detnet N. Finn
Internet-Draft P. Thubert
Intended status: Standards Track Cisco
Expires: April 21, 2016 October 19, 2015

Deterministic Networking Problem Statement


This paper documents the needs in various industries to establish multi-hop paths for characterized flows with deterministic properties .

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Table of Contents

1. Introduction

Operational Technology (OT) refers to industrial networks that are specifically deployed in order to monitor production systems and support control loops and movement detection operations for process control (i.e., continuous manufacturing) and factory automation (i.e., discrete manufacturing), as well as protection systems used in power distribution automation (the SmartGrid). Due to its different goals, OT has evolved in parallel but in a manner that is radically different from Information Technology/Information and Communications Technology (IT/ICT), focusing on highly secure, reliable and deterministic networks, with limited scalability over a bounded and closed area.

In OT environments, deterministic networks are characterized as providing a guaranteed bandwidth with extremely low packet loss rates, bounded latency, and low jitter.

The convergence of IT and OT technologies, also called the Industrial Internet, represents a major evolution for both sides. For IT, it means a new level of Quality of Service whereby the transfer of packets is completely controlled and repeatable, different flows are perfectly isolated from one another, and packet loss and system downtimes are reduced drastically; for OT, it means sharing IT resources between deterministic and stochastic flows in order to retrieve vasts amounts of so-far unmeasured data and enable additional optimizations.

The work has already started; in particular, the industrial automation space has been developing a number of Ethernet-based replacements for existing digital control systems (DCS), often not packet-based (fieldbus technologies). These replacements are meant to provide similar behavior as the incumbent protocols, and their common focus is to transport a fully characterized flow over a well-controlled environment (i.e., a factory floor), with a bounded latency, extraordinarily low frame loss, and a very narrow jitter. Examples of such protocols include PROFINET [Profinet], ODVA Ethernet/IP [EIP], and EtherCAT.

As an example, Industrial Automation segregates the network along the broad lines of the Purdue Enterprise Reference Architecture (PERA) [ISA95], using different technologies at each level, and public infrastructures such as the power distribution grid require deterministic properties over the Wide Area. To fully serve an industrial application between a wireless sensor and a virtualized control system operating from the carpeted floor, a deterministic path may span, for instance, across a (limited) number of 802.1 bridges and then a (limited) number of IP routers. In that example, the IEEE802.1 bridges may be operating at Layer-2 over Ethernet whereas the IP routers may be 6TiSCH [TiSCH] nodes operating at Layer-2 and/or Layer-3 over the IEEE802.15.4 MAC.

In parallel, the need for determinism in professional and home audio/video markets drove the formation of the Audio/Video Bridging (AVB) standards efforts in IEEE 802.1. With the demand for connectivity and multimedia in transportation, AVB is being evaluated for application in vehicle head units, rear seat entertainment modules, amplifiers, camera modules, and engine control systems. Automotive AVB networks share the OT requirements for deterministic networks characteristics.

Other instances of in-vehicle deterministic networks have arisen as well for control networks in cars, trains and buses, as well as avionics, with, for instance, the mission-critical "Avionics Full-Duplex Switched Ethernet" (AFDX) that was designed as part of the ARINC 664 standards. Existing automotive control networks such as the LIN, CAN and FlexRay standards were not designed to cover the increasing demands in terms of bandwidth and scalability that we see with various kinds of Driver Assistance Systems (DAS); it results that new multiplexing technologies based on Ethernet are now getting traction.

Other industries where strong needs for deterministic networks are now emerging include: radio access networks [I-D.korhonen-detnet-telreq], the SmartGrid [I-D.wetterwald-detnet-utilities-reqs], and ProAudio networks [I-D.gunther-detnet-proaudio-req].

This wider application scope for deterministic networks has led to the IEEE802.1 AVB Task Group becoming the Time-Sensitive Networking (TSN) Task Group (TG) [IEEE802.1TSNTG], additionally covering industrial and vehicular applications.

The networks in consideration are now extending beyond the LAN boundaries and require secure deterministic forwarding and connectivity over a mixed Layer-2/Layer-3 network. The properties of deterministic networks will have specific requirements for the use of routed networks to support these applications and a new model must be proposed to integrate determinism in IT technology.

The proposed model should enable a fully scheduled operation orchestrated by a central controller, and may support a more distributed operation with probably lesser capabilities. In any fashion, the model should not compromise the ability of a network to keep carrying the sorts of traffic that is already carried today in conjunction with new, more deterministic flows.

Once the abstract model is agreed upon, the IETF will need to specify the signaling elements to be used to establish a path and the tagging elements to be used identify the flows that are to be forwarded along that path. The IETF will also need to specify the necessary protocols, or protocol additions, based on relevant IETF technologies such as PCE [PCE], TEAS [TEAS], CCAMP [CCAMP] and MPLS [MPLS], to implement the selected model.

As a result of this work, it will be possible to establish a multi-hop path over the IP network, for a particular flow with given timing and precise throughput requirements, and carry this particular flow along the multi-hop path with such characteristics as low latency and ultra-low jitter, duplication and elimination of packets over non-congruent paths for a higher delivery ratio, and/or zero congestion loss, regardless of the amount of other flows in the network. Depending on the network capabilities and on the current state, requests to establish a path by an end-node or a network management entity may be granted or rejected, an existing path may be moved or removed, and flows exceeding their contract may face packet declassification and drop.

2. Terminology

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 [RFC2119].

3. On Deterministic Networking

The Internet is not the only digital network that has grown dramatically over the last 30-40 years. Video and audio entertainment, and control systems for machinery, manufacturing processes, and vehicles are also ubiquitous, and are now based almost entirely on digital technologies. Over the past 10 years, engineers in these fields have come to realize that significant advantages in both cost and in the ability to accelerate growth can be obtained by basing all of these disparate digital technologies on packet networks.

The goals of Deterministic Networking are to enable the migration of applications that use special-purpose fieldbus technologies (HDMI, CANbus, ProfiBus, etc... even RS-232!) to packet technologies in general, and the Internet Protocol in particular, and to support both these new applications, and existing packet network applications, over the same physical network.

Considerable experience ([ODVA],[AVnu], [Profinet],[IEC62439], etc...) has shown that these applications need a some or all of a suite of features that includes:

  1. Time synchronization of all host and network nodes (routers and/or bridges), accurate to something between 10 nanoseconds and 10 microseconds, depending on the application.
  2. Support for critical packet flows that:

  3. Multiple methods to schedule, shape, limit, and otherwise control the transmission of critical packets at each hop through the network data plane;
  4. Robust defenses against misbehaving hosts, routers, or bridges, both in the data and control planes, with guarantees that a critical flow within its guaranteed resources cannot be affected by other flows whatever the pressures on the network;
  5. One or more methods to reserve resources in bridges and routers to carry these flows.

Time synchronization techniques need not be addressed by an IETF Working Group; there are a number of standards available for this purpose, including IEEE 1588, IEEE 802.1AS, and more.

The multicast, latency, loss ratio, and non-throttling needs are made necessary by the algorithms employed by the applications. They are not simply the transliteration of fieldbus needs to a packet-based fieldbus simulation, but reflect fundamental mathematics of the control of a physical system.

With classical forwarding latency- and loss-sensitive packets across a network, interactions among different critical flows introduce fundamental uncertainties in delivery schedules. The details of the queuing, shaping, and scheduling algorithms employed by each bridge or router to control the output sequence on a given port affect the detailed makeup of the output stream, e.g. how finely a given flow's packets are mixed among those of other flows.

This, in turn, has a strong effect on the buffer requirements, and hence the latency guarantees deliverable, by the next bridge or router along the path. For this reason, the IEEE 802.1 Time-Sensitive Networking Task Group has defined a new set of queuing, shaping, and scheduling algorithms (see Section 5.2) that enable each bridge or router to compute the exact number of buffers to be allocated for each flow or class of flows. The present authors assume that these techniques will be used by the DetNet Working Group.

Robustness is a common need for networking protocols, but plays a more important part in real-time control networks, where expensive equipment, and even lives, can be lost due to misbehaving equipment.

Reserving resources before packet transmission is the one fundamental shift in the behavior of network applications that is impossible to avoid. In the first place, a network cannot deliver finite latency and practically zero packet loss to an arbitrarily high offered load. Secondly, achieving practically zero packet loss for un-throttled (though bandwidth limited) flows means that bridges and routers have to dedicate buffer resources to specific flows or to classes of flows. The requirements of each reservation have to be translated into the parameters that control each host's, bridge's, and router's queuing, shaping, and scheduling functions and delivered to the hosts, bridges, and routers.

4. Related IETF work

4.1. Deterministic PHB

[I-D.svshah-tsvwg-deterministic-forwarding] defines a Differentiated Services Per-Hop-Behavior (PHB) Group called Deterministic Forwarding (DF). The document describes the purpose and semantics of this PHB. It also describes creation and forwarding treatment of the service class, and how the code-point can be mapped into one of the aggregated Diffserv service classes [RFC5127].

4.2. 6TiSCH

Industrial process control already leverages deterministic wireless Low power and Lossy Networks (LLNs) to interconnect critical resource-constrained devices and form wireless mesh networks, with standards such as [ISA100.11a] and [WirelessHART].

These standards rely on variations of the [IEEE802154] timeSlotted Channel Hopping (TSCH) [RFC7554] Medium Access Control (MAC), and a form of centralized Path Computation Element (PCE), to deliver deterministic capabilities.

The TSCH MAC benefits include high reliability against interference, low power consumption on characterized flows, and Traffic Engineering capabilities. Typical applications are open and closed control loops, as well as supervisory control flows and management.

The 6TiSCH Working Group focuses only on the TSCH mode of the IEEE802.15.4e standard. The WG currently defines a framework for managing the TSCH schedule. Future work will standardize deterministic operations over so-called tracks as described in [I-D.ietf-6tisch-architecture]. Tracks are an instance of a deterministic path, and the DetNet work is a prerequisite to specify track operations and serve process control applications. The dependencies that 6TiSCH has on PCE and DetNet work are further discussed in [I-D.thubert-6tisch-4detnet].

[RFC5673] and [I-D.ietf-roll-rpl-industrial-applicability] section 2.1.3 and next discuss application-layer paradigms, such as Source-sink (SS) that is a Multipeer to Multipeer (MP2MP) model that is primarily used for alarms and alerts, Publish-subscribe (PS, or pub/sub) that is typically used for sensor data, as well as Peer-to-peer (P2P) and Peer-to-multipeer (P2MP) communications. Additional considerations on Duocast and its N-cast generalization are also provided for improved reliability.

5. Related work in other standards organizations

5.1. Bridged solutions

Completed and ongoing work in other standards bodies have, to date, produced viable solutions, suitable for carrying IP traffic for a subset of the applications of interest to DetNet, but only over bridged networks, not through routers. Among these are:

5.2. Queuing and shaping

A number of standards are completed or in progress in the IEEE 802.1 (bridging) and IEEE 802.3 (Ethernet) Working Groups related to the queuing and transmission of Ethernet frames. Most of these standards could be applied to non-Ethernet or non-802 media with equal facility, and so will likely be of use to DetNet. See the DetNet architecture draft [I-D.finn-detnet-architecture] for a detailed list.

6. Problem Statement

6.1. DetNet architecture

An architecture that defines the space in which the various parts of the DetNet solution operate is required. A start has been made with [I-D.finn-detnet-architecture]. The main consideration is to build on art that is deployed in existing OT networks.

These networks are systematically designed around a central controller that has a God's view on the devices, their capabilities, and their links to neighbors. The controller gets requests to establish flows with certain Traffic Specifications, and programs the necessary resources in the network to support those flows.

This design, referred to as Software Defined Networking (SDN), simplifies the computation and the setup of paths, and ensures a better view and an easier control of the network by an operator. To inherit from this art, it has been determined early in DetNet discussions that the work would initially focus on an SDN model as well.

DetNet should thus produce the complete SDN architecture with describes at a high level the interaction and data models to:

The related concepts are already laid out at the IETF with [RFC7426], which introduces the following elements:

SDN Layers and Architecture Terminology per RFC 7426

                   |                                |
                   | +-------------+   +----------+ |
                   | | Application |   |  Service | |
                   | +-------------+   +----------+ |
                   |       Application Plane        |
     |           Network Services Abstraction Layer (NSAL)           |
            |                                                |
            |               Service Interface                |
            |                                                |
     o------Y------------------o       o---------------------Y------o
     |      |    Control Plane |       | Management Plane    |      |
     | +----Y----+   +-----+   |       |  +-----+       +----Y----+ |
     | | Service |   | App |   |       |  | App |       | Service | |
     | +----Y----+   +--Y--+   |       |  +--Y--+       +----Y----+ |
     |      |           |      |       |     |               |      |
     | *----Y-----------Y----* |       | *---Y---------------Y----* |
     | | Control Abstraction | |       | | Management Abstraction | |
     | |     Layer (CAL)     | |       | |      Layer (MAL)       | |
     | *----------Y----------* |       | *----------Y-------------* |
     |            |            |       |            |               |
     o------------|------------o       o------------|---------------o
                  |                                 |
                  | CP                              | MP
                  | Southbound                      | Southbound
                  | Interface                       | Interface
                  |                                 |
     |         Device and resource Abstraction Layer (DAL)           |
     |            |                                 |                |
     |    o-------Y----------o   +-----+   o--------Y----------o     |
     |    | Forwarding Plane |   | App |   | Operational Plane |     |
     |    o------------------o   +-----+   o-------------------o     |
     |                       Network Device                          |

Figure 1

6.2. Flow Characterization

Deterministic forwarding can only apply on flows with well-defined characteristics such as periodicity and burstiness. Before a path can be established to serve them, the expression of those characteristics, and how the network can serve them, for instance in shaping and forwarding operations, must be specified.

6.3. Centralized Path Computation and Installation

A centralized routing model, such as provided with a PCE, enables global and per-flow optimizations. The model is attractive but a number of issues are left to be solved. In particular:

6.4. Distributed Path Setup

Whether a distributed alternative without a PCE can be valuable could be studied as well. Such an alternative could for instance inherit from the Resource ReSerVation Protocol [RFC5127] (RSVP) flows. But the focus of the work should be to deliver the centralized approach first.

6.5. Duplicated data format

In some cases the duplication and elimination of packets over non-congruent paths is required to achieve a sufficiently high delivery ratio to meet application needs. In these cases, a small number of packet formats and supporting protocols are required (preferably, just one) to serialize the packets of a DetNet stream at one point in the network, replicate them at one or more points in the network, and discard duplicates at one or more other points in the network, including perhaps the destination host. Using an existing solution would be preferable to inventing a new one.

7. Security Considerations

Security in the context of Deterministic Networking has an added dimension; the time of delivery of a packet can be just as important as the contents of the packet, itself. A man-in-the-middle attack, for example, can impose, and then systematically adjust, additional delays into a link, and thus disrupt or subvert a real-time application without having to crack any encryption methods employed. See [RFC7384] for an exploration of this issue in a related context.

Typical control networks today rely on complete physical isolation to prevent rogue access to network resources. DetNet enables the virtualization of those networks over a converged IT/OT infrastructure. Doing so, DetNet introduces an additional risk that flows interact and interfere with one another as they share physical resources such as Ethernet trunks and radio spectrum. The requirement is that there is no possible data leak from and into a deterministic flow, and in a more general fashion there is no possible influence whatsoever from the outside on a deterministic flow. The expectation is that physical resources are effectively associated with a given flow at a given point of time. In that model, Time Sharing of physical resources becomes transparent to the individual flows which have no clue whether the resources are used by other flows at other times.

Security must cover:

8. IANA Considerations

This document does not require an action from IANA.

9. Acknowledgments

The authors wish to thank Jouni Korhonen, Erik Nordmark, George Swallow, Rudy Klecka, Anca Zamfir, David Black, Thomas Watteyne, Shitanshu Shah, Craig Gunther, Rodney Cummings, Wilfried Steiner, Marcel Kiessling, Karl Weber, Ethan Grossman, Patrick Wetterwald, Subha Dhesikan, Rudy Klecka and Pat Thaler for their various contribution to this work.

10. References

10.1. Normative References

[I-D.gunther-detnet-proaudio-req] Gunther, C. and E. Grossman, "Deterministic Networking Professional Audio Requirements", Internet-Draft draft-gunther-detnet-proaudio-req-01, March 2015.
[I-D.korhonen-detnet-telreq] Korhonen, J., "Deterministic networking for radio access networks", Internet-Draft draft-korhonen-detnet-telreq-00, May 2015.
[I-D.thubert-6tisch-4detnet] Thubert, P., "6TiSCH requirements for DetNet", Internet-Draft draft-thubert-6tisch-4detnet-01, June 2015.
[I-D.wetterwald-detnet-utilities-reqs] Wetterwald, P. and J. Raymond, "Deterministic Networking Uitilities requirements", Internet-Draft draft-wetterwald-detnet-utilities-reqs-02, June 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

10.2. Informative References

[AVnu], "The AVnu Alliance tests and certifies devices for interoperability, providing a simple and reliable networking solution for AV network implementation based on the IEEE Audio Video Bridging (AVB) and Time-Sensitive Networking (TSN) standards."
[CCAMP] IETF, "Common Control and Measurement Plane"
[EIP], "EtherNet/IP provides users with the network tools to deploy standard Ethernet technology (IEEE 802.3 combined with the TCP/IP Suite) for industrial automation applications while enabling Internet and enterprise connectivity data anytime, anywhere."
[HART], "Highway Addressable Remote Transducer, a group of specifications for industrial process and control devices administered by the HART Foundation"
[I-D.finn-detnet-architecture] Finn, N., Thubert, P. and M. Teener, "Deterministic Networking Architecture", Internet-Draft draft-finn-detnet-architecture-01, March 2015.
[I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-08, May 2015.
[I-D.ietf-roll-rpl-industrial-applicability] Phinney, T., Thubert, P. and R. Assimiti, "RPL applicability in industrial networks", Internet-Draft draft-ietf-roll-rpl-industrial-applicability-02, October 2013.
[I-D.svshah-tsvwg-deterministic-forwarding] Shah, S. and P. Thubert, "Deterministic Forwarding PHB", Internet-Draft draft-svshah-tsvwg-deterministic-forwarding-04, August 2015.
[IEC62439] IEC, "Industrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR) - IEC62439-3", 2012.
[IEEE802.1AS-2011] IEEE, "Timing and Synchronizations (IEEE 802.1AS-2011)", 2011.
[IEEE802.1BA-2011] IEEE, "AVB Systems (IEEE 802.1BA-2011)", 2011.
[IEEE802.1Q-2011] IEEE, "MAC Bridges and VLANs (IEEE 802.1Q-2011", 2011.
[IEEE802.1Qat-2010] IEEE, "Stream Reservation Protocol (IEEE 802.1Qat-2010)", 2010.
[IEEE802.1Qav] IEEE, "Forwarding and Queuing (IEEE 802.1Qav-2009)", 2009.
[IEEE802.1TSNTG] IEEE Standards Association, "IEEE 802.1 Time-Sensitive Networks Task Group", 2013.
[IEEE802154] IEEE standard for Information Technology, "IEEE std. 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks"
[IEEE802154e] IEEE standard for Information Technology, "IEEE std. 802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs) Amendment 1: MAC sublayer", April 2012.
[ISA100.11a] ISA/IEC, "ISA100.11a, Wireless Systems for Automation, also IEC 62734", 2011.
[ISA95] ANSI/ISA, "Enterprise-Control System Integration Part 1: Models and Terminology", 2000.
[MPLS] IETF, "Multiprotocol Label Switching"
[ODVA], "The organization that supports network technologies built on the Common Industrial Protocol (CIP) including EtherNet/IP."
[PCE] IETF, "Path Computation Element"
[Profinet], "PROFINET is a standard for industrial networking in automation."
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, DOI 10.17487/RFC2205, September 1997.
[RFC5127] Chan, K., Babiarz, J. and F. Baker, "Aggregation of Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127, February 2008.
[RFC5673] Pister, K., Thubert, P., Dwars, S. and T. Phinney, "Industrial Routing Requirements in Low-Power and Lossy Networks", RFC 5673, DOI 10.17487/RFC5673, October 2009.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, October 2014.
[RFC7426] Haleplidis, E., Pentikousis, K., Denazis, S., Hadi Salim, J., Meyer, D. and O. Koufopavlou, "Software-Defined Networking (SDN): Layers and Architecture Terminology", RFC 7426, DOI 10.17487/RFC7426, January 2015.
[RFC7554] Watteyne, T., Palattella, M. and L. Grieco, "Using IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the Internet of Things (IoT): Problem Statement", RFC 7554, DOI 10.17487/RFC7554, May 2015.
[TEAS] IETF, "Traffic Engineering Architecture and Signaling"
[TiSCH] IETF, "IPv6 over the TSCH mode over 802.15.4"
[WirelessHART], "Industrial Communication Networks - Wireless Communication Network and Communication Profiles - WirelessHART - IEC 62591", 2010.

Authors' Addresses

Norm Finn Cisco Systems 510 McCarthy Blvd SJ-24 Milpitas, California 95035 USA Phone: +1 408 526 4495 EMail:
Pascal Thubert Cisco Systems Village d'Entreprises Green Side 400, Avenue de Roumanille Batiment T3 Biot - Sophia Antipolis, 06410 FRANCE Phone: +33 4 97 23 26 34 EMail: