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Operational Guidance for Deployment of L4S in the Internet

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

       Operational Guidance for Deployment of L4S in the Internet


   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on 6 November 2021.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Per-Flow Fairness . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Detection of Classic ECN Bottlenecks  . . . . . . . . . . . .   6
     3.1.  Recent Studies  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Future Experiments  . . . . . . . . . . . . . . . . . . .   7
   4.  Operator of an L4S host . . . . . . . . . . . . . . . . . . .   8
     4.1.  Edge Servers  . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Other hosts . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . .  12
     5.3.  Configure Non-Coupled Dual Queue  . . . . . . . . . . . .  12
     5.4.  WRED with ECT(1) Differentation . . . . . . . . . . . . .  13
     5.5.  Disable RFC3168 Support . . . . . . . . . . . . . . . . .  13
     5.6.  Re-mark ECT(1) to NotECT Prior to AQM . . . . . . . . . .  14
   6.  Operator of a Network Employing RFC3168 FQ Bottlenecks  . . .  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  . . . . .  15
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  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 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

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   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])
   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

   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 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

   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

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   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 could be deployed/
   enabled in the future, hence 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

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   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

   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

   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 for classic senders, the RFC3168 issue

   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 [Cubic].  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 [Harm] to classic flows need to be
   understood.  Most importantly, if there are situations in which the

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   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.  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.  Recent Studies

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

   In 2020, Akamai conducted a study
   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].  They concluded that prevalence of CE-marking was low
   across the ~800 ASNs observed, but it was growing, and that they
   could not determine whether the CE marking was due to a single queue
   or FQ.  There were a small handful (5-7) of ASNs showing evidence of
   CE-marking across more than 10% of their client IPs, and the global
   baseline was CE-marking across 0.3% of IPs.

   In 2017, Apple reported [TCPECN] on their observations of ECN marking
   by networks, broken out by country.  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, leaving

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   three countries: China 1% of paths, Mexico 3.2% of paths, France 6%
   of paths.  The percentage in France appears consistent with reports
   UyvpwUiNw0obd_EylBBV7kDRIHs/) that fq_codel has been implemented in
   DSL home routers deployed by

   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].  This ISP had deployed
   RFC3168 fq_codel equipment in some of their subnets, but in other
   subnets there were 33 IPs where CE-marking was possibly observed,
   corresponding to approximately 10% of paths, significantly greater
   than the baseline reported by Akamai.  It was agreed
   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 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])
   have examined ECN traversal, but have not reported data on prevalence
   of CE-marking by networks.

3.2.  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 (
   5.3) of [RFC8290]) or because of VPN tunneling (see 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

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   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 are upgraded to support L4S
   (see Section 6) 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] contains recommendations on some of the mechanisms that
   can be used to detect RFC3168 bottlenecks.  In particular, Section 4
   of [Detection] outlines an approach for out-band-detection of RFC3168

4.  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, via lab testing, limited field testing, large scale
   field testing, etc.  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

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

   *  General purpose servers (e.g. web servers)

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      -  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]).

      -  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], 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

      -  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.

   *  Specialized servers handling long-running sessions (e.g. cloud

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

      -  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] could be
         performed during the session

   TODO: discussion of risk of incorrectly classifying a path

   In addition, the responsibilities of and actions taken by a sender
   may 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.

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4.1.  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.

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4.2.  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) 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.

5.  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 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.

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5.1.  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.

5.2.  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

   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

5.3.  Configure Non-Coupled Dual Queue

   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 Classic

      -  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

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   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.

   Option 2:

   *  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

   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.  WRED with ECT(1) Differentation

   This configuration is similar to Option 2 in the previous section,
   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.  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.  This
   alternative is only mentioned in case there is no other way to
   reconfigure an RFC3168 AQM.

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5.6.  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.  This
   alternative is only mentioned 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.

6.  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) 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, 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 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.  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

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   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

7.  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 of the 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

7.2.  Unsuccessful termination of the 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] 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).

8.  Contributors

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

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9.  IANA Considerations


10.  Security Considerations

   For further study.

11.  Informative References

   [Akamai]   Holland, J., "Latency & AQM Observations on the Internet",
              IETF MAPRG interim-2020-maprg-01, August 2020,

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

   [COBALT]   Palmei, J. and et al., "Design and Evaluation of COBALT
              Queue Discipline", IEEE International Symposium on Local
              and Metropolitan Area Networks 2019, 2019,

   [Cubic]    Ha, S., Rhee, I., and L. Xu, "CUBIC: A New TCP-Friendly
              High-Speed TCP Variant", ACM SIGOPS Operating Systems
              Review , 2008,

              Briscoe, B. and A.S. Ahmed, "TCP Prague Fall-back on
              Detection of a Classic ECN AQM", ArXiv , February 2021,

              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,

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              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,

   [Harm]     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,

              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, <

              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,

              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, <

              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,

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   [IANA-ECN] Internet Assigned Numbers Authority, "IANA ECN Field
              Assignments", 2018, <

              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,

   [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,

   [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,

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <>.

   [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,

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

   [TCPECN]   Bhooma, P., "TCP ECN: Experience with enabling ECN on the
              Internet", 98th IETF MAPRG Presentation , 2017,

Author's Address

   Greg White (editor)


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