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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: January 9, 2009                                   Cisco Systems
                                                                S. Dwars
                                                                   Shell
                                                              T. Phinney
                                                            July 8, 2008


    Industrial Routing Requirements in Low Power and Lossy Networks
                 draft-ietf-roll-indus-routing-reqs-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on January 9, 2009.

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



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   reliability, and easy installation and maintenance.  The aim of this
   document is to analyze the requirements for the routing protocol used
   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
     2.2.  Network Topology of Industrial Applications  . . . . . . .  7
       2.2.1.  The Physical Topology  . . . . . . . . . . . . . . . .  9
       2.2.2.  Logical Topologies . . . . . . . . . . . . . . . . . . 10
   3.  Service Requirements . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  Configurable Application Requirement . . . . . . . . . . . 13
     3.2.  Different Routes for Different Flows . . . . . . . . . . . 14
   4.  Reliability Requirements . . . . . . . . . . . . . . . . . . . 14
   5.  Device-Aware Routing Requirements  . . . . . . . . . . . . . . 15
   6.  Broadcast/Multicast  . . . . . . . . . . . . . . . . . . . . . 17
   7.  Route Establishment Time . . . . . . . . . . . . . . . . . . . 17
   8.  Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  Manageability  . . . . . . . . . . . . . . . . . . . . . . . . 19
   10. Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     13.2. Informative References . . . . . . . . . . . . . . . . . . 22
     13.3. External Informative References  . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 24














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1.  Terminology

   Actuator: a field device that moves or controls plant equipment.

   Closed Loop Control: A process whereby a device controller controls
   an actuator based on information sensed by one or more field devices.

   Downstream: Data direction traveling from the plant application to
   the field device.

   PCD: Process Control Domain.  The 'legacy' wired plant Network.

   OD: Office Domain.  The office Network.

   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 L2N
   access points in the plant.

   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.

   L2N Access Point: The L2N access point is an infrastructure device
   that connects the low power and lossy network system to a plant's
   backbone network.

   Open Loop Control: A process whereby a plant operator manually
   manipulates an actuator over the network where the decision is
   influenced by information sensed by field devices.

   Plant Application: The plant application is a computer process
   running in the plant that communicates with field devices to perform
   tasks that may include control, monitoring and data gathering.

   Upstream: Data direction traveling from the field device to the plant
   application.

   RL2N: Routing in Low power and Lossy Networks.



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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 also carries an overhead cost associated with
   planning the installation, determining where the cable has to run,
   and interfacing with the various organizations required 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 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



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   same low power and lossy wireless network technology.  This may mean
   several disconnected, autonomous L2N 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 L2N 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.

   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)





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      *  Class 5: Logging and downloading / uploading - No immediate
         operational consequence (e.g., history collection, sequence-of-
         events, preventive maintenance)

   Safety critical functions affect 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.

   In the near future, most low power and lossy network systems 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 L2N access point and traveling
   to particular sensors.  During diagnostics, however, a technician may



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   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 L2N 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 L2N 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.

   More likely though is that loops will be closed in the field
   entirely, which in most cases eliminates the need for having wireless
   links within the control loop.  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 and L2N-ish, this architecture reduces the tight
   latency and availability requirements for the wireless links.

   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
   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 from two to twenty.  It is assumed
   that the field devices themselves will provide routing capability for



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   the network, and additional repeaters/routers will not be required in
   most cases.

   For most industrial applications, a manager, gateway or backbone
   router acts as a sink for the wireless sensor network.  The vast
   majority of the traffic is real time publish/subscribe sensor data
   from the field devices over a L2N 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 Low Power and Lossy Network 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.

   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
   important function of 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 L2N 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
   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 will be best served with direct Internet
   connectivity.  Examples include: third-party-maintained luminaries;
   vendor-managed inventory systems, where a supplier of chemicals needs
   access to tank level readings at his customer's site; temporary



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   'Babysitting sensors' deployed for just a few days, perhaps during
   startup, troubleshooting, or ad-hoc measurement campaigns for R&D
   purposes.  In these cases, the sensor data naturally flows to the
   Internet, and other domains such as office and plant should be
   circumvented.  This will allow quick deployment without impacting
   plant safety integrity.

   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, L2N 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
   L2N 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
   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



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   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.  From 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
                           L2N

                      Figure 1: The Physical Topology

2.2.2.  Logical Topologies

   Most of the traffic over the LLN is publish/subscribe of sensor data
   from the field device towards the backbone router or gateway that
   acts as the sink for the WSN.  The destination of the sensor data is
   an Infrastructure device 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 events, if that second topology has



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   lesser constraints for the security policy.  This isolation might be
   implemented as Virtual LANs and Virtual Routing Tables in shared
   nodes 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, it makes sense to build proactively a set of default routes
   between the sensors and one or more backbone router and maintain
   those routes at all times.  Also, because of the lossy nature of the
   network, the routing in place should attempt to propose multiple
   forwarding solutions, building forwarding topologies in the form of
   Directed Acyclic Graphs oriented towards the sinks.

   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-
   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.

   For these reasons, the ROLL routing infrastructure MUST be able to
   compute and update constrained routes on demand (that is reactively),
   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.




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3.  Service Requirements

   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 L2N 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.

   For industrial applications Service parameters include but might not
   be limited to:

   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.





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   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.  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 - 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).

   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 set up unidirectional or asymmetrical cost routes
   that are composed of one or more non congruent paths.

3.1.  Configurable Application Requirement

   Time-varying user requirements for latency and bandwidth will 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
   underlying link characteristics that may change dynamically.



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3.2.  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 for different flows.


4.  Reliability Requirements

   There are a variety of different ways to look at reliability in an
   industrial low power lossy network:

   1)  Availability of source to sink connectivity when the application
       needs it, expressed in #fail / #success

   2)  Availability of source to sink connectivity when the application
       might need it, expressed in #potential fail / available
       bandwidth,

   3)  Probability of failure on demand,

   4)  Ability, expressed in #failures divided by #successes to get data
       delivered from source to sink within a capped time,

   5)  How well a network (serving many applications) achieves end-to-
       end delivery of packets within a bounded latency

   The common theme running through all reliability requirements from a
   user perspective is that it be end-to-end, usually with a time bound.

   The impact of not receiving sensor data due to sporadic network
   outages can be devastating if this happens unnoticed.  However, if
   sinks 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.  Depending on 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.

   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



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   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, reliability is usually defined with
   respect to end-to-end delivery of packets within a bounded latency.
   Reliability 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 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 up either reliability, or thrustworthiness
   requirements.

   Hop-by-hop path diversity is used to improve latency-bounded
   reliability.  Additionally, bicasting or pluricasting may be used
   over multiple non congruent / non overlapping paths to increase the
   likelihood that at least one instance of a critical packet be
   delivered error free.

   Because data from field devices are aggregated and funneled at the
   L2N access point before they are routed to plant applications, L2N
   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 L2N
   access point.  The routing protocol MUST support multiple L2N access
   points and load distribution among L2N access points.  The routing
   protocol MUST support multiple L2N access points when L2N access
   point redundancy is required.  Because L2Ns are lossy in nature,
   multiple paths in a L2N route MUST be supported.  The availability of
   each path in a 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 L2N 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



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   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.

   Example 1: solar panel, lead-acid battery sized for two weeks of
   rain.  In this system, the average power consumption over any two
   week period must be kept below a threshhold defined by the solar
   panel.  The peak power over minutes or hours could be dramatically
   higher.

   Example 2: 100uA vibration scavenger, 1mF tantalum capacitor.  With
   very limited storage capability, even the short-term average power
   consumption of this system must be low.  If the cost of sending or
   receiving a packet is 100uC, and a maximum tolerable capacitor
   voltage droop of 1V is allowed, then the long term average must be
   less than 1 packet sent or received per second, and no more than 5
   packets may be forwarded in any given second.

   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



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   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.  This
       is byproduct of having potentially physically separated backbone
       routers that can inject messages into different portions of the
       same larger LLN.

   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.

   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



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   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
   optimizing the system in response to changing link statistics.  The
   routing algorithm MUST re-optimize the paths when field devices
   change due to insertion, removal or failure, and this re-optimization
   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
   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 L2Ns 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 L2N 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 L2N access points, to
   applications, and to field devices.  The routing protocol SHOULD also



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   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
   methods, whereby automatically any new device will connect and report
   at the L2N 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 L2N 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 L2Ns is expected to be easy to deploy and
   manage.  Because the number of field devices in a network is large,



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   provisioning the devices manually would not make sense.  Therefore,
   the routing protocol MUST support auto-provisioning of field devices.
   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
   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 be 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
   L2Ns.  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 L2N 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 doesn't complicate life for those that
   deploy and commission.  Given average skill levels in the field, and



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   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 L2N 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 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.

   Wireless typically forces us to abandon classical 'by perimeter'
   thinking when trying to secure network domains.  Wireless nodes in
   L2N 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.  Standards exist that address those
   vulnerabilities.


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



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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.culler-rl2n-routing-reqs]
              Vasseur, J. and D. Cullerot, "Routing Requirements for Low
              Power And Lossy Networks",
              draft-culler-rl2n-routing-reqs-01 (work in progress),
              July 2007.

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".

   [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





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   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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Full Copyright Statement

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