set of non-exclusive effects:
1. packet errors due to wireless channel effects;
2. packet errors due to medium access control;
3. packet errors due to interference from other systems;
4. link unavailability due to network dynamicity; etc.
The wireless channel causes the received power to drop below a given
threshold in a random fashion, thereby causing detection errors in
the receiving node. The underlying effects are path loss, shadowing
and fading.
Since the wireless medium is broadcast in nature, nodes in their
communication radios require suitable medium access control protocols
which are capable of resolving any arising contention. Some
available protocols may cause packets of neighbouring nodes to
collide and hence cause a link outage.
Furthermore, the outdoors deployment of U-LLNs also has implications
for the interference temperature and hence link reliability and range
if ISM bands are to be used. For instance, if the 2.4GHz ISM band is
used to facilitate communication between U-LLN nodes, then heavily
loaded WLAN hot-spots become a detrimental performance factor
jeopardizing the functioning of the U-LLN.
Finally, nodes appearing and disappearing causes dynamics in the
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network which can yield link outages and changes of topologies.
4. Urban LLN Application Scenarios
Urban applications represent a special segment of LLNs with its
unique set of requirements. To facilitate the requirements
discussion in Section 4, this section lists a few typical but not
exhaustive deployment problems and usage cases of U-LLN.
4.1. Deployment of Nodes
Contrary to other LLN applications, deployment of nodes is likely to
happen in batches out of a box. Typically, hundreds to thousands of
nodes are being shipped by the manufacturer with pre-programmed
functionalities which are then rolled-out by a service provider or
subcontracted entities. Prior or after roll-out, the network needs
to be ramped-up. This initialization phase may include, among
others, allocation of addresses, (possibly hierarchical) roles in the
network, synchronization, determination of schedules, etc.
If initialization is performed prior to roll-out, all nodes are
likely to be in one another's 1-hop radio neighborhood. Pre-
programmed MAC and routing protocols may hence fail to function
properly, thereby wasting a large amount of energy. Whilst the major
burden will be on resolving MAC conflicts, any proposed U-LLN routing
protocol needs to cater for such a case. For instance,
0-configuration and network address allocation needs to be properly
supported, etc.
After roll-out, nodes will have a finite set of one-hop neighbors,
likely of low cardinality (in the order of 5- 10). However, some
nodes may be deployed in areas where there are hundreds of
neighboring devices. In the resulting topology there may be regions
where many (redundant) paths are possible through the network. Other
regions may be dependant on critical links to achieve connectivity
with the rest of the network. Any proposed LLN routing protocol
ought to support the autonomous organization and configuration of the
network at lowest possible energy cost [Lu2007], where autonomy is
understood to be the ability of the network to operate without
external influence. For example, nodes in urban sensor nodes SHOULD
be able to:
o Dynamically adapt to ever-changing conditions of communication
(possible degradation of QoS, variable nature of the traffic (real
time vs. non real time, sensed data vs. alerts, node mobility, a
combination thereof, etc.),
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o Dynamically provision the service-specific (if not traffic-
specific) resources that will comply with the QoS and security
requirements of the service,
o Dynamically compute, select and possibly optimize the (multiple)
path(s) that will be used by the participating devices to forward
the traffic towards the actuators and/or the access point
according to the service-specific and traffic-specific QoS,
traffic engineering and security policies that will have to be
enforced at the scale of a routing domain (that is, a set of
networking devices administered by a globally unique entity), or a
region of such domain (e.g. a metropolitan area composed of
clusters of sensors).
The result of such organization SHOULD be that each node or set of
nodes is uniquely addressable so as to facilitate the set up of
schedules, etc.
The U-LLN routing protocol(s) MUST accommodate both unicast and
multicast forwarding schemes. The U-LLN routing protocol(s) SHOULD
support anycast forwarding schemes. Unless exceptionally needed,
broadcast forwarding schemes are not advised in urban sensor
networking environments.
4.2. Association and Disassociation/Disappearance of Nodes
After the initialization phase and possibly some operational time,
new nodes may be injected into the network as well as existing nodes
removed from the network. The former might be because a removed node
is replaced or denser readings/actuations are needed or routing
protocols report connectivity problems. The latter might be because
a node's battery is depleted, the node is removed for maintenance,
the node is stolen or accidentally destroyed, etc. Differentiation
SHOULD be made between node disappearance, where the node disappears
without prior notification, and user or node-initiated disassociation
("phased-out"), where the node has enough time to inform the network
about its removal.
The protocol(s) hence SHOULD support the pinpointing of problematic
routing areas as well as an organization of the network which
facilitates reconfiguration in the case of association and
disassociation/disappearance of nodes at lowest possible energy and
delay. The latter may include the change of hierarchies, routing
paths, packet forwarding schedules, etc. Furthermore, to inform the
access point(s) of the node's arrival and association with the
network as well as freshly associated nodes about packet forwarding
schedules, roles, etc, appropriate (link state) updating mechanisms
SHOULD be supported.
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4.3. Regular Measurement Reporting
The majority of sensing nodes will be configured to report their
readings on a regular basis. The frequency of data sensing and
reporting may be different but is generally expected to be fairly
low, i.e. in the range of once per hour, per day, etc. The ratio
between data sensing and reporting frequencies will determine the
memory and data aggregation capabilities of the nodes. Latency of an
end-to-end delivery and acknowledgements of a successful data
delivery may not be vital as sensing outages can be observed at the
access point(s) - when, for instance, there is no reading arriving
from a given sensor or cluster of sensors within a day. In this
case, a query can be launched to check upon the state and
availability of a sensing node or sensing cluster.
The protocol(s) hence MUST support a large number of highly
directional unicast flows from the sensing nodes or sensing clusters
towards the access point or highly directed multicast or anycast
flows from the nodes towards multiple access points.
Route computation and selection may depend on the transmitted
information, the frequency of reporting, the amount of energy
remaining in the nodes, the recharging pattern of energy-scavenged
nodes, etc. For instance, temperature readings could be reported
every hour via one set of battery-powered nodes, whereas air quality
indicators are reported only during daytime via nodes powered by
solar energy. More generally, entire routing areas may be avoided at
e.g. night but heavily used during the day when nodes are scavenging
from sunlight.
4.4. Queried Measurement Reporting
Occasionally, network external data queries can be launched by one or
several access points. For instance, it is desirable to know the
level of pollution at a specific point or along a given road in the
urban environment. The queries' rates of occurrence are not regular
but rather random, where heavy-tail distributions seem appropriate to
model their behavior. Queries do not necessarily need to be reported
back to the same access point from where the query was launched.
Round-trip times, i.e. from the launch of a query from an access
point towards the delivery of the measured data to an access point,
are of importance. However, they are not very stringent where
latencies SHOULD simply be sufficiently smaller than typical
reporting intervals; for instance, in the order of seconds or minute.
To facilitate the query process, U-LLN network devices SHOULD support
unicast and multicast routing capabilities.
The same approach is also applicable for schedule update,
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provisioning of patches and upgrades, etc. In this case, however,
the provision of acknowledgements and the support of unicast,
multicast, and anycast are of importance.
4.5. Alert Reporting
Rarely, the sensing nodes will measure an event which classifies as
alarm where such a classification is typically done locally within
each node by means of a pre-programmed or prior diffused threshold.
Note that on approaching the alert threshold level, nodes may wish to
change their sensing and reporting cycles. An alarm is likely being
registered by a plurality of sensing nodes where the delivery of a
single alert message with its location of origin suffices in most
cases. One example of alert reporting is if the level of toxic gases
rises above a threshold, thereupon the sensing nodes in the vicinity
of this event report the danger. Another example of alert reporting
is when a recycling glass container - equipped with a sensor
measuring its level of occupancy - reports that the container is full
and hence needs to be emptied.
Routing within urban sensor networks SHOULD require the U-LLN nodes
to dynamically compute, select and install different paths towards a
same destination, depending on the nature of the traffic. From this
perspective, such nodes SHOULD inspect the contents of traffic
payload for making routing and forwarding decisions: for example, the
analysis of the traffic payload SHOULD be derived into aggregation
capabilities for the sake of forwarding efficiency.
Routes clearly need to be unicast (towards one access point) or
multicast (towards multiple access points). Delays and latencies are
important; however, again, deliveries within seconds SHOULD suffice
in most of the cases.
5. Traffic Pattern
Unlike traditional ad hoc networks, the information flow in U-LLNs is
highly directional. There are three main flows to be distinguished:
1. sensed information from the sensing nodes towards one or a subset
of the access point(s);
2. query requests from the access point(s) towards the sensing
nodes;
3. control information from the access point(s) towards the
actuators.
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Some of the flows may need the reverse route for delivering
acknowledgements. Finally, in the future, some direct information
flows between field devices without access points may also occur.
Sensed data is likely to be highly correlated in space, time and
observed events; an example of the latter is when temperature
increase and humidity decrease as the day commences. Data may be
sensed and delivered at different rates with both rates being
typically fairly low, i.e. in the range of minutes, hours, days, etc.
Data may be delivered regularly according to a schedule or a regular
query; it may also be delivered irregularly after an externally
triggered query; it may also be triggered after a sudden network-
internal event or alert. Data delivery may trigger acknowledgements
or maintenance traffic in the reverse direction. The network hence
needs to be able to adjust to the varying activity duty cycles, as
well as to periodic and sporadic traffic. Also, sensed data ought to
be secured and locatable.
Some data delivery may have tight latency requirements, for example
in a case such as a live meter reading for customer service in a
smart-metering application, or in a case where a sensor reading
response must arrive within a certain time in order to be useful.
The network SHOULD take into consideration that different application
traffic may require different priorities when traversing the network,
and that some traffic may be more sensitive to latency.
An U-LLN SHOULD support occasional large scale traffic flows from
sensing nodes to access points, such as system-wide alerts. In the
example of an AMI U-LLN this could be in response to events such as a
city wide power outage. In this scenario all powered devices in a
large segment of the network may have lost power and are running off
of a temporary `last gasp' source such as a capacitor or small
battery. A node MUST be able to send its own alerts toward an access
point while continuing to forward traffic on behalf of other devices
who are also experiencing an alert condition. The network MUST be
able to manage this sudden large traffic flow. It may be useful for
the routing layer to collaborate with the application layer to
perform data aggregation, in order to reduce the total volume of a
large traffic flow, and make more efficient use of the limited energy
available.
An U-LLN may also need to support efficient large scale messaging to
groups of actuators. For example, an AMI U-LLN supporting a city-
wide demand response system will need to efficiently broadcast demand
response control information to a large subset of actuators in the
system.
Some scenarios will require internetworking between the U-LLN and
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another network, such as a home network. For example, an AMI
application that implements a demand-response system may need to
forward traffic from a utility, across the U-LLN, into a home
automation network. A typical use case would be to inform a customer
of incentives to reduce demand during peaks, or to automatically
adjust the thermostat of customers who have enrolled in such a demand
management program. Subsequent traffic may be triggered to flow back
through the U-LLN to the utility. The network SHOULD support
internetworking, while giving attention to security implications of
interfacing, for example, a home network with a utility U-LLN.
6. Requirements of Urban LLN Applications
Urban low power and lossy network applications have a number of
specific requirements related to the set of operating conditions, as
exemplified in the previous section.
6.1. Scalability
The large and diverse measurement space of U-LLN nodes - coupled with
the typically large urban areas - will yield extremely large network
sizes. Current urban roll-outs are composed of sometimes more than a
hundred nodes; future roll-outs, however, may easily reach numbers in
the tens of thousands to millions. One of the utmost important LLN
routing protocol design criteria is hence scalability.
The routing protocol(s) MUST be scalable so as to accommodate a very
large and increasing number of nodes without deteriorating to-be-
specified performance parameters below to-be-specified thresholds.
The routing protocols(s) SHOULD support the organization of a large
number of nodes into regions of to-be-specified size.
6.2. Parameter Constrained Routing
Batteries in some nodes may deplete quicker than in others; the
existence of one node for the maintenance of a routing path may not
be as important as of another node; the battery scavenging methods
may recharge the battery at regular or irregular intervals; some
nodes may have a constant power source; some nodes may have a larger
memory and are hence be able to store more neighborhood information;
some nodes may have a stronger CPU and are hence able to perform more
sophisticated data aggregation methods; etc.
To this end, the routing protocol(s) MUST support parameter
constrained routing, where examples of such parameters (CPU, memory
size, battery level, etc.) have been given in the previous paragraph.
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6.3. Support of Autonomous and Alien Configuration
With the large number of nodes, manually configuring and
troubleshooting each node is not efficient. The scale and the large
number of possible topologies that may be encountered in the U-LLN
encourages the development of automated management capabilities that
may (partly) rely upon self-organizing techniques. The network is
expected to self-organize and self-configure according to some prior
defined rules and protocols, as well as to support externally
triggered configurations (for instance through a commissioning tool
which may facilitate the organization of the network at a minimum
energy cost).
To this end, the routing protocol(s) MUST provide a set of features
including 0-configuration at network ramp-up, (network-internal)
self- organization and configuration due to topological changes,
ability to support (network-external) patches and configuration
updates. For the latter, the protocol(s) MUST support multi- and
any-cast addressing. The protocol(s) SHOULD also support the
formation and identification of groups of field devices in the
network.
6.4. Support of Highly Directed Information Flows
The reporting of the data readings by a large amount of spatially
dispersed nodes towards a few access points will lead to highly
directed information flows. For instance, a suitable addressing
scheme can be devised which facilitates the data flow. Also, as one
gets closer to the access point, the traffic concentration increases
which may lead to high load imbalances in node usage.
To this end, the routing protocol(s) SHOULD support and utilize the
fact of highly directed traffic flow to facilitate scalability and
parameter constrained routing.
6.5. Support of Heterogeneous Field Devices
The sheer amount of different field devices will unlikely be provided
by a single manufacturer. A heterogeneous roll-out with nodes using
different physical and medium access control layers is hence likely.
To mandate fully interoperable implementations, the routing
protocol(s) proposed in U-LLN MUST support different devices and
underlying technologies without compromising the operability and
energy efficiency of the network.
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6.6. Support of Multicast, Anycast, and Implementation of Groupcast
Some urban sensing systems require low-level addressing of a group of
nodes in the same subnet, or for a node representative of a group of
nodes, without any prior creation of multicast groups, simply
carrying a list of recipients in the subnet
[I-D.brandt-roll-home-routing-reqs].
Routing protocols activated in urban sensor networks MUST support
unicast (traffic is sent to a single field device), multicast
(traffic is sent to a set of devices that are subscribed to the same
multicast group), and anycast (where multiple field devices are
configured to accept traffic sent on a single IP anycast address)
transmission schemes [RFC4291] [RFC1546]. Routing protocols
activated in urban sensor networks SHOULD accommodate "groupcast"
forwarding schemes, where traffic is sent to a set of devices that
implicitly belong to the same group/cast.
The support of unicast, groupcast, multicast, and anycast also has an
implication on the addressing scheme but is beyond the scope of this
document that focuses on the routing requirements aspects.
Note: with IP multicast, signaling mechanisms are used by a receiver
to join a group and the sender does not know the receivers of the
group. What is required is the ability to address a group of
receivers known by the sender even if the receivers do not need to
know that they have been grouped by the sender (since requesting each
individual node to join a multicast group would be very energy-
consuming).
6.7. Network Dynamicity
Although mobility is assumed to be low in urban LLNs, network
dynamicity due to node association, disassociation and disappearance,
as well as long-term link perturbations is not negligible. This in
turn impacts re-organization and re-configuration convergence as well
as routing protocol convergence.
To this end, local network dynamics SHOULD NOT impact the entire
network to be re-organized or re-reconfigured; however, the network
SHOULD be locally optimized to cater for the encountered changes.
Convergence and route establishment times SHOULD be significantly
lower than the smallest reporting interval.
6.8. Latency
With the exception of alert reporting solutions and to a certain
extent queried reporting, U-LLN are delay tolerant as long as the
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information arrives within a fraction of the smallest reporting
interval, e.g. a few seconds if reporting is done every 4 hours.
To this end, the routing protocol(s) SHOULD support minimum latency
for alert reporting and time-critical data queries. For regular data
reporting, it SHOULD support latencies not exceeding a fraction of
the smallest reporting interval. Due to the different latency
requirements, the routing protocol(s) SHOULD support the ability of
dealing with different latency requirements. The routing protocol(s)
SHOULD also support the ability to route according to different
metrics (one of which could e.g. be latency).
7. Security Considerations
As every network, U-LLNs are exposed to security threats that MUST be
addressed. The wireless and distributed nature of these networks
increases the spectrum of potential security threats. This is
further amplified by the resource constraints of the nodes, thereby
preventing resource intensive security approaches from being
deployed. A viable security approach SHOULD be sufficiently
lightweight that it may be implemented across all nodes in a U-LLN.
These issues require special attention during the design process, so
as to facilitate a commercially attractive deployment.
A secure communication in a wireless network encompasses three main
elements, i.e. confidentiality (encryption of data), integrity
(correctness of data), and authentication (legitimacy of data).
U-LLN networks SHOULD support mechanisms to preserve the
confidentiality of the traffic that they forward. The U-LLN network
SHOULD NOT prevent an application from employing additional
confidentiality mechanisms.
Authentication can e.g. be violated if external sources insert
incorrect data packets; integrity can e.g. be violated if nodes start
to break down and hence commence measuring and relaying data
incorrectly. Nonetheless, some sensor readings as well as the
actuator control signals need to be confidential.
The U-LLN network MUST deny all routing services to any node who has
not been authenticated to the U-LLN and authorized for the use of
routing services.
The U-LLN MUST be protected against attempts to inject false or
modified packets. For example, an attacker SHOULD be prevented from
manipulating or disabling the routing function by compromising
routing update messages. Moreover, it SHOULD NOT be possible to
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coerce the network into routing packets which have been modified in
transit. To this end the routing protocol(s) MUST support message
integrity.
Further example security issues which may arise are the abnormal
behavior of nodes which 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, external attacks aimed at
injecting dummy traffic to drain the network power, etc.
The properties of self-configuration and self-organization which are
desirable in a U-LLN introduce additional security considerations.
Mechanisms MUST be in place to deny any rogue node which attempts to
take advantage of self-configuration and self-organization
procedures. Such attacks may attempt, for example, to cause denial
of service, 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 U-LLN before any
self-configuration or self-organization is allowed to proceed. A
node that has already authenticated and associated with the U-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 which has not clearly established trust with
the U-LLN.
Consideration SHOULD be given to cases where the U-LLN may interface
with other networks such as a home network. The U-LLN SHOULD NOT
interface with any external network which has not established trust.
The U-LLN SHOULD be capable of limiting the resources granted in
support of an external network so as not to be vulnerable to denial
of service.
With low computation power and scarce energy resources, U-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 have total access to, or be
able to completely deny service to the whole network.
In general, the routing protocol(s) SHOULD support the implementation
of security best practices across the U-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 security solutions will have an impact onto routing
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protocol(s). To this end, routing protocol(s) proposed in the
context of U-LLNs MUST support integrity measures and SHOULD support
confidentiality (security) measures.
8. Open Issues
Other items to be addressed in further revisions of this document
include:
o node mobility
9. IANA Considerations
This document makes no request of IANA.
10. Acknowledgements
The in-depth feedback of JP Vasseur, Cisco, and Jonathan Hui, Arch
Rock, is greatly appreciated.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.brandt-roll-home-routing-reqs]
Brandt, A., "Home Automation Routing Requirement in Low
Power and Lossy Networks",
draft-brandt-roll-home-routing-reqs-01 (work in progress),
May 2008.
[I-D.culler-rl2n-routing-reqs]
Vasseur, J. and D. Cullerot, "Routing Requirements for Low
Power And Lossy Networks",
draft-culler-rl2n-routing-reqs-01 (work in progress),
July 2007.
[Lu2007] J.L. Lu, F. Valois, D. Barthel, M. Dohler, "FISCO: A Fully
Integrated Scheme of Self-Configuration and Self-
Organization for WSN", IEEE WCNC 2007, Hong Kong, China,
11-15 March 2007, pp. 3370-3375.
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[RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
Authors' Addresses
Mischa Dohler (editor)
CTTC
Parc Mediterrani de la Tecnologia, Av. Canal Olimpic S/N
08860 Castelldefels, Barcelona
Spain
Email: mischa.dohler@cttc.es
Thomas Watteyne (editor)
France Telecom R&D
28 Chemin du Vieux Chene
38243 Meylan Cedex
France
Email: thomas.watteyne@orange-ftgroup.com
Tim Winter (editor)
Eka Systems
20201 Century Blvd. Suite 250
Germantown, MD 20874
USA
Email: tim.winter@ekasystems.com
Christian Jacquenet
France Telecom R&D
4 rue du Clos Courtel BP 91226
35512 Cesson Sevigne
France
Email: christian.jacquenet@orange-ftgroup.com
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Giyyarpuram Madhusudan
France Telecom R&D
28 Chemin du Vieux Chene
38243 Meylan Cedex
France
Email: giyyarpuram.madhusudan@orange-ftgroup.com
Gabriel Chegaray
France Telecom R&D
28 Chemin du Vieux Chene
38243 Meylan Cedex
France
Email: gabriel.chegaray@orange-ftgroup.com
Dominique Barthel
France Telecom R&D
28 Chemin du Vieux Chene
38243 Meylan Cedex
France
Email: Dominique.Barthel@orange-ftgroup.com
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