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.
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Finn & Thubert Informational [Page 1]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.
Finn & Thubert Informational [Page 3]RFC 8557 Deterministic Networking Problem Statement May 2019
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).
Finn & Thubert Informational [Page 4]RFC 8557 Deterministic Networking Problem Statement May 2019
* 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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RFC 8557 Deterministic Networking Problem Statement May 2019
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>.
Finn & Thubert Informational [Page 10]RFC 8557 Deterministic Networking Problem Statement May 2019
[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
Finn & Thubert Informational [Page 11]
