draft-ietf-tsvwg-aqm-dualq-coupled-25.original   draft-ietf-tsvwg-aqm-dualq-coupled-26v2.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: 2 March 2023 Independent Expires: April 24, 2023 Independent
G. White G. White
CableLabs CableLabs
29 August 2022 October 21, 2022
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-25 draft-ietf-tsvwg-aqm-dualq-coupled-26
Abstract Abstract
This specification defines a framework for coupling the Active Queue This specification defines a framework for coupling the Active Queue
Management (AQM) algorithms in two queues intended for flows with Management (AQM) algorithms in two queues intended for flows with
different responses to congestion. This provides a way for the different responses to congestion. This provides a way for the
Internet to transition from the scaling problems of standard TCP Internet to transition from the scaling problems of standard TCP
Reno-friendly ('Classic') congestion controls to the family of Reno-friendly ('Classic') congestion controls to the family of
'Scalable' congestion controls. These are designed for consistently 'Scalable' congestion controls. These are designed for consistently
very Low queuing Latency, very Low congestion Loss and Scaling of very Low queuing Latency, very Low congestion Loss and Scaling of
skipping to change at page 1, line 49 skipping to change at page 1, line 49
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Outline of the Problem . . . . . . . . . . . . . . . . . 3 1.1. Outline of the Problem . . . . . . . . . . . . . . . . . 3
1.2. Context, Scope & Applicability . . . . . . . . . . . . . 6 1.2. Context, Scope & Applicability . . . . . . . . . . . . . 6
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. Features . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4. Features . . . . . . . . . . . . . . . . . . . . . . . . 9
2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 11 2. DualQ Coupled AQM . . . . . . . . . . . . . . . . . . . . . . 11
2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 11 2.1. Coupled AQM . . . . . . . . . . . . . . . . . . . . . . . 11
2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 12 2.2. Dual Queue . . . . . . . . . . . . . . . . . . . . . . . 12
2.3. Traffic Classification . . . . . . . . . . . . . . . . . 12 2.3. Traffic Classification . . . . . . . . . . . . . . . . . 12
2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 13 2.4. Overall DualQ Coupled AQM Structure . . . . . . . . . . . 13
2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 17 2.5. Normative Requirements for a DualQ Coupled AQM . . . . . 17
2.5.1. Functional Requirements . . . . . . . . . . . . . . . 17 2.5.1. Functional Requirements . . . . . . . . . . . . . . . 17
2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 18 2.5.1.1. Requirements in Unexpected Cases . . . . . . . . 18
2.5.2. Management Requirements . . . . . . . . . . . . . . . 19 2.5.2. Management Requirements . . . . . . . . . . . . . . . 19
2.5.2.1. Configuration . . . . . . . . . . . . . . . . . . 19 2.5.2.1. Configuration . . . . . . . . . . . . . . . . . . 19
2.5.2.2. Monitoring . . . . . . . . . . . . . . . . . . . 21 2.5.2.2. Monitoring . . . . . . . . . . . . . . . . . . . 21
2.5.2.3. Anomaly Detection . . . . . . . . . . . . . . . . 22 2.5.2.3. Anomaly Detection . . . . . . . . . . . . . . . . 21
2.5.2.4. Deployment, Coexistence and Scaling . . . . . . . 22 2.5.2.4. Deployment, Coexistence and Scaling . . . . . . . 22
3. IANA Considerations (to be removed by RFC Editor) . . . . . . 22 3. IANA Considerations (to be removed by RFC Editor) . . . . . . 22
4. Security Considerations . . . . . . . . . . . . . . . . . . . 22 4. Security Considerations . . . . . . . . . . . . . . . . . . . 22
4.1. Low Delay without Requiring Per-Flow Processing . . . . . 22 4.1. Low Delay without Requiring Per-Flow Processing . . . . . 22
4.2. Handling Unresponsive Flows and Overload . . . . . . . . 23 4.2. Handling Unresponsive Flows and Overload . . . . . . . . 23
4.2.1. Unresponsive Traffic without Overload . . . . . . . . 24 4.2.1. Unresponsive Traffic without Overload . . . . . . . . 24
4.2.2. Avoiding Short-Term Classic Starvation: Sacrifice L4S 4.2.2. Avoiding Short-Term Classic Starvation: Sacrifice L4S
Throughput or Delay? . . . . . . . . . . . . . . . . 25 Throughput or Delay? . . . . . . . . . . . . . . . . 24
4.2.3. L4S ECN Saturation: Introduce Drop or Delay? . . . . 26 4.2.3. L4S ECN Saturation: Introduce Drop or Delay? . . . . 26
4.2.3.1. Protecting against Overload by Unresponsive 4.2.3.1. Protecting against Overload by Unresponsive ECN-
ECN-Capable Traffic . . . . . . . . . . . . . . . . 28 Capable Traffic . . . . . . . . . . . . . . . . . 27
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1. Normative References . . . . . . . . . . . . . . . . . . 28 5.1. Normative References . . . . . . . . . . . . . . . . . . 28
5.2. Informative References . . . . . . . . . . . . . . . . . 29 5.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 35
Appendix A. Example DualQ Coupled PI2 Algorithm . . . . . . . . 34
A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 35 A.1. Pass #1: Core Concepts . . . . . . . . . . . . . . . . . 35
A.2. Pass #2: Edge-Case Details . . . . . . . . . . . . . . . 46 A.2. Pass #2: Edge-Case Details . . . . . . . . . . . . . . . 45
Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 51 Appendix B. Example DualQ Coupled Curvy RED Algorithm . . . . . 50
B.1. Curvy RED in Pseudocode . . . . . . . . . . . . . . . . . 51 B.1. Curvy RED in Pseudocode . . . . . . . . . . . . . . . . . 50
B.2. Efficient Implementation of Curvy RED . . . . . . . . . . 57 B.2. Efficient Implementation of Curvy RED . . . . . . . . . . 56
Appendix C. Choice of Coupling Factor, k . . . . . . . . . . . . 59 Appendix C. Choice of Coupling Factor, k . . . . . . . . . . . . 58
C.1. RTT-Dependence . . . . . . . . . . . . . . . . . . . . . 59 C.1. RTT-Dependence . . . . . . . . . . . . . . . . . . . . . 58
C.2. Guidance on Controlling Throughput Equivalence . . . . . 60 C.2. Guidance on Controlling Throughput Equivalence . . . . . 59
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 64 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 63
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 64
1. Introduction 1. Introduction
This document specifies a framework for DualQ Coupled AQMs, which can This document specifies a framework for DualQ Coupled AQMs, which can
serve as the network part of the L4S serve as the network part of the L4S
architecture [I-D.ietf-tsvwg-l4s-arch]. A Coupled DualQ AQM consists architecture [I-D.ietf-tsvwg-l4s-arch]. A Coupled DualQ AQM consists
of two queues; L4S and Classic. The L4S queue is intended for of two queues; L4S and Classic. The L4S queue is intended for
Scalable congestion controls that can maintain very low queuing Scalable congestion controls that can maintain very low queuing
latency (sub-millisecond on average) and high throughput at the same latency (sub-millisecond on average) and high throughput at the same
time. The Coupled DualQ acts like a semi-permeable membrane: the L4S time. The Coupled DualQ acts like a semi-permeable membrane: the L4S
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now it has not been possible to allow any number of low latency, high now it has not been possible to allow any number of low latency, high
throughput applications to seek to fully utilize available capacity, throughput applications to seek to fully utilize available capacity,
because the capacity-seeking process itself causes too much queuing because the capacity-seeking process itself causes too much queuing
delay. delay.
To reduce this queuing delay caused by the capacity seeking process, To reduce this queuing delay caused by the capacity seeking process,
changes either to the network alone or to end-systems alone are in changes either to the network alone or to end-systems alone are in
progress. L4S involves a recognition that both approaches are progress. L4S involves a recognition that both approaches are
yielding diminishing returns: yielding diminishing returns:
* Recent state-of-the-art active queue management (AQM) in the o Recent state-of-the-art active queue management (AQM) in the
network, e.g. FQ-CoDel [RFC8290], PIE [RFC8033], Adaptive network, e.g. FQ-CoDel [RFC8290], PIE [RFC8033], Adaptive
RED [ARED01] ) has reduced queuing delay for all traffic, not just RED [ARED01] ) has reduced queuing delay for all traffic, not just
a select few applications. However, no matter how good the AQM, a select few applications. However, no matter how good the AQM,
the capacity-seeking (sawtoothing) rate of TCP-like congestion the capacity-seeking (sawtoothing) rate of TCP-like congestion
controls represents a lower limit that will either cause queuing controls represents a lower limit that will either cause queuing
delay to vary or cause the link to be under-utilized. These AQMs delay to vary or cause the link to be under-utilized. These AQMs
are tuned to allow a typical capacity-seeking Reno-friendly flow are tuned to allow a typical capacity-seeking Reno-friendly flow
to induce an average queue that roughly doubles the base RTT, to induce an average queue that roughly doubles the base RTT,
adding 5-15 ms of queuing on average (cf. 500 microseconds with adding 5-15 ms of queuing on average (cf. 500 microseconds with
L4S for the same mix of long-running and web traffic). However, L4S for the same mix of long-running and web traffic). However,
for many applications low delay is not useful unless it is for many applications low delay is not useful unless it is
consistently low. With these AQMs, 99th percentile queuing delay consistently low. With these AQMs, 99th percentile queuing delay
is 20-30 ms (cf. 2 ms with the same traffic over L4S). is 20-30 ms (cf. 2 ms with the same traffic over L4S).
* Similarly, recent research into using e2e congestion control o Similarly, recent research into using e2e congestion control
without needing an AQM in the network (e.g. BBR without needing an AQM in the network (e.g. BBR
[I-D.cardwell-iccrg-bbr-congestion-control]) seems to have hit a [I-D.cardwell-iccrg-bbr-congestion-control]) seems to have hit a
similar lower limit to queuing delay of about 20ms on average, but similar lower limit to queuing delay of about 20ms on average, but
there are also regular 25ms delay spikes due to bandwidth probes there are also regular 25ms delay spikes due to bandwidth probes
and 60ms spikes due to flow-starts. and 60ms spikes due to flow-starts.
L4S learns from the experience of Data Center TCP [RFC8257], which L4S learns from the experience of Data Center TCP [RFC8257], which
shows the power of complementary changes both in the network and on shows the power of complementary changes both in the network and on
end-systems. DCTCP teaches us that two small but radical changes to end-systems. DCTCP teaches us that two small but radical changes to
congestion control are needed to cut the two major outstanding causes congestion control are needed to cut the two major outstanding causes
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For the public Internet, nearly all the benefit will typically be For the public Internet, nearly all the benefit will typically be
achieved by deploying the Coupled AQM into either end of the access achieved by deploying the Coupled AQM into either end of the access
link between a 'site' and the Internet, which is invariably the link between a 'site' and the Internet, which is invariably the
bottleneck (see section 6.4 of[I-D.ietf-tsvwg-l4s-arch] about bottleneck (see section 6.4 of[I-D.ietf-tsvwg-l4s-arch] about
deployment, which also defines the term 'site' to mean a home, an deployment, which also defines the term 'site' to mean a home, an
office, a campus or mobile user equipment). office, a campus or mobile user equipment).
Latency is not the only concern of L4S: Latency is not the only concern of L4S:
* The "Low Loss" part of the name denotes that L4S generally o The "Low Loss" part of the name denotes that L4S generally
achieves zero congestion loss (which would otherwise cause achieves zero congestion loss (which would otherwise cause
retransmission delays), due to its use of ECN. retransmission delays), due to its use of ECN.
* The "Scalable throughput" part of the name denotes that the per- o The "Scalable throughput" part of the name denotes that the per-
flow throughput of Scalable congestion controls should scale flow throughput of Scalable congestion controls should scale
indefinitely, avoiding the imminent scaling problems with 'TCP- indefinitely, avoiding the imminent scaling problems with 'TCP-
Friendly' congestion control algorithms [RFC3649]. Friendly' congestion control algorithms [RFC3649].
The former is clearly in scope of this AQM document. However, the The former is clearly in scope of this AQM document. However, the
latter is an outcome of the end-system behaviour, and therefore latter is an outcome of the end-system behaviour, and therefore
outside the scope of this AQM document, even though the AQM is an outside the scope of this AQM document, even though the AQM is an
enabler. enabler.
The overall L4S architecture [I-D.ietf-tsvwg-l4s-arch] gives more The overall L4S architecture [I-D.ietf-tsvwg-l4s-arch] gives more
detail, including on wider deployment aspects such as backwards detail, including on wider deployment aspects such as backwards
compatibility of Scalable congestion controls in bottlenecks where a compatibility of Scalable congestion controls in bottlenecks where a
DualQ Coupled AQM has not been deployed. The supporting papers DualQ Coupled AQM has not been deployed. The supporting papers
[DualPI2Linux], [PI2], [DCttH19] and [PI2param] give the full [L4Seval22], [DualPI2Linux], [PI2] and [PI2param] give the full
rationale for the AQM's design, both discursively and in more precise rationale for the AQM's design, both discursively and in more precise
mathematical form, as well as the results of performance evaluations. mathematical form, as well as the results of performance evaluations.
The main results have been validated independently when using the The main results have been validated independently when using the
Prague congestion control [Boru20] (experiments are run using Prague Prague congestion control [Boru20] (experiments are run using Prague
and DCTCP, but only the former are relevant for validation, because and DCTCP, but only the former are relevant for validation, because
Prague fixes a number of problems with the Linux DCTCP code that make Prague fixes a number of problems with the Linux DCTCP code that make
it unsuitable for the public Internet). it unsuitable for the public Internet).
1.3. Terminology 1.3. Terminology
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averages 2 congestion signals per round-trip whatever the flow averages 2 congestion signals per round-trip whatever the flow
rate, as do other recently developed scalable congestion controls, rate, as do other recently developed scalable congestion controls,
e.g. Relentless TCP [I-D.mathis-iccrg-relentless-tcp], TCP Prague e.g. Relentless TCP [I-D.mathis-iccrg-relentless-tcp], TCP Prague
[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux], [I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux],
BBRv2 [BBRv2], [I-D.cardwell-iccrg-bbr-congestion-control] and the BBRv2 [BBRv2], [I-D.cardwell-iccrg-bbr-congestion-control] and the
L4S variant of SCREAM for real-time media [SCReAM], [RFC8298]). L4S variant of SCREAM for real-time media [SCReAM], [RFC8298]).
For the public Internet a Scalable transport has to comply with For the public Internet a Scalable transport has to comply with
the requirements in Section 4 of [I-D.ietf-tsvwg-ecn-l4s-id] the requirements in Section 4 of [I-D.ietf-tsvwg-ecn-l4s-id]
(aka. the 'Prague L4S requirements'). (aka. the 'Prague L4S requirements').
C: Abbreviation for Classic, e.g. when used as a subscript. C: Abbreviation for Classic, e.g. when used as a subscript.
L: Abbreviation for L4S, e.g. when used as a subscript. L: Abbreviation for L4S, e.g. when used as a subscript.
The terms Classic or L4S can also qualify other nouns, such as The terms Classic or L4S can also qualify other nouns, such as
'codepoint', 'identifier', 'classification', 'packet', 'flow'. 'codepoint', 'identifier', 'classification', 'packet', 'flow'.
For example: an L4S packet means a packet with an L4S identifier For example: an L4S packet means a packet with an L4S identifier
sent from an L4S congestion control. sent from an L4S congestion control.
Both Classic and L4S services can cope with a proportion of Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well, but in the L4S unresponsive or less-responsive traffic as well, but in the L4S
case its rate has to be smooth enough or low enough not to build a case its rate has to be smooth enough or low enough not to build a
queue (e.g. DNS, VoIP, game sync datagrams, etc.). The DualQ queue (e.g. DNS, VoIP, game sync datagrams, etc.). The DualQ
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traffic. traffic.
Thousands of tests have been conducted in a typical fixed residential Thousands of tests have been conducted in a typical fixed residential
broadband setting. Experiments used a range of base round trip broadband setting. Experiments used a range of base round trip
delays up to 100ms and link rates up to 200 Mb/s between the data delays up to 100ms and link rates up to 200 Mb/s between the data
centre and home network, with varying amounts of background traffic centre and home network, with varying amounts of background traffic
in both queues. For every L4S packet, the AQM kept the average in both queues. For every L4S packet, the AQM kept the average
queuing delay below 1ms (or 2 packets where serialization delay queuing delay below 1ms (or 2 packets where serialization delay
exceeded 1ms on slower links), with 99th percentile no worse than exceeded 1ms on slower links), with 99th percentile no worse than
2ms. No losses at all were introduced by the L4S AQM. Details of 2ms. No losses at all were introduced by the L4S AQM. Details of
the extensive experiments are available [DualPI2Linux], [PI2], the extensive experiments are available [L4Seval22], [DualPI2Linux].
[DCttH19]. Subjective testing using very demanding high bandwidth Subjective testing using very demanding high bandwidth low latency
low latency applications over a single shared access link is also applications over a single shared access link is also described
described in [L4Sdemo16] and summarized in the section about in [L4Sdemo16] and summarized in the section about applications in
applications in the L4S architecture [I-D.ietf-tsvwg-l4s-arch] . the L4S architecture [I-D.ietf-tsvwg-l4s-arch] .
In all these experiments, the host was connected to the home network In all these experiments, the host was connected to the home network
by fixed Ethernet, in order to quantify the queuing delay that can be by fixed Ethernet, in order to quantify the queuing delay that can be
achieved by a user who cares about delay. It should be emphasized achieved by a user who cares about delay. It should be emphasized
that L4S support at the bottleneck link cannot 'undelay' bursts that L4S support at the bottleneck link cannot 'undelay' bursts
introduced by another link on the path, for instance by legacy Wi-Fi introduced by another link on the path, for instance by legacy Wi-Fi
equipment. However, if L4S support is added to the queue feeding the equipment. However, if L4S support is added to the queue feeding the
_outgoing_ WAN link of a home gateway, it would be counterproductive _outgoing_ WAN link of a home gateway, it would be counterproductive
not to also reduce the burstiness of the _incoming_ Wi-Fi. Also, not to also reduce the burstiness of the _incoming_ Wi-Fi. Also,
trials of Wi-Fi equipment with an L4S DualQ Coupled AQM on the trials of Wi-Fi equipment with an L4S DualQ Coupled AQM on the
skipping to change at page 10, line 37 skipping to change at page 10, line 37
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 (TCP Prague etc.) and still achieve low delay. capacity-seeking flows (TCP Prague etc.) and still achieve low delay.
The L4S queue does not rely on the presence of other traffic in the 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. The tail latency of traffic whether or not there is Classic traffic. The tail latency of
traffic served by the Classic AQM is sometimes a little better traffic served by the Classic AQM is sometimes a little better
sometimes a little worse, when a proportion of the traffic is L4S. sometimes a little worse, when a proportion of the traffic is L4S.
The two queues are only necessary because: The two queues are only necessary because:
* the large variations (sawteeth) of Classic flows need roughly a o the large variations (sawteeth) of Classic flows need roughly a
base RTT of queuing delay to ensure full utilization base RTT of queuing delay to ensure full utilization
* Scalable flows do not need a queue to keep utilization high, but o Scalable flows do not need a queue to keep utilization high, but
they cannot keep latency predictably low if they are mixed with they cannot keep latency predictably low if they are mixed with
Classic traffic, Classic traffic,
The L4S queue has latency priority within sub-round trip timescales, The L4S queue has latency priority within sub-round trip timescales,
but over longer periods the coupling from the Classic to the L4S AQM but over longer periods the coupling from the Classic to the L4S AQM
(explained below) ensures that it does not have bandwidth priority (explained below) ensures that it does not have bandwidth priority
over the Classic queue. over the Classic queue.
2. DualQ Coupled AQM 2. DualQ Coupled AQM
There are two main aspects to the approach: There are two main aspects to the approach:
* The Coupled AQM that addresses throughput equivalence between o The Coupled AQM that addresses throughput equivalence between
Classic (e.g. Reno, Cubic) flows and L4S flows (that satisfy the Classic (e.g. Reno, Cubic) flows and L4S flows (that satisfy the
Prague L4S requirements). Prague L4S requirements).
* 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 the steady-state congestion window, cwnd, and the drop between the steady-state congestion window, cwnd, and the drop
probability, p of standard Reno congestion control [RFC5681]. To a probability, p of standard Reno congestion control [RFC5681]. To a
first order approximation, the steady-state cwnd of Reno is inversely first order approximation, the steady-state cwnd of Reno is inversely
proportional to the square root of p. proportional to the square root of p.
skipping to change at page 15, line 28 skipping to change at page 15, line 28
`----------'\\ | AQM |---->: ,'|`-.___.-' `----------'\\ | AQM |---->: ,'|`-.___.-'
\\ | |p' | <' | \\ | |p' | <' |
\\ `-------' (p'^2) //`' \\ `-------' (p'^2) //`'
\\ ^ | // \\ ^ | //
\\,. | v p_C // \\,. | v p_C //
< | _________ .------.// < | _________ .------.//
`\| | | | Drop |/ `\| | | | Drop |/
Classic (C) |queue |===>|/mark | Classic (C) |queue |===>|/mark |
__|______| `------' __|______| `------'
Figure 1: DualQ Coupled AQM Schematic
Legend: ===> traffic flow; ---> control dependency. Legend: ===> traffic flow; ---> control dependency.
Figure 1: DualQ Coupled AQM Schematic
After the AQMs have applied their dropping or marking, the scheduler After the AQMs have applied their dropping or marking, the scheduler
forwards their packets to the link. Even though the scheduler gives forwards their packets to the link. Even though the scheduler gives
priority to the L queue, it is not as strong as the coupling from the priority to the L queue, it is not as strong as the coupling from the
C queue. This is because, as the C queue grows, the base AQM applies C queue. This is because, as the C queue grows, the base AQM applies
more congestion signals to L traffic (as well as C). As L flows more congestion signals to L traffic (as well as C). As L flows
reduce their rate in response, they use less than the scheduling reduce their rate in response, they use less than the scheduling
share for L traffic. So, because the scheduler is work preserving, share for L traffic. So, because the scheduler is work preserving,
it schedules any C traffic in the gaps. it schedules any C traffic in the gaps.
Giving priority to the L queue has the benefit of very low L queue Giving priority to the L queue has the benefit of very low L queue
skipping to change at page 18, line 41 skipping to change at page 18, line 41
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 expected that the AQM in each queue will inspect the ECN field It is expected that the AQM in each queue will inspect the ECN field
to determine what sort of congestion notification to signal, then it to determine what sort of congestion notification to signal, then it
will decide whether to apply congestion notification to this will decide whether to apply congestion notification to this
particular packet, as follows: particular packet, as follows:
* If a packet that does not carry an ECT(1) or CE codepoint is o If a packet that does not carry an ECT(1) or CE codepoint is
classified into the L queue: classified into the L queue:
- if the packet is ECT(0), the L AQM SHOULD apply CE-marking * if the packet is ECT(0), the L AQM SHOULD apply CE-marking
using a probability appropriate to Classic congestion control using a probability appropriate to Classic congestion control
and appropriate to the target delay in the L queue and appropriate to the target delay in the L queue
- if the packet is Not-ECT, the appropriate action depends on * if the packet is Not-ECT, the appropriate action depends on
whether some other function is protecting the L queue from whether some other function is protecting the L queue from
misbehaving flows (e.g. per-flow queue protection misbehaving flows (e.g. per-flow queue protection
[I-D.briscoe-docsis-q-protection] or latency policing): [I-D.briscoe-docsis-q-protection] or latency policing):
o If separate queue protection is provided, the L AQM SHOULD + If separate queue protection is provided, the L AQM SHOULD
ignore the packet and forward it unchanged, meaning it ignore the packet and forward it unchanged, meaning it
should not calculate whether to apply congestion should not calculate whether to apply congestion
notification and it should neither drop nor CE-mark the notification and it should neither drop nor CE-mark the
packet (for instance, the operator might classify EF traffic packet (for instance, the operator might classify EF traffic
that is unresponsive to drop into the L queue, alongside that is unresponsive to drop into the L queue, alongside
responsive L4S-ECN traffic) responsive L4S-ECN traffic)
o if separate queue protection is not provided, the L AQM + if separate queue protection is not provided, the L AQM
SHOULD apply drop using a drop probability appropriate to SHOULD apply drop using a drop probability appropriate to
Classic congestion control and appropriate to the target Classic congestion control and appropriate to the target
delay in the L queue delay in the L queue
* If a packet that carries an ECT(1) codepoint is classified into o If a packet that carries an ECT(1) codepoint is classified into
the C queue: the C queue:
- the C AQM SHOULD apply CE-marking using the coupled AQM * the C AQM SHOULD apply CE-marking using the coupled AQM
probability p_CL (= k*p'). probability p_CL (= k*p').
The above requirements are worded as "SHOULDs", because operator- The above requirements are worded as "SHOULDs", because operator-
specific classifiers are for flexibility, by definition. Therefore, specific classifiers are for flexibility, by definition. Therefore,
alternative actions might be appropriate in the operator's specific alternative actions might be appropriate in the operator's specific
circumstances. An example would be where the operator knows that circumstances. An example would be where the operator knows that
certain legacy traffic marked with one codepoint actually has a certain legacy traffic marked with one codepoint actually has a
congestion response associated with another codepoint. congestion response associated with another codepoint.
If the DualQ Coupled AQM has detected overload, it MUST introduce If the DualQ Coupled AQM has detected overload, it MUST introduce
skipping to change at page 20, line 5 skipping to change at page 19, line 47
2.5.2. Management Requirements 2.5.2. Management Requirements
2.5.2.1. Configuration 2.5.2.1. Configuration
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:
* 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);
* Expected typical RTT, which can be used to determine the queuing o Expected typical RTT, which can be used to determine the queuing
delay of the Classic AQM at its operating point, in order to delay of the Classic AQM at its operating point, in order to
prevent typical lone flows from under-utilizing capacity. For prevent typical lone flows from under-utilizing capacity. For
example: example:
- for the PI2 algorithm (Appendix A) the queuing delay target is * for the PI2 algorithm (Appendix A) the queuing delay target is
dependent on the typical RTT; dependent on the typical RTT;
- for the Curvy RED algorithm (Appendix B) the queuing delay at * for the Curvy RED algorithm (Appendix B) the queuing delay at
the desired operating point of the curvy ramp is configured to the desired operating point of the curvy ramp is configured to
encompass a typical RTT; encompass a typical RTT;
- if another Classic AQM was used, it would be likely to need an * if another Classic AQM was used, it would be likely to need an
operating point for the queue based on the typical RTT, and if operating point for the queue based on the typical RTT, and if
so it SHOULD be expressed in units of time. so it SHOULD be expressed in units of time.
An operating point that is manually calculated might be directly An operating point that is manually calculated might be directly
configurable instead, e.g. for links with large numbers of flows configurable instead, e.g. for links with large numbers of flows
where under-utilization by a single flow would be unlikely. where under-utilization by a single flow would be unlikely.
* Expected maximum RTT, which can be used to set the stability o Expected maximum RTT, which can be used to set the stability
parameter(s) of the Classic AQM. For example: parameter(s) of the Classic AQM. For example:
- for the PI2 algorithm (Appendix A), the gain parameters of the * for the PI2 algorithm (Appendix A), the gain parameters of the
PI algorithm depend on the maximum RTT. PI algorithm depend on the maximum RTT.
- for the Curvy RED algorithm (Appendix B) the smoothing * for the Curvy RED algorithm (Appendix B) the smoothing
parameter is chosen to filter out transients in the queue parameter is chosen to filter out transients in the queue
within a maximum RTT. within a maximum RTT.
Stability parameter(s) that are manually calculated assuming a Stability parameter(s) that are manually calculated assuming a
maximum RTT might be directly configurable instead. maximum RTT might be directly configurable instead.
* Coupling factor, k (see Appendix C.2); o Coupling factor, k (see Appendix C.2);
* A limit to the conditional priority of L4S. This is scheduler- o A limit to the conditional priority of L4S. This is scheduler-
dependent, but it SHOULD be expressed as a relation between the dependent, but it SHOULD be expressed as a relation between the
max delay of a C packet and an L packet. For example: max delay of a C packet and an L packet. For example:
- for a WRR scheduler a weight ratio between L and C of w:1 means * for a WRR scheduler a weight ratio between L and C of w:1 means
that the maximum delay to a C packet is w times that of an L that the maximum delay to a C packet is w times that of an L
packet. packet.
- for a time-shifted FIFO (TS-FIFO) scheduler (see Section 4.2.2) * for a time-shifted FIFO (TS-FIFO) scheduler (see Section 4.2.2)
a time-shift of tshift means that the maximum delay to a C a time-shift of tshift means that the maximum delay to a C
packet is tshift greater than that of an L packet. tshift could packet is tshift greater than that of an L packet. tshift could
be expressed as a multiple of the typical RTT rather than as an be expressed as a multiple of the typical RTT rather than as an
absolute delay. absolute delay.
* The maximum Classic ECN marking probability, p_Cmax, before o The maximum Classic ECN marking probability, p_Cmax, before
introducing drop. introducing drop.
2.5.2.2. Monitoring 2.5.2.2. Monitoring
An experimental DualQ Coupled AQM SHOULD allow the operator to An experimental DualQ Coupled AQM SHOULD allow the operator to
monitor each of the following operational statistics on demand, per monitor each of the following operational statistics on demand, per
queue and per configurable sample interval, for performance queue and per configurable sample interval, for performance
monitoring and perhaps also for accounting in some cases: monitoring and perhaps also for accounting in some cases:
* Bits forwarded, from which utilization can be calculated; o Bits forwarded, from which utilization can be calculated;
* Total packets in the three categories: arrived, presented to the o Total packets in the three categories: arrived, presented to the
AQM, and forwarded. The difference between the first two will AQM, and forwarded. The difference between the first two will
measure any non-AQM tail discard. The difference between the last measure any non-AQM tail discard. The difference between the last
two will measure proactive AQM discard; two will measure proactive AQM discard;
* ECN packets marked, non-ECN packets dropped, ECN packets dropped, o ECN packets marked, non-ECN packets dropped, ECN packets dropped,
which can be combined with the three total packet counts above to which can be combined with the three total packet counts above to
calculate marking and dropping probabilities; calculate marking and dropping probabilities;
* Queue delay (not including serialization delay of the head packet o Queue delay (not including serialization delay of the head packet
or medium acquisition delay) - see further notes below. or medium acquisition delay) - see further notes below.
Unlike the other statistics, queue delay cannot be captured in a Unlike the other statistics, queue delay cannot be captured in a
simple accumulating counter. Therefore, the type of queue delay simple accumulating counter. Therefore, the type of queue delay
statistics produced (mean, percentiles, etc.) will depend on statistics produced (mean, percentiles, etc.) will depend on
implementation constraints. To facilitate comparative evaluation implementation constraints. To facilitate comparative evaluation
of different implementations and approaches, an implementation of different implementations and approaches, an implementation
SHOULD allow mean and 99th percentile queue delay to be derived SHOULD allow mean and 99th percentile queue delay to be derived
(per queue per sample interval). A relatively simple way to do (per queue per sample interval). A relatively simple way to do
this would be to store a coarse-grained histogram of queue delay. this would be to store a coarse-grained histogram of queue delay.
skipping to change at page 22, line 10 skipping to change at page 21, line 49
a sample interval, each bin would accumulate a count of the number a sample interval, each bin would accumulate a count of the number
of packets that had fallen within each range. The maximum queue of packets that had fallen within each range. The maximum queue
delay per queue per interval MAY also be recorded, to aid delay per queue per interval MAY also be recorded, to aid
diagnosis of faults and anomalous events. diagnosis of faults and anomalous events.
2.5.2.3. Anomaly Detection 2.5.2.3. Anomaly Detection
An experimental DualQ Coupled AQM SHOULD asynchronously report the An experimental DualQ Coupled AQM SHOULD asynchronously report the
following data about anomalous conditions: following data about anomalous conditions:
* Start-time and duration of overload state. o Start-time and duration of overload state.
A hysteresis mechanism SHOULD be used to prevent flapping in and A hysteresis mechanism SHOULD be used to prevent flapping in and
out of overload causing an event storm. For instance, exit from out of overload causing an event storm. For instance, exit from
overload state could trigger one report, but also latch a timer. overload state could trigger one report, but also latch a timer.
Then, during that time, if the AQM enters and exits overload state Then, during that time, if the AQM enters and exits overload state
any number of times, the duration in overload state is any number of times, the duration in overload state is
accumulated, but no new report is generated until the first time accumulated, but no new report is generated until the first time
the AQM is out of overload once the timer has expired. the AQM is out of overload once the timer has expired.
2.5.2.4. Deployment, Coexistence and Scaling 2.5.2.4. Deployment, Coexistence and Scaling
skipping to change at page 24, line 5 skipping to change at page 23, line 38
A trade-off needs to be made between complexity and the risk of A trade-off needs to be made between complexity and the risk of
either traffic class harming the other. In overloaded conditions the either traffic class harming the other. In overloaded conditions the
higher priority L4S service will have to sacrifice some aspect of its higher priority L4S service will have to sacrifice some aspect of its
performance. Depending on the degree of overload, alternative performance. Depending on the degree of overload, alternative
solutions may relax a different factor: e.g. throughput, delay, drop. solutions may relax a different factor: e.g. throughput, delay, drop.
These choices need to be made either by the developer or by operator These choices need to be made either by the developer or by operator
policy, rather than by the IETF. Subsequent subsections discuss policy, rather than by the IETF. Subsequent subsections discuss
aspects relating to handling of different degrees of overload: aspects relating to handling of different degrees of overload:
* Unresponsive flows (L and/or C) but not overloaded, i.e. the sum o Unresponsive flows (L and/or C) but not overloaded, i.e. the sum
of unresponsive load before adding any responsive traffic is below of unresponsive load before adding any responsive traffic is below
capacity; capacity;
This case is handled by the regular Coupled DualQ (Section 2.1) This case is handled by the regular Coupled DualQ (Section 2.1)
but not discussed there. So below, Section 4.2.1 explains the but not discussed there. So below, Section 4.2.1 explains the
design goal, and how it is achieved in practice; design goal, and how it is achieved in practice;
* Unresponsive flows (L and/or C) causing persistent overload, o Unresponsive flows (L and/or C) causing persistent overload,
i.e. the sum of unresponsive load even before adding any i.e. the sum of unresponsive load even before adding any
responsive traffic persistently exceeds capacity; responsive traffic persistently exceeds capacity;
This case is not covered by the regular Coupled DualQ mechanism This case is not covered by the regular Coupled DualQ mechanism
(Section 2.1) but the last para in Section 2.5.1.1 sets out a (Section 2.1) but the last para in Section 2.5.1.1 sets out a
requirement to handle the case where ECN-capable traffic could requirement to handle the case where ECN-capable traffic could
starve non-ECN-capable traffic. Section 4.2.3 below discusses starve non-ECN-capable traffic. Section 4.2.3 below discusses
the general options and gives specific examples. the general options and gives specific examples.
* Short-term overload that lies between the 'not overloaded' and o Short-term overload that lies between the 'not overloaded' and
'persistently overloaded' cases. 'persistently overloaded' cases.
For the period before overload is deemed persistent, For the period before overload is deemed persistent,
Section 4.2.2 discusses options for more immediate mechanisms Section 4.2.2 discusses options for more immediate mechanisms
at the scheduler timescale. These prevent short-term at the scheduler timescale. These prevent short-term
starvation of the C queue by making the priority of the L queue starvation of the C queue by making the priority of the L queue
conditional, as required in Section 2.5.1. conditional, as required in Section 2.5.1.
4.2.1. Unresponsive Traffic without Overload 4.2.1. Unresponsive Traffic without Overload
When one or more L flows and/or C flows are unresponsive, but their When one or more L flows and/or C flows are unresponsive, but their
total load is within the link capacity so that they do not saturate total load is within the link capacity so that they do not saturate
the coupled marking (below 100%), the goal of a DualQ AQM is to the coupled marking (below 100%), the goal of a DualQ AQM is to
behave no worse than a single-queue AQM. behave no worse than a single-queue AQM.
Tests have shown that this is indeed the case with no additional Tests have shown that this is indeed the case with no additional
mechanism beyond the regular Coupled DualQ of Section 2.1 (see the mechanism beyond the regular Coupled DualQ of Section 2.1 (see the
results of 'overload experiments' in [DCttH19]). Perhaps counter- results of 'overload experiments' in [L4Seval22]). Perhaps counter-
intuitively, whether the unresponsive flow classifies itself into the intuitively, whether the unresponsive flow classifies itself into the
L or the C queue, the DualQ system behaves as if it has subtracted L or the C queue, the DualQ system behaves as if it has subtracted
from the overall link capacity. Then, the coupling shares out the from the overall link capacity. Then, the coupling shares out the
remaining capacity between any competing responsive flows (in either remaining capacity between any competing responsive flows (in either
queue). See also Section 4.2.2, which discusses scheduler-specific queue). See also Section 4.2.2, which discusses scheduler-specific
details. details.
4.2.2. Avoiding Short-Term Classic Starvation: Sacrifice L4S Throughput 4.2.2. Avoiding Short-Term Classic Starvation: Sacrifice L4S Throughput
or Delay? or Delay?
skipping to change at page 25, line 19 skipping to change at page 24, line 47
Section 2.5.1) to avoid short-term starvation of Classic. Otherwise, Section 2.5.1) to avoid short-term starvation of Classic. Otherwise,
as explained in Section 2.4, even a lone responsive L4S flow could as explained in Section 2.4, even a lone responsive L4S flow could
temporarily block a small finite set of C packets (e.g. an initial temporarily block a small finite set of C packets (e.g. an initial
window or DNS request). The blockage would only be brief, but it window or DNS request). The blockage would only be brief, but it
could be longer for certain AQM implementations that can only could be longer for certain AQM implementations that can only
increase the congestion signal coupled from the C queue when C increase the congestion signal coupled from the C queue when C
packets are actually being dequeued. There is then the question of packets are actually being dequeued. There is then the question of
whether to sacrifice L4S throughput or L4S delay (or some other whether to sacrifice L4S throughput or L4S delay (or some other
policy) to make the priority conditional: policy) to make the priority conditional:
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
throughput during overload. This can either be thought of as throughput during overload. This can either be thought of as
guaranteeing a minimum throughput service for Classic traffic, or guaranteeing a minimum throughput service for Classic traffic, or
as guaranteeing a maximum delay for a packet at the head of the as guaranteeing a maximum delay for a packet at the head of the
Classic queue. Classic queue.
Cautionary note: a WRR scheduler can only guarantee Classic Cautionary note: a WRR scheduler can only guarantee Classic
throughput if Classic sources are sending enough to use it -- throughput if Classic sources are sending enough to use it --
congestion signals can undermine scheduling because they determine congestion signals can undermine scheduling because they determine
how much responsive traffic of each class arrives for scheduling how much responsive traffic of each class arrives for scheduling
skipping to change at page 28, line 30 skipping to change at page 28, line 10
As an example, experiments with the DualPI2 AQM (Appendix A) have As an example, experiments with the DualPI2 AQM (Appendix A) have
shown that introducing 'drop on saturation' at 100% coupled L4S shown that introducing 'drop on saturation' at 100% coupled L4S
marking addresses this problem with unresponsive ECN as well as marking addresses this problem with unresponsive ECN as well as
addressing the saturation problem. At saturation, DualPI2 switches addressing the saturation problem. At saturation, DualPI2 switches
into overload mode, where the base AQM is driven by the max delay of into overload mode, where the base AQM is driven by the max delay of
both queues and it introduces probabilistic drop to both queues both queues and it introduces probabilistic drop to both queues
equally. It leaves only a small range of congestion levels just equally. It leaves only a small range of congestion levels just
below saturation where unresponsive traffic gains any advantage from below saturation where unresponsive traffic gains any advantage from
using the ECN capability (relative to being unresponsive without using the ECN capability (relative to being unresponsive without
ECN), and the advantage is hardly detectable (see [DualQ-Test] and ECN), and the advantage is hardly detectable (see [DualQ-Test] and
section IV-E of [DCttH19]. Also overload with an unresponsive ECT(1) section IV-G of [L4Seval22]. Also overload with an unresponsive
flow gets no more bandwidth advantage than with ECT(0). ECT(1) flow gets no more bandwidth advantage than with ECT(0).
5. References 5. References
5.1. Normative References 5.1. Normative References
[I-D.ietf-tsvwg-ecn-l4s-id] [I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. D. and B. Briscoe, "Explicit Congestion Schepper, K. and B. Briscoe, "Explicit Congestion
Notification (ECN) Protocol for Very Low Queuing Delay Notification (ECN) Protocol for Ultra-Low Queuing Delay
(L4S)", Work in Progress, Internet-Draft, draft-ietf- (L4S)", draft-ietf-tsvwg-ecn-l4s-id-14 (work in progress),
tsvwg-ecn-l4s-id-28, 8 August 2022, March 2021.
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-tsvwg-ecn-l4s-id/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001, RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>. <https://www.rfc-editor.org/info/rfc3168>.
skipping to change at page 29, line 32 skipping to change at page 29, line 7
[AQMmetrics] [AQMmetrics]
Kwon, M. and S. Fahmy, "A Comparison of Load-based and Kwon, M. and S. Fahmy, "A Comparison of Load-based and
Queue- based Active Queue Management Algorithms", Proc. Queue- based Active Queue Management Algorithms", Proc.
Int'l Soc. for Optical Engineering (SPIE) 4866:35--46 DOI: Int'l Soc. for Optical Engineering (SPIE) 4866:35--46 DOI:
10.1117/12.473021, 2002, 10.1117/12.473021, 2002,
<https://www.cs.purdue.edu/homes/fahmy/papers/ldc.pdf>. <https://www.cs.purdue.edu/homes/fahmy/papers/ldc.pdf>.
[ARED01] Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An [ARED01] Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
Algorithm for Increasing the Robustness of RED's Active Algorithm for Increasing the Robustness of RED's Active
Queue Management", ACIRI Technical Report , August 2001, Queue Management", ACIRI Technical Report 301, August
<https://www.icir.org/floyd/red.html>. 2001, <http://www.icsi.berkeley.edu/icsi/node/2032>.
[BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", GitHub [BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", GitHub
repository; Linux congestion control module, repository; Linux congestion control module,
<https://github.com/google/bbr/blob/v2alpha/README.md>. <https://github.com/google/bbr/blob/v2alpha/README.md>.
[Boru20] Boru Oljira, D., Grinnemo, K-J., Brunstrom, A., and J. [Boru20] Boru Oljira, D., Grinnemo, K-J., Brunstrom, A., and J.
Taheri, "Validating the Sharing Behavior and Latency Taheri, "Validating the Sharing Behavior and Latency
Characteristics of the L4S Architecture", ACM CCR Characteristics of the L4S Architecture", ACM CCR
50(2):37--44, May 2020, 50(2):37--44, May 2020,
<https://dl.acm.org/doi/abs/10.1145/3402413.3402419>. <https://dl.acm.org/doi/abs/10.1145/3402413.3402419>.
skipping to change at page 30, line 15 skipping to change at page 29, line 37
[CoDel] Nichols, K. and V. Jacobson, "Controlling Queue Delay", [CoDel] Nichols, K. and V. Jacobson, "Controlling Queue Delay",
ACM Queue 10(5), May 2012, ACM Queue 10(5), May 2012,
<https://queue.acm.org/issuedetail.cfm?issue=2208917>. <https://queue.acm.org/issuedetail.cfm?issue=2208917>.
[CRED_Insights] [CRED_Insights]
Briscoe, B., "Insights from Curvy RED (Random Early Briscoe, B., "Insights from Curvy RED (Random Early
Detection)", BT Technical Report TR-TUB8-2015-003 Detection)", BT Technical Report TR-TUB8-2015-003
arXiv:1904.07339 [cs.NI], July 2015, arXiv:1904.07339 [cs.NI], July 2015,
<https://arxiv.org/abs/1904.07339>. <https://arxiv.org/abs/1904.07339>.
[DCttH19] De Schepper, K., Bondarenko, O., Tilmans, O., and B.
Briscoe, "`Data Centre to the Home': Ultra-Low Latency for
All", Updated RITE project Technical Report , July 2019,
<https://bobbriscoe.net/pubs.html#DCttH_TR>.
[DOCSIS3.1] [DOCSIS3.1]
CableLabs, "MAC and Upper Layer Protocols Interface CableLabs, "MAC and Upper Layer Protocols Interface
(MULPI) Specification, CM-SP-MULPIv3.1", Data-Over-Cable (MULPI) Specification, CM-SP-MULPIv3.1", Data-Over-Cable
Service Interface Specifications DOCSIS® 3.1 Version i17 Service Interface Specifications DOCSIS(R) 3.1 Version i17
or later, 21 January 2019, <https://specification- or later, January 2019, <https://specification-
search.cablelabs.com/CM-SP-MULPIv3.1>. search.cablelabs.com/CM-SP-MULPIv3.1>.
[DualPI2Linux] [DualPI2Linux]
Albisser, O., De Schepper, K., Briscoe, B., Tilmans, O., Albisser, O., De Schepper, K., Briscoe, B., Tilmans, O.,
and H. Steen, "DUALPI2 - Low Latency, Low Loss and and H. Steen, "DUALPI2 - Low Latency, Low Loss and
Scalable (L4S) AQM", Proc. Linux Netdev 0x13 , March 2019, Scalable (L4S) AQM", Proc. Linux Netdev 0x13 , March 2019,
<https://www.netdevconf.org/0x13/session.html?talk- <https://www.netdevconf.org/0x13/session.html?talk-
DUALPI2-AQM>. DUALPI2-AQM>.
[DualQ-Test] [DualQ-Test]
Steen, H., "Destruction Testing: Ultra-Low Delay using Steen, H., "Destruction Testing: Ultra-Low Delay using
Dual Queue Coupled Active Queue Management", Master's Dual Queue Coupled Active Queue Management", Master's
Thesis, Dept of Informatics, Uni Oslo , May 2017, Thesis, Dept of Informatics, Uni Oslo , May 2017.
<https://www.duo.uio.no/bitstream/handle/10852/57424/
thesis-henrste.pdf?sequence=1>.
[Dukkipati06] [Dukkipati06]
Dukkipati, N. and N. McKeown, "Why Flow-Completion Time is Dukkipati, N. and N. McKeown, "Why Flow-Completion Time is
the Right Metric for Congestion Control", ACM CCR the Right Metric for Congestion Control", ACM CCR
36(1):59--62, January 2006, 36(1):59--62, January 2006,
<https://dl.acm.org/doi/10.1145/1111322.1111336>. <https://dl.acm.org/doi/10.1145/1111322.1111336>.
[Heist21] Heist, P. and J. Morton, "L4S Tests", GitHub README, [Heist21] Heist, P. and J. Morton, "L4S Tests", GitHub README,
August 2021, <https://github.com/heistp/l4s- August 2021, <https://github.com/heistp/l4s-
tests/#underutilization-with-bursty-traffic>. tests/#underutilization-with-bursty-traffic>.
[I-D.briscoe-docsis-q-protection] [I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection Briscoe, B. and G. White, "Queue Protection to Preserve
Algorithm to Preserve Low Latency", Work in Progress, Low Latency", draft-briscoe-docsis-q-protection-00 (work
Internet-Draft, draft-briscoe-docsis-q-protection-06, 13 in progress), July 2019.
May 2022,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
briscoe-docsis-q-protection/>.
[I-D.briscoe-iccrg-prague-congestion-control] [I-D.briscoe-iccrg-prague-congestion-control]
Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague
Congestion Control", Work in Progress, Internet-Draft, Congestion Control", draft-briscoe-iccrg-prague-
draft-briscoe-iccrg-prague-congestion-control-01, 11 July congestion-control-01 (work in progress), July 2022,
2022, <https://datatracker.ietf.org/api/v1/doc/document/ <https://www.ietf.org/archive/id/draft-briscoe-iccrg-
draft-briscoe-iccrg-prague-congestion-control/>. prague-congestion-control-01.txt>.
[I-D.briscoe-tsvwg-l4s-diffserv] [I-D.briscoe-tsvwg-l4s-diffserv]
Briscoe, B., "Interactions between Low Latency, Low Loss, Briscoe, B., "Interactions between Low Latency, Low Loss,
Scalable Throughput (L4S) and Differentiated Services", Scalable Throughput (L4S) and Differentiated Services",
Work in Progress, Internet-Draft, draft-briscoe-tsvwg-l4s- draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
diffserv-02, 2 July 2018, November 2018.
<https://datatracker.ietf.org/api/v1/doc/document/draft-
briscoe-tsvwg-l4s-diffserv/>.
[I-D.cardwell-iccrg-bbr-congestion-control] [I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V. Cardwell, N., Cheng, Y., Yeganeh, S., and V. Jacobson,
Jacobson, "BBR Congestion Control", Work in Progress, "BBR Congestion Control", draft-cardwell-iccrg-bbr-
Internet-Draft, draft-cardwell-iccrg-bbr-congestion- congestion-control-00 (work in progress), July 2017.
control-02, 7 March 2022,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
cardwell-iccrg-bbr-congestion-control/>.
[I-D.ietf-tsvwg-l4s-arch] [I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White, Briscoe, B., Schepper, K., Bagnulo, M., and G. White, "Low
"Low Latency, Low Loss, Scalable Throughput (L4S) Internet Latency, Low Loss, Scalable Throughput (L4S) Internet
Service: Architecture", Work in Progress, Internet-Draft, Service: Architecture", draft-ietf-tsvwg-l4s-arch-08 (work
draft-ietf-tsvwg-l4s-arch-19, 27 July 2022, in progress), November 2020.
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-tsvwg-l4s-arch/>.
[I-D.mathis-iccrg-relentless-tcp] [I-D.mathis-iccrg-relentless-tcp]
Mathis, M., "Relentless Congestion Control", Work in Mathis, M., "Relentless Congestion Control", draft-mathis-
Progress, Internet-Draft, draft-mathis-iccrg-relentless- iccrg-relentless-tcp-00 (work in progress), March 2009.
tcp-00, 4 March 2009, <https://www.ietf.org/archive/id/
draft-mathis-iccrg-relentless-tcp-00.txt>. [L4S_5G] Willars, P., Wittenmark, E., Ronkainen, H., Oestberg, C.,
Johansson, I., Strand, J., Ledl, P., and D. Schnieders,
"Enabling time-critical applications over 5G with rate
adaptation", Ericsson - Deutsche Telekom White Paper BNEW-
21:025455 Uen, May 2021, <https://www.ericsson.com/en/
reports-and-papers/white-papers/enabling-time-critical-
applications-over-5g-with-rate-adaptation>.
[L4Sdemo16] [L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B. Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live Briscoe, "Ultra-Low Delay for All: Live Experience, Live
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016, Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<https//dl.acm.org/citation.cfm?doid=2910017.2910633 <https//dl.acm.org/citation.cfm?doid=2910017.2910633
(videos of demos: (videos of demos:
https://riteproject.eu/dctth/#1511dispatchwg )>. https://riteproject.eu/dctth/#1511dispatchwg )>.
[L4S_5G] Willars, P., Wittenmark, E., Ronkainen, H., Östberg, C., [L4Seval22]
Johansson, I., Strand, J., Lédl, P., and D. Schnieders, De Schepper, K., Albisser, O., Tilmans, O., and B.
"Enabling time-critical applications over 5G with rate Briscoe, "Dual Queue Coupled AQM: Deployable Very Low
adaptation", Ericsson - Deutsche Telekom White Paper BNEW- Queuing Delay for All", Preprint submitted to IEEE/ACM
21:025455 Uen, May 2021, <https://www.ericsson.com/en/ Transactions on Networking arXiv:2209.01078 [cs.NI],
reports-and-papers/white-papers/enabling-time-critical- September 2022, <https://arxiv.org/abs/2209.01078>.
applications-over-5g-with-rate-adaptation>.
[Labovitz10] [Labovitz10]
Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide, Labovitz, C., Iekel-Johnson, S., McPherson, D., Oberheide,
J., and F. Jahanian, "Internet Inter-Domain Traffic", Proc J., and F. Jahanian, "Internet Inter-Domain Traffic", Proc
ACM SIGCOMM; ACM CCR 40(4):75--86, August 2010, ACM SIGCOMM; ACM CCR 40(4):75--86, August 2010,
<https://doi.org/10.1145/1851275.1851194>. <https://doi.org/10.1145/1851275.1851194>.
[LLD] White, G., Sundaresan, K., and B. Briscoe, "Low Latency [LLD] White, G., Sundaresan, K., and B. Briscoe, "Low Latency
DOCSIS: Technology Overview", CableLabs White Paper , DOCSIS: Technology Overview", CableLabs White Paper ,
February 2019, <https://cablela.bs/low-latency-docsis- February 2019, <https://cablela.bs/low-latency-docsis-
technology-overview-february-2019>. technology-overview-february-2019>.
[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.
<https://infocom2003.ieee-infocom.org/papers/27_04.PDF>.
[PI2] De Schepper, K., Bondarenko, O., Briscoe, B., and I. [PI2] De Schepper, K., Bondarenko, O., Briscoe, B., and I.
Tsang, "PI2: A Linearized AQM for both Classic and Tsang, "PI2: A Linearized AQM for both Classic and
Scalable TCP", ACM CoNEXT'16 , December 2016, Scalable TCP", ACM CoNEXT'16 , December 2016,
<https://riteproject.files.wordpress.com/2015/10/ <https://dl.acm.org/doi/10.1145/2999572.2999578>.
pi2_conext.pdf>.
[PI2param] Briscoe, B., "PI2 Parameters", Technical Report TR-BB- [PI2param]
Briscoe, B., "PI2 Parameters", Technical Report TR-BB-
2021-001 arXiv:2107.01003 [cs.NI], July 2021, 2021-001 arXiv:2107.01003 [cs.NI], July 2021,
<https://arxiv.org/abs/2107.01003>. <https://arxiv.org/abs/2107.01003>.
[PragueLinux] [PragueLinux]
Briscoe, B., De Schepper, K., Albisser, O., Misund, J., Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
Tilmans, O., Kühlewind, M., and A.S. Ahmed, "Implementing Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing
the `TCP Prague' Requirements for Low Latency Low Loss the `TCP Prague' Requirements for Low Latency Low Loss
Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 , Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
March 2019, <https://www.netdevconf.org/0x13/ March 2019, <https://www.netdevconf.org/0x13/
session.html?talk-tcp-prague-l4s>. session.html?talk-tcp-prague-l4s>.
[RFC0970] Nagle, J., "On Packet Switches With Infinite Storage", [RFC0970] Nagle, J., "On Packet Switches With Infinite Storage",
RFC 970, DOI 10.17487/RFC0970, December 1985, RFC 970, DOI 10.17487/RFC0970, December 1985,
<https://www.rfc-editor.org/info/rfc970>. <https://www.rfc-editor.org/info/rfc970>.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, [RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
skipping to change at page 33, line 21 skipping to change at page 32, line 34
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the Queue Management and Congestion Avoidance in the
Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998, Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
<https://www.rfc-editor.org/info/rfc2309>. <https://www.rfc-editor.org/info/rfc2309>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000, RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>. <https://www.rfc-editor.org/info/rfc2914>.
[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le [RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D. J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002, Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<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>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion [RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033, Control Algorithms", BCP 133, RFC 5033,
skipping to change at page 35, line 9 skipping to change at page 34, line 19
[RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of [RFC8404] Moriarty, K., Ed. and A. Morton, Ed., "Effects of
Pervasive Encryption on Operators", RFC 8404, Pervasive Encryption on Operators", RFC 8404,
DOI 10.17487/RFC8404, July 2018, DOI 10.17487/RFC8404, July 2018,
<https://www.rfc-editor.org/info/rfc8404>. <https://www.rfc-editor.org/info/rfc8404>.
[SCReAM] Johansson, I., "SCReAM", GitHub repository; , [SCReAM] Johansson, I., "SCReAM", GitHub repository; ,
<https://github.com/EricssonResearch/scream/blob/master/ <https://github.com/EricssonResearch/scream/blob/master/
README.md>. README.md>.
[SigQ-Dyn] Briscoe, B., "Rapid Signalling of Queue Dynamics", [SigQ-Dyn]
Briscoe, B., "Rapid Signalling of Queue Dynamics",
Technical Report TR-BB-2017-001 arXiv:1904.07044 [cs.NI], Technical Report TR-BB-2017-001 arXiv:1904.07044 [cs.NI],
September 2017, <https://arxiv.org/abs/1904.07044>. September 2017, <https://arxiv.org/abs/1904.07044>.
Appendix A. Example DualQ Coupled PI2 Algorithm Appendix A. Example DualQ Coupled PI2 Algorithm
As a first concrete example, the pseudocode below gives the DualPI2 As a first concrete example, the pseudocode below gives the DualPI2
algorithm. DualPI2 follows the structure of the DualQ Coupled AQM algorithm. DualPI2 follows the structure of the DualQ Coupled AQM
framework in Figure 1. A simple ramp function (configured in units framework in Figure 1. A simple ramp function (configured in units
of queuing time) with unsmoothed ECN marking is used for the Native of queuing time) with unsmoothed ECN marking is used for the Native
L4S AQM. The ramp can also be configured as a step function. The L4S AQM. The ramp can also be configured as a step function. The
skipping to change at page 36, line 5 skipping to change at page 35, line 13
[DualPI2Linux]. The specification of the DualQ Coupled AQM for [DualPI2Linux]. The specification of the DualQ Coupled AQM for
DOCSIS cable modems and CMTSs is available in [DOCSIS3.1] and DOCSIS cable modems and CMTSs is available in [DOCSIS3.1] and
explained in [LLD]. explained in [LLD].
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 six functions: pseudocode consists of the following six functions:
* The initialization function dualpi2_params_init(...) (Figure 2) o The initialization function dualpi2_params_init(...) (Figure 2)
that sets parameter defaults (the API for setting non-default that sets parameter defaults (the API for setting non-default
values is omitted for brevity) values is omitted for brevity)
* The enqueue function dualpi2_enqueue(lq, cq, pkt) (Figure 3) o The enqueue function dualpi2_enqueue(lq, cq, pkt) (Figure 3)
* The dequeue function dualpi2_dequeue(lq, cq, pkt) (Figure 4) o The dequeue function dualpi2_dequeue(lq, cq, pkt) (Figure 4)
* The recurrence function recur(q, likelihood) for de-randomized ECN o The recurrence function recur(q, likelihood) for de-randomized ECN
marking (shown at the end of Figure 4). marking (shown at the end of Figure 4).
* The L4S AQM function laqm(qdelay) (Figure 5) used to calculate the o The L4S AQM function laqm(qdelay) (Figure 5) used to calculate the
ECN-marking probability for the L4S queue ECN-marking probability for the L4S queue
* The base AQM function that implements the PI algorithm o The base AQM function that implements the PI algorithm
dualpi2_update(lq, cq) (Figure 6) used to regularly update the dualpi2_update(lq, cq) (Figure 6) used to regularly update the
base probability (p'), which is squared for the Classic AQM as base probability (p'), which is squared for the Classic AQM as
well as being coupled across to the L4S queue. well as being coupled across to the L4S queue.
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:
* 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;
* cq.byt() or lq.byt() returns the current length (aka. backlog) of o cq.byt() or lq.byt() returns the current length (aka. backlog) of
the relevant queue in bytes; the relevant queue in bytes;
* cq.len() or lq.len() returns the current length of the relevant o cq.len() or lq.len() returns the current length of the relevant
queue in packets; queue in packets;
* cq.time() or lq.time() returns the current queuing delay of the o cq.time() or lq.time() returns the current queuing delay of the
relevant queue in units of time (see Note a); relevant queue in units of time (see Note a);
* mark(pkt) and drop(pkt) for ECN-marking and dropping a packet; o mark(pkt) and drop(pkt) for ECN-marking and dropping a packet;
In experiments so far (building on experiments with PIE) on broadband In experiments so far (building on experiments with PIE) on broadband
access links ranging from 4 Mb/s to 200 Mb/s with base RTTs from 5 ms access links ranging from 4 Mb/s to 200 Mb/s with base RTTs from 5 ms
to 100 ms, DualPI2 achieves good results with the default parameters to 100 ms, DualPI2 achieves good results with the default parameters
in Figure 2. The parameters are categorised by whether they relate in Figure 2. The parameters are categorised by whether they relate
to the Base PI2 AQM, the L4S AQM or the framework coupling them to the Base PI2 AQM, the L4S AQM or the framework coupling them
together. Constants and variables derived from these parameters are together. Constants and variables derived from these parameters are
also included at the end of each category. Each parameter is also included at the end of each category. Each parameter is
explained as it is encountered in the walk-through of the pseudocode explained as it is encountered in the walk-through of the pseudocode
below, and the rationale for the chosen defaults are given so that below, and the rationale for the chosen defaults are given so that
skipping to change at page 38, line 16 skipping to change at page 37, line 16
2: if ( lq.byt() + cq.byt() + MTU > limit) 2: if ( lq.byt() + cq.byt() + MTU > limit)
3: drop(pkt) % drop packet if buffer is full 3: drop(pkt) % drop packet if buffer is full
4: timestamp(pkt) % only needed if using the sojourn technique 4: timestamp(pkt) % only needed if using the sojourn technique
5: % Packet classifier 5: % Packet classifier
6: if ( ecn(pkt) modulo 2 == 1 ) % ECN bits = ECT(1) or CE 6: if ( ecn(pkt) modulo 2 == 1 ) % ECN bits = ECT(1) or CE
7: lq.enqueue(pkt) 7: lq.enqueue(pkt)
8: else % ECN bits = not-ECT or ECT(0) 8: else % ECN bits = not-ECT or ECT(0)
9: cq.enqueue(pkt) 9: cq.enqueue(pkt)
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.byt() + cq.byt() > 0 ) { 2: while ( lq.byt() + cq.byt() > 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 LAQM 5: p'_L = laqm(lq.time()) % Native LAQM
6: p_L = max(p'_L, p_CL) % Combining function 6: p_L = max(p'_L, p_CL) % Combining function
7: if ( recur(lq, p_L) ) % Linear marking 7: if ( recur(lq, p_L) ) % Linear marking
8: mark(pkt) 8: mark(pkt)
9: } else { 9: } else {
skipping to change at page 38, line 50 skipping to change at page 37, line 50
23: recur(q, likelihood) { % Returns TRUE with a certain likelihood 23: recur(q, likelihood) { % Returns TRUE with a certain likelihood
24: q.count += likelihood 24: q.count += likelihood
25: if (q.count > 1) { 25: if (q.count > 1) {
26: q.count -= 1 26: q.count -= 1
27: return TRUE 27: return TRUE
28: } 28: }
29: return FALSE 29: return FALSE
30: } 30: }
Figure 4: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM Figure 4: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM
When packets arrive, first a common queue limit is checked as shown When packets arrive, first a common queue limit is checked as shown
in line 2 of the enqueuing pseudocode in Figure 3. This assumes a in line 2 of the enqueuing pseudocode in Figure 3. This assumes a
shared buffer for the two queues (Note b discusses the merits of shared buffer for the two queues (Note b discusses the merits of
separate buffers). In order to avoid any bias against larger separate buffers). In order to avoid any bias against larger
packets, 1 MTU of space is always allowed, and the limit is packets, 1 MTU of space is always allowed, and the limit is
deliberately tested before enqueue. deliberately tested before enqueue.
If limit is not exceeded, the packet is timestamped in line 4 (only If limit is not exceeded, the packet is timestamped in line 4 (only
if the sojourn time technique is being used to measure queue delay; if the sojourn time technique is being used to measure queue delay;
skipping to change at page 39, line 43 skipping to change at page 38, line 43
loop sloppier (for a typical RTT it would double the Classic queue's loop sloppier (for a typical RTT it would double the Classic queue's
feedback delay). feedback delay).
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).
* If an L4S packet is scheduled, in lines 7 and 8 the packet is ECN- o If an L4S packet is scheduled, in lines 7 and 8 the packet is ECN-
marked with likelihood p_L. The recur() function at the end of marked with likelihood p_L. The recur() function at the end of
Figure 4 is used, which is preferred over random marking because Figure 4 is used, which is preferred over random marking because
it avoids delay due to randomization when interpreting congestion it avoids delay due to randomization when interpreting congestion
signals, but it still desynchronizes the saw-teeth of the flows. signals, but it still desynchronizes the saw-teeth of the flows.
Line 6 calculates p_L as the maximum of the coupled L4S Line 6 calculates p_L as the maximum of the coupled L4S
probability p_CL and the probability from the native L4S AQM p'_L. probability p_CL and the probability from the native L4S AQM p'_L.
This implements the max() function shown in Figure 1 to couple the This implements the max() function shown in Figure 1 to couple the
outputs of the two AQMs together. Of the two probabilities input outputs of the two AQMs together. Of the two probabilities input
to p_L in line 6: to p_L in line 6:
- p'_L is calculated per packet in line 5 by the laqm() function * p'_L is calculated per packet in line 5 by the laqm() function
(see Figure 5), (see Figure 5),
- Whereas p_CL is maintained by the dualpi2_update() function * Whereas p_CL is maintained by the dualpi2_update() function
which runs every Tupdate (Tupdate is set in line 12 of which runs every Tupdate (Tupdate is set in line 12 of
Figure 2). Figure 2).
* If a Classic packet is scheduled, lines 10 to 17 drop or mark the o If a Classic packet is scheduled, lines 10 to 17 drop or mark the
packet with probability p_C. packet with probability p_C.
The Native L4S AQM algorithm (Figure 5) is a ramp function, similar The Native L4S AQM algorithm (Figure 5) is a ramp function, similar
to the RED algorithm, but simplified as follows: to the RED algorithm, but simplified as follows:
* The extent of the ramp is defined in units of queuing delay, not o The extent of the ramp is defined in units of queuing delay, not
bytes, so that configuration remains invariant as the queue bytes, so that configuration remains invariant as the queue
departure rate varies. departure rate varies.
* It uses instantaneous queueing delay, which avoids the complexity o It uses instantaneous queueing delay, which avoids the complexity
of smoothing, but also avoids embedding a worst-case RTT of of smoothing, but also avoids embedding a worst-case RTT of
smoothing delay in the network (see Section 2.1). smoothing delay in the network (see Section 2.1).
* The ramp rises linearly directly from 0 to 1, not to an o The ramp rises linearly directly from 0 to 1, not to 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.
* Marking does not have to be randomized. Determinism is used o Marking does not have to be randomized. Determinism is used
instead of randomness; to reduce the delay necessary to smooth out instead of randomness; to reduce the delay necessary to smooth out
the noise of randomness from the signal. the noise of randomness from the signal.
The ramp function requires two configuration parameters, the minimum The ramp function requires two configuration parameters, the minimum
threshold (minTh) and the width of the ramp (range), both in units of threshold (minTh) and the width of the ramp (range), both in units of
queuing time, as shown in lines 17 & 18 of the initialization queuing time, as shown in lines 17 & 18 of the initialization
function in Figure 2. The ramp function can be configured as a step function in Figure 2. The ramp function can be configured as a step
(see Note c). (see Note c).
Although the DCTCP paper [Alizadeh-stability] recommends an ECN Although the DCTCP paper [Alizadeh-stability] recommends an ECN
skipping to change at page 41, line 24 skipping to change at page 40, line 24
Figure 5: Example Pseudocode for the Native L4S AQM Figure 5: Example Pseudocode for the Native L4S AQM
1: dualpi2_update(lq, cq) { % Update p' every Tupdate 1: dualpi2_update(lq, cq) { % 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 * (curq - target) + beta * (curq - prevq) 3: p' = p' + alpha * (curq - target) + beta * (curq - prevq)
4: p_CL = k * p' % Coupled L4S prob = base prob * coupling factor 4: p_CL = k * p' % 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 6: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM
(Clamping p' within the range [0,1] omitted for clarity - see text) (Clamping p' within the range [0,1] omitted for clarity - see text)
Figure 6: Example PI-Update Pseudocode for DualQ Coupled PI2 AQM
The coupled marking probability, p_CL depends on the base probability The coupled marking probability, p_CL depends on the base probability
(p'), which is kept up to date by the core PI algorithm in Figure 6 (p'), which is kept up to date by the core PI algorithm in Figure 6
executed every Tupdate. 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 (see Note a) and implicitly takes the current queuing current time (see Note a) and implicitly takes the current queuing
delay as 0 if the queue is empty. delay as 0 if the queue is empty.
skipping to change at page 42, line 13 skipping to change at page 41, line 12
[PI2param], the target queuing delay on line 9 of Figure 2 is related [PI2param], the target queuing delay on line 9 of Figure 2 is related
to the typical base RTT worldwide, RTT_typ, by two factors: target = to the typical base RTT worldwide, RTT_typ, by two factors: target =
RTT_typ * g * f. Below we summarize the rationale behind these RTT_typ * g * f. Below we summarize the rationale behind these
factors and introduce a further adjustment. The two factors ensure factors and introduce a further adjustment. The two factors ensure
that, in a large proportion of cases (say 90%), the sawtooth that, in a large proportion of cases (say 90%), the sawtooth
variations in RTT of a single flow will fit within the buffer without variations in RTT of a single flow will fit within the buffer without
underutilizing the link. Frankly, these factors are educated underutilizing the link. Frankly, these factors are educated
guesses, but with the emphasis closer to 'educated' than to 'guess' guesses, but with the emphasis closer to 'educated' than to 'guess'
(see [PI2param] for full background): (see [PI2param] for full background):
* RTT_typ is taken as 25 ms. This is based on an average CDN o RTT_typ is taken as 25 ms. This is based on an average CDN
latency measured in each country weighted by the number of latency measured in each country weighted by the number of
Internet users in that country to produce an overall weighted Internet users in that country to produce an overall weighted
average for the Internet [PI2param]. Countries were ranked by average for the Internet [PI2param]. Countries were ranked by
number of Internet users, and once 90% of Internet users were number of Internet users, and once 90% of Internet users were
covered, smaller countries were excluded to avoid covered, smaller countries were excluded to avoid
unrepresentatively small sample sizes. Also, importantly, the unrepresentatively small sample sizes. Also, importantly, the
data for the average CDN latency in China (with the largest number data for the average CDN latency in China (with the largest number
of Internet users) has been removed, because the CDN latency was a of Internet users) has been removed, because the CDN latency was a
significant outlier and, on reflection, the experimental technique significant outlier and, on reflection, the experimental technique
seemed inappropriate to the CDN market in China. seemed inappropriate to the CDN market in China.
* g is taken as 0.38. The factor g is a geometry factor that o g is taken as 0.38. The factor g is a geometry factor that
characterizes the shape of the sawteeth of prevalent Classic characterizes the shape of the sawteeth of prevalent Classic
congestion controllers. The geometry factor is the fraction of congestion controllers. The geometry factor is the fraction of
the amplitude of the sawtooth variability in queue delay that lies the amplitude of the sawtooth variability in queue delay that lies
below the AQM's target. For instance, at low bit rate, the below the AQM's target. For instance, at low bit rate, the
geometry factor of standard Reno is 0.5, but at higher rates it geometry factor of standard Reno is 0.5, but at higher rates it
tends to just under 1. According to the census of congestion tends to just under 1. According to the census of congestion
controllers conducted by Mishra et al. in Jul-Oct controllers conducted by Mishra et al. in Jul-Oct
2019 [CCcensus19], most Classic TCP traffic uses Cubic. And, 2019 [CCcensus19], most Classic TCP traffic uses Cubic. And,
according to the analysis in [PI2param], if running over a PI2 according to the analysis in [PI2param], if running over a PI2
AQM, a large proportion of this Cubic traffic would be in its AQM, a large proportion of this Cubic traffic would be in its
Reno-Friendly mode, which has a geometry factor of ~0.39 (all Reno-Friendly mode, which has a geometry factor of ~0.39 (all
known implementations). The rest of the Cubic traffic would be in known implementations). The rest of the Cubic traffic would be in
true Cubic mode, which has a geometry factor of ~0.36. Without true Cubic mode, which has a geometry factor of ~0.36. Without
modelling the sawtooth profiles from all the other less prevalent modelling the sawtooth profiles from all the other less prevalent
congestion controllers, we estimate a 7:3 weighted average of congestion controllers, we estimate a 7:3 weighted average of
these two, resulting in an average geometry factor of 0.38. these two, resulting in an average geometry factor of 0.38.
* f is taken as 2. The factor f is a safety factor that increases o f is taken as 2. The factor f is a safety factor that increases
the target queue to allow for the distribution of RTT_typ around the target queue to allow for the distribution of RTT_typ around
its mean. Otherwise, the target queue would only avoid its mean. Otherwise, the target queue would only avoid
underutilization for those users below the mean. It also provides underutilization for those users below the mean. It also provides
a safety margin for the proportion of paths in use that span a safety margin for the proportion of paths in use that span
beyond the distance between a user and their local CDN. beyond the distance between a user and their local CDN.
Currently, no data is available on the variance of queue delay Currently, no data is available on the variance of queue delay
around the mean in each region, so there is plenty of room for around the mean in each region, so there is plenty of room for
this guess to become more educated. this guess to become more educated.
* [PI2param] recommends target = RTT_typ * g * f = 25ms * 0.38 * 2 = o [PI2param] recommends target = RTT_typ * g * f = 25ms * 0.38 * 2 =
19 ms. However, a further adjustment is warranted, because target 19 ms. However, a further adjustment is warranted, because target
is moving year-on-year. The paper is based on data collected in is moving year-on-year. The paper is based on data collected in
2019, and it mentions evidence from speedtest.net that suggests 2019, and it mentions evidence from speedtest.net that suggests
RTT_typ reduced by 17% (fixed) or 12% (mobile) between 2020 and RTT_typ reduced by 17% (fixed) or 12% (mobile) between 2020 and
2021. Therefore, we recommend a default of target = 15 ms at the 2021. Therefore, we recommend a default of target = 15 ms at the
time of writing (2021). time of writing (2021).
Operators can always use the data and discussion in [PI2param] to Operators can always use the data and discussion in [PI2param] to
configure a more appropriate target for their environment. For configure a more appropriate target for their environment. For
instance, an operator might wish to question the assumptions called instance, an operator might wish to question the assumptions called
skipping to change at page 45, line 5 skipping to change at page 43, line 48
Tupdate dependent on p'. Instead, in PI2, alpha and beta are Tupdate dependent on p'. Instead, in PI2, alpha and beta are
independent of p' because the squaring applied to Classic traffic 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.
Nonetheless, an implementer might wish to add selected details to Nonetheless, an implementer might wish to add selected details to
either AQM. For instance the Linux reference DualPI2 implementation either AQM. For instance the Linux reference DualPI2 implementation
includes the following (not shown in the pseudocode above): includes the following (not shown in the pseudocode above):
* Classic and coupled marking or dropping (i.e. based on p_C and o Classic and coupled marking or dropping (i.e. based on p_C and
p_CL from the PI controller) is not applied to a packet if the p_CL from the PI controller) is not applied to a packet if the
aggregate queue length in bytes is < 2 MTU (prior to enqueuing the aggregate queue length in bytes is < 2 MTU (prior to enqueuing the
packet or dequeuing it, depending on whether the AQM is configured packet or dequeuing it, depending on whether the AQM is configured
to be applied at enqueue or dequeue); to be applied at enqueue or dequeue);
* In the WRR scheduler, the 'credit' indicating which queue should o In the WRR scheduler, the 'credit' indicating which queue should
transmit is only changed if there are packets in both queues transmit is only changed if there are packets in both queues
(i.e. if there is actual resource contention). This means that a (i.e. if there is actual resource contention). This means that a
properly paced L flow might never be delayed by the WRR. The WRR properly paced L flow might never be delayed by the WRR. The WRR
credit is reset in favour of the L queue when the link is idle. credit is reset in favour of the L queue when the link is idle.
An implementer might also wish to add other heuristics, e.g. burst An implementer might also wish to add other heuristics, e.g. burst
protection [RFC8033] or enhanced burst protection [RFC8034]. protection [RFC8033] or enhanced burst protection [RFC8034].
Notes: Notes:
skipping to change at page 47, line 23 skipping to change at page 46, line 20
The pseudocode given here applies where the link rate is unknown, The pseudocode given here applies where the link rate is unknown,
which is more common for software implementations that might be which is more common for software implementations that might be
deployed in scenarios where the link is shared with other queues. In deployed in scenarios where the link is shared with other queues. In
lines 5a to 5d in Figure 7 the native L4S marking probability, p'_L, lines 5a to 5d in Figure 7 the native L4S marking probability, p'_L,
is zeroed if the queue is only 1 packet (in the default is zeroed if the queue is only 1 packet (in the default
configuration). configuration).
Linux implementation note: Linux implementation note:
* In Linux, the check that the queue exceeds Th_len before marking o In Linux, the check that the queue exceeds Th_len before marking
with the native L4S AQM is actually at enqueue, not dequeue, with the native L4S AQM is actually at enqueue, not dequeue,
otherwise it would exempt the last packet of a burst from being otherwise it would exempt the last packet of a burst from being
marked. The result of the check is conveyed from enqueue to the marked. The result of the check is conveyed from enqueue to the
dequeue function via a boolean in the packet metadata. dequeue function via a boolean in the packet metadata.
Persistent overload is deemed to have occurred when Classic drop/ Persistent overload is deemed to have occurred when Classic drop/
marking probability reaches p_Cmax. Above this point, the Classic marking probability reaches p_Cmax. Above this point, the Classic
drop probability is applied to both L and C queues, irrespective of drop probability is applied to both L and C queues, irrespective of
whether any packet is ECN-capable. ECT packets that are not dropped whether any packet is ECN-capable. ECT packets that are not dropped
can still be ECN-marked. can still be ECN-marked.
skipping to change at page 49, line 41 skipping to change at page 48, line 41
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 7: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM Figure 7: Example Dequeue Pseudocode for DualQ Coupled PI2 AQM
(Including Code for Edge-Cases) (Including Code for Edge-Cases)
1: dualpi2_update(lq, cq) { % Update p' every Tupdate 1: dualpi2_update(lq, cq) { % Update p' every Tupdate
2a: curq = max(cq.time(), lq.time()) % use greatest queuing time 2a: curq = max(cq.time(), lq.time()) % use greatest queuing time
3: p' = p' + alpha * (curq - target) + beta * (curq - prevq) 3: p' = p' + alpha * (curq - target) + beta * (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 8: 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 4.2.2). (see Section 4.2.2).
* A well-understood weighted scheduler such as weighted round-robin o A well-understood weighted scheduler such as weighted round-robin
(WRR) is recommended. As long as the scheduler weight for Classic (WRR) is recommended. As long as the scheduler weight for Classic
is small (e.g. 1/16), its exact value is unimportant because it is small (e.g. 1/16), its exact value is unimportant because it
does not normally determine capacity shares. The weight is only does not normally determine capacity shares. The weight is only
important to prevent unresponsive L4S traffic starving Classic important to prevent unresponsive L4S traffic starving Classic
traffic in the short term (see Section 4.2.2). This is because traffic in the short term (see Section 4.2.2). This is because
capacity sharing between the queues is normally determined by the capacity sharing between the queues is normally determined by the
coupled congestion signal, which overrides the scheduler, by coupled congestion signal, which overrides the scheduler, by
making L4S sources leave roughly equal per-flow capacity available making L4S sources leave roughly equal per-flow capacity available
for Classic flows. for Classic flows.
* Alternatively, a time-shifted FIFO (TS-FIFO) could be used. It o Alternatively, a time-shifted FIFO (TS-FIFO) could be used. It
works by selecting the head packet that has waited the longest, works by selecting the head packet that has waited the longest,
biased against the Classic traffic by a time-shift of tshift. To biased against the Classic traffic by a time-shift of tshift. To
implement time-shifted FIFO, the scheduler() function in line 3 of implement time-shifted FIFO, the scheduler() function in line 3 of
the dequeue code would simply be implemented as the scheduler() the dequeue code would simply be implemented as the scheduler()
function at the bottom of Figure 10 in Appendix B. For the public function at the bottom of Figure 10 in Appendix B. For the public
Internet a good value for tshift is 50ms. For private networks Internet a good value for tshift is 50ms. For private networks
with smaller diameter, about 4*target would be reasonable. TS- with smaller diameter, about 4*target would be reasonable. TS-
FIFO is a very simple scheduler, but complexity might need to be FIFO is a very simple scheduler, but complexity might need to be
added to address some deficiencies (which is why it is not added to address some deficiencies (which is why it is not
recommended over WRR): recommended over WRR):
- TS-FIFO does not fully isolate latency in the L4S queue from * TS-FIFO does not fully isolate latency in the L4S queue from
uncontrolled bursts in the Classic queue; uncontrolled bursts in the Classic queue;
- Using sojourn time for TS-FIFO is only appropriate if time- * Using sojourn time for TS-FIFO is only appropriate if time-
stamping of packets is feasible; stamping of packets is feasible;
- Even if time-stamping is supported, the sojourn time of the * Even if time-stamping is supported, the sojourn time of the
head packet is always stale, so a more instantaneous measure of head packet is always stale, so a more instantaneous measure of
queue delay could be used (see Note a in Appendix A.1). queue delay could be used (see Note a in Appendix A.1).
* A strict priority scheduler would be inappropriate as discussed in o A strict priority scheduler would be inappropriate as discussed in
Section 4.2.2. Section 4.2.2.
Appendix B. Example DualQ Coupled Curvy RED Algorithm Appendix B. Example DualQ Coupled Curvy RED Algorithm
As another example of a DualQ Coupled AQM algorithm, the pseudocode As another example of a DualQ Coupled AQM algorithm, the pseudocode
below gives the Curvy RED based algorithm. Although the AQM was below gives the Curvy RED based algorithm. Although the AQM was
designed to be efficient in integer arithmetic, to aid understanding designed to be efficient in integer arithmetic, to aid understanding
it is first given using floating point arithmetic (Figure 10). Then, it is first given using floating point arithmetic (Figure 10). Then,
one possible optimization for integer arithmetic is given, also in one possible optimization for integer arithmetic is given, also in
pseudocode (Figure 11). To aid comparison, the line numbers are kept pseudocode (Figure 11). To aid comparison, the line numbers are kept
in step between the two by using letter suffixes where the longer in step between the two by using letter suffixes where the longer
code needs extra lines. code needs extra lines.
B.1. Curvy RED in Pseudocode B.1. Curvy RED in Pseudocode
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) and packet (pkt), the L4S queue (lq) and the Classic queue (cq) and
consists of the following five functions: consists of the following five functions:
* The initialization function cred_params_init(...) (Figure 2) that o The initialization function cred_params_init(...) (Figure 2) that
sets parameter defaults (the API for setting non-default values is sets parameter defaults (the API for setting non-default values is
omitted for brevity); omitted for brevity);
* The dequeue function cred_dequeue(lq, cq, pkt) (Figure 4); o The dequeue function cred_dequeue(lq, cq, pkt) (Figure 4);
* The scheduling function scheduler(), which selects between the o The scheduling function scheduler(), which selects between the
head packets of the two queues. head packets of the two queues.
It also uses the following functions that are either shown elsewhere, It also uses the following functions that are either shown elsewhere,
or not shown in full here: or not shown in full here:
* The enqueue function, which is identical to that used for DualPI2, o The enqueue function, which is identical to that used for DualPI2,
dualpi2_enqueue(lq, cq, pkt) in Figure 3; dualpi2_enqueue(lq, cq, pkt) in Figure 3;
* mark(pkt) and drop(pkt) for ECN-marking and dropping a packet; o mark(pkt) and drop(pkt) for ECN-marking and dropping a packet;
* cq.byt() or lq.byt() returns the current length (aka. backlog) of o cq.byt() or lq.byt() returns the current length (aka. backlog) of
the relevant queue in bytes; the relevant queue in bytes;
* cq.time() or lq.time() returns the current queuing delay of the o cq.time() or lq.time() returns the current queuing delay of the
relevant queue in units of time (see Note a in Appendix A.1). relevant queue in units of time (see Note a in Appendix A.1).
Because Curvy RED was evaluated before DualPI2, certain improvements Because Curvy RED was evaluated before DualPI2, certain improvements
introduced for DualPI2 were not evaluated for Curvy RED. In the introduced for DualPI2 were not evaluated for Curvy RED. In the
pseudocode below, the straightforward improvements have been added on pseudocode below, the straightforward improvements have been added on
the assumption they will provide similar benefits, but that has not the assumption they will provide similar benefits, but that has not
been proven experimentally. They are: i) a conditional priority been proven experimentally. They are: i) a conditional priority
scheduler instead of strict priority ii) a time-based threshold for scheduler instead of strict priority ii) a time-based threshold for
the native L4S AQM; iii) ECN support for the Classic AQM. A recent the native L4S AQM; iii) ECN support for the Classic AQM. A recent
evaluation has proved that a minimum ECN-marking threshold (minTh) evaluation has proved that a minimum ECN-marking threshold (minTh)
skipping to change at page 58, line 45 skipping to change at page 57, line 37
13: continue % continue to the top of the while loop 13: continue % continue to the top of the while loop
14: } 14: }
15: mark(pkt) 15: mark(pkt)
16: } 16: }
17: } 17: }
18: return(pkt) % return the packet and stop here 18: return(pkt) % return the packet and stop here
19: } 19: }
20: return(NULL) % no packet to dequeue 20: return(NULL) % no packet to dequeue
21: } 21: }
Figure 11: Optimised Example Dequeue Pseudocode for DualQ Coupled Figure 11: Optimised Example Dequeue Pseudocode for DualQ Coupled AQM
AQM using Integer Arithmetic using Integer Arithmetic
The two ranges, range_L and range_C are expressed as powers of 2 so The two ranges, range_L and range_C are expressed as powers of 2 so
that division can be implemented as a right bit-shift (>>) in lines 5 that division can be implemented as a right bit-shift (>>) in lines 5
and 10 of the integer variant of the pseudocode (Figure 11). and 10 of the integer variant of the pseudocode (Figure 11).
For the integer variant of the pseudocode, an integer version of the For the integer variant of the pseudocode, an integer version of the
rand() function used at line 25 of the maxrand(function) in Figure 10 rand() function used at line 25 of the maxrand(function) in Figure 10
would be arranged to return an integer in the range 0 <= maxrand() < would be arranged to return an integer in the range 0 <= maxrand() <
2^32 (not shown). This would scale up all the floating point 2^32 (not shown). This would scale up all the floating point
probabilities in the range [0,1] by 2^32. probabilities in the range [0,1] by 2^32.
skipping to change at page 60, line 38 skipping to change at page 59, line 34
operator needs to decide at what base RTT it wants L4S and Classic operator needs to decide at what base RTT it wants L4S and Classic
flows to have roughly equal throughput, once the effect of the flows to have roughly equal throughput, once the effect of the
additional Classic queue on Classic throughput has been taken into additional Classic queue on Classic throughput has been taken into
account. With this approach, a network operator can determine a good account. With this approach, a network operator can determine a good
coupling factor without knowing the precise L4S algorithm for coupling factor without knowing the precise L4S algorithm for
reducing RTT-dependence - or even in the absence of any algorithm. reducing RTT-dependence - or even in the absence of any algorithm.
The following additional terminology will be used, with appropriate The following additional terminology will be used, with appropriate
subscripts: subscripts:
r: Packet rate [pkt/s] r: Packet rate [pkt/s]
R: RTT [s/round] R: RTT [s/round]
p: ECN marking probability [] p: ECN marking probability []
On the Classic side, we consider Reno as the most sensitive and On the Classic side, we consider Reno as the most sensitive and
therefore worst-case Classic congestion control. We will also therefore worst-case Classic congestion control. We will also
consider Cubic in its Reno-friendly mode ('CReno'), as the most consider Cubic in its Reno-friendly mode ('CReno'), as the most
prevalent congestion control, according to the references and prevalent congestion control, according to the references and
analysis in [PI2param]. In either case, the Classic packet rate in analysis in [PI2param]. In either case, the Classic packet rate in
steady state is given by the well-known square root formula for Reno steady state is given by the well-known square root formula for Reno
congestion control: congestion control:
r_C = 1.22 / (R_C * p_C^0.5) (5) r_C = 1.22 / (R_C * p_C^0.5) (5)
skipping to change at page 65, line 7 skipping to change at page 64, line 7
Henderson <tomh@tomh.org> updated that earlier model and created a Henderson <tomh@tomh.org> updated that earlier model and created a
model for the DualQ variant specified as part of the Low Latency model for the DualQ variant specified as part of the Low Latency
DOCSIS specification, as well as conducting extensive evaluations. DOCSIS specification, as well as conducting extensive evaluations.
Ing Jyh (Inton) Tsang of Nokia, Belgium built the End-to-End Data Ing Jyh (Inton) Tsang of Nokia, Belgium built the End-to-End Data
Centre to the Home broadband testbed on which DualQ Coupled AQM Centre to the Home broadband testbed on which DualQ Coupled AQM
implementations were tested. implementations were tested.
Authors' Addresses Authors' Addresses
Koen De Schepper Koen De Schepper
Nokia Bell Labs Nokia Bell Labs
Antwerp Antwerp
Belgium Belgium
Email: koen.de_schepper@nokia.com
URI: https://www.bell-labs.com/about/researcher-profiles/ Email: koen.de_schepper@nokia.com
koende_schepper/ URI: https://www.bell-labs.com/about/researcher-profiles/koende_schepper/
Bob Briscoe (editor) Bob Briscoe (editor)
Independent Independent
United Kingdom UK
Email: ietf@bobbriscoe.net Email: ietf@bobbriscoe.net
URI: https://bobbriscoe.net/ URI: https://bobbriscoe.net/
Greg White Greg White
CableLabs CableLabs
Louisville, CO, Louisville, CO
United States of America US
Email: G.White@CableLabs.com Email: G.White@CableLabs.com
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