| Internet-Draft | Deterministic Networking | April 2022 |
| Liu, et al. | Expires 13 October 2022 | [Page] |
Aiming at the large-scale deterministic network, this document describes the technical and operational requirements when the different deterministic levels of applications co-exist and are transported over a wide area. This document also describes the corresponding Deterministic Networking (DetNet) data plane enhancement requirements.¶
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Packet networks are evolving from bandwidth-guaranteed Quality of Service (QoS) to latency-guaranteed QoS that guarantees bounded latency and definite latency. Bounded latency and definite latency can be further understood as in-time delivery, in which a packet arrives without exceeding a predetermined time, and on-time delivery, in which a packet arrives at a predetermined time, respectively. In addition, network survivability, which typically guarantees traffic recovery within 50 ms in the event of a network failure, is evolving to a level that guarantees lossless recovery. In order to realize the evolution of QoS and network survivability of these networks, Time-Sensitive Networking (TSN) technology and Deterministic Networking (DetNet) technology are considered to be essential.¶
TSN is a set of standards developed by the IEEE 802.1 TSN Task Group (TG) [IEEE802.1TSN] and specifies mechanisms and protocols necessary to realize highly available IEEE 802.1 networks with bounded latency to carry time-sensitive, real-time application traffic.¶
DetNet, of which architecture is defined in RFC 8655 [RFC8655], provides a capability to carry specified unicast or multicast data flows for real-time applications with extremely low data loss rates and bounded latency within a network domain. The overall framework for DetNet data plane is provided in [RFC8938], and various documents on different data plane technologies and their interworking technologies to extend the service range of data that TSN intends to deliver to the IP (Internet Protocol) and MPLS (Multi-Protocol Label Switching) networks have been standardized.¶
Since TSN and DetNet were proposed, application use cases have always been one of the hottest topics. As documented in RFC 8578 [RFC8578], the scope of networks addressed by the current DetNet is limited to networks that can be centrally controlled, i.e., an "enterprise" (aka "corporate") network, excluding "the open Internet," explicitly. After years of development, TSN has been used in several industries, and has enough public awareness of the industry for its scope. DetNet also has done a lot of work and the standards are mature, and people become concerned about how to meet deterministic service demand in large-scale networks. The current DetNet is limited to a single administrative domain network, and there are technical elements necessary for application to a large-scale network spanning multiple domains.¶
This document describes requirements for large-scale deterministic networks where different deterministic levels of applications co- exist and large-scale deterministic networking across multiple administrative domains is possible. This document also describes the requirements for enhancing the DetNet data plane defined prior to this document.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
While [RFC2119] and [RFC8174] describe interpretations of these key words in terms of protocol specifications and implementations, they are used in this document to describe technical and operational requirements to realize large-scale deterministic networks.¶
When deterministic network services are introduced, network providers always face the problem of how to match application needs to the technology, so more works are needed for network service providers to successfully sell DetNet type services to customers. The providers are in need of the following:¶
Service level objective definitions, considering absolute or relative latency and jitter bounds, flows types and physical network scale¶
Suitable queuing mechanisms, considering more options for queuing mechanisms for different service level, and¶
Deployment strategies, considering how to integrate into existing networks, service, and control plane.¶
[RFC8578]provides various use cases and their requirements in the areas of industry, electricity, buildings, etc. Some of them clearly specify the requirements for latency and jitter, while some others do not for the jitter. Different types of users have different demands, just as a network provider provides different network services for personal business or enterprise business.¶
One kind has critical SLA requirement, such as remote control or cloud Programmable Logic Controller (PLC) of manufacturing and differential protection of electricity. If these services exceed the boundaries of latency and jitter, it will bring property losses and security risks, so they cannot tolerate with any non-deterministic situation and can pay more on the network service.¶
Another kind has relatively losse levels of SLA requirement, such as cloud gaming, cloud VR and online meeting for "consumer" networks. The users of these applications hope to have a better network experience, but they can tolerate it to a certain extent. If the network quality is not good sometime, they might be willing to spend more money for high-quality network services. In some aspects, because such services have no industry barriers and can tolerate exceeding the upper boundary of latency within a small probability, they have relatively lower requirements for the network and may be easier to deploy.¶
Different application demands are actually related to cost. For strict deterministic services, strict technologies need to be used, and all network devices may need to be upgraded. For non-strict deterministic services, it may only be necessary to upgrade some network devices (maybe edge nodes) or share corresponding network resources. From the perspective of deployment, it is helpful if there is a clear classification of application demands, including latency, jitter, reliability, etc. In this way, the corresponding technology to implement could be chosen, taking into account both performance and cost, but how to make choice is not within the scope of this document.¶
Critical latency requirements:
| <->| Industrial, tight jitter, hard latency limit
|<------->| Industrial, hard latency limit
|
|<-------------.....> Relatively lower latency requirements
|
|<-------------........................> Best effort
|
+---------------------------------------------------------->
latency
Due to the different kinds of application requirements in large-scale networks, the corresponding technical requirements should be considered.¶
A large-scale network may span over multiple networks with one or more administrative domains. One of DetNet's objectives is to stitch TSN islands together. All devices inside a TSN domain are time- synchronized, and most of TSN technologies rely on precise time synchronization[IEEE802.1Qbv][IEEE802.1Qch][IEEE802.1Qav]However, different TSN islands may have different clocks which are not synchronized as shown in Figure 2, where the time difference of two TSN domains is D. DetNet needs to connect these two TSN domains together and provide end-to-end deterministic latency service. The mechanism adopted by a large-scale deterministic network MUST support the interaction across time domains, so that time domains are synchronized. This can be done, for example, by putting extra buffer space at the ingress of a new domain, increasing the dead time as a guard band, or using some timing compensation mechanism. This document does not intend to list all the potential ways.¶
+--------------+ +--------------+
| | DetNet Connection | |
| TSN Domain I +-----------------------------+ TSN Domain II|
| | | |
+--------------+ +--------------+
| | | | |
Clock of TSN +--------+--------+--------+--------+
Domain I =
=
= | | | | |
Clock of TSN = +--------+--------+--------+--------+
Domain II = =
=<==D==>=
= =
Within a single time synchronization domain, different clock accuracy is expected, for example the crystal oscillator in Ethernet is specified at 100 ppm[Fast-Ethernet-MII-clock], Synchronous Ethernet (SyncE) can achieve 50 ppb[G.8262], and more precise time synchronization[G.8273] is expected in 5G mobile backhaul. The clocks experience different jitter and wander. It may cause different level of asymmetry of the path. The large-scale networks SHOULD be able to recover or absorb such time variance within a domain and across multiple domains.¶
Some networks like mobile backhaul use frequency synchronization, such as SyncE, instead of the strict time synchronization. It is usually hard to achieve the full time synchronization in large-scale networks when considering the size of the network topology. It is desired that the same deterministic performance in term of the bounded latency and jitter SHOULD be achieved when full time synchronization is not available, that is to say, when only partial synchronization (SyncE is one of the examples) is in use.¶
There are a large number of traffic flows in a large-scale network and some of them are acyclic. Asynchronization based methods can meet the requirements of those traffic flows. Moreover, The mechanisms not requiring the time and/or frequency synchronization eliminate the hardware cost and difficulty at the network nodes.[IEEE802.1Qcr] conceptually uses per-flow based asynchronous shaper to achieve bounded latency. The formula proof shows its effectiveness. It can naturally tolerate the time variance, but it exhibits the concerns of per-flow state buffer management as shown in[I-D.eckert-detnet-bounded-latency-problems]When it is in use, the requirement in Section 4.3 SHOULD be carefully met.¶
In a large-scale network, a single hop distance is enough to generate large latency. The speed of optical transmission in fiber is 200 km/ ms. Thus, the propagation delay of a single hop can be in the order of a few milliseconds. It is much greater than that of a LAN, and introduces impacts on queuing mechanisms, such as cyclic or time aware scheduling method.¶
For a cyclic based method, suppose a large-scale network wants to keep using the simple cycle mapping relationship, however the link distance between two nodes is longer. Moreover, a downstream node may have many upstream nodes each with different link propagation delays (e.g., 9 us, 10 us, 11 us, 15 us and 20 us). In order to absorb the longest link propagation delay, the length of cycle must be set to at least 20 us. However, since packet's arrival time varies within the receiving cycle, larger cycle length means larger delay variance.¶
Upstream Node X |sending cycle | |
+--"------------+------------+
= "\ = =
= " \ = =
= " \ = =
= " \ = =
= " V = =
Downstream Node Y |receiving cycle| |
+--"----"-------+----\-------+
= " " = \ =
= " " = V resent out
= " " = =
Time Line -=--"----"-------=------------=----->
(in us) 0 | | 10 20
v v
Transmission Latency
A large-scale network normally uses higher speed links, especially for its backbone. Current deterministic mechanisms used in a local network is usually deployed in link speed of 10 Mbps or 1 Gbps, or possibly 10 Gbps. The data rate of 10G, 100G, 400G and even higher is commonly used in wide area networks. With the increasing of the data rate, the network scheduling cycle can be reduced if the same amount of the data is required to be sent each cycle for each application. Or more data can be sent if the network cycle time remains the same. For the former, it requires the more precise time control (e.g. cycle in the order of a few microseconds or sub- microseconds) for the input stream gate and the timed output buffer. For the latter, more buffer space is required which imposes more complex buffer or queue management and larger memory consumption.¶
Another aspect to consider is the aggregation of the flows. In the large-scale network, the number of flows can be hundreds or tens of thousands. They can be aggregated into a small number of deterministic path or tunnels. It is practical to have a few flow- based or aggregated-flow based status in the local network. But in higher speed and larger scale networks, it is hardly feasible. If[IEEE802.1Qcr]is in use, it requires more buffers comparing to the other full/partial time synchronized mechanisms. Therefore, it requires optimizations to support higher link speeds.¶
Comparing to a LAN, a large-scale network may have more network devices and traffic flows, and there is a greater possibility of adding or removing network devices and traffic flows. The deterministic latency forwarding mechanisms MUST scale to networks of significant size with numerous network devices and a massive traffic flows.¶
The increase or decrease of network devices in large-scale networks is more frequent than that in LANs. The change of the number of devices may affect the implementation and adjustment of deterministic network mechanism, such as the topology discovery, queuing mechanism and packet replication and elimination. A simple use case to understand is ultra-low-latency (public) 5G transport networks, which would require DetNet extend to every 5G base station. For some network operators, their networks may need to connect to ~100 K base stations (serving multiple mobile networks operators), and this number will only increase with 5G.¶
It is almost impossible to identify individual IP flows at the DetNet data plane because of the large overhead and resource reservation for a massive number of flows. DetNet allows the leverage of the flow aggregation. With the large scaling of the network, proper provision at the control plane to accommodate such higher aggregation is required. Individual flows may join and exit the aggregated flow rapidly which causes the dynamic in identification of the aggregated DetNet flow. The wildcards and value ranges used in the identification may have to change in order to ensure the aggregated flows have compatible deterministic characteristics.¶
The micro-burst will happen more often due to the massive traffic flows, so some methods to decrease it are needed.[I-D.du-detnet-layer3-low-latency]introduces a reference method requiring a scalable buffer to adjust the speed of sending the packets, so as to keep a uniform transmission rate, and it also support the flow aggregation.¶
Network link failures are more common in large-scale networks. Path switching or re-convergence of routing will cause high latency of packet loss and retransmission, which is usually in seconds before the network becomes stable again. It is necessary to support certain mechanisms to adapt to failures of links or nodes and topology changes.¶
The change of path or topology poses a higher challenge to packet replication and elimination. The full disjoint paths when implementing the Packet Replication, Elimination, and Ordering Functions (PREOF) gives a better chance of survival when one of the nodes or links in the path fails. At the same time, it brings the challenges of finding paths with similar distance and/or number of hops so that there is enough buffer space to absorb the latency difference caused by different paths when the scale is large.¶
It is required to provide diversified deterministic service for various applications in a large-scale network and to support the corresponding diversified queueing mechanisms (possibly at multiple DetNet QoS levels). Different queueing mechanisms can provide different levels of latency, jitter and other guarantees, and there may be situations where a network device provides multiple queueing mechanisms at the same time. For example, a network aggregation device may use the mechanisms specified in [IEEE802.1Qbv] and [IEEE802.1Qcr], and other mechanisms to forward traffic to different paths at the same time. By providing a variety of queueing mechanisms to meet diversified deterministic service Requirements, compared with LAN environment, this demand is particularly prominent in large-scale networks. There are usually eight traffic classes in TSN enabled networks. The different queueing mechanisms can be employed to the queues of one or more of those traffic class. In practice, there may be more than eight queues or sub-queues to support more complicated queueing mechanisms.¶
Accordingly, the configuration for multiple queueing mechanisms is complicated in large-scale deterministic networks and MUST support the unified or simplified scheduling and management of multiple queue mechanisms. For example, in the distributed scenario where there is no controller, flooding the related information of the queue mechanism, including the types and related algorithms, queue forwarding capability, etc. In the centralized scenario, the queueing mechanisms and other information could be reported to the controller to build a deterministic network resource topology pool for path calculation.¶
In large-scale deterministic networks, it may across multiple network domains and adopt a variety of different queueing mechanisms within each domain. It is required to support the inter-domain deterministic mechanism at the inter-domain boundary nodes such as the priority redefinition and rescheduling of queues to achieve the end-to-end latency, bounded jitter and packet loss ratio.¶
Moreover, changing from one queueing mechanism to another may generate additional end-to-end latency and/or jitter which should be taken into consideration. For example, when a flow is forwarded across multiple network domains based on different queueing mechanisms, such as a time synchronous Qbv mechanism[IEEE802.1Qbv] and an asynchronous Qcr mechanism [IEEE802.1Qcr], a collaboration mechanism crossing multi-domains MUST be considered, such as increasing the buffer of inter-domain devices to provide enough adjustment space for the flow to cross different queueing mechanisms, so as to provide end-to-end deterministic services across multiple network domains.¶
According to[RFC8938], the DetNet data plane can provide or carry two metadata in MPLS and IP data planes: Flow-ID and sequence number. The Flow-ID could be used for identification of the DetNet flow or aggregate flow, and the sequence number could be used for PREOF for each DetNet flow. The Flow-ID is used by both the service and forwarding sub-layers, but the sequence number is only used by the service layer. Metadata can also be used for OAM indications and instrumentation of DetNet data plane operation.¶
Generally speaking, more data plane metadata and related processing SHOULD be supported in the large-scale networks. Native IPv6 data plane should be supported. This section lists the data plane enhancement requirements based on but not limited to the technical requirements in Section 4.¶
Current IPv6 aggregated flow identification is generally based on 5 or 6 tuples, IP prefixes, or wildcards as indicated in [RFC8938]. However, in large-scale deterministic networks the number of individual flows is huge, and they may randomly join and leave the aggregated flow at each hop. Such behaviours lead to the difficulty in identifying aggregated flows by relying on the prefixes or wildcards.¶
In addition, flow identification is also used to quickly push a packet to a suitable queue. In a large-scale network, there are mix of flows requiring deterministic latency service and normal forwarding service. Explicit flow identification makes it easier to quickly distinguish the DetNet flows without requiring the longest match rule on multiple tuples in IP data plane. Therefore, explicit aggregated flow identification SHOULD be supported.¶
According to Section 4.1, a large-scale network should support synchronized or asynchronized queuing mechanisms. Different queueing mechanisms require different metadata to be defined to help regulation and queue management. For instance, the data plane MUST support the identification of cycle for cyclic queuing or the timing related information for time based queuing.¶
Sequence number is the only metadata currently defined for redundancy feature of Detnet. MPLS data plane uses Detnet-over-MPLS label stack to carry it. At the same time, native IPv6 data plane should be able to carry this information too. If specific IP encapsulation or tunnel is in use, this meta data should be defined explicitly for that data plane.¶
Explicit route at the control plane and/or management is required so that the "best" path can be selected to meet the latency requirement for DetNet flows. At the data planes, MPLS label stack can be used for this purpose. IP data plane enhancement is required to support the explicit path selection based on IP source routing or SRv6.¶
This document specifies the technical requirements when ensuring the deterministic features in the large-scale networks, and the corresponding data plane enhancement requirements to support the them. Some of the proposed queueing mechanisms and trials are cited and the authors of the document think those proposals give reasonably sound insights to enhancement the current queueing mechanisms to meet the deterministic requirements of the large-scale networks.¶
There are no IANA actions required by this document.¶
This section will be described later.¶
The authors would like to thank Yaakov Stein for helpful suggestions. The authors also would like to thank Liang Geng, Peter Willis, Shunsuke Homma and Li Qiang for their previous works.¶
The following people have substantially contributed to this document:¶
Zongpeng Du
China Mobile
EMail: duzongpeng@chinamobile.com
¶
Some trials have been carried out to verify the concept of large-scale deterministic networks.¶
In order to verify the deterministic technology of large-scale networks, a trial of Deterministic IP on China Environment for Network Innovations (CENI), which is a network built for new network technology trial, was deployed. A network with a distance of 3,000 km over 13 hops was tested, and the jitter was controlled within 100us.¶
In order to verify the remote control on Deterministic IP, which required that the latency should be controlled within 4 ms and jitter should be controlled within 20 us. A trial cooperated with Baosteel spanned 600 km was deployed. Baosteel is a Chinese steel company and put forward this demand. Both of the first and second trials are based on a frequency synchronization solution. The mechanism details could be found in . [I-D.dang-queuing-with-multiple-cyclic-buffers][I-D.qiang-detnet-large-scale-detnet].¶
In order to realize multi flows synchronization on an inter- provincial network in an exhibition, Emergen proposed the requirement that two flows of video and virtual reality (VR) were sent from province A, and arrived at province B together, so people can see the synchronization of video collected by camera and the VR model. This requirement was proposed to facilitate the virtual industry product deployment. Due to time and other problems, it was realized by the edge network device for a relatively lower levels of service level agreement (SLA).¶
Teaming up with a smart factory operator, network operators, equipment companies, and universities, ETRI demonstrated an ultra-low latency, high-reliability 5G wired and wireless network-based remote industrial Internet of Things (IIoT) service by connecting a control center and a smart factory through three different operators' networks at a distance of 280 km. In this trail, it was demonstrated that real-time remote smart manufacturing service is possible by making round-trip delay below 3 ms within a smart factory and below 10 ms between remote 5G industrial devices. In the future, the team plans to examine feasibility of large-scale deterministic networking by connecting smart factories in Gyeongsan, South Korea and Oulu, Finland.¶
These trials show that both operators and enterprise users begin to put forward requirements for the certainty of large-scale networks, but the implementation technologies are not exactly the same.¶