Network Working Group Z. Du
Internet-Draft P. Liu
Intended status: Informational China Mobile
Expires: August 26, 2021 February 22, 2021
Micro-burst Decreasing in Layer3 Network for Low-Latency Traffic
draft-du-detnet-layer3-low-latency-02
Abstract
This document introduces the problem of micro-bursts in layer3
network, and proposed a method to decrease the micro-bursts in layer3
network for low-latency traffic.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
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This Internet-Draft will expire on August 26, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Gaps for Large-scale Layer 3 Deterministic Network . . . . . 3
3. Rethinking the Problem in IP Forwarding . . . . . . . . . . . 3
4. Method to Decrease Micro-bursts . . . . . . . . . . . . . . . 5
4.1. Working Flow of the Method . . . . . . . . . . . . . . . 5
4.2. Process of Edge Node . . . . . . . . . . . . . . . . . . 5
4.3. Process of Forwarding Node . . . . . . . . . . . . . . . 6
5. Analysis of the Proposed Method . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The DetNet architecture in RFC 8655 [RFC8655] is supposed to work in
campus-wide networks and private WANs, including the large-scale ISP
network scenario, such as the 5G bearing network mentioned in RFC
8578 [RFC8578]. It is essential for the large-scale ISP network to
be able to provide the low-latency service. The low-latency
requirement exists in both L2 and L3 networks, and in both small and
large networks.
However, as talked in [I-D.qiang-detnet-large-scale-detnet],
deploying deterministic services in a large-scale network brings a
lot of new challenges. A novel method called LDN (Large-scale
Deterministic Network) is introduced in
[I-D.qiang-detnet-large-scale-detnet], which explores the
deterministic forwarding over a large-scale network.
This document also explores the deterministic service in the large-
scale layer 3 network, and proposed a method based on micro-burst
decreasing, which can benefit the forwarding of low-latency traffic
in a large-scale network.
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2. Gaps for Large-scale Layer 3 Deterministic Network
According to RFC 8655 [RFC8655], DetNet operates at the IP layer and
delivers service over lower-layer technologies such as MPLS and IEEE
802.1 Time-Sensitive Networking (TSN). However, the TSN mechanisms
are designed for L2 network originally, and cannot be directly used
in the large-scale layer 3 network because of various reasons. Some
of them are described as below.
Some TSN mechanisms need synchronization of the network equipments,
which is easier in a small network, but hard in a large network. It
brings in some complex maintenance jobs across a large distance that
are not needed before.
Some TSN mechanisms need a per-flow state in the forwarding plane,
which is un-scalable. Aggregation methods need to be considered.
Some TSN mechanisms need a constant and forecastable traffic
characteristics, which is more complicated in a large network which
includes much more flows joining in or leaving randomly and the
traffic characteristics are more dynamic.
The main aspects of the problems are the simplicity and the
scalability. The former can ensure that the mechanism is easy to
deploy, and the second can ensure that the mechanism is able to bear
a large number of deterministic services.
3. Rethinking the Problem in IP Forwarding
As a comparison, the current IP forwarding mechanism is considered to
be a good example fulfilling the requirements of simplicity and
scalability. However, traditional IP network is based on statistical
multiplexing, and can only provide Best Effort service, short of SLA
guaranteed mechanisms.
When we rethink the problem in the current IP forwarding mechanism,
we can find that in the current IP network, a long delay in queuing,
or some packet losses due to burst are acceptable; however, it is
unacceptable in the deterministic forwarding. Therefore, they have
different design principles in a low layer.
The current forwarding mechanism in an IP router, which is based on
statistical multiplexing, cannot provide the deterministic service
because of various reasons. Even be given a high priority, a
deterministic packet can experience a long congestion delay or be
lost in a relatively light-loaded network, which is caused by micro-
burst in the network.
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Micro-burst is a special case of network congestion, which typically
lasts a short period, at the granularity of millisecond. In a micro-
burst, a lot of data are received on the interface suddenly, and the
temporary bandwidth requirement would be tens of or hundreds of the
average bandwidth requirement, or even exceed the interface
bandwidth.
In most cases, the buffer on the equipment can handle the micro-
bursts. However, in some corner cases, micro-bursts bring in a long
delay (at the granularity of millisecond) or even packet loss.
The following paragraphs introduce the causes of the micro-burst.
Firstly, IP traffic has a instinct of burstiness no matter in the
macro or micro aspect, i.e., it does not have a constant traffic
model even after aggregations.
Secondly, IP network has a flexible topology, where the incoming
traffic may exceed the bandwidth of the outgoing interface. For
example, an interface with a large bandwidth may need to send traffic
to an interface with a smaller bandwidth, and multiple flows from
several incoming interfaces may need to occupy the same outgoing
interface.
Thirdly, the IP node has been designed to send traffic as quickly as
possible, and it is not aware whether the downstream node's buffer
can handle the traffic. For example, Figure 1 below shows the
problem of the current IP scheduling mechanism. Before the
scheduling in an IP network, the packets are well paced, but after
the scheduling, the packets will be gathered even the total traffic
rate is unchanged. When an IP outgoing interface receives multiple
critical flows from several incoming interfaces, the situation
becomes worse. However, an IP router will try to send them as soon
as possible, so occasionally, in some later hops, micro-bursts will
emerge.
_ _ _ _ _ _ _ _ _ _ _
| | | | | | | | | | | | | | | | | | | | | |
---------------------------------------------------------------------
Before scheduling in an IP network
_ _ _ _ _ _ _ _ _ _ _
| || || || || || | | || || || || |
---------------------------------------------------------------------
After scheduling in an IP network
Figure 1: Change of the traffic characteristics in an IP network
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This document proposes a method to support the low latency traffic
bearing in an IP network, such as the 5G bearing network, by avoiding
micro-bursts in the network as much as possible. The principle in
this method is to forward critical and BE traffic separately, and do
not distinguish different critical flows in the intermediate nodes on
the forwarding plane.
4. Method to Decrease Micro-bursts
The method needs the cooperation of the edge nodes and the
forwarding/core nodes in an IP network.
4.1. Working Flow of the Method
Generally, the method contains two steps:
Step1: per flow schedule in the edge node. The purpose is to make
sure that each critical traffic has a constant traffic model.
Step2: per interface schedule in the core node. Traffic are
aggregated to ensure the scalability, and the pacing also makes sure
that they do not gather. The purpose is to make the critical traffic
be forwarded as the shape when outgoing the edge, not as quickly as
possible. We assume that the sending rate of the buffer for the
critical traffic is the same as the receiving rate (maybe an
algorithm is needed here). If all work good, the buffer will be
maintained with a proper depth.
Other requirements include an RSVP liked mechanism with a good
scalability, which should be used to make sure the bandwidth is not
exceeded on the interface.
4.2. Process of Edge Node
The edge node of the IP network can recognize each critical flows
just as in the TSN network, and then give them individually a good
shaping. In fact, in TSN mechanisms, no micro-busrt will emerge for
critical traffic, and each TSN mechanism is proved to be effective
under certain conditions.
This document suggests the edge node to shape the critical traffic by
using the CBS method in IEEE 802.1Qav, or the shaping methods in IEEE
802.1Qcr. Generally, the shaping methods can generate a paced
traffic for each critical flow.
The parameters of the shaper, such as the sending rate, can be
configured for each flow by some means.
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4.3. Process of Forwarding Node
For the forwarding node, it is uneasy to recognize each critical flow
because of the high pressure of forwarding a large amount of packets.
It is suggested that no per-flow state is maintained in the
forwarding node. It is to say that, in the forwarding node, the
critical flows should be aggregated and handled together.
This document suggests that the forwarding node can deploy a specific
queue at each outgoing interface. The queue will buffer all critical
traffic that need to go out through that interface, and will pace
them by using methods mentioned in the last section.
The shaping method in TSN is used here instead of the original
forwarding method in an IP router, which can make the critical
traffic be forwarded orderly instead of as soon as possible.
Therefore, micro-bursts can be decreased in the network.
If all the forwarding nodes can do their jobs properly, i.e., they
can well pace the critical traffic, no or rare micro-bursts for the
critical traffic would take place. In this way, the critical traffic
will have a relatively low latency in the IP network with less
uncertainties of micro-bursts.
As no per-flow state is maintained in the forwarding node, the
sending rate of the shaper is hard to decide. In this document, the
sending rate is suggested to be generated referring to the incoming
rate of the queue. The purpose is to maintain a proper buffer depth
for the queue.
5. Analysis of the Proposed Method
The method proposed does not need synchronization, just as the
asynchronous mechanisms studied in IEEE 802.1 Qcr. Furthermore, the
method has a larger aggregation granularity, which can fulfill the
requirements of simplicity and scalability. However, it has a larger
uncertainty in the forwarding than the TSN mechanisms, which needs to
be further studied.
6. IANA Considerations
TBD.
7. Security Considerations
TBD.
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8. Acknowledgements
TBD.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases",
RFC 8578, DOI 10.17487/RFC8578, May 2019,
<https://www.rfc-editor.org/info/rfc8578>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
9.2. Informative References
[I-D.qiang-detnet-large-scale-detnet]
Qiang, L., Geng, X., Liu, B., Eckert, T., Geng, L., and G.
Li, "Large-Scale Deterministic IP Network", draft-qiang-
detnet-large-scale-detnet-05 (work in progress), September
2019.
Authors' Addresses
Zongpeng Du
China Mobile
No.32 XuanWuMen West Street
Beijing 100053
China
Email: duzongpeng@foxmail.com
Peng Liu
China Mobile
No.32 XuanWuMen West Street
Beijing 100053
China
Email: liupengyjy@chinamobile.com
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