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.
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RFC 8557 Deterministic Networking Problem Statement May 2019
"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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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.
* 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.
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.
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
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.
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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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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[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-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/>.
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