< draft-ietf-tsvwg-l4s-arch-19.txt   draft-ietf-tsvwg-l4s-arch-20d.txt >
Transport Area Working Group B. Briscoe, Ed. Transport Area Working Group B. Briscoe, Ed.
Internet-Draft Independent Internet-Draft Independent
Intended status: Informational K. De Schepper Intended status: Informational K. De Schepper
Expires: 28 January 2023 Nokia Bell Labs Expires: 28 February 2023 Nokia Bell Labs
M. Bagnulo Braun M. Bagnulo Braun
Universidad Carlos III de Madrid Universidad Carlos III de Madrid
G. White G. White
CableLabs CableLabs
27 July 2022 27 August 2022
Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture Architecture
draft-ietf-tsvwg-l4s-arch-19 draft-ietf-tsvwg-l4s-arch-20
Abstract Abstract
This document describes the L4S architecture, which enables Internet This document describes the L4S architecture, which enables Internet
applications to achieve Low queuing Latency, Low Loss, and Scalable applications to achieve Low queuing Latency, Low Loss, and Scalable
throughput (L4S). The insight on which L4S is based is that the root throughput (L4S). L4S is based on the insight that the root cause of
cause of queuing delay is in the congestion controllers of senders, queuing delay is in the capacity-seeking congestion controllers of
not in the queue itself. With the L4S architecture all Internet senders, not in the queue itself. With the L4S architecture all
applications could (but do not have to) transition away from Internet applications could (but do not have to) transition away from
congestion control algorithms that cause substantial queuing delay, congestion control algorithms that cause substantial queuing delay,
to a new class of congestion controls that induce very little to a new class of congestion controls that can seek capacity with
queuing, aided by explicit congestion signalling from the network. very little queuing. These are aided by a modified form of explicit
This new class of congestion controls can provide low latency for congestion notification (ECN) from the network. With this new
capacity-seeking flows, so applications can achieve both high architecture, applications can have both low latency and high
bandwidth and low latency. throughput.
The architecture primarily concerns incremental deployment. It The architecture primarily concerns incremental deployment. It
defines mechanisms that allow the new class of L4S congestion defines mechanisms that allow the new class of L4S congestion
controls to coexist with 'Classic' congestion controls in a shared controls to coexist with 'Classic' congestion controls in a shared
network. These mechanisms aim to ensure that the latency and network. The aim is for L4S latency and throughput to be usually
throughput performance using an L4S-compliant congestion controller much better (and rarely worse), while typically not impacting Classic
is usually much better (and rarely worse) than performance would have performance.
been using a 'Classic' congestion controller, and that competing
flows continuing to use 'Classic' controllers are typically not
impacted by the presence of L4S. These characteristics are important
to encourage adoption of L4S congestion control algorithms and L4S
compliant network elements.
The L4S architecture consists of three components: network support to
isolate L4S traffic from classic traffic; protocol features that
allow network elements to identify L4S traffic; and host support for
L4S congestion controls. The protocol is defined separately as an
experimental change to Explicit Congestion Notification (ECN).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 January 2023. This Internet-Draft will expire on 28 February 2023.
Copyright Notice Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 3, line 4 skipping to change at page 2, line 43
4.1. Protocol Mechanisms . . . . . . . . . . . . . . . . . . . 9 4.1. Protocol Mechanisms . . . . . . . . . . . . . . . . . . . 9
4.2. Network Components . . . . . . . . . . . . . . . . . . . 10 4.2. Network Components . . . . . . . . . . . . . . . . . . . 10
4.3. Host Mechanisms . . . . . . . . . . . . . . . . . . . . . 13 4.3. Host Mechanisms . . . . . . . . . . . . . . . . . . . . . 13
5. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Why These Primary Components? . . . . . . . . . . . . . . 15 5.1. Why These Primary Components? . . . . . . . . . . . . . . 15
5.2. What L4S adds to Existing Approaches . . . . . . . . . . 18 5.2. What L4S adds to Existing Approaches . . . . . . . . . . 18
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 21 6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 21 6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 22 6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 22
6.3. Applicability with Specific Link Technologies . . . . . . 24 6.3. Applicability with Specific Link Technologies . . . . . . 24
6.4. Deployment Considerations . . . . . . . . . . . . . . . . 24 6.4. Deployment Considerations . . . . . . . . . . . . . . . . 25
6.4.1. Deployment Topology . . . . . . . . . . . . . . . . . 25 6.4.1. Deployment Topology . . . . . . . . . . . . . . . . . 25
6.4.2. Deployment Sequences . . . . . . . . . . . . . . . . 26 6.4.2. Deployment Sequences . . . . . . . . . . . . . . . . 26
6.4.3. L4S Flow but Non-ECN Bottleneck . . . . . . . . . . . 29 6.4.3. L4S Flow but Non-ECN Bottleneck . . . . . . . . . . . 29
6.4.4. L4S Flow but Classic ECN Bottleneck . . . . . . . . . 30 6.4.4. L4S Flow but Classic ECN Bottleneck . . . . . . . . . 30
6.4.5. L4S AQM Deployment within Tunnels . . . . . . . . . . 30 6.4.5. L4S AQM Deployment within Tunnels . . . . . . . . . . 30
7. IANA Considerations (to be removed by RFC Editor) . . . . . . 30 7. IANA Considerations (to be removed by RFC Editor) . . . . . . 30
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30 8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
8.1. Traffic Rate (Non-)Policing . . . . . . . . . . . . . . . 30 8.1. Traffic Rate (Non-)Policing . . . . . . . . . . . . . . . 31
8.1.1. (Non-)Policing Rate per Flow . . . . . . . . . . . . 31
8.1.2. (Non-)Policing L4S Service Rate . . . . . . . . . . . 31
8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 32 8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 32
8.3. Interaction between Rate Policing and L4S . . . . . . . . 34 8.3. Interaction between Rate Policing and L4S . . . . . . . . 34
8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 34 8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 35
8.5. Privacy Considerations . . . . . . . . . . . . . . . . . 35 8.5. Privacy Considerations . . . . . . . . . . . . . . . . . 35
9. Informative References . . . . . . . . . . . . . . . . . . . 36 9. Informative References . . . . . . . . . . . . . . . . . . . 36
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 45 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction 1. Introduction
At any one time, it is increasingly common for all of the traffic in At any one time, it is increasingly common for all of the traffic in
a bottleneck link (e.g. a household's Internet access) to come from a bottleneck link (e.g. a household's Internet access) to come from
applications that prefer low delay: interactive Web, Web services, applications that prefer low delay: interactive Web, Web services,
voice, conversational video, interactive video, interactive remote voice, conversational video, interactive video, interactive remote
presence, instant messaging, online gaming, remote desktop, cloud- presence, instant messaging, online gaming, remote desktop, cloud-
based applications and video-assisted remote control of machinery and based applications and video-assisted remote control of machinery and
industrial processes. In the last decade or so, much has been done industrial processes. In the last decade or so, much has been done
skipping to change at page 3, line 48 skipping to change at page 3, line 40
important because, for interactive applications, losses translate important because, for interactive applications, losses translate
into even longer retransmission delays. into even longer retransmission delays.
It has been demonstrated that, once access network bit rates reach It has been demonstrated that, once access network bit rates reach
levels now common in the developed world, increasing link capacity levels now common in the developed world, increasing link capacity
offers diminishing returns if latency (delay) is not addressed offers diminishing returns if latency (delay) is not addressed
[Dukkipati06], [Rajiullah15]. Therefore, the goal is an Internet [Dukkipati06], [Rajiullah15]. Therefore, the goal is an Internet
service with very Low queueing Latency, very Low Loss and Scalable service with very Low queueing Latency, very Low Loss and Scalable
throughput (L4S). Very low queuing latency means less than throughput (L4S). Very low queuing latency means less than
1 millisecond (ms) on average and less than about 2 ms at the 99th 1 millisecond (ms) on average and less than about 2 ms at the 99th
percentile. This document describes the L4S architecture for percentile. End-to-end delay above 50 ms [Raaen14] or even above
achieving these goals. 20 ms [NASA04] starts to feel unnatural for more demanding
interactive applications. So removing unnecessary delay variability
increases the reach of these applications (the distance over which
they are comfortable to use). This document describes the L4S
architecture for achieving these goals.
Differentiated services (Diffserv) offers Expedited Forwarding Differentiated services (Diffserv) offers Expedited Forwarding
(EF [RFC3246]) for some packets at the expense of others, but this (EF [RFC3246]) for some packets at the expense of others, but this
makes no difference when all (or most) of the traffic at a bottleneck makes no difference when all (or most) of the traffic at a bottleneck
at any one time requires low latency. In contrast, L4S still works at any one time requires low latency. In contrast, L4S still works
well when all traffic is L4S - a service that gives without taking well when all traffic is L4S - a service that gives without taking
needs none of the configuration or management baggage (traffic needs none of the configuration or management baggage (traffic
policing, traffic contracts) associated with favouring some traffic policing, traffic contracts) associated with favouring some traffic
flows over others. flows over others.
skipping to change at page 4, line 28 skipping to change at page 4, line 22
is itself a large capacity-seeking or adaptive rate (e.g. interactive is itself a large capacity-seeking or adaptive rate (e.g. interactive
video) flow. At these times, the performance improvement from L4S video) flow. At these times, the performance improvement from L4S
must be sufficient that network operators will be motivated to deploy must be sufficient that network operators will be motivated to deploy
it. it.
Active Queue Management (AQM) is part of the solution to queuing Active Queue Management (AQM) is part of the solution to queuing
under load. AQM improves performance for all traffic, but there is a under load. AQM improves performance for all traffic, but there is a
limit to how much queuing delay can be reduced by solely changing the limit to how much queuing delay can be reduced by solely changing the
network; without addressing the root of the problem. network; without addressing the root of the problem.
The root of the problem is the presence of standard TCP congestion The root of the problem is the presence of standard congestion
control (Reno [RFC5681]) or compatible variants (e.g. TCP control (Reno [RFC5681]) or compatible variants
Cubic [RFC8312]). We shall use the term 'Classic' for these Reno- (e.g. CUBIC [RFC8312]) that are used in TCP and in other transports
friendly congestion controls. Classic congestion controls induce such as QUIC [RFC9000]. We shall use the term 'Classic' for these
relatively large saw-tooth-shaped excursions up the queue and down Reno-friendly congestion controls. Classic congestion controls
again, which have been growing as flow rate scales [RFC3649]. So if induce relatively large saw-tooth-shaped excursions up the queue and
a network operator naively attempts to reduce queuing delay by down again, which have been growing as flow rate scales [RFC3649].
So if a network operator naively attempts to reduce queuing delay by
configuring an AQM to operate at a shallower queue, a Classic configuring an AQM to operate at a shallower queue, a Classic
congestion control will significantly underutilize the link at the congestion control will significantly underutilize the link at the
bottom of every saw-tooth. bottom of every saw-tooth.
It has been demonstrated that if the sending host replaces a Classic It has been demonstrated that if the sending host replaces a Classic
congestion control with a 'Scalable' alternative, when a suitable AQM congestion control with a 'Scalable' alternative, when a suitable AQM
is deployed in the network the performance under load of all the is deployed in the network the performance under load of all the
above interactive applications can be significantly improved. For above interactive applications can be significantly improved. For
instance, queuing delay under heavy load with the example DCTCP/DualQ instance, queuing delay under heavy load with the example DCTCP/DualQ
solution cited below on a DSL or Ethernet link is roughly 1 to 2 solution cited below on a DSL or Ethernet link is roughly 1 to 2
skipping to change at page 5, line 18 skipping to change at page 5, line 12
start using it as soon as the sender's stack is updated. Access start using it as soon as the sender's stack is updated. Access
networks are typically designed with one link as the bottleneck for networks are typically designed with one link as the bottleneck for
each site (which might be a home, small enterprise or mobile device), each site (which might be a home, small enterprise or mobile device),
so deployment at either or both ends of this link should give nearly so deployment at either or both ends of this link should give nearly
all the benefit in the respective direction. With some transport all the benefit in the respective direction. With some transport
protocols, namely TCP and SCTP, the sender has to check that the protocols, namely TCP and SCTP, the sender has to check that the
receiver has been suitably updated to give more accurate feedback, receiver has been suitably updated to give more accurate feedback,
whereas with more recent transport protocols such as QUIC and DCCP, whereas with more recent transport protocols such as QUIC and DCCP,
all receivers have always been suitable. all receivers have always been suitable.
This document presents the L4S architecture, by describing and This document presents the L4S architecture. It consists of three
justifying the component parts and how they interact to provide the components: network support to isolate L4S traffic from classic
scalable, low latency, low loss Internet service. It also details traffic; protocol features that allow network elements to identify
the approach to incremental deployment, as briefly summarized above. L4S traffic; and host support for L4S congestion controls. The
protocol is defined separately [I-D.ietf-tsvwg-ecn-l4s-id] as an
experimental change to Explicit Congestion Notification (ECN). This
document describes and justifies the component parts and how they
interact to provide the scalable, low latency, low loss Internet
service. It also details the approach to incremental deployment, as
briefly summarized above.
1.1. Document Roadmap 1.1. Document Roadmap
This document describes the L4S architecture in three passes. First This document describes the L4S architecture in three passes. First
this brief overview gives the very high level idea and states the this brief overview gives the very high level idea and states the
main components with minimal rationale. This is only intended to main components with minimal rationale. This is only intended to
give some context for the terminology definitions that follow in give some context for the terminology definitions that follow in
Section 3, and to explain the structure of the rest of the document. Section 3, and to explain the structure of the rest of the document.
Then Section 4 goes into more detail on each component with some Then Section 4 goes into more detail on each component with some
rationale, but still mostly stating what the architecture is, rather rationale, but still mostly stating what the architecture is, rather
than why. Finally Section 5 justifies why each element of the than why. Finally, Section 5 justifies why each element of the
solution was chosen (Section 5.1) and why these choices were solution was chosen (Section 5.1) and why these choices were
different from other solutions (Section 5.2). different from other solutions (Section 5.2).
Having described the architecture, Section 6 clarifies its Having described the architecture, Section 6 clarifies its
applicability; that is, the applications and use-cases that motivated applicability; that is, the applications and use-cases that motivated
the design, the challenges applying the architecture to various link the design, the challenges applying the architecture to various link
technologies, and various incremental deployment models: including technologies, and various incremental deployment models: including
the two main deployment topologies, different sequences for the two main deployment topologies, different sequences for
incremental deployment and various interactions with pre-existing incremental deployment and various interactions with pre-existing
approaches. The document ends with the usual tail pieces, including approaches. The document ends with the usual tailpieces, including
extensive discussion of traffic policing and other security extensive discussion of traffic policing and other security
considerations in Section 8. considerations in Section 8.
2. L4S Architecture Overview 2. L4S Architecture Overview
Below we outline the three main components to the L4S architecture; Below we outline the three main components to the L4S architecture;
1) the scalable congestion control on the sending host; 2) the AQM at 1) the scalable congestion control on the sending host; 2) the AQM at
the network bottleneck; and 3) the protocol between them. the network bottleneck; and 3) the protocol between them.
But first, the main point to grasp is that low latency is not But first, the main point to grasp is that low latency is not
skipping to change at page 6, line 25 skipping to change at page 6, line 25
essential for L4S, senders use the ECN field as the protocol that essential for L4S, senders use the ECN field as the protocol that
allows the network to identify which packets are L4S and which are allows the network to identify which packets are L4S and which are
Classic. Classic.
1) Host: Scalable congestion controls already exist. They solve the 1) Host: Scalable congestion controls already exist. They solve the
scaling problem with Classic congestion controls, such as Reno or scaling problem with Classic congestion controls, such as Reno or
Cubic. Because flow rate has scaled since TCP congestion control Cubic. Because flow rate has scaled since TCP congestion control
was first designed in 1988, assuming the flow lasts long enough, was first designed in 1988, assuming the flow lasts long enough,
it now takes hundreds of round trips (and growing) to recover it now takes hundreds of round trips (and growing) to recover
after a congestion signal (whether a loss or an ECN mark) as shown after a congestion signal (whether a loss or an ECN mark) as shown
in the examples in Section 5.1 and [RFC3649]. Therefore control in the examples in Section 5.1 and [RFC3649]. Therefore, control
of queuing and utilization becomes very slack, and the slightest of queuing and utilization becomes very slack, and the slightest
disturbances (e.g. from new flows starting) prevent a high rate disturbances (e.g. from new flows starting) prevent a high rate
from being attained. from being attained.
With a scalable congestion control, the average time from one With a scalable congestion control, the average time from one
congestion signal to the next (the recovery time) remains congestion signal to the next (the recovery time) remains
invariant as the flow rate scales, all other factors being equal. invariant as the flow rate scales, all other factors being equal.
This maintains the same degree of control over queueing and This maintains the same degree of control over queueing and
utilization whatever the flow rate, as well as ensuring that high utilization whatever the flow rate, as well as ensuring that high
throughput is more robust to disturbances. The scalable control throughput is more robust to disturbances. The scalable control
skipping to change at page 7, line 6 skipping to change at page 7, line 6
(QUIC, SCTP, RTP/RTCP, RMCAT, etc.). Indeed, between the present (QUIC, SCTP, RTP/RTCP, RMCAT, etc.). Indeed, between the present
document being drafted and published, the following scalable document being drafted and published, the following scalable
congestion controls were implemented: TCP Prague [PragueLinux], congestion controls were implemented: TCP Prague [PragueLinux],
QUIC Prague, an L4S variant of the RMCAT SCReAM QUIC Prague, an L4S variant of the RMCAT SCReAM
controller [SCReAM] and the L4S ECN part of BBRv2 [BBRv2] intended controller [SCReAM] and the L4S ECN part of BBRv2 [BBRv2] intended
for TCP and QUIC transports. for TCP and QUIC transports.
2) Network: L4S traffic needs to be isolated from the queuing 2) Network: L4S traffic needs to be isolated from the queuing
latency of Classic traffic. One queue per application flow (FQ) latency of Classic traffic. One queue per application flow (FQ)
is one way to achieve this, e.g. FQ-CoDel [RFC8290]. However, is one way to achieve this, e.g. FQ-CoDel [RFC8290]. However,
just two queues is sufficient and does not require inspection of using just two queues is sufficient and does not require
transport layer headers in the network, which is not always inspection of transport layer headers in the network, which is not
possible (see Section 5.2). With just two queues, it might seem always possible (see Section 5.2). With just two queues, it might
impossible to know how much capacity to schedule for each queue seem impossible to know how much capacity to schedule for each
without inspecting how many flows at any one time are using each. queue without inspecting how many flows at any one time are using
And it would be undesirable to arbitrarily divide access network each. And it would be undesirable to arbitrarily divide access
capacity into two partitions. The Dual Queue Coupled AQM was network capacity into two partitions. The Dual Queue Coupled AQM
developed as a minimal complexity solution to this problem. It was developed as a minimal complexity solution to this problem.
acts like a 'semi-permeable' membrane that partitions latency but It acts like a 'semi-permeable' membrane that partitions latency
not bandwidth. As such, the two queues are for transition from but not bandwidth. As such, the two queues are for transition
Classic to L4S behaviour, not bandwidth prioritization. from Classic to L4S behaviour, not bandwidth prioritization.
Section 4 gives a high level explanation of how the per-flow-queue Section 4 gives a high level explanation of how the per-flow-queue
(FQ) and DualQ variants of L4S work, and (FQ) and DualQ variants of L4S work, and
[I-D.ietf-tsvwg-aqm-dualq-coupled] gives a full explanation of the [I-D.ietf-tsvwg-aqm-dualq-coupled] gives a full explanation of the
DualQ Coupled AQM framework. A specific marking algorithm is not DualQ Coupled AQM framework. A specific marking algorithm is not
mandated for L4S AQMs. Appendices of mandated for L4S AQMs. Appendices of
[I-D.ietf-tsvwg-aqm-dualq-coupled] give non-normative examples [I-D.ietf-tsvwg-aqm-dualq-coupled] give non-normative examples
that have been implemented and evaluated, and give recommended that have been implemented and evaluated, and give recommended
default parameter settings. It is expected that L4S experiments default parameter settings. It is expected that L4S experiments
will improve knowledge of parameter settings and whether the set will improve knowledge of parameter settings and whether the set
of marking algorithms needs to be limited. of marking algorithms needs to be limited.
3) Protocol: A host needs to distinguish L4S and Classic packets 3) Protocol: A sending host needs to distinguish L4S and Classic
with an identifier so that the network can classify them into packets with an identifier so that the network can classify them
their separate treatments. The L4S identifier into their separate treatments. The L4S identifier
spec. [I-D.ietf-tsvwg-ecn-l4s-id] concludes that all alternatives spec. [I-D.ietf-tsvwg-ecn-l4s-id] concludes that all alternatives
involve compromises, but the ECT(1) and CE codepoints of the ECN involve compromises, but the ECT(1) and CE codepoints of the ECN
field represent a workable solution. As already explained, the field represent a workable solution. As already explained, the
network also uses ECN to immediately signal the very start of network also uses ECN to immediately signal the very start of
queue growth to the transport. queue growth to the transport.
3. Terminology 3. Terminology
Note: The following definitions are copied from the L4S ECN [Note to the RFC Editor (to be removed before publication as an RFC):
spec [I-D.ietf-tsvwg-ecn-l4s-id] for convenience. If there are The following definitions are copied from the L4S ECN
accidental differences, those in [I-D.ietf-tsvwg-ecn-l4s-id] take spec [I-D.ietf-tsvwg-ecn-l4s-id] for the reader's convenience.
precedence. Except, here, Classic CC and Scalable CC are condensed because they
refer to Section 5.1 later. Also the definition of Traffic Policing
is not needed in [I-D.ietf-tsvwg-ecn-l4s-id].]
Classic Congestion Control: A congestion control behaviour that can Classic Congestion Control: A congestion control behaviour that can
co-exist with standard Reno [RFC5681] without causing co-exist with standard Reno [RFC5681] without causing
significantly negative impact on its flow rate [RFC5033]. The significantly negative impact on its flow rate [RFC5033]. The
scaling problem with Classic congestion control is explained, with scaling problem with Classic congestion control is explained, with
examples, in Section 5.1 and in [RFC3649]. examples, in Section 5.1 and in [RFC3649].
Scalable Congestion Control: A congestion control where the average Scalable Congestion Control: A congestion control where the average
time from one congestion signal to the next (the recovery time) time from one congestion signal to the next (the recovery time)
remains invariant as the flow rate scales, all other factors being remains invariant as the flow rate scales, all other factors being
skipping to change at page 8, line 26 skipping to change at page 8, line 28
congestion control behaviours that co-exist with Reno [RFC5681] congestion control behaviours that co-exist with Reno [RFC5681]
(e.g. Reno itself, Cubic [RFC8312], (e.g. Reno itself, Cubic [RFC8312],
Compound [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]). The term Compound [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]). The term
'Classic queue' means a queue providing the Classic service. 'Classic queue' means a queue providing the Classic service.
Low-Latency, Low-Loss Scalable throughput (L4S) service: The 'L4S' Low-Latency, Low-Loss Scalable throughput (L4S) service: The 'L4S'
service is intended for traffic from scalable congestion control service is intended for traffic from scalable congestion control
algorithms, such as the Prague congestion algorithms, such as the Prague congestion
control [I-D.briscoe-iccrg-prague-congestion-control], which was control [I-D.briscoe-iccrg-prague-congestion-control], which was
derived from DCTCP [RFC8257]. The L4S service is for more derived from DCTCP [RFC8257]. The L4S service is for more
general traffic than just TCP Prague -- it allows the set of general traffic than just Prague -- it allows the set of
congestion controls with similar scaling properties to Prague to congestion controls with similar scaling properties to Prague to
evolve, such as the examples listed above (Relentless, SCReAM). evolve, such as the examples listed above (Relentless, SCReAM).
The term 'L4S queue' means a queue providing the L4S service. The term 'L4S queue' means a queue providing the L4S service.
The terms Classic or L4S can also qualify other nouns, such as The terms Classic or L4S can also qualify other nouns, such as
'queue', 'codepoint', 'identifier', 'classification', 'packet', 'queue', 'codepoint', 'identifier', 'classification', 'packet',
'flow'. For example: an L4S packet means a packet with an L4S 'flow'. For example: an L4S packet means a packet with an L4S
identifier sent from an L4S congestion control. identifier 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 to not build a case its rate has to be smooth enough or low enough to not build a
queue (e.g. DNS, VoIP, game sync datagrams, etc). queue (e.g. DNS, VoIP, game sync datagrams, etc.).
Reno-friendly: The subset of Classic traffic that is friendly to the Reno-friendly: The subset of Classic traffic that is friendly to the
standard Reno congestion control defined for TCP in [RFC5681]. standard Reno congestion control defined for TCP in [RFC5681].
The TFRC spec. [RFC5348] indirectly implies that 'friendly' is The TFRC spec. [RFC5348] indirectly implies that 'friendly' is
defined as "generally within a factor of two of the sending rate defined as "generally within a factor of two of the sending rate
of a TCP flow under the same conditions". Reno-friendly is used of a TCP flow under the same conditions". Reno-friendly is used
here in place of 'TCP-friendly', given the latter has become here in place of 'TCP-friendly', given the latter has become
imprecise, because the TCP protocol is now used with so many imprecise, because the TCP protocol is now used with so many
different congestion control behaviours, and Reno is used in non- different congestion control behaviours, and Reno is used in non-
TCP transports such as QUIC [RFC9000]. TCP transports such as QUIC [RFC9000].
skipping to change at page 9, line 28 skipping to change at page 9, line 30
network bottleneck is typically the access link to the site. Not network bottleneck is typically the access link to the site. Not
all network arrangements fit this model but it is a useful, widely all network arrangements fit this model but it is a useful, widely
applicable generalization. applicable generalization.
Traffic policing: Limiting traffic by dropping packets or shifting Traffic policing: Limiting traffic by dropping packets or shifting
them to lower service class (as opposed to introducing delay, them to lower service class (as opposed to introducing delay,
which is termed traffic shaping). Policing can involve limiting which is termed traffic shaping). Policing can involve limiting
average rate and/or burst size. Policing focused on limiting average rate and/or burst size. Policing focused on limiting
queuing but not average flow rate is termed congestion policing, queuing but not average flow rate is termed congestion policing,
latency policing, burst policing or queue protection in this latency policing, burst policing or queue protection in this
document. Otherwise the term rate policing is used. document. Otherwise, the term rate policing is used.
4. L4S Architecture Components 4. L4S Architecture Components
The L4S architecture is composed of the elements in the following The L4S architecture is composed of the elements in the following
three subsections. three subsections.
4.1. Protocol Mechanisms 4.1. Protocol Mechanisms
The L4S architecture involves: a) unassignment of an identifier; b) The L4S architecture involves: a) unassignment of the previous use of
reassignment of the same identifier; and c) optional further the identifier; b) reassignment of the same identifier; and c)
identifiers: optional further identifiers:
a. An essential aspect of a scalable congestion control is the use a. An essential aspect of a scalable congestion control is the use
of explicit congestion signals. 'Classic' ECN [RFC3168] requires of explicit congestion signals. 'Classic' ECN [RFC3168] requires
an ECN signal to be treated as equivalent to drop, both when it an ECN signal to be treated as equivalent to drop, both when it
is generated in the network and when it is responded to by hosts. is generated in the network and when it is responded to by hosts.
L4S needs networks and hosts to support a more fine-grained L4S needs networks and hosts to support a more fine-grained
meaning for each ECN signal that is less severe than a drop, so meaning for each ECN signal that is less severe than a drop, so
that the L4S signals: that the L4S signals:
* can be much more frequent; * can be much more frequent;
* can be signalled immediately, without the significant delay * can be signalled immediately, without the significant delay
required to smooth out fluctuations in the queue. required to smooth out fluctuations in the queue.
To enable L4S, the standards track Classic ECN spec. [RFC3168] To enable L4S, the standards track Classic ECN spec. [RFC3168]
has had to be updated to allow L4S packets to depart from the has had to be updated to allow L4S packets to depart from the
'equivalent to drop' constraint. [RFC8311] is a standards track 'equivalent to drop' constraint. [RFC8311] is a standards track
update to relax specific requirements in RFC 3168 (and certain update to relax specific requirements in RFC 3168 (and certain
other standards track RFCs), which clears the way for the other standards track RFCs), which clears the way for the
experimental changes proposed for L4S. [RFC8311] also experimental changes proposed for L4S. Also, the ECT(1)
reclassifies the original experimental assignment of the ECT(1) codepoint was previously assigned as the experimental ECN
codepoint as an ECN nonce [RFC3540] as historic. nonce [RFC3540], which RFC 8311 recategorizes as historic to make
the codepoint available again.
b. [I-D.ietf-tsvwg-ecn-l4s-id] specifies that ECT(1) is used as the b. [I-D.ietf-tsvwg-ecn-l4s-id] specifies that ECT(1) is used as the
identifier to classify L4S packets into a separate treatment from identifier to classify L4S packets into a separate treatment from
Classic packets. This satisfies the requirement for identifying Classic packets. This satisfies the requirement for identifying
an alternative ECN treatment in [RFC4774]. an alternative ECN treatment in [RFC4774].
The CE codepoint is used to indicate Congestion Experienced by The CE codepoint is used to indicate Congestion Experienced by
both L4S and Classic treatments. This raises the concern that a both L4S and Classic treatments. This raises the concern that a
Classic AQM earlier on the path might have marked some ECT(0) Classic AQM earlier on the path might have marked some ECT(0)
packets as CE. Then these packets will be erroneously classified packets as CE. Then these packets will be erroneously classified
skipping to change at page 13, line 12 skipping to change at page 13, line 12
Scalable Sending Host; 2) Isolation in separate network Scalable Sending Host; 2) Isolation in separate network
queues; and 3) Packet Identification Protocol queues; and 3) Packet Identification Protocol
b. Per-Flow Queues and AQMs: A scheduler with per-flow queues such b. Per-Flow Queues and AQMs: A scheduler with per-flow queues such
as FQ-CoDel or FQ-PIE can be used for L4S. For instance within as FQ-CoDel or FQ-PIE can be used for L4S. For instance within
each queue of an FQ-CoDel system, as well as a CoDel AQM, there each queue of an FQ-CoDel system, as well as a CoDel AQM, there
is typically also the option of ECN marking at an immediate is typically also the option of ECN marking at an immediate
(unsmoothed) shallow threshold to support use in data centres (unsmoothed) shallow threshold to support use in data centres
(see Sec.5.2.7 of the FQ-CoDel spec [RFC8290]). In Linux, this (see Sec.5.2.7 of the FQ-CoDel spec [RFC8290]). In Linux, this
has been modified so that the shallow threshold can be solely has been modified so that the shallow threshold can be solely
applied to ECT(1) packets [FQ_CoDel_Thresh]. Then if there is a applied to ECT(1) packets [FQ_CoDel_Thresh]. Then, if there is a
flow of non-ECN or ECT(0) packets in the per-flow-queue, the flow of non-ECN or ECT(0) packets in the per-flow-queue, the
Classic AQM (e.g. CoDel) is applied; while if there is a flow of Classic AQM (e.g. CoDel) is applied; while if there is a flow of
ECT(1) packets in the queue, the shallower (typically sub- ECT(1) packets in the queue, the shallower (typically sub-
millisecond) threshold is applied. In addition, ECT(0) and not- millisecond) threshold is applied. In addition, ECT(0) and not-
ECT packets could potentially be classified into a separate flow- ECT packets could potentially be classified into a separate flow-
queue from ECT(1) and CE packets to avoid them mixing if they queue from ECT(1) and CE packets to avoid them mixing if they
share a common flow-identifier (e.g. in a VPN). share a common flow-identifier (e.g. in a VPN).
c. Dual-queues, but per-flow AQMs: It should also be possible to use c. Dual-queues, but per-flow AQMs: It should also be possible to use
dual queues for isolation, but with per-flow marking to control dual queues for isolation, but with per-flow marking to control
skipping to change at page 14, line 21 skipping to change at page 14, line 21
Transport protocols other than TCP use various congestion Transport protocols other than TCP use various congestion
controls that are designed to be friendly with Reno. Before they controls that are designed to be friendly with Reno. Before they
can use the L4S service, they will need to be updated to can use the L4S service, they will need to be updated to
implement a scalable congestion response, which they will have to implement a scalable congestion response, which they will have to
indicate by using the ECT(1) codepoint. Scalable variants are indicate by using the ECT(1) codepoint. Scalable variants are
under consideration for more recent transport protocols, under consideration for more recent transport protocols,
e.g. QUIC, and the L4S ECN part of BBRv2 [BBRv2], e.g. QUIC, and the L4S ECN part of BBRv2 [BBRv2],
[I-D.cardwell-iccrg-bbr-congestion-control] is a scalable [I-D.cardwell-iccrg-bbr-congestion-control] is a scalable
congestion control intended for the TCP and QUIC transports, congestion control intended for the TCP and QUIC transports,
amongst others. Also an L4S variant of the RMCAT SCReAM amongst others. Also, an L4S variant of the RMCAT SCReAM
controller [RFC8298] has been implemented [SCReAM] for media controller [RFC8298] has been implemented [SCReAM] for media
transported over RTP. transported over RTP.
Section 4.3 of the L4S ECN spec [I-D.ietf-tsvwg-ecn-l4s-id] Section 4.3 of the L4S ECN spec [I-D.ietf-tsvwg-ecn-l4s-id]
defines scalable congestion control in more detail, and specifies defines scalable congestion control in more detail, and specifies
the requirements that an L4S scalable congestion control has to the requirements that an L4S scalable congestion control has to
comply with. comply with.
b. The ECN feedback in some transport protocols is already b. The ECN feedback in some transport protocols is already
sufficiently fine-grained for L4S (specifically DCCP [RFC4340] sufficiently fine-grained for L4S (specifically DCCP [RFC4340]
skipping to change at page 14, line 43 skipping to change at page 14, line 43
the process of being updated: the process of being updated:
* For the case of TCP, the feedback protocol for ECN embeds the * For the case of TCP, the feedback protocol for ECN embeds the
assumption from Classic ECN [RFC3168] that an ECN mark is assumption from Classic ECN [RFC3168] that an ECN mark is
equivalent to a drop, making it unusable for a scalable TCP. equivalent to a drop, making it unusable for a scalable TCP.
Therefore, the implementation of TCP receivers will have to be Therefore, the implementation of TCP receivers will have to be
upgraded [RFC7560]. Work to standardize and implement more upgraded [RFC7560]. Work to standardize and implement more
accurate ECN feedback for TCP (AccECN) is in accurate ECN feedback for TCP (AccECN) is in
progress [I-D.ietf-tcpm-accurate-ecn], [PragueLinux]. progress [I-D.ietf-tcpm-accurate-ecn], [PragueLinux].
* ECN feedback is only roughly sketched in an appendix of the * ECN feedback was only roughly sketched in an appendix of the
SCTP specification [RFC4960]. A fuller specification has been now obsoleted second specification of SCTP [RFC4960], while a
proposed in a long-expired draft [I-D.stewart-tsvwg-sctpecn], fuller specification was proposed in a long-expired
which would need to be implemented and deployed before SCTP draft [I-D.stewart-tsvwg-sctpecn]. A new design would need to
could support L4S. be implemented and deployed before SCTP could support L4S.
* For RTP, sufficient ECN feedback was defined in [RFC6679], but * For RTP, sufficient ECN feedback was defined in [RFC6679], but
[RFC8888] defines the latest standards track improvements. [RFC8888] defines the latest standards track improvements.
5. Rationale 5. Rationale
5.1. Why These Primary Components? 5.1. Why These Primary Components?
Explicit congestion signalling (protocol): Explicit congestion Explicit congestion signalling (protocol): Explicit congestion
signalling is a key part of the L4S approach. In contrast, use of signalling is a key part of the L4S approach. In contrast, use of
drop as a congestion signal creates a tension because drop is both drop as a congestion signal creates a tension because drop is both
an impairment (less would be better) and a useful signal (more an impairment (less would be better) and a useful signal (more
would be better): would be better):
* Explicit congestion signals can be used many times per round * Explicit congestion signals can be used many times per round
trip, to keep tight control, without any impairment. Under trip, to keep tight control, without any impairment. Under
heavy load, even more explicit signals can be applied so the heavy load, even more explicit signals can be applied, so that
queue can be kept short whatever the load. In contrast, the queue can be kept short whatever the load. In contrast,
Classic AQMs have to introduce very high packet drop at high Classic AQMs have to introduce very high packet drop at high
load to keep the queue short. By using ECN, an L4S congestion load to keep the queue short. By using ECN, an L4S congestion
control's sawtooth reduction can be smaller and therefore control's sawtooth reduction can be smaller and therefore
return to the operating point more often, without worrying that return to the operating point more often, without worrying that
more sawteeth will cause more signals. The consequent smaller more sawteeth will cause more signals. The consequent smaller
amplitude sawteeth fit between an empty queue and a very amplitude sawteeth fit between an empty queue and a very
shallow marking threshold (~1 ms in the public Internet), so shallow marking threshold (~1 ms in the public Internet), so
queue delay variation can be very low, without risk of under- queue delay variation can be very low, without risk of under-
utilization. utilization.
skipping to change at page 17, line 30 skipping to change at page 17, line 30
proportionately by 8x as well, from 422 ms to 3.38 s. It is proportionately by 8x as well, from 422 ms to 3.38 s. It is
clearly problematic for a congestion control to take multiple clearly problematic for a congestion control to take multiple
seconds to recover from each congestion event. Cubic [RFC8312] seconds to recover from each congestion event. Cubic [RFC8312]
was developed to be less unscalable, but it is approaching its was developed to be less unscalable, but it is approaching its
scaling limit; with the same max RTT of 30 ms, at 120 Mb/s Cubic scaling limit; with the same max RTT of 30 ms, at 120 Mb/s Cubic
is still fully in its Reno-friendly mode, so it takes about 4.3 s is still fully in its Reno-friendly mode, so it takes about 4.3 s
to recover. However, once the flow rate scales by 8x again to to recover. However, once the flow rate scales by 8x again to
960 Mb/s it enters true Cubic mode, with a recovery time of 960 Mb/s it enters true Cubic mode, with a recovery time of
12.2 s. From then on, each further scaling by 8x doubles Cubic's 12.2 s. From then on, each further scaling by 8x doubles Cubic's
recovery time (because the cube root of 8 is 2), e.g. at 7.68 Gb/s recovery time (because the cube root of 8 is 2), e.g. at 7.68 Gb/s
the recovery time is 24.3 s. In contrast a scalable congestion the recovery time is 24.3 s. In contrast, a scalable congestion
control like DCTCP or TCP Prague induces 2 congestion signals per control like DCTCP or TCP Prague induces 2 congestion signals per
round trip on average, which remains invariant for any flow rate, round trip on average, which remains invariant for any flow rate,
keeping dynamic control very tight. keeping dynamic control very tight.
For a feel of where the global average lone-flow download sits on For a feel of where the global average lone-flow download sits on
this scale at the time of writing (2021), according to [BDPdata] this scale at the time of writing (2021), according to [BDPdata]
globally averaged fixed access capacity was 103 Mb/s in 2020 and globally averaged fixed access capacity was 103 Mb/s in 2020 and
averaged base RTT to a CDN was 25-34ms in 2019. Averaging of per- averaged base RTT to a CDN was 25-34ms in 2019. Averaging of per-
country data was weighted by Internet user population (data country data was weighted by Internet user population (data
collected globally is necessarily of variable quality, but the collected globally is necessarily of variable quality, but the
skipping to change at page 20, line 33 skipping to change at page 20, line 33
control. Then, flow rate policing can be added separately if control. Then, flow rate policing can be added separately if
desired. This allows application control up to a point, but desired. This allows application control up to a point, but
the network can still choose to set the point at which it the network can still choose to set the point at which it
intervenes to prevent one flow completely starving another. intervenes to prevent one flow completely starving another.
Note: Note:
1. It might seem that self-inflicted queuing delay within a per- 1. It might seem that self-inflicted queuing delay within a per-
flow queue should not be counted, because if the delay wasn't flow queue should not be counted, because if the delay wasn't
in the network it would just shift to the sender. However, in the network it would just shift to the sender. However,
modern adaptive applications, e.g. HTTP/2 [RFC7540] or some modern adaptive applications, e.g. HTTP/2 [RFC9113] or some
interactive media applications (see Section 6.1), can keep low interactive media applications (see Section 6.1), can keep low
latency objects at the front of their local send queue by latency objects at the front of their local send queue by
shuffling priorities of other objects dependent on the shuffling priorities of other objects dependent on the
progress of other transfers (for example see [lowat]). They progress of other transfers (for example see [lowat]). They
cannot shuffle objects once they have released them into the cannot shuffle objects once they have released them into the
network. network.
Alternative Back-off ECN (ABE): Here again, L4S is not an Alternative Back-off ECN (ABE): Here again, L4S is not an
alternative to ABE but a complement that introduces much lower alternative to ABE but a complement that introduces much lower
queuing delay. ABE [RFC8511] alters the host behaviour in queuing delay. ABE [RFC8511] alters the host behaviour in
response to ECN marking to utilize a link better and give ECN response to ECN marking to utilize a link better and give ECN
flows faster throughput. It uses ECT(0) and assumes the network flows faster throughput. It uses ECT(0) and assumes the network
still treats ECN and drop the same. Therefore ABE exploits any still treats ECN and drop the same. Therefore, ABE exploits any
lower queuing delay that AQMs can provide. But as explained lower queuing delay that AQMs can provide. But, as explained
above, AQMs still cannot reduce queuing delay too far without above, AQMs still cannot reduce queuing delay too far without
losing link utilization (to allow for other, non-ABE, flows). losing link utilization (to allow for other, non-ABE, flows).
BBR: Bottleneck Bandwidth and Round-trip propagation time BBR: Bottleneck Bandwidth and Round-trip propagation time
(BBR [I-D.cardwell-iccrg-bbr-congestion-control]) controls queuing (BBR [I-D.cardwell-iccrg-bbr-congestion-control]) controls queuing
delay end-to-end without needing any special logic in the network, delay end-to-end without needing any special logic in the network,
such as an AQM. So it works pretty-much on any path. BBR keeps such as an AQM. So it works pretty-much on any path. BBR keeps
queuing delay reasonably low, but perhaps not quite as low as with queuing delay reasonably low, but perhaps not quite as low as with
state-of-the-art AQMs such as PIE or FQ-CoDel, and certainly state-of-the-art AQMs such as PIE or FQ-CoDel, and certainly
nowhere near as low as with L4S. Queuing delay is also not nowhere near as low as with L4S. Queuing delay is also not
consistently low, due to BBR's regular bandwidth probing spikes consistently low, due to BBR's regular bandwidth probing spikes
and its aggressive flow start-up phase. and its aggressive flow start-up phase.
L4S complements BBR. Indeed BBRv2 can use L4S ECN where available L4S complements BBR. Indeed, BBRv2 can use L4S ECN where
and a scalable L4S congestion control behaviour in response to any available and a scalable L4S congestion control behaviour in
ECN signalling from the path [BBRv2]. The L4S ECN signal response to any ECN signalling from the path [BBRv2]. The L4S ECN
complements the delay based congestion control aspects of BBR with signal complements the delay based congestion control aspects of
an explicit indication that hosts can use, both to converge on a BBR with an explicit indication that hosts can use, both to
fair rate and to keep below a shallow queue target set by the converge on a fair rate and to keep below a shallow queue target
network. Without L4S ECN, both these aspects need to be assumed set by the network. Without L4S ECN, both these aspects need to
or estimated. be assumed or estimated.
6. Applicability 6. Applicability
6.1. Applications 6.1. Applications
A transport layer that solves the current latency issues will provide A transport layer that solves the current latency issues will provide
new service, product and application opportunities. new service, product and application opportunities.
With the L4S approach, the following existing applications also With the L4S approach, the following existing applications also
experience significantly better quality of experience under load: experience significantly better quality of experience under load:
skipping to change at page 22, line 14 skipping to change at page 22, line 14
The above two applications have been successfully demonstrated with The above two applications have been successfully demonstrated with
L4S, both running together over a 40 Mb/s broadband access link L4S, both running together over a 40 Mb/s broadband access link
loaded up with the numerous other latency sensitive applications in loaded up with the numerous other latency sensitive applications in
the previous list as well as numerous downloads - all sharing the the previous list as well as numerous downloads - all sharing the
same bottleneck queue simultaneously [L4Sdemo16]. For the former, a same bottleneck queue simultaneously [L4Sdemo16]. For the former, a
panoramic video of a football stadium could be swiped and pinched so panoramic video of a football stadium could be swiped and pinched so
that, on the fly, a proxy in the cloud could generate a sub-window of that, on the fly, a proxy in the cloud could generate a sub-window of
the match video under the finger-gesture control of each user. For the match video under the finger-gesture control of each user. For
the latter, a virtual reality headset displayed a viewport taken from the latter, a virtual reality headset displayed a viewport taken from
a 360 degree camera in a racing car. The user's head movements a 360-degree camera in a racing car. The user's head movements
controlled the viewport extracted by a cloud-based proxy. In both controlled the viewport extracted by a cloud-based proxy. In both
cases, with 7 ms end-to-end base delay, the additional queuing delay cases, with 7 ms end-to-end base delay, the additional queuing delay
of roughly 1 ms was so low that it seemed the video was generated of roughly 1 ms was so low that it seemed the video was generated
locally. locally.
Using a swiping finger gesture or head movement to pan a video are Using a swiping finger gesture or head movement to pan a video are
extremely latency-demanding actions -- far more demanding than VoIP. extremely latency-demanding actions -- far more demanding than VoIP.
Because human vision can detect extremely low delays of the order of Because human vision can detect extremely low delays of the order of
single milliseconds when delay is translated into a visual lag single milliseconds when delay is translated into a visual lag
between a video and a reference point (the finger or the orientation between a video and a reference point (the finger or the orientation
of the head sensed by the balance system in the inner ear -- the of the head sensed by the balance system in the inner ear -- the
vestibular system). vestibular system). With an alternative AQM, the video noticeably
lagged behind the finger gestures and head movements.
Without the low queuing delay of L4S, cloud-based applications like Without the low queuing delay of L4S, cloud-based applications like
these would not be credible without significantly more access these would not be credible without significantly more access
bandwidth (to deliver all possible video that might be viewed) and bandwidth (to deliver all possible video that might be viewed) and
more local processing, which would increase the weight and power more local processing, which would increase the weight and power
consumption of head-mounted displays. When all interactive consumption of head-mounted displays. When all interactive
processing can be done in the cloud, only the data to be rendered for processing can be done in the cloud, only the data to be rendered for
the end user needs to be sent. the end user needs to be sent.
Other low latency high bandwidth applications such as: Other low latency high bandwidth applications such as:
skipping to change at page 24, line 8 skipping to change at page 24, line 8
budget can communicate over longer distances, or via a longer budget can communicate over longer distances, or via a longer
chain of service functions [RFC7665] or onion routers. chain of service functions [RFC7665] or onion routers.
* If delay jitter is minimized, it is possible to reduce the * If delay jitter is minimized, it is possible to reduce the
dejitter buffers on the receive end of video streaming, which dejitter buffers on the receive end of video streaming, which
should improve the interactive experience should improve the interactive experience
6.3. Applicability with Specific Link Technologies 6.3. Applicability with Specific Link Technologies
Certain link technologies aggregate data from multiple packets into Certain link technologies aggregate data from multiple packets into
bursts, and buffer incoming packets while building each burst. WiFi, bursts, and buffer incoming packets while building each burst. Wi-
PON and cable all involve such packet aggregation, whereas fixed Fi, PON and cable all involve such packet aggregation, whereas fixed
Ethernet and DSL do not. No sender, whether L4S or not, can do Ethernet and DSL do not. No sender, whether L4S or not, can do
anything to reduce the buffering needed for packet aggregation. So anything to reduce the buffering needed for packet aggregation. So
an AQM should not count this buffering as part of the queue that it an AQM should not count this buffering as part of the queue that it
controls, given no amount of congestion signals will reduce it. controls, given no amount of congestion signals will reduce it.
Certain link technologies also add buffering for other reasons, Certain link technologies also add buffering for other reasons,
specifically: specifically:
* Radio links (cellular, WiFi, satellite) that are distant from the * Radio links (cellular, Wi-Fi, satellite) that are distant from the
source are particularly challenging. The radio link capacity can source are particularly challenging. The radio link capacity can
vary rapidly by orders of magnitude, so it is considered desirable vary rapidly by orders of magnitude, so it is considered desirable
to hold a standing queue that can utilize sudden increases of to hold a standing queue that can utilize sudden increases of
capacity; capacity;
* Cellular networks are further complicated by a perceived need to * Cellular networks are further complicated by a perceived need to
buffer in order to make hand-overs imperceptible; buffer in order to make hand-overs imperceptible;
L4S cannot remove the need for all these different forms of L4S cannot remove the need for all these different forms of
buffering. However, by removing 'the longest pole in the tent' buffering. However, by removing 'the longest pole in the tent'
(buffering for the large sawteeth of Classic congestion controls), (buffering for the large sawteeth of Classic congestion controls),
L4S exposes all these 'shorter poles' to greater scrutiny. L4S exposes all these 'shorter poles' to greater scrutiny.
Until now, the buffering needed for these additional reasons tended Until now, the buffering needed for these additional reasons tended
to be over-specified - with the excuse that none were 'the longest to be over-specified - with the excuse that none were 'the longest
pole in the tent'. But having removed the 'longest pole', it becomes pole in the tent'. But having removed the 'longest pole', it becomes
worthwhile to minimize them, for instance reducing packet aggregation worthwhile to minimize them, for instance reducing packet aggregation
burst sizes and MAC scheduling intervals. burst sizes and MAC scheduling intervals.
Also certain link types, particularly radio-based links, are far more
prone to transmission losses. Section 6.4.3 explains how an L4S
response to loss has to be as drastic as a Classic response.
Nonetheless, research referred to in the same section has
demonstrated potential for considerably more effective loss repair at
the link layer, due to the relaxed ordering constraints of L4S
packets.
6.4. Deployment Considerations 6.4. Deployment Considerations
L4S AQMs, whether DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] or FQ, L4S AQMs, whether DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] or FQ,
e.g. [RFC8290] are, in themselves, an incremental deployment e.g. [RFC8290] are, in themselves, an incremental deployment
mechanism for L4S - so that L4S traffic can coexist with existing mechanism for L4S - so that L4S traffic can coexist with existing
Classic (Reno-friendly) traffic. Section 6.4.1 explains why only Classic (Reno-friendly) traffic. Section 6.4.1 explains why only
deploying an L4S AQM in one node at each end of the access link will deploying an L4S AQM in one node at each end of the access link will
realize nearly all the benefit of L4S. realize nearly all the benefit of L4S.
L4S involves both end systems and the network, so Section 6.4.2 L4S involves both end systems and the network, so Section 6.4.2
skipping to change at page 26, line 14 skipping to change at page 26, line 33
Deployment in mesh topologies depends on how overbooked the core is. Deployment in mesh topologies depends on how overbooked the core is.
If the core is non-blocking, or at least generously provisioned so If the core is non-blocking, or at least generously provisioned so
that the edges are nearly always the bottlenecks, it would only be that the edges are nearly always the bottlenecks, it would only be
necessary to deploy an L4S AQM at the edge bottlenecks. For example, necessary to deploy an L4S AQM at the edge bottlenecks. For example,
some data-centre networks are designed with the bottleneck in the some data-centre networks are designed with the bottleneck in the
hypervisor or host NICs, while others bottleneck at the top-of-rack hypervisor or host NICs, while others bottleneck at the top-of-rack
switch (both the output ports facing hosts and those facing the switch (both the output ports facing hosts and those facing the
core). core).
An L4S AQM would often next be needed where the WiFi links in a home An L4S AQM would often next be needed where the Wi-Fi links in a home
sometimes become the bottleneck. And an L4S AQM would eventually sometimes become the bottleneck. And an L4S AQM would eventually
also need to be deployed at any other persistent bottlenecks such as also need to be deployed at any other persistent bottlenecks such as
network interconnections, e.g. some public Internet exchange points network interconnections, e.g. some public Internet exchange points
and the ingress and egress to WAN links interconnecting data-centres. and the ingress and egress to WAN links interconnecting data-centres.
6.4.2. Deployment Sequences 6.4.2. Deployment Sequences
For any one L4S flow to provide benefit, it requires three (or For any one L4S flow to provide benefit, it requires three (or
sometimes two) parts to have been deployed: i) the congestion control sometimes two) parts to have been deployed: i) the congestion control
at the sender; ii) the AQM at the bottleneck; and iii) older at the sender; ii) the AQM at the bottleneck; and iii) older
skipping to change at page 27, line 31 skipping to change at page 28, line 4
+-+--------------------+----------------------+---------------------+ +-+--------------------+----------------------+---------------------+
|2| Upgrade DCTCP to | |Replace DCTCP feedb'k| |2| Upgrade DCTCP to | |Replace DCTCP feedb'k|
| | TCP Prague | | with AccECN | | | TCP Prague | | with AccECN |
| | FULLY WORKS DOWNSTREAM | | | FULLY WORKS DOWNSTREAM |
+-+--------------------+----------------------+---------------------+ +-+--------------------+----------------------+---------------------+
| | | | Upgrade DCTCP to | | | | | Upgrade DCTCP to |
|3| | Add L4S AQM upstream | TCP Prague | |3| | Add L4S AQM upstream | TCP Prague |
| | | | | | | | | |
| | FULLY WORKS UPSTREAM AND DOWNSTREAM | | | FULLY WORKS UPSTREAM AND DOWNSTREAM |
+-+--------------------+----------------------+---------------------+ +-+--------------------+----------------------+---------------------+
Figure 3: Example L4S Deployment Sequence Figure 3: Example L4S Deployment Sequence
Figure 3 illustrates some example sequences in which the parts of L4S Figure 3 illustrates some example sequences in which the parts of L4S
might be deployed. It consists of the following stages: might be deployed. It consists of the following stages, preceded by
a presumption that DCTCP is already installed at both ends:
1. Here, the immediate benefit of a single AQM deployment can be 1. DCTCP is not applicable for use over the public Internet, so it
seen, but limited to a controlled trial or controlled deployment. is emphasized here that any DCTCP flow has to be completely
In this example downstream deployment is first, but in other contained within a controlled trial environment.
scenarios the upstream might be deployed first. If no AQM at all
was previously deployed for the downstream access, an L4S AQM Within this trial environment, once an L4S AQM has been deployed,
greatly improves the Classic service (as well as adding the L4S the trial DCTCP flow will experience immediate benefit, without
service). If an AQM was already deployed, the Classic service any other deployment being needed. In this example downstream
will be unchanged (and L4S will add an improvement on top). deployment is first, but in other scenarios the upstream might be
deployed first. If no AQM at all was previously deployed for the
downstream access, an L4S AQM greatly improves the Classic
service (as well as adding the L4S service). If an AQM was
already deployed, the Classic service will be unchanged (and L4S
will add an improvement on top).
2. In this stage, the name 'TCP 2. In this stage, the name 'TCP
Prague' [I-D.briscoe-iccrg-prague-congestion-control] is used to Prague' [I-D.briscoe-iccrg-prague-congestion-control] is used to
represent a variant of DCTCP that is designed to be used in a represent a variant of DCTCP that is designed to be used in a
production Internet environment (assuming it complies with the production Internet environment (that is, it has to comply with
requirements in Section 4 of the L4S ECN all the requirements in Section 4 of the L4S ECN
spec [I-D.ietf-tsvwg-ecn-l4s-id]). If the application is spec [I-D.ietf-tsvwg-ecn-l4s-id], which then means it can be used
primarily unidirectional, 'TCP Prague' at one end will provide over the public Internet). If the application is primarily
all the benefit needed. unidirectional, 'TCP Prague' at one end will provide all the
benefit needed.
For TCP transports, Accurate ECN feedback For TCP transports, Accurate ECN feedback
(AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the other end, (AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the other end,
but it is a generic ECN feedback facility that is already planned but it is a generic ECN feedback facility that is already planned
to be deployed for other purposes, e.g. DCTCP, BBR. The two ends to be deployed for other purposes, e.g. DCTCP, BBR. The two ends
can be deployed in either order, because, in TCP, an L4S can be deployed in either order, because, in TCP, an L4S
congestion control only enables itself if it has negotiated the congestion control only enables itself if it has negotiated the
use of AccECN feedback with the other end during the connection use of AccECN feedback with the other end during the connection
handshake. Thus, deployment of TCP Prague on a server enables handshake. Thus, deployment of TCP Prague on a server enables
L4S trials to move to a production service in one direction, L4S trials to move to a production service in one direction,
skipping to change at page 28, line 35 skipping to change at page 29, line 7
Prague relative to DCTCP (see Appendix A.2 of the L4S ECN Prague relative to DCTCP (see Appendix A.2 of the L4S ECN
spec [I-D.ietf-tsvwg-ecn-l4s-id]). spec [I-D.ietf-tsvwg-ecn-l4s-id]).
Unlike TCP, from the outset, QUIC ECN feedback [RFC9000] has Unlike TCP, from the outset, QUIC ECN feedback [RFC9000] has
supported L4S. Therefore, if the transport is QUIC, one-ended supported L4S. Therefore, if the transport is QUIC, one-ended
deployment of a Prague congestion control at this stage is simple deployment of a Prague congestion control at this stage is simple
and sufficient. and sufficient.
For QUIC, if a proxy sits in the path between multiple origin For QUIC, if a proxy sits in the path between multiple origin
servers and the access bottlenecks to multiple clients, then servers and the access bottlenecks to multiple clients, then
upgrading the proxy to a Scalable CC would provide the benefits upgrading the proxy with a Scalable congestion control would
of L4S over all the clients' downstream bottlenecks in one go --- provide the benefits of L4S over all the clients' downstream
whether or not all the origin servers were upgraded. Conversely, bottlenecks in one go --- whether or not all the origin servers
where a proxy has not been upgraded, the clients served by it were upgraded. Conversely, where a proxy has not been upgraded,
will not benefit from L4S at all in the downstream, even when any the clients served by it will not benefit from L4S at all in the
origin server behind the proxy has been upgraded to support L4S. downstream, even when any origin server behind the proxy has been
upgraded to support L4S.
For TCP, a proxy upgraded to support 'TCP Prague' would provide For TCP, a proxy upgraded to support 'TCP Prague' would provide
the benefits of L4S downstream to all clients that support AccECN the benefits of L4S downstream to all clients that support AccECN
(whether or not they support L4S as well). And in the upstream, (whether or not they support L4S as well). And in the upstream,
the proxy would also support AccECN as a receiver, so that any the proxy would also support AccECN as a receiver, so that any
client deploying its own L4S support would benefit in the client deploying its own L4S support would benefit in the
upstream direction, irrespective of whether any origin server upstream direction, irrespective of whether any origin server
beyond the proxy supported AccECN. beyond the proxy supported AccECN.
3. This is a two-move stage to enable L4S upstream. An L4S AQM or 3. This is a two-move stage to enable L4S upstream. An L4S AQM or
skipping to change at page 30, line 39 skipping to change at page 31, line 9
[I-D.ietf-tsvwg-ecn-encap-guidelines]. [I-D.ietf-tsvwg-ecn-encap-guidelines].
7. IANA Considerations (to be removed by RFC Editor) 7. IANA Considerations (to be removed by RFC Editor)
This specification contains no IANA considerations. This specification contains no IANA considerations.
8. Security Considerations 8. Security Considerations
8.1. Traffic Rate (Non-)Policing 8.1. Traffic Rate (Non-)Policing
In the current Internet, scheduling usually enforces separation 8.1.1. (Non-)Policing Rate per Flow
between 'sites' (e.g. households, businesses or mobile
users [RFC0970]) and various techniques like redirection to traffic
scrubbing facilities deal with flooding attacks. However, there has
never been a universal need to police the rate of individual
application flows - the Internet has generally always relied on self-
restraint of congestion controls at senders for sharing intra-'site'
capacity.
As explained in Section 5.2, the DualQ variant of L4S provides low In the current Internet, ISPs usually enforce separation between the
delay without prejudging the issue of flow-rate control. Then, if capacity of shared links assigned to different 'sites'
flow-rate control is needed, per-flow-queuing (FQ) can be used (e.g. households, businesses or mobile users - see terminology in
instead, or flow rate policing can be added as a modular addition to Section 3) using some form of scheduler [RFC0970]. And they use
a DualQ. various techniques like redirection to traffic scrubbing facilities
to deal with flooding attacks. However, there has never been a
universal need to police the rate of individual application flows -
the Internet has generally always relied on self-restraint of
congestion controls at senders for sharing intra-'site' capacity.
Because the L4S service reduces delay without increasing the delay of L4S has been designed not to upset this status quo. If a DualQ is
Classic traffic, it should not be necessary to rate-police access to used to provide L4S service, section 4.2 of
the L4S service. In contrast, Section 5.2 explains how Diffserv only [I-D.ietf-tsvwg-aqm-dualq-coupled] explains how it is designed to
makes a difference if some packets get less favourable treatment than give no more rate advantage to unresponsive flows than a single-queue
others, which typically requires traffic rate policing, which can, in AQM would, whether or not there is traffic overload.
turn, lead to further complexity such as traffic contracts at trust
boundaries. Because L4S avoids this management complexity, it is Also, in case per-flow rate policing is ever required, it can be
more likely to work end-to-end. added because it is orthogonal to the distinction between L4S and
Classic. As explained in Section 5.2, the DualQ variant of L4S
provides low delay without prejudging the issue of flow-rate control.
So, if flow-rate control is needed, per-flow-queuing (FQ) with L4S
support can be used instead, or flow rate policing can be added as a
modular addition to a DualQ. However, per-flow rate control is not
usually deployed as a security mechanism, because an active attacker
can just shard its traffic over more flow IDs if the rate of each is
restricted.
8.1.2. (Non-)Policing L4S Service Rate
Section 5.2 explains how Diffserv only makes a difference if some
packets get less favourable treatment than others, which typically
requires traffic rate policing for a low latency class. In contrast,
it should not be necessary to rate-police access to the L4S service
to protect the Classic service, because L4S is designed to reduce
delay without harming the delay or rate of any Classic traffic.
During early deployment (and perhaps always), some networks will not During early deployment (and perhaps always), some networks will not
offer the L4S service. In general, these networks should not need to offer the L4S service. In general, these networks should not need to
police L4S traffic. They are required (by both the ECN police L4S traffic. They are required (by both the ECN
spec [RFC3168] and the L4S ECN spec [I-D.ietf-tsvwg-ecn-l4s-id]) not spec [RFC3168] and the L4S ECN spec [I-D.ietf-tsvwg-ecn-l4s-id]) not
to change the L4S identifier, which would interfere with end-to-end to change the L4S identifier, which would interfere with end-to-end
congestion control. If they already treat ECN traffic as Not-ECT, congestion control. If they already treat ECN traffic as Not-ECT,
they can merely treat L4S traffic as Not-ECT too. At a bottleneck, they can merely treat L4S traffic as Not-ECT too. At a bottleneck,
such networks will introduce some queuing and dropping. When a such networks will introduce some queuing and dropping. When a
scalable congestion control detects a drop it will have to respond scalable congestion control detects a drop it will have to respond
skipping to change at page 32, line 6 skipping to change at page 32, line 24
In cases that are expected to be rare, networks that solely support In cases that are expected to be rare, networks that solely support
Classic ECN [RFC3168] in a single queue bottleneck might opt to Classic ECN [RFC3168] in a single queue bottleneck might opt to
police L4S traffic so as to protect competing Classic ECN traffic police L4S traffic so as to protect competing Classic ECN traffic
(for instance, see Section 6.1.3 of the L4S operational (for instance, see Section 6.1.3 of the L4S operational
guidance [I-D.ietf-tsvwg-l4sops]). However, Section 4.3 of the L4S guidance [I-D.ietf-tsvwg-l4sops]). However, Section 4.3 of the L4S
ECN spec [I-D.ietf-tsvwg-ecn-l4s-id] recommends that the sender ECN spec [I-D.ietf-tsvwg-ecn-l4s-id] recommends that the sender
adapts its congestion response to properly coexist with Classic ECN adapts its congestion response to properly coexist with Classic ECN
flows, i.e. reverting to the self-restraint approach. flows, i.e. reverting to the self-restraint approach.
Certain network operators might choose to restrict access to the L4S Certain network operators might choose to restrict access to the L4S
class, perhaps only to selected premium customers as a value-added service, perhaps only to selected premium customers as a value-added
service. Their packet classifier (item 2 in Figure 1) could identify service. Their packet classifier (item 2 in Figure 1) could identify
such customers against some other field (e.g. source address range) such customers against some other field (e.g. source address range)
as well as classifying on the ECN field. If only the ECN L4S as well as classifying on the ECN field. If only the ECN L4S
identifier matched, but not the source address (say), the classifier identifier matched, but not the source address (say), the classifier
could direct these packets (from non-premium customers) into the could direct these packets (from non-premium customers) into the
Classic queue. Explaining clearly how operators can use an Classic queue. Explaining clearly how operators can use additional
additional local classifiers (see section 5.4 of the L4S ECN local classifiers (see section 5.4 of the L4S ECN
spec [I-D.ietf-tsvwg-ecn-l4s-id]) is intended to remove any spec [I-D.ietf-tsvwg-ecn-l4s-id]) is intended to remove any
motivation to clear the L4S identifier. Then at least the L4S ECN motivation to clear the L4S identifier. Then at least the L4S ECN
identifier will be more likely to survive end-to-end even though the identifier will be more likely to survive end-to-end even though the
service may not be supported at every hop. Such local arrangements service may not be supported at every hop. Such local arrangements
would only require simple registered/not-registered packet would only require simple registered/not-registered packet
classification, rather than the managed, application-specific traffic classification, rather than the managed, application-specific traffic
policing against customer-specific traffic contracts that Diffserv policing against customer-specific traffic contracts that Diffserv
uses. uses.
8.2. 'Latency Friendliness' 8.2. 'Latency Friendliness'
Like the Classic service, the L4S service relies on self-restraint - Like the Classic service, the L4S service relies on self-restraint -
limiting rate in response to congestion. In addition, the L4S limiting rate in response to congestion. In addition, the L4S
service requires self-restraint in terms of limiting latency service requires self-restraint in terms of limiting latency
(burstiness). It is hoped that self-interest and guidance on dynamic (burstiness). It is hoped that self-interest and guidance on dynamic
behaviour (especially flow start-up, which might need to be behaviour (especially flow start-up, which might need to be
standardized) will be sufficient to prevent transports from sending standardized) will be sufficient to prevent transports from sending
excessive bursts of L4S traffic, given the application's own latency excessive bursts of L4S traffic, given the application's own latency
will suffer most from such behaviour. will suffer most from such behaviour.
Whether burst policing becomes necessary remains to be seen. Without Because the L4S service can reduce delay without discernibly
it, there will be potential for attacks on the low latency of the L4S increasing the delay of any Classic traffic, it should not be
service. necessary to police L4S traffic to protect the delay of Classic.
However, whether burst policing becomes necessary to protect other
L4S traffic remains to be seen. Without it, there will be potential
for attacks on the low latency of the L4S service.
If needed, various arrangements could be used to address this If needed, various arrangements could be used to address this
concern: concern:
Local bottleneck queue protection: A per-flow (5-tuple) queue Local bottleneck queue protection: A per-flow (5-tuple) queue
protection function [I-D.briscoe-docsis-q-protection] has been protection function [I-D.briscoe-docsis-q-protection] has been
developed for the low latency queue in DOCSIS, which has adopted developed for the low latency queue in DOCSIS, which has adopted
the DualQ L4S architecture. It protects the low latency service the DualQ L4S architecture. It protects the low latency service
from any queue-building flows that accidentally or maliciously from any queue-building flows that accidentally or maliciously
classify themselves into the low latency queue. It is designed to classify themselves into the low latency queue. It is designed to
skipping to change at page 33, line 18 skipping to change at page 33, line 40
malware, in a similar way to how traffic from flooding attack malware, in a similar way to how traffic from flooding attack
sources is rerouted via scrubbing facilities. sources is rerouted via scrubbing facilities.
Local bottleneck per-flow scheduling: Per-flow scheduling should Local bottleneck per-flow scheduling: Per-flow scheduling should
inherently isolate non-bursty flows from bursty (see Section 5.2 inherently isolate non-bursty flows from bursty (see Section 5.2
for discussion of the merits of per-flow scheduling relative to for discussion of the merits of per-flow scheduling relative to
per-flow policing). per-flow policing).
Distributed access subnet queue protection: Per-flow queue Distributed access subnet queue protection: Per-flow queue
protection could be arranged for a queue structure distributed protection could be arranged for a queue structure distributed
across a subnet inter-communicating using lower layer control across a subnet intercommunicating using lower layer control
messages (see Section 2.1.4 of [QDyn]). For instance, in a radio messages (see Section 2.1.4 of [QDyn]). For instance, in a radio
access network, user equipment already sends regular buffer status access network, user equipment already sends regular buffer status
reports to a radio network controller, which could use this reports to a radio network controller, which could use this
information to remotely police individual flows. information to remotely police individual flows.
Distributed Congestion Exposure to Ingress Policers: The Congestion Distributed Congestion Exposure to Ingress Policers: The Congestion
Exposure (ConEx) architecture [RFC7713] uses egress audit to Exposure (ConEx) architecture [RFC7713] uses egress audit to
motivate senders to truthfully signal path congestion in-band motivate senders to truthfully signal path congestion in-band
where it can be used by ingress policers. An edge-to-edge variant where it can be used by ingress policers. An edge-to-edge variant
of this architecture is also possible. of this architecture is also possible.
skipping to change at page 33, line 45 skipping to change at page 34, line 18
Distributed core network queue protection: The policing function Distributed core network queue protection: The policing function
could be divided between per-flow mechanisms at the network could be divided between per-flow mechanisms at the network
ingress that characterize the burstiness of each flow into a ingress that characterize the burstiness of each flow into a
signal carried with the traffic, and per-class mechanisms at signal carried with the traffic, and per-class mechanisms at
bottlenecks that act on these signals if queuing actually occurs bottlenecks that act on these signals if queuing actually occurs
once the traffic converges. This would be somewhat similar to once the traffic converges. This would be somewhat similar to
[Nadas20], which is in turn similar to the idea behind core [Nadas20], which is in turn similar to the idea behind core
stateless fair queuing. stateless fair queuing.
None of these possible queue protection capabilities are considered a No single one of these possible queue protection capabilities is
necessary part of the L4S architecture, which works without them (in considered an essential part of the L4S architecture, which works
a similar way to how the Internet works without per-flow rate without any of them under non-attack conditions (much as the Internet
policing). Indeed, even where latency policers are deployed, under normally works without per-flow rate policing). Indeed, even where
normal circumstances they would not intervene, and if operators found latency policers are deployed, under normal circumstances they would
they were not necessary they could disable them. Part of the L4S not intervene, and if operators found they were not necessary they
experiment will be to see whether such a function is necessary, and could disable them. Part of the L4S experiment will be to see
which arrangements are most appropriate to the size of the problem. whether such a function is necessary, and which arrangements are most
appropriate to the size of the problem.
8.3. Interaction between Rate Policing and L4S 8.3. Interaction between Rate Policing and L4S
As mentioned in Section 5.2, L4S should remove the need for low As mentioned in Section 5.2, L4S should remove the need for low
latency Diffserv classes. However, those Diffserv classes that give latency Diffserv classes. However, those Diffserv classes that give
certain applications or users priority over capacity, would still be certain applications or users priority over capacity, would still be
applicable in certain scenarios (e.g. corporate networks). Then, applicable in certain scenarios (e.g. corporate networks). Then,
within such Diffserv classes, L4S would often be applicable to give within such Diffserv classes, L4S would often be applicable to give
traffic low latency and low loss as well. Within such a Diffserv traffic low latency and low loss as well. Within such a Diffserv
class, the bandwidth available to a user or application is often class, the bandwidth available to a user or application is often
limited by a rate policer. Similarly, in the default Diffserv class, limited by a rate policer. Similarly, in the default Diffserv class,
rate policers are used to partition shared capacity. rate policers are sometimes used to partition shared capacity.
A classic rate policer drops any packets exceeding a set rate, A classic rate policer drops any packets exceeding a set rate,
usually also giving a burst allowance (variants exist where the usually also giving a burst allowance (variants exist where the
policer re-marks non-compliant traffic to a discard-eligible Diffserv policer re-marks non-compliant traffic to a discard-eligible Diffserv
codepoint, so they can be dropped elsewhere during contention). codepoint, so they can be dropped elsewhere during contention).
Whenever L4S traffic encounters one of these rate policers, it will Whenever L4S traffic encounters one of these rate policers, it will
experience drops and the source will have to fall back to a Classic experience drops and the source will have to fall back to a Classic
congestion control, thus losing the benefits of L4S (Section 6.4.3). congestion control, thus losing the benefits of L4S (Section 6.4.3).
So, in networks that already use rate policers and plan to deploy So, in networks that already use rate policers and plan to deploy
L4S, it will be preferable to redesign these rate policers to be more L4S, it will be preferable to redesign these rate policers to be more
skipping to change at page 34, line 48 skipping to change at page 35, line 25
possible to design scalable congestion controls to respond less possible to design scalable congestion controls to respond less
catastrophically to loss that has not been preceded by a period of catastrophically to loss that has not been preceded by a period of
increasing delay. increasing delay.
The design of L4S-friendly rate policers will require a separate The design of L4S-friendly rate policers will require a separate
dedicated document. For further discussion of the interaction dedicated document. For further discussion of the interaction
between L4S and Diffserv, see [I-D.briscoe-tsvwg-l4s-diffserv]. between L4S and Diffserv, see [I-D.briscoe-tsvwg-l4s-diffserv].
8.4. ECN Integrity 8.4. ECN Integrity
Receiving hosts can fool a sender into downloading faster by Various ways have been developed to protect the integrity of the
suppressing feedback of ECN marks (or of losses if retransmissions congestion feedback loop (whether signalled by loss, Classic ECN or
are not necessary or available otherwise). Various ways to protect L4S ECN) against misbehaviour by the receiver, sender or network (or
transport feedback integrity have been developed. For instance: all three). Brief details of each including applicability, pros and
cons is given in Appendix C.1 of the L4S ECN
* The sender can test the integrity of the receiver's feedback by spec [I-D.ietf-tsvwg-ecn-l4s-id].
occasionally setting the IP-ECN field to the congestion
experienced (CE) codepoint, which is normally only set by a
congested link. Then the sender can test whether the receiver's
feedback faithfully reports what it expects (see 2nd para of
Section 20.2 of the Classic ECN spec [RFC3168]).
* A network can enforce a congestion response to its ECN markings
(or packet losses) by auditing congestion exposure
(ConEx) [RFC7713].
* Transport layer authentication such as the TCP authentication
option (TCP-AO [RFC5925]) or QUIC's use of TLS [RFC9001] can
detect any tampering with congestion feedback.
* The ECN Nonce [RFC3540] was proposed to detect tampering with
congestion feedback, but it has been reclassified as
historic [RFC8311].
Appendix C.1 of the L4S ECN spec [I-D.ietf-tsvwg-ecn-l4s-id] gives
more details of these techniques including their applicability and
pros and cons.
8.5. Privacy Considerations 8.5. Privacy Considerations
As discussed in Section 5.2, the L4S architecture does not preclude As discussed in Section 5.2, the L4S architecture does not preclude
approaches that inspect end-to-end transport layer identifiers. For approaches that inspect end-to-end transport layer identifiers. For
instance, L4S support has been added to FQ-CoDel, which classifies by instance, L4S support has been added to FQ-CoDel, which classifies by
application flow ID in the network. However, the main innovation of application flow ID in the network. However, the main innovation of
L4S is the DualQ AQM framework that does not need to inspect any L4S is the DualQ AQM framework that does not need to inspect any
deeper than the outermost IP header, because the L4S identifier is in deeper than the outermost IP header, because the L4S identifier is in
the IP-ECN field. the IP-ECN field.
skipping to change at page 36, line 13 skipping to change at page 36, line 17
privacy. privacy.
9. Informative References 9. Informative References
[AFCD] Xue, L., Kumar, S., Cui, C., Kondikoppa, P., Chiu, C-H., [AFCD] Xue, L., Kumar, S., Cui, C., Kondikoppa, P., Chiu, C-H.,
and S-J. Park, "Towards fair and low latency next and S-J. Park, "Towards fair and low latency next
generation high speed networks: AFCD queuing", Journal of generation high speed networks: AFCD queuing", Journal of
Network and Computer Applications 70:183--193, July 2016, Network and Computer Applications 70:183--193, July 2016,
<https://doi.org/10.1016/j.jnca.2016.03.021>. <https://doi.org/10.1016/j.jnca.2016.03.021>.
[BBRv2] Cardwell, N., "TCP BBR v2 Alpha/Preview Release", github [BBRv2] Cardwell, N., "TCP 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>.
[BDPdata] Briscoe, B., "PI2 Parameters", Technical Report TR-BB- [BDPdata] 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>.
[BufferSize] [BufferSize]
Appenzeller, G., Keslassy, I., and N. McKeown, "Sizing Appenzeller, G., Keslassy, I., and N. McKeown, "Sizing
Router Buffers", In Proc. SIGCOMM'04 34(4):281--292, Router Buffers", In Proc. SIGCOMM'04 34(4):281--292,
skipping to change at page 36, line 49 skipping to change at page 37, line 5
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® 3.1 Version i17
or later, 21 January 2019, <https://specification- or later, 21 January 2019, <https://specification-
search.cablelabs.com/CM-SP-MULPIv3.1>. search.cablelabs.com/CM-SP-MULPIv3.1>.
[DOCSIS3AQM] [DOCSIS3AQM]
White, G., "Active Queue Management Algorithms for DOCSIS White, G., "Active Queue Management Algorithms for DOCSIS
3.0; A Simulation Study of CoDel, SFQ-CoDel and PIE in 3.0; A Simulation Study of CoDel, SFQ-CoDel and PIE in
DOCSIS 3.0 Networks", CableLabs Technical Report , April DOCSIS 3.0 Networks", CableLabs Technical Report , April
2013, <{http://www.cablelabs.com/wp- 2013, <{https://www.cablelabs.com/wp-
content/uploads/2013/11/ content/uploads/2013/11/
Active_Queue_Management_Algorithms_DOCSIS_3_0.pdf>. Active_Queue_Management_Algorithms_DOCSIS_3_0.pdf>.
[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>.
skipping to change at page 37, line 29 skipping to change at page 37, line 33
Høiland-Jørgensen, T., "fq_codel: generalise ce_threshold Høiland-Jørgensen, T., "fq_codel: generalise ce_threshold
marking for subset of traffic", Linux Patch Commit ID: marking for subset of traffic", Linux Patch Commit ID:
dfcb63ce1de6b10b, 20 October 2021, dfcb63ce1de6b10b, 20 October 2021,
<https://git.kernel.org/pub/scm/linux/kernel/git/netdev/ <https://git.kernel.org/pub/scm/linux/kernel/git/netdev/
net-next.git/commit/?id=dfcb63ce1de6b10b>. net-next.git/commit/?id=dfcb63ce1de6b10b>.
[Hohlfeld14] [Hohlfeld14]
Hohlfeld, O., Pujol, E., Ciucu, F., Feldmann, A., and P. Hohlfeld, O., Pujol, E., Ciucu, F., Feldmann, A., and P.
Barford, "A QoE Perspective on Sizing Network Buffers", Barford, "A QoE Perspective on Sizing Network Buffers",
Proc. ACM Internet Measurement Conf (IMC'14) hmm, November Proc. ACM Internet Measurement Conf (IMC'14) hmm, November
2014, <http://doi.acm.org/10.1145/2663716.2663730>. 2014, <https://doi.acm.org/10.1145/2663716.2663730>.
[I-D.briscoe-conex-policing] [I-D.briscoe-conex-policing]
Briscoe, B., "Network Performance Isolation using Briscoe, B., "Network Performance Isolation using
Congestion Policing", Work in Progress, Internet-Draft, Congestion Policing", Work in Progress, Internet-Draft,
draft-briscoe-conex-policing-01, 14 February 2014, draft-briscoe-conex-policing-01, 14 February 2014,
<https://datatracker.ietf.org/doc/html/draft-briscoe- <https://www.ietf.org/archive/id/draft-briscoe-conex-
conex-policing-01>. policing-01.txt>.
[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, "The DOCSIS(r) Queue Protection
Algorithm to Preserve Low Latency", Work in Progress, Algorithm to Preserve Low Latency", Work in Progress,
Internet-Draft, draft-briscoe-docsis-q-protection-06, 13 Internet-Draft, draft-briscoe-docsis-q-protection-06, 13
May 2022, <https://datatracker.ietf.org/doc/html/draft- May 2022,
briscoe-docsis-q-protection-06>. <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", Work in Progress, Internet-Draft,
draft-briscoe-iccrg-prague-congestion-control-01, 11 July draft-briscoe-iccrg-prague-congestion-control-01, 11 July
2022, <https://datatracker.ietf.org/doc/html/draft- 2022, <https://datatracker.ietf.org/api/v1/doc/document/
briscoe-iccrg-prague-congestion-control-01>. draft-briscoe-iccrg-prague-congestion-control/>.
[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- Work in Progress, Internet-Draft, draft-briscoe-tsvwg-l4s-
diffserv-02, 4 November 2018, diffserv-02, 2 July 2018,
<https://datatracker.ietf.org/doc/html/draft-briscoe- <https://datatracker.ietf.org/api/v1/doc/document/draft-
tsvwg-l4s-diffserv-02>. 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. H., Swett, I., and V.
Jacobson, "BBR Congestion Control", Work in Progress, Jacobson, "BBR Congestion Control", Work in Progress,
Internet-Draft, draft-cardwell-iccrg-bbr-congestion- Internet-Draft, draft-cardwell-iccrg-bbr-congestion-
control-02, 7 March 2022, control-02, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-cardwell- <https://datatracker.ietf.org/api/v1/doc/document/draft-
iccrg-bbr-congestion-control-02>. cardwell-iccrg-bbr-congestion-control/>.
[I-D.ietf-tcpm-accurate-ecn] [I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", Work in Progress, Internet- Accurate ECN Feedback in TCP", Work in Progress, Internet-
Draft, draft-ietf-tcpm-accurate-ecn-20, 25 July 2022, Draft, draft-ietf-tcpm-accurate-ecn-20, 25 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm- <https://datatracker.ietf.org/api/v1/doc/document/draft-
accurate-ecn-20>. ietf-tcpm-accurate-ecn/>.
[I-D.ietf-tsvwg-aqm-dualq-coupled] [I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", Work in Progress, Internet-Draft, draft-ietf- (L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-aqm-dualq-coupled-24, 7 July 2022, tsvwg-aqm-dualq-coupled-24, 7 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg- <https://datatracker.ietf.org/api/v1/doc/document/draft-
aqm-dualq-coupled-24>. ietf-tsvwg-aqm-dualq-coupled/>.
[I-D.ietf-tsvwg-ecn-encap-guidelines] [I-D.ietf-tsvwg-ecn-encap-guidelines]
Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
Congestion Notification to Protocols that Encapsulate IP", Congestion Notification to Protocols that Encapsulate IP",
Work in Progress, Internet-Draft, draft-ietf-tsvwg-ecn- Work in Progress, Internet-Draft, draft-ietf-tsvwg-ecn-
encap-guidelines-17, 11 July 2022, encap-guidelines-17, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg- <https://datatracker.ietf.org/api/v1/doc/document/draft-
ecn-encap-guidelines-17>. ietf-tsvwg-ecn-encap-guidelines/>.
[I-D.ietf-tsvwg-ecn-l4s-id] [I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. D. and B. Briscoe, "Explicit Congestion Schepper, K. D. and B. Briscoe, "Explicit Congestion
Notification (ECN) Protocol for Very Low Queuing Delay Notification (ECN) Protocol for Very Low Queuing Delay
(L4S)", Work in Progress, Internet-Draft, draft-ietf- (L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-ecn-l4s-id-26, 7 July 2022, tsvwg-ecn-l4s-id-28, 8 August 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg- <https://datatracker.ietf.org/api/v1/doc/document/draft-
ecn-l4s-id-26>. ietf-tsvwg-ecn-l4s-id/>.
[I-D.ietf-tsvwg-l4sops] [I-D.ietf-tsvwg-l4sops]
White, G., "Operational Guidance for Deployment of L4S in White, G., "Operational Guidance for Deployment of L4S in
the Internet", Work in Progress, Internet-Draft, draft- the Internet", Work in Progress, Internet-Draft, draft-
ietf-tsvwg-l4sops-03, 28 April 2022, ietf-tsvwg-l4sops-03, 28 April 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg- <https://datatracker.ietf.org/api/v1/doc/document/draft-
l4sops-03>. ietf-tsvwg-l4sops/>.
[I-D.ietf-tsvwg-nqb] [I-D.ietf-tsvwg-nqb]
White, G. and T. Fossati, "A Non-Queue-Building Per-Hop White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
Behavior (NQB PHB) for Differentiated Services", Work in Behavior (NQB PHB) for Differentiated Services", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-nqb-10, 4 March Progress, Internet-Draft, draft-ietf-tsvwg-nqb-10, 4 March
2022, <https://datatracker.ietf.org/doc/html/draft-ietf- 2022, <https://datatracker.ietf.org/api/v1/doc/document/
tsvwg-nqb-10>. draft-ietf-tsvwg-nqb/>.
[I-D.ietf-tsvwg-rfc6040update-shim] [I-D.ietf-tsvwg-rfc6040update-shim]
Briscoe, B., "Propagating Explicit Congestion Notification Briscoe, B., "Propagating Explicit Congestion Notification
Across IP Tunnel Headers Separated by a Shim", Work in Across IP Tunnel Headers Separated by a Shim", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-rfc6040update- Progress, Internet-Draft, draft-ietf-tsvwg-rfc6040update-
shim-15, 11 July 2022, shim-15, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg- <https://datatracker.ietf.org/api/v1/doc/document/draft-
rfc6040update-shim-15>. ietf-tsvwg-rfc6040update-shim/>.
[I-D.morton-tsvwg-codel-approx-fair] [I-D.morton-tsvwg-codel-approx-fair]
Morton, J. and P. G. Heist, "Controlled Delay Approximate Morton, J. and P. G. Heist, "Controlled Delay Approximate
Fairness AQM", Work in Progress, Internet-Draft, draft- Fairness AQM", Work in Progress, Internet-Draft, draft-
morton-tsvwg-codel-approx-fair-01, 9 March 2020, morton-tsvwg-codel-approx-fair-01, 9 March 2020,
<https://datatracker.ietf.org/doc/html/draft-morton-tsvwg- <https://www.ietf.org/archive/id/draft-morton-tsvwg-codel-
codel-approx-fair-01>. approx-fair-01.txt>.
[I-D.sridharan-tcpm-ctcp] [I-D.sridharan-tcpm-ctcp]
Sridharan, M., Tan, K., Bansal, D., and D. Thaler, Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
"Compound TCP: A New TCP Congestion Control for High-Speed "Compound TCP: A New TCP Congestion Control for High-Speed
and Long Distance Networks", Work in Progress, Internet- and Long Distance Networks", Work in Progress, Internet-
Draft, draft-sridharan-tcpm-ctcp-02, 11 November 2008, Draft, draft-sridharan-tcpm-ctcp-02, 29 October 2007,
<https://datatracker.ietf.org/doc/html/draft-sridharan- <https://datatracker.ietf.org/api/v1/doc/document/draft-
tcpm-ctcp-02>. sridharan-tcpm-ctcp/>.
[I-D.stewart-tsvwg-sctpecn] [I-D.stewart-tsvwg-sctpecn]
Stewart, R. R., Tuexen, M., and X. Dong, "ECN for Stream Stewart, R. R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", Work in Progress, Control Transmission Protocol (SCTP)", Work in Progress,
Internet-Draft, draft-stewart-tsvwg-sctpecn-05, 15 January Internet-Draft, draft-stewart-tsvwg-sctpecn-05, 15 January
2014, <https://datatracker.ietf.org/doc/html/draft- 2014, <https://www.ietf.org/archive/id/draft-stewart-
stewart-tsvwg-sctpecn-05>. tsvwg-sctpecn-05.txt>.
[L4Sdemo16] [L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B. Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live Briscoe, "Ultra-Low Delay for All: Live Experience, Live
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016, Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<http://dl.acm.org/citation.cfm?doid=2910017.2910633 <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 )>.
[LEDBAT_AQM] [LEDBAT_AQM]
Al-Saadi, R., Armitage, G., and J. But, "Characterising Al-Saadi, R., Armitage, G., and J. But, "Characterising
LEDBAT Performance Through Bottlenecks Using PIE, FQ-CoDel LEDBAT Performance Through Bottlenecks Using PIE, FQ-CoDel
and FQ-PIE Active Queue Management", Proc. IEEE 42nd and FQ-PIE Active Queue Management", Proc. IEEE 42nd
Conference on Local Computer Networks (LCN) 278--285, Conference on Local Computer Networks (LCN) 278--285,
2017, <https://ieeexplore.ieee.org/document/8109367>. 2017, <https://ieeexplore.ieee.org/document/8109367>.
skipping to change at page 40, line 37 skipping to change at page 40, line 45
McIlroy, M.D., Pinson, E. N., and B. A. Tague, "UNIX Time- McIlroy, M.D., Pinson, E. N., and B. A. Tague, "UNIX Time-
Sharing System: Foreword", The Bell System Technical Sharing System: Foreword", The Bell System Technical
Journal 57:6(1902--1903), July 1978, Journal 57:6(1902--1903), July 1978,
<https://archive.org/details/bstj57-6-1899>. <https://archive.org/details/bstj57-6-1899>.
[Nadas20] Nádas, S., Gombos, G., Fejes, F., and S. Laki, "A [Nadas20] Nádas, S., Gombos, G., Fejes, F., and S. Laki, "A
Congestion Control Independent L4S Scheduler", Proc. Congestion Control Independent L4S Scheduler", Proc.
Applied Networking Research Workshop (ANRW '20) 45--51, Applied Networking Research Workshop (ANRW '20) 45--51,
July 2020, <https://doi.org/10.1145/3404868.3406669>. July 2020, <https://doi.org/10.1145/3404868.3406669>.
[NASA04] Bailey, R.R., Trey Arthur III, J.J., and S.P. Williams,
"Latency Requirements for Head-Worn Display S/EVS
Applications", SPIE Defense and Security
Symposium LF99-1955, April 2004,
<https://ntrs.nasa.gov/api/citations/20120009198/
downloads/20120009198.pdf?attachment=true>.
[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., Kühlewind, M., and A.S. 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>.
[QDyn] Briscoe, B., "Rapid Signalling of Queue Dynamics", [QDyn] Briscoe, B., "Rapid Signalling of Queue Dynamics",
bobbriscoe.net Technical Report TR-BB-2017-001; bobbriscoe.net Technical Report TR-BB-2017-001;
arXiv:1904.07044 [cs.NI], September 2017, arXiv:1904.07044 [cs.NI], September 2017,
<https://arxiv.org/abs/1904.07044>. <https://arxiv.org/abs/1904.07044>.
[Raaen14] Raaen, K. and T-M. Grønli, "Latency thresholds for
usability in games: A survey", Norsk IKT-konferanse for
forskning og utdanning , 2014,
<http://ojs.bibsys.no/index.php/NIK/article/view/9/6>.
[Rajiullah15] [Rajiullah15]
Rajiullah, M., "Towards a Low Latency Internet: Rajiullah, M., "Towards a Low Latency Internet:
Understanding and Solutions", Masters Thesis; Karlstad Understanding and Solutions", Master's Thesis; Karlstad
Uni, Dept of Maths & CS 2015:41, 2015, <https://www.diva- Uni, Dept of Maths & CS 2015:41, 2015, <https://www.diva-
portal.org/smash/get/diva2:846109/FULLTEXT01.pdf>. portal.org/smash/get/diva2:846109/FULLTEXT01.pdf>.
[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>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
skipping to change at page 43, line 11 skipping to change at page 43, line 29
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817, "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012, DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>. <https://www.rfc-editor.org/info/rfc6817>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013, DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>. <https://www.rfc-editor.org/info/rfc6973>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe, [RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy "Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback", in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC7560, August 2015, RFC 7560, DOI 10.17487/RFC7560, August 2015,
<https://www.rfc-editor.org/info/rfc7560>. <https://www.rfc-editor.org/info/rfc7560>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>. <https://www.rfc-editor.org/info/rfc7567>.
skipping to change at page 45, line 5 skipping to change at page 45, line 24
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000, Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021, DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>. <https://www.rfc-editor.org/info/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>. <https://www.rfc-editor.org/info/rfc9001>.
[SCReAM] Johansson, I., "SCReAM", github repository; , [RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[SCReAM] Johansson, I., "SCReAM", GitHub repository; ,
<https://github.com/EricssonResearch/scream/blob/master/ <https://github.com/EricssonResearch/scream/blob/master/
README.md>. README.md>.
[TCP-CA] Jacobson, V. and M.J. Karels, "Congestion Avoidance and [TCP-CA] Jacobson, V. and M.J. Karels, "Congestion Avoidance and
Control", Laurence Berkeley Labs Technical Report , Control", Laurence Berkeley Labs Technical Report ,
November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>. November 1988, <https://ee.lbl.gov/papers/congavoid.pdf>.
[UnorderedLTE] [UnorderedLTE]
Austrheim, M.V., "Implementing immediate forwarding for 4G Austrheim, M.V., "Implementing immediate forwarding for 4G
in a network simulator", Masters Thesis, Uni Oslo , June in a network simulator", Master's Thesis, Uni Oslo , June
2019. 2019.
Acknowledgements Acknowledgements
Thanks to Richard Scheffenegger, Wes Eddy, Karen Nielsen, David Thanks to Richard Scheffenegger, Wes Eddy, Karen Nielsen, David
Black, Jake Holland, Vidhi Goel, Ermin Sakic, Praveen Black, Jake Holland, Vidhi Goel, Ermin Sakic, Praveen
Balasubramanian, Gorry Fairhurst, Mirja Kuehlewind, Philip Eardley, Balasubramanian, Gorry Fairhurst, Mirja Kuehlewind, Philip Eardley,
Neal Cardwell, Pete Heist and Martin Duke for their useful review Neal Cardwell, Pete Heist and Martin Duke for their useful review
comments. Thanks also to the area reviewers: Marco Tiloca. comments. Thanks also to the area reviewers: Marco Tiloca, Lars
Eggert, Roman Danyliw and Eric Vyncke.
Bob Briscoe and Koen De Schepper were part-funded by the European Bob Briscoe and Koen De Schepper were part-funded by the European
Community under its Seventh Framework Programme through the Reducing Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700). The Internet Transport Latency (RITE) project (ICT-317700). The
contribution of Koen De Schepper was also part-funded by the 5Growth contribution of Koen De Schepper was also part-funded by the 5Growth
and DAEMON EU H2020 projects. Bob Briscoe was also part-funded by and DAEMON EU H2020 projects. Bob Briscoe was also part-funded by
the Research Council of Norway through the TimeIn project, partly by the Research Council of Norway through the TimeIn project, partly by
CableLabs and partly by the Comcast Innovation Fund. The views CableLabs and partly by the Comcast Innovation Fund. The views
expressed here are solely those of the authors. expressed here are solely those of the authors.
Authors' Addresses Authors' Addresses
Bob Briscoe (editor) Bob Briscoe (editor)
Independent Independent
United Kingdom United Kingdom
Email: ietf@bobbriscoe.net Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/ URI: https://bobbriscoe.net/
Koen De Schepper Koen De Schepper
Nokia Bell Labs Nokia Bell Labs
Antwerp Antwerp
Belgium Belgium
Email: koen.de_schepper@nokia.com Email: koen.de_schepper@nokia.com
URI: https://www.bell-labs.com/usr/koen.de_schepper URI: https://www.bell-labs.com/about/researcher-profiles/
koende_schepper/
Marcelo Bagnulo Marcelo Bagnulo
Universidad Carlos III de Madrid Universidad Carlos III de Madrid
Av. Universidad 30 Av. Universidad 30
Leganes, Madrid 28911 Leganes, Madrid 28911
Spain Spain
Phone: 34 91 6249500 Phone: 34 91 6249500
Email: marcelo@it.uc3m.es Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es URI: https://www.it.uc3m.es
Greg White Greg White
CableLabs CableLabs
United States of America United States of America
Email: G.White@CableLabs.com Email: G.White@CableLabs.com
 End of changes. 84 change blocks. 
237 lines changed or deleted 270 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/