Time-Sensitive Networking’s Journey from Layer 2 to Layer 3



Why a completely new model is needed to integrate determinism into routed networks.

Introduction
In the last decade, Ethernet has made its case as the communication medium for industrial and automotive networks. The applications in these types of networks require time-critical delivery of traffic —not what Ethernet was originally designed for. A new set of technologies has evolved to ensure existing applications run on new Ethernet-based communication systems. New technologies like audio/video bridging (AVB) and time-sensitive networking (TSN) have matured and secured their application in the car network. Likewise, as these technologies matured, they became more widely used in factories’ industrial floor- and control-networks.

TSN handles three basic traffic types found in automotive or industrial networks:

  • Scheduled Traffic such as hard-real-time message or control messages that are periodic in nature—an example can be the control messages coming from a sensor in an Advanced Driver Assistance System (ADAS).
  • Best Effort Traffic, i.e. traffic that is not sensitive to any other Quality of Service metrics, for example the analytics data uploaded from a factory network.
  • Reserved Traffic for frames allocated in different time slots but with a specified bandwidth reservation for each priority type, such as audio or video traffic for a car infotainment system or a professional broadcasting network.

To handle workloads such as those just cited, the IEEE 802.1TSN working group defined two pillars of traffic shaping: the time-aware shaper (IEEE 802.1Qbv) and the credit-based shaper (IEEE 802.1Qav).

The time aware shaper defines the transmission path into fixed length, repeating time cycles. These cycles are divided into time slots according to the TSN configuration agreed between the talkers, forwarders, and listeners. The different time slots can be configured and assigned to one or more of the eight Ethernet priorities. This design provides dedicated time slots for time-critical traffic to flow through without being contested by other lower priority traffic. The reserved traffic and the best effort traffic can be assigned to other time slots not occupied by the hard-real-time traffic. The credit-based shaper assigns the reserved traffic to a higher priority position than the best-effort traffic.

Extending TSN to Layer 3 Networks
As adopting Ethernet for closed and small-sized networks has proven successful, increasing interest is surfacing from multiple industries with relatively similar needs for latency guarantees and ultra-low packet loss. The new ecosystem of traffic shaping and time synchronization standards under the IEEE 802.1TSN working group focuses on providing the determinism in the Ethernet-based bridged networks which are within the boundary of a single LAN segment or broadcast domain. The emerging application areas will require latency guarantees over a routed network connecting several geographical locations, i.e. across multiple LAN segments. Applications include professional audio/video, electrical utilities, building automation systems, wireless for industrial communication, 5G fronthaul, machine-to-machine communication, mining, private blockchain, and 5G network slicing. The IETF Deterministic Networking (DetNet) working group is focused on identifying use cases, defining the problem, and finding solutions, working in collaboration with the IEEE 802.1TSN working group.

Blockchain and M2M Examples
Blockchain is a digitized and decentralized method of storing data in public networks. In a blockchain consensus process, communication happens between all the blocks which are stored in different nodes separated by a public network. In general, these nodes are connected by L2 or L3 VPN connections today. The network treats these blockchain consensus messages as best effort because of the inherent nature of Ethernet traffic. This whole process could be made more efficient with deterministic and low-latency behavior.

Taking another example from the industrial machine-to-machine communication space, there can be multiple machine sections working together to comprise a single machine. Figure 1 describes a representative architecture of an industrial control application and associated network design (Referring from Industrial theory of Operation by Avnu Alliance). In this system, a single machine, which consists of four different sections, controls an industrial process. A manufacturing site could include multiple machines like this. Each section of a machine is a subnet with a unique VLAN, and they are connected through an L3 network. The whole system is synchronized and coordinated to produce the final product. In this type of a scenario, there are existing proprietary technologies, TSN profiles within the L2 bridges, and the DetNet profile within the L3 router working in tandem.

A Probable Solution Space for Layer 3 Deterministic Networks
There are challenges with respect to using the 802.1TSN techniques in the routed domains. Some of the fundamental TSN technologies, such as the time-aware shaper, require re-computation of the entire path whenever a new flow is added or an existing flow is modified. This type of approach may not be suitable for a larger-scale routed network.

On the other hand, a geographically separated application, a professional broadcasting network say, requires the Internet as a L3 routed network to serve as a connectivity medium between sites. There are a great number of heterogeneous devices in a large-scale network such as the Internet. It is difficult and costly to keep precise time synchronization among all these devices. In the absence of a universal sense of a clock it’s not possible for the network to use mechanisms such as scheduled traffic. Therefore, a completely new model is needed to integrate determinism into the routed networks.

There are different points of views which are in discussion between the IETF DetNet working group members. Some of the basic building blocks for providing deterministic networking in a routed network requires:

  • A global view of the network for assigning appropriate path for a flow
  • An end-to-end connectivity mechanism
  • Ability to assign QoS for individual flows and redundant paths.

A Network Management Entity ( ME) role may be important for managing the timeslot for data transmission and device resources like available bandwidth. A Path Computation Element (PCE) function can compute and assign an end-to-end path for the TSN flows. For the end-to-end path, MPLS Pseudowire is an established technology which can provide connectivity across L2 and L3 domains. Figure 2 shows an example modelling of a time-critical flow, which originates from a bridged TSN domain 1, crosses a public routed network and terminates in another bridged TSN domain 2. In this example, the time-critical flow carries its TSN metadata for the bridged domain in the “TSN Encap” header. In the routed domain, the flow is encapsulated with a MPLS Pseudowire label stack for reachability to the TSN domain 2.

Figure 2: Modeling a time-critical flow

Conclusion
Connecting time-critical components over a public network is an extremely complex technology and getting things to work at scale is a long shot. The interplay of various types of traffic adds to the complexity. From ixia’s (Now part of Keysight) huge experience of validating the service provider networks, data centers, and other types of networks we have learned how even a very robustly designed system can fail under stress conditions and negative scenarios. The designer of deterministic networks will have to keep these aspects in mind. The outlook is promising, but implementers face a huge challenge to cover every corner case. Therefore, a robust validation strategy will be key.


Avik Bhattacharya is the Product Manager for the Automotive and Industrial Ethernet validation portfolio in Ixia Solution Group now part of Keysight. He has more than 12 years’ experience working on cutting-edge networking technologies.

 

 

 

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