    Internet Engineering Task Force (IETF)                           N. Finn
    Request for Comments: 8557                   Huawei Technologies Co. Ltd
    Category: Informational                                       P. Thubert
    ISSN: 2070-1721                                                    Cisco
                                                                    May 2019

                   Deterministic Networking Problem Statement

    Abstract

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

    Status of This Memo

       This document is not an Internet Standards Track specification; it is
       published for informational purposes.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Not all documents
       approved by the IESG are candidates for any level of Internet
       Standard; see Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8557.

    Copyright Notice

       Copyright (c) 2019 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

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       described in the Simplified BSD License.

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    RFC 8557       Deterministic Networking Problem Statement       May 2019

    Table of Contents

       1. Introduction ....................................................2
       2. On Deterministic Networking .....................................4
       3. Problem Statement ...............................................6
          3.1. Supported Topologies .......................................6
          3.2. Flow Characterization ......................................6
          3.3. Centralized Path Computation and Installation ..............7
          3.4. Distributed Path Setup .....................................8
          3.5. Duplicated Data Format .....................................8
       4. Security Considerations .........................................9
       5. IANA Considerations .............................................9
       6. Informative References .........................................10
       Acknowledgments ...................................................11
       Authors' Addresses ................................................11

    1.  Introduction

       "Deterministic Networking Use Cases" [RFC8578] illustrates that
       beyond the classical case of Industrial Automation and Control
       Systems (IACSs) there are in fact multiple industries with strong,
       and relatively similar, needs for deterministic network services with
       latency guarantees and ultra-low packet loss.

       The generalization of the needs for more deterministic networks has
       led to the IEEE 802.1 Audio Video Bridging (AVB) Task Group becoming
       the Time-Sensitive Networking (TSN) [IEEE-802.1TSNTG] Task Group
       (TG), with a much-expanded constituency from the industrial and
       vehicular markets.

       Along with this expansion, the networks considered here are becoming
       larger and structured, requiring deterministic forwarding beyond the
       LAN boundaries.  For instance, an IACS segregates the network along
       the broad lines of the Purdue Enterprise Reference Architecture
       (PERA) [ISA95], typically using deterministic LANs for Purdue level 2
       control systems, whereas public infrastructures such as electricity
       automation require deterministic properties over the wide area.
       Implementers have come to realize that the convergence of IT and
       Operation Technology (OT) networks requires Layer 3, as well as
       Layer 2, capabilities.

       While the initial user base has focused almost entirely on Ethernet
       physical media and Ethernet-based bridging protocols from several
       Standards Development Organizations (SDOs), the need for Layer 3, as
       expressed above, must not be confined to Ethernet and Ethernet-like
       media.  While such media must be encompassed by any useful
       Deterministic Networking (DetNet) architecture, cooperation between
       the IETF and other SDOs must not be limited to the IEEE or the

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       IEEE 802 organizations.  Furthermore, while both completed and
       ongoing work in other SDOs, and in IEEE 802 in particular, provides
       an obvious starting point for a DetNet architecture, we must not
       assume that these other SDOs' work confines the space in which the
       DetNet architecture progresses.

       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 this
       determinism in IT implementations.  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.  At any rate, 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.  Note: "Deterministic Networking Architecture" [DetNet-Arch]
       was produced by the DetNet Working Group to describe that model.

       At the time of this writing, it is expected that

       o  once the abstract model is agreed upon, the IETF will specify
          (1) the signaling elements to be used to establish a path and
          (2) the tagging elements to be used to identify the flows that are
          to be forwarded along that path

       o  the IETF will specify the necessary protocols or protocol
          additions, based on relevant IETF technologies, to implement the
          selected model

       A desirable outcome of the work is the ability to establish a
       multi-hop path over the IP or MPLS network for a particular flow with
       given timing and precise throughput requirements and to carry this
       particular flow along the multi-hop path with such characteristics as
       low latency and ultra-low jitter, reordering and/or replication 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 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
       DetNet flows exceeding their contract may face packet
       declassification and drop.

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    2.  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, as well as 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 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 (1) enable the migration
       of applications with critical timing and reliability issues that
       currently use special-purpose fieldbus technologies (High-Definition
       Multimedia Interface (HDMI), Controller Area Network (CAN bus),
       PROFIBUS [PROFIBUS], etc. ... even RS-232!) to packet technologies in
       general and to IP in particular and (2) support both these new
       applications and existing packet network applications over the same
       physical network.  In other words, a deterministic network is
       backwards compatible with (capable of transporting) statistically
       multiplexed traffic while preserving the properties of the accepted
       deterministic flows.

       [RFC8578] indicates that applications in multiple fields need 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 deterministic packet flows that:

           *  Can be unicast or multicast.

           *  Need absolute guarantees of minimum and maximum latency
              end to end across the network; sometimes a tight jitter is
              required as well.

           *  Need a packet loss ratio beyond the classical range for a
              particular medium, in the range of 10^-9 to 10^-12 or better
              on Ethernet and on the order of 10^-5 in wireless sensor mesh
              networks.

           *  Can, in total, absorb more than half of the network's
              available bandwidth (that is, massive over-provisioning is
              ruled out as a solution).

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           *  Cannot suffer throttling, congestion feedback, or any other
              network-imposed transmission delay, although the flows can be
              meaningfully characterized by either (1) a fixed, repeating
              transmission schedule or (2) a maximum bandwidth and packet
              size.

       3.  Multiple methods for scheduling, shaping, limiting, and otherwise
           controlling the transmission of critical packets at each hop
           through the network data plane.

       4.  Robust defenses against misbehaving hosts, routers, or bridges,
           in both the data plane and the control plane, with guarantees
           that a critical flow within its guaranteed resources cannot be
           affected by other flows, whatever the pressures on the network.
           For more on the specific threats against DetNet, see
           "Deterministic Networking (DetNet) Security Considerations"
           [DetNet-Security].

       5.  One or more methods for reserving 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-1588], IEEE 802.1AS [IEEE-8021AS],
       and more.

       The needs related to multicast, latency, loss ratio, and throttling
       avoidance exist because the algorithms employed by the applications
       demand it.  They are not simply the transliteration of fieldbus needs
       to a packet-based fieldbus simulation; they also reflect fundamental
       mathematics of the control of a physical system.

       With classical forwarding of latency-sensitive 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 TSN TG has
       defined a new set of queuing, shaping, and scheduling algorithms that
       enable each bridge or router to compute the exact number of buffers
       to be allocated for each flow or class of flows.

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       Networking protocols commonly need robustness.  Note that robustness
       plays a particularly 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 unthrottled
       (though bandwidth-limited) flows means that bridges and routers have
       to dedicate buffer resources to specific flows or 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.

    3.  Problem Statement

    3.1.  Supported Topologies

       In some use cases, the end point that runs the application is
       involved in the Deterministic Networking operation -- for instance,
       by controlling certain aspects of its throughput, such as rate or
       precise time of emission.  In such a case, the deterministic path is
       end to end from application host to application host.

       On the other end, the deterministic portion of a path may be a tunnel
       between an ingress point and an egress router.  In any case, routers
       and switches in between should not need to be aware of whether the
       path is end to end or a tunnel.

       While it is clear that DetNet does not aim to set up deterministic
       paths over the global Internet, there is still a lack of clarity
       regarding the limits of a domain where a deterministic path can be
       set up.  These limits may depend on the technology that is used to
       set the path up, whether it is centralized or distributed.

    3.2.  Flow Characterization

       Deterministic forwarding can only apply to flows with such
       well-defined characteristics 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.

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    3.3.  Centralized Path Computation and Installation

       A centralized routing model, such as that provided with a Path
       Computation Element (PCE) (see [RFC4655]), enables global and
       per-flow optimizations.  This type of model is attractive, but a
       number of issues remain to be solved -- in particular:

       o  whether and how the path computation can be installed by

          *  an end device or

          *  a network management entity

          and

       o  how the path is set up -- either

          *  by installing state at each hop with a direct interaction
             between the forwarding device and the PCE or

          *  along a path by injecting a source-routed request at one end of
             the path, following classical Traffic Engineering (TE) models

       To enable a centralized model, DetNet should produce a description of
       the high-level interaction and data models to:

       o  report the topology and device capabilities to the central
          controller

       o  establish a direct interface between the centralized PCE and each
          device under its control in order to enable vertical signaling

       o  request a path setup for a new flow with particular
          characteristics over the service interface and control it through
          its life cycle

       o  provide support for life-cycle management for a path
          (instantiate/modify/update/delete)

       o  provide support for adaptability to cope with such various events
          as loss of a link

       o  expose the status of the path to the end devices (User-Network
          Interfaces (UNIs))

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       o  provide additional reliability through redundancy, particularly
          with Packet Replication, Elimination, and Ordering Functions
          (PREOF), where redundant paths may deliver packets out of order
          and PREOF may need to correct the ordering

       o  indicate the flows and packet sequences in-band with the flows.
          This is needed for flows that require PREOF in order to isolate
          duplicates and reorder packets at the end of the sequence

    3.4.  Distributed Path Setup

       Whether a distributed alternative without a PCE can be valuable could
       be studied as well.  Such an alternative could, for instance, build
       upon Resource Reservation Protocol - TE (RSVP-TE) flows [RFC3209].
       But the focus of the work should be to deliver the centralized
       approach first.

       To enable functionality similar to that of RSVP-TE, the following
       steps would take place:

       1.  Neighbors and their capabilities would be discovered and exposed
           to compute a path that would fit the DetNet constraints --
           typically those of latency, time precision, and resource
           availability.

       2.  A constrained path would be calculated with an improved version
           of Constrained Shortest Path First (CSPF) that is aware of
           DetNet.

       3.  The path may be installed using a control protocol such as
           RSVP-TE, extended to enable flow identification and install new
           per-hop behavior such as Packet Replication, Elimination, and
           Ordering, and to reserve physical resources for the flow.  In
           that case, traffic flows could be transported through an MPLS-TE
           tunnel, using the reserved resources for this flow at each hop.

    3.5.  Duplicated Data Format

       In some cases, the duplication and elimination of packets over
       non-congruent paths are 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 of each) 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.

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    4.  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 of
       flows interacting and interfering with one another as they share
       physical resources such as Ethernet trunks and the radio spectrum.
       The requirement is that there is no possible data leak from and into
       a deterministic flow.  Stated more generally, 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 in time.  In that model, the
       time-sharing of physical resources becomes transparent to the
       individual flows, as these flows have no clue regarding whether or
       not the resources are used by other flows at other times.

       The overall security of a deterministic system must cover:

       o  the protection of the signaling protocol

       o  the authentication and authorization of the controlling nodes,
          including plug-and-play participating end systems

       o  the identification and shaping of the flows

       o  the isolation of flows from leakage and other influences from any
          activity sharing physical resources

       The specific threats against DetNet are further discussed in
       [DetNet-Security].

    5.  IANA Considerations

       This document has no IANA actions.

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    6.  Informative References

       [DetNet-Arch]
                  Finn, N., Thubert, P., Varga, B., and J. Farkas,
                  "Deterministic Networking Architecture", Work in
                  Progress, draft-ietf-detnet-architecture-13, May 2019.

       [DetNet-Security]
                  Mizrahi, T., Grossman, E., Ed., Hacker, A., Das, S.,
                  Dowdell, J., Austad, H., Stanton, K., and N. Finn,
                  "Deterministic Networking (DetNet) Security
                  Considerations", Work in Progress,
                  draft-ietf-detnet-security-04, March 2019.

       [IEEE-1588]
                  IEEE, "IEEE Standard for a Precision Clock Synchronization
                  Protocol for Networked Measurement and Control Systems",
                  IEEE Standard 1588-2008, <https://standards.ieee.org/
                  findstds/standard/1588-2008.html>.

       [IEEE-802.1TSNTG]
                  IEEE Standards Association, "IEEE 802.1 Time-Sensitive
                  Networking Task Group",
                  <http://www.ieee802.org/1/pages/avbridges.html>.

       [IEEE-8021AS]
                  IEEE, "IEEE Standard for Local and Metropolitan Area
                  Networks - Timing and Synchronization for Time-Sensitive
                  Applications in Bridged Local Area Networks",
                  IEEE 802.1AS-2011,
                  <http://www.ieee802.org/1/pages/802.1as.html>.

       [ISA95]    ANSI/ISA, "Enterprise-Control System Integration - Part 1:
                  Models and Terminology", <https://www.isa.org/isa95/>.

       [PROFIBUS] IEC, "PROFIBUS Standard - DP Specification (IEC 61158
                  Type 3)", <https://www.profibus.com/>.

       [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
                  and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
                  Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
                  <https://www.rfc-editor.org/info/rfc3209>.

       [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                  Computation Element (PCE)-Based Architecture", RFC 4655,
                  DOI 10.17487/RFC4655, August 2006,
                  <https://www.rfc-editor.org/info/rfc4655>.

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       [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
                  Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
                  October 2014, <https://www.rfc-editor.org/info/rfc7384>.

       [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
                  RFC 8578, DOI 10.17487/RFC8578, May 2019,
                  <https://www.rfc-editor.org/info/rfc8578>.

    Acknowledgments

       The authors wish to thank Lou Berger, Pat Thaler, Jouni Korhonen,
       Janos Farkas, Stewart Bryant, Andrew Malis, Ethan Grossman, Patrick
       Wetterwald, Subha Dhesikan, Matthew Miller, Erik Nordmark, George
       Swallow, Rodney Cummings, Ines Robles, Shwetha Bhandari, Rudy Klecka,
       Anca Zamfir, David Black, Thomas Watteyne, Shitanshu Shah, Kiran
       Makhijani, Craig Gunther, Warren Kumari, Wilfried Steiner, Marcel
       Kiessling, Karl Weber, Alissa Cooper, and Benjamin Kaduk for their
       various contributions to this work.

    Authors' Addresses

       Norman Finn
       Huawei Technologies Co. Ltd
       3755 Avocado Blvd.
       PMB 436
       La Mesa, California  91941
       United States of America

       Phone: +1 925 980 6430
       Email: norman.finn@mail01.huawei.com

       Pascal Thubert
       Cisco Systems, Inc.
       Building D, 45 Allee des Ormes - BP1200
       Mougins - Sophia Antipolis  06254
       France

       Phone: +33 4 97 23 26 34
       Email: pthubert@cisco.com

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