DetNet Enhancements for Large-Scale Deterministic Networks

4.1.1. Support the Differentiated DetNet QoS of Multiple Services

As defined in [RFC8655], the DetNet QoS can be expressed in terms of : Minimum and maximum end-to-end latency, bounded jitter (packet delay variation), packet loss ratio and an upper bound on out-of-order packet delivery. As described in [RFC8578], DetNet applications differ in their network topologies and specific desired behavior and different services requires differentiated DetNet QoS. In the large-scale networks, multiple services with differentiated DetNet QoS is co-existed in the same DetNet network. The classification of the deterministic flows within different levels is should be taken into considerations. It is required to provide Latency, bounded jitter and packet loss dynamically and flexibly in all scenarios for each characterized flow.¶

As the Figure 1 shows, the services can be divided into 5 levels and level 2~5 is the DetNet flows and level-1 is non-DetNet flow. DetNet applications and DetNet QoS is differentiated within each level.¶

   +-------------+-----------+----------+----------+----------+-----------+
   | Item        | Level-1   | Level-2  | Level-3  | Level-4  |  Level-5  |
   +-------------+-----------+----------+----------+----------+-----------+
   | Applications| Broadcast |  Voice   | Audio and| AR/VR    | Industrial|
   | Examples    |           |          | Video    |          |           |
   +-------------+-----------+----------+----------+----------+-----------+
   | DetNet QoS  | Bandwidth | Jitter   | Latency  | Low      | Ultra-low |
   |             | Guarantee | Guarantee| Guarantee| latency  |latency and|
   |             |           |          |          |and jitter| jitter    |
   +-------------+-----------+----------+----------+----------+-----------+

From the perspective of deterministic service requirements, deterministic Quality of Service (QoS) in the network can be divided into five types or levels:¶

Level-1: bandwidth guarantee. The indicator requirements include basic bandwidth guarantee and certain packet loss tolerance. There is no requirement for the upper bound of the latency, and no requirement for the jitter. Typical services include download and FTP services.¶

Level-2: jitter guarantee. The indicator requirements include: jitter 50ms, delay 300ms. Typical services include synchronous voice services, such as voice call.¶

Level-3: Latency guarantee. The indicator requirements include: delay 50ms, jitter 50ms. Typical services include real-time communication services, such as video, production monitoring, and communication services.¶

Level-4: low delay and low jitter guarantee. The indicator requirements include: delay 20ms, jitter 5ms. Typical services include video interaction services, such as AR/VR, holographic communication, cloud video and cloud games.¶

Level-5: ultra-low delay and jitter guarantee. The indicator requirements include: delay 10ms, jitter 100us. Typical services include production control services, such as power protection and remote control.¶

Moreover, different DetNet services is required to tolerate different percentage of packet loss ratio such as 99.9%, 99.99%, 99.999%, and so on. It is also required to provide service isolation. In some scenarios, such as intelligent electrical network, the isolation requirements are very important. For example, the automatic operation or control of a process or isochronous data and service with different priorities need to meet the requirements of hard isolation. In addition to the requirements of delay and jitter, the differential protection (DP) service needs to be isolated from other services and hard isolated tunnel is required.¶

4.1.2. Guarantees of Multiple Dynamic Deterministic Flows

As described in [RFC8557], deterministic forwarding can only apply to flows with such well-defined characteristics as periodicity and burstiness. As defined in DetNet architecture [RFC8655], the traffic characteristics of an App-flow can be CBR (constant bit rate) or VBR (variable bit rate) of L1, L2 and L3 layers (VBR takes the maximum value when reserving resources). But the current scenarios and technical solutions only consider CBR flow, without considering the coexistence of VBR and CBR, the burst and aperiodicity of flows. The operations such as shaping or scheduling have not been specified. Even TSN mechanisms are based on a constant and forecastable traffic characteristics.¶

It will be more complicated in WAN applications where much more flows coexist and the traffic characteristics is more dynamic. A huge number of flows with different DetNet QoS requirements is dynamically concurrent and the state of each flow cannot be maintained. It is required to offer reliable delivery and SLA guarantee for dynamic flows. For example, periodic flow and aperiodic flow (including micro burst flow, etc.), CBR and VBR flow, flow with different periods or phases, etc. When the network needs to forward these deterministic flows at the same time, it must solve the problems of time micro bursts, queue processing and aggregation of multiple flows. It is required to guarantee the deterministic QoS of multiple dynamic flows. Flow shaping and concurrent and micro-burst control should be provided.¶

4.2.1. Support the Distributed Deterministic Routes

In large-scale deterministic networks, the distributed scenario with no controller should be taken into consideration. It is required to support the distributed deterministic routes which are established by distributed protocols such as IGP.¶

4.2.2. Support the Inter-domain Deterministic Routes

In large-scale deterministic networks, it may across multiple network domains, it is required to support the inter-domain deterministic routes to achieve the end-to-end latency, bounded jitter. And the deadline of latency and jitter of each domain and segment should be determined and controlled. The inter-domain mechanism MUST be considered at the boundary nodes such as BGP configurations.¶

4.2.3. Support the Replication and Elimination Deterministic Routes

As described in [RFC8557], the packet replication and elimination service protection should be provided to achieve the low packet loss ratio. It will copy the flows and spread the data over multiple disjoint forwarding paths. The bounded latency and jitter of each path should be meet service deterministic requirement. And the difference of latency within these paths should be limited. So the replication and elimination deterministic routes with configured latency and jitter policy should be supported.¶

4.3.1. Management and Scheduling of the Network Resources

Traditional Ethernet, IP and MPLS networks which is based on statistical multiplexing provides best-effort packet service and offers no delivery and SLA guarantee. As described in [RFC8655], the primary technique by which DetNet achieves its QoS is to allocate sufficient resources. But it can not be achieved by not sufficient resource which can be allocated due to practical and cost reason. So it is required to achieve the high-efficiency of resources utilization when provide the DetNet service.¶

Network resources include nodes, links, ports, bandwidth, queues, etc. The congestion control, shaping and queue scheduling and other traffic mechanisms which have been proposed in IEEE 802.1 TSN such as IEEE802.1Qbv, IEEE802.1Qch, IEEE802.1Qav, IEEE802.1Qcr and so on.¶

Resource classification and modeling is required along with the explicit path with more SLA guarantee parameters like bandwidth, latency, jitter, packet loss and so on.¶

4.3.2. Support the Utilization of Heterogeneous Resources

In large-scale application, a large-scale number of nodes and long-distance transmission in the network will lead to latency and jitter, such as increasing transmission latency, jitter and packet loss. It is required to reduce the scale of the network topology by establishing cut-through channels. The existing technologies such as FlexE and SR tunnels should be taken into consideration. And multiple capabilities is also provided by the nodes and links within the network topology such as FlexE tunnels, TSN sub-network and IP/MPLS/SRv6 tunnels. It is required to integrate the multi-capability resources to achieve the optimal DetNet QoS.¶

Heterogeneous resource should be used and unified and simplified resources mechanism under the selection of existing multiple technical methods to realize the elastic of deterministic capability.¶

5.2.1. Deterministic Resources

Differentiated deterministic service requirements require the networks to provide different deterministic capabilities. The resources related to deterministic capabilities are also differentiated. The networks need to shield the differences between network capabilities. Deterministic resource is the basis for providing deterministic network services. It refers to the resources that meet the deterministic indicators of a node and link processing as well as the corresponding resource processing mechanisms (such as link bandwidth, queues, and scheduling algorithms). It is necessary to make overall resource planning for the network and make unified modeling for heterogeneous deterministic resources to form unified deterministic links to provide guarantee for the deterministic forwarding capabilities at different levels. A deterministic link can be a sub-network that provides deterministic transmission or a Point-to-Point (P2P) link. When the existing resources in the network are insufficient to meet the SLA requirements, virtual networks need to be reconstructed.¶

5.2.2. Deterministic Routes

To meet the requirements of different types and levels of deterministic services, deterministic route is to create deterministic routes with different SLA levels based on the deterministic link resources after unified modeling.¶

Deterministic routes can be based on strict explicit paths or loose routes. The former is applicable to centralized scenarios with controllers, and the latter is applicable to distributed scenarios without controllers. In the centralized scenario, when the source and sink PEs of a deterministic service are located at the two ends of a WAN with a limited physical range, one controller (single domain) or multiple controllers (cross domain) compute one or more paths with deterministic SLA in advance according to the typical Traffic Specification (T-SPEC) based on the collected deterministic resources, or compute dynamically according to the service T-SPEC as required by the services. It is suggested to generate two non-intersecting paths with very close delay to form 1+1 protection and perform concurrent transmission and dual reception, and make replication and elimination on the egress PE. In the distributed scenario, intrinsic deterministic loose routes are computed on the device side through routing protocols. Interior Gateway Protocol (IGP) is used to compute deterministic routes based on deterministic-delay inside a domain, and Border Gateway Protocol (BGP) is used to compute deterministic routes based on accurate delay/jitter across domains.¶

5.2.3. Deterministic Services

Deterministic services provide unified planning and scheduling mechanisms for service flows and perform end-to-end delay and jitter control. It is necessary to implement admission control and traffic policing at the ingress PE node based on the SLA of deterministic service flows, and map the service flows to deterministic routes to achieve the final goal of deterministic QoS.¶

Deterministic services support that the end-to-end delay/jitter of the traffic with a specific T-SPEC in the network will be strictly limited within a bounded range on the basis of deterministic resource and route . As different service levels have different requirements for delay and jitter, the resources and routing mechanisms used for mapping services to deterministic routes are also different. For example, the extremely low delay and jitter can be guaranteed by multiplexing the rigid pipes at L1, so as to avoid the excessive intra-node delay contributed by too many hops of intermediate nodes at L3. Or in the customized virtual network, the bounded delay and jitter can be guaranteed by forwarding along the paths composed of links based on the ATS or CQF scheduling algorithm. Traffic policing on the ingress PE ensures that the service traffic does not exceed the reserved bandwidth, and then performs traffic shaping on the egress node. Different scheduling algorithms have different shaping effects.¶