| ROLL | T. Phinney, Ed. |
| Internet-Draft | consultant |
| Intended status: Informational | P. Thubert |
| Expires: March 13, 2014 | cisco |
| RA. Assimiti | |
| Nivis | |
| September 09, 2013 |
RPL applicability in industrial networks
draft-ietf-roll-rpl-industrial-applicability-01
The wide deployment of wireless devices, with their low installed cost (compared to wired devices), will significantly improve the productivity and safety of industrial plants. It will simultaneously increase the efficiency and safety of the plant's workers, by extending and making more timely the information set available about plant operations. The new Routing Protocol for Low Power and Lossy Networks (RPL) defines a Distance Vector protocol that is designed for such networks. The aim of this document is to analyze the applicability of that routing protocol in industrial LLNs formed of field devices.
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Information Technology (IT) is already, and increasingly will be applied to Industrial Automation and Control System (IACS) technology in application areas where those IT technologies can be constrained sufficiently by Service Level Agreements (SLA) or other modest change that they are able to meet the operational needs of IACS. When that happens, the IACS benefits from the large intellectual, experiential and training investment that has already occurred in those IT precursors. One can conclude that future reuse of additional IT protocols for IACS will continue to occur due to the significant intellectual, experiential and training economies which result from that reuse.
Following that logic, many vendors are already extending or replacing their local field-bus technology with Ethernet and IP-based solutions. Examples of this evolution include CIP EtherNet/IP, Modbus/TCP, Foundation Fieldbus HSE, PROFInet and Invensys/Foxboro FOXnet. At the same time, wireless, low power field devices are being introduced that facilitate a significant increase in the amount of information which industrial users can collect and the number of control points that can be remotely managed.
IPv6 appears as a core technology at the conjunction of both trends, as illustrated by the current [ISA100.11a] industrial Wireless Sensor Networking (WSN) specification, where layers 1-4 technologies developed for end uses other than IACS - IEEE 802.15.4 PHY and MAC, 6LoWPAN and IPv6, and UDP - are adapted to IACS use. But due to the lack of open standards for routing in Low power and Lossy Networks (LLN) at the time ISA100.11a was crafted, routing was accomplished at the link layer and is specific to that standard.
The IETF ROLL Working Group has defined application-specific routing requirements for a LLN routing protocol, specified in:
The Routing Protocol for Low Power and Lossy Networks (RPL) [RFC6550] specification and its point to point extension/optimization [RFC6997] define a generic Distance Vector protocol that is adapted to a variety of Low Power and Lossy Networks (LLN) types by the application of specific Objective Functions (OFs). RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) within instances of the protocol, each instance being associated with an Objective Function to form a routing topology.
A field device that belongs to an instance uses the OF to determine which DODAG and which Version of that DODAG the device should join. The device also uses the OF to select a number of routers within the DODAG current and subsequent Versions to serve as parents or as feasible successors. A new Version of the DODAG is periodically reconstructed to enable a global reoptimization of the graph.
A RPL OF states the outcome of the process used by a RPL node to select and optimize routes within a RPL Instance based on the information objects available. The separation of OFs from the core protocol specification allows RPL to be adapted to meet the different optimization criteria required by the wide range of industrial classes of traffic and applications.
This document provides information on how RPL can accommodate the industrial requirements for LLNs, in particular as specified in [RFC5673].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
Additionally, this document uses terminology from [I-D.ietf-roll-terminology], and uses usual terminology from the Process Control and Factory Automation industries, some of which is recapitulated below:
This applicability statement does not address requirements related to wireless LLNs employed in factory automation and related applications.
[RFC5673] describes in detail the routing requirements for industrial LLNs. This RFC provides information on the varying deployment scenarios for such LLNs and how RPL assists in meeting those requirements.
Large industrial plants, or major operating areas within such plants, repeatedly go through four major phases, each of which typically lasts from months to years:
followed eventually by an (at least theoretical)
It is also likely, after a major catastrophe at a plant, to have a
The deployment scenarios for wireless LLN devices may be different in each of these phases. In particular, during the Construction or major modification phase (P1), LLN devices may be installed months before the intended LLN can become usefully operational (because needed routers and infrastructure devices are not yet installed or active), and there are likely to be many personnel in whom the plant owner/operator has only limited trust, such as subcontractors and others in the plant area who have undergone only a cursory background investigation (if any at all). In general, during this phase, plant instrumentation is not yet operational, so could be removed and replaced by a Trojaned device without much likelihood of physical detection of the substitution. Thus physical security of LLN devices is generally a more significant risk factor during this phase than once the plant is operational, where simple replacement of device electronics is detectable.
Extra LLN devices and even extra LLN subnets may be employed during Planned startup (P2) and Planned shutdown (P4) phases, in support of the task of transitioning the plant or plant area between operational and shutdown states. The extra devices typically provide extra monitoring as the plant transitions infrequent activity states. (In many continuous process plants, up to 2x extra staff are employed at monitoring and control workstations during these two phases, precisely because the plant is undergoing extraordinary behavior as it transitions to or from its steady-state operational condition.)
Similar transient devices and subnets may be used during an unscheduled Post-emergency recovery and repair phase (P6) of operation, but in that case the extra devices usually are routers substituting for plant LLN devices that have been damaged by the incident (such as a fire, explosion, flood, tornado or hurricane) that induced the emergency.
The Planned startup (P2) and Planned shutdown (P4) phases are similar in many respects, but the LLN environment of the two can be quite different, since the Planned shutdown phase can assume that the stable LLN environment used for Normal operation (P3) is functional during shutdown, whereas that stable environment usually is still being established during startup.
The Post-emergency recovery and repair phase (P6) typically operates in an LLN environment that is somewhere between that of the Planned startup (P2) and Normal operation (P3) phases, but with an indeterminate number of temporary routers placed to facilitate communication across and around the area affected by the catastrophe.
Smaller industrial plants and sites may go through similar phases, but often commingle the phases because, in those smaller plants, the phases require less planning and structuring of personnel responsibilities and thus permit less formalization and partitioning of the operating scenarios. For example, it is much simpler, and usually requires much less planning, to bring new equipment on a skid into a plant, using a forklift, than to lay temporary railroad track or employ an extended-axle heavy haul tractor-trailer to deliver a multi-ton process vessel, and temporarily deploy and use very large heavy-lift cranes to install it. In the former cases, nearby equipment usually can continue normal operation while the installation proceeds; in the latter case that is almost always impossible, due to safety and other concerns.
The domain of applicability for the RPL protocol may include all phases but the Normal Operation phase, where the bandwidth allocation and the routes are usually optimized by an external Path Computing Engine (PCE), e.g. an ISA100.11a System Manager.
Additionally, it could be envisioned to include RPL in the normal operation provided that a new Objective Function is defined that actually interacts with the PCE is order to establish the reference topology, in which case RPL operations would only apply to emergency repair actions. when the reference topology becomes unusable for some failure, and as long as the problem persists.
The industrial market classifies process applications into three broad categories and six classes.
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.
The domain of applicability for the RPL protocol probably matches the range of classes where industrial users are interested in deploying wireless networks. This domain includes monitoring classes (4 and 5), and the non-critical portions of control classes (2 and 3). RPL might also be considered as an additional repair mechanism in all situations, and independently of the flow classification and the medium type.
+---------------------+------------------------------------------------+
| Phase \ Class | 0 1 2 3 4 5 |
+=====================+================================================+
| Construction | X X X X |
+---------------------+------------------------------------------------+
| Planned startup | X X X X |
+---------------------+------------------------------------------------+
| Normal operation | ? ? ? |
+---------------------+------------------------------------------------+
| Planned shutdown | X X X X |
+---------------------+------------------------------------------------+
|Plant decommissioning| X X X X |
+---------------------+------------------------------------------------+
| Recovery and repair | X X X X X X |
+---------------------+------------------------------------------------+
? : typically usable for all but higher-rate classes 0,1 PS traffic
Figure 1: RPL applicability matrix
It appears from the above sections that whether and the way RPL can be applied for a given flow depends both on the deployment scenario and on the class of application / traffic. At a high level, this can be summarized by the following matrix:
In an IACS, high-rate communications flows (e.g., 1 Hz or 4 Hz for a traditional process automation network) typically are such that only a single wireless LLN hop separates the source device from a LLN Border Router (LBR) to a significantly higher data-rate backbone network, typically based on IEEE 802.3, IEEE 802.11, or IEEE 802.16, as illustrated in Figure 2.
---+------------------------
| Plant Network
|
+-----+
| | Gateway
| |
+-----+
|
| Backbone
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | LLN border | | LLN border | | LLN border
o | | router | | router | | router
+-----+ +-----+ +-----+
o o o o
o o o o o o o o o o o
LLN
o : stationary wireless field device, seldom acting as an LLN router
Figure 2: High-rate low-delay low-variance IACS topology
For factory automation networks, the basic communications cycle for control is typically much faster, on the order of 100 Hz or more. In this case the LLN itself may be based on high-data-rate IEEE 802.11 or a 100 Mbit/s or faster optical link, and the higher-rate network used by the LBRs to connect the LLN to superior automation equipment typically might be based on fiber-optic IEEE 802.3, with multiple LBRs around the periphery of the factory area, so that most high-rate communications again requires only a single wireless LLN hop.
Multi-hop LLN routing is used within the LLN portion of such networks to provide backup communications paths when primary single-hop LLN paths fail, or for lower repetition rate communications where longer LLN transit times and higher variance are not an issue. Typically, the majority of devices in an IACS can tolerate such higher-delay higher-variance paths, so routing choices often are driven by energy considerations for the affected devices, rather than simply by IACS performance requirements, as illustrated in Figure 3.
---+------------------------
| 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
o : stationary wireless field device, often acting as an LLN router
M : mobile wireless device
Figure 3: Low-rate higher-delay higher-variance IACS topology
Two decades of experience with digital fieldbuses has shown that four communications paradigms dominate in IACS:
In SS, the source-sink communication paradigm, each of many devices in one set, S1, sends UDP-like messages, usually infrequently and intermittently, to a second set of devices, S2, determined by a common multicast address. A typical example would be that all devices within a given process unit N are configured to send process alarm messages to the multicast address Receivers_of_process_alarms_for_unit_N. Receiving devices, typically on non-LLN networks accessed via LBRs, are configured to receive such multicast messages if their work assignment covers process unit N, and not otherwise.
Timeliness of message delivery is a significant aspect of some SS communication. When the SS traffic conveys process alarms or device alerts, there is often a contractual requirement, and sometimes even a regulatory requirement, on the maximum end-to-end transit delay of the SS message, including both the LLN and non-LLN components of that delay. However, there is no requirement on relative jitter in the delivery of multiple SS messages from the same source, and message reordering during transit is irrelevant.
Within the LLN, the SS paradigm simply requires that messages so addressed be forwarded to the responsible LBR (or set of equivalent LBRs) for further forwarding outside the LLN. Within the LLN such traffic typically is device-to-LBR or device-to-redundant-set-of-equivalent-LBRs. In general, SS traffic may be aggregated before forwarding when both the multicast destination address and other QoS attributes are identical. If information on the target delivery times for SS messages is available to the aggregating forwarding device, that device may intentionally delay forwarding somewhat to facilitate further aggregation, which can significantly reduce LLN alarm-reporting traffic during major plant upset events.
In PS, the publish-subscribe communication paradigm, a device sends UDP-like messages, usually periodically or cyclicly (i.e., repetitively but without fixed periodicity), to a single multicast address derived from or correlated with the device's own address. A typical example would be that each sensor and actuator device within a given process unit N is configured to send process state messages to the multicast address that designates its specific publications. In essence the derived multicast address for device D is Receivers_of_publications_by_device_D. Typically those receivers are in two categories: controllers (C) for control loops in which device D participates, and devices accessed via the LLN's LBRs that monitor and/or accumulate historical information about device D's status and outputs.
If the controller(s) that receive device D's publication are all outside the LLN and accessed by LBRs, then within the LLN such traffic typically is device-to-LBR or device-to-redundant-set-of-equivalent-LBRs. But if a controller (Cn) is within the LLN, then a number of different LLN-local traffic patterns may be employed, depending on the capabilities of the underlying link technology and on configured performance requirements for such reporting. Typically in such a case, publication by device D is forwarded up a DODAG to an LLN router that is also on a downward DODAG to a destination controller Cn, then forwarded down that second DODAG to that destination controller Cn. Of course, if the LLN router (or even the LBR) is itself the intended destination controller, which will often be the case, then no downward forwarding occurs.
Timeliness of message delivery is a critical aspect of PS communication. Individual messages can be lost without significant impact on the controlled physical process, but typically a sequence of four consecutive lost messages will trigger fallback behavior of the control algorithms, which is considered a system failure by most system owner/operators. (In general, and unless a local catastrophic event such as a major explosion or a tornado occurs in the plant, invocation of more than one instance of such fallback handling per year, per plant, is considered unacceptable.)
Message loss, delay and jitter in delivery of PS messaging is a relative matter. PS messaging is used for transfer of process measurements and associated status from sensors to control computation elements, from control computation elements to actuators, and of current commanded position and status from actuators back to control computation elements. The actual time interval of interest is that which starts with sensing of the physical process (which necessarily occurs before the sensed value can be sent in the first message) and which ends when the computed control correction is applied to the physical process by the appropriate actuator (which cannot occur until after the second message containing the computed control output has been received by that actuator). With rare exception, the control algorithms used with PS messaging in the process automation industries - those managing continuous material flows - rely on fixed-period sampling, computation and transfer of outputs, while those in the factory automation industries - those managing discrete manufacturing operations - rely on bounded delay between sampling of inputs, control computation and transfer of outputs to physical actuators that affect the controlled process.
Deliberately manipulated message delay and jitter in delivery of PS messaging has the potential to destabilize control loops. It is the responsibility of conveyed higher-level protocols to protect against such potential security attacks by detecting overly delayed or jittered messages at delivery, converting them into instances of message loss. Thus network and data-link protocols such as IPv6 and Ethernet need not themselves address such issues, although their selection and employment should take the existence (or lack) of such higher-layer protection mechanisms, and the resulting consequences due to excessive delay and jitter, into consideration in their parameterization.
In general, PS traffic within the LLN is not aggregated before forwarding, to minimize message loss and delay in reception by any relevant controller(s) that are outside the LLN. However, if all intended destination controllers are within the LLN, and at least one of those intended controllers also serves as an LLN router on a DODAG to off-LLN destinations that all are not controllers, then the router functions in that device may aggregate PS traffic before forwarding when the required routing and other QoS attributes are identical. If information on the target delivery times for PS messages to non-controller devices is available to the aggregating forwarding device, that device may intentionally delay forwarding somewhat to facilitate further aggregation.
In some system architectures, message streams that use PS to convey current process measurements and status are compressed at the source through a 2‑dimensional winnowing process that compares
If the interval since that last-sent message is less than a predefined maximum time, and the status is unchanged, and the process measurement(s) conveyed in the message is within predefined deadband(s) of the last-sent measurement value(s), then transmission of the new message is suppressed. Often this suppression takes the form of not queuing the new message for transmission, but in some protocols a brief placeholder message indicating "no significant change" is queued in its stead.
In P2P, the peer-to-peer communication paradigm, a device sends UDP-like or TCP-like messages from one device (D1) to a second device (D2), usually with bidirectional but asymmetric flow of application data, where the amount of data is significantly greater in one direction than the other. Typical examples are transfer of configuration information to or from a process field device, or transfer of captured process diagnostics (e.g., time-stamped noise signatures from a coriolis flowmeter) to an off-LLN higher-level asset management system. Unicast addressing is used in both directions of data flow.
In general, specific P2P traffic has only loose timeliness requirements, typically just those required so that response times to human-operator-initiated actions meet human factors requirements. As a consequence, in general, message aggregation is permitted, although few opportunities are likely to present themselves for such aggregation due to the sporadic nature of such messaging to a single destination, and/or due to the large message payloads that often occur in at least one direction of transmission.
In P2MP, the peer-to-multipeer communication paradigm, a device sends UDP-like messages downward, from one device (D1) to a set of other devices (Dn). Typical examples are bulk downloads to a set of devices that use identical code image segments or identically-structured database segments; group commands to enable device state transitions that are quasi-synchronized across all or part of the local network (e.g., switch to the next set of point-to-point downloaded session keys, or notifying that the network is switching to an emergency repair and recovery mode); etc. Multicast addressing is used in the downward direction of data flow.
Devices can be assigned to a number of multicast groups, for instance by device type. Then, if it becomes necessary to reflash all devices of a given type with a new load image, a multicast distribution mechanism can be leveraged to optimize the distribution operation.
In general, P2MP traffic has only loose timeliness requirements. As a consequence, in general, message aggregation is permitted, although few opportunities are likely to present themselves for such aggregation due to the sporadic nature of such messaging to a single multicast group destination, and/or due to the large message payloads that often occur when P2MP is used for group downloads. However, in general, message aggregation negatively impacts the delivery success rate for each of the aggregated messages, since the probability of error in a received message increases with message length> Together these considerations often lead to a policy of non-aggregation for P2MP messaging.
Note: Reliable group download protocols, such as the no-longer-published IEEE 802.1E (ISO/IEC 15802‑4) system load protocol, and reliable multicast protocols based on the guidance of [RFC2887], are instructive in how P2MP can be used for initial bulk download, followed by either P2MP or P2P selective retransmissions for missed download segments.
In industrial automation systems, some traffic is from (relatively) high-rate monitoring and control loops, of Class 0 and Class 1 as described in [RFC5673]. In such systems, the wireless link protocol, which typically uses immediate in-band acknowledgement to confirm delivery (or, on failure, conclude that a retransmission is required), can be adapted to attempt simultaneous delivery to more than one receiving device, with separated, sequenced immediate in-band acknowledgement by each of those intended receivers. (This mechanism is known colloquially as "duocast" (for two intended receivers), or more generically as "N‑cast" (for N intended receivers).) Transmission is deemed successful if at least one such immediate acknowledgement is received by the sending device; otherwise the device queues the message for retransmission, up until the maximum configured number of retries has been attempted.
The logic behind duocast/N‑cast is very simple: In wireless systems without FEC (forward error correction), the overall rate of success for transactions consisting of an initial transmission and an immediate acknowledgement is typically 95%. In other words, 5% of such transactions fail, either because the initial message of the transaction is not received correctly by the intended receiver, or because the immediate acknowledgment by that receiver is not received correctly by the transaction initiator.
In the generalized case of N‑cast, where any received acknowledgement serves to complete the transaction, and where the N intended receivers are spatially diverse, physically separated from each other by multiple wavelengths, the probability that all such receivers fail to receive the initial message of the transaction, or that all generated immediate acknowledgements are not received by the transaction initiator, is typically approximately (5%)^N. Thus, for duocast, the expected success rate for a single transaction goes from 95% (1.0 ‑ 0.05) to 99.75% (1.0 ‑ 0.05^2), to 99.9875% (1.0 ‑ 0.05^3) when N=3, and even higher when N>3.
From the above analysis, it is obvious that the primary benefit of N‑cast occurs when N goes from N=1 (unicast) to N=2 (duocast); the reduction in transaction loss rate for increasing N>2 is quite small, and for N>3 it is infinitesimal. In the typical industrial automation environment of class 1 process control loops, which typically repeat at a 1 Hz or 4 Hz rate, in a very large process plant with thousands of field devices reporting at that rate, the maximum number of transmission retries that must be planned, and for which capacity must be scheduled (within the requisite 250 ms or 1 s interval) is seven (7) retries for unicast PS reporting, but only three (3) retries with duocast PS reporting. (This is determined by the requirement to not miss four successive reports more than once per year, across the entire plant, as such a loss typically triggers fallback behavior in the controlled loop, which is considered a failure of the wireless system by the plant owner/operator.) In practice, the enormous reduction in both planned and used retransmission capacity provided by duocast/N‑cast is what enables 4 Hz loops to be supported in large wireless systems.
When available, duocast/N‑cast typically is used only for one-hop PS traffic on Class 1 and Class 0 control loops. It may also be employed for rapid, reliable one-hop delivery of Class 0 and sometimes Class 1 process alarms and device alerts, which use the SS paradigm. Because it requires scheduling of multiple receivers that are prepared to acknowledge the received message during the transaction, in general it is not appropriate for the other types of traffic in such systems - P2P and P2MP - and is not needed for other classes of control loops or other types of traffic, which do not have such stringent reporting requirements.
Note: Although there are known patent applications for duocast and N‑cast, at the time of this writing the patent assignee, Honeywell International, has offered to permit cost-free RAND use in those industrial wireless standards that have chosen to employee the technology, under a reciprocal licensing requirement relative to that use. Since duocast and N‑cast provide performance and energy optimizations, they are not essential for use in wireless systems. However, in practice, their use makes it possible to support 4 Hz wireless loops and meet sub-second safety alarm reporting requirements in large plants, where that might otherwise be impractical without use of a wired network. When duocast/N‑cast is not employed, the wireless retransmission capacity that is needed to support such fast loops often is excessive, typically over 100x that actually used for retransmission (i.e., providing for seven retries per transaction when the mean number used is only 0.06 retries).
To match the requirements above, RPL provides a number of RPL Modes of Operation (MOP):
The RPL MOP that can be applied for a given flow depends on the communication paradigm. It must be noted that a DODAG that is used for PS traffic can also be used for SS traffic since the MOP 2 extends the MOP 0, and that a DODAG that is used for P2MP distribution can also be used for downward PS since the MOP 3 extends the MOP 2.
On the other hand, an Objective Function (OF) that optimizes metrics for a pure upwards DODAG might differ from the OF that optimizes a mixed upward and downward DODAG.
As a result, it can be expected that different RPL instances are installed with different OFs, different channel allocations, etc... that result in different routing and forwarding topologies, sometimes with differing delay vs. energy profiles, optimized separately for the different flows at hand.
+---------------------+------------+-----------------------------------+
| Paradigm\RPL MOP | RPL spec | Mode of operation |
+=====================+============+===================================+
| Peer-to-peer | RPL P2P | reactive (on-demand) |
+---------------------+------------+-----------------------------------+
| P2P line-of-sight | RPL base | 2 (storing) with multicast DAO |
+---------------------+------------+-----------------------------------+
| P2MP distribution | RPL base | 3 (storing with multicast) |
+---------------------+------------+-----------------------------------+
| Publish-subscribe | RPL base | 1 or 2 (storing or not-storing) |
+---------------------+------------+-----------------------------------+
| Source-sink | RPL base | 0 (no downward route) |
+---------------------+------------+-----------------------------------+
| N-cast publish | RPL base | 0 (no downward route) |
+---------------------+------------+-----------------------------------+
Figure 4: RPL applicability per communication paradigm
This can be broadly summarized in the following table:
To be completed.
The functional requirements for most industrial automation deployments are similar to those listed in [RFC5673]
RPL supports:
For wireless installations of small size with undemanding communication requirements, RPL is likely to generate satisfactory routing without any special effort. However, in larger installations or where timeliness considerations do not permit multi-second wireless-subnet transit times, then flow labeling is likely required so that forwarding routers can make informed tradeoffs between conserving their own energy resources and meeting overall system needs.
This section outlines a RPL profile for a representative deployment in a process control application. Process monitoring without control is typically less demanding, so a subset of this profile generally will suffice.
RPL allows formation of multiple instances that operate independently of each other. Each instance may use a different objective function and different modes of operation. It is highly recommended that wireless field devices participate in different instances that utilize objective functions that meet different optimization goals. These optimization goals target:
Some of these optimization goals will have to be met concurrently in a single instance by imposing various constraints.
Each wireless field device should participate in a set composed of a minimum of three instances that meet optimization goals associated with three traffic flows which need to be supported by all industrial LLNs.
Participation of each wireless field device in at least one instance that hosts a DODAG with a virtual root is highly recommended.
Wireless industrial networks are typically composed of multiple LLNs that terminate in a LLN Border Router (LBR). The LBRs communicate with each other and with other entities present on the backbone (such as the Gateway and the System Manager) over a wired or wireless backbone infrastructure. When a device A that operates in LLN 1 sends a packet to a device B that operates in LLN2, the packets egresses LLN1 through LBR1 and ingresses LLN2 through LBR2 after travelling over the backbone infrastructure that connects the LBRs. In order to accommodate this packet flow that travels from one LLN to another, it is highly recommended that wireless field devices participate in at least one instance that has a DODAG with a virtual root.
In general, storing mode is required for high-reporting-rate devices (where "high rate" is with respect to the underlying link data conveyance capability). Such devices, in the absence of path failure, are typically only one hop from the LBR(s) that convey their messaging to other parts of the system. Fortunately, in such cases, the routing tables required by such nodes are small, even when they include information on DODAGs that are used as backup alternate routes.
Deeper multi-hop wireless LLNs (hop count > 1) should support storing mode in order to minimize the overhead associated with source routing given the limited header capacity associated with typical physical layers employed in wireless LLNs. Support for storing mode requires additional RAM resources be present in the constrained wireless fielde devices. Typical wireless LLNs scale to a maximum of one hundred field devices. Hence the appropriate RAM resources for supporting storing mode should be part of the hardware requirements imposed upon wireless field devices during the design phase.
The ISA100.11a standard mandates that all LBRs maintain routing tables with enough capacity to accomodate operation in storing mode. The standard also mandates that all wireless field devices maintain routing tables but it does not make any capacity assumptions, allowing for null routing tables. The System Manager should read the routing table capacity of each wireless field router and LBR during their join phase, and determine if support for storing mode in a particular LLN is feasible.
Lack of support for storing mode is also detrimental to battery operated wireless field devices due to the power consumption associated with transporting the hefty headers associated with source routing. Support for storing mode also ensures path redundancy which in turn allows for better prediction of the latency associated with downward traffic flows. Guaranteed latencies are of paramount importance for various traffic flows in wireless industrial LLNs.
Support for both upward and downward traffic flows is a requirement in industrial automation systems. As a result, nodes send DAO messages to establish downward paths from the root to themselves. DAO messages are not acknowledged in wireless industrial LLNs that are composed of battery operated field devices in order to minimize the power consumption overhead associated with path discovery. Given that wireless field devices in LLNs will typically participate in multiple RPL instances and DODAGs, it is highly recommended that both the RPLInstance ID and the DODAGID be included in the DAO.
RPL relies on an Objective Function for selecting parents and computing path costs and rank. This objective function is decoupled from the core RPL mechanisms and also from the metrics in use in the network. Two objective functions for RPL have been defined at the time of this writing, the RPL Objective Function 0 [RFC6552] and the Minimum Rank with Hysteresis Objective Function [RFC6719], both of which define a selection method for a preferred parent and backup parents, and are suitable for industrial automation network deployments.
Industrial wireless LLNs are subject to swift variations in terms of the propagation of the wireless signal, variations that can affect the quality of the links between field devices. This is due to the nature of the environment in which they operate which can be characterized as metal jungles that cause wireles propagation distortions, multi-path fading and scattering. Hence support for hysteresis is needed in order to ensure relative link stability which in turn ensures route stability.
As mentioned in previous sections of this document, different traffic flows require different optimization goals. Wireless field devices should participate in multiple instances associated with multiple objective functions.
More complex objective functions are needed that take in consideration multiple constraints and utilize weighted sums of multiple additive and multiplicative metrics. Additional objective functions specifically designed for such networks may be defined in companion RFCs.
To effectively handle time-varying link characteristics and availability, industrial automation network deployments SHOULD utilize the local repair mechanisms in RPL.
Local repair is triggered by broken link detection, and in storing mode also by loop detection.
The first local repair mechanism consists of a node detaching from a DODAG and then re-attaching to the same or to a different DODAG at a later time. While detached, a node advertises an infinite rank value so that its children can select a different parent. This process is known as poisoning and is described in Section 8.2.2.5 of [RFC6550]. While RPL provides an option to form a local DODAG, doing so in industrial automation network deployments is of little benefit since applications typically communicate through a LBR. After the detached node has made sufficient effort to send notification to its children that it is detached, the node can rejoin the same DODAG with a higher rank value. The configured duration of the poisoning mechanism needs to take into account the disconnection time applications running over the network can tolerate. Note that when joining a different DODAG, the node need not perform poisoning.
The second local repair mechanism controls how much a node can increase its rank within a given DODAG Version (e.g., after detaching from the DODAG as a result of broken link or loop detection). Setting the DAGMaxRankIncrease to a non-zero value enables this mechanism, and setting it to a value of less than infinity limits the cost of count-to-infinity scenarios when they occur, thus controlling the duration of disconnection applications may experience.
Industrial automation network deployments typically operate in areas that provide limited physical security (relative to the risk of attack). For this reason, the link layer, transport layer and application layer technologies utilized within such networks typically provide security mechanisms to ensure authentication, confidentiality, integrity, timeliness and freshness. As a result, such deployments may not need to implement RPL's security mechanisms and could rely on link layer and higher layer security features.
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Trickle was designed to be density-aware and perform well in networks characterized by a wide range of node densities. The combination of DIO packet suppression and adaptive timers for sending updates allows Trickle to perform well in both sparse and dense environments.
Node densities in industrial automation network deployments can vary greatly, from nodes having only one or a handful of neighbors to nodes having several hundred neighbors. In high density environments, relatively low values for Imin may cause a short period of congestion when an inconsistency is detected and DIO updates are sent by a large number of neighboring nodes nearly simultaneously. While the Trickle timer will exponentially backoff, some time may elapse before the congestion subsides. Although some link layers employ contention mechanisms that attempt to avoid congestion, relying solely on the link layer to avoid congestion caused by a large number of DIO updates can result in increased communication latency for other control and data traffic in the network.
To mitigate this kind of short-term congestion, this document recommends a more conservative set of values for the Trickle parameters than those specified in [RFC6206]. In particular, DIOIntervalMin is set to a larger value to avoid periods of congestion in dense environments, and DIORefundancyConstant is parameterized accordingly as described below. These values are appropriate for the timely distribution of DIO updates in both sparse and dense scenarios while avoiding the short-term congestion that might arise in dense scenarios.
Because the actual link capacity depends on the particular link technology used within an industrial automation network deployment, the Trickle parameters are specified in terms of the link's maximum capacity for conveying link-local multicast messages. If the link can convey m link-local multicast packets per second on average, the expected time it takes to transmit a link-local multicast packet is 1/m seconds.
DIOIntervalMin: Industrial automation network deployments SHOULD set DIOIntervalMin such that the Trickle Imin is at least 50 times as long as it takes to convey a link-local multicast packet. This value is larger than that recommended in [RFC6206] to avoid congestion in dense plant deployments as described above.
DIOIntervalDoublings: Industrial automation network deployments SHOULD set DIOIntervalDoublings such that the Trickle Imax is at least TBD minutes or more.
DIORedundancyConstant: Industrial automation network deployments SHOULD set DIORedundancyConstant to a value of at least 10. This is due to the larger chosen value for DIOIntervalMin and the proportional relationship between Imin and k suggested in [RFC6206]. This increase is intended to compensate for the increased communication latency of DIO updates caused by the increase in the DIOIntervalMin value, though the proportional relationship between Imin and k suggested in [RFC6206] is not preserved. Instead, DIORedundancyConstant is set to a lower value in order to reduce the number of packet transmissions in dense environments.
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RPL enables automatic and consistent configuration of RPL routers through parameters specified by the DODAG root and disseminated through DIO packets. The use of Trickle for scheduling DIO transmissions ensures lightweight yet timely propagation of important network and parameter updates and allows network operators to choose the trade-off point they are comfortable with respect to overhead vs. reliability and timeliness of network updates.
The metrics in use in the network along with the Trickle Timer parameters used to control the frequency and redundancy of network updates can be dynamically varied by the root during the lifetime of the network. To that end, all DIO messages SHOULD contain a Metric Container option for disseminating the metrics and metric values used for DODAG setup. In addition, DIO messages SHOULD contain a DODAG Configuration option for disseminating the Trickle Timer parameters throughout the network.
The possibility of dynamically updating the metrics in use in the network as well as the frequency of network updates allows deployment characteristics (e.g., network density) to be discovered during network bring-up and to be used to tailor network parameters once the network is operational rather than having to rely on precise pre- configuration. This also allows the network parameters and the overall routing protocol behavior to evolve during the lifetime of the network.
RPL specifies a number of variables and events that can be tracked for purposes of network fault and performance monitoring of RPL routers. Depending on the memory and processing capabilities of each smart grid device, various subsets of these can be employed in the field.
Industrial automation network deployments typically operate in areas that provide limited physical security (relative to the risk of attack). For this reason, the link layer, transport layer and application layer technologies utilized within such networks typically provide security mechanisms to ensure authentication, confidentiality, integrity, timeliness and freshness. As a result, such deployments may not need to implement RPL's security mechanisms and could rely on link layer and higher layer security features.
This document does not specify operations that could introduce new threats. Security considerations for RPL deployments are to be developed in accordance with recommendations laid out in, for example, [I-D.tsao-roll-security-framework].
Industrial automation networks are subject to stringent security requirements as they are considered a critical infrastructure component. At the same time, since they are composed of large numbers of resource- constrained devices inter-connected with limited-throughput links, many available security mechanisms are not practical for use in such networks. As a result, the choice of security mechanisms is highly dependent on the device and network capabilities characterizing a particular deployment.
In contrast to other types of LLNs, in industrial automation networks centralized administrative control and access to a permanent secure infrastructure is available. As a result link-layer, transport-layer and/or application-layer security mechanisms are typically in place and may make use of RPL's secure mode unnecessary.
This specification has no requirement on IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |