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Versions: (draft-pister-roll-indus-routing-reqs) 00 01 02 03 04 05 06 RFC 5673

Networking Working Group                                  K. Pister, Ed.
Internet-Draft                                             Dust Networks
Intended status: Informational                           P. Thubert, Ed.
Expires: July 26, 2009                                     Cisco Systems
                                                                S. Dwars
                                                                   Shell
                                                              T. Phinney
                                                        January 22, 2009


    Industrial Routing Requirements in Low Power and Lossy Networks
                 draft-ietf-roll-indus-routing-reqs-04

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on July 26, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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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 by extending the information set available from
   wired systems.  In an industrial environment, low power, high
   reliability, and easy installation and maintenance are mandatory
   qualities for wireless devices.  The aim of this document is to
   analyze the requirements for the routing protocol used for Low power
   and Lossy Networks (LLN) 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].
































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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Applications and Traffic Patterns  . . . . . . . . . . . .  5
     2.2.  Network Topology of Industrial Applications  . . . . . . .  9
       2.2.1.  The Physical Topology  . . . . . . . . . . . . . . . . 10
       2.2.2.  Logical Topologies . . . . . . . . . . . . . . . . . . 12
   3.  Traffic Characteristics  . . . . . . . . . . . . . . . . . . . 13
     3.1.  Service Parameters . . . . . . . . . . . . . . . . . . . . 14
     3.2.  Configurable Application Requirement . . . . . . . . . . . 15
     3.3.  Different Routes for Different Flows . . . . . . . . . . . 15
   4.  Reliability Requirements . . . . . . . . . . . . . . . . . . . 15
   5.  Device-Aware Routing Requirements  . . . . . . . . . . . . . . 17
   6.  Broadcast/Multicast  . . . . . . . . . . . . . . . . . . . . . 18
   7.  Route Establishment Time . . . . . . . . . . . . . . . . . . . 19
   8.  Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  Manageability  . . . . . . . . . . . . . . . . . . . . . . . . 20
   10. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     13.2. Informative References . . . . . . . . . . . . . . . . . . 23
     13.3. External Informative References  . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24

























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1.  Terminology

   This document employes terminology defined in the ROLL terminology
   document [I-D.ietf-roll-terminology].  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 Foundation (see [HART]).  The latest version
   for the specifications is HART7 which includes the additions for
   WirelessHART.

   ISA: "International Society of Automation".  ISA is 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.


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.  IPv6 is perceived as a key technology to
   provide the scalability and interoperability that are required in
   that space and is being more and more present in standards and
   products under development and early deployments.

   Cable is perceived as a more proven, safer techhnology, 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 LLN 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 low
   power and lossy networks (LLN) 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



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   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 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 LLN networks 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 1,200 baud (e.g.
   wired HART) to the one to two hundred Kbps range for most of the
   others.  The existing protocols are often master/slave with command/
   response.

2.1.  Applications and Traffic Patterns

   The industrial market classifies process applications into three
   broad categories and six classes.





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   o  Safety

      *  Class 0: Emergency action - Always a critical function

   o  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)

   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 becomes 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 3 through 2.

   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.



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   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
   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.  Some
   sensors are bursty, such as vibration monitors that 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 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 Kbytes 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 ms but many control loops are tolerant of latencies above
   1 s.

   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 field busses.  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 a wireless, this architecture reduces the tight



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   latency and availability requirements for the wireless links.

   Closing loops in the field:

   o  does not prevent the same loop from being closed through a remote
      multi-variable 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, and 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 which 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 mean-time between shutdowns target of 30 years,
   the latency requirement implies nine 9s of reliability.  Given such
   exposure, given the intrinsic vulnerability of wireless link



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   availability, and given the emergence of control in the field
   architectures, most users tend to not aim for fast closed loop
   control with wireless links within that fast loop.

2.2.  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 maximum number of hops of twenty.  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 a 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 is a 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 change 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, the plant network, also known as the 'Process Control
   Domain', the PCD.  These plant automation networks are domain wise
   segregated from the office network or office domain (OD), which in
   itself is typically segregated from the Internet.

   Sinks for LLN sensor data reside on both the plant network PCD, the
   business network 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 office domain.
   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 R and D purposes are other examples where
   Internet would be the domain where wireless sensor data shall land,
   and other domains such as office and plant should preferably be
   circumvented if quick deployment without potentially impacting plant
   safety integrity is required.

   This multiple domain multiple applications 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 co-existence challenges, yet
   logically segregated to avoid creation of intolerable short cuts
   between existing wired domains.

   Given this challenge, LLN networks 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 identity and
   trust established by the gateways in the authenticity of message
   originators.

   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.

2.2.1.  The Physical Topology

   There is no specific physical topology for an industrial process
   control network.  One extreme example is a multi-square-kilometer
   refinery where isolated tanks, some of them with power but most with



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   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 one or two hops.  The backbone is
   in the refinery proper, many hops away.  Even there, powered
   infrastructure is also typically several hops away.  So hopping to/
   from the powered infrastructure will in general 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
   router.  Most communication between field devices and infrastructure
   devices as well as field device to field device occurs across the
   backbone.  Form afar, this model resembles the WIFI ESS (Extended
   Service Set).  But from a layer 3 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
                  |     |
                  +-----+
                     |
                     |      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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2.2.2.  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 devices that sits on the backbone and is
   reachable via one or more backbone router.

   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 router 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
   connectivity is very specific and rare, though the traffic associated
   might be of foremost importance.  Field device to field device routes
   are often the most critical, optimized and well-maintained routes.  A
   class 0 control loop requires guaranteed delivery and extremely tight
   response times.  Both the respect of criteria in the route
   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 back up route only in case of loss
   of the wired path.

   Considering that though each field device to field device 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 workers to tens
   of years for a command and control closed loop.  Finally, time-



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   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.


3.  Traffic Characteristics

   The industrial applications fall into four large service categories
   [ISA100.11a]:

   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.

   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 period of time (related to
       file size and data rate) to meet the bulk transfers service
       requirements.





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3.1.  Service Parameters

   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
      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).

   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 field device
   to field device connectivity as well as for some field device to
   Infrastructure devices over time.

   Industrial application data flows between field devices are not



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   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.

3.2.  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 route on paths that are changed to
   appropriately provision the application requirements.  The routing
   protocol MUST support the ability to recompute paths based on Network
   Layer abstractions of the underlying link attributes/metric that may
   change dynamically.

3.3.  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
   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 characteritics (e.g.
   Optimized according to different cost, etc...).


4.  Reliability Requirements

   LLN reliability constitutes several unrelated aspects:

   1)  Availability of source to destination connectivity when the
       application needs it, expressed in number of succeses / 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)  ...

   This makes quantifying reliability the equivalent of plotting it on a
   three plus dimensional graph.  Different applications have different
   requirements, and expressing reliability as a one dimensional
   parameter, like 'reliability my wireless network is 99.9%' is often
   creating 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
   shut down 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, 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, so 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 maintainance.  The maximum
   disruption time is the time it takes at most for a specific path to
   be restored when broken.  Route maintainance ensures that a path is
   monitored to be restored when broken within 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 a availability of
   less than 90% with hours of latency.  Most industrial standards, such
   as HART7, 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 thrustworthiness
   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.


5.  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, 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 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 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.


6.  Broadcast/Multicast

   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 results 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.

   Both 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 broadcast or multicast
   addressing.


7.  Route Establishment Time

   During network formation, installers with no networking skill must be
   able to determine if their devices are "in the network" with
   sufficient connectivity to perform their function.  Installers will
   have sufficient skill to provision the devices with a sample rate or
   activity profile.  The routing algorithm MUST find the appropriate
   route(s) and report success or failure within several minutes, and
   SHOULD report success or failure within tens of seconds.

   Network connectivity in real deployments is always time varying, with
   time constants from seconds to months.  So long as the underlying
   connectivity has not been compromised, this link churn should not
   substantially affect network operation.  The routing algorithm MUST
   respond to normal link failure rates with routes that meet the
   Service requirements (especially latency) throughout the routing
   response.  The routing algorithm SHOULD always be in the process of
   recalculating the route in response to changing link statistics.  The
   routing algorithm MUST recalculate the paths when field devices
   change due to insertion, removal or failure, and this recalculation
   MUST NOT cause latencies greater than the specified constraints
   (typically seconds to minutes).


8.  Mobility

   Various economic factors have contributed to a reduction of trained



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   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
   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.

   Undecided yet is if these PDAs will use the LLN network directly to
   talk to field sensors, or if they will rather use other wireless
   connectivity that proxys back into the field, or to anywhere else,
   the user interfaces typically used for plant historians, asset
   management systems, and the likes.

   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.


9.  Manageability

   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.

   Most users in fact demand even much further simplified provisioning



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   methods, whereby automatically any new device will connect and report
   at the LLN access point.  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 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 a LLN access point.

   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 crypto
   capabilities, to connect anyway.  Examples are 3rd party road
   tankers, rail cargo containers with overfill protection sensors, or
   consumer cars that need to be refueled with hydrogen by robots at
   future petrol 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 routing
   MAY require commissioning of information about the node 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
   also MUST support the distribution of configuration from a
   centralized management controller if operator-initiated configuration
   change is allowed.


10.  Security

   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 temporary losing connectivity, say due to a



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   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 attacks that have little to do with the wireless devices in the
   LLNs.  The second type of attack, access points that might be
   wireless backdoors that may 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.

   Current generation industrial wireless device manufactures are
   specifying security at the MAC 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 unique randomly-
   generated end-to-end Session keys.  HART7 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.  It is vital that also 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 average skill levels in the field, and
   given 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 in 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 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



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   managed using secure messages and protocols that prevent outsider
   attacks and limit insider attacks from field devices installed in
   insecure locations in the plant.

   Wireless typically forces abandonance 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 this document.


11.  IANA Considerations

   This document includes no request to IANA.


12.  Acknowledgements

   Many thanks to Rick Enns, Alexander Chernoguzov and Chol Su Kang for
   their contributions.


13.  References

13.1.  Normative References

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

13.2.  Informative References

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-00 (work in
              progress), October 2008.

13.3.  External Informative References

   [HART]     www.hartcomm.org, "Highway Addressable Remote Transducer",
              a group of specifications for industrial process and
              control devices administered by the HART Foundation".




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   [ISA100.11a]
              ISA, "ISA100, Wireless Systems for Automation", May 2008,
              <     http://www.isa.org/Community/
              SP100WirelessSystemsforAutomation>.


Authors' Addresses

   Kris Pister (editor)
   Dust Networks
   30695 Huntwood Ave.
   Hayward,   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
   5012 W. Torrey Pines Circle
   Glendale, AZ  85308-3221
   USA

   Phone: +1 602 938 3163
   Email: tom.phinney@cox.net





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