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INFORMATIONAL
Network Working Group K. Pister, Ed.
Request for Comments: 5673 Dust Networks
Category: Informational P. Thubert, Ed.
Cisco Systems
S. Dwars
Shell
T. Phinney
Consultant
October 2009
Industrial Routing Requirements in Low-Power and Lossy Networks
Abstract
The wide deployment of lower-cost wireless devices will significantly
improve the productivity and safety of industrial plants while
increasing the efficiency of plant workers by extending the
information set available about the plant operations. The aim of
this document is to analyze the functional requirements for a routing
protocol used in industrial Low-power and Lossy Networks (LLNs) of
field devices.
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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Applications and Traffic Patterns . . . . . . . . . . . . 5
3.2. Network Topology of Industrial Applications . . . . . . . 9
3.2.1. The Physical Topology . . . . . . . . . . . . . . . . 10
3.2.2. Logical Topologies . . . . . . . . . . . . . . . . . . 12
4. Requirements Related to Traffic Characteristics . . . . . . . 13
4.1. Service Requirements . . . . . . . . . . . . . . . . . . . 14
4.2. Configurable Application Requirement . . . . . . . . . . . 15
4.3. Different Routes for Different Flows . . . . . . . . . . . 15
5. Reliability Requirements . . . . . . . . . . . . . . . . . . . 16
6. Device-Aware Routing Requirements . . . . . . . . . . . . . . 18
7. Broadcast/Multicast Requirements . . . . . . . . . . . . . . . 19
8. Protocol Performance Requirements . . . . . . . . . . . . . . 20
9. Mobility Requirements . . . . . . . . . . . . . . . . . . . . 21
10. Manageability Requirements . . . . . . . . . . . . . . . . . . 21
11. Antagonistic Requirements . . . . . . . . . . . . . . . . . . 22
12. Security Considerations . . . . . . . . . . . . . . . . . . . 23
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
14.1. Normative References . . . . . . . . . . . . . . . . . . . 25
14.2. Informative References . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Information Technology (IT) is already, and increasingly will be
applied to industrial Control Technology (CT) in application areas
where those IT technologies can be constrained sufficiently by
Service Level Agreements (SLA) or other modest changes that they are
able to meet the operational needs of industrial CT. When that
happens, the CT benefits from the large intellectual, experiential,
and training investment that has already occurred in those IT
precursors. One can conclude that future reuse of additional IT
protocols for industrial CT will continue to occur due to the
significant intellectual, experiential, and training economies that
result from that reuse.
Following that logic, many vendors are already extending or replacing
their local fieldbus [IEC61158] technology with Ethernet and IP-based
solutions. Examples of this evolution include Common Industrial
Protocol (CIP) EtherNet/IP, Modbus/TCP, Fieldbus Foundation High
Speed Ethernet (HSE), PROFInet, and Invensys/Foxboro FOXnet. At the
same time, wireless, low-power field devices are being introduced
that facilitate a significant increase in the amount of information
that industrial users can collect and the number of control points
that can be remotely managed.
IPv6 appears as a core technology at the conjunction of both trends,
as illustrated by the current [ISA100.11a] industrial Wireless Sensor
Networking specification, where technologies for layers 1-4 that were
developed for purposes other than industrial CT -- [IEEE802.15.4] PHY
and MAC, IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs) [RFC4919], and UDP -- are adapted to industrial CT use.
But due to the lack of open standards for routing in Low-power and
Lossy Networks (LLNs), even ISA100.11a leaves the routing operation
to proprietary methods.
The aim of this document is to analyze the requirements from the
industrial environment for a routing protocol in Low power and Lossy
Networks (LLNs) based on IPv6 to power the next generation of Control
Technology.
1.1. 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].
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2. Terminology
This document employs terminology defined in the ROLL (Routing Over
Low-power and Lossy networks) terminology document [ROLL-TERM]. This
document also refers to industrial standards:
HART: Highway Addressable Remote Transducer, a group of
specifications for industrial process and control devices
administered by the HART Communication Foundation (see [HART]). The
latest version for the specifications is HART7, which includes the
additions for WirelessHART [IEC62591].
ISA: International Society of Automation, an ANSI-accredited
standards-making society. ISA100 is an ISA committee whose charter
includes defining a family of standards for industrial automation.
[ISA100.11a] is a working group within ISA100 that is working on a
standard for monitoring and non-critical process control
applications.
3. Overview
Wireless, low-power field devices enable industrial users to
significantly increase the amount of information collected and the
number of control points that can be remotely managed. The
deployment of these wireless devices will significantly improve the
productivity and safety of the plants while increasing the efficiency
of the plant workers. IPv6 is perceived as a key technology to
provide the scalability and interoperability that are required in
that space, and it is more and more present in standards and products
under development and early deployments.
Cable is perceived as a more proven, safer technology, and existing,
operational deployments are very stable in time. For these reasons,
it is not expected that wireless will replace wire in any foreseeable
future; the consensus in the industrial space is rather that wireless
will tremendously augment the scope and benefits of automation by
enabling the control of devices that were not connected in the past
for reasons of cost and/or deployment complexities. But for LLNs to
be adopted in the industrial environment, the wireless network needs
to have three qualities: low power, high reliability, and easy
installation and maintenance. The routing protocol used for LLNs is
important to fulfilling these goals.
Industrial automation is segmented into two distinct application
spaces, known as "process" or "process control" and "discrete
manufacturing" or "factory automation". In industrial process
control, the product is typically a fluid (oil, gas, chemicals,
etc.). In factory automation or discrete manufacturing, the products
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are individual elements (screws, cars, dolls). While there is some
overlap of products and systems between these two segments, they are
surprisingly separate communities. The specifications targeting
industrial process control tend to have more tolerance for network
latency than what is needed for factory automation.
Irrespective of this different 'process' and 'discrete' plant nature,
both plant types will have similar needs for automating the
collection of data that used to be collected manually, or was not
collected before. Examples are wireless sensors that report the
state of a fuse, report the state of a luminary, HVAC status, report
vibration levels on pumps, report man-down, and so on.
Other novel application arenas that equally apply to both 'process'
and 'discrete' involve mobile sensors that roam in and out of plants,
such as active sensor tags on containers or vehicles.
Some if not all of these applications will need to be served by the
same low-power and lossy wireless network technology. This may mean
several disconnected, autonomous LLNs connecting to multiple hosts,
but sharing the same ether. Interconnecting such networks, if only
to supervise channel and priority allocations, or to fully
synchronize, or to share path capacity within a set of physical
network components may be desired, or may not be desired for
practical reasons, such as e.g., cyber security concerns in relation
to plant safety and integrity.
All application spaces desire battery-operated networks of hundreds
of sensors and actuators communicating with LLN access points. In an
oil refinery, the total number of devices might exceed one million,
but the devices will be clustered into smaller networks that in most
cases interconnect and report to an existing plant network
infrastructure.
Existing wired sensor networks in this space typically use
communication protocols with low data rates, from 1200 baud (e.g.,
wired HART) to the 100-200 kbps range for most of the others. The
existing protocols are often master/slave with command/response.
3.1. Applications and Traffic Patterns
The industrial market classifies process applications into three
broad categories and six classes.
o Safety
* Class 0: Emergency action - Always a critical function
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o Control
* Class 1: Closed-loop regulatory control - Often a critical
function
* Class 2: Closed-loop supervisory control - Usually a non-
critical function
* Class 3: Open-loop control - Operator takes action and controls
the actuator (human in the loop)
o Monitoring
* Class 4: Alerting - Short-term operational effect (for example,
event-based maintenance)
* Class 5: Logging and downloading / uploading - No immediate
operational consequence (e.g., history collection, sequence-of-
events, preventive maintenance)
Safety-critical functions effect the basic safety integrity of the
plant. These normally dormant functions kick in only when process
control systems, or their operators, have failed. By design and by
regular interval inspection, they have a well-understood probability
of failure on demand in the range of typically once per 10-1000
years.
In-time deliveries of messages become more relevant as the class
number decreases.
Note that for a control application, the jitter is just as important
as latency and has a potential of destabilizing control algorithms.
Industrial users are interested in deploying wireless networks for
the monitoring classes 4 and 5, and in the non-critical portions of
classes 2 through 3.
Classes 4 and 5 also include asset monitoring and tracking, which
include equipment monitoring and are essentially separate from
process monitoring. An example of equipment monitoring is the
recording of motor vibrations to detect bearing wear. However,
similar sensors detecting excessive vibration levels could be used as
safeguarding loops that immediately initiate a trip, and thus end up
being class 0.
In the near future, most LLN systems in industrial automation
environments will be for low-frequency data collection. Packets
containing samples will be generated continuously, and 90% of the
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market is covered by packet rates of between 1/second and 1/hour,
with the average under 1/minute. In industrial process, these
sensors include temperature, pressure, fluid flow, tank level, and
corrosion. Some sensors are bursty, such as vibration monitors that
may generate and transmit tens of kilobytes (hundreds to thousands of
packets) of time-series data at reporting rates of minutes to days.
Almost all of these sensors will have built-in microprocessors that
may detect alarm conditions. Time-critical alarm packets are
expected to be granted a lower latency than periodic sensor data
streams.
Some devices will transmit a log file every day, again with typically
tens of kilobytes of data. For these applications, there is very
little "downstream" traffic coming from the LLN access point and
traveling to particular sensors. During diagnostics, however, a
technician may be investigating a fault from a control room and
expect to have "low" latency (human tolerable) in a command/response
mode.
Low-rate control, often with a "human in the loop" (also referred to
as "open loop"), is implemented via communication to a control room
because that's where the human in the loop will be. The sensor data
makes its way through the LLN access point to the centralized
controller where it is processed, the operator sees the information
and takes action, and the control information is then sent out to the
actuator node in the network.
In the future, it is envisioned that some open-loop processes will be
automated (closed loop) and packets will flow over local loops and
not involve the LLN access point. These closed-loop controls for
non-critical applications will be implemented on LLNs. Non-critical
closed-loop applications have a latency requirement that can be as
low as 100 milliseconds but many control loops are tolerant of
latencies above 1 second.
More likely though is that loops will be closed in the field
entirely, and in such a case, having wireless links within the
control loop does not usually present actual value. Most control
loops have sensors and actuators within such proximity that a wire
between them remains the most sensible option from an economic point
of view. This 'control in the field' architecture is already common
practice with wired fieldbusses. An 'upstream' wireless link would
only be used to influence the in-field controller settings and to
occasionally capture diagnostics. Even though the link back to a
control room might be wireless, this architecture reduces the tight
latency and availability requirements for the wireless links.
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Closing loops in the field:
o does not prevent the same loop from being closed through a remote
multivariable controller during some modes of operation, while
being closed directly in the field during other modes of operation
(e.g., fallback, or when timing is more critical)
o does not imply that the loop will be closed with a wired
connection, or that the wired connection is more energy efficient
even when it exists as an alternate to the wireless connection.
A realistic future scenario is for a field device with a battery or
ultra-capacitor power storage to have both wireless and unpowered
wired communications capability (e.g., galvanically isolated RS-485),
where the wireless communication is more flexible and, for local loop
operation, more energy efficient. The wired communication capability
serves as a backup interconnect among the loop elements, but without
a wired connection back to the operations center blockhouse. In
other words, the loop elements are interconnected through wiring to a
nearby junction box, but the 2 km home-run link from the junction box
to the control center does not exist.
When wireless communication conditions are good, devices use wireless
for loop interconnect, and either one wireless device reports alarms
and other status to the control center for all elements of the loop,
or each element reports independently. When wireless communications
are sporadic, the loop interconnect uses the self-powered
galvanically isolated RS-485 link and one of the devices with good
wireless communications to the control center serves as a router for
those devices that are unable to contact the control center directly.
The above approach is particularly attractive for large storage tanks
in tank farms, where devices may not all have good wireless
visibility of the control center, and where a home-run cable from the
tank to the control center is undesirable due to the electro-
potential differences between the tank location and the distant
control center that arise during lightning storms.
In fast control, tens of milliseconds of latency is typical. In many
of these systems, if a packet does not arrive within the specified
interval, the system enters an emergency shutdown state, often with
substantial financial repercussions. For a one-second control loop
in a system with a target of 30 years for the mean time between
shutdowns, the latency requirement implies nine 9s of reliability
(aka 99.9999999% reliability). Given such exposure, given the
intrinsic vulnerability of wireless link availability, and given the
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emergence of control in the field architectures, most users tend not
to aim for fast closed-loop control with wireless links within that
fast loop.
3.2. Network Topology of Industrial Applications
Although network topology is difficult to generalize, the majority of
existing applications can be met by networks of 10 to 200 field
devices and a maximum number of hops of 20. It is assumed that the
field devices themselves will provide routing capability for the
network, and additional repeaters/routers will not be required in
most cases.
For the vast majority of industrial applications, the traffic is
mostly composed of real-time publish/subscribe sensor data also
referred to as buffered, from the field devices over an LLN towards
one or more sinks. Increasingly over time, these sinks will be a
part of a backbone, but today they are often fragmented and isolated.
The wireless sensor network (WSN) is an LLN of field devices for
which two logical roles are defined, the field routers and the non-
routing devices. It is acceptable and even probable that the
repartition of the roles across the field devices changes over time
to balance the cost of the forwarding operation amongst the nodes.
In order to scale a control network in terms of density, one possible
architecture is to deploy a backbone as a canopy that aggregates
multiple smaller LLNs. The backbone is a high-speed infrastructure
network that may interconnect multiple WSNs through backbone routers.
Infrastructure devices can be connected to the backbone. A gateway/
manager that interconnects the backbone to the plant network of the
corporate network can be viewed as collapsing the backbone and the
infrastructure devices into a single device that operates all the
required logical roles. The backbone is likely to become an option
in the industrial network.
Typically, such backbones interconnect to the 'legacy' wired plant
infrastructure, which is known as the plant network or Process
Control Domain (PCD). These plant automation networks are segregated
domain-wise from the office network or office domain (OD), which in
itself is typically segregated from the Internet.
Sinks for LLN sensor data reside on the plant network (the PCD), the
business network (the OD), and on the Internet. Applications close
to existing plant automation, such as wired process control and
monitoring systems running on fieldbusses, that require high
availability and low latencies, and that are managed by 'Control and
Automation' departments typically reside on the PCD. Other
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applications such as automated corrosion monitoring, cathodic
protection voltage verification, or machine condition (vibration)
monitoring where one sample per week is considered over-sampling,
would more likely deliver their sensor readings in the OD. Such
applications are 'owned' by, e.g., maintenance departments.
Yet other applications like third-party-maintained luminaries, or
vendor-managed inventory systems, where a supplier of chemicals needs
access to tank level readings at his customer's site, will be best
served with direct Internet connectivity all the way to its sensor at
his customer's site. Temporary 'babysitting sensors' deployed for
just a few days, say during startup or troubleshooting or for ad hoc
measurement campaigns for research and development purposes, are
other examples where Internet would be the domain where wireless
sensor data would land, and other domains such as the OD and PCD
should preferably be circumvented if quick deployment without
potentially impacting plant safety integrity is required.
This multiple-domain multiple-application connectivity creates a
significant challenge. Many different applications will all share
the same medium, the ether, within the fence, preferably sharing the
same frequency bands, and preferably sharing the same protocols,
preferably synchronized to optimize coexistence challenges, yet
logically segregated to avoid creation of intolerable shortcuts
between existing wired domains.
Given this challenge, LLNs are best to be treated as all sitting on
yet another segregated domain, segregated from all other wired
domains where conventional security is organized by perimeter.
Moving away from the traditional perimeter-security mindset means
moving towards stronger end-device identity authentication, so that
LLN access points can split the various wireless data streams and
interconnect back to the appropriate domain (pending the gateways'
establishment of the message originators' identity and trust).
Similar considerations are to be given to how multiple applications
may or may not be allowed to share routing devices and their
potentially redundant bandwidth within the network. Challenges here
are to balance available capacity, required latencies, expected
priorities, and (last but not least) available (battery) energy
within the routing devices.
3.2.1. The Physical Topology
There is no specific physical topology for an industrial process
control network.
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One extreme example is a multi-square-kilometer refinery where
isolated tanks, some of them with power but most with no backbone
connectivity, compose a farm that spans over of the surface of the
plant. A few hundred field devices are deployed to ensure the global
coverage using a wireless self-forming self-healing mesh network that
might be 5 to 10 hops across. Local feedback loops and mobile
workers tend to be only 1 or 2 hops. The backbone is in the refinery
proper, many hops away. Even there, powered infrastructure is also
typically several hops away. In that case, hopping to/from the
powered infrastructure may often be more costly than the direct
route.
In the opposite extreme case, the backbone network spans all the
nodes and most nodes are in direct sight of one or more backbone
routers. Most communication between field devices and infrastructure
devices, as well as field device to field device, occurs across the
backbone. From afar, this model resembles the WiFi ESS (Extended
Service Set). But from a layer-3 (L3) perspective, the issues are
the default (backbone) router selection and the routing inside the
backbone, whereas the radio hop towards the field device is in fact a
simple local delivery.
---------+----------------------------
| Plant Network
|
+-----+
| | Gateway M : Mobile device
| | o : Field device
+-----+
|
| Backbone
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
| | router | | router | | router
+-----+ +-----+ +-----+
o o o o o o o o o o o o o
o o o o o o o o o o o o o o o o o o
o o o o o o o o o o o M o o o o o
o o M o o o o o o o o o o o o o
o o o o o o o o o
o o o o o
LLN
Figure 1: Backbone-Based Physical Topology
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An intermediate case is illustrated in Figure 1 with a backbone that
spans the Wireless Sensor Network in such a fashion that any WSN node
is only a few wireless hops away from the nearest backbone router.
WSN nodes are expected to organize into self-forming, self-healing,
self-optimizing logical topologies that enable leveraging the
backbone when it is most efficient to do so.
It must be noted that the routing function is expected to be so
simple that any field device could assume the role of a router,
depending on the self-discovery of the topology and the power status
of the neighbors. On the other hand, only devices equipped with the
appropriate hardware and software combination could assume the role
of an endpoint for a given purpose, such as sensor or actuator.
3.2.2. Logical Topologies
Most of the traffic over the LLN is publish/subscribe of sensor data
from the field device towards a sink that can be a backbone router, a
gateway, or a controller/manager. The destination of the sensor data
is an infrastructure device that sits on the backbone and is
reachable via one or more backbone routers.
For security, reliability, availability, or serviceability reasons,
it is often required that the logical topologies are not physically
congruent over the radio network; that is, they form logical
partitions of the LLN. For instance, a routing topology that is set
up for control should be isolated from a topology that reports the
temperature and the status of the vents, if that second topology has
lesser constraints for the security policy. This isolation might be
implemented as Virtual LANs and Virtual Routing Tables in shared
nodes in the backbone, but correspond effectively to physical nodes
in the wireless network.
Since publishing the data is the raison d'etre for most of the
sensors, in some cases it makes sense to build proactively a set of
routes between the sensors and one or more backbone routers and
maintain those routes at all time. Also, because of the lossy nature
of the network, the routing in place should attempt to propose
multiple paths in the form of Directed Acyclic Graphs oriented
towards the destination.
In contrast with the general requirement of maintaining default
routes towards the sinks, the need for field device to field device
(FD-to-FD) connectivity is very specific and rare, though the traffic
associated might be of foremost importance. FD-to-FD routes are
often the most critical, optimized, and well-maintained routes. A
class 0 safeguarding loop requires guaranteed delivery and extremely
tight response times. Both the respect of criteria in the route
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computation and the quality of the maintenance of the route are
critical for the field devices' operation. Typically, a control loop
will be using a dedicated direct wire that has very different
capabilities, cost, and constraints than the wireless medium, with
the need to use a wireless path as a backup route only in case of
loss of the wired path.
Considering that each FD-to-FD route computation has specific
constraints in terms of latency and availability, it can be expected
that the shortest path possible will often be selected and that this
path will be routed inside the LLN as opposed to via the backbone.
It can also be noted that the lifetimes of the routes might range
from minutes for a mobile worker to tens of years for a command and
control closed loop. Finally, time-varying user requirements for
latency and bandwidth will change the constraints on the routes,
which might either trigger a constrained route recomputation, a
reprovisioning of the underlying L2 protocols, or both in that order.
For instance, a wireless worker may initiate a bulk transfer to
configure or diagnose a field device. A level sensor device may need
to perform a calibration and send a bulk file to a plant.
4. Requirements Related to Traffic Characteristics
[ISA100.11a] selected IPv6 as its network layer for a number of
reasons, including the huge address space and the large potential
size of a subnet, which can range up to 10K nodes in a plant
deployment. In the ISA100 model, industrial applications fall into
four large service categories:
1. Periodic data (aka buffered). Data that is generated
periodically and has a well understood data bandwidth
requirement, both deterministic and predictable. Timely delivery
of such data is often the core function of a wireless sensor
network and permanent resources are assigned to ensure that the
required bandwidth stays available. Buffered data usually
exhibits a short time to live, and the newer reading obsoletes
the previous. In some cases, alarms are low-priority information
that gets repeated over and over. The end-to-end latency of this
data is not as important as the regularity with which the data is
presented to the plant application.
2. Event data. This category includes alarms and aperiodic data
reports with bursty data bandwidth requirements. In certain
cases, alarms are critical and require a priority service from
the network.
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3. Client/Server. Many industrial applications are based on a
client/server model and implement a command response protocol.
The data bandwidth required is often bursty. The acceptable
round-trip latency for some legacy systems was based on the time
to send tens of bytes over a 1200 baud link. Hundreds of
milliseconds is typical. This type of request is statistically
multiplexed over the LLN and cost-based, fair-share, best-effort
service is usually expected.
4. Bulk transfer. Bulk transfers involve the transmission of blocks
of data in multiple packets where temporary resources are
assigned to meet a transaction time constraint. Transient
resources are assigned for a limited time (related to file size
and data rate) to meet the bulk transfers service requirements.
4.1. Service Requirements
The following service parameters can affect routing decisions in a
resource-constrained network:
o Data bandwidth - the bandwidth might be allocated permanently or
for a period of time to a specific flow that usually exhibits
well-defined properties of burstiness and throughput. Some
bandwidth will also be statistically shared between flows in a
best-effort fashion.
o Latency - the time taken for the data to transit the network from
the source to the destination. This may be expressed in terms of
a deadline for delivery. Most monitoring latencies will be in
seconds to minutes.
o Transmission phase - process applications can be synchronized to
wall clock time and require coordinated transmissions. A common
coordination frequency is 4 Hz (250 ms).
o Service contract type - revocation priority. LLNs have limited
network resources that can vary with time. This means the system
can become fully subscribed or even over-subscribed. System
policies determine how resources are allocated when resources are
over-subscribed. The choices are blocking and graceful
degradation.
o Transmission priority - the means by which limited resources
within field devices are allocated across multiple services. For
transmissions, a device has to select which packet in its queue
will be sent at the next transmission opportunity. Packet
priority is used as one criterion for selecting the next packet.
For reception, a device has to decide how to store a received
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packet. The field devices are memory-constrained and receive
buffers may become full. Packet priority is used to select which
packets are stored or discarded.
The routing protocol MUST also support different metric types for
each link used to compute the path according to some objective
function (e.g., minimize latency) depending on the nature of the
traffic.
For these reasons, the ROLL routing infrastructure is REQUIRED to
compute and update constrained routes on demand, and it can be
expected that this model will become more prevalent for FD-to-FD
connectivity as well as for some FD-to-infrastructure-device
connectivity over time.
Industrial application data flows between field devices are not
necessarily symmetric. In particular, asymmetrical cost and
unidirectional routes are common for published data and alerts, which
represent the most part of the sensor traffic. The routing protocol
MUST be able to compute a set of unidirectional routes with
potentially different costs that are composed of one or more non-
congruent paths.
As multiple paths are set up and a variety of flows traverse the
network towards a same destination (for instance, a node acting as a
sink for the LLN), the use of an additional marking/tagging mechanism
based on upper-layer information will be REQUIRED for intermediate
routers to discriminate the flows and perform the appropriate routing
decision using only the content of the IPv6 packet (e.g., use of
DSCP, Flow Label).
4.2. Configurable Application Requirement
Time-varying user requirements for latency and bandwidth may require
changes in the provisioning of the underlying L2 protocols. A
technician may initiate a query/response session or bulk transfer to
diagnose or configure a field device. A level sensor device may need
to perform a calibration and send a bulk file to a plant. The
routing protocol MUST support the ability to recompute paths based on
network-layer abstractions of the underlying link attributes/metrics
that may change dynamically.
4.3. Different Routes for Different Flows
Because different services categories have different service
requirements, it is often desirable to have different routes for
different data flows between the same two endpoints. For example,
alarm or periodic data from A to Z may require path diversity with
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specific latency and reliability. A file transfer between A and Z
may not need path diversity. The routing algorithm MUST be able to
generate different routes with different characteristics (e.g.,
optimized according to different costs, etc.).
Dynamic or configured states of links and nodes influence the
capability of a given path to fulfill operational requirements such
as stability, battery cost, or latency. Constraints such as battery
lifetime derive from the application itself, and because industrial
applications data flows are typically well-defined and well-
controlled, it is usually possible to estimate the battery
consumption of a router for a given topology.
The routing protocol MUST support the ability to (re)compute paths
based on network-layer abstractions of upper-layer constraints to
maintain the level of operation within required parameters. Such
information MAY be advertised by the routing protocol as metrics that
enable routing algorithms to establish appropriate paths that fit the
upper-layer constraints.
The handling of an IPv6 packet by the network layer operates on the
standard properties and the settings of the IPv6 packet header
fields. These fields include the 3-tuple of the Flow Label and the
Source and Destination Address that can be used to identify a flow
and the Traffic Class octet that can be used to influence the Per Hop
Behavior in intermediate routers.
An application MAY choose how to set those fields for each packet or
for streams of packets, and the routing protocol specification SHOULD
state how different field settings will be handled to perform
different routing decisions.
5. Reliability Requirements
LLN reliability constitutes several unrelated aspects:
1) Availability of source-to-destination connectivity when the
application needs it, expressed in number of successes divided by
number of attempts.
2) Availability of source-to-destination connectivity when the
application might need it, expressed in number of potential
failures / available bandwidth,
3) Ability, expressed in number of successes divided by number of
attempts to get data delivered from source to destination within
a capped time,
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4) How well a network (serving many applications) achieves end-to-
end delivery of packets within a bounded latency,
5) Trustworthiness of data that is delivered to the sinks,
6) and others depending on the specific case.
This makes quantifying reliability the equivalent of plotting it on a
three (or more) dimensional graph. Different applications have
different requirements, and expressing reliability as a one
dimensional parameter, like 'reliability on my wireless network is
99.9%' often creates more confusion than clarity.
The impact of not receiving sensor data due to sporadic network
outages can be devastating if this happens unnoticed. However, if
destinations that expect periodic sensor data or alarm status updates
fail to get them, then automatically these systems can take
appropriate actions that prevent dangerous situations. Pending the
wireless application, appropriate action ranges from initiating a
shutdown within 100 ms, to using a last known good value for as much
as N successive samples, to sending out an operator into the plant to
collect monthly data in the conventional way, i.e., some portable
sensor, or paper and a clipboard.
The impact of receiving corrupted data, and not being able to detect
that received data is corrupt, is often more dangerous. Data
corruption can either come from random bit errors due to white noise,
or from occasional bursty interference sources like thunderstorms or
leaky microwave ovens, but also from conscious attacks by
adversaries.
Another critical aspect for the routing is the capability to ensure
maximum disruption time and route maintenance. The maximum
disruption time is the time it takes at most for a specific path to
be restored when broken. Route maintenance ensures that a path is
monitored cannot stay disrupted for more than the maximum disruption
time. Maintenance should also ensure that a path continues to
provide the service for which it was established, for instance, in
terms of bandwidth, jitter, and latency.
In industrial applications, availability is usually defined with
respect to end-to-end delivery of packets within a bounded latency.
Availability requirements vary over many orders of magnitude. Some
non-critical monitoring applications may tolerate an availability of
less than 90% with hours of latency. Most industrial standards, such
as HART7 [IEC62591], have set user availability expectations at
99.9%. Regulatory requirements are a driver for some industrial
applications. Regulatory monitoring requires high data integrity
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because lost data is assumed to be out of compliance and subject to
fines. This can drive up either availability or trustworthiness
requirements.
Because LLN link stability is often low, path diversity is critical.
Hop-by-hop link diversity is used to improve latency-bounded
reliability by sending data over diverse paths.
Because data from field devices are aggregated and funneled at the
LLN access point before they are routed to plant applications, LLN
access point redundancy is an important factor in overall
availability. A route that connects a field device to a plant
application may have multiple paths that go through more than one LLN
access point. The routing protocol MUST be able to compute paths of
not-necessarily-equal cost toward a given destination so as to enable
load-balancing across a variety of paths. The availability of each
path in a multipath route can change over time. Hence, it is
important to measure the availability on a per-path basis and select
a path (or paths) according to the availability requirements.
6. Device-Aware Routing Requirements
Wireless LLN nodes in industrial environments are powered by a
variety of sources. Battery-operated devices with lifetime
requirements of at least five years are the most common. Battery
operated devices have a cap on their total energy, and typically can
report an estimate of remaining energy, and typically do not have
constraints on the short-term average power consumption. Energy-
scavenging devices are more complex. These systems contain both a
power-scavenging device (such as solar, vibration, or temperature
difference) and an energy storage device, such as a rechargeable
battery or a capacitor. These systems, therefore, have limits on
both long-term average power consumption (which cannot exceed the
average scavenged power over the same interval) as well as the short-
term limits imposed by the energy storage requirements. For solar-
powered systems, the energy storage system is generally designed to
provide days of power in the absence of sunlight. Many industrial
sensors run off of a 4-20 mA current loop, and can scavenge on the
order of milliwatts from that source. Vibration monitoring systems
are a natural choice for vibration scavenging, which typically only
provides tens or hundreds of microwatts. Due to industrial
temperature ranges and desired lifetimes, the choices of energy
storage devices can be limited, and the resulting stored energy is
often comparable to the energy cost of sending or receiving a packet
rather than the energy of operating the node for several days. And
of course, some nodes will be line-powered.
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Example 1: solar panel, lead-acid battery sized for two weeks of
rain.
Example 2: vibration scavenger, 1 mF tantalum capacitor.
Field devices have limited resources. Low-power, low-cost devices
have limited memory for storing route information. Typical field
devices will have a finite number of routes they can support for
their embedded sensor/actuator application and for forwarding other
devices packets in a mesh network slotted-link.
Users may strongly prefer that the same device have different
lifetime requirements in different locations. A sensor monitoring a
non-critical parameter in an easily accessed location may have a
lifetime requirement that is shorter and may tolerate more
statistical variation than a mission-critical sensor in a hard-to-
reach place that requires a plant shutdown in order to replace.
The routing algorithm MUST support node-constrained routing (e.g.,
taking into account the existing energy state as a node constraint).
Node constraints include power and memory, as well as constraints
placed on the device by the user, such as battery life.
7. Broadcast/Multicast Requirements
Some existing industrial plant applications do not use broadcast or
multicast addressing to communicate to field devices. Unicast
address support is sufficient for them.
In some other industrial process automation environments, multicast
over IP is used to deliver to multiple nodes that may be functionally
similar or not. Example usages are:
1) Delivery of alerts to multiple similar servers in an automation
control room. Alerts are multicast to a group address based on
the part of the automation process where the alerts arose (e.g.,
the multicast address "all-nodes-interested-in-alerts-for-
process-unit-X"). This is always a restricted-scope multicast,
not a broadcast.
2) Delivery of common packets to multiple routers over a backbone,
where the packets result in each receiving router initiating
multicast (sometimes as a full broadcast) within the LLN. For
instance, this can be a byproduct of having potentially
physically separated backbone routers that can inject messages
into different portions of the same larger LLN.
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3) Publication of measurement data to more than one subscriber.
This feature is useful in some peer-to-peer control applications.
For example, level position may be useful to a controller that
operates the flow valve and also to the overfill alarm indicator.
Both controller and alarm indicator would receive the same
publication sent as a multicast by the level gauge.
All of these uses require an 1:N security mechanism as well; they
aren't of any use if the end-to-end security is only point-to-point.
It is quite possible that first-generation wireless automation field
networks can be adequately useful without either of these
capabilities, but in the near future, wireless field devices with
communication controllers and protocol stacks will require control
and configuration, such as firmware downloading, that may benefit
from broadcast or multicast addressing.
The routing protocol SHOULD support multicast addressing.
8. Protocol Performance Requirements
The routing protocol MUST converge after the addition of a new device
within several minutes, and SHOULD converge within tens of seconds
such that a device is able to establish connectivity to any other
point in the network or determine that there is a connectivity issue.
Any routing algorithm used to determine how to route packets in the
network, MUST be capable of routing packets to and from a newly added
device within several minutes of its addition, and SHOULD be able to
perform this function within tens of seconds.
The routing protocol MUST distribute sufficient information about
link failures to enable traffic to be routed such that all service
requirements (especially latency) continue to be met. This places a
requirement on the speed of distribution and convergence of this
information as well as the responsiveness of any routing algorithms
used to determine how to route packets. This requirement only
applies at normal link failure rates (see Section 5) and MAY degrade
during failure storms.
Any algorithm that computes routes for packets in the network MUST be
able to perform route computations in advance of needing to use the
route. Since such algorithms are required to react to link failures,
link usage information, and other dynamic link properties as the
information is distributed by the routing protocol, the algorithms
SHOULD recompute route based on the receipt of new information.
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9. Mobility Requirements
Various economic factors have contributed to a reduction of trained
workers in the industrial plant. A very common problem is that of
the "wireless worker". Carrying a PDA or something similar, this
worker will be able to accomplish more work in less time than the
older, better-trained workers that he or she replaces. Whether the
premise is valid, the use case is commonly presented: the worker will
be wirelessly connected to the plant IT system to download
documentation, instructions, etc., and will need to be able to
connect "directly" to the sensors and control points in or near the
equipment on which he or she is working. It is possible that this
"direct" connection could come via the normal LLNs data collection
network. This connection is likely to require higher bandwidth and
lower latency than the normal data collection operation.
PDAs are typically used as the user interfaces for plant historians,
asset management systems, and the like. It is undecided if these
PDAs will use the LLN directly to talk to field sensors, or if they
will rather use other wireless connectivity that proxies back into
the field or to anywhere else.
The routing protocol SHOULD support the wireless worker with fast
network connection times of a few of seconds, and low command and
response latencies to the plant behind the LLN access points, to
applications, and to field devices. The routing protocol SHOULD also
support the bandwidth allocation for bulk transfers between the field
device and the handheld device of the wireless worker. The routing
protocol SHOULD support walking speeds for maintaining network
connectivity as the handheld device changes position in the wireless
network.
Some field devices will be mobile. These devices may be located on
moving parts such as rotating components, or they may be located on
vehicles such as cranes or fork lifts. The routing protocol SHOULD
support vehicular speeds of up to 35 kmph.
10. Manageability Requirements
The process and control industry is manpower constrained. The aging
demographics of plant personnel are causing a looming manpower
problem for industry across many markets. The goal for the
industrial networks is to have the installation process not require
any new skills for the plant personnel. The person would install the
wireless sensor or wireless actuator the same way the wired sensor or
wired actuator is installed, except the step to connect wire is
eliminated.
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Most users in fact demand even much further simplified provisioning
methods, a plug and play operation that would be fully transparent to
the user. This requires availability of open and untrusted side
channels for new joiners, and it requires strong and automated
authentication so that networks can automatically accept or reject
new joiners. Ideally, for a user, adding new routing devices should
be as easy as dragging and dropping an icon from a pool of
authenticated new joiners into a pool for the wired domain that this
new sensor should connect to. Under the hood, invisible to the user,
auditable security mechanisms should take care of new device
authentication, and secret join key distribution. These more
sophisticated 'over the air' secure provisioning methods should
eliminate the use of traditional configuration tools for setting up
devices prior to being ready to securely join an LLN access point.
The routing protocol SHOULD be fully configurable over the air as
part of the joining process of a new routing device.
There will be many new applications where even without any human
intervention at the plant, devices that have never been on site
before, should be allowed, based on their credentials and
cryptographic capabilities, to connect anyway. Examples are third-
party road tankers, rail cargo containers with overfill protection
sensors, or consumer cars that need to be refueled with hydrogen by
robots at future fueling stations.
The routing protocol for LLNs is expected to be easy to deploy and
manage. Because the number of field devices in a network is large,
provisioning the devices manually may not make sense. The proper
operation of the routing protocol MAY require that the node be
commissioned with information about itself, like identity, security
tokens, radio standards and frequencies, etc.
The routing protocol SHOULD NOT require to preprovision information
about the environment where the node will be deployed. The routing
protocol MUST enable the full discovery and setup of the environment
(available links, selected peers, reachable network). The protocol
MUST enable the distribution of its own configuration to be performed
by some external mechanism from a centralized management controller.
11. Antagonistic Requirements
This document contains a number of strongly required constraints on
the ROLL routing protocol. Some of those strong requirements might
appear antagonistic and, as such, impossible to fulfill at the same
time.
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For instance, the strong requirement of power economy applies on
general routing but is variant since it is reasonable to spend more
energy on ensuring the availability of a short emergency closed-loop
path than it is to maintain an alert path that is used for regular
updates on the operating status of the device. In the same fashion,
the strong requirement on easy provisioning does not match easily the
strong security requirements that can be needed to implement a
factory policy. Then again, a non-default non-trivial setup can be
acceptable as long as the default configuration enables a device to
join with some degree of security.
Convergence time and network size are also antagonistic. The values
expressed in Section 8 ("Protocol Performance Requirements") apply to
an average network with tens of devices. The use of a backbone can
maintain that level of performance and still enable to grow the
network to thousands of node. In any case, it is acceptable to grow
reasonably the convergence time with the network size.
12. Security Considerations
Given that wireless sensor networks in industrial automation operate
in systems that have substantial financial and human safety
implications, security is of considerable concern. Levels of
security violation that are tolerated as a "cost of doing business"
in the banking industry are not acceptable when in some cases
literally thousands of lives may be at risk.
Security is easily confused with guarantee for availability. When
discussing wireless security, it's important to distinguish clearly
between the risks of temporarily losing connectivity, say due to a
thunderstorm, and the risks associated with knowledgeable adversaries
attacking a wireless system. The conscious attacks need to be split
between 1) attacks on the actual application served by the wireless
devices and 2) attacks that exploit the presence of a wireless access
point that may provide connectivity onto legacy wired plant networks,
so these are attacks that have little to do with the wireless devices
in the LLNs. In the second type of attack, access points that might
be wireless backdoors that allow an attacker outside the fence to
access typically non-secured process control and/or office networks
are typically the ones that do create exposures where lives are at
risk. This implies that the LLN access point on its own must possess
functionality that guarantees domain segregation, and thus prohibits
many types of traffic further upstream.
The current generation of industrial wireless device manufacturers is
specifying security at the MAC (Media Access Control) layer and the
transport layer. A shared key is used to authenticate messages at
the MAC layer. At the transport layer, commands are encrypted with
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statistically unique randomly generated end-to-end session keys.
HART7 [IEC62591] and ISA100.11a are examples of security systems for
industrial wireless networks.
Although such symmetric key encryption and authentication mechanisms
at MAC and transport layers may protect reasonably well during the
lifecycle, the initial network boot (provisioning) step in many cases
requires more sophisticated steps to securely land the initial secret
keys in field devices. Also, it is vital that during these steps,
the ease of deployment and the freedom of mixing and matching
products from different suppliers does not complicate life for those
that deploy and commission. Given the average skill levels in the
field and the serious resource constraints in the market, investing a
little bit more in sensor-node hardware and software so that new
devices automatically can be deemed trustworthy, and thus
automatically join the domains that they should join, with just one
drag-and-drop action for those in charge of deploying, will yield
faster adoption and proliferation of the LLN technology.
Industrial plants may not maintain the same level of physical
security for field devices that is associated with traditional
network sites such as locked IT centers. In industrial plants, it
must be assumed that the field devices have marginal physical
security and might be compromised. The routing protocol SHOULD limit
the risk incurred by one node being compromised, for instance by
proposing a non-congruent path for a given route and balancing the
traffic across the network.
The routing protocol SHOULD compartmentalize the trust placed in
field devices so that a compromised field device does not destroy the
security of the whole network. The routing MUST be configured and
managed using secure messages and protocols that prevent outsider
attacks and limit insider attacks from field devices installed in
insecure locations in the plant.
The wireless environment typically forces the abandonment of
classical 'by perimeter' thinking when trying to secure network
domains. Wireless nodes in LLN networks should thus be regarded as
little islands with trusted kernels, situated in an ocean of
untrusted connectivity, an ocean that might be full of pirate ships.
Consequently, confidence in node identity and ability to challenge
authenticity of source node credentials gets more relevant.
Cryptographic boundaries inside devices that clearly demark the
border between trusted and untrusted areas need to be drawn.
Protection against compromise of the cryptographic boundaries inside
the hardware of devices is outside of the scope of this document.
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Note that because nodes are usually expected to be capable of
routing, the end-node security requirements are usually a superset of
the router requirements, in order to prevent a end node from being
used to inject forged information into the network that could alter
the plant operations.
Additional details of security across all application scenarios are
provided in the ROLL security framework [ROLL-SEC-FMWK].
Implications of these security requirements for the routing protocol
itself are a topic for future work.
13. Acknowledgements
Many thanks to Rick Enns, Alexander Chernoguzov, and Chol Su Kang for
their contributions.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[HART] HART (Highway Addressable Remote Transducer)
Communication Foundation, "HART Communication
Protocol and Foundation - Home Page",
<http://www.hartcomm.org>.
[IEC61158] IEC, "Industrial communication networks - Fieldbus
specifications", IEC 61158 series.
[IEC62591] IEC, "Industrial communication networks - Wireless
communication network and communication profiles -
WirelessHART", IEC 62591.
[IEEE802.15.4] IEEE, "Telecommunications and information exchange
between systems -- Local and metropolitan area
networks -- Specific requirements Part 15.4:
Wireless Medium Access Control (MAC) and Physical
Layer (PHY) Specifications for Low-Rate Wireless
Personal Area Networks (WPANs)", IEEE 802.15.4,
2006.
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[ISA100.11a] ISA, "Wireless systems for industrial automation:
Process control and related applications",
ISA 100.11a, May 2008, <http://www.isa.org/
Community/SP100WirelessSystemsforAutomation>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher,
"IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs): Overview, Assumptions, Problem
Statement, and Goals", RFC 4919, August 2007.
[ROLL-SEC-FMWK] Tsao, T., Alexander, R., Dohler, M., Daza, V., and
A. Lozano, "A Security Framework for Routing over
Low Power and Lossy Networks", Work in Progress,
September 2009.
[ROLL-TERM] Vasseur, JP., "Terminology in Low power And Lossy
Networks", Work in Progress, October 2009.
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Authors' Addresses
Kris Pister (editor)
Dust Networks
30695 Huntwood Ave.
Hayward, CA 94544
USA
EMail: kpister@dustnetworks.com
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 497 23 26 34
EMail: pthubert@cisco.com
Sicco Dwars
Shell Global Solutions International B.V.
Sir Winston Churchilllaan 299
Rijswijk 2288 DC
Netherlands
Phone: +31 70 447 2660
EMail: sicco.dwars@shell.com
Tom Phinney
Consultant
5012 W. Torrey Pines Circle
Glendale, AZ 85308-3221
USA
Phone: +1 602 938 3163
EMail: tom.phinney@cox.net
Pister, et al. Informational [Page 27]
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