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RFC 5548

Routing Requirements for Urban Low-Power and Lossy Networks

Pages: 21
Informational

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Network Working Group                                     M. Dohler, Ed.
Request for Comments: 5548                                          CTTC
Category: Informational                                 T. Watteyne, Ed.
                                                       BSAC, UC Berkeley
                                                          T. Winter, Ed.
                                                             Eka Systems
                                                         D. Barthel, Ed.
                                                      France Telecom R&D
                                                                May 2009


      Routing Requirements for Urban Low-Power and Lossy Networks

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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 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 (for example, the data required by applications that perform smart-metering or that monitor meteorological, pollution, and allergy conditions). The majority of these nodes are expected to communicate wirelessly over a variety of links such as IEEE 802.15.4, low-power IEEE 802.11, or IEEE 802.15.1 (Bluetooth), which 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 outdoor urban application scenarios. As such, for a wireless
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   solution for Routing Over Low-Power and Lossy (ROLL) networks to be
   useful, the protocol(s) ought to be energy-efficient, scalable, and
   autonomous.  This documents aims to specify a set of IPv6 routing
   requirements reflecting these and further U-LLNs' tailored
   characteristics.

Table of Contents

1. Introduction ....................................................3 2. Terminology .....................................................3 2.1. Requirements Language ......................................4 3. Overview of Urban Low-Power and Lossy Networks ..................4 3.1. Canonical Network Elements .................................4 3.1.1. Sensors .............................................4 3.1.2. Actuators ...........................................5 3.1.3. Routers .............................................6 3.2. Topology ...................................................6 3.3. Resource Constraints .......................................7 3.4. Link Reliability ...........................................7 4. Urban LLN Application Scenarios .................................8 4.1. Deployment of Nodes ........................................8 4.2. Association and Disassociation/Disappearance of Nodes ......9 4.3. Regular Measurement Reporting ..............................9 4.4. Queried Measurement Reporting .............................10 4.5. Alert Reporting ...........................................11 5. Traffic Pattern ................................................11 6. Requirements of Urban-LLN Applications .........................13 6.1. Scalability ...............................................13 6.2. Parameter-Constrained Routing .............................13 6.3. Support of Autonomous and Alien Configuration .............14 6.4. Support of Highly Directed Information Flows ..............15 6.5. Support of Multicast and Anycast ..........................15 6.6. Network Dynamicity ........................................16 6.7. Latency ...................................................16 7. Security Considerations ........................................16 8. References .....................................................18 8.1. Normative References ......................................18 8.2. Informative References ....................................18 Appendix A. Acknowledgements .....................................20 Appendix B. Contributors .........................................20
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1. Introduction

This document details application-specific IPv6 routing requirements for Urban Low-Power and Lossy Networks (U-LLNs). Note that this document details the set of IPv6 routing requirements for U-LLNs in strict compliance with the layered IP architecture. 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 overlap with or be derived from other application-specific requirements documents [ROLL-HOME] [ROLL-INDUS] [ROLL-BUILD]. Section 7 provides an overview of routing security considerations of U-LLN implementations.

2. Terminology

The terminology used in this document is consistent with and incorporates that described in "Terminology in Low power And Lossy Networks" [ROLL-TERM]. This terminology is extended in this document as follows: Anycast: Addressing and Routing scheme for forwarding packets to at least one of the "nearest" interfaces from a group, as described in RFC4291 [RFC4291] and RFC1546 [RFC1546]. Autonomous: Refers to the ability of a routing protocol to independently function without requiring any external influence or guidance. Includes self-configuration and self-organization capabilities. DoS: Denial of Service, a class of attack that attempts to cause resource exhaustion to the detriment of a node or network.
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   ISM band:  Industrial, Scientific, and Medical band.  This is a
              region of radio spectrum where low-power, unlicensed
              devices may generally be used, with specific guidance from
              an applicable local radio spectrum authority.

   U-LLN:  Urban Low-Power and Lossy Network.

   WLAN: Wireless Local Area Network.

2.1. 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].

3. Overview of Urban Low-Power and Lossy Networks

3.1. Canonical Network Elements

A U-LLN is understood to be a network composed of three key elements, i.e., 1. sensors, 2. actuators, and 3. routers that communicate wirelessly. The aim of the following sections (3.1.1, 3.1.2, and 3.1.3) is to illustrate the functional nature of a sensor, actuator, and router in this context. That said, it must be understood that these functionalities are not exclusive. A particular device may act as a simple router or may alternatively be a router equipped with a sensing functionality, in which case it will be seen as a "regular" router as far as routing is concerned.

3.1.1. 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, UV index, strength and direction of wind, etc.;
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   3.  pollution data, such as gases (sulfur dioxide, nitrogen oxide,
       carbon monoxide, ozone), heavy metals (e.g., mercury), pH,
       radioactivity, etc.;

   4.  ambient data, such as levels of allergens (pollen, dust),
       electromagnetic pollution, noise, etc.

   Sensor nodes run applications that typically gather the measurement
   data and send it to data collection and processing application(s) on
   other node(s) (often outside the U-LLN).

   Sensor nodes are capable of forwarding data.  Sensor nodes are
   generally not mobile in the majority of near-future roll-outs.  In
   many anticipated roll-outs, sensor nodes may suffer from long-term
   resource constraints.

   A prominent example is a "smart grid" application that 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 the quality of
   electrical service provided to a customer, providing granular
   interval data, or automating the detection of alarm conditions.  In
   addition, they may be capable of advanced interactive
   functionalities, which may invoke an actuator component, 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 that
   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).

3.1.2. Actuators

Actuator nodes are capable of controlling urban devices; examples are street or traffic lights. They run applications that receive instructions from control applications on other nodes (possibly outside the U-LLN). 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 of forwarding data. Actuators are not likely to be mobile in the majority of near-future roll-outs. Actuator nodes may also suffer from long-term resource constraints, e.g., in the case where they are battery powered.
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3.1.3. Routers

Routers generally act to close coverage and routing gaps within the interior of the U-LLN; examples of their use are: 1. prolong the U-LLN's lifetime, 2. balance nodes' energy depletion, and 3. build advanced sensing infrastructures. There can be several routers supporting the same U-LLN; however, the number of routers is well below the amount of sensing nodes. The routers are generally not mobile, i.e., fixed to a random or pre- planned location. Routers may, but generally do not, suffer from any form of (long-term) resource constraint, except that they need to be small and sufficiently cheap. Routers differ from actuator and sensing nodes in that they neither control nor sense. That being said, a sensing node or actuator may also be a router within the U-LLN. Some routers provide access to wider infrastructures, such as the Internet, and are named Low-Power and Lossy Network Border Routers (LBRs) in that context. LBR nodes in particular may also run applications that communicate with sensor and actuator nodes (e.g., collecting and processing data from sensor applications, or sending instructions to actuator applications).

3.2. Topology

Whilst millions of sensing nodes may very well be deployed in an urban area, they are likely to be associated with more than one network. These networks may or may not communicate between one another. 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 to 10^7; this is still very large and unprecedented in current roll-outs. 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.
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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); 4. capacitive/inductive energy provision (e.g., passive Radio Frequency IDentification (RFID)); 5. always on (e.g., powered electricity meter). In the case of a battery-powered sensing node, the battery shelf life is usually in the order of 10 to 15 years, rendering network lifetime maximization with battery-powered nodes beyond this lifespan useless. The physical and electromagnetic distances between the three key elements, i.e., sensors, actuators, and routers, can generally be very large, i.e., from several hundreds of meters to one kilometer. Not every field node is likely to reach the LBR in a single hop, thereby requiring suitable routing protocols that manage the information flow in an energy-efficient manner.

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 MAC (Medium Access Control) (e.g., collision); 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.
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   Since the wireless medium is broadcast in nature, nodes in their
   communication radios require suitable medium access control protocols
   that are capable of resolving any arising contention.  Some available
   protocols may not be able to prevent packets of neighboring nodes
   from colliding, possibly leading to a high Packet Error Rate (PER)
   and causing a link outage.

   Furthermore, the outdoor deployment of U-LLNs also has implications
   for the interference temperature and hence link reliability and range
   if Industrial, Scientific, and Medical (ISM) bands are to be used.
   For instance, if the 2.4 GHz ISM band is used to facilitate
   communication between U-LLN nodes, then heavily loaded Wireless Local
   Area Network (WLAN) hot-spots may become a detrimental performance
   factor, leading to high PER and jeopardizing the functioning of the
   U-LLN.

   Finally, nodes appearing and disappearing causes dynamics in the
   network that 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 6, 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 to 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 one-hop radio neighborhood. Pre- programmed Media Access Control (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, zero-configuration and network address allocation needs to be properly supported, etc.
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   After roll-out, nodes will have a finite set of one-hop neighbors,
   likely of low cardinality (in the order of 5 to 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 dependent on critical links to achieve connectivity
   with the rest of the network.  Any proposed LLN routing protocol
   ought to support the autonomous self-organization and self-
   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.  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.

   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 as part of maintenance, or new nodes are added because more sensors for denser readings/actuations are needed, or because 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. The protocol(s) hence should be able to convey information about malfunctioning nodes that may affect or jeopardize the overall routing efficiency, so that self-organization and self-configuration capabilities of the sensor network might be solicited to facilitate the appropriate reconfiguration. This information may include, e.g., exact or relative geographical position, etc. The reconfiguration may include the change of hierarchies, routing paths, packet forwarding schedules, etc. Furthermore, to inform the LBR(s) of the node's arrival and association with the network as well as freshly associated nodes about packet forwarding schedules, roles, etc., appropriate updating mechanisms should be supported.

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
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   end-to-end delivery and acknowledgements of a successful data
   delivery may not be vital as sensing outages can be observed at data
   collection applications -- 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.

   It is not uncommon to gather data on a few servers located outside of
   the U-LLN.  In such cases, a large number of highly directional
   unicast flows from the sensing nodes or sensing clusters are likely
   to transit through a LBR.  Thus, the protocol(s) should be optimized
   to support a large number of unicast flows from the sensing nodes or
   sensing clusters towards a LBR, or highly directed multicast or
   anycast flows from the nodes towards multiple LBRs.

   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 the daytime via nodes powered by
   solar energy.  More generally, entire routing areas may be avoided
   (e.g., at night) but heavily used during the day when nodes are
   scavenging energy from sunlight.

4.4. Queried Measurement Reporting

Occasionally, network-external data queries can be launched by one or several applications. 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 node from where the query was launched. Round-trip times, i.e., from the launch of a query from a node until the delivery of the measured data to a node, 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 minutes. The routing protocol(s) should consider the selection of paths with appropriate (e.g., latency) metrics to support queried measurement reporting. To facilitate the query process, U-LLN devices should support unicast and multicast routing capabilities. The same approach is also applicable for schedule update, 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.
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4.5. Alert Reporting

Rarely, the sensing nodes will measure an event that classifies as an 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, but not all, 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. Routes clearly need to be unicast (towards one LBR) or multicast (towards multiple LBRs). Delays and latencies are important; however, for a U-LLN deployed in support of a typical application, 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 to applications outside the U-LLN, going through one or a subset of the LBR(s); 2. query requests from applications outside the U-LLN, going through the LBR(s) towards the sensing nodes; 3. control information from applications outside the U-LLN, going through the LBR(s) towards the actuators. Some of the flows may need the reverse route for delivering acknowledgements. Finally, in the future, some direct information flows between field devices without LBRs 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. Schedules may be driven by, for
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   example, a smart-metering application where data is expected to be
   delivered every hour, or an environmental monitoring application
   where a battery-powered node is expected to report its status at a
   specific time once a day.  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 in the selection of a route
   when traversing the network, and that some traffic may be more
   sensitive to latency.

   A U-LLN should support occasional large-scale traffic flows from
   sensing nodes through LBRs (to nodes outside the U-LLN), 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 be 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 LBR while continuing to forward traffic on
   behalf of other devices that are also experiencing an alert
   condition.  The network needs to be able to manage this sudden large
   traffic flow.

   A 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
   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.
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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 sections.

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 one 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 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. The routing protocol(s) MUST be scalable so as to accommodate a very large and increasing number of nodes without deteriorating selected performance parameters below configurable thresholds. The routing protocols(s) SHOULD support the organization of a large number of nodes into regions of configurable 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 energy-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. In other words, the routing protocol MUST be able to advertise node capabilities that will be exclusively used by the routing protocol engine for routing decision. For the sake of example, such a capability could be related to the node capability itself (e.g., remaining power) or some application that could influence routing (e.g., capability to aggregate data).
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   Routing within urban sensor networks SHOULD require the U-LLN nodes
   to dynamically compute, select, and install different paths towards
   the same destination, depending on the nature of the traffic.  Such
   functionality in support of, for example, data aggregation, may imply
   use of some mechanisms to mark/tag the traffic for appropriate
   routing decision using the IPv6 packet format (e.g., use of Diffserv
   Code Point (DSCP), Flow Label) based on an upper-layer marking
   decision.  From this perspective, such nodes MAY use node
   capabilities (e.g., to act as an aggregator) in conjunction with the
   anycast endpoints and packet marking to route the traffic.

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 that 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 zero-configuration at network ramp-up, (network-internal) self-organization and configuration due to topological changes, and the ability to support (network-external) patches and configuration updates. For the latter, the protocol(s) MUST support multicast and anycast addressing. The protocol(s) SHOULD also support the formation and identification of groups of field devices in the network. The routing protocol(s) SHOULD be able to dynamically adapt, e.g., through the application of appropriate routing metrics, to ever- changing conditions of communication (possible degradation of quality of service (QoS), variable nature of the traffic (real-time versus non-real-time, sensed data versus alerts), node mobility, a combination thereof, etc.). The routing protocol(s) SHOULD be able to 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 a LBR according to the service-specific and traffic- specific QoS, traffic engineering, and routing security policies that
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   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).

6.4. Support of Highly Directed Information Flows

As pointed out in Section 4.3, it is not uncommon to gather data on a few servers located outside of the U-LLN. In this case, the reporting of the data readings by a large amount of spatially dispersed nodes towards a few LBRs will lead to highly directed information flows. For instance, a suitable addressing scheme can be devised that facilitates the data flow. Also, as one gets closer to the LBR, 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 large number of highly directed traffic flows to facilitate scalability and parameter-constrained routing. The routing protocol MUST be able to accommodate traffic bursts by dynamically computing and selecting multiple paths towards the same destination.

6.5. Support of Multicast and Anycast

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. The support of unicast, multicast, and anycast also has an implication on the addressing scheme, but it is beyond the scope of this document that focuses on the routing requirements. Some urban sensing systems may 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. Such addressing schemes, where a sender can form an addressable group of receivers, are not currently supported by IPv6, and not further discussed in this specification [ROLL-HOME]. The network SHOULD support internetworking when identical protocols are used, while giving attention to routing security implications of interfacing, for example, a home network with a utility U-LLN. The
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   network may support the ability to interact with another network
   using a different protocol, for example, by supporting route
   redistribution.

6.6. 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 reorganization and reconfiguration convergence as well as routing protocol convergence. To this end, local network dynamics SHOULD NOT impact the entire network to be reorganized or re-reconfigured; however, the network SHOULD be locally optimized to cater for the encountered changes. The routing protocol(s) SHOULD support appropriate mechanisms in order to be informed of the association, disassociation, and disappearance of nodes. The routing protocol(s) SHOULD support appropriate updating mechanisms in order to be informed of changes in connectivity. The routing protocol(s) SHOULD use this information to initiate protocol-specific mechanisms for reorganization and reconfiguration as necessary to maintain overall routing efficiency. Convergence and route establishment times SHOULD be significantly lower than the smallest reporting interval. 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 pending removal.

6.7. Latency

With the exception of alert-reporting solutions and (to a certain extent) queried reporting, U-LLNs are delay tolerant as long as the information arrives within a fraction of the smallest reporting interval, e.g., a few seconds if reporting is done every 4 hours. 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 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
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   approaches from being deployed.  A viable routing 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.

   The U-LLN MUST deny any node that has not been authenticated to the
   U-LLN and authorized to participate to the routing decision process.

   An attacker SHOULD be prevented from manipulating or disabling the
   routing function, for example, by compromising routing control
   messages.  To this end, the routing protocol(s) MUST support message
   integrity.

   Further examples of routing security issues that may arise are 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 U-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 U-LLN before the
   former can take part in self-configuration or self-organization.  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 that 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 that 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 DoS.
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   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 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 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 routing security solutions will have an impact on
   the routing protocol(s).  To this end, routing protocol(s) proposed
   in the context of U-LLNs MUST support authentication and integrity
   measures and SHOULD support confidentiality (routing security)
   measures.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2. Informative References

[Lu2007] Lu, JL., Valois, F., Barthel, D., and M. Dohler, "FISCO: A Fully Integrated Scheme of Self-Configuration and Self-Organization for WSN", 11-15 March 2007, pp. 3370-3375, IEEE WCNC 2007, Hong Kong, China. [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. [ROLL-BUILD] Martocci, J., Ed., De Mil, P., Vermeylen, W., and N. Riou, "Building Automation Routing Requirements in Low Power and Lossy Networks", Work in Progress, February 2009. [ROLL-HOME] Brandt, A. and G. Porcu, "Home Automation Routing Requirements in Low Power and Lossy Networks", Work in Progress, November 2008.
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   [ROLL-INDUS]  Pister, K., Ed., Thubert, P., Ed., Dwars, S., and T.
                 Phinney, "Industrial Routing Requirements in Low Power
                 and Lossy Networks", Work in Progress, April 2009.

   [ROLL-TERM]   Vasseur, J., "Terminology in Low power And Lossy
                 Networks", Work in Progress, October 2008.
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Appendix A. Acknowledgements

The in-depth feedback of JP Vasseur, Jonathan Hui, Iain Calder, and Pasi Eronen is greatly appreciated.

Appendix B. Contributors

Christian Jacquenet France Telecom R&D 4 rue du Clos Courtel BP 91226 35512 Cesson Sevigne France EMail: christian.jacquenet@orange-ftgroup.com 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
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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) Berkeley Sensor & Actuator Center, University of California, Berkeley 497 Cory Hall #1774 Berkeley, CA 94720-1774 USA EMail: watteyne@eecs.berkeley.edu Tim Winter (editor) Eka Systems 20201 Century Blvd. Suite 250 Germantown, MD 20874 USA EMail: wintert@acm.org Dominique Barthel (editor) France Telecom R&D 28 Chemin du Vieux Chene 38243 Meylan Cedex France EMail: Dominique.Barthel@orange-ftgroup.com