draft-ietf-tsvwg-aqm-dualq-coupled-15.txt   draft-ietf-tsvwg-aqm-dualq-coupled-16.txt 
Transport Area working group (tsvwg) K. De Schepper Transport Area working group (tsvwg) K. De Schepper
Internet-Draft Nokia Bell Labs Internet-Draft Nokia Bell Labs
Intended status: Experimental B. Briscoe, Ed. Intended status: Experimental B. Briscoe, Ed.
Expires: November 22, 2021 Independent Expires: January 7, 2022 Independent
G. White G. White
CableLabs CableLabs
May 21, 2021 July 6, 2021
DualQ Coupled AQMs for Low Latency, Low Loss and Scalable Throughput DualQ Coupled AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S) (L4S)
draft-ietf-tsvwg-aqm-dualq-coupled-15 draft-ietf-tsvwg-aqm-dualq-coupled-16
Abstract Abstract
The Low Latency Low Loss Scalable Throughput (L4S) architecture The Low Latency Low Loss Scalable Throughput (L4S) architecture
allows data flows over the public Internet to achieve consistent low allows data flows over the public Internet to achieve consistent low
queuing latency, generally zero congestion loss and scaling of per- queuing latency, generally zero congestion loss and scaling of per-
flow throughput without the scaling problems of standard TCP Reno- flow throughput without the scaling problems of standard TCP Reno-
friendly congestion controls. To achieve this, L4S data flows have friendly congestion controls. To achieve this, L4S data flows have
to use one of the family of 'Scalable' congestion controls (TCP to use one of the family of 'Scalable' congestion controls (TCP
Prague and Data Center TCP are examples) and a form of Explicit Prague and Data Center TCP are examples) and a form of Explicit
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 22, 2021. This Internet-Draft will expire on January 7, 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Outline of the Problem . . . . . . . . . . . . . . . . . 3 1.1. Outline of the Problem . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. Features . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4. Features . . . . . . . . . . . . . . . . . . . . . . . . 9
2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 10 2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 10
2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 10 2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 11
2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 12 2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 12
2.3. Traffic Classification . . . . . . . . . . . . . . . . . 12 2.3. Traffic Classification . . . . . . . . . . . . . . . . . 12
2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 13 2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 13
2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 16 2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 16
2.5.1. Functional Requirements . . . . . . . . . . . . . . . 16 2.5.1. Functional Requirements . . . . . . . . . . . . . . . 16
2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 17 2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 18
2.5.2. Management Requirements . . . . . . . . . . . . . . . 18 2.5.2. Management Requirements . . . . . . . . . . . . . . . 19
2.5.2.1. Configuration . . . . . . . . . . . . . . . . . . 18 2.5.2.1. Configuration . . . . . . . . . . . . . . . . . . 19
2.5.2.2. Monitoring . . . . . . . . . . . . . . . . . . . 20 2.5.2.2. Monitoring . . . . . . . . . . . . . . . . . . . 20
2.5.2.3. Anomaly Detection . . . . . . . . . . . . . . . . 20 2.5.2.3. Anomaly Detection . . . . . . . . . . . . . . . . 21
2.5.2.4. Deployment, Coexistence and Scaling . . . . . . . 21 2.5.2.4. Deployment, Coexistence and Scaling . . . . . . . 21
3. IANA Considerations (to be removed by RFC Editor) . . . . . . 21 3. IANA Considerations (to be removed by RFC Editor) . . . . . . 22
4. Security Considerations . . . . . . . . . . . . . . . . . . . 21 4. Security Considerations . . . . . . . . . . . . . . . . . . . 22
4.1. Overload Handling . . . . . . . . . . . . . . . . . . . . 21 4.1. Overload Handling . . . . . . . . . . . . . . . . . . . . 22
4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput 4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput
or Delay? . . . . . . . . . . . . . . . . . . . . . . 22 or Delay? . . . . . . . . . . . . . . . . . . . . . . 22
4.1.2. Congestion Signal Saturation: Introduce L4S Drop or 4.1.2. Congestion Signal Saturation: Introduce L4S Drop or
Delay? . . . . . . . . . . . . . . . . . . . . . . . 23 Delay? . . . . . . . . . . . . . . . . . . . . . . . 24
4.1.3. Protecting against Unresponsive ECN-Capable Traffic . 24 4.1.3. Protecting against Unresponsive ECN-Capable Traffic . 25
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Normative References . . . . . . . . . . . . . . . . . . 25 7.1. Normative References . . . . . . . . . . . . . . . . . . 26
7.2. Informative References . . . . . . . . . . . . . . . . . 26 7.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 30 Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 32
A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 31 A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 32
A.2. Pass #2: Overload Details . . . . . . . . . . . . . . . . 40 A.2. Pass #2: Overload Details . . . . . . . . . . . . . . . . 42
Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 44 Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 46
B.1. Curvy RED in Pseudocode . . . . . . . . . . . . . . . . . 44 B.1. Curvy RED in Pseudocode . . . . . . . . . . . . . . . . . 46
B.2. Efficient Implementation of Curvy RED . . . . . . . . . . 50 B.2. Efficient Implementation of Curvy RED . . . . . . . . . . 52
Appendix C. Choice of Coupling Factor, k . . . . . . . . . . . . 52 Appendix C. Choice of Coupling Factor, k . . . . . . . . . . . . 54
C.1. RTT-Dependence . . . . . . . . . . . . . . . . . . . . . 52 C.1. RTT-Dependence . . . . . . . . . . . . . . . . . . . . . 54
C.2. Guidance on Controlling Throughput Equivalence . . . . . 53 C.2. Guidance on Controlling Throughput Equivalence . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 56
1. Introduction 1. Introduction
This document specifies a framework for DualQ Coupled AQMs, which is This document specifies a framework for DualQ Coupled AQMs, which is
the network part of the L4S architecture [I-D.ietf-tsvwg-l4s-arch]. the network part of the L4S architecture [I-D.ietf-tsvwg-l4s-arch].
L4S enables both very low queuing latency (sub-millisecond on L4S enables both very low queuing latency (sub-millisecond on
average) and high throughput at the same time, for ad hoc numbers of average) and high throughput at the same time, for ad hoc numbers of
capacity-seeking applications all sharing the same capacity. capacity-seeking applications all sharing the same capacity.
1.1. Outline of the Problem 1.1. Outline of the Problem
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environment could be arranged, such as in private data centres (hence environment could be arranged, such as in private data centres (hence
the name DCTCP). the name DCTCP).
This document specifies a `DualQ Coupled AQM' extension that solves This document specifies a `DualQ Coupled AQM' extension that solves
the problem of coexistence between Scalable and Classic flows, the problem of coexistence between Scalable and Classic flows,
without having to inspect flow identifiers. It is not like flow- without having to inspect flow identifiers. It is not like flow-
queuing approaches [RFC8290] that classify packets by flow identifier queuing approaches [RFC8290] that classify packets by flow identifier
into separate queues in order to isolate sparse flows from the higher into separate queues in order to isolate sparse flows from the higher
latency in the queues assigned to heavier flows. If a flow needs latency in the queues assigned to heavier flows. If a flow needs
both low delay and high throughput, having a queue to itself does not both low delay and high throughput, having a queue to itself does not
isolate it from the harm it causes to itself. In contrast, L4S isolate it from the harm it causes to itself. In contrast, DualQ
addresses the root cause of the latency problem --- it is an enabler Coupled AQMs addresses the root cause of the latency problem --- they
for the smooth low latency scalable behaviour of Scalable congestion are an enabler for the smooth low latency scalable behaviour of
controls, so that every packet in every flow can enjoy very low Scalable congestion controls, so that every packet in every flow can
latency, then there is no need to isolate each flow into a separate enjoy very low latency, then there is no need to isolate each flow
queue. into a separate queue.
1.2. Scope 1.2. Scope
L4S involves complementary changes in the network and on end-systems: L4S involves complementary changes in the network and on end-systems:
Network: A DualQ Coupled AQM (defined in the present document); Network: A DualQ Coupled AQM (defined in the present document) or a
modification to flow-queue AQMs (described in section 4.2.b of
[I-D.ietf-tsvwg-l4s-arch]);
End-system: A Scalable congestion control (defined in Section 2.1). End-system: A Scalable congestion control (defined in section 4 of
[I-D.ietf-tsvwg-ecn-l4s-id]).
Packet identifier: The network and end-system parts of L4S can be Packet identifier: The network and end-system parts of L4S can be
deployed incrementally, because they both identify L4S packets deployed incrementally, because they both identify L4S packets
using the experimentally assigned explicit congestion notification using the experimentally assigned explicit congestion notification
(ECN) codepoints in the IP header: ECT(1) and CE [RFC8311] (ECN) codepoints in the IP header: ECT(1) and CE [RFC8311]
[I-D.ietf-tsvwg-ecn-l4s-id]. [I-D.ietf-tsvwg-ecn-l4s-id].
Data Center TCP (DCTCP [RFC8257]) is an example of a Scalable Data Center TCP (DCTCP [RFC8257]) is an example of a Scalable
congestion control that has been deployed for some time in Linux, congestion control for controlled environments that has been deployed
Windows and FreeBSD operating systems and Relentless TCP [Mathis09] for some time in Linux, Windows and FreeBSD operating systems.
is another example. During the progress of this document through the During the progress of this document through the IETF a number of
IETF a number of other Scalable congestion controls were implemented, other Scalable congestion controls were implemented, e.g. TCP
e.g. TCP Prague [PragueLinux], QUIC Prague and the L4S variant of Prague [I-D.briscoe-iccrg-prague-congestion-control] [PragueLinux],
SCREAM for real-time media [RFC8298]. (Note: after the v3.19 Linux BBRv2 [BBRv2], QUIC Prague and the L4S variant of SCREAM for real-
kernel, bugs were introduced into DCTCP's scalable behaviour and not time media [RFC8298].
all the patches applied for L4S evaluation had been applied to the
mainline Linux kernel, which was at v5.5 at the time of writing. TCP
Prague includes these patches and is available for all these Linux
kernels).
The focus of this specification is to enable deployment of the The focus of this specification is to enable deployment of the
network part of the L4S service. Then, without any management network part of the L4S service. Then, without any management
intervention, applications can exploit this new network capability as intervention, applications can exploit this new network capability as
their operating systems migrate to Scalable congestion controls, their operating systems migrate to Scalable congestion controls,
which can then evolve _while_ their benefits are being enjoyed by which can then evolve _while_ their benefits are being enjoyed by
everyone on the Internet. everyone on the Internet.
The DualQ Coupled AQM framework can incorporate any AQM designed for The DualQ Coupled AQM framework can incorporate any AQM designed for
a single queue that generates a statistical or deterministic mark/ a single queue that generates a statistical or deterministic mark/
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Coupled AQM would be applicable and easy to deploy in all types of Coupled AQM would be applicable and easy to deploy in all types of
buffers; buffers in cost-reduced mass-market residential equipment; buffers; buffers in cost-reduced mass-market residential equipment;
buffers in end-system stacks; buffers in carrier-scale equipment buffers in end-system stacks; buffers in carrier-scale equipment
including remote access servers, routers, firewalls and Ethernet including remote access servers, routers, firewalls and Ethernet
switches; buffers in network interface cards, buffers in virtualized switches; buffers in network interface cards, buffers in virtualized
network appliances, hypervisors, and so on. network appliances, hypervisors, and so on.
For the public Internet, nearly all the benefit will typically be For the public Internet, nearly all the benefit will typically be
achieved by deploying the Coupled AQM into either end of the access achieved by deploying the Coupled AQM into either end of the access
link between a 'site' and the Internet, which is invariably the link between a 'site' and the Internet, which is invariably the
bottleneck. Here, the term 'site' is used loosely to mean a home, an bottleneck (see section 6.4 of[I-D.ietf-tsvwg-l4s-arch] about
office, a campus or mobile user equipment. deployment, which also defines the term 'site' to mean a home, an
office, a campus or mobile user equipment).
Latency is not the only concern of L4S: Latency is not the only concern of L4S:
o The 'Low Loss" part of the name denotes that L4S generally o The 'Low Loss" part of the name denotes that L4S generally
achieves zero congestion loss (which would otherwise cause achieves zero congestion loss (which would otherwise cause
retransmission delays), due to its use of ECN. retransmission delays), due to its use of ECN.
o The "Scalable throughput" part of the name denotes that the per- o The "Scalable throughput" part of the name denotes that the per-
flow throughput of Scalable congestion controls should scale flow throughput of Scalable congestion controls should scale
indefinitely, avoiding the imminent scaling problems with 'TCP- indefinitely, avoiding the imminent scaling problems with 'TCP-
Friendly' congestion control algorithms [RFC3649]. Friendly' congestion control algorithms [RFC3649].
The former is clearly in scope of this AQM document. However, the The former is clearly in scope of this AQM document. However, the
latter is an outcome of the end-system behaviour, and therefore latter is an outcome of the end-system behaviour, and therefore
outside the scope of this AQM document, even though the AQM is an outside the scope of this AQM document, even though the AQM is an
enabler. enabler.
The overall L4S architecture [I-D.ietf-tsvwg-l4s-arch] gives more The overall L4S architecture [I-D.ietf-tsvwg-l4s-arch] gives more
detail, including on wider deployment aspects such as backwards detail, including on wider deployment aspects such as backwards
compatibility of Scalable congestion controls in bottlenecks where a compatibility of Scalable congestion controls in bottlenecks where a
DualQ Coupled AQM has not been deployed. The supporting papers [PI2] DualQ Coupled AQM has not been deployed. The supporting papers
and [DCttH15] give the full rationale for the AQM's design, both [DualPI2Linux], [PI2] and [DCttH15] give the full rationale for the
discursively and in more precise mathematical form. AQM's design, both discursively and in more precise mathematical
form, as well as the results of performance evaluations.
1.3. Terminology 1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] when, and document are to be interpreted as described in [RFC2119] when, and
only when, they appear in all capitals, as shown here. only when, they appear in all capitals, as shown here.
The DualQ Coupled AQM uses two queues for two services. Each of the The DualQ Coupled AQM uses two queues for two services. Each of the
following terms identifies both the service and the queue that following terms identifies both the service and the queue that
provides the service: provides the service:
Classic service/queue: The Classic service is intended for all the Classic service/queue: The Classic service is intended for all the
congestion control behaviours that co-exist with Reno [RFC5681] congestion control behaviours that co-exist with Reno [RFC5681]
(e.g. Reno itself, Cubic [RFC8312], TFRC [RFC5348]). (e.g. Reno itself, Cubic [RFC8312], TFRC [RFC5348]).
Low-Latency, Low-Loss Scalable throughput (L4S) service/queue: The Low-Latency, Low-Loss Scalable throughput (L4S) service/queue: The
'L4S' service is intended for traffic from scalable congestion 'L4S' service is intended for traffic from scalable congestion
control algorithms, such as Data Center TCP [RFC8257]. The L4S control algorithms, such as TCP Prague
service is for more general traffic than just DCTCP--it allows the [I-D.briscoe-iccrg-prague-congestion-control], which was derived
set of congestion controls with similar scaling properties to from Data Center TCP [RFC8257]. The L4S service is for more
DCTCP to evolve (e.g. Relentless TCP [Mathis09], TCP general traffic than just TCP Prague--it allows the set of
Prague [PragueLinux] and the L4S variant of SCREAM for real-time congestion controls with similar scaling properties to Prague to
media [RFC8298]). evolve, such as the examples listed earlier (Relentless, SCReAM,
etc.).
Classic Congestion Control: A congestion control behaviour that can Classic Congestion Control: A congestion control behaviour that can
co-exist with standard TCP Reno [RFC5681] without causing co-exist with standard TCP Reno [RFC5681] without causing
significantly negative impact on its flow rate [RFC5033]. With significantly negative impact on its flow rate [RFC5033]. With
Classic congestion controls, as flow rate scales, the number of Classic congestion controls, such as Reno or Cubic, because flow
round trips between congestion signals (losses or ECN marks) rises rate has scaled since TCP congestion control was first designed in
with the flow rate. So it takes longer and longer to recover 1988, it now takes hundreds of round trips (and growing) to
after each congestion event. Therefore control of queuing and recover after a congestion signal (whether a loss or an ECN mark)
utilization becomes very slack, and the slightest disturbance as shown in the examples in section 5.1 of
prevents a high rate from being attained [RFC3649]. [I-D.ietf-tsvwg-l4s-arch] and in [RFC3649]. Therefore control of
queuing and utilization becomes very slack, and the slightest
disturbances (e.g. from new flows starting) prevent a high rate
from being attained.
Scalable Congestion Control: A congestion control where the average Scalable Congestion Control: A congestion control where the average
time from one congestion signal to the next (the recovery time) time from one congestion signal to the next (the recovery time)
remains invariant as the flow rate scales, all other factors being remains invariant as the flow rate scales, all other factors being
equal. This maintains the same degree of control over queueing equal. This maintains the same degree of control over queueing
and utilization whatever the flow rate, as well as ensuring that and utilization whatever the flow rate, as well as ensuring that
high throughput is robust to disturbances. For instance, DCTCP high throughput is robust to disturbances. For instance, DCTCP
averages 2 congestion signals per round-trip whatever the flow averages 2 congestion signals per round-trip whatever the flow
rate. For the public Internet a Scalable transport has to comply rate, as do other recently developed scalable congestion controls,
with the requirements in Section 4 of [I-D.ietf-tsvwg-ecn-l4s-id] e.g. Relentless TCP [Mathis09], TCP Prague
(aka. the 'Prague L4S requirements'). [I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux],
BBRv2 [BBRv2] and the L4S variant of SCREAM for real-time
media [SCReAM], [RFC8298]). For the public Internet a Scalable
transport has to comply with the requirements in Section 4 of
[I-D.ietf-tsvwg-ecn-l4s-id] (aka. the 'Prague L4S requirements').
C: Abbreviation for Classic, e.g. when used as a subscript. C: Abbreviation for Classic, e.g. when used as a subscript.
L: Abbreviation for L4S, e.g. when used as a subscript. L: Abbreviation for L4S, e.g. when used as a subscript.
The terms Classic or L4S can also qualify other nouns, such as The terms Classic or L4S can also qualify other nouns, such as
'codepoint', 'identifier', 'classification', 'packet', 'flow'. 'codepoint', 'identifier', 'classification', 'packet', 'flow'.
For example: an L4S packet means a packet with an L4S identifier For example: an L4S packet means a packet with an L4S identifier
sent from an L4S congestion control. sent from an L4S congestion control.
Both Classic and L4S queues can cope with a proportion of Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well (e.g. DNS, VoIP, unresponsive or less-responsive traffic as well, but in the L4S
game sync datagrams), just as a single queue AQM can if this case its rate has to be smooth enough or low enough not to build a
traffic makes minimal contribution to queuing. The DualQ Coupled queue (e.g. DNS, VoIP, game sync datagrams, etc). The DualQ
AQM behaviour is defined to be similar to a single FIFO queue with Coupled AQM behaviour is defined to be similar to a single FIFO
respect to unresponsive and overload traffic. queue with respect to unresponsive and overload traffic.
Reno-friendly: The subset of Classic traffic that excludes Reno-friendly: The subset of Classic traffic that is friendly to the
unresponsive traffic and excludes experimental congestion controls standard Reno congestion control defined for TCP in [RFC5681].
intended to coexist with Reno but without always being strictly Reno-friendly is used in place of 'TCP-friendly', given the latter
friendly to it (as allowed by [RFC5033]). Reno-friendly is used has become imprecise, because the TCP protocol is now used with so
in place of 'TCP-friendly', given that friendliness is a property many different congestion control behaviours, and Reno is used in
of the congestion controller (Reno), not the wire protocol (TCP), non-TCP transports such as QUIC.
which is used with many different congestion control behaviours.
Classic ECN: The original Explicit Congestion Notification (ECN) Classic ECN: The original Explicit Congestion Notification (ECN)
protocol [RFC3168], which requires ECN signals to be treated the protocol [RFC3168], which requires ECN signals to be treated the
same as drops, both when generated in the network and when same as drops, both when generated in the network and when
responded to by the sender. responded to by the sender.
The names used for the four codepoints of the 2-bit IP-ECN field For L4S, the names used for the four codepoints of the 2-bit IP-
are as defined in [RFC3168]: Not ECT, ECT(0), ECT(1) and CE, where ECN field are unchanged from those defined in [RFC3168]: Not ECT,
ECT stands for ECN-Capable Transport and CE stands for Congestion ECT(0), ECT(1) and CE, where ECT stands for ECN-Capable Transport
Experienced. and CE stands for Congestion Experienced. A packet marked with
the CE codepoint is termed 'ECN-marked' or sometimes just 'marked'
where the context makes ECN obvious.
1.4. Features 1.4. Features
The AQM couples marking and/or dropping from the Classic queue to the The AQM couples marking and/or dropping from the Classic queue to the
L4S queue in such a way that a flow will get roughly the same L4S queue in such a way that a flow will get roughly the same
throughput whichever it uses. Therefore both queues can feed into throughput whichever it uses. Therefore both queues can feed into
the full capacity of a link and no rates need to be configured for the full capacity of a link and no rates need to be configured for
the queues. The L4S queue enables Scalable congestion controls like the queues. The L4S queue enables Scalable congestion controls like
DCTCP or TCP Prague to give very low and predictably low latency, DCTCP or TCP Prague to give very low and predictably low latency,
without compromising the performance of competing 'Classic' Internet without compromising the performance of competing 'Classic' Internet
traffic. traffic.
Thousands of tests have been conducted in a typical fixed residential Thousands of tests have been conducted in a typical fixed residential
broadband setting. Experiments used a range of base round trip broadband setting. Experiments used a range of base round trip
delays up to 100ms and link rates up to 200 Mb/s between the data delays up to 100ms and link rates up to 200 Mb/s between the data
centre and home network, with varying amounts of background traffic centre and home network, with varying amounts of background traffic
in both queues. For every L4S packet, the AQM kept the average in both queues. For every L4S packet, the AQM kept the average
queuing delay below 1ms (or 2 packets where serialization delay queuing delay below 1ms (or 2 packets where serialization delay
exceeded 1ms on slower links), with 99th percentile no worse than exceeded 1ms on slower links), with 99th percentile no worse than
2ms. No losses at all were introduced by the L4S AQM. Details of 2ms. No losses at all were introduced by the L4S AQM. Details of
the extensive experiments are available [PI2] [DCttH15]. the extensive experiments are available [DualPI2Linux], [PI2],
[DCttH15].
Subjective testing was also conducted by multiple people all Subjective testing was also conducted by multiple people all
simultaneously using very demanding high bandwidth low latency simultaneously using very demanding high bandwidth low latency
applications over a single shared access link [L4Sdemo16]. In one applications over a single shared access link [L4Sdemo16]. In one
application, each user could use finger gestures to pan or zoom their application, each user could use finger gestures to pan or zoom their
own high definition (HD) sub-window of a larger video scene generated own high definition (HD) sub-window of a larger video scene generated
on the fly in 'the cloud' from a football match. Another user on the fly in 'the cloud' from a football match. Another user
wearing VR goggles was remotely receiving a feed from a 360-degree wearing VR goggles was remotely receiving a feed from a 360-degree
camera in a racing car, again with the sub-window in their field of camera in a racing car, again with the sub-window in their field of
vision generated on the fly in 'the cloud' dependent on their head vision generated on the fly in 'the cloud' dependent on their head
skipping to change at page 10, line 31 skipping to change at page 10, line 39
The two queues are only necessary because: The two queues are only necessary because:
o the large variations (sawteeth) of Classic flows need roughly a o the large variations (sawteeth) of Classic flows need roughly a
base RTT of queuing delay to ensure full utilization base RTT of queuing delay to ensure full utilization
o Scalable flows do not need a queue to keep utilization high, but o Scalable flows do not need a queue to keep utilization high, but
they cannot keep latency predictably low if they are mixed with they cannot keep latency predictably low if they are mixed with
Classic traffic, Classic traffic,
The L4S queue has latency priority, but the coupling from the Classic The L4S queue has latency priority within sub-round trip timescales,
to the L4S AQM (explained below) ensures that it does not have but over longer periods the coupling from the Classic to the L4S AQM
bandwidth priority over the Classic queue. (explained below) ensures that it does not have bandwidth priority
over the Classic queue.
2. DualQ Coupled AQM 2. DualQ Coupled AQM
There are two main aspects to the approach: There are two main aspects to the approach:
o The Coupled AQM that addresses throughput equivalence between o The Coupled AQM that addresses throughput equivalence between
Classic (e.g. Reno, Cubic) flows and L4S flows (that satisfy the Classic (e.g. Reno, Cubic) flows and L4S flows (that satisfy the
Prague L4S requirements). Prague L4S requirements).
o The Dual Queue structure that provides latency separation for L4S o The Dual Queue structure that provides latency separation for L4S
skipping to change at page 11, line 24 skipping to change at page 11, line 33
term 'Classic' will be used for the collection of Reno-friendly term 'Classic' will be used for the collection of Reno-friendly
traffic including Cubic and potentially other experimental congestion traffic including Cubic and potentially other experimental congestion
controls intended not to significantly impact the flow rate of Reno. controls intended not to significantly impact the flow rate of Reno.
A supporting paper [PI2] includes the derivation of the equivalent A supporting paper [PI2] includes the derivation of the equivalent
rate equation for DCTCP, for which cwnd is inversely proportional to rate equation for DCTCP, for which cwnd is inversely proportional to
p (not the square root), where in this case p is the ECN marking p (not the square root), where in this case p is the ECN marking
probability. DCTCP is not the only congestion control that behaves probability. DCTCP is not the only congestion control that behaves
like this, so the term 'Scalable' will be used for all similar like this, so the term 'Scalable' will be used for all similar
congestion control behaviours (see examples in Section 1.2). The congestion control behaviours (see examples in Section 1.2). The
term 'L4S' is also used for traffic driven by a Scalable congestion term 'L4S' is used for traffic driven by a Scalable congestion
control that also complies with the additional 'Prague L4S' control that also complies with the additional 'Prague L4S'
requirements [I-D.ietf-tsvwg-ecn-l4s-id]. requirements [I-D.ietf-tsvwg-ecn-l4s-id].
For safe co-existence, under stationary conditions, a Scalable flow For safe co-existence, under stationary conditions, a Scalable flow
has to run at roughly the same rate as a Reno TCP flow (all other has to run at roughly the same rate as a Reno TCP flow (all other
factors being equal). So the drop or marking probability for Classic factors being equal). So the drop or marking probability for Classic
traffic, p_C has to be distinct from the marking probability for L4S traffic, p_C has to be distinct from the marking probability for L4S
traffic, p_L. The original ECN specification [RFC3168] required traffic, p_L. The original ECN specification [RFC3168] required
these probabilities to be the same, but [RFC8311] updates RFC 3168 to these probabilities to be the same, but [RFC8311] updates RFC 3168 to
enable experiments in which these probabilities are different. enable experiments in which these probabilities are different.
skipping to change at page 19, line 8 skipping to change at page 19, line 38
o Optional packet classifier(s) to use in addition to the ECN field o Optional packet classifier(s) to use in addition to the ECN field
(see Section 2.3); (see Section 2.3);
o Expected typical RTT, which can be used to determine the queuing o Expected typical RTT, which can be used to determine the queuing
delay of the Classic AQM at its operating point, in order to delay of the Classic AQM at its operating point, in order to
prevent typical lone flows from under-utilizing capacity. For prevent typical lone flows from under-utilizing capacity. For
example: example:
* for the PI2 algorithm (Appendix A) the queuing delay target is * for the PI2 algorithm (Appendix A) the queuing delay target is
set to the typical RTT; dependent on the typical RTT;
* for the Curvy RED algorithm (Appendix B) the queuing delay at * for the Curvy RED algorithm (Appendix B) the queuing delay at
the desired operating point of the curvy ramp is configured to the desired operating point of the curvy ramp is configured to
encompass a typical RTT; encompass a typical RTT;
* if another Classic AQM was used, it would be likely to need an * if another Classic AQM was used, it would be likely to need an
operating point for the queue based on the typical RTT, and if operating point for the queue based on the typical RTT, and if
so it SHOULD be expressed in units of time. so it SHOULD be expressed in units of time.
An operating point that is manually calculated might be directly An operating point that is manually calculated might be directly
skipping to change at page 26, line 41 skipping to change at page 27, line 29
Algorithm for Increasing the Robustness of RED's Active Algorithm for Increasing the Robustness of RED's Active
Queue Management", ACIRI Technical Report , August 2001, Queue Management", ACIRI Technical Report , August 2001,
<http://www.icir.org/floyd/red.html>. <http://www.icir.org/floyd/red.html>.
[BBRv1] Cardwell, N., Cheng, Y., Hassas Yeganeh, S., and V. [BBRv1] Cardwell, N., Cheng, Y., Hassas Yeganeh, S., and V.
Jacobson, "BBR Congestion Control", Internet Draft draft- Jacobson, "BBR Congestion Control", Internet Draft draft-
cardwell-iccrg-bbr-congestion-control-00, July 2017, cardwell-iccrg-bbr-congestion-control-00, July 2017,
<https://tools.ietf.org/html/draft-cardwell-iccrg-bbr- <https://tools.ietf.org/html/draft-cardwell-iccrg-bbr-
congestion-control-00>. congestion-control-00>.
[BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", github
repository; Linux congestion control module,
<https://github.com/google/bbr/blob/v2alpha/README.md>.
[CCcensus19]
Mishra, A., Sun, X., Jain, A., Pande, S., Joshi, R., and
B. Leong, "The Great Internet TCP Congestion Control
Census", Proc. ACM on Measurement and Analysis of
Computing Systems 3(3), December 2019,
<https://doi.org/10.1145/3366693>.
[CoDel] Nichols, K. and V. Jacobson, "Controlling Queue Delay", [CoDel] Nichols, K. and V. Jacobson, "Controlling Queue Delay",
ACM Queue 10(5), May 2012, ACM Queue 10(5), May 2012,
<http://queue.acm.org/issuedetail.cfm?issue=2208917>. <http://queue.acm.org/issuedetail.cfm?issue=2208917>.
[CRED_Insights] [CRED_Insights]
Briscoe, B., "Insights from Curvy RED (Random Early Briscoe, B., "Insights from Curvy RED (Random Early
Detection)", BT Technical Report TR-TUB8-2015-003 Detection)", BT Technical Report TR-TUB8-2015-003
arXiv:1904.07339 [cs.NI], July 2015, arXiv:1904.07339 [cs.NI], July 2015,
<https://arxiv.org/abs/1904.07339>. <https://arxiv.org/abs/1904.07339>.
skipping to change at page 27, line 34 skipping to change at page 28, line 34
[DualQ-Test] [DualQ-Test]
Steen, H., "Destruction Testing: Ultra-Low Delay using Steen, H., "Destruction Testing: Ultra-Low Delay using
Dual Queue Coupled Active Queue Management", Masters Dual Queue Coupled Active Queue Management", Masters
Thesis, Dept of Informatics, Uni Oslo , May 2017. Thesis, Dept of Informatics, Uni Oslo , May 2017.
[I-D.briscoe-docsis-q-protection] [I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "Queue Protection to Preserve Briscoe, B. and G. White, "Queue Protection to Preserve
Low Latency", draft-briscoe-docsis-q-protection-00 (work Low Latency", draft-briscoe-docsis-q-protection-00 (work
in progress), July 2019. in progress), July 2019.
[I-D.briscoe-iccrg-prague-congestion-control]
Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague
Congestion Control", draft-briscoe-iccrg-prague-
congestion-control-00 (work in progress), March 2021.
[I-D.briscoe-tsvwg-l4s-diffserv] [I-D.briscoe-tsvwg-l4s-diffserv]
Briscoe, B., "Interactions between Low Latency, Low Loss, Briscoe, B., "Interactions between Low Latency, Low Loss,
Scalable Throughput (L4S) and Differentiated Services", Scalable Throughput (L4S) and Differentiated Services",
draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress), draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
November 2018. November 2018.
[I-D.cardwell-iccrg-bbr-congestion-control] [I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S. H., and V. Jacobson, Cardwell, N., Cheng, Y., Yeganeh, S. H., and V. Jacobson,
"BBR Congestion Control", draft-cardwell-iccrg-bbr- "BBR Congestion Control", draft-cardwell-iccrg-bbr-
congestion-control-00 (work in progress), July 2017. congestion-control-00 (work in progress), July 2017.
skipping to change at page 28, line 18 skipping to change at page 29, line 24
ietf-tsvwg-nqb-05 (work in progress), March 2021. ietf-tsvwg-nqb-05 (work in progress), March 2021.
[L4Sdemo16] [L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B. Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live Briscoe, "Ultra-Low Delay for All: Live Experience, Live
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016, Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<http://dl.acm.org/citation.cfm?doid=2910017.2910633 <http://dl.acm.org/citation.cfm?doid=2910017.2910633
(videos of demos: (videos of demos:
https://riteproject.eu/dctth/#1511dispatchwg )>. https://riteproject.eu/dctth/#1511dispatchwg )>.
[Labovitz10]
Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide,
J., and F. Jahanian, "Internet Inter-Domain Traffic", Proc
ACM SIGCOMM; ACM CCR 40(4):75--86, August 2010,
<https://doi.org/10.1145/1851275.1851194>.
[LLD] White, G., Sundaresan, K., and B. Briscoe, "Low Latency [LLD] White, G., Sundaresan, K., and B. Briscoe, "Low Latency
DOCSIS: Technology Overview", CableLabs White Paper , DOCSIS: Technology Overview", CableLabs White Paper ,
February 2019, <https://cablela.bs/low-latency-docsis- February 2019, <https://cablela.bs/low-latency-docsis-
technology-overview-february-2019>. technology-overview-february-2019>.
[Mathis09] [Mathis09]
Mathis, M., "Relentless Congestion Control", PFLDNeT'09 , Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
May 2009, <http://www.hpcc.jp/pfldnet2009/ May 2009, <http://www.hpcc.jp/pfldnet2009/
Program_files/1569198525.pdf>. Program_files/1569198525.pdf>.
skipping to change at page 28, line 40 skipping to change at page 30, line 5
service classes with application in the UTRAN", Proc. IEEE service classes with application in the UTRAN", Proc. IEEE
Conference on Computer Communications (INFOCOM'03) Vol.2 Conference on Computer Communications (INFOCOM'03) Vol.2
pp.1116-1122, March 2003. pp.1116-1122, March 2003.
[PI2] De Schepper, K., Bondarenko, O., Briscoe, B., and I. [PI2] De Schepper, K., Bondarenko, O., Briscoe, B., and I.
Tsang, "PI2: A Linearized AQM for both Classic and Tsang, "PI2: A Linearized AQM for both Classic and
Scalable TCP", ACM CoNEXT'16 , December 2016, Scalable TCP", ACM CoNEXT'16 , December 2016,
<https://riteproject.files.wordpress.com/2015/10/ <https://riteproject.files.wordpress.com/2015/10/
pi2_conext.pdf>. pi2_conext.pdf>.
[PI2param]
Briscoe, B., "PI2 Parameters", Technical Report TR-BB-
2021-001 arXiv:2107.01003 [cs.NI], July 2021,
<https://arxiv.org/abs/2107.01003>.
[PragueLinux] [PragueLinux]
Briscoe, B., De Schepper, K., Albisser, O., Misund, J., Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing
the `TCP Prague' Requirements for Low Latency Low Loss the `TCP Prague' Requirements for Low Latency Low Loss
Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 , Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
March 2019, <https://www.netdevconf.org/0x13/ March 2019, <https://www.netdevconf.org/0x13/
session.html?talk-tcp-prague-l4s>. session.html?talk-tcp-prague-l4s>.
[RFC0970] Nagle, J., "On Packet Switches With Infinite Storage", [RFC0970] Nagle, J., "On Packet Switches With Infinite Storage",
RFC 970, DOI 10.17487/RFC0970, December 1985, RFC 970, DOI 10.17487/RFC0970, December 1985,
skipping to change at page 30, line 31 skipping to change at page 31, line 47
[RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation [RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
2017, <https://www.rfc-editor.org/info/rfc8298>. 2017, <https://www.rfc-editor.org/info/rfc8298>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and [RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018, RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>. <https://www.rfc-editor.org/info/rfc8312>.
[SCReAM] Johansson, I., "SCReAM", github repository; ,
<https://github.com/EricssonResearch/scream/blob/master/
README.md>.
[SigQ-Dyn] [SigQ-Dyn]
Briscoe, B., "Rapid Signalling of Queue Dynamics", Briscoe, B., "Rapid Signalling of Queue Dynamics",
Technical Report TR-BB-2017-001 arXiv:1904.07044 [cs.NI], Technical Report TR-BB-2017-001 arXiv:1904.07044 [cs.NI],
September 2017, <https://arxiv.org/abs/1904.07044>. September 2017, <https://arxiv.org/abs/1904.07044>.
Appendix A. Example DualQ Coupled PI2 Algorithm Appendix A. Example DualQ Coupled PI2 Algorithm
As a first concrete example, the pseudocode below gives the DualPI2 As a first concrete example, the pseudocode below gives the DualPI2
algorithm. DualPI2 follows the structure of the DualQ Coupled AQM algorithm. DualPI2 follows the structure of the DualQ Coupled AQM
framework in Figure 1. A simple ramp function (configured in units framework in Figure 1. A simple ramp function (configured in units
skipping to change at page 32, line 19 skipping to change at page 33, line 39
o mark(pkt) and drop(pkt) for ECN-marking and dropping a packet; o mark(pkt) and drop(pkt) for ECN-marking and dropping a packet;
In experiments so far (building on experiments with PIE) on broadband In experiments so far (building on experiments with PIE) on broadband
access links ranging from 4 Mb/s to 200 Mb/s with base RTTs from 5 ms access links ranging from 4 Mb/s to 200 Mb/s with base RTTs from 5 ms
to 100 ms, DualPI2 achieves good results with the default parameters to 100 ms, DualPI2 achieves good results with the default parameters
in Figure 2. The parameters are categorised by whether they relate in Figure 2. The parameters are categorised by whether they relate
to the Base PI2 AQM, the L4S AQM or the framework coupling them to the Base PI2 AQM, the L4S AQM or the framework coupling them
together. Constants and variables derived from these parameters are together. Constants and variables derived from these parameters are
also included at the end of each category. Each parameter is also included at the end of each category. Each parameter is
explained as it is encountered in the walk-through of the pseudocode explained as it is encountered in the walk-through of the pseudocode
below. below, and the rationale for the chosen defaults are given so that
sensible values can be used in scenarios other than the regular
public Internet.
1: dualpi2_params_init(...) { % Set input parameter defaults 1: dualpi2_params_init(...) { % Set input parameter defaults
2: % DualQ Coupled framework parameters 2: % DualQ Coupled framework parameters
5: limit = MAX_LINK_RATE * 250 ms % Dual buffer size 5: limit = MAX_LINK_RATE * 250 ms % Dual buffer size
3: k = 2 % Coupling factor 3: k = 2 % Coupling factor
4: % NOT SHOWN % scheduler-dependent weight or equival't parameter 4: % NOT SHOWN % scheduler-dependent weight or equival't parameter
6: 6:
7: % PI2 AQM parameters 7: % PI2 Classic AQM parameters
8: RTT_max = 100 ms % Worst case RTT expected 8: % Typical RTT, RTT_typ = 34 ms
9: RTT_typ = 15 ms % Typical RTT 9: target = 15 ms % Queue delay target = RTT_typ * 0.22 * 2
10: RTT_max = 100 ms % Worst case RTT expected
11: % PI2 constants derived from above PI2 parameters 11: % PI2 constants derived from above PI2 parameters
10: p_Cmax = min(1/k^2, 1) % Max Classic drop/mark prob 12: p_Cmax = min(1/k^2, 1) % Max Classic drop/mark prob
12: target = RTT_typ % PI AQM Classic queue delay target 13: Tupdate = min(target, RTT_max/3) % PI sampling interval
13: Tupdate = min(RTT_typ, RTT_max/3) % PI sampling interval
14: alpha = 0.1 * Tupdate / RTT_max^2 % PI integral gain in Hz 14: alpha = 0.1 * Tupdate / RTT_max^2 % PI integral gain in Hz
15: beta = 0.3 / RTT_max % PI proportional gain in Hz 15: beta = 0.3 / RTT_max % PI proportional gain in Hz
16: 16:
17: % L4S ramp AQM parameters 17: % L4S ramp AQM parameters
18: minTh = 800 us % L4S min marking threshold in time units 18: minTh = 800 us % L4S min marking threshold in time units
19: range = 400 us % Range of L4S ramp in time units 19: range = 400 us % Range of L4S ramp in time units
20: Th_len = 2 * MTU % Min L4S marking threshold in bytes 20: Th_len = 2 * MTU % Min L4S marking threshold in bytes
21: % L4S constants incl. those derived from other parameters 21: % L4S constants incl. those derived from other parameters
22: p_Lmax = 1 % Max L4S marking prob 22: p_Lmax = 1 % Max L4S marking prob
23: floor = Th_len / MIN_LINK_RATE 23: floor = Th_len / MIN_LINK_RATE
skipping to change at page 35, line 44 skipping to change at page 37, line 10
probability p_CL and the probability from the native L4S AQM p'_L. probability p_CL and the probability from the native L4S AQM p'_L.
This implements the max() function shown in Figure 1 to couple the This implements the max() function shown in Figure 1 to couple the
outputs of the two AQMs together. Of the two probabilities input outputs of the two AQMs together. Of the two probabilities input
to p_L in line 6: to p_L in line 6:
* p'_L is calculated per packet in line 5 by the laqm() function * p'_L is calculated per packet in line 5 by the laqm() function
(see Figure 5), (see Figure 5),
* Whereas p_CL is maintained by the dualpi2_update() function * Whereas p_CL is maintained by the dualpi2_update() function
which runs every Tupdate (Tupdate is set in line 13 of which runs every Tupdate (Tupdate is set in line 13 of
Figure 2. It defaults to 16 ms in the reference Linux Figure 2).
implementation because it has to be rounded to a multiple of 4
ms).
o If a Classic packet is scheduled, lines 10 to 17 drop or mark the o If a Classic packet is scheduled, lines 10 to 17 drop or mark the
packet with probability p_C. packet with probability p_C.
The Native L4S AQM algorithm (Figure 5) is a ramp function, similar The Native L4S AQM algorithm (Figure 5) is a ramp function, similar
to the RED algorithm, but simplified as follows: to the RED algorithm, but simplified as follows:
o The extent of the ramp is defined in units of queuing delay, not o The extent of the ramp is defined in units of queuing delay, not
bytes, so that configuration remains invariant as the queue bytes, so that configuration remains invariant as the queue
departure rate varies. departure rate varies.
skipping to change at page 37, line 41 skipping to change at page 38, line 43
Note that p' solely depends on the queuing time in the Classic queue. Note that p' solely depends on the queuing time in the Classic queue.
In line 2, the current queuing delay (curq) is evaluated from how In line 2, the current queuing delay (curq) is evaluated from how
long the head packet was in the Classic queue (cq). The function long the head packet was in the Classic queue (cq). The function
cq.time() (not shown) subtracts the time stamped at enqueue from the cq.time() (not shown) subtracts the time stamped at enqueue from the
current time (see Note a) and implicitly takes the current queuing current time (see Note a) and implicitly takes the current queuing
delay as 0 if the queue is empty. delay as 0 if the queue is empty.
The algorithm centres on line 3, which is a classical Proportional- The algorithm centres on line 3, which is a classical Proportional-
Integral (PI) controller that alters p' dependent on: a) the error Integral (PI) controller that alters p' dependent on: a) the error
between the current queuing delay (curq) and the target queuing delay between the current queuing delay (curq) and the target queuing
('target' - see [RFC8033]); and b) the change in queuing delay since delay, 'target'; and b) the change in queuing delay since the last
the last sample. The name 'PI' represents the fact that the second sample. The name 'PI' represents the fact that the second factor
factor (how fast the queue is growing) is _P_roportional to load (how fast the queue is growing) is _P_roportional to load while the
while the first is the _I_ntegral of the load (so it removes any first is the _I_ntegral of the load (so it removes any standing queue
standing queue in excess of the target). in excess of the target).
The two 'gain factors' in line 3, alpha and beta, respectively weight The target parameter can be set based on local knowledge, but the aim
how strongly each of these elements ((a) and (b)) alters p'. They is for the default to be a good compromise for anywhere in the
are in units of 'per second of delay' or Hz, because they transform intended deployment environment---the public Internet. The target
differences in queueing delay into changes in probability (assuming queuing delay is related to the typical base RTT, RTT_typ, by two
probability has a value from 0 to 1). factors, shown in the comment on line 9 of Figure 2 as target =
RTT_typ * 0.22 * 2. These factors ensure that, in a large proportion
of cases (say 90%), the sawtooth variations in RTT will fit within
the buffer without underutilizing the link. Frankly, these factors
are educated guesses, but with the emphasis closer to 'educated' than
to 'guess' (see [PI2param] for background investigations):
o RTT_typ is taken as 34 ms. This is based on an average CDN
latency measured in each country weighted by the number of
Internet users in that country to produce an overall weighted
average for the Internet [PI2param].
o The factor 0.22 is a geometry factor that characterizes the shape
of the sawteeth of prevalent Classic congestion controllers. The
geometry factor is the difference between the minimum and the
average queue delays of the sawteeth, relative to the base RTT.
For instance, the geometry factor of standard Reno is 0.5.
According to the census of congestion controllers conducted by
Mishra _et al_ in Jul-Oct 2019 [CCcensus19], most Classic TCP
traffic uses Cubic. And, according to the analysis in [PI2param],
if running over a PI2 AQM, a large proportion of this Cubic
traffic would be in its Reno-Friendly mode, which has a geometry
factor of 0.21 (Linux implementation). The rest of the Cubic
traffic would be in true Cubic mode, which has a geometry factor
of 0.32. Without modelling the sawtooth profiles from all the
other less prevalent congestion controllers, we estimate a 9:1
weighted average of these two, resulting in an average geometry
factor of 0.22.
o The factor 2, is a safety factor that increases the target queue
to allow for the distribution of RTT_typ around its mean.
Otherwise the target queue would only avoid underutilization for
those users below the mean. It also provides a safety margin for
the proportion of paths in use that span beyond the distance
between a user and their local CDN. Currently no data is
available on the variance of queue delay around the mean in each
region, so there is plenty of room for this guess to become more
educated.
The two 'gain factors' in line 3 of Figure 6, alpha and beta,
respectively weight how strongly each of the two elements (Integral
and Proportional) alters p'. They are in units of 'per second of
delay' or Hz, because they transform differences in queueing delay
into changes in probability (assuming probability has a value from 0
to 1).
alpha and beta determine how much p' ought to change after each alpha and beta determine how much p' ought to change after each
update interval (Tupdate). For smaller Tupdate, p' should change by update interval (Tupdate). For smaller Tupdate, p' should change by
the same amount per second, but in finer more frequent steps. So the same amount per second, but in finer more frequent steps. So
alpha depends on Tupdate (see line 14 of the initialization function alpha depends on Tupdate (see line 13 of the initialization function
in Figure 2). It is best to update p' as frequently as possible, but in Figure 2). It is best to update p' as frequently as possible, but
Tupdate will probably be constrained by hardware performance. As Tupdate will probably be constrained by hardware performance. As
shown in line 13, the update interval should be at least as frequent shown in line 13, the update interval should be frequent enough to
as once per the RTT of a typical flow (RTT_typ) as long as it does update at least once in the time taken for the target queue to drain
not exceed roughly RTT_max/3. For link rates from 4 - 200 Mb/s, a ('target') as long as it updates at least three times per maximum
target RTT of 15ms and a maximum RTT of 100ms, it has been verified RTT. Tupdate defaults to 16 ms in the reference Linux implementation
through extensive testing that Tupdate=16ms (as recommended in because it has to be rounded to a multiple of 4 ms. For link rates
from 4 to 200 Mb/s and a maximum RTT of 100ms, it has been verified
through extensive testing that Tupdate=16ms (as also recommended in
[RFC8033]) is sufficient. [RFC8033]) is sufficient.
The choice of alpha and beta also determines the AQM's stable The choice of alpha and beta also determines the AQM's stable
operating range. The AQM ought to change p' as fast as possible in operating range. The AQM ought to change p' as fast as possible in
response to changes in load without over-compensating and therefore response to changes in load without over-compensating and therefore
causing oscillations in the queue. Therefore, the values of alpha causing oscillations in the queue. Therefore, the values of alpha
and beta also depend on the RTT of the expected worst-case flow and beta also depend on the RTT of the expected worst-case flow
(RTT_max). (RTT_max).
The maximum RTT of a PI controller (RTT_max in line 10 of Figure 2)
is not an absolute maximum, but more instability (more queue
variability) sets in for long-running flows with an RTT above this
value. The propagation delay half way round the planet and back in
glass fibre is 200 ms. However, hardly any traffic traverses such
extreme paths and, since the significant consolidation of Internet
traffic between 2007 and 2009 [Labovitz10], a high and growing
proportion of all Internet traffic (roughly two-thirds at the time of
writing) has been served from content distribution networks (CDNs) or
'cloud' services distributed close to end-users. The Internet might
change again, but for now, designing for a maximum RTT of 100ms is a
good compromise between faster queue control at low RTT and some
instability on the occasions when a longer path is necessary.
Recommended derivations of the gain constants alpha and beta can be Recommended derivations of the gain constants alpha and beta can be
approximated for Reno over a PI2 AQM as: alpha = 0.1 * Tupdate / approximated for Reno over a PI2 AQM as: alpha = 0.1 * Tupdate /
RTT_max^2; beta = 0.3 / RTT_max, as shown in lines 14 & 15 of RTT_max^2; beta = 0.3 / RTT_max, as shown in lines 14 & 15 of
Figure 2. These are derived from the stability analysis in [PI2]. Figure 2. These are derived from the stability analysis in [PI2].
For the default values of Tupdate=16 ms and RTT_max = 100 ms, they For the default values of Tupdate=16 ms and RTT_max = 100 ms, they
result in alpha = 0.16; beta = 3.2 (discrepancies are due to result in alpha = 0.16; beta = 3.2 (discrepancies are due to
rounding). These defaults have been verified with a wide range of rounding). These defaults have been verified with a wide range of
link rates, target delays and a range of traffic models with mixed link rates, target delays and a range of traffic models with mixed
and similar RTTs, short and long flows, etc. and similar RTTs, short and long flows, etc.
 End of changes. 41 change blocks. 
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