wdiff draft-ietf-tsvwg-l4sops-00.txt draft-ietf-tsvwg-l4sops-01.txt

Transport Area Working Group G. White, Ed.
Internet-Draft CableLabs
Intended status: Informational 5 May 12 July 2021
Expires: 6 November 2021 13 January 2022

Operational Guidance for Deployment of L4S in the Internet
draft-ietf-tsvwg-l4sops-00
draft-ietf-tsvwg-l4sops-01

Abstract

This document is intended to provide guidance in order to ensure
successful deployment of Low Latency Low Loss Scalable throughput
(L4S) in the Internet. Other L4S documents provide guidance for
running an L4S experiment, but this document is focused solely on
potential interactions between L4S flows and flows using the original
('Classic') ECN over a Classic ECN bottleneck link. The document
discusses the potential outcomes of these interactions, describes
mechanisms to detect the presence of Classic ECN bottlenecks, and
identifies opportunities to prevent and/or detect and resolve
fairness problems in such networks. This guidance is aimed at
operators of end-systems, operators of networks, and researchers.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 6 November 2021. 13 January 2022.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3
2. Per-Flow Fairness . . . . . . . . . . . . . . . . . . . . . . 4 5
3. Flow Queuing Systems . . . . . . . . . . . . . . . . . . . . 7
4. Detection of Classic ECN Bottlenecks . . . . . . . . . . . . 6
3.1. 7
4.1. Recent Studies . . . . . . . . . . . . . . . . . . . . . 6
3.2. 7
4.2. Future Experiments . . . . . . . . . . . . . . . . . . . 7
4. 9
5. Operator of an L4S host . . . . . . . . . . . . . . . . . . . 8
4.1. Edge Servers 9
5.1. Server Type . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Other hosts
5.1.1. General purpose servers (e.g. web servers) . . . . . 10
5.1.2. Specialized servers handling long-running sessions
(e.g. cloud gaming) . . . . . . . . . . . . . . . . . 10
5.2. Server deployment environment . 11
5. Operator of a Network Employing RFC3168 FIFO Bottlenecks . . 11
5.1. Configure AQM to treat ECT(1) as NotECT . . . . . . . . . 12
5.2. ECT(1) Tunnel Bypass . . 11
5.2.1. Edge Servers . . . . . . . . . . . . . . . . 12
5.3. Configure Non-Coupled Dual Queue . . . . 11
5.2.2. Other hosts . . . . . . . . 12
5.4. WRED with ECT(1) Differentation . . . . . . . . . . . . . 13
5.5. Disable 12
6. Operator of a Network Employing RFC3168 Support FIFO Bottlenecks . . 13
6.1. Preferred Options . . . . . . . . . . . . . . . . . . . . 13
5.6. Re-mark ECT(1) to NotECT Prior
6.1.1. Upgrade AQMs to an L4S-aware AQM . . . . . . . . . . 14
6. Operator of a Network Employing RFC3168 FQ Bottlenecks 13
6.1.2. Configure Non-Coupled Dual Queue with Shallow
Target . . . 14
7. Conclusion of the L4S experiment . . . . . . . . . . . . . . 15
7.1. Successful termination of the L4S experiment . . . . . . 15
7.2. Unsuccessful termination of the L4S experiment 13
6.1.3. Approximate Fair Dropping . . . . . 15
8. Contributors . . . . . . . . . 14
6.1.4. Replace RFC3168 FIFO with RFC3168 FQ . . . . . . . . 14
6.1.5. Do Nothing . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . 14
6.2. Less Preferred Options . . . . . . . 16
10. Security Considerations . . . . . . . . . . 14
6.2.1. Configure Non-Coupled Dual Queue Treating ECT(1) as
NotECT . . . . . . . . . 16
11. Informative References . . . . . . . . . . . . . . 14
6.2.2. WRED with ECT(1) Differentation . . . . . 16
Author's Address . . . . . . 15
6.2.3. Configure AQM to treat ECT(1) as NotECT . . . . . . . 15
6.2.4. ECT(1) Tunnel Bypass . . . . . . . . . . . 18

1. Introduction

Low-latency, low-loss, scalable throughput (L4S)
[I-D.ietf-tsvwg-l4s-arch] traffic is designed to provide lower
queuing delay than conventional traffic via a new network service
based on . . . . . 15
6.3. Last Resort Options . . . . . . . . . . . . . . . . . . . 15
6.3.1. Disable RFC3168 Support . . . . . . . . . . . . . . . 16
6.3.2. Re-mark ECT(1) to NotECT Prior to AQM . . . . . . . . 16
7. Operator of a Network Employing RFC3168 FQ Bottlenecks . . . 16
8. Conclusion of the L4S experiment . . . . . . . . . . . . . . 17
8.1. Termination of a successful L4S experiment . . . . . . . 17
8.2. Termination of an unsuccessful L4S experiment . . . . . . 18
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Informative References . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21

1. Introduction

Low-latency, low-loss, scalable throughput (L4S)
[I-D.ietf-tsvwg-l4s-arch] traffic is designed to provide lower
queuing delay than conventional traffic via a new network service
based on a modified Explicit Congestion Notification (ECN) response
from the network. L4S traffic is identified by the ECT(1) codepoint,
and network bottlenecks that support L4S should congestion-mark
ECT(1) packets to enable L4S congestion feedback. However, L4S
traffic is also expected to coexist well with classic congestion
controlled traffic even if the bottleneck queue does not support L4S.
This includes paths where the bottleneck link utilizes packet drops
in response to congestion (either due to buffer overrun or active
queue management), as well as paths that implement a 'flow-queuing'
scheduler such as fq_codel [RFC8290]. A potential area of poor
interoperability lies in network bottlenecks employing a shared queue
that implements an Active Queue Management (AQM) algorithm that
provides Explicit Congestion Notification signaling according to
[RFC3168]. RFC3168 has been updated (via [RFC8311]) to reserve
ECT(1) for experimental use only (also see [IANA-ECN]), and its use
for L4S has been specified in [I-D.ietf-tsvwg-ecn-l4s-id]. However,
any deployed RFC3168 AQMs might not be updated, and RFC8311 still
prefers that routers not involved in L4S experimentation treat ECT(1)
and ECT(0) as equivalent. It has been demonstrated ([Detection]) [Briscoe] that
when a set of long-running flows comprising both classic congestion
controlled flows and L4S-compliant congestion controlled flows
compete for bandwidth in such a legacy shared RFC3168 queue, the
classic congestion controlled flows may achieve lower throughput than
they would have if all of the flows had been classic congestion
controlled flows. This 'unfairness' between the two classes is more
pronounced on longer RTT paths (e.g. 50ms and above) and/or at higher
link rates (e.g. 50 Mbps and above). The lower the capacity per
flow, the less pronounced the problem becomes. Thus the imbalance is
most significant when the slowest flow rate is still high in absolute
terms.

The root cause of the unfairness is that the L4S architecture
redefines the congestion signal (CE mark) and congestion response in
the case of packets marked ECT(1) (used by L4S senders), whereas a
RFC3168 queue does not differentiate between packets marked ECT(0)
(used by classic senders) and those marked ECT(1), and provides
identical CE
marks identically to both types. The result is that the two classes
respond differently to the CE congestion signal. The classic senders expect that CE
marks are sent very rarely (e.g. approximately 1 CE mark every 200
round trips on a 50 Mbps x 50ms path) while the L4S senders expect
very frequent CE marking (e.g. approximately 2 CE marks per round
trip). The result is that the classic senders respond to the CE
marks provided by the bottleneck by yielding capacity to the L4S
flows. The resulting rate imbalance can be demonstrated, and could
be a cause of concern in some cases.

This concern primarily relates to single-queue (FIFO) bottleneck
links that implement RFC3168 ECN, but the situation can also
potentially occur with per-flow queuing, e.g. fq_codel [RFC8290],
when flow isolation is imperfect due to hash collisions or VPN
tunnels.

While the above mentioned unfairness has been demonstrated in
laboratory testing, it has not been observed in operational networks,
in part because members of the Transport Working group are not aware
of any deployments of single-queue Classic ECN bottlenecks in the
Internet.

This issue was considered in November 2015 (and reaffirmed in April
2020) when the WG decided on the identifier to use for L4S, as
recorded in Appendix B.1 of [I-D.ietf-tsvwg-ecn-l4s-id]. It was
recognized that compromises would have to be made because IP header
space is extremely limited. A number of alternative codepoint
schemes were compared for their ability to traverse most Internet
paths, to work over tunnels, to work at lower layers, to work with
TCP, etc. It was decided to progress on the basis that robust
performance in presence of these single-queue RFC3168 bottlenecks is
not the most critical issue, since it was believed that they are
rare.

Nonetheless, there is the possibility that such deployments exist,
and there is the possibility that more they could be deployed/
enabled deployed/enabled in
the future, hence future. Since any negative impact of this coexistence issue
would not be directly experienced by the party experimenting with L4S
endpoints, but rather by the other users of the bottleneck, there is
an interest in providing guidance to ensure that measures can be
taken to address the potential issues, should they arise in practice.

TODO: further discussion on severity and who might be impacted?

2. Per-Flow Fairness

There are a number of factors that influence the relative rates
achieved by a set of users or a set of applications sharing a queue
in a bottleneck link. Notably the response that each application has
to congestion signals (whether loss or explicit signaling) can play a
large role in determining whether the applications share the
bandwidth in an equitable manner. In the Internet, ISPs typically
control capacity sharing between their customers using a scheduler at
the access bottleneck rather than relying on the congestion responses
of end-systems. So in that context this question primarily concerns
capacity sharing between the applications used by one customer site.
Nonetheless, there are many networks on the Internet where capacity
sharing relies, at least to some extent, on congestion control in the
end-systems. The traditional norm for congestion response has been
that it is handled on a per-connection basis, and that (all else
being equal) it results in each connection in the bottleneck
achieving a data rate inversely proportional to the average RTT of
the connection. The end result (in the case of steady-state behavior
of a set of like connections) is that each user or application
achieves a data rate proportional to N/RTT, where N is the number of
simultaneous connections that the user or application creates, and
RTT is the harmonic mean of the average round-trip-times for those
connections. Thus, users or applications that create a larger number
of connections and/or that have a lower RTT achieve a larger share of
the bottleneck link rate than others.

While this may not be considered fair by many, it nonetheless has
been the typical starting point for discussions around fairness. In
fact it has been common when evaluating new congestion responses to
actually set aside N & RTT as variables in the equation, and just
compare per-flow rates between flows with the same RTT. For example
[RFC5348] defines the congestion response for a flow to be
'"reasonably fair" if its sending rate is generally within a factor
of two of the sending rate of a [Reno] TCP flow under the same
conditions.' Given that RTTs can vary by roughly two orders of
magnitude and flow counts can vary by at least an order of magnitude
between applications, it seems that the accepted definition of
reasonable fairness leaves quite a bit of room for different levels
of performance between users or applications, and so perhaps isn't
the gold standard, but is rather a metric that is used because of its
convenience.

In practice, the effect of this RTT dependence has historically been
muted by the fact that many networks were deployed with very large
("bloated") drop-tail buffers that would introduce queuing delays
well in excess of the base RTT of the flows utilizing the link, thus
equalizing (to some degree) the effective RTTs of those flows.

Recently, as network equipment suppliers and operators have worked to
improve the latency performance of the network by the use of smaller
buffers and/or AQM algorithms, this has had the side-effect of
uncovering the inherent RTT bias in classic congestion control
algorithms.

The L4S architecture aims to significantly improve this situation, by
requiring senders to adopt a congestion response that eliminates RTT
bias as much as possible (see [I-D.ietf-tsvwg-ecn-l4s-id]). As a
result, L4S promotes a level of per-flow fairness beyond what is
ordinarily considered per-flow fairness beyond what is
ordinarily considered for classic senders, the RFC3168 issue
notwithstanding.

It is also worth noting that the congestion control algorithms
deployed currently on the internet tend toward (RTT-weighted)
fairness only over long timescales. For example, the cubic algorithm
can take minutes to converge to fairness when a new flow joins an
existing flow on a link [Ha]. Since the vast majority of TCP
connections don't last for minutes, it is unclear to what degree per-
flow, same-RTT fairness, even when demonstrated in the lab,
translates to the real world.

So, in real networks, where per-application, per-end-host or per-
customer fairness may be more important than long-term, same-RTT,
per-flow fairness, it may not be that instructive to focus on the
latter as being a necessary end goal.

Nonetheless, situations in which the presence of an L4S flow has the
potential to cause harm [Ware] to classic flows need to be
understood. Most importantly, if there are situations in which the
introduction of L4S traffic would degrade both the absolute and
relative performance of classic traffic significantly, i.e. to the
point that it would be considered starvation while L4S was not
starved, these situations need to be understood and either remedied
or avoided.

Aligned with this context, the guidance provided in this document is
aimed not at monitoring the relative performance of L4S senders
compared against classic senders on a per-flow basis, but rather at
identifying instances where RFC3168 bottlenecks are deployed so that
operators of L4S senders can have the opportunity to assess whether
any actions need to be taken. Additionally this document provides
guidance for network operators around configuring any RFC3168
bottlenecks to minimize the potential for negative interactions
between L4S and classic senders, senders.

3. Flow Queuing Systems

As noted above, the concern around RFC3168 issue
notwithstanding.

It coexistence mainly
concerns single-queue systems where classic and L4S traffic are
mixed. In a flow-queuing system, when flow isolation is also worth noting that successful,
the FQ scheduling of such queues isolates classic congestion control algorithms
deployed currently on the internet tend toward (RTT-weighted)
fairness only over long timescales. For example, the cubic algorithm
can take minutes to converge to fairness when a new flow joins an
existing flow on a link [Cubic]. Since
traffic from L4S traffic, and thus eliminates the vast majority of TCP
connections don't last potential for minutes, it is unclear
unfairness. But, these systems are known to what degree per-
flow, same-RTT fairness, even when demonstrated sometimes result in the lab,
translates
imperfect isolation, either due to the real world.

So, in real networks, where per-application, per-end-host hash collisions (see Section 5.3
of [RFC8290]), because of VPN tunneling (see Section 6.2 of
[RFC8290]), or per-
customer fairness may be more important than long-term, same-RTT,
per-flow fairness, it may not be that instructive due to focus on deliberate configuration (see Section 7,
Paragraph 5).

It is believed that the
latter as being a necessary end goal.

Nonetheless, situations majority of FQ deployments in which bottleneck
links today (e.g. Cake [Hoiland-Jorgensen]) employ hashing
algorithms that virtually eliminate the presence possibility of collisions,
making this a non-issue for those deployments. But, VPN tunnels
remain an L4S flow has the
potential to cause harm [Harm] to classic flows need to be
understood. Most importantly, if there are situations in which issue for FQ deployments, and the introduction of L4S
traffic would degrade both raises the absolute and
relative performance of possibility that tunnels containing mixed classic
and L4S traffic significantly, i.e. to the
point that it would be considered starvation while L4S was exist, in which case FQ implementations that
have not
starved, these situations need been updated to be understood and either remedied
or avoided.

Aligned with this context, the guidance provided in this document is
aimed not at monitoring the relative performance of L4S senders
compared against classic senders on a per-flow basis, but rather at
identifying instances where RFC3168 bottlenecks are deployed so L4S-aware could exhibit similar
unfairness properties as single queue AQMs. Section 7 discusses some
remedies that
operators of L4S senders can have the opportunity to assess whether
any actions need to be taken. Additionally this document provides
guidance for network implemented by operators around configuring any RFC3168
bottlenecks of FQ equipment in
order to minimize the potential for negative interactions
between this risk. Additionally, end-host mitigations such
as separating L4S and classic senders.

3. Classic traffic into distinct VPN tunnels could
be employed.

4. Detection of Classic ECN Bottlenecks

The IETF encourages researchers, end system deployers and network
operators to conduct experiments to identify to what degree RFC3168
bottlecks exist in networks. These types of measurement campaigns,
even if each is conducted over a limited set of paths, could be
useful to further understand the scope of any potential issues, to
guide end system deployers on where to examine performance more
closely (or possibly delay L4S deployment), and to help network
operators identify nodes where remediation may be necessary to
provide the best performance.

3.1.

4.1. Recent Studies

A small number of recent studies have attempted to gauge the level of
RFC3168 AQM deployment in the internet.

In 2020, Akamai conducted a study
(https://mailarchive.ietf.org/arch/msg/tsvwg/2tbRHphJ8K_CE6is9n7iQy-
VAZM/) of "downstream" (server to client) CE marking broken out by
ASN on two separate days, one in late March, the other in mid July
[Akamai].

[Holland]. They concluded that prevalence of CE-marking was low
across the ~800 ASNs observed, observed (0.19% - 0.30% of ECT client IPs ever
saw a CE mark), but it was growing, and that they could not determine
whether the CE marking was due to a single queue or FQ. They also
observed that RFC3168 AQMs are not uniformly distributed. There were a
three small handful (5-7) of ASNs showing evidence ISPs where prevalence of CE-marking across more than was above ~70%,
indicating a likely deployment by the ISP. There were another four
small ASNs where the prevalence was between 10% of their client IPs, and 20%, which may
also indicate deployment by the ISP. There were also roughly six
larger ASNs (and perhaps 20 small ASNs) where the global
baseline prevalence was CE-marking across 0.3% of IPs.
between 3% and 8%.

In 2017, Apple reported [TCPECN] on their observations of ECN marking by
networks, broken out by country. country [Bhooma]. They reported four
countries that exceeded the global baseline seen by Akamai, but one
of these (Argentine Republic) was later discovered to be due to a bug, bug
(https://datatracker.ietf.org/meeting/106/materials/slides-106-tsvwg-
sessa-72-l4s-drafts-00#page=15), leaving three countries: China 1% of
paths, Mexico 3.2% of paths, France 6% of paths. The percentage in
France appears consistent with reports
(https://mailarchive.ietf.org/arch/msg/tsvwg/
UyvpwUiNw0obd_EylBBV7kDRIHs/) that fq_codel has been implemented in
DSL home routers deployed by Free.fr.

In December 2020 - January 2021, Pete Heist worked with a small
cooperative WISP in the Czech Republic to collect data on CE-marking
[I-D.heist-tsvwg-ecn-deployment-observations]. Overall, 18.6% of
paths saw possible RFC3168 AQM activity, which appears to place this
ISP in the small group with moderately high RFC3168 prevalence
reported by Akamai. This ISP had was known to have deployed RFC3168
fq_codel equipment in some of their subnets, but and in other subnets
there were 33 IPs where CE-marking possible AQM activity was possibly observed, observed via CE-
marks and/or ECE flags, corresponding to approximately 10% of paths, significantly greater
than the baseline reported by Akamai. paths.
It was agreed
(https://mailarchive.ietf.org/arch/msg/tsvwg/Rj7GylByZuFa3_LTCMvEfb-
CYpw/) (https://mailarchive.ietf.org/arch/msg/tsvwg/
Rj7GylByZuFa3_LTCMvEfb-CYpw/) that these were likely to be due to
fq_codel implementations in home routers deployed by members of the
cooperative.

The interpretation of these studies seems to be that there are no
known deployments of FIFO RFC3168, all of the known RFC3168
deployments are fq_codel, the majority of the currently unknown
deployments are likely to be fq_codel, and there may be a small
number of networks where CE-marking is prevalent (and thus likely
ISP-managed) where it is currently unknown as to whether the source
is a FIFO or an FQ system.

Other studies (e.g. [EnablingECN], [ECNreadiness], [MeasuringECN]) [Trammel], [Bauer], [Mandalari]) have examined
ECN traversal, but have not reported data on prevalence of CE-marking
by networks.

3.2. Another [Roddav] examined traces from a Tier 1 ISP link
in 2018 and observed that 94% of the non-zero ECN marked packets were
CE, which appears to reflect a misconfiguration of equipment using
that link, as opposed to providing evidence of RFC3168 AQM
deployment.

4.2. Future Experiments

The design of future experiments should consider not only the
detection of RFC3168 ECN marking, but also the determination whether
the bottleneck AQM is a single queue (FIFO) or a flow-queuing (FQ)
system. It is believed that the vast majority, if not all, of the
RFC3168 AQMs in use at bottleneck links are flow-queuing systems
(e.g. fq_codel [RFC8290] or [COBALT]). When flow isolation is
successful, the FQ scheduling of such queues isolates classic
congestion control traffic from L4S traffic, and thus eliminates the
potential for unfairness. But, these systems are known to sometimes
result in imperfect isolation, either due to hash collisions (see
Section 5.3 (https://datatracker.ietf.org/doc/html/rfc8290#section-
5.3) of [RFC8290]) or because of VPN tunneling (see Section 6.2
(https://datatracker.ietf.org/doc/html/rfc8290#section-6.2) of
[RFC8290]). It is believed that the majority of FQ deployments in
bottleneck links today (e.g. [Cake]) employ hashing algorithms that
virtually eliminate the possibility of collisions, making this a non-
issue for those deployments. But, VPN tunnels remain an issue for FQ
deployments, and the introduction of L4S traffic raises the
possibility that tunnels containing mixed classic and L4S traffic
would exist, in which case FQ implementations that have not been
updated to be L4S-aware could exhibit similar unfairness properties
as single queue AQMs. Until such queues majority, if not all, of the
RFC3168 AQMs in use at bottleneck links are upgraded to support L4S
(see Section 6) flow-queuing systems
(e.g. fq_codel [RFC8290] or treat ECT(1) as not-ECT traffic, end-host
mitigations such as separating L4S and Classic traffic into distinct
VPN tunnels could be employed.

[Detection] COBALT [Palmei]).

[Briscoe] contains recommendations on some of the mechanisms that can
be used to detect RFC3168 bottlenecks. In particular, Section 4 of [Detection]
[Briscoe] outlines an approach for out-band-detection of RFC3168
bottlenecks.

5. Operator of an L4S host

From a host's perspective, support for L4S only involves the sender
via ECT(1) marking & L4S-compatible congestion control. The receiver
is involved in ECN feedback but can generally be agnostic to whether
ECN is being used for L4S [I-D.ietf-tsvwg-l4s-arch]. Between these
two entities, it is primarily incumbent upon the sender to evaluate
the potential for presence of RFC3168 FIFO bottlenecks and make
decisions whether or not to use L4S congestion control. While is is
possible for a receiver to disable L4S functionality by not
negotiating ECN, a general purpose receiver is not expected to
perform any testing or monitoring for RFC3168, and is also not
expected to invoke any active response in the case that such a
bottleneck exists.

Prior to deployment of any new technology, it is commonplace for the
parties involved in the deployment to validate the performance of the
new technology, technology via lab testing, limited field testing, large scale
field testing, etc. etc., usually in a progressive manner. The same is
expected for deployers of L4S technology. As part of that
validation, it is recommended that deployers consider the issue of
RFC3168 FIFO bottlenecks and conduct experiments as described in the
previous section, or otherwise assess the impact that the L4S
technology will have in the networks in which it is to be deployed,
and take action as is described further in this section. This sort
of progressive (incremental) deployment helps to ensure that any
issues are discovered when the scale of those issues is relatively
small.

TODO: discussion of risk of incorrectly classifying a path

5.1. Server Type

If pre-deployment testing raises concerns about issues with RFC3168
bottlenecks, the actions taken may depend on the server type:

* type.

5.1.1. General purpose servers (e.g. web servers)
-

* Out-of-band active testing could be performed by the server. For
example, a javascript application could run simultaneous downloads
(i.e. with and without L4S) during page reading time in order to
survey for presence of RFC3168 FIFO bottlenecks on paths to users
(e.g. as described in Section 4 of [Detection]).

- [Briscoe]).

* In-band testing could be built in to the transport protocol
implementation at the sender in order to perform detection (see
Section 5 of [Detection], [Briscoe], though note that this mechanism does not
differentiate between FIFO and FQ).

* Discontinuing use of L4S based on the detection of RFC3168 FIFO
bottlenecks is likely not needed for short transactional transfers
(e.g. sub 10 seconds) since these are unlikely to achieve the
steady-state conditions where unfairness has been observed.

* For longer file transfers, it may be possible to fall-back to
Classic behavior in real-time (i.e. when doing in-band testing),
or to cache those destinations where RFC3168 has been detected,
and disable L4S for subsequent long file transfers to those
destinations.

5.1.2. Specialized servers handling long-running sessions (e.g. cloud
gaming)

* Out-of-band active testing could be performed at each session
startup

* Out-of-band active testing could be integrated into a "pre-
validation" of the service, done when the user signs up, and
periodically thereafter

* In-band detection as described in [Detection] [Briscoe] could be performed
during the session

TODO: discussion of risk of incorrectly classifying a path

In addition, the

5.2. Server deployment environment

The responsibilities of and actions taken by a sender may
additionally depend on the environment in which it is deployed. The
following sub-sections discuss two scenarios: senders serving a limited
limited, known target audience and those that serve an unknown target
audience.

4.1.

5.2.1. Edge Servers

Some hosts (such as CDN leaf nodes and servers internal to an ISP)
are deployed in environments in which they serve content to a
constrained set of networks or clients. The operator of such hosts
may be able to determine whether there is the possibility of
[RFC3168] FIFO bottlenecks being present, and utilize this
information to make decisions on selectively deploying L4S and/or
disabling it (e.g. bleaching ECN). Furthermore, such an operator may
be able to determine the likelihood of an L4S bottleneck being
present, and use this information as well.

For example, if a particular network is known to have deployed legacy
[RFC3168] FIFO bottlenecks, usage of L4S for long capacity-seeking
file transfers on that network could be delayed until those
bottlenecks can be upgraded to mitigate any potential issues as
discussed in the next section.

Prior to deploying L4S on edge servers a server operator should:

* Consult with network operators on presence of legacy [RFC3168]
FIFO bottlenecks

* Consult with network operators on presence of L4S bottlenecks

* Perform pre-deployment testing per network

If a particular network offers connectivity to other networks (e.g.
in the case of an ISP offering service to their customer's networks),
the lack of RFC3168 FIFO bottleneck deployment in the ISP network
can't be taken as evidence that RFC3168 FIFO bottlenecks don't exist
end-to-end (because one may have been deployed by the end-user
network). In these cases, deployment of L4S will need to take
appropriate steps to detect the presence of such bottlenecks. At
present, it is believed that the vast majority of RFC3168 bottlenecks
in end-user networks are implementations that utilize fq_codel or
Cake, where the unfairness problem is less likely to be a concern.
While this doesn't completely eliminate the possibility that a legacy
[RFC3168] FIFO bottleneck could exist, it nonetheless provides useful
information that can be utilized in the decision making around the
potential risk for any unfairness to be experienced by end users.

4.2.

5.2.2. Other hosts

Hosts that are deployed in locations that serve a wide variety of
networks face a more difficult prospect in terms of handling the
potential presence of RFC3168 FIFO bottlenecks. Nonetheless, the
steps listed in the ealier section (based on server type) can be
taken to minimize the risk of unfairness.

The interpretation of studies on ECN usage and their deployment
context (see Section 3.1) 4.1) has so far concluded that RFC3168 FIFO
bottlenecks are likely to be rare, and so detections using these
techniques may also prove to be rare. Therefore, it may be possible
for a host to cache a list of end host ip addresses where a RFC3168
bottleneck has been detected. Entries in such a cache would need to
age-out after a period of time to account for IP address changes,
path changes, equipment upgrades, etc. [TODO: more info on ways to
cache/maintain such a list]

It has been suggested that a public block-list of domains that
implement RFC3168 FIFO bottlenecks could be maintained. There are a
number of significant issues that would seem to make this idea
infeasible, not the least of which is the fact that presence of
RFC3168 FIFO bottlenecks or L4S bottlenecks is not a property of a
domain, it is the property of a link, and therefore of the particular
current path between two endpoints.

It has also been suggested that a public allow-list of domains that
are participating in the L4S experiment could be maintained. This
approach would not be useful, given the presence of an L4S domain on
the path does not imply the absence of RFC3168 AQMs upstream or
downstream of that domain. Also, the approach cannot cater for
domains with a mix of L4S and RFC3168 AQMs.

6. Operator of a Network Employing RFC3168 FIFO Bottlenecks

While it is, of course, preferred for networks to deploy L4S-capable
high fidelity congestion signaling, and while it is more preferable for L4S senders to detect problems
themselves, a network operator who has deployed equipment in a likely
bottleneck link location (i.e. a link that is expected to frequently
be fully saturated) that is configured with a legacy [RFC3168] FIFO
AQM can take certain steps in order to improve rate fairness between
classic traffic and L4S traffic, and thus enable L4S to be deployed
in a greater number of paths.

Some of the options listed in this section may not be feasible in all
networking equipment.

5.1. Configure AQM

6.1. Preferred Options

6.1.1. Upgrade AQMs to treat ECT(1) as NotECT an L4S-aware AQM

If equipment is configurable in such a way as to only supply CE marks
to ECT(0) packets, and treat ECT(1) packets identically to NotECT, or
is upgradable to support this capability, doing so will eliminate the
risk of unfairness.

5.2. ECT(1) Tunnel Bypass

Tunnel ECT(1) traffic through the RFC3168 bottleneck with the outer
header indicating Not-ECT, by using AQM implementation can be upgraded to enable support
for L4S, either an ECN tunnel ingress in
Compatibility Mode [RFC6040] via [I-D.ietf-tsvwg-aqm-dualq-coupled] or a Limited Functionality ECN tunnel
[RFC3168].

Two variants exist for via an L4S-
aware FQ implementation, this is the preferred approach

1. per-domain: tunnel ECT(1) pkts to domain edge towards dst

2. per-dst: tunnel ECT(1) pkts to dst

5.3. addressing
potential unfairness, because it additionally enables all of the
benefits of L4S.

6.1.2. Configure Non-Coupled Dual Queue with Shallow Target

Equipment supporting [RFC3168] may be configurable to enable two
parallel queues for the same traffic class, with classification done
based on the ECN field.

Option 1:

* Configure 2 queues, both with ECN; 50:50 WRR scheduler

- Queue #1: ECT(1) & CE packets - Shallow immediate AQM target

- Queue #2: ECT(0) & NotECT packets - Classic AQM target

* Outcome in the case of n L4S flows and m long-running m long-running Classic
flows

- if m & n are non-zero, flows get 1/2n and 1/2m of the capacity,
otherwise 1/n or 1/m

- never < 1/2 each flow's rate if all had been Classic

This option would allow L4S flows to achieve low latency, low loss
and scalable throughput, but would sacrifice the more precise flow
balance offered by [I-D.ietf-tsvwg-aqm-dualq-coupled]. This option
would be expected to result in some reordering of previously CE
marked packets sent by Classic ECN senders, which is a trait shared
with [I-D.ietf-tsvwg-aqm-dualq-coupled]. As is discussed in
[I-D.ietf-tsvwg-ecn-l4s-id], this reordering would be either zero
risk or very low risk.

6.1.3. Approximate Fair Dropping

The Approximate Fair Dropping ([AFD]) algorithm tracks individual
flow rates and introduces either packet drops or CE-marks to each
flow in proportion to the amount by which the flow rate exceeds a
computed per-flow fair-share rate. Where an implementation of AFD or
an equivalent algorithm is available, it could be enabled on an
interface with a single-queue RFC3168 AQM as a fairly lightweight way
to inject additional ECN marks into any significantly higher rate
flows. See also [Cisco-N9000].

6.1.4. Replace RFC3168 FIFO with RFC3168 FQ

As discussed in Section XREF, implementations of RFC3168 with an FQ
scheduler (e.g. fq_codel or Cake) significantly reduce the likelihood
of experiencing any unfairness between Classic and L4S traffic.

6.1.5. Do Nothing

If it is infeasible to implement any of the above options, it may be
preferable for an operator of RFC3168 FIFO bottlenecks to leave them
unchanged. In many deployment situations the risk of fairness issues
may be very low, and the impact if they occur may not be particularly
troublesome. This could, for instance, be true in bottlenecks where
there is a high degree of flow aggregation or in high-speed
bottlenecks (e.g. greater than 100 Mbps).

6.2. Less Preferred Options

In the case that there is a concern about per-flow fairness between
L4S flows and Classic flows

- if m & n are non-zero, flows get 1/2n in an RFC3168 FIFO bottleneck, and 1/2m none
of the capacity,
otherwise 1/n or 1/m

- never < 1/2 each flow's rate if all had been Classic

This option would allow L4S flows to achieve low latency, low loss
and scalable throughput, but would sacrifice remedies in the more precise flow
balance offered by [I-D.ietf-tsvwg-aqm-dualq-coupled]. This option
would previous section can be expected to result in some reordering of previously CE
marked packets sent by Classic ECN senders, which is a trait shared
with [I-D.ietf-tsvwg-aqm-dualq-coupled]. As is discussed implemented, the
options listed in
[I-D.ietf-tsvwg-ecn-l4s-id], this reordering would section could be either zero
risk or very low risk.

Option 2: considered.

6.2.1. Configure Non-Coupled Dual Queue Treating ECT(1) as NotECT

* Configure 2 queues, both with AQM; 50:50 WRR scheduler

- Queue #1: ECT(1) & NotECT packets - ECN disabled

- Queue #2: ECT(0) & CE packets - ECN enabled

* Outcome
- ECT(1) treated as NotECT

- Flow balance for the 2 queues is the same as in option 1 Section 6.1.2

This option would not allow L4S flows to achieve low latency, low
loss and scalable throughput in this bottleneck link. As a result it
is the less preferred option.

5.4.

6.2.2. WRED with ECT(1) Differentation

This configuration is similar to Option 2 in the previous section, option described in
Section 6.2.1, but uses a single queue with WRED functionality.

* Configure the queue with two WRED classes

- Class #1: ECT(1) & NotECT packets - ECN disabled

- Class #2: ECT(0) & CE packets - ECN enabled

5.5.

6.2.3. Configure AQM to treat ECT(1) as NotECT

6.2.4. ECT(1) Tunnel Bypass

Tunnel ECT(1) traffic through the RFC3168 bottleneck with the outer
header indicating Not-ECT, by using either an ECN tunnel ingress in
Compatibility Mode [RFC6040] or a Limited Functionality ECN tunnel
[RFC3168].

Two variants exist for this approach

1. per-domain: tunnel ECT(1) pkts to domain edge towards dst

2. per-dst: tunnel ECT(1) pkts to dst

6.3. Last Resort Options

If serious issues are detected, where the presence of L4S flows is
determined to be the likely cause, and none of the above options are
implementable, the options in this section can be considered as a
last resort. These options are not recommended.

6.3.1. Disable RFC3168 Support

Disabling an [RFC3168] AQM from CE marking both ECT(0) traffic and
ECT(1) traffic eliminates the unfairness issue. A downside to this
approach is that classic senders will no longer get the benefits of
Explict Congestion Notification at this bottleneck link. link either. This
alternative is only mentioned in case there is no other way to
reconfigure an RFC3168 AQM.

5.6.

6.3.2. Re-mark ECT(1) to NotECT Prior to AQM

Remarking ECT(1) packets as NotECT (i.e. bleaching ECT(1)) ensures
that they are treated identically to classic NotECT senders.
However, this action is not recommended because a) it would also
prevent downstream L4S bottlenecks from providing high fidelity
congestion signals; and b) it could lead to problems with future
experiments that use ECT(1) in alternative ways to L4S. L4S; and c) it
would violate requirements in [I-D.ietf-tsvwg-ecn-l4s-id]. This
alternative is only mentioned as an absolute last resort in case there is
no other way to reconfigure an RFC3168 AQM.

Note that the CE codepoint must never be bleached, otherwise it would
black-hole congestion indications.

7. Operator of a Network Employing RFC3168 FQ Bottlenecks

A network operator who has deployed flow-queuing systems that
implement RFC3168 (e.g. fq_codel or CAKE) CAKE using default hashing) at
network bottlenecks will likely see fewer potential issues when L4S
traffic is present on their network as compared to operators of
RFC3168 FIFOs. As discussed in previous sections, Section 3, the flow queuing mechanism
will typically isolate L4S flows and Classic flows into separate
queues, and the scheduler will then enforce per-flow fairness. As a
result, the potential fairness issues between Classic and L4S traffic
that can occur in FIFOs will typically not occur in FQ systems. That
said, FQ systems commonly treat a tunneled traffic aggregate as a
single flow, and thus a tunneled traffic aggregate that contains a
mix of Classic and L4S traffic will utilize a single queue, and the
traffic within the tunnel could experience the same fairness issue as
has been described for RFC3168 FIFOs. This unfairness is compounded
by the fact that the FQ scheduler will already be causing unfairness
to flows within the tunnel relative to flows that are not tunneled. tunneled
(each of which gets the same bandwidth share as does the tunnel).
Additionally, many of the deployed RFC3168 FQ systems currently
implement an AQM algorithm (either CoDel or COBALT) that is designed
for Classic traffic and reacts sluggishly to L4S (or unresponsive)
traffic, with the result being that L4S senders could in some cases
see worse latency performance than Classic senders.

While the potential unfairness result is arguably less impactful in
the case of RFC3168 FQ bottlenecks, it is believed that RFC3168 FQ
bottlenecks are currently more common than RFC3168 FIFO bottlenecks.
The most common deployments of RFC3168 FQ bottlenecks are in home
routers running OpenWRT firmware where the user has turned the
feature on.

As is the case with RFC3168 FIFOs, the preferred remedy for a network
operator that wishes to enable the best performance possible with
regard to L4S, is for the network operator to update RFC3168 FQ
bottlenecks to be L4S-aware. In cases where that is infeasible,
several of the remedies described in the previous section can be used
to reduce or eliminate these issues.

* Configure AQM to treat ECT(1) as NotECT

* ECT(1) Tunnel Bypass

* Disable RFC3168 Support

* Re-mark ECT(1) to NotECT Prior to AQM

Note that some FQ schedulers can be configured to intentionally
aggregate multiple flows into each queue. This might be used, for
instance, to implement per-user or per-host fairness rather than per-
flow fairness. In this case, if the flow aggregates contain a mix of
Classic and L4S traffic, one would expect to see the same potential
unfairness as is seen in the FIFO case. The same remedies mentioned
above would apply in this case as well.

8. Conclusion of the L4S experiment

This section gives guidance on how L4S-deploying networks and
endpoints should respond to either of the two possible outcomes of
the IETF-supported L4S experiment.

7.1. Successful termination

8.1. Termination of the a successful L4S experiment

If the L4S experiment is deemed successful, the IETF would be
expected to move the L4S specifications to standards track. Networks
would then be encouraged to continue/begin deploying L4S-aware nodes
and to replace all non-L4S-aware RFC3168 AQMs already deployed as far
as feasible, or at least restrict RFC3168 AQM to interpret ECT(1)
equal to NotECT. Networks that participated in the experiment would
be expected to track the evolution of the L4S standards and adapt
their implementations accordingly (e.g. if as part of switching from
experimental to standards track, changes in the L4S RFCs become
necessary).

7.2. Unsuccessful termination

8.2. Termination of the an unsuccessful L4S experiment

If the L4S experiment is deemed unsuccessful due to lack of
deployment of compliant end-systems or AQMs, it might need to be
terminated: any L4S network nodes should then be un-deployed and the
ECT(1) codepoint usage should be released/recycled as quickly as
possible, recognizing that this process may take some time. To
facilitate this potential outcome, [draft-ecn-l4s-id] [I-D.ietf-tsvwg-ecn-l4s-id]
requires L4S hosts to be configurable to revert to non-L4S congestion
control, and networks to be configurable to treat ECT(1) the same as
ECT(0).

9. Contributors

Thanks to Bob Briscoe, Jake Holland, Koen De Schepper, Olivier
Tilmans, Tom Henderson, Asad Ahmed, Gorry Fairhurst, Sebastian
Moeller, Pete Heist, and members of the TSVWG mailing list for their
contributions to this document.

10. IANA Considerations

None.

10.

11. Security Considerations

For further study.

11.

12. Informative References

[Akamai] Holland, J., "Latency & AQM Observations on the Internet",
IETF MAPRG interim-2020-maprg-01, August 2020,
<https://www.ietf.org/proceedings/interim-2020-maprg-
01/slides/slides-interim-2020-maprg-01-sessa-latency-aqm-
observations-on-the-internet-01.pdf>.

[Cake] Hoiland-Jorgensen, T., Taht, D., and J. Morton, "Piece of
CAKE: A Comprehensive Queue Management Solution for Home
Gateways", 2018, <https://arxiv.org/abs/1804.07617>.

[COBALT] Palmei, J.

[AFD] Pan, R., Breslau, L., Prabhakar, B., and et al., "Design S. Shenker,
"Approximate Fairness through Differential Dropping",
Computer Comm. Rev. vol.33, no.1, January 2003,
<https://people.eecs.berkeley.edu/~istoica/classes/
cs268/10/papers/afd.pdf>.

[Bauer] Bauer, S., Beverly, R., and Evaluation A. Berger, "Measuring the
State of COBALT
Queue Discipline", IEEE International Symposium on Local
and Metropolitan Area Networks 2019, 2019,
<https://ieeexplore.ieee.org/abstract/document/8847054>.

[Cubic] Ha, S., Rhee, I., ECN Readiness in Servers, Clients, and L. Xu, "CUBIC: A New TCP-Friendly
High-Speed TCP Variant", Routers",
Proc ACM SIGOPS Operating Systems
Review SIGCOMM Internet Measurement Conference IMC'11,
2011,
<http://conferences.sigcomm.org/imc/2011/docs/p171.pdf>.

[Bhooma] Bhooma, P., "TCP ECN: Experience with enabling ECN on the
Internet", 98th IETF MAPRG Presentation , 2008,
<https://www.cs.princeton.edu/courses/archive/fall16/
cos561/papers/Cubic08.pdf>.

[Detection] 2017,
<https://datatracker.ietf.org/meeting/98/materials/slides-
98-maprg-tcp-ecn-experience-with-enabling-ecn-on-the-
internet-padma-bhooma-00>.

[Briscoe] Briscoe, B. and A.S. Ahmed, "TCP Prague Fall-back on
Detection of a Classic ECN AQM", ArXiv , February 2021,
<https://arxiv.org/abs/1911.00710>.

[ECNreadiness]
Bauer,

[Cisco-N9000]
Cisco, "Intelligent Buffer Management on Cisco Nexus 9000
Series Switches White Paper", Cisco Product
Document 1486580292771926, 6 June 2017,
<https://www.cisco.com/c/en/us/products/collateral/
switches/nexus-9000-series-switches/white-paper-
c11-738488.html>.

[Ha] Ha, S., Beverly, R., and A. Berger, "Measuring the
State of ECN Readiness in Servers, Clients, Rhee, I., and Routers",
Proc L. Xu, "CUBIC: A New TCP-Friendly
High-Speed TCP Variant", ACM SIGCOMM Internet Measurement Conference IMC'11,
2011,
<http://conferences.sigcomm.org/imc/2011/docs/p171.pdf>.

[EnablingECN]
Trammel, B., Kuehlewind, M., Boppart, SIGOPS Operating Systems
Review , 2008,
<https://www.cs.princeton.edu/courses/archive/fall16/
cos561/papers/Cubic08.pdf>.

[Hoiland-Jorgensen]
Hoiland-Jorgensen, T., Taht, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
Wide Deployment J. Morton, "Piece of Explicit Congestion Notification", Proc
Passive
CAKE: A Comprehensive Queue Management Solution for Home
Gateways", 2018, <https://arxiv.org/abs/1804.07617>.

[Holland] Holland, J., "Latency & Active Measurement Conference PAM15, 2015,
<https://link.springer.com/
chapter/10.1007%2F978-3-319-15509-8_15>.

[Harm] Ware, R., Mukerjee, M., Seshan, S., and J. Sherry, "Beyond
Jain's Fairness Index: Setting AQM Observations on the Bar For The Deployment
of Congestion Control Algorithms", Hotnets'19 , 2019,
<https://www.cs.cmu.edu/~rware/assets/pdf/ware-
hotnets19.pdf>. Internet",
IETF MAPRG interim-2020-maprg-01, August 2020,
<https://www.ietf.org/proceedings/interim-2020-maprg-
01/slides/slides-interim-2020-maprg-01-sessa-latency-aqm-
observations-on-the-internet-01.pdf>.

[I-D.heist-tsvwg-ecn-deployment-observations]
Heist, P. and J. Morton, "Explicit Congestion Notification
(ECN) Deployment Observations", Work in Progress,
Internet-Draft, draft-heist-tsvwg-ecn-deployment-
observations-02, 8 March 2021, <http://www.ietf.org/
internet-drafts/draft-heist-tsvwg-ecn-deployment-
observations-02.txt>.

[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-aqm-dualq-coupled-13, 15 November 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-aqm-
dualq-coupled-13.txt>.

[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay (L4S)", Work in Progress,
Internet-Draft, draft-ietf-tsvwg-ecn-l4s-id-12, 15
November 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-tsvwg-ecn-l4s-id-12.txt>.

[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K., Bagnulo, M., and G. White, "Low
Latency, Low Loss, Scalable Throughput (L4S) Internet
Service: Architecture", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-l4s-arch-08, 15 November 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-l4s-
arch-08.txt>.

[IANA-ECN] Internet Assigned Numbers Authority, "IANA ECN Field
Assignments", 2018, <https://www.iana.org/assignments/
dscp-registry/dscp-registry.xhtml#ecn-field>.

[MeasuringECN]

[Mandalari]
Mandalari, AM., Lutu, A., Briscoe, B., Bagnulo, M., and O.
Alay, "Measuring ECN++: Good News for ++, Bad News for ECN
over Mobile", DOI 10.1109/MCOM.2018.1700739, IEEE
Communications Magazine vol. 56, no. 3, March 2018,
<https://ieeexplore.ieee.org/document/8316790>.

[Palmei] Palmei, J. and X. et al., "Design and Evaluation of COBALT
Queue Discipline", IEEE International Symposium on Local
and Metropolitan Area Networks 2019, 2019,
<https://ieeexplore.ieee.org/abstract/document/8847054>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
<https://www.rfc-editor.org/info/rfc5348>.

[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.

[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.

[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.

[TCPECN] Bhooma, P., "TCP ECN: Experience with enabling

[Roddav] Roddav, N., Streit, K., Rodosek, G.D., and A. Pras, "On
the Usage of DSCP and ECN on Codepoints in Internet Backbone
Traffic Traces for IPv4 and IPv6",
DOI 10.1109/ISNCC.2019.8909187, ISNCC 2019, 2019,
<https://ieeexplore.ieee.org/document/8909187>.

[Trammel] Trammel, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
Wide Deployment of Explicit Congestion Notification", Proc
Passive & Active Measurement Conference PAM15, 2015,
<https://link.springer.com/
chapter/10.1007%2F978-3-319-15509-8_15>.

[Ware] Ware, R., Mukerjee, M., Seshan, S., and J. Sherry, "Beyond
Jain's Fairness Index: Setting the
Internet", 98th IETF MAPRG Presentation Bar For The Deployment
of Congestion Control Algorithms", Hotnets'19 , 2017,
<https://datatracker.ietf.org/meeting/98/materials/slides-
98-maprg-tcp-ecn-experience-with-enabling-ecn-on-the-
internet-padma-bhooma-00>. 2019,
<https://www.cs.cmu.edu/~rware/assets/pdf/ware-
hotnets19.pdf>.

Author's Address

Greg White (editor)
CableLabs

Email: g.white@cablelabs.com