Transport Area Working Group G. White, Ed.
Internet-Draft CableLabs
Intended status: Informational 15 March 2025
Expires: 16 September 2025
Operational Guidance on Coexistence with Classic ECN during L4S
Deployment
draft-ietf-tsvwg-l4sops-07
Abstract
This document provides 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. 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 16 September 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Per-Flow Fairness . . . . . . . . . . . . . . . . . . . . . . 4
3. Flow Queuing Systems . . . . . . . . . . . . . . . . . . . . 6
4. Detection of Classic ECN Bottlenecks . . . . . . . . . . . . 7
4.1. Recent Studies . . . . . . . . . . . . . . . . . . . . . 7
4.2. Future Experiments . . . . . . . . . . . . . . . . . . . 9
5. Operator of an L4S host . . . . . . . . . . . . . . . . . . . 9
5.1. Server Type . . . . . . . . . . . . . . . . . . . . . . . 10
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
5.2.1. Edge Servers . . . . . . . . . . . . . . . . . . . . 11
5.2.2. Other hosts . . . . . . . . . . . . . . . . . . . . . 12
6. Operator of a Network Employing RFC3168 FIFO Bottlenecks . . 13
6.1. Preferred Options . . . . . . . . . . . . . . . . . . . . 13
6.1.1. Upgrade AQMs to an L4S-aware AQM . . . . . . . . . . 13
6.1.2. Configure Non-Coupled Dual Queue with Shallow
Target . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.3. Approximate Fair Dropping . . . . . . . . . . . . . . 14
6.1.4. Replace RFC3168 FIFO with RFC3168 FQ . . . . . . . . 14
6.1.5. Do Nothing . . . . . . . . . . . . . . . . . . . . . 15
6.2. Non-Preferred Options . . . . . . . . . . . . . . . . . . 15
6.2.1. Configure Non-Coupled Dual Queue Treating ECT(1) as
NotECT . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.2. WRED with ECT(1) Differentiation . . . . . . . . . . 15
6.2.3. Configure AQM to treat ECT(1) as NotECT . . . . . . . 16
6.2.4. ECT(1) Tunnel Bypass . . . . . . . . . . . . . . . . 16
6.3. Last Resort Options . . . . . . . . . . . . . . . . . . . 16
6.3.1. Disable RFC3168 Support . . . . . . . . . . . . . . . 16
6.3.2. Re-mark ECT(1) to NotECT Prior to AQM . . . . . . . . 17
7. Operator of a Network Employing RFC3168 FQ Bottlenecks . . . 17
8. Conclusion of the L4S experiment . . . . . . . . . . . . . . 18
8.1. Termination of a successful L4S experiment . . . . . . . 18
8.2. Termination of an unsuccessful L4S experiment . . . . . . 18
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
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11. Security Considerations . . . . . . . . . . . . . . . . . . . 19
12. Normative References . . . . . . . . . . . . . . . . . . . . 19
13. Informative References . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Low-latency, low-loss, scalable throughput (L4S) [RFC9330] 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 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) (also see [IANA-ECN]), and its use for
L4S has been specified in [RFC9331]. 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 [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 bandwidth delay
product of a flow, the less pronounced the problem becomes. Thus the
imbalance is often 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 CE
marks identically to both types. 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
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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) bottlenecks
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 [RFC9331]. 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 they could be deployed/enabled in
the 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.
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
bottleneck queue. 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.
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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 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 converges
"towards a rate that is as independent of RTT as is possible without
compromising stability or utilization" (see [RFC9331]). As a result,
L4S promotes a level of per-flow fairness beyond what is ordinarily
considered for classic senders, the RFC3168 issue notwithstanding.
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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 sharing a bottleneck queue [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 might be more important than long-term, same-RTT,
per-flow fairness, it might not be helpful 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.
3. Flow Queuing Systems
As noted above, the concern around RFC3168 coexistence mainly
concerns single-queue systems where classic and L4S traffic are
mixed. In a flow-queuing system, 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
of [RFC8290]), because of VPN tunneling (see Section 6.2 of
[RFC8290]), or due to deliberate configuration (see Section 7,
Paragraph 5).
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It is believed that the majority of FQ deployments in bottlenecks
today (e.g., Cake [Hoiland-Jorgensen]) 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. Section 7 discusses some remedies that can be
implemented by operators of FQ equipment in order to minimize this
risk. Additionally, end-host mitigations such as separating L4S and
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
bottlenecks 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.
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
[Holland]. They concluded that prevalence of CE-marking was low
across the ~800 ASNs 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
three small ISPs where prevalence of CE-marking was above ~70%,
indicating a likely deployment by the ISP. There were another four
small ASNs where the prevalence was between 10% and 20%, which may
also indicate deployment by the ISP. There were also roughly six
larger ASNs (and perhaps 20 small ASNs) where the prevalence was
between 3% and 8%.
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In 2017, Apple reported on their observations of ECN marking by
networks, broken out by 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
(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 was known to have deployed RFC3168
fq_codel equipment in some of their subnets, and in other subnets
there were 33 IPs where possible AQM activity was observed via CE-
marks and/or ECE flags, corresponding to approximately 10% of paths.
It was agreed (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., [Trammel], [Bauer], [Mandalari]) have examined
ECN traversal, but have not reported data on prevalence of CE-marking
by networks. 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.
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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 bottlenecks are flow-queuing systems (e.g.,
fq_codel [RFC8290] or COBALT [Palmei]).
[Briscoe] contains recommendations on some of the mechanisms that can
be used to detect RFC3168 bottlenecks. In particular, Section 4 of
[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 [RFC9330]. 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 via lab testing, limited field testing, large scale
field testing, 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.
Some of the recommendations in this section involve the sender
determining (through various means) the likelihood of a particular
path having a bottleneck that implements single queue RFC3168 AQM.
Since this determination can be imprecise, there exists some risk
that a path is incorrectly classified. In the case of false-
positives (where a path is erroneously believed to contain RFC3168),
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discontinuing the use of L4S on that path would result in a lost
opportunity for low-latency low-loss service, and thus likely an
unnecessary degradation in the quality of experience for the user.
In the case of false-negatives, the use of L4S has the potential to
result in a reduction in the throughput of non-L4S flows while the
L4S flow is active. In environments where the risk of false-
negatives is significant, it is recommended that hosts limit the use
of L4S congestion control to application-limited flows that are
especially sensitive to latency, latency variation and loss.
5.1. Server Type
If pre-deployment testing raises concerns about issues with RFC3168
bottlenecks, the actions taken may depend on the server 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 [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 [Briscoe], though note that this mechanism does not
differentiate between FIFO and FQ).
Depending on the details of the L4S congestion control
implementation, taking action based on the detection of RFC3168 FIFO
bottlenecks may not be needed for short transactional transfers that
are unlikely to achieve the steady-state conditions where unfairness
is likely to occur. 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
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* In-band detection as described in [Briscoe] could be performed
during the session
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, known target audience and those that serve an unknown target
audience.
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.
It is recommended that L4S experimental deployments begin with such
servers.
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
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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.
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 earlier section (based on server type) can be
taken to minimize the risk of unfairness.
It is recommended that operators of such hosts consider carefully
whether these hosts are appropriate for early experimentation with
L4S.
The interpretation of studies on ECN usage and their deployment
context (see Section 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. Additionally, the most recent
large scale study [Holland] indicated that there were a small number
of networks in which RFC3168 bottlenecks are more prevalent than the
global average. Therefore, it may be possible for a host to maintain
a list of networks where L4S should not be enabled, and, for other
networks, 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.
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6. Operator of a Network Employing RFC3168 FIFO Bottlenecks
While it is more preferable for L4S senders to detect problems
themselves, a network operator who has deployed equipment in a likely
bottleneck 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.
6.1. Preferred Options
The options in this section preserve the ability of the bottleneck to
CE-mark ECT(1) packets as well as ECT(0) packets. The result of
these options is that hosts utilizing classic (RFC3168) ECN and hosts
utilizing L4S ECN receive the benefit of ECN. Further with these
options, the hosts that choose to use L4S ECN see the benefit of
reduced latency and latency-variation compared to hosts that choose
instead to use classic ECN.
6.1.1. Upgrade AQMs to an L4S-aware AQM
If the RFC3168 AQM implementation can be upgraded to enable support
for L4S, either via [RFC9332] or via an L4S-aware FQ implementation,
this is the preferred approach to addressing potential unfairness,
because it additionally enables all of the benefits of L4S.
Section 4.2 of [RFC9330] contains a description of the options
available, including a discussion about L4S-aware FQ implementations.
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.
* 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 Classic
flows
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- 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 [RFC9332]. 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 [RFC9332]. As is
discussed in [RFC9331], this reordering would be either zero risk or
very low risk.
If classification based on the ECN field isn't possible in the
bottleneck, this option may still be useful if an external system can
be configured to reflect the ECN codepoint to another field that
could then be used as an alternative identifier to classify traffic
into Queue #1. For example, if at network ingress an edge router can
apply a local-use DSCP to ECT(1) & CE packets, the bottleneck can
then utilize a DSCP classifier. Similarly, in MPLS networks, ECT(1)
& CE packets could use a different EXP value [RFC5129] than classic
packets. More generally, any tunneling protocol can be used to proxy
the ECN value of the encapsulated packet to its outer header,
enabling bottlenecks to classify packets based on their input virtual
interface.
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.
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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. Non-Preferred Options
The options in this section come with a downside that they treat
ECT(1) packets as NotECT, and thus don't provide the latency/loss
benefit to flows marked ECT(1) (i.e. L4S flows). In the case that
there is a strong concern about per-flow fairness between L4S flows
and Classic flows in an RFC3168 FIFO bottleneck, and none of the
remedies in the previous section can be implemented, the options
listed in this section could be considered. These options are non-
preferred because bottlenecks that implement them create a dilemma
for operators of hosts, in that the application could see better
performance if it uses classic (RFC3168) ECN rather than L4S ECN.
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 Section 6.1.2
This option could potentially be implemented using an identifier
other than the ECN field, as discussed in Section 6.1.2.
6.2.2. WRED with ECT(1) Differentiation
This configuration is similar to the 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
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- Class #2: ECT(0) & CE packets - ECN enabled
This option could potentially be implemented using an identifier
other than the ECN field, as discussed in Section 6.1.2.
6.2.3. Configure AQM to treat ECT(1) as NotECT
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.
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
Explicit Congestion Notification at this bottleneck either. This
alternative is only mentioned in case there is no other way to
reconfigure an RFC3168 AQM.
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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; b) it could lead to problems with future
experiments that use ECT(1) in alternative ways to L4S; and c) it
would violate requirements in [RFC9331]. This alternative is
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 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 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
(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.
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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
* 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.
8.1. Termination of 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).
8.2. Termination of an unsuccessful L4S experiment
If the L4S experiment is deemed unsuccessful (e.g., 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, [RFC9331] requires L4S hosts to be
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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.
11. Security Considerations
For further study.
12. Normative References
[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>.
[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>.
[RFC9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC9330, January 2023,
<https://www.rfc-editor.org/info/rfc9330>.
[RFC9331] De Schepper, K. and B. Briscoe, Ed., "The Explicit
Congestion Notification (ECN) Protocol for Low Latency,
Low Loss, and Scalable Throughput (L4S)", RFC 9331,
DOI 10.17487/RFC9331, January 2023,
<https://www.rfc-editor.org/info/rfc9331>.
[RFC9332] De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
Queue Coupled Active Queue Management (AQM) for Low
Latency, Low Loss, and Scalable Throughput (L4S)",
RFC 9332, DOI 10.17487/RFC9332, January 2023,
<https://www.rfc-editor.org/info/rfc9332>.
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13. Informative References
[AFD] Pan, R., Breslau, L., Prabhakar, B., and 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 A. Berger, "Measuring the
State of ECN Readiness in Servers, Clients, and Routers",
Proc ACM 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 , 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>.
[Cisco-N9000]
"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., Rhee, I., and L. Xu, "CUBIC: A New TCP-Friendly
High-Speed TCP Variant", ACM SIGOPS Operating Systems
Review , 2008,
<https://www.cs.princeton.edu/courses/archive/fall16/
cos561/papers/Cubic08.pdf>.
[Hoiland-Jorgensen]
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>.
[Holland] 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>.
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[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,
<https://www.ietf.org/archive/id/draft-heist-tsvwg-ecn-
deployment-observations-02.html>.
[IANA-ECN] Internet Assigned Numbers Authority, "IANA ECN Field
Assignments", 2018, <https://www.iana.org/assignments/
dscp-registry/dscp-registry.xhtml#ecn-field>.
[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., Gupta, S., Imputato, P., Morton, J.,
Tahiliani, M., Avallone, S., and D. Taht, "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>.
[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>.
[Roddav] Roddav, N., Streit, K., Rodosek, G.D., and A. Pras, "On
the Usage of DSCP and ECN 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>.
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[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 Bar For The Deployment
of Congestion Control Algorithms", Hotnets'19 , 2019,
<https://www.cs.cmu.edu/~rware/assets/pdf/ware-
hotnets19.pdf>.
Author's Address
Greg White (editor)
CableLabs
Email: g.white@cablelabs.com
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