draft-ietf-tsvwg-aqm-dualq-coupled-07.txt   draft-ietf-tsvwg-aqm-dualq-coupled-08.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: April 25, 2019 CableLabs Expires: May 8, 2019 CableLabs
O. Bondarenko O. Bondarenko
Simula Research Lab Simula Research Lab
I. Tsang I. Tsang
Nokia Nokia
October 22, 2018 November 04, 2018
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-07 draft-ietf-tsvwg-aqm-dualq-coupled-08
Abstract Abstract
Data Centre TCP (DCTCP) was designed to provide predictably low The Low Latency Low Loss Scalable Throughput (L4S) architecture
queuing latency, near-zero loss, and throughput scalability using allows data flows over the public Internet to predictably achieve
explicit congestion notification (ECN) and an extremely simple ultra-low queuing latency, generally zero congestion loss and scaling
marking behaviour on switches. However, DCTCP does not co-exist with of per-flow throughput without the problems of traditional TCP. To
existing TCP traffic---DCTCP is so aggressive that existing TCP achieve this, L4S data flows use a 'scalable' congestion control
algorithms approach starvation. So, until now, DCTCP could only be similar to Data Centre TCP (DCTCP) and a form of Explicit Congestion
deployed where a clean-slate environment could be arranged, such as Notification (ECN) with modified behaviour. However, until now,
in private data centres. This specification defines `DualQ Coupled scalable congestion controls did not co-exist with existing TCP Reno/
Active Queue Management (AQM)' to allow scalable congestion controls Cubic traffic---scalable controls are so aggressive that 'Classic'
like DCTCP to safely co-exist with classic Internet traffic. The TCP algorithms drive themselves to starvation. Therefore, until now,
Coupled AQM ensures that a flow runs at about the same rate whether L4S controls could only be deployed where a clean-slate environment
it uses DCTCP or TCP Reno/Cubic, but without inspecting transport could be arranged, such as in private data centres (hence the name
layer flow identifiers. When tested in a residential broadband DCTCP). This specification defines `DualQ Coupled Active Queue
setting, DCTCP achieved sub-millisecond average queuing delay and Management (AQM)', which enables these scalable congestion controls
zero congestion loss under a wide range of mixes of DCTCP and to safely co-exist with Classic Internet traffic.
`Classic' broadband Internet traffic, without compromising the
performance of the Classic traffic. The solution also reduces The Coupled AQM ensures that a flow runs at about the same rate
network complexity and eliminates network configuration. whether it uses DCTCP or TCP Reno/Cubic. It achieves this
indirectly, without having to inspect transport layer flow
identifiers, When tested in a residential broadband setting, DCTCP
also achieves sub-millisecond average queuing delay and zero
congestion loss under a wide range of mixes of DCTCP and `Classic'
broadband Internet traffic, without compromising the performance of
the Classic traffic. The solution also reduces network complexity
and eliminates network configuration.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 25, 2019. This Internet-Draft will expire on May 8, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 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
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publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 31 skipping to change at page 2, line 39
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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. Problem and Scope . . . . . . . . . . . . . . . . . . . . 3 1.1. Problem and Scope . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 6
2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 7 2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Traffic Classification . . . . . . . . . . . . . . . . . 8 2.3. Traffic Classification . . . . . . . . . . . . . . . . . 9
2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 9 2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 10
2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 11 2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 12
2.5.1. Functional Requirements . . . . . . . . . . . . . . . 11 2.5.1. Functional Requirements . . . . . . . . . . . . . . . 12
2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 13 2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 13
2.5.2. Management Requirements . . . . . . . . . . . . . . . 14 2.5.2. Management Requirements . . . . . . . . . . . . . . . 15
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15 4. Security Considerations . . . . . . . . . . . . . . . . . . . 16
4.1. Overload Handling . . . . . . . . . . . . . . . . . . . . 15 4.1. Overload Handling . . . . . . . . . . . . . . . . . . . . 16
4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput 4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput
or Delay? . . . . . . . . . . . . . . . . . . . . . . 15 or Delay? . . . . . . . . . . . . . . . . . . . . . . 17
4.1.2. Congestion Signal Saturation: Introduce L4S Drop or 4.1.2. Congestion Signal Saturation: Introduce L4S Drop or
Delay? . . . . . . . . . . . . . . . . . . . . . . . 16 Delay? . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.3. Protecting against Unresponsive ECN-Capable Traffic . 17 4.1.3. Protecting against Unresponsive ECN-Capable Traffic . 19
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Normative References . . . . . . . . . . . . . . . . . . 18 6.1. Normative References . . . . . . . . . . . . . . . . . . 20
6.2. Informative References . . . . . . . . . . . . . . . . . 18 6.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 21 Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 23
A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 21 A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 23
A.2. Pass #2: Overload Details . . . . . . . . . . . . . . . . 27 A.2. Pass #2: Overload Details . . . . . . . . . . . . . . . . 30
Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 30 Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 33
Appendix C. Guidance on Controlling Throughput Equivalence . . . 36 Appendix C. Guidance on Controlling Throughput Equivalence . . . 39
Appendix D. Open Issues . . . . . . . . . . . . . . . . . . . . 37 Appendix D. Open Issues . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction 1. Introduction
1.1. Problem and Scope 1.1. Problem and Scope
Latency is becoming the critical performance factor for many (most?) Latency is becoming the critical performance factor for many (most?)
applications on the public Internet, e.g. interactive Web, Web applications on the public Internet, e.g. interactive Web, Web
services, voice, conversational video, interactive video, interactive services, voice, conversational video, interactive video, interactive
remote presence, instant messaging, online gaming, remote desktop, remote presence, instant messaging, online gaming, remote desktop,
cloud-based applications, and video-assisted remote control of cloud-based applications, and video-assisted remote control of
machinery and industrial processes. In the developed world, further machinery and industrial processes. In the developed world, further
increases in access network bit-rate offer diminishing returns, increases in access network bit-rate offer diminishing returns,
whereas latency is still a multi-faceted problem. In the last decade whereas latency is still a multi-faceted problem. In the last decade
or so, much has been done to reduce propagation time by placing or so, much has been done to reduce propagation time by placing
caches or servers closer to users. However, queuing remains a major caches or servers closer to users. However, queuing remains a major
component of latency. intermittent component of latency.
The Diffserv architecture provides Expedited Forwarding [RFC3246], so The Diffserv architecture provides Expedited Forwarding [RFC3246], so
that low latency traffic can jump the queue of other traffic. that low latency traffic can jump the queue of other traffic.
However, on access links dedicated to individual sites (homes, small However, on access links dedicated to individual sites (homes, small
enterprises or mobile devices), often all traffic at any one time enterprises or mobile devices), often all traffic at any one time
will be latency-sensitive and, if all the traffic on a link is marked will be latency-sensitive and, if all the traffic on a link is marked
as EF, Diffserv cannot reduce the delay of any of it. In contrast, as EF, Diffserv cannot reduce the delay of any of it. In contrast,
the Low Latency Low Loss Scalable throughput (L4S) approach removes the Low Latency Low Loss Scalable throughput (L4S) approach removes
the causes of any unnecessary queuing delay. the causes of any unnecessary queuing delay.
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Active queue management (AQM) was originally developed to solve this Active queue management (AQM) was originally developed to solve this
problem (and others). Unlike Diffserv, which gives low latency to problem (and others). Unlike Diffserv, which gives low latency to
some traffic at the expense of others, AQM controls latency for _all_ some traffic at the expense of others, AQM controls latency for _all_
traffic in a class. In general, AQMs introduce an increasing level traffic in a class. In general, AQMs introduce an increasing level
of discard from the buffer the longer the queue persists above a of discard from the buffer the longer the queue persists above a
shallow threshold. This gives sufficient signals to capacity-seeking shallow threshold. This gives sufficient signals to capacity-seeking
(aka. greedy) flows to keep the buffer empty for its intended (aka. greedy) flows to keep the buffer empty for its intended
purpose: absorbing bursts. However, RED [RFC2309] and other purpose: absorbing bursts. However, RED [RFC2309] and other
algorithms from the 1990s were sensitive to their configuration and algorithms from the 1990s were sensitive to their configuration and
hard to set correctly. So, AQM was not widely deployed. hard to set correctly. So, AQM was not widely deployed in the 1990s.
More recent state-of-the-art AQMs, e.g. fq_CoDel [RFC8290], More recent state-of-the-art AQMs, e.g. fq_CoDel [RFC8290],
PIE [RFC8033], Adaptive RED [ARED01], are easier to configure, PIE [RFC8033], Adaptive RED [ARED01], are easier to configure,
because they define the queuing threshold in time not bytes, so it is because they define the queuing threshold in time not bytes, so it is
invariant for different link rates. However, no matter how good the invariant for different link rates. However, no matter how good the
AQM, the sawtoothing rate of TCP will either cause queuing delay to AQM, the sawtoothing rate of TCP will either cause queuing delay to
vary or cause the link to be under-utilized. Even with a perfectly vary or cause the link to be under-utilized. Even with a perfectly
tuned AQM, the additional queuing delay will be of the same order as tuned AQM, the additional queuing delay will be of the same order as
the underlying speed-of-light delay across the network. Flow-queuing the underlying speed-of-light delay across the network. Flow-queuing
can isolate one flow from another, but it cannot isolate a TCP flow can isolate one flow from another, but it cannot isolate a TCP flow
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The former causes a flow's round trip time (RTT) to vary from about 1 The former causes a flow's round trip time (RTT) to vary from about 1
to 2 times the base RTT between the machines in question. The latter to 2 times the base RTT between the machines in question. The latter
delays the system's response to change by a worst-case delays the system's response to change by a worst-case
(transcontinental) RTT, which could be hundreds of times the actual (transcontinental) RTT, which could be hundreds of times the actual
RTT of typical traffic from localized CDNs. RTT of typical traffic from localized CDNs.
Latency is not our only concern: Latency is not our only concern:
3. It was known when TCP was first developed that it would not scale 3. It was known when TCP was first developed that it would not scale
to high bandwidth-delay products. to high bandwidth-delay products [TCP-CA].
Given regular broadband bit-rates over WAN distances are Given regular broadband bit-rates over WAN distances are
already [RFC3649] beyond the scaling range of `classic' TCP Reno, already [RFC3649] beyond the scaling range of `classic' TCP Reno,
`less unscalable' Cubic [RFC8312] and `less unscalable' Cubic [RFC8312] and
Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been
successfully deployed. However, these are now approaching their successfully deployed. However, these are now approaching their
scaling limits. Unfortunately, fully scalable TCPs such as DCTCP scaling limits. Unfortunately, fully scalable TCPs such as DCTCP
cause `classic' TCP to starve itself, which is why they have been cause `classic' TCP to starve itself, which is why they have been
confined to private data centres or research testbeds (until now). confined to private data centres or research testbeds (until now).
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. The AQM is not like without having to inspect flow identifiers. The AQM is not like
flow-queuing approaches [RFC8290] that classify packets by flow flow-queuing approaches [RFC8290] that classify packets by flow
identifier into numerous separate queues in order to isolate sparse identifier into numerous separate queues in order to isolate sparse
flows from the higher latency in the queues assigned to heavier flow. flows from the higher latency in the queues assigned to heavier
In contrast, the AQM exploits the behaviour of scalable congestion flows. In contrast, the AQM exploits the behaviour of scalable
controls like DCTCP so that every packet in every flow sharing the congestion controls like DCTCP so that every packet in every flow
queue for DCTCP-like traffic can be served with very low latency. sharing the queue for DCTCP-like traffic can be served with very low
latency.
This AQM extension can be combined with any single queue AQM that This AQM extension can be combined with any AQM designed for a single
generates a statistical or deterministic mark/drop probability driven queue that generates a statistical or deterministic mark/drop
by the queue dynamics. In many cases it simplifies the basic control probability driven by the queue dynamics. In many cases it
algorithm, and requires little extra processing. Therefore it is simplifies the basic control algorithm, and requires little extra
believed the Coupled AQM would be applicable and easy to deploy in processing. Therefore it is believed the Coupled AQM would be
all types of buffers; buffers in cost-reduced mass-market residential applicable and easy to deploy in all types of buffers; buffers in
equipment; buffers in end-system stacks; buffers in carrier-scale cost-reduced mass-market residential equipment; buffers in end-system
equipment including remote access servers, routers, firewalls and stacks; buffers in carrier-scale equipment including remote access
Ethernet switches; buffers in network interface cards, buffers in servers, routers, firewalls and Ethernet switches; buffers in network
virtualized network appliances, hypervisors, and so on. interface cards, buffers in virtualized network appliances,
hypervisors, and so on.
The overall L4S architecture is described in For the public Internet, nearly all the benefit will typically be
[I-D.ietf-tsvwg-l4s-arch]. The supporting papers [PI2] and [DCttH15] achieved by deploying the Coupled AQM into either end of the access
give the full rationale for the AQM's design, both discursively and link between a 'site' and the Internet, which is invariably the
in more precise mathematical form. bottleneck. Here, the term 'site' is used loosely to mean a home, an
office, a campus or mobile user equipment.
The overall L4S architecture [I-D.ietf-tsvwg-l4s-arch] gives more
detail, including on wider deployment aspects such as coexistence in
bottlenecks where a DualQ Coupled AQM has not been deployed. The
supporting papers [PI2] and [DCttH15] give the full rationale for the
AQM's design, both discursively and in more precise mathematical
form.
1.2. Terminology 1.2. 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]. In this document are to be interpreted as described in [RFC2119] when, and
document, these words will appear with that interpretation only when only when, they appear in all capitals, as shown here.
in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
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 (denoted by subscript C): The `Classic' service is intended Classic (denoted by subscript C): The `Classic' service is intended
for all the behaviours that currently co-exist with TCP Reno (TCP for all the behaviours that currently co-exist with TCP Reno (TCP
Cubic, Compound, SCTP, etc). Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S, denoted by subscript L): Low-Latency, Low-Loss and Scalable (L4S, denoted by subscript L):
The `L4S' service is intended for a set of congestion controls The `L4S' service is intended for a set of congestion controls
with scalable properties such as DCTCP (e.g. with scalable properties (e.g. DCTCP [RFC8257], Relentless
Relentless [Mathis09]). TCP [Mathis09], the L4S variant of SCREAM for real-time
media {ToDo: ref}). For the public Internet a scalable control
has to comply with the requirements in [I-D.ietf-tsvwg-ecn-l4s-id]
(aka. the 'TCP Prague requirements').
Either service can cope with a proportion of unresponsive or less- Either service can cope with a proportion of unresponsive or less-
responsive traffic as well (e.g. DNS, VoIP, etc), just as a single responsive traffic as well, as long (e.g. DNS, VoIP, game sync
queue AQM can. The DualQ Coupled AQM behaviour is similar to a datagrams, etc), just as a single queue AQM can if this traffic makes
single FIFO queue with respect to unresponsive and overload traffic. minimal contribution to queuing. The DualQ Coupled AQM behaviour
below is defined to be similar to a single FIFO queue with respect to
unresponsive and overload traffic.
1.3. Features 1.3. Features
The AQM couples marking and/or dropping across the two queues such The AQM couples marking and/or dropping across the two queues such
that a flow will get roughly the same throughput whichever it uses. that a flow will get roughly the same throughput whichever it uses.
Therefore both queues can feed into the full capacity of a link and Therefore both queues can feed into the full capacity of a link and
no rates need to be configured for the queues. The L4S queue enables no rates need to be configured for the queues. The L4S queue enables
scalable congestion controls like DCTCP to give stunningly low and scalable congestion controls like DCTCP to give stunningly low and
predictably low latency, without compromising the performance of predictably low latency, without compromising the performance of
competing 'Classic' Internet traffic. Thousands of tests have been competing 'Classic' Internet traffic. Thousands of tests have been
conducted in a typical fixed residential broadband setting. Typical conducted in a typical fixed residential broadband setting. Typical
experiments used base round trip delays up to 100ms between the data experiments used base round trip delays up to 100ms between the data
centre and home network, and large amounts of background traffic in centre and home network, and large amounts of background traffic in
both queues. For every L4S packet, the AQM kept the average queuing both queues. For every L4S packet, the AQM kept the average queuing
delay below 1ms (or 2 packets if serialization delay is bigger for delay below 1ms (or 2 packets if serialization delay is bigger for
slow links), and no losses at all were introduced by the AQM. slow links), and no losses at all were introduced by the AQM.
Details of the extensive experiments will be made available [PI2] Details of the extensive experiments are available [PI2] [DCttH15].
[DCttH15].
Subjective testing was also conducted using a demanding panoramic Subjective testing was also conducted by multiple people all
interactive video application run over a stack with DCTCP enabled and simultaneously using very demanding high bandwidth low latency
deployed on the testbed. Each user could pan or zoom their own high applications over a single shared access link [L4Sdemo16]. In one
definition (HD) sub-window of a larger video scene from a football application, each user could use finger gestures to pan or zoom their
match. Even though the user was also downloading large amounts of own high definition (HD) sub-window of a larger video scene generated
L4S and Classic data, latency was so low that the picture appeared to on the fly in 'the cloud' from a football match. Another user
stick to their finger on the touchpad (all the L4S data achieved the wearing VR goggles was remotely receiving a feed from a 360-degree
same ultra-low latency). With an alternative AQM, the video camera in a racing car, again with the sub-window in their field of
noticeably lagged behind the finger gestures. vision generated on the fly in 'the cloud' dependent on their head
movements. Even though other users were also downloading large
amounts of L4S and Classic data, playing a gaming benchmark and
watchings videos over the same 40Mb/s downstream broadband link,
latency was so low that the football picture appeared to stick to the
user's finger on the touchpad and the experience fed from the remote
camera did not noticeably lag head movements. All the L4S data (even
including the downloads) achieved the same ultra-low latency. With
an alternative AQM, the video noticeably lagged behind the finger
gestures and head movements.
Unlike Diffserv Expedited Forwarding, the L4S queue does not have to Unlike Diffserv Expedited Forwarding, the L4S queue does not have to
be limited to a small proportion of the link capacity in order to be limited to a small proportion of the link capacity in order to
achieve low delay. The L4S queue can be filled with a heavy load of achieve low delay. The L4S queue can be filled with a heavy load of
capacity-seeking flows like DCTCP and still achieve low delay. The capacity-seeking flows like DCTCP and still achieve low delay. The
L4S queue does not rely on the presence of other traffic in the L4S queue does not rely on the presence of other traffic in the
Classic queue that can be 'overtaken'. It gives low latency to L4S Classic queue that can be 'overtaken'. It gives low latency to L4S
traffic whether or not there is Classic traffic, and the latency of traffic whether or not there is Classic traffic, and the latency of
Classic traffic does not suffer when a proportion of the traffic is Classic traffic does not suffer when a proportion of the traffic is
L4S. The two queues are only necessary because DCTCP-like flows L4S. The two queues are only necessary because DCTCP-like flows
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Then, without any management intervention, applications can exploit Then, without any management intervention, applications can exploit
it by migrating to scalable controls like DCTCP, which can then it by migrating to scalable controls like DCTCP, which can then
evolve _while_ their benefits are being enjoyed by everyone on the evolve _while_ their benefits are being enjoyed by everyone on the
Internet. Internet.
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 (e.g. DCTCP) flows Classic (e.g. Reno, Cubic) flows and L4S flows (that satisfy the
TCP Prague requirements).
o the Dual Queue structure that provides latency separation for L4S o the Dual Queue structure that provides latency separation for L4S
flows to isolate them from the typically large Classic queue. flows to isolate them from the typically large Classic queue.
2.1. Coupled AQM 2.1. Coupled AQM
In the 1990s, the `TCP formula' was derived for the relationship In the 1990s, the `TCP formula' was derived for the relationship
between TCP's congestion window, cwnd, and its drop probability, p. between TCP's congestion window, cwnd, and its drop probability, p.
To a first order approximation, cwnd of TCP Reno is inversely To a first order approximation, cwnd of TCP Reno is inversely
proportional to the square root of p. proportional to the square root of p.
TCP Cubic implements a Reno-compatibility mode, which is the only We focus on Reno as the worst case, because if we do not harm Reno,
relevant mode for typical RTTs under 20ms as long as the throughput we will not harm Cubic. Nonetheless, TCP Cubic implements a Reno-
of a single flow is less than about 500Mb/s. Therefore it can be compatibility mode, which is the only relevant mode for typical RTTs
assumed that Cubic traffic behaves similarly to Reno (but with a under 20ms as long as the throughput of a single flow is less than
slightly different constant of proportionality), and the term about 500Mb/s. Therefore it can be assumed that Cubic traffic
'Classic' will be used for the collection of Reno-friendly traffic behaves similarly to Reno (but with a slightly different constant of
including Cubic in Reno mode. proportionality). The term 'Classic' will be used for the collection
of Reno-friendly traffic including Cubic in Reno mode.
The supporting paper [PI2] includes the derivation of the equivalent The 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 'L4S' traffic will be used for all similar like this, so the term 'L4S' traffic will be used for all similar
behaviour. behaviour.
In order to make a DCTCP flow run at roughly the same rate as a Reno For safe co-existence, under stationary conditions, a DCTCP flow has
TCP flow (all other factors being equal), the drop or marking to run at roughly the same rate as a Reno TCP flow (all other factors
probability for Classic traffic, p_C has to be distinct from the being equal). So the drop or marking probability for Classic
marking probability for L4S traffic, p_L (in contrast to RFC3168 traffic, p_C has to be distinct from the marking probability for L4S
which requires them to be the same). To remain stable, Classic traffic, p_L. [RFC8311] updates the original ECN specification
traffic needs p_C to change relatively slowly, whereas L4S traffic [RFC3168] to allow these probabilities to be distinct, because RFC
needs to be controlled rapidly by a probability p_L that track the 3168 required them to be the same.
instantaneous queue. It is necessary to make the Classic drop
probability p_C proportional to the square of a variable we shall Also, to remain stable, Classic sources need the network to smooth
call p_CL, which is an input to the instantaneous L4S marking p_C so it changes relatively slowly. In contrast, L4S avoids
probability p_L but changes as slowly as p_C. This makes the Reno smoothing in the network, because it delays all signals for a worst-
flow rate roughly equal the DCTCP flow rate, because it squares the case RTT. So instead, L4S sources smooth the ECN marking probability
square root of p_C in the Reno rate equation to make it proportional themselves, so they expect the network to generate ECN marks with a
to the smoothed value of p_L used in the DCTCP rate equation. probability p_L that tracks the instantaneous unsmoothed queue.
The Coupled AQM achieves safe coexistence by making the Classic drop
probability p_C proportional to the square of the coupled L4S
probability p_CL. p_CL is an input to the instantaneous L4S marking
probability p_L but it changes as slowly as p_C. This makes the Reno
flow rate roughly equal the DCTCP flow rate, because the squaring of
p_CL counterbalances the square root of p_C in the Classic 'TCP
formula'.
Stating this as a formula, the relation between Classic drop Stating this as a formula, the relation between Classic drop
probability, p_C, and the input variable p_CL to the L4S marking probability, p_C, and the coupled L4S probability p_CL needs to take
probability p_L needs to take the form: the form:
p_C = ( p_CL / k )^2 (1) p_C = ( p_CL / k )^2 (1)
where k is the constant of proportionality. where k is the constant of proportionality, which we shall call the
coupling factor.
2.2. Dual Queue 2.2. Dual Queue
Classic traffic typically builds a large queue to prevent under- Classic traffic typically builds a large queue to prevent under-
utilization. Therefore a separate queue is provided for L4S traffic, utilization. Therefore a separate queue is provided for L4S traffic,
and it is scheduled with priority over Classic. Priority is and it is scheduled with priority over Classic. Priority is
conditional to prevent starvation of Classic traffic. conditional to prevent starvation of Classic traffic.
Nonetheless, coupled marking ensures that giving priority to L4S Nonetheless, coupled marking ensures that giving priority to L4S
traffic still leaves the right amount of spare scheduling time for traffic still leaves the right amount of spare scheduling time for
Classic flows to each get equivalent throughput to DCTCP flows (all Classic flows to each get equivalent throughput to DCTCP flows (all
other factors such as RTT being equal). The algorithm achieves this other factors such as RTT being equal).
without having to inspect flow identifiers.
2.3. Traffic Classification 2.3. Traffic Classification
Both the Coupled AQM and DualQ mechanisms need an identifier to Both the Coupled AQM and DualQ mechanisms need an identifier to
distinguish L and C packets. A separate draft distinguish L and C packets. Then the coupling algorithm can achieve
[I-D.ietf-tsvwg-ecn-l4s-id] recommends using the ECT(1) codepoint of coexistence without having to inspect flow identifiers, because it
the ECN field as this identifier, having assessed various can apply the appropriate marking or dropping probability to all
alternatives. An additional process document has proved necessary to flows of each type. A separate
make the ECT(1) codepoint available for experimentation [RFC8311]. specification [I-D.ietf-tsvwg-ecn-l4s-id] requires the sender to use
the ECT(1) codepoint of the ECN field as this identifier, having
assessed various alternatives. An additional process document has
proved necessary to make the ECT(1) codepoint available for
experimentation [RFC8311].
For policy reasons, an operator might choose to steer certain packets For policy reasons, an operator might choose to steer certain packets
(e.g. from certain flows or with certain addresses) out of the L (e.g. from certain flows or with certain addresses) out of the L
queue, even though they identify themselves as L4S by their ECN queue, even though they identify themselves as L4S by their ECN
codepoints. In such cases, the classifier MUST NOT alter the ECN codepoints. In such cases, the device MUST NOT alter the ECN field,
field, so that it is preserved end-to-end. The aim is that each so that it is preserved end-to-end. The aim is that each operator
operator can choose how it treats L4S traffic locally, but an can choose how it treats L4S traffic locally, but an individual
individual operator does not alter the identification of L4S packets, operator does not alter the identification of L4S packets, which
which would prevent other operators downstream from making their own would prevent other operators downstream from making their own
choices on how to treat L4S traffic. choices on how to treat L4S traffic.
In addition, other identifiers could be used to classify certain In addition, other identifiers could be used to classify certain
additional packet types into the L queue, that are deemed not to risk additional packet types into the L queue, that are deemed not to risk
harming the L4S service. For instance addresses of specific harming the L4S service. For instance addresses of specific
applications or hosts (see [I-D.ietf-tsvwg-ecn-l4s-id]), specific applications or hosts (see [I-D.ietf-tsvwg-ecn-l4s-id]), specific
Diffserv codepoints such as EF (Expedited Forwarding) and Voice-Admit Diffserv codepoints such as EF (Expedited Forwarding) and Voice-Admit
service classes (see [I-D.briscoe-tsvwg-l4s-diffserv]) or certain service classes (see [I-D.briscoe-tsvwg-l4s-diffserv]) or certain
protocols (e.g. ARP, DNS). protocols (e.g. ARP, DNS).
Note that the DualQ Coupled AQM only reads these classifiers, it MUST Note that the mechanism only reads these classifiers, it MUST NOT re-
NOT re-mark or alter these identifiers (except for marking the ECN mark or alter these identifiers (except for marking the ECN field
field with the CE codepoint - with increasing frequency to indicate with the CE codepoint - with increasing frequency to indicate
increasing congestion). increasing congestion).
2.4. Overall DualQ Coupled AQM Structure 2.4. Overall DualQ Coupled AQM Structure
Figure 1 shows the overall structure that any DualQ Coupled AQM is Figure 1 shows the overall structure that any DualQ Coupled AQM is
likely to have. This schematic is intended to aid understanding of likely to have. This schematic is intended to aid understanding of
the current designs of DualQ Coupled AQMs. However, it is not the current designs of DualQ Coupled AQMs. However, it is not
intended to preclude other innovative ways of satisfying the intended to preclude other innovative ways of satisfying the
normative requirements in Section 2.5 that minimally define a DualQ normative requirements in Section 2.5 that minimally define a DualQ
Coupled AQM. Coupled AQM.
The classifier on the left separates incoming traffic between the two The classifier on the left separates incoming traffic between the two
queues (L and C). Each queue has its own AQM that determines the queues (L and C). Each queue has its own AQM that determines the
likelihood of marking or dropping (p_L and p_C). It has been proved likelihood of marking or dropping (p_L and p_C). It has been
[PI2] that it is preferable to control TCP with a linear PI proved [PI2] that it is preferable to control load with a linear
controller, then square the output before applying it as a drop controller, then square the output before applying it as a drop
probability to TCP. So, the AQM for Classic traffic needs to be probability to TCP (because TCP decreases its load proportional to
implemented in two stages: i) a base stage that outputs an internal the square-root of the increase in drop). So, the AQM for Classic
probability p' (pronounced p-prime); and ii) a squaring stage that traffic needs to be implemented in two stages: i) a base stage that
outputs p_C, where outputs an internal probability p' (pronounced p-prime); and ii) a
squaring stage that outputs p_C, where
p_C = (p')^2. (2) p_C = (p')^2. (2)
Substituting for p_C in Eqn (1) gives: Substituting for p_C in Eqn (1) gives:
p' = p_CL / k p' = p_CL / k
So we get our slow-moving input to ECN marking in the L queue as: So the slow-moving input to ECN marking in the L queue (the coupled
L4S probability) is:
p_CL = k*p', (3) p_CL = k*p', (3)
where k is the constant coupling factor (see Appendix C). where k is the constant coupling factor (see Appendix C).
It can be seen that these two transformations of p' implement the It can be seen that these two transformations of p' implement the
required coupling given in equation (1) earlier. Substituting for p' required coupling given in equation (1) earlier.
from equation (3) into (2):
p_C = ( p_CL / k )^2.
The actual probability p_L that we apply to the L queue needs to The actual probability p_L that we apply to the L queue needs to
track the immediate L queue delay, as well as track p_CL under track the immediate L queue delay, as well as track p_CL under
stationary conditions. So we use a native AQM in the L queue that stationary conditions. So we use a native AQM in the L queue that
calculates a marking probability p'L as a function of the calculates a probability p'_L as a function of the instantaneous L
instantaneous L queue. And, given the L queue has conditional strict queue. And, given the L queue has conditional strict priority over
priority over the C queue, whenever the L queue grows, we should the C queue, whenever the L queue grows, we should apply marking
apply marking probability p'_L, but p_L should not fall below p_CL. probability p'_L, but p_L should not fall below p_CL. This suggests:
This suggests:
p_L = max(p'L, p_CL), p_L = max(p'_L, p_CL), (4)
which has also been found to work very well in practice. which has also been found to work very well in practice.
This allows p_L to be coupled to p_C by marking L4S traffic
proportionately to the intermediate output from the first stage.
Specifically, the output of the base AQM is coupled across to the L
queue in proportion to the output of the base AQM
_________ _________
| | ,------. | | ,------.
L4S queue | |===>| ECN | L4S queue | |===>| ECN |
,'| _______|_| |marker|\ ,'| _______|_| |marker|\
<' | | `------'\\ <' | | `------'\\
//`' v ^ p_L \\ //`' v ^ p_L \\
// ,-------. | \\ // ,-------. | \\
// |Native |p'L | \\,. // |Native |p'_L | \\,.
// | L4S |-->(MAX) < | ___ // | L4S |--->(MAX) < | ___
,----------.// | AQM | ^ p_CL `\|.'Cond-`. ,----------.// | AQM | ^ p_CL `\|.'Cond-`.
| IP-ECN |/ `-------' | / itional \ | IP-ECN |/ `-------' | / itional \
==>|Classifier| ,-------. (k*p') [ priority]==> ==>|Classifier| ,-------. (k*p') [ priority]==>
| |\ | Base | | \scheduler/ | |\ | Base | | \scheduler/
`----------'\\ | AQM |--->: ,'|`-.___.-' `----------'\\ | AQM |---->: ,'|`-.___.-'
\\ | |p' | <' | \\ | |p' | <' |
\\ `-------' (p'^2) //`' \\ `-------' (p'^2) //`'
\\ ^ | // \\ ^ | //
\\,. | v p_C // \\,. | v p_C //
< | _________ .------.// < | _________ .------.//
`\| | | | Drop |/ `\| | | | Drop |/
Classic |queue |===>|/mark | Classic |queue |===>|/mark |
__|______| `------' __|______| `------'
Legend: ===> traffic flow; ---> control dependency. Legend: ===> traffic flow; ---> control dependency.
Figure 1: DualQ Coupled AQM Schematic Figure 1: DualQ Coupled AQM Schematic
skipping to change at page 11, line 50 skipping to change at page 12, line 38
capitals) in Section 2.5 are observed. capitals) in Section 2.5 are observed.
2.5. Normative Requirements for a DualQ Coupled AQM 2.5. Normative Requirements for a DualQ Coupled AQM
The following requirements are intended to capture only the essential The following requirements are intended to capture only the essential
aspects of a DualQ Coupled AQM. They are intended to be independent aspects of a DualQ Coupled AQM. They are intended to be independent
of the particular AQMs used for each queue. of the particular AQMs used for each queue.
2.5.1. Functional Requirements 2.5.1. Functional Requirements
In the Dual Queue, L4S packets MUST be given priority over Classic, A Dual Queue Coupled AQM implementation MUST utilize two queues, each
although priority MUST be bounded in order not to starve Classic with an AQM algorithm. The two queues can be part of a larger
traffic. queuing hierarchy [I-D.briscoe-tsvwg-l4s-diffserv].
The AQM algorithm for the low latency (L) queue MUST apply ECN
marking.
The scheduler draining the two queues MUST give L4S packets priority
over Classic, although priority MUST be bounded in order not to
starve Classic traffic.
Whatever identifier is used for L4S experiments,
[I-D.ietf-tsvwg-ecn-l4s-id] defines the meaning of an ECN marking on [I-D.ietf-tsvwg-ecn-l4s-id] defines the meaning of an ECN marking on
L4S traffic, relative to drop of Classic traffic. In order to L4S traffic, relative to drop of Classic traffic. In order to
prevent starvation of Classic traffic by scalable L4S traffic, it prevent starvation of Classic traffic by scalable L4S traffic, it
says, "The likelihood that an AQM drops a Not-ECT Classic packet says, "The likelihood that an AQM drops a Not-ECT Classic packet
(p_C) MUST be roughly proportional to the square of the likelihood (p_C) MUST be roughly proportional to the square of the likelihood
that it would have marked it if it had been an L4S packet (p_L)." In that it would have marked it if it had been an L4S packet (p_L)."
other words, in any DualQ Coupled AQM, the power to which p_L is The term 'likelihood' is used to allow for marking and dropping to be
raised in Eqn. (1) MUST be 2. The term 'likelihood' is used to allow either probabilistic or deterministic.
for marking and dropping to be either probabilistic or deterministic.
For the current specification, this translates into the following
requirement. A DualQ Coupled AQM MUST apply ECN marking to traffic
in the L queue that is no lower than that derived from the likelihood
of drop (or ECN marking) in the Classic queue using Eqn. (1).
The constant of proportionality, k, in Eqn (1) determines the The constant of proportionality, k, in Eqn (1) determines the
relative flow rates of Classic and L4S flows when the AQM concerned relative flow rates of Classic and L4S flows when the AQM concerned
is the bottleneck (all other factors being equal). is the bottleneck (all other factors being equal).
[I-D.ietf-tsvwg-ecn-l4s-id] says, "The constant of proportionality [I-D.ietf-tsvwg-ecn-l4s-id] says, "The constant of proportionality
(k) does not have to be standardised for interoperability, but a (k) does not have to be standardised for interoperability, but a
value of 2 is RECOMMENDED." value of 2 is RECOMMENDED."
Assuming scalable congestion controls for the Internet will be as Assuming scalable congestion controls for the Internet will be as
aggressive as DCTCP, this will ensure their congestion window will be aggressive as DCTCP, this will ensure their congestion window will be
roughly the same as that of a standards track TCP congestion control roughly the same as that of a standards track TCP congestion control
(Reno) [RFC5681] and other so-called TCP-friendly controls, such as (Reno) [RFC5681] and other so-called TCP-friendly controls, such as
TCP Cubic in its TCP-friendly mode. TCP Cubic in its TCP-friendly mode.
{ToDo: The requirements for scalable congestion controls on the
Internet (termed the TCP Prague requirements)
[I-D.ietf-tsvwg-ecn-l4s-id] are not necessarily final. If the
aggressiveness of DCTCP is not defined as the benchmark for scalable
controls on the Internet, the recommended value of k will also be
subject to change.}
The choice of k is a matter of operator policy, and operators MAY The choice of k is a matter of operator policy, and operators MAY
choose a different value using Table 1 and the guidelines in choose a different value using Table 1 and the guidelines in
Appendix C. Appendix C.
If multiple users share capacity at a bottleneck (e.g. in the If multiple users share capacity at a bottleneck (e.g. in the
Internet access link of a campus network), the operator's choice of k Internet access link of a campus network), the operator's choice of k
will determine capacity sharing between the flows of different users. will determine capacity sharing between the flows of different users.
However, on the public Internet, access network operators typically However, on the public Internet, access network operators typically
isolate customers from each other with some form of layer-2 isolate customers from each other with some form of layer-2
multiplexing (TDM in DOCSIS, CDMA in 3G) or L3 scheduling (WRR in multiplexing (OFDM(A) in DOCSIS3.1, CDMA in 3G, SC-FDMA in LTE) or L3
DSL), rather than relying on TCP to share capacity between customers scheduling (WRR in DSL), rather than relying on TCP to share capacity
[RFC0970]. In such cases, the choice of k will solely affect between customers [RFC0970]. In such cases, the choice of k will
relative flow rates within each customer's access capacity, not solely affect relative flow rates within each customer's access
between customers. Also, k will not affect relative flow rates at capacity, not between customers. Also, k will not affect relative
any times when all flows are Classic or all L4S, and it will not flow rates at any times when all flows are Classic or all L4S, and it
affect small flows. will not affect the relative throughput of small flows.
2.5.1.1. Requirements in Unexpected Cases 2.5.1.1. Requirements in Unexpected Cases
The flexibility to allow operator-specific classifiers (Section 2.3) The flexibility to allow operator-specific classifiers (Section 2.3)
leads to the need to specify what the AQM in each queue ought to do leads to the need to specify what the AQM in each queue ought to do
with packets that do not carry the ECN field expected for that queue. with packets that do not carry the ECN field expected for that queue.
It is recommended that the AQM in each queue inspects the ECN field It is recommended that the AQM in each queue inspects the ECN field
to determine what sort of congestion notification to signal, then to determine what sort of congestion notification to signal, then
decides whether to apply congestion notification to this particular decides whether to apply congestion notification to this particular
packet, as follows: packet, as follows:
skipping to change at page 14, line 18 skipping to change at page 15, line 16
By default, a DualQ Coupled AQM SHOULD NOT need any configuration for By default, a DualQ Coupled AQM SHOULD NOT need any configuration for
use at a bottleneck on the public Internet [RFC7567]. The following use at a bottleneck on the public Internet [RFC7567]. The following
parameters MAY be operator-configurable, e.g. to tune for non- parameters MAY be operator-configurable, e.g. to tune for non-
Internet settings: Internet settings:
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 (a parameter for typical or target queuing o Expected typical RTT (a parameter for typical or target queuing
delay in each queue might be configurable instead); delay in each queue might be configurable instead; if so it MUST
be expressed in units of time);
o Expected maximum RTT (a stability parameter that depends on o Expected maximum RTT (a stability parameter that depends on
maximum RTT might be configurable instead); maximum RTT might be configurable instead);
o Coupling factor, k; o Coupling factor, k;
o The limit to the conditional priority of L4S (scheduler-dependent, o The limit to the conditional priority of L4S (scheduler-dependent,
e.g. the scheduler weight for WRR, or the time-shift for time- e.g. the scheduler weight for WRR, or the time-shift for time-
shifted FIFO); shifted FIFO);
o The maximum Classic ECN marking probability, p_Cmax, before o The maximum Classic ECN marking probability, p_Cmax, before
switching over to drop. switching over to drop.
An experimental DualQ Coupled AQM SHOULD allow the operator to An experimental DualQ Coupled AQM SHOULD allow the operator to
monitor the following operational statistics: monitor each of the following operational statistics on demand, per
queue and per configurable sample interval, for performance
monitoring and perhaps also for accounting in some cases:
o Bits forwarded (total and per queue per sample interval), from o Bits forwarded, from which utilization can be calculated;
which utilization can be calculated
o Q delay (per queue over sample interval) {ToDo: max per interval, o Total packets arriving, enqueued and dequeued to distinguish tail
histogram with configurable edges (from which percentile(s) can be discard from proactive AQM discard;
derived), not incl. medium access delay}
o Total packets arriving, enqueued and dequeued (per queue per o ECN packets marked, non-ECN packets dropped, ECN packets dropped,
sample interval) from which marking and dropping probabilities can be calculated;
o ECN packets marked, non-ECN packets dropped, ECN packets dropped o Queue delay (not including serialization delay of the head packet
(per queue per sample interval), from which marking and dropping or medium acquisition delay) - see further notes below.
probabilities can be calculated
o Time and duration of each overload event. Unlike the other statistics, queue delay cannot be captured in a
simple accumulating counter. Therefore the type of queue delay
statistics produced (mean, percentiles, etc.) will depend on
implementation constraints. To facilitate comparative evaluation
of different implementations and approaches, an implementation
SHOULD allow mean and 99th percentile queue delay to be derived
(per queue per sample interval). A relatively simple way to do
this would be to store a coarse-grained histogram of queue delay.
This could be done with a small number of bins with configurable
edges that represent contiguous ranges of queue delay. Then, over
a sample interval, each bin would accumulate a count of the number
of packets that had fallen within each range. The maximum queue
delay per queue per interval MAY also be recorded.
The type of statistics produced for variables like Q delay (mean, An experimental DualQ Coupled AQM SHOULD asynchronously report the
percentiles, etc.) will depend on implementation constraints. following data about anomalous conditions:
o Start-time and duration of overload state.
A hysteresis mechanism SHOULD be used to prevent flapping in and
out of overload causing an event storm. For instance, exit from
overload state could trigger one report, but also latch a timer.
Then, during that time, if the AQM enters and exits overload state
any number of times, the duration in overload state is accumulated
but no new report is generated until the first time the AQM is out
of overload once the timer has expired.
[RFC5706] suggests that deployment, coexistence and scaling should
also be covered as management requirements. The raison d'etre of the
DualQ Couple AQM is to enable deployment and coexistence of scalable
congestion controls - as incremental replacements for today's TCP-
friendly controls that do not scale with bandwidth-delay product.
Therefore, these motivating issues are explained in the Introduction
and detailed in the L4S architecture [I-D.ietf-tsvwg-l4s-arch].
Also, the descriptions of specific DualQ Coupled AQM algorithms in
the appendices cover scaling of their configuration parameters, e.g.
with respect to RTT and sampling frequency.
3. IANA Considerations 3. IANA Considerations
This specification contains no IANA considerations. This specification contains no IANA considerations.
4. Security Considerations 4. Security Considerations
4.1. Overload Handling 4.1. Overload Handling
Where the interests of users or flows might conflict, it could be Where the interests of users or flows might conflict, it could be
skipping to change at page 15, line 34 skipping to change at page 17, line 16
useful objective would be for the overload behaviour of the DualQ AQM useful objective would be for the overload behaviour of the DualQ AQM
to be at least no worse than a single queue AQM. However, a trade- to be at least no worse than a single queue AQM. However, a trade-
off needs to be made between complexity and the risk of either off needs to be made between complexity and the risk of either
traffic class harming the other. In each of the following three traffic class harming the other. In each of the following three
subsections, an overload issue specific to the DualQ is described, subsections, an overload issue specific to the DualQ is described,
followed by proposed solution(s). followed by proposed solution(s).
Under overload the higher priority L4S service will have to sacrifice Under overload the higher priority L4S service will have to sacrifice
some aspect of its performance. Alternative solutions are provided some aspect of its performance. Alternative solutions are provided
below that each relax a different factor: e.g. throughput, delay, below that each relax a different factor: e.g. throughput, delay,
drop. Some of these choices might need to be determined by operator drop. These choices need to be made either by the developer or by
policy or by the developer, rather than by the IETF. {ToDo: Reach operator policy, rather than by the IETF.
consensus on which it is to be in each case.}
4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput or Delay? 4.1.1. Avoiding Classic Starvation: Sacrifice L4S Throughput or Delay?
Priority of L4S is required to be conditional to avoid total Priority of L4S is required to be conditional to avoid total
throughput starvation of Classic by heavy L4S traffic. This raises throughput starvation of Classic by heavy L4S traffic. This raises
the question of whether to sacrifice L4S throughput or L4S delay (or the question of whether to sacrifice L4S throughput or L4S delay (or
some other policy) to mitigate starvation of Classic: some other policy) to mitigate starvation of Classic:
Sacrifice L4S throughput: By using weighted round robin as the Sacrifice L4S throughput: By using weighted round robin as the
conditional priority scheduler, the L4S service can sacrifice some conditional priority scheduler, the L4S service can sacrifice some
skipping to change at page 16, line 45 skipping to change at page 18, line 26
packets relative to L4S. packets relative to L4S.
The example implementation in Appendix A can implement either policy. The example implementation in Appendix A can implement either policy.
4.1.2. Congestion Signal Saturation: Introduce L4S Drop or Delay? 4.1.2. Congestion Signal Saturation: Introduce L4S Drop or Delay?
To keep the throughput of both L4S and Classic flows roughly equal To keep the throughput of both L4S and Classic flows roughly equal
over the full load range, a different control strategy needs to be over the full load range, a different control strategy needs to be
defined above the point where one AQM first saturates to a defined above the point where one AQM first saturates to a
probability of 100% leaving no room to push back the load any harder. probability of 100% leaving no room to push back the load any harder.
If k>1, L4S will saturate first, but saturation can be caused by If k>1, L4S will saturate first, even though saturation could be
unresponsive traffic in either queue. caused by unresponsive traffic in either queue.
The term 'unresponsive' includes cases where a flow becomes The term 'unresponsive' includes cases where a flow becomes
temporarily unresponsive, for instance, a real-time flow that takes a temporarily unresponsive, for instance, a real-time flow that takes a
while to adapt its rate in response to congestion, or a TCP-like flow while to adapt its rate in response to congestion, or a TCP-like flow
that is normally responsive, but above a certain congestion level it that is normally responsive, but above a certain congestion level it
will not be able to reduce its congestion window below the minimum of will not be able to reduce its congestion window below the minimum of
2 segments, effectively becoming unresponsive. (Note that L4S 2 segments [RFC5681], effectively becoming unresponsive. (Note that
traffic ought to remain responsive below a window of 2 segments (see L4S traffic ought to remain responsive below a window of 2 segments
[I-D.ietf-tsvwg-ecn-l4s-id]). (see [I-D.ietf-tsvwg-ecn-l4s-id]).
Saturation raises the question of whether to relieve congestion by Saturation raises the question of whether to relieve congestion by
introducing some drop into the L4S queue or by allowing delay to grow introducing some drop into the L4S queue or by allowing delay to grow
in both queues (which could eventually lead to tail drop too): in both queues (which could eventually lead to tail drop too):
Drop on Saturation: Saturation can be avoided by setting a maximum Drop on Saturation: Saturation can be avoided by setting a maximum
threshold for L4S ECN marking (assuming k>1) before saturation threshold for L4S ECN marking (assuming k>1) before saturation
starts to make the flow rates of the different traffic types starts to make the flow rates of the different traffic types
diverge. Above that the drop probability of Classic traffic is diverge. Above that the drop probability of Classic traffic is
applied to all packets of all traffic types. Then experiments applied to all packets of all traffic types. Then experiments
have shown that queueing delay can be kept at the target in any have shown that queueing delay can be kept at the target in any
overload situation, including with unresponsive traffic, and no overload situation, including with unresponsive traffic, and no
further measures are required. further measures are required [DualQ-Test].
Delay on Saturation: When L4S marking saturates, instead of Delay on Saturation: When L4S marking saturates, instead of
switching to drop, the drop and marking probabilities could be switching to drop, the drop and marking probabilities could be
capped. Beyond that, delay will grow either solely in the queue capped. Beyond that, delay will grow either solely in the queue
with unresponsive traffic (if WRR is used), or in both queues (if with unresponsive traffic (if WRR is used), or in both queues (if
time-shifted FIFO is used). In either case, the higher delay time-shifted FIFO is used). In either case, the higher delay
ought to control temporary high congestion. If the overload is ought to control temporary high congestion. If the overload is
more persistent, eventually the combined DualQ will overflow and more persistent, eventually the combined DualQ will overflow and
tail drop will control congestion. tail drop will control congestion.
The example implementation in Appendix A applies only the "drop on The example implementation in Appendix A solely applies the "drop on
saturation" policy. saturation" policy.
4.1.3. Protecting against Unresponsive ECN-Capable Traffic 4.1.3. Protecting against Unresponsive ECN-Capable Traffic
Unresponsive traffic has a greater advantage if it is also ECN- Unresponsive traffic has a greater advantage if it is also ECN-
capable. The advantage is undetectable at normal low levels of drop/ capable. The advantage is undetectable at normal low levels of drop/
marking, but it becomes significant with the higher levels of drop/ marking, but it becomes significant with the higher levels of drop/
marking typical during overload. This is an issue whether the ECN- marking typical during overload. This is an issue whether the ECN-
capable traffic is L4S or Classic. capable traffic is L4S or Classic.
skipping to change at page 18, line 10 skipping to change at page 19, line 41
problem with unresponsive ECN as well as addressing the saturation problem with unresponsive ECN as well as addressing the saturation
problem. It leaves only a small range of congestion levels where problem. It leaves only a small range of congestion levels where
unresponsive traffic gains any advantage from using the ECN unresponsive traffic gains any advantage from using the ECN
capability, and the advantage is hardly detectable [DualQ-Test]. capability, and the advantage is hardly detectable [DualQ-Test].
5. Acknowledgements 5. Acknowledgements
Thanks to Anil Agarwal, Sowmini Varadhan's and Gabi Bracha for Thanks to Anil Agarwal, Sowmini Varadhan's and Gabi Bracha for
detailed review comments particularly of the appendices and detailed review comments particularly of the appendices and
suggestions on how to make our explanation clearer. Thanks also to suggestions on how to make our explanation clearer. Thanks also to
Greg White and Tom Henderson for insights on the choice of schedulers Greg White for improving the normative requirements and both Greg and
and queue delay measurement techniques. Tom Henderson for insights on the choice of schedulers, queue delay
measurement techniques.
The authors' contributions were originally part-funded by the The authors' contributions were originally part-funded by the
European Community under its Seventh Framework Programme through the European Community under its Seventh Framework Programme through the
Reducing Internet Transport Latency (RITE) project (ICT-317700). Bob Reducing Internet Transport Latency (RITE) project (ICT-317700). Bob
Briscoe's contribution was also part-funded by the Research Council Briscoe's contribution was also part-funded by the Research Council
of Norway through the TimeIn project. The views expressed here are of Norway through the TimeIn project. The views expressed here are
solely those of the authors. solely those of the authors.
6. References 6. References
skipping to change at page 19, line 34 skipping to change at page 21, line 23
Low Loss, Scalable Throughput (L4S) Internet Service: Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture", draft-ietf-tsvwg-l4s-arch-02 (work in Architecture", draft-ietf-tsvwg-l4s-arch-02 (work in
progress), March 2018. progress), March 2018.
[I-D.sridharan-tcpm-ctcp] [I-D.sridharan-tcpm-ctcp]
Sridharan, M., Tan, K., Bansal, D., and D. Thaler, Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
"Compound TCP: A New TCP Congestion Control for High-Speed "Compound TCP: A New TCP Congestion Control for High-Speed
and Long Distance Networks", draft-sridharan-tcpm-ctcp-02 and Long Distance Networks", draft-sridharan-tcpm-ctcp-02
(work in progress), November 2008. (work in progress), November 2008.
[L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<http://dl.acm.org/citation.cfm?doid=2910017.2910633
(videos of demos: https://riteproject.eu/
dctth/#1511dispatchwg )>.
[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>.
[MEDF] Menth, M., Schmid, M., Heiss, H., and T. Reim, "MEDF - a [MEDF] Menth, M., Schmid, M., Heiss, H., and T. Reim, "MEDF - a
simple scheduling algorithm for two real-time transport simple scheduling algorithm for two real-time transport
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.
skipping to change at page 20, line 36 skipping to change at page 22, line 32
<https://www.rfc-editor.org/info/rfc3246>. <https://www.rfc-editor.org/info/rfc3246>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003, RFC 3649, DOI 10.17487/RFC3649, December 2003,
<https://www.rfc-editor.org/info/rfc3649>. <https://www.rfc-editor.org/info/rfc3649>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. <https://www.rfc-editor.org/info/rfc5681>.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, DOI 10.17487/RFC5706, November 2009,
<https://www.rfc-editor.org/info/rfc5706>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>. <https://www.rfc-editor.org/info/rfc7567>.
[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White, [RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A "Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017, Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>. <https://www.rfc-editor.org/info/rfc8033>.
skipping to change at page 21, line 26 skipping to change at page 23, line 26
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion [RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311, Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018, DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>. <https://www.rfc-editor.org/info/rfc8311>.
[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>.
[TCP-CA] Jacobson, V. and M. Karels, "Congestion Avoidance and
Control", Laurence Berkeley Labs Technical Report ,
November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>.
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 step threshold (in units of queuing framework in Figure 1. A simple step threshold (in units of queuing
time) is used for the Native L4S AQM, but a ramp is also described as time) is used for the Native L4S AQM, but a ramp is also described as
an alternative. And the PI2 algorithm [PI2] is used for the Classic an alternative. And the PI2 algorithm [PI2] is used for the Classic
AQM. PI2 is an improved variant of the PIE AQM [RFC8033]. AQM. PI2 is an improved variant of the PIE AQM [RFC8033].
We will introduce the pseudocode in two passes. The first pass We will introduce the pseudocode in two passes. The first pass
skipping to change at page 21, line 48 skipping to change at page 23, line 52
between the two passes by using letter suffixes where the longer code between the two passes by using letter suffixes where the longer code
needs extra lines. needs extra lines.
A full open source implementation for Linux is available at: A full open source implementation for Linux is available at:
https://github.com/olgabo/dualpi2. https://github.com/olgabo/dualpi2.
A.1. Pass #1: Core Concepts A.1. Pass #1: Core Concepts
The pseudocode manipulates three main structures of variables: the The pseudocode manipulates three main structures of variables: the
packet (pkt), the L4S queue (lq) and the Classic queue (cq). The packet (pkt), the L4S queue (lq) and the Classic queue (cq). The
pseudocode consists of the following four functions: pseudocode consists of the following five functions:
o initialization code (Figure 2) that sets parameter defaults (the o initialization code (Figure 2) that sets parameter defaults (the
API for setting non-default values is omitted for brevity) API for setting non-default values is omitted for brevity)
o enqueue code (Figure 3) o enqueue code (Figure 3)
o dequeue code (Figure 4) o dequeue code (Figure 4)
o a ramp function (Figure 5) used to calculate the ECN-marking
probability for the L4S queue
o code to regularly update the base probability (p) used in the o code to regularly update the base probability (p) used in the
dequeue code (Figure 5). dequeue code (Figure 6).
It also uses the following functions that are not shown in full here: It also uses the following functions that are not shown in full here:
o scheduler(), which selects between the head packets of the two o scheduler(), which selects between the head packets of the two
queues; the choice of scheduler technology is discussed later; queues; the choice of scheduler technology is discussed later;
o cq.len() or lq.len() returns the current length (aka. backlog) of o cq.len() or lq.len() returns the current length (aka. backlog) of
the relevant queue in bytes; the relevant queue in bytes;
o cq.time() or lq.time() returns the current queuing delay (aka. o cq.time() or lq.time() returns the current queuing delay (aka.
skipping to change at page 23, line 21 skipping to change at page 25, line 21
7: p_Cmax = 1/4 % Max Classic drop/mark prob 7: p_Cmax = 1/4 % Max Classic drop/mark prob
8: % Constants derived from PI2 AQM parameters 8: % Constants derived from PI2 AQM parameters
9: alpha_U = alpha *Tupdate % PI integral gain per update interval 9: alpha_U = alpha *Tupdate % PI integral gain per update interval
10: beta_U = beta * Tupdate % PI prop'nal gain per update interval 10: beta_U = beta * Tupdate % PI prop'nal gain per update interval
11: 11:
12: % DualQ Coupled framework parameters 12: % DualQ Coupled framework parameters
13: k = 2 % Coupling factor 13: k = 2 % Coupling factor
14: % scheduler weight or equival't parameter (scheduler-dependent) 14: % scheduler weight or equival't parameter (scheduler-dependent)
15: limit = MAX_LINK_RATE * 250 ms % Dual buffer size 15: limit = MAX_LINK_RATE * 250 ms % Dual buffer size
16: 16:
17: % L4S AQM parameters 17: % L4S ramp AQM parameters
18: T_time = 1 ms % L4S marking threshold in time 18: minTh = 475 us % L4S min marking threshold in time units
19: T_len = 2 * MTU % Min L4S marking threshold in bytes 19: range = 525 us % Range of L4S ramp in time units
20: % Constants derived from L4S AQM parameters 20: Th_len = 2 * MTU % Min L4S marking threshold in bytes
21: p_Lmax = min(k*sqrt(p_Cmax), 1) % Max L4S marking prob 21: % Constants derived from L4S AQM parameters
22: } 22: p_Lmax = min(k*sqrt(p_Cmax), 1) % Max L4S marking prob
23: floor = Th_len * 8 / MIN_LINK_RATE % MIN_LINK_RATE is in Mb/s
24: if (minTh < floor) {
25: % Adjust ramp to exceed serialization time of 2 MTU
26: range = max(range - (floor-minTh), 1) % 1us avoids /0 error
27: minTh = floor
28: }
29: maxTh = minTh+range % L4S min marking threshold in time units
30: }
Figure 2: Example Header Pseudocode for DualQ Coupled PI2 AQM Figure 2: Example Header Pseudocode for DualQ Coupled PI2 AQM
For brevity the pseudocode shows some parameters in units of
microseconds (us), but a real implementation would probably use
nanoseconds.
The overall goal of the code is to maintain the base probability (p), The overall goal of the code is to maintain the base probability (p),
which is an internal variable from which the marking and dropping which is an internal variable from which the marking and dropping
probabilities for L4S and Classic traffic (p_L and p_C) are derived. probabilities for L4S and Classic traffic (p_L and p_C) are derived.
The variable named p in the pseudocode and in this walk-through is The variable named p in the pseudocode and in this walk-through is
the same as p' (p-prime) in Section 2.4. The probabilities p_L and the same as p' (p-prime) in Section 2.4. The probabilities p_L and
p_C are derived in lines 3, 4 and 5 of the dualpi2_update() function p_C are derived in lines 3, 4 and 5 of the dualpi2_update() function
(Figure 5) then used in the dualpi2_dequeue() function (Figure 4). (Figure 6) then used in the dualpi2_dequeue() function (Figure 4).
The code walk-through below builds up to explaining that part of the The code walk-through below builds up to explaining that part of the
code eventually, but it starts from packet arrival. code eventually, but it starts from packet arrival.
1: dualpi2_enqueue(lq, cq, pkt) { % Test limit and classify lq or cq 1: dualpi2_enqueue(lq, cq, pkt) { % Test limit and classify lq or cq
2: if ( lq.len() + cq.len() > limit ) 2: if ( lq.len() + cq.len() > limit )
3: drop(pkt) % drop packet if buffer is full 3: drop(pkt) % drop packet if buffer is full
4: else { % Packet classifier 4: else { % Packet classifier
5: if ( ecn(pkt) modulo 2 == 1 ) % ECN bits = ECT(1) or CE 5: if ( ecn(pkt) modulo 2 == 1 ) % ECN bits = ECT(1) or CE
6: lq.enqueue(pkt) 6: lq.enqueue(pkt)
7: else % ECN bits = not-ECT or ECT(0) 7: else % ECN bits = not-ECT or ECT(0)
8: cq.enqueue(pkt) 8: cq.enqueue(pkt)
9: } 9: }
10: } 10: }
Figure 3: Example Enqueue Pseudocode for DualQ Coupled PI2 AQM Figure 3: Example Enqueue Pseudocode for DualQ Coupled PI2 AQM
1: dualpi2_dequeue(lq, cq, pkt) { % Couples L4S & Classic queues 1: dualpi2_dequeue(lq, cq, pkt) { % Couples L4S & Classic queues
2: while ( lq.len() + cq.len() > 0 ) 2: while ( lq.len() + cq.len() > 0 )
3: if ( scheduler() == lq ) { 3: if ( scheduler() == lq ) {
4: lq.dequeue(pkt) % Scheduler chooses lq 4: lq.dequeue(pkt) % Scheduler chooses lq
5: p'_L = laqm(lq.time()) % Native L4S AQM
{ToDo: Generalize 5-7 for any L AQM (see email to Tom 9-Aug-18)} 6: p_L = max(p'_L, p_CL) % Combining function
7: if ( p_L > rand() ) % Linear marking
5: if ( ((lq.time() > T_time) % step marking ...
6: AND (lq.len() > T_len))
7: OR (p_CL > rand()) ) % ...or linear marking
8: mark(pkt) 8: mark(pkt)
9: } else { 9: } else {
10: cq.dequeue(pkt) % Scheduler chooses cq 10: cq.dequeue(pkt) % Scheduler chooses cq
11: if ( p_C > rand() ) { % probability p_C = p^2 11: if ( p_C > rand() ) { % probability p_C = p^2
12: if ( ecn(pkt) == 0 ) { % if ECN field = not-ECT 12: if ( ecn(pkt) == 0 ) { % if ECN field = not-ECT
13: drop(pkt) % squared drop 13: drop(pkt) % squared drop
14: continue % continue to the top of the while loop 14: continue % continue to the top of the while loop
15: } 15: }
16: mark(pkt) % squared mark 16: mark(pkt) % squared mark
17: } 17: }
skipping to change at page 25, line 28 skipping to change at page 27, line 38
add a whole queue of delay to the control signals, making the control add a whole queue of delay to the control signals, making the control
loop very sloppy. loop very sloppy.
All the dequeue code is contained within a large while loop so that All the dequeue code is contained within a large while loop so that
if it decides to drop a packet, it will continue until it selects a if it decides to drop a packet, it will continue until it selects a
packet to schedule. Line 3 of the dequeue pseudocode is where the packet to schedule. Line 3 of the dequeue pseudocode is where the
scheduler chooses between the L4S queue (lq) and the Classic queue scheduler chooses between the L4S queue (lq) and the Classic queue
(cq). Detailed implementation of the scheduler is not shown (see (cq). Detailed implementation of the scheduler is not shown (see
discussion later). discussion later).
o If an L4S packet is scheduled, lines 5 to 8 mark the packet if o If an L4S packet is scheduled, lines 7 and 8 ECN-mark the packet
either the L4S threshold (T_time) is exceeded, or if a random if a random marking decision is drawn according to p_L. Line 6
marking decision is drawn according to p_CL (maintained by the calculates p_L as the maximum of the coupled L4S probability p_CL
dualpi2_update() function discussed below). This logical 'OR' on and the probability from the native L4S AQM p'_L. This implements
a per-packet basis implements the max() function shown in Figure 1 the max() function shown in Figure 1 to couple the outputs of the
to couple the outputs of the two AQMs together. The L4S threshold two AQMs together. Of the two probabilities input to p_L in line
is usually in units of time (default T_time = 1 ms). However, on 6:
slow links the packet serialization time can approach the
threshold T_time, so line 6 sets a floor of T_len (=2 MTU) to the * p'_L is calculated per packet in line 5 by the laqm() function
threshold, otherwise marking is always too frequent on slow links. (see Figure 5),
* whereas p_CL is maintained by the dualpi2_update() function
which runs every Tupdate (default 16ms) (see Figure 2).
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 based on the squared probability p_C. packet based on the squared probability p_C.
There is some concern that using a step function for the Native L4S The Native L4S AQM algorithm (Figure 5) is a ramp function, similar
AQM requires end-systems to smooth the signal for a lot longer - to the RED algorithm, but simpler due to the following differences:
until its fidelity is sufficient. The latency benefits of a ramp are
being investigated as a simple alternative to the step. This ramp
would be similar to the RED algorithm, with the following
differences:
o The min and max of the ramp are defined in units of queuing delay, o The min and max of the ramp are defined in units of queuing delay,
not bytes, so that configuration remains invariant as the queue not bytes, so that configuration remains invariant as the queue
departure rate varies. departure rate varies.
o It uses instantaneous queueing delay without smoothing (smoothing o It uses instantaneous queueing delay to remove smoothing delay
is done in the end-systems). (L4S senders smooth incoming ECN feedback when necessary).
o Determinism is being experimented with instead of randomness; to
reduce the delay necessary to smooth out the noise of randomness
from the signal. For each packet, the algorithm would accumulate
p'_L in a counter and mark the packet that took the counter over
1, then subtract 1 from the counter and continue.
o The ramp rises linearly directly from 0 to 1, not to a an o The ramp rises linearly directly from 0 to 1, not to a an
intermediate value of p'_L as RED would, because there is no need intermediate value of p'_L as RED would, because there is no need
to keep ECN marking probability low. to keep ECN marking probability low.
This ramp algorithm would require two configuration parameters (min o Marking does not have to be randomized. Determinism is being
and max threshold in units of queuing time), in contrast to the experimented with instead of randomness; to reduce the delay
single parameter of a step. necessary to smooth out the noise of randomness from the signal.
In this case, for each packet, the algorithm would accumulate p_L
in a counter and mark the packet that took the counter over 1,
then subtract 1 from the counter and continue.
This ramp function requires two configuration parameters, the minimum
threshold (minTh) and the width of the ramp (range), both in units of
queuing time), as shown in the parameter initialization code in
Figure 2. A minimum marking threshold parameter (Th_len) in
transmission units (default 2 MTU) is also necessary to ensure that
the ramp does not trigger excessive marking on slow links. The code
in lines 23-28 of Figure 2 converts 2 MTU into time units and adjusts
the ramp thresholds to be no shallower than this floor.
An operator can effectively turn the ramp into a step function, as
used by DCTCP, by setting the range to its minimum value (e.g. 1 ns).
Then the condition for the ramp calculation will hardly ever arise.
There is some concern that using the step function of DCTCP for the
Native L4S AQM requires end-systems to smooth the signal for an
unnecessarily large number of round trips to ensure sufficient
fidelity. A ramp seems to be no worse than a step in initial
experiments with existing DCTCP. Therefore, it is recommended that a
ramp is configured in place of a step, which will allow congestion
control algorithms to investigate faster smoothing algorithms.
1: laqm(qdelay) { % Returns native L4S AQM probability
2: if (qdelay >= maxTh)
3: return 1
4: else if (qdelay > minTh)
5: return (qdelay - minTh)/range % Divide would use a bit-shift
6: else
7: return 0
8: }
Figure 5: Example Pseudocode for the Native L4S AQM
1: dualpi2_update(lq, cq, target) { % Update p every Tupdate 1: dualpi2_update(lq, cq, target) { % Update p every Tupdate
2: curq = cq.time() % use queuing time of first-in Classic packet 2: curq = cq.time() % use queuing time of first-in Classic packet
3: p = p + alpha_U * (curq - target) + beta_U * (curq - prevq) 3: p = p + alpha_U * (curq - target) + beta_U * (curq - prevq)
4: p_CL = p * k % Coupled L4S prob = base prob * coupling factor 4: p_CL = p * k % Coupled L4S prob = base prob * coupling factor
5: p_C = p^2 % Classic prob = (base prob)^2 5: p_C = p^2 % Classic prob = (base prob)^2
6: prevq = curq 6: prevq = curq
7: } 7: }
Figure 5: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM Figure 6: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM
The base probability (p) is kept up to date by the core PI algorithm p_CL depends on the base probability (p), which is kept up to date by
in Figure 5, which is executed every Tupdate. the core PI algorithm in Figure 6 executed every Tupdate.
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 and implicitly takes the current queuing delay as 0 if current time and implicitly takes the current queuing delay as 0 if
the queue is empty. 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
skipping to change at page 27, line 22 skipping to change at page 30, line 15
without over-compensating and therefore causing oscillations in the without over-compensating and therefore causing oscillations in the
queue. queue.
alpha and beta determine how much p ought to change if it was updated alpha and beta determine how much p ought to change if it was updated
every second. It is best to update p as frequently as possible, but every second. It is best to update p as frequently as possible, but
the update interval (Tupdate) will probably be constrained by the update interval (Tupdate) will probably be constrained by
hardware performance. For link rates from 4 - 200 Mb/s, we found hardware performance. For link rates from 4 - 200 Mb/s, we found
Tupdate=16ms (as recommended in [RFC8033]) is sufficient. However Tupdate=16ms (as recommended in [RFC8033]) is sufficient. However
small the chosen value of Tupdate, p should change by the same amount small the chosen value of Tupdate, p should change by the same amount
per second, but in finer more frequent steps. So the gain factors per second, but in finer more frequent steps. So the gain factors
used for updating p in Figure 5 need to be scaled by (Tupdate/1s), used for updating p in Figure 6 need to be scaled by (Tupdate/1s),
which is done in lines 9 and 10 of Figure 2). The suffix '_U' which is done in lines 9 and 10 of Figure 2). The suffix '_U'
represents 'per update time' (Tupdate). represents 'per update time' (Tupdate).
In corner cases, p can overflow the range [0,1] so the resulting In corner cases, p can overflow the range [0,1] so the resulting
value of p has to be bounded (omitted from the pseudocode). Then, as value of p has to be bounded (omitted from the pseudocode). Then, as
already explained, the coupled and Classic probabilities are derived already explained, the coupled and Classic probabilities are derived
from the new p in lines 4 and 5 as p_CL = k*p and p_C = p^2. from the new p in lines 4 and 5 as p_CL = k*p and p_C = p^2.
Because the coupled L4S marking probability (p_CL) is factored up by Because the coupled L4S marking probability (p_CL) is factored up by
k, the dynamic gain parameters alpha and beta are also inherently k, the dynamic gain parameters alpha and beta are also inherently
skipping to change at page 27, line 50 skipping to change at page 30, line 43
are independent of p because the squaring applied to Classic traffic are independent of p because the squaring applied to Classic traffic
tunes them inherently. This is explained in [PI2], which also tunes them inherently. This is explained in [PI2], which also
explains why this more principled approach removes the need for most explains why this more principled approach removes the need for most
of the heuristics that had to be added to PIE. of the heuristics that had to be added to PIE.
{ToDo: Scaling beta with Tupdate and scaling both alpha & beta with {ToDo: Scaling beta with Tupdate and scaling both alpha & beta with
RTT} RTT}
A.2. Pass #2: Overload Details A.2. Pass #2: Overload Details
Figure 6 repeats the dequeue function of Figure 4, but with overload Figure 7 repeats the dequeue function of Figure 4, but with overload
details added. Similarly Figure 7 repeats the core PI algorithm of details added. Similarly Figure 8 repeats the core PI algorithm of
Figure 5 with overload details added. The initialization and enqueue Figure 6 with overload details added. The initialization, enqueue
functions are unchanged. and L4S AQM functions are unchanged.
In line 7 of the initialization function (Figure 2), the default In line 7 of the initialization function (Figure 2), the default
maximum Classic drop probability p_Cmax = 1/4 or 25%. This is the maximum Classic drop probability p_Cmax = 1/4 or 25%. This is the
point at which it is deemed that the Classic queue has become point at which it is deemed that the Classic queue has become
persistently overloaded, so it switches to using solely drop, even persistently overloaded, so it switches to using solely drop, even
for ECN-capable packets. This protects the queue against any for ECN-capable packets. This protects the queue against any
unresponsive traffic that falsely claims that it is responsive to ECN unresponsive traffic that falsely claims that it is responsive to ECN
marking, as required by [RFC3168] and [RFC7567]. marking, as required by [RFC3168] and [RFC7567].
Line 21 of the initialization function translates this into a maximum Line 22 of the initialization function translates this into a maximum
L4S marking probability (p_Lmax) by rearranging Equation (1). With a L4S marking probability (p_Lmax) by rearranging Equation (1). With a
coupling factor of k=2 (the default) or greater, this translates to a coupling factor of k=2 (the default) or greater, this translates to a
maximum L4S marking probability of 1 (or 100%). This is intended to maximum L4S marking probability of 1 (or 100%). This is intended to
ensure that the L4S queue starts to introduce dropping once marking ensure that the L4S queue starts to introduce dropping once marking
saturates and can rise no further. The 'TCP Prague' requirements saturates and can rise no further. The 'TCP Prague' requirements
[I-D.ietf-tsvwg-ecn-l4s-id] state that, when an L4S congestion [I-D.ietf-tsvwg-ecn-l4s-id] state that, when an L4S congestion
control detects a drop, it falls back to a response that coexists control detects a drop, it falls back to a response that coexists
with 'Classic' TCP. So it is correct that the L4S queue drops with 'Classic' TCP. So it is correct that the L4S queue drops
packets proportional to p^2, as if they are Classic packets. packets proportional to p^2, as if they are Classic packets.
Both these switch-overs are triggered by the tests for overload Both these switch-overs are triggered by the tests for overload
introduced in lines 4b and 12b of the dequeue function (Figure 6). introduced in lines 4b and 12b of the dequeue function (Figure 7).
Lines 8c to 8g drop L4S packets with probability p^2. Lines 8h to 8i Lines 8c to 8g drop L4S packets with probability p^2. Lines 8h to 8i
mark the remaining packets with probability p_CL. If p_Lmax = 1, mark the remaining packets with probability p_CL. If p_Lmax = 1,
which is the suggested default configuration, all remaining packets which is the suggested default configuration, all remaining packets
will be marked because, to have reached the else block at line 8b, will be marked because, to have reached the else block at line 8b,
p_CL >= 1. p_CL >= 1.
Lines 2c to 2d in the core PI algorithm (Figure 7) deal with overload Lines 2c to 2d in the core PI algorithm (Figure 8) deal with overload
of the L4S queue when there is no Classic traffic. This is of the L4S queue when there is no Classic traffic. This is
necessary, because the core PI algorithm maintains the appropriate necessary, because the core PI algorithm maintains the appropriate
drop probability to regulate overload, but it depends on the length drop probability to regulate overload, but it depends on the length
of the Classic queue. If there is no Classic queue the naive of the Classic queue. If there is no Classic queue the naive
algorithm in Figure 5 drops nothing, even if the L4S queue is algorithm in Figure 6 drops nothing, even if the L4S queue is
overloaded - so tail drop would have to take over (lines 3 and 4 of overloaded - so tail drop would have to take over (lines 3 and 4 of
Figure 3). Figure 3).
If the test at line 2a finds that the Classic queue is empty, line 2d If the test at line 2a finds that the Classic queue is empty, line 2d
measures the current queue delay using the L4S queue instead. While measures the current queue delay using the L4S queue instead. While
the L4S queue is not overloaded, its delay will always be tiny the L4S queue is not overloaded, its delay will always be tiny
compared to the target Classic queue delay. So p_L will be driven to compared to the target Classic queue delay. So p_L will be driven to
zero, and the L4S queue will naturally be governed solely by zero, and the L4S queue will naturally be governed solely by
threshold marking (lines 5 and 6 of the dequeue algorithm in threshold marking (lines 5 and 6 of the dequeue algorithm in
Figure 6). But, if unresponsive L4S source(s) cause overload, the Figure 7). But, if unresponsive L4S source(s) cause overload, the
DualQ transitions smoothly to L4S marking based on the PI algorithm. DualQ transitions smoothly to L4S marking based on the PI algorithm.
And as overload increases, it naturally transitions from marking to And as overload increases, it naturally transitions from marking to
dropping by the switch-over mechanism already described. dropping by the switch-over mechanism already described.
1: dualpi2_dequeue(lq, cq) { % Couples L4S & Classic queues, lq & cq 1: dualpi2_dequeue(lq, cq) { % Couples L4S & Classic queues, lq & cq
2: while ( lq.len() + cq.len() > 0 ) 2: while ( lq.len() + cq.len() > 0 )
3: if ( scheduler() == lq ) { 3: if ( scheduler() == lq ) {
4a: lq.dequeue(pkt) 4a: lq.dequeue(pkt)
4b: if ( p_CL < p_Lmax ) { % Check for overload saturation 4b: if ( p_CL < p_Lmax ) { % Check for overload saturation
5: if ( ((lq.time() > T_time) % step marking ... 5: p'_L = laqm(lq.time()) % Native L4S AQM
6: AND (lq.len > T_len)) 6: p_L = max(p'_L, p_CL) % Combining function
7: OR (p_CL > rand()) ) % ...or linear marking 7: if ( p_L > rand() ) % Linear marking
8a: mark(pkt) 8a: mark(pkt)
8b: } else { % overload saturation 8b: } else { % overload saturation
8c: if ( p_C > rand() ) { % probability p_C = p^2 8c: if ( p_C > rand() ) { % probability p_C = p^2
8e: drop(pkt) % revert to Classic drop due to overload 8e: drop(pkt) % revert to Classic drop due to overload
8f: continue % continue to the top of the while loop 8f: continue % continue to the top of the while loop
8g: } 8g: }
8h: if ( p_CL > rand() ) % probability p_CL = k * p 8h: if ( p_CL > rand() ) % probability p_CL = k * p
8i: mark(pkt) % linear marking of remaining packets 8i: mark(pkt) % linear marking of remaining packets
8j: } 8j: }
9: } else { 9: } else {
10: cq.dequeue(pkt) 10: cq.dequeue(pkt)
skipping to change at page 29, line 41 skipping to change at page 32, line 38
14: continue % continue to the top of the while loop 14: continue % continue to the top of the while loop
15: } 15: }
16: mark(pkt) % squared mark 16: mark(pkt) % squared mark
17: } 17: }
18: } 18: }
19: return(pkt) % return the packet and stop 19: return(pkt) % return the packet and stop
20: } 20: }
21: return(NULL) % no packet to dequeue 21: return(NULL) % no packet to dequeue
22: } 22: }
Figure 6: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM Figure 7: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM
(Including Integer Arithmetic and Overload Code) (Including Integer Arithmetic and Overload Code)
1: dualpi2_update(lq, cq, target) { % Update p every Tupdate 1: dualpi2_update(lq, cq, target) { % Update p every Tupdate
2a: if ( cq.len() > 0 ) 2a: if ( cq.len() > 0 )
2b: curq = cq.time() %use queuing time of first-in Classic packet 2b: curq = cq.time() %use queuing time of first-in Classic packet
2c: else % Classic queue empty 2c: else % Classic queue empty
2d: curq = lq.time() % use queuing time of first-in L4S packet 2d: curq = lq.time() % use queuing time of first-in L4S packet
3: p = p + alpha_U * (curq - target) + beta_U * (curq - prevq) 3: p = p + alpha_U * (curq - target) + beta_U * (curq - prevq)
4: p_CL = p * k % Coupled L4S prob = base prob * coupling factor 4: p_CL = p * k % Coupled L4S prob = base prob * coupling factor
5: p_C = p^2 % Classic prob = (base prob)^2 5: p_C = p^2 % Classic prob = (base prob)^2
6: prevq = curq 6: prevq = curq
7: } 7: }
Figure 7: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM Figure 8: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM
(Including Overload Code) (Including Overload Code)
The choice of scheduler technology is critical to overload protection The choice of scheduler technology is critical to overload protection
(see Section 4.1). (see Section 4.1).
o A well-understood weighted scheduler such as weighted round robin o A well-understood weighted scheduler such as weighted round robin
(WRR) is recommended. The scheduler weight for Classic should be (WRR) is recommended. The scheduler weight for Classic should be
low, e.g. 1/16. low, e.g. 1/16.
o Alternatively, a time-shifted FIFO could be used. This is a very o Alternatively, a time-shifted FIFO could be used. This is a very
skipping to change at page 31, line 33 skipping to change at page 34, line 33
14: return(NULL) % no packet to dequeue 14: return(NULL) % no packet to dequeue
15: } 15: }
16: maxrand(u) { % return the max of u random numbers 16: maxrand(u) { % return the max of u random numbers
17: maxr=0 17: maxr=0
18: while (u-- > 0) 18: while (u-- > 0)
19: maxr = max(maxr, rand()) % 0 <= rand() < 1 19: maxr = max(maxr, rand()) % 0 <= rand() < 1
20: return(maxr) 20: return(maxr)
21: } 21: }
Figure 8: Example Dequeue Pseudocode for DualQ Coupled Curvy RED AQM Figure 9: Example Dequeue Pseudocode for DualQ Coupled Curvy RED AQM
Packet classification code is not shown, as it is no different from Packet classification code is not shown, as it is no different from
Figure 3. Potential classification schemes are discussed in Figure 3. Potential classification schemes are discussed in
Section 2.3. The Curvy RED algorithm has not been maintained to the Section 2.3. The Curvy RED algorithm has not been maintained to the
same degree as the DualPI2 algorithm. Some ideas used in DualPI2 same degree as the DualPI2 algorithm. Some ideas used in DualPI2
would need to be translated into Curvy RED, such as i) the would need to be translated into Curvy RED, such as i) the
conditional priority scheduler instead of strict priority ii) the conditional priority scheduler instead of strict priority ii) the
time-based L4S threshold; iii) turning off ECN as overload time-based L4S threshold; iii) turning off ECN as overload
protection; iv) Classic ECN support. These are not shown in the protection; iv) Classic ECN support. These are not shown in the
Curvy RED pseudocode, but would need to be implemented for Curvy RED pseudocode, but would need to be implemented for
skipping to change at page 32, line 42 skipping to change at page 35, line 42
Specifically, in line 3a the marking probability p_L is set to the Specifically, in line 3a the marking probability p_L is set to the
Classic queueing time qc.sec() in seconds divided by the L4S Classic queueing time qc.sec() in seconds divided by the L4S
scaling parameter 2^S_L, which represents the queuing time (in scaling parameter 2^S_L, which represents the queuing time (in
seconds) at which marking probability would hit 100%. Then in line seconds) at which marking probability would hit 100%. Then in line
3d (if U=1) the result is compared with a uniformly distributed 3d (if U=1) the result is compared with a uniformly distributed
random number between 0 and 1, which ensures that marking random number between 0 and 1, which ensures that marking
probability will linearly increase with queueing time. The probability will linearly increase with queueing time. The
scaling parameter is expressed as a power of 2 so that division scaling parameter is expressed as a power of 2 so that division
can be implemented as a right bit-shift (>>) in line 3 of the can be implemented as a right bit-shift (>>) in line 3 of the
integer variant of the pseudocode (Figure 9). integer variant of the pseudocode (Figure 10).
Classic: If the test at line 7 determines that there is at least one Classic: If the test at line 7 determines that there is at least one
Classic packet to dequeue, the test at line 9b determines whether Classic packet to dequeue, the test at line 9b determines whether
to drop it. But before that, line 8b updates Q_C, which is an to drop it. But before that, line 8b updates Q_C, which is an
exponentially weighted moving average (Note 5) of the queuing time exponentially weighted moving average (Note 5) of the queuing time
in the Classic queue, where pkt.sec() is the instantaneous in the Classic queue, where pkt.sec() is the instantaneous
queueing time of the current Classic packet and alpha is the EWMA queueing time of the current Classic packet and alpha is the EWMA
constant for the classic queue. In line 8a, alpha is represented constant for the classic queue. In line 8a, alpha is represented
as an integer power of 2, so that in line 8 of the integer code as an integer power of 2, so that in line 8 of the integer code
the division needed to weight the moving average can be the division needed to weight the moving average can be
skipping to change at page 35, line 31 skipping to change at page 38, line 31
7: while ( cq.dequeue(pkt) ) { 7: while ( cq.dequeue(pkt) ) {
8: Q_C += (pkt.ns() - Q_C) >> f_C % Classic Q EWMA 8: Q_C += (pkt.ns() - Q_C) >> f_C % Classic Q EWMA
9: if ( (Q_C >> (S_C-2) ) > maxrand(2*U) ) 9: if ( (Q_C >> (S_C-2) ) > maxrand(2*U) )
10: drop(pkt) % Squared drop, redo loop 10: drop(pkt) % Squared drop, redo loop
11: else 11: else
12: return(pkt) % return the packet and stop here 12: return(pkt) % return the packet and stop here
13: } 13: }
14: return(NULL) % no packet to dequeue 14: return(NULL) % no packet to dequeue
15: } 15: }
Figure 9: Optimised Example Dequeue Pseudocode for Coupled DualQ AQM Figure 10: Optimised Example Dequeue Pseudocode for Coupled DualQ AQM
using Integer Arithmetic using Integer Arithmetic
Notes: Notes:
1. The drain rate of the queue can vary if it is scheduled relative 1. The drain rate of the queue can vary if it is scheduled relative
to other queues, or to cater for fluctuations in a wireless to other queues, or to cater for fluctuations in a wireless
medium. To auto-adjust to changes in drain rate, the queue must medium. To auto-adjust to changes in drain rate, the queue must
be measured in time, not bytes or packets [CoDel]. In our Linux be measured in time, not bytes or packets [CoDel]. In our Linux
implementation, it was easiest to measure queuing time at implementation, it was easiest to measure queuing time at
dequeue. Queuing time can be estimated when a packet is enqueued dequeue. Queuing time can be estimated when a packet is enqueued
skipping to change at page 36, line 26 skipping to change at page 39, line 26
adaptive smoothing methods could be valid and it might be adaptive smoothing methods could be valid and it might be
appropriate to decrease the EWMA faster than it increases. appropriate to decrease the EWMA faster than it increases.
6. In practice at line 10 the Classic queue would probably test for 6. In practice at line 10 the Classic queue would probably test for
ECN capability on the packet to determine whether to drop or mark ECN capability on the packet to determine whether to drop or mark
the packet. However, for brevity such detail is omitted. All the packet. However, for brevity such detail is omitted. All
packets classified into the L4S queue have to be ECN-capable, so packets classified into the L4S queue have to be ECN-capable, so
no dropping logic is necessary at line 3. Nonetheless, L4S no dropping logic is necessary at line 3. Nonetheless, L4S
packets could be dropped by overload code (see Section 4.1). packets could be dropped by overload code (see Section 4.1).
7. In the integer variant of the pseudocode (Figure 9) real numbers 7. In the integer variant of the pseudocode (Figure 10) real numbers
are all represented as integers scaled up by 2^32. In lines 3 & are all represented as integers scaled up by 2^32. In lines 3 &
9 the function maxrand() is arranged to return an integer in the 9 the function maxrand() is arranged to return an integer in the
range 0 <= maxrand() < 2^32. Queuing times are also scaled up by range 0 <= maxrand() < 2^32. Queuing times are also scaled up by
2^32, but in two stages: i) In lines 3 and 8 queuing times 2^32, but in two stages: i) In lines 3 and 8 queuing times
cq.ns() and pkt.ns() are returned in integer nanoseconds, making cq.ns() and pkt.ns() are returned in integer nanoseconds, making
the values about 2^30 times larger than when the units were the values about 2^30 times larger than when the units were
seconds, ii) then in lines 3 and 9 an adjustment of -2 to the seconds, ii) then in lines 3 and 9 an adjustment of -2 to the
right bit-shift multiplies the result by 2^2, to complete the right bit-shift multiplies the result by 2^2, to complete the
scaling by 2^32. scaling by 2^32.
skipping to change at page 37, line 46 skipping to change at page 40, line 46
For a typical mix of RTTs from local data centres and across the For a typical mix of RTTs from local data centres and across the
general Internet, a value of k'=1 (equivalent to k=2) is recommended general Internet, a value of k'=1 (equivalent to k=2) is recommended
as a good workable compromise. as a good workable compromise.
Appendix D. Open Issues Appendix D. Open Issues
Most of the following open issues are also tagged '{ToDo}' at the Most of the following open issues are also tagged '{ToDo}' at the
appropriate point in the document: appropriate point in the document:
Operational guidance to monitor L4S experiment
PI2 appendix: scaling of alpha & beta, esp. dependence of beta_U PI2 appendix: scaling of alpha & beta, esp. dependence of beta_U
on Tupdate on Tupdate
Curvy RED appendix: complete the unfinished parts Curvy RED appendix: complete the unfinished parts
Authors' Addresses Authors' Addresses
Koen De Schepper Koen De Schepper
Nokia Bell Labs Nokia Bell Labs
Antwerp Antwerp
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