Networking Working Group K. Pister
Internet-Draft R. Enns
Intended status: Informational Dust Networks
Expires: September 29, 2008 P. Thubert
Cisco Systems, Inc
March 28, 2008
Industrial Routing Requirements in Low Power and Lossy Networks
draft-pister-roll-indus-routing-reqs-00
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Abstract
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. For wireless devices to have a significant
advantage over wired devices in an industrial environment the
wireless network needs to have three qualities: low power, high
reliability, and easy installation and maintenance. The aim of this
document is to analyze the requirements for the routing protocol used
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for low power and lossy networks (L2N) in industrial environments.
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].
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Applications and Traffic Patterns . . . . . . . . . . . . 5
3. Quality of Service (QoS) Routing requirements . . . . . . . . 7
3.1. Configurable Application Requirement . . . . . . . . . . . 9
4. Network Topology . . . . . . . . . . . . . . . . . . . . . . . 9
5. Device-Aware Routing Requirements . . . . . . . . . . . . . . 10
6. Broadcast/Multicast . . . . . . . . . . . . . . . . . . . . . 11
7. Route Establishment Time . . . . . . . . . . . . . . . . . . . 11
8. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Manageability and Ease Of Use . . . . . . . . . . . . . . . . 12
10. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11. Informative Reference . . . . . . . . . . . . . . . . . . . . 13
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
14.1. Normative References . . . . . . . . . . . . . . . . . . . 13
14.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Terminology
Access Point: The access point is an infrastructure device that
connects the low power and lossy network system to a plant's backbone
network.
Actuator: a field device that moves or controls plant equipment.
Channel Hopping: An algorithm by which field devices synchronously
change channels during operation.
Channel: RF frequency band used to transmit a modulated signal
carrying packets.
Closed Loop Control: A process whereby a host application controls an
actuator based on information sensed by field devices.
Downstream: Data direction traveling from the host application to the
field device.
Field Device: physical devices placed in the plant's operating
environment (both RF and environmental). Field devices include
sensors and actuators as well as network routing devices and access
points in the plant. Superframe: A collection of timeslots repeating
at a constant rate.
HART: "Highway Addressable Remote Transducer", a group of
specifications for industrial process and control devices
administered by the HART Foundation (see [HART]). The latest version
for the specifications is HART7 which includes the additions for
WirelessHART.
Host Application: The host application is a process running in the
plant that communicates with field devices to perform tasks on that
may include control, monitoring and data gathering.
ISA: "International Society of Automation". ISA is an ANSI
accredited standards making society. ISA100 is an ISA working group
whose charter includes defining a family of standards for industrial
automation. ISA100.11a is a work group within ISA100 that is working
on a standard for non-critical process and control applications.
L2N: Low Power and Lossy Network Slotted-Link: a data structure that
is associated with a superframe that contains a connection between
field devices that comprises a timeslot assignment, and channel and
usage information.
Open Loop Control: A process whereby a plant technician controls an
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actuator over the network where the decision is influenced by
information sensed by field devices.
Timeslot: A fixed time interval that may be used for the transmission
or reception of a packet between two field devices. A timeslot used
for communications is associated with a slotted-link. Upstream: Data
direction traveling from the field device to the host application.
RF: Radio Frequency Sensor: A field device that measures data and/or
detects an event.
RL2N: Routing in Low power and Lossy Networks
2. Introduction
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.
Wireless field devices enable expansion of networked points by
appreciably reducing cost of installing a device. The cost
reductions come from eliminating cabling costs and simplified
planning. Cabling for a field device can run from $100s/ft to
$1,000s/ft. depending on the safety regulations of the plant.
Cabling also caries an overhead cost associated with planning the
installation, where the cable has to run, and the various
organizations that have to coordinate its deployment. Doing away
with the network and power cables reduces the planning and
administrative overhead of installing a device.
For wireless devices to have a significant advantage over wired
devices in an industrial environment the wireless network needs to
have three qualities: low power, high reliability, and easy
installation and maintenance. The routing protocol used for low
power and lossy networks (L2N) 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 ...).
In factory automation or discrete manufacturing, the products are
individual elements (screws, cars, dolls). While there is some
overlap between products and systems between these two segments, they
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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.
Both application spaces desire battery operated networks of hundreds
of sensors and actuators communicating with wired access points. In
an oil refinery, the total number of devices is likely to exceed one
million, but the devices will be clustered into smaller networks
reporting to existing wired infrastructure.
Existing wired sensor networks in this space typically use
communication protocols with low data rates - from 1,200 baud (wired
HART) into the one to two hundred kbps range for most of the others.
The existing protocols are often master/slave with command/response.
Note that the total low power and lossy network system capacity for
devices using the IEEE802.15.4-2006 2.4 GHz radio is at most 1.6 Mbps
when spatial reuse of channels is not utilized.
2.1. 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 Control
* Class 1: Closed loop regulatory control - Often a critical
function
* Class 2: Closed loop supervisory control - Usually 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)
Critical functions are effect the basic safety or integrity of the
plant. Timely deliveries of messages are more important as class
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numbers decrease.
Industrial customers are interested in deploying wireless networks
for the monitoring classes 4 and 5 and in the non-critical portions
of classes 3 through 1.
Classes 4 and 5 also include equipment monitoring which is strictly
speaking separate from process monitoring. An example of equipment
monitoring is the recording of motor vibrations to detect bearing
wear.
Most low power and lossy network systems in the near future will be
for low frequency data collection. Packets containing samples will
be generated continuously, and 90% of the market is covered by packet
rates of between 1/s and 1/hour, with the average under 1/min. In
industrial process these sensors include temperature, pressure, fluid
flow, tank level, and corrosion. There are some sensors which are
bursty, such as vibration monitors which may generate and transmit
tens of kilo-bytes (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 which
may detect alarm conditions. Time crucial alarm packets are expected
to have lower latency than sensor data, often requiring substantially
more bandwidth.
Some devices will transmit a log file every day, again with typically
tens of Kbytes of data. For these applications there is very little
"downstream" traffic coming from the 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" or "open loop" is
implemented today via communication to a centralized controller, i.e.
sensor data makes its way through the access point to the centralized
controller where it is processed, the operator sees the information
and takes action, and control information is 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 access point. These closed loop controls for non-
critical applications will be implemented on L2Ns. Non-critical
closed loop applications have a latency requirement that can be as
low as 100 ms but many control loops are tolerant of latencies above
1 s.
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In critical control, 10's of milliseconds of latencies are typical.
In many of these systems, if a packet does not arrive within the
specified interval, the system will enter an emergency shutdown
state, often with substantial financial repercussions. For a 1
second control loop in a system with a mean-time between shutdowns
target of 30 years, the latency requirement implies nine 9s of
reliability.
Thus, the routing protocol for L2Ns MUST support multi-topology
routing (e.g especially critical for critical control applications).
The routing protocol MUST provide the ability to color slotted-links
(where the color corresponds to a user defined slotted-link
attribute) and can be used to include/exclude slotted-links from a
logical topology.
For all but the most latency-tolerant applications, route discovery
is likely to be too slow a process to initiate when a route failure
is detected.
The routing protocol MUST support multiple paths (a tree-based
solution is not sufficient).
3. Quality of Service (QoS) Routing requirements
The industrial applications fall into four large service categories:
1. Published data. Data that is generated per periodically and has
a well understood data bandwidth requirement. The end-to-end
latency of this data is not as important as regularity with which
it is presented to the host application.
2. Event data. This category includes alarms and aperiodic data
reports with bursty data bandwidth requirements
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 round trip
latency for some operations can be 200 ms.
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. Bulk transfers
assign resources for a limited period of time to meet the QoS
requirements.
For industrial applications QoS parameters include:
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o Data bandwidth - periodic, burst statistics
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
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 Reliability - the end-to-end data delivery statistic. All
applications have latency and reliability requirements. In
industrial applications, these vary over many orders of magnitude.
Some non-critical monitoring applications may tolerate latencies
of days and reliability of less than 90%. Most monitoring
latencies will be in seconds to minutes, and industrial standard
such as HART7 has set user reliability expectations at 99.9%.
Regulatory requirements are a driver for some industrial
applications. Regulatory monitoring requires high data integrity
because lost data is assumed to be out of compliance and subject
to fines. This can drive reliability requirements to higher then
99.9%.
o QoS contract type - revocation priority. L2Ns 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 - within field devices there are limited
resources need to be 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
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.
In industrial wireless L2Ns a time slotting technology is used. A
time slotted media access protocol synchronizes channel hopping which
is one of the means that is used to make the wireless network
reliable. Timeslots also are employed to reduce the power by
minimizing the active duty cycle for field devices. Communications
between devices are assigned a combination of timeslot and channel
assignment called a slotted-link.
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The routing protocol MUST also support different metric types for
each slotted-link used to compute the path according to some
objective function (e.g. minimize latency, maximize reliability,
...).
Industrial application data flows between field devices are not
necessarily symmetric. The routing protocol MUST be able to set up
routes that are directional.
3.1. Configurable Application Requirement
Time-varying user requirements for latency and bandwidth will require
changes in the provisioning of the underlying L2 protocols. The
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 host. The routing protocol
MUST route on paths which are changed to appropriately provision the
application requirements. The routing protocol MUST support the
ability to recompute paths based on slotted-link characteristics that
may change dynamically.
4. Network Topology
Network topology is very tough to generalize, but networks of 10 to
200 field devices and maximum number of hops from two to twenty
covers the majority of existing applications. It is assumed that the
field devices themselves will provide routing capability for the
network, and in most cases additional repeaters/routers will not be
required.
Timeslot size is about 10 ms and timeslot synchronization
requirements are on the order of +/-1 ms for non-critical process and
control data (some L2 protocols provide/require tighter
synchronization). Wall clock time *accuracy* requirements vary
substantially, but are generally about 100ms. Some applications that
time stamp data require 1 ms accuracy to determine the sequence of
events reported. (Note that data time stamping does not translate to
a latency requirement.)
In low power and lossy network systems using the IEEE802.15.4-2006
2.4 GHz radio the total raw throughput per radio is 250 kbps. 10 ms
timeslots reduces this to 101.6 Kbps for maximum sized packets. This
constrains the typical throughput of a single radio access point to
less 100 kbps. Therefore an access point with one IEEE 802.15.4
radio has a maximum aggregate throughput 100 packets per second and
no more then about 100 Kbps.
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A graph that connects a field device to a host application may have
more than one access point. The routing protocol MUST support
multiple access points and load distribution when aggregate network
throughputs need to exceed 100 kbps. The routing protocol MUST
support multiple access points when access point redundancy is
required.
5. Device-Aware Routing Requirements
Wireless L2N nodes in industrial environments are powered by a
variety of sources. Battery operated devices with lifetime
requirements of at least 5 years are the most common. Battery
operated devices have a cap on their total energy, and typically can
report some 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. Therefore these systems have limits on both
the 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 4-20mA current loop, and can scavenge on the
order of mW 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.
Example 1: solar panel, lead-acid battery sized for two weeks of
rain.
Example 2: vibration scavenger, 1mF 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 have strong preferences on lifetime that is different for
the same device in different locations. A sensor monitoring a non-
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critical parameter in an easily-accessed location may have a lifetime
requirement that is shorter and tolerate more statistical variation
than a mission-critical sensor in a hard to reach place that requires
shutdown of the plant 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.
6. Broadcast/Multicast
Existing industrial host applications do not use broadcast or
multicast addressing to communicate to field devices. Unicast
address support is sufficient. However wireless field devices with
communication controllers and protocol stacks will require control
and configuration such as firmware downloading that may benefit from
broadcast and multicast addressing.
The routing protocol SHOULD support broadcast and multicast
addressing.
7. Route Establishment Time
Network connectivity in real deployments is always time-varying, with
time constants from seconds to months. Optimization is perhaps not
the right word to use, in that network optimization will need to run
continuously, and single-slotted-link failures that cause loss of
connectivity are not likely to be tolerated. Once the network is
formed, it should never need to "optimized" to a new configuration in
response to a lost slotted-link. The routing algorithm SHOULD not
have to re-optimize in response to the loss of a slotted-link. The
routing algorithms SHOULD always be in the process of plesio-
optimizing the system for the changing RF environment. The routing
algorithm MUST re-optimize the path when field devices change due to
insertion, removal or failure.
8. Mobility
Various economic factors have contributed to a reduction of trained
workers in the plant. The industry as a whole appears to be trying
to solve this problem with what is called 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
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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 L2Ns data collection network. This
connection is likely to require higher bandwidth and lower latency
than the normal data collection operation.
The routing protocol SHOULD support the wireless worker with fast
network connection times of a few of seconds, low latency command and
response latencies to host behind the access points and to
applications and to field devices. The routing protocol SHOULD also
support configuring graphs for bulk transfers. The routing protocol
MUST 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.
9. Manageability and Ease Of Use
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 make the installation process not require
any new skills for the plant personnel. The industrial customers do
not even want to require the current level of networking knowledge
needed for do-it-yourself home network installations.
The routing protocol for L2Ns must be easy to deploy and manage. In
a further revision of this document, metrics to measure ease of
deployment for the routing protocol will be detailed.
10. Security
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
which 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.
Industrial wireless device manufactures are specifying security at
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the MAC layer and the Transport layer. A shared "Network Key" is
used to authenticate messages at the MAC layer. At the transport
layer, commands are encrypted with unique randomly-generated end-to-
end Session keys. HART7 and ISA100.11a are examples of security
systems for industrial wireless networks.
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 the security system needs to have limited trust in them.
The routing protocol SHOULD place limited trust in the field devices
deployed in the plant 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.
11. Informative Reference
[HART] "Highway Addressable Remote Transducer", a group of
specifications for industrial process and control devices
administered by the HART Foundation, www.hartcomm.org.
12. IANA Considerations
This document includes no request to IANA.
13. Acknowledgements
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
[I-D.culler-rl2n-routing-reqs]
Vasseur, J. and D. Cullerot, "Routing Requirements for Low
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Power And Lossy Networks",
draft-culler-rl2n-routing-reqs-01 (work in progress),
July 2007.
Authors' Addresses
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: kpister@dustnetworks.com
Rick Enns
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: enns@stanfordalumni.org
Pascal Thubert
Cisco Systems, Inc
Village d'Entreprises Green Side - 400, Avenue de Roumanille
Sophia Antipolis, 06410
Email: pthubert@cisco.com
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