Explicit Congestion Notification (ECN) Protocol for Very Low Queuing Delay (L4S)
draft-ietf-tsvwg-ecn-l4s-id-17

Document Type Active Internet-Draft (tsvwg WG)
Authors Koen De Schepper  , Bob Briscoe 
Last updated 2021-05-21
Replaces draft-briscoe-tsvwg-ecn-l4s-id
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Document shepherd Wesley Eddy
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Send notices to Wesley Eddy <wes@mti-systems.com>
Transport Services (tsv)                                  K. De Schepper
Internet-Draft                                           Nokia Bell Labs
Intended status: Experimental                            B. Briscoe, Ed.
Expires: November 22, 2021                                   Independent
                                                            May 21, 2021

  Explicit Congestion Notification (ECN) Protocol for Very Low Queuing
                              Delay (L4S)
                     draft-ietf-tsvwg-ecn-l4s-id-17

Abstract

   This specification defines the protocol to be used for a new network
   service called low latency, low loss and scalable throughput (L4S).
   L4S uses an Explicit Congestion Notification (ECN) scheme at the IP
   layer that is similar to the original (or 'Classic') ECN approach,
   except as specified within.  L4S uses 'scalable' congestion control,
   which induces much more frequent control signals from the network and
   it responds to them with much more fine-grained adjustments, so that
   very low (typically sub-millisecond on average) and consistently low
   queuing delay becomes possible for L4S traffic without compromising
   link utilization.  Thus even capacity-seeking (TCP-like) traffic can
   have high bandwidth and very low delay at the same time, even during
   periods of high traffic load.

   The L4S identifier defined in this document distinguishes L4S from
   'Classic' (e.g. TCP-Reno-friendly) traffic.  It gives an incremental
   migration path so that suitably modified network bottlenecks can
   distinguish and isolate existing traffic that still follows the
   Classic behaviour, to prevent it degrading the low queuing delay and
   low loss of L4S traffic.  This specification defines the rules that
   L4S transports and network elements need to follow with the intention
   that L4S flows neither harm each other's performance nor that of
   Classic traffic.  Examples of new active queue management (AQM)
   marking algorithms and examples of new transports (whether TCP-like
   or real-time) are specified separately.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on November 22, 2021.

Copyright Notice

   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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Latency, Loss and Scaling Problems  . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   2.  Choice of L4S Packet Identifier: Requirements . . . . . . . .   9
   3.  L4S Packet Identification . . . . . . . . . . . . . . . . . .  10
   4.  Transport Layer Behaviour (the 'Prague Requirements') . . . .  11
     4.1.  Codepoint Setting . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Prerequisite Transport Feedback . . . . . . . . . . . . .  11
     4.3.  Prerequisite Congestion Response  . . . . . . . . . . . .  12
     4.4.  Filtering or Smoothing of ECN Feedback  . . . . . . . . .  15
   5.  Network Node Behaviour  . . . . . . . . . . . . . . . . . . .  15
     5.1.  Classification and Re-Marking Behaviour . . . . . . . . .  15
     5.2.  The Strength of L4S CE Marking Relative to Drop . . . . .  17
     5.3.  Exception for L4S Packet Identification by Network Nodes
           with Transport-Layer Awareness  . . . . . . . . . . . . .  18
     5.4.  Interaction of the L4S Identifier with other Identifiers   18
       5.4.1.  DualQ Examples of Other Identifiers Complementing L4S
               Identifiers . . . . . . . . . . . . . . . . . . . . .  18
         5.4.1.1.  Inclusion of Additional Traffic with L4S  . . . .  18
         5.4.1.2.  Exclusion of Traffic From L4S Treatment . . . . .  20
         5.4.1.3.  Generalized Combination of L4S and Other
                   Identifiers . . . . . . . . . . . . . . . . . . .  21
       5.4.2.  Per-Flow Queuing Examples of Other Identifiers

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               Complementing L4S Identifiers . . . . . . . . . . . .  22
     5.5.  Limiting Packet Bursts from Links Supporting L4S AQMs . .  22
   6.  Behaviour of Tunnels and Encapsulations . . . . . . . . . . .  23
     6.1.  No Change to ECN Tunnels and Encapsulations in General  .  23
     6.2.  VPN Behaviour to Avoid Limitations of Anti-Replay . . . .  24
   7.  L4S Experiments . . . . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Open Questions  . . . . . . . . . . . . . . . . . . . . .  25
     7.2.  Open Issues . . . . . . . . . . . . . . . . . . . . . . .  26
     7.3.  Future Potential  . . . . . . . . . . . . . . . . . . . .  27
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     11.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  The 'Prague L4S Requirements'  . . . . . . . . . . .  38
     A.1.  Requirements for Scalable Transport Protocols . . . . . .  39
       A.1.1.  Use of L4S Packet Identifier  . . . . . . . . . . . .  39
       A.1.2.  Accurate ECN Feedback . . . . . . . . . . . . . . . .  39
       A.1.3.  Capable of Replacement by Classic Congestion Control   39
       A.1.4.  Fall back to Classic Congestion Control on Packet
               Loss  . . . . . . . . . . . . . . . . . . . . . . . .  40
       A.1.5.  Coexistence with Classic Congestion Control at
               Classic ECN bottlenecks . . . . . . . . . . . . . . .  41
       A.1.6.  Reduce RTT dependence . . . . . . . . . . . . . . . .  44
       A.1.7.  Scaling down to fractional congestion windows . . . .  44
       A.1.8.  Measuring Reordering Tolerance in Time Units  . . . .  46
     A.2.  Scalable Transport Protocol Optimizations . . . . . . . .  48
       A.2.1.  Setting ECT in Control Packets and Retransmissions  .  48
       A.2.2.  Faster than Additive Increase . . . . . . . . . . . .  49
       A.2.3.  Faster Convergence at Flow Start  . . . . . . . . . .  49
   Appendix B.  Compromises in the Choice of L4S Identifier  . . . .  50
   Appendix C.  Potential Competing Uses for the ECT(1) Codepoint  .  55
     C.1.  Integrity of Congestion Feedback  . . . . . . . . . . . .  55
     C.2.  Notification of Less Severe Congestion than CE  . . . . .  56
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56

1.  Introduction

   This specification defines the protocol to be used for a new network
   service called low latency, low loss and scalable throughput (L4S).
   L4S uses an Explicit Congestion Notification (ECN) scheme at the IP
   layer that is similar to the original (or 'Classic') Explicit
   Congestion Notification (ECN [RFC3168]).  RFC 3168 required an ECN
   mark to be equivalent to a drop, both when applied in the network and
   when responded to by a transport.  Unlike Classic ECN marking, the
   network applies L4S marking more immediately and more aggressively
   than drop, and the transport response to each mark is reduced and

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   smoothed relative to that for drop.  The two changes counterbalance
   each other so that the throughput of an L4S flow will be roughly the
   same as a comparable non-L4S flow under the same conditions.
   Nonetheless, the much more frequent control signals and the finer
   responses to them result in very low queuing delay without
   compromising link utilization, and this low delay can be maintained
   during high load.  Very low queuing delay means that queuing due to
   the e2e congestion control is less than 1 millisecond (ms) on average
   and less than about 2 ms at the 99th percentile.

   L4S relies on 'scalable' congestion controls for these delay
   properties and for preserving low delay as flow rate scales, hence
   the name.  The congestion control used in Data Center TCP (DCTCP) is
   an example of a scalable congestion control, but DCTCP is applicable
   solely to controlled environments like data centres [RFC8257],
   because it is too aggressive to co-exist with existing TCP-Reno-
   friendly traffic.  The DualQ Coupled AQM, which is defined in a
   complementary experimental specification
   [I-D.ietf-tsvwg-aqm-dualq-coupled], is an AQM framework that enables
   scalable congestion controls derived from DCTCP to co-exist with
   existing traffic, each getting roughly the same flow rate when they
   compete under similar conditions.  Note that a scalable congestion
   control is still not safe to deploy on the Internet unless it
   satisfies the requirements listed in Section 4.

   L4S is not only for elastic (TCP-like) traffic - there are scalable
   congestion controls for real-time media, such as the L4S variant of
   the SCReAM [RFC8298] real-time media congestion avoidance technique
   (RMCAT).  The factor that distinguishes L4S from Classic traffic is
   its behaviour in response to congestion.  The transport wire
   protocol, e.g. TCP, QUIC, SCTP, DCCP, RTP/RTCP, is orthogonal (and
   therefore not suitable for distinguishing L4S from Classic packets).

   The L4S identifier defined in this document is the key piece that
   distinguishes L4S from 'Classic' (e.g. Reno-friendly) traffic.  It
   gives an incremental migration path so that suitably modified network
   bottlenecks can distinguish and isolate existing Classic traffic from
   L4S traffic to prevent the former from degrading the very low delay
   and loss of the new scalable transports, without harming Classic
   performance at these bottlenecks.  Initial implementation of the
   separate parts of the system has been motivated by the performance
   benefits.

1.1.  Latency, Loss and Scaling Problems

   Latency is becoming the critical performance factor for many (most?)
   applications on the public Internet, e.g. interactive Web, Web
   services, voice, conversational video, interactive video, interactive

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   remote presence, instant messaging, online gaming, remote desktop,
   cloud-based applications, and video-assisted remote control of
   machinery and industrial processes.  In the 'developed' world,
   further increases in access network bit-rate offer diminishing
   returns, whereas latency is still a multi-faceted problem.  In the
   last decade or so, much has been done to reduce propagation time by
   placing caches or servers closer to users.  However, queuing remains
   a major intermittent component of latency.

   The Diffserv architecture provides Expedited Forwarding [RFC3246], so
   that low latency traffic can jump the queue of other traffic.  If
   growth in high-throughput latency-sensitive applications continues,
   periods with solely latency-sensitive traffic will become
   increasingly common on links where traffic aggregation is low.  For
   instance, on the access links dedicated to individual sites (homes,
   small enterprises or mobile devices).  These links also tend to
   become the path bottleneck under load.  During these periods, if all
   the traffic were marked for the same treatment at these bottlenecks,
   Diffserv would make no difference.  Instead, it becomes imperative to
   remove the underlying causes of any unnecessary delay.

   The bufferbloat project has shown that excessively-large buffering
   ('bufferbloat') has been introducing significantly more delay than
   the underlying propagation time.  These delays appear only
   intermittently--only when a capacity-seeking (e.g. TCP) flow is long
   enough for the queue to fill the buffer, making every packet in other
   flows sharing the buffer sit through the queue.

   Active queue management (AQM) was originally developed to solve this
   problem (and others).  Unlike Diffserv, which gives low latency to
   some traffic at the expense of others, AQM controls latency for _all_
   traffic in a class.  In general, AQM methods introduce an increasing
   level of discard from the buffer the longer the queue persists above
   a shallow threshold.  This gives sufficient signals to capacity-
   seeking (aka. greedy) flows to keep the buffer empty for its intended
   purpose: absorbing bursts.  However, RED [RFC2309] and other
   algorithms from the 1990s were sensitive to their configuration and
   hard to set correctly.  So, this form of AQM was not widely deployed.

   More recent state-of-the-art AQM methods, e.g. FQ-CoDel [RFC8290],
   PIE [RFC8033], Adaptive RED [ARED01], are easier to configure,
   because they define the queuing threshold in time not bytes, so it is
   invariant for different link rates.  However, no matter how good the
   AQM, the sawtoothing sending window of a Classic congestion control
   will either cause queuing delay to vary or cause the link to be
   under-utilized.  Even with a perfectly tuned AQM, the additional
   queuing delay will be of the same order as the underlying speed-of-
   light delay across the network.

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   If a sender's own behaviour is introducing queuing delay variation,
   no AQM in the network can 'un-vary' the delay without significantly
   compromising link utilization.  Even flow-queuing (e.g. [RFC8290]),
   which isolates one flow from another, cannot isolate a flow from the
   delay variations it inflicts on itself.  Therefore those applications
   that need to seek out high bandwidth but also need low latency will
   have to migrate to scalable congestion control.

   Altering host behaviour is not enough on its own though.  Even if
   hosts adopt low latency behaviour (scalable congestion controls),
   they need to be isolated from the behaviour of existing Classic
   congestion controls that induce large queue variations.  L4S enables
   that migration by providing latency isolation in the network and
   distinguishing the two types of packets that need to be isolated: L4S
   and Classic.  L4S isolation can be achieved with a queue per flow
   (e.g. [RFC8290]) but a DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] is
   sufficient, and actually gives better tail latency.  Both approaches
   are addressed in this document.

   The DualQ solution was developed to make very low latency available
   without requiring per-flow queues at every bottleneck.  This was
   because FQ has well-known downsides - not least the need to inspect
   transport layer headers in the network, which makes it incompatible
   with privacy approaches such as IPSec VPN tunnels, and incompatible
   with link layer queue management, where transport layer headers can
   be hidden, e.g. 5G.

   Latency is not the only concern addressed by L4S: It was known when
   TCP congestion avoidance was first developed that it would not scale
   to high bandwidth-delay products (footnote 6 of Jacobson and Karels
   [TCP-CA]).  Given regular broadband bit-rates over WAN distances are
   already [RFC3649] beyond the scaling range of Reno congestion
   control, 'less unscalable' Cubic [RFC8312] and
   Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been
   successfully deployed.  However, these are now approaching their
   scaling limits.  Unfortunately, fully scalable congestion controls
   such as DCTCP [RFC8257] outcompete Classic ECN congestion controls
   sharing the same queue, which is why they have been confined to
   private data centres or research testbeds.

   It turns out that these scalable congestion control algorithms that
   solve the latency problem can also solve the scalability problem of
   Classic congestion controls.  The finer sawteeth in the congestion
   window have low amplitude, so they cause very little queuing delay
   variation and the average time to recover from one congestion signal
   to the next (the average duration of each sawtooth) remains
   invariant, which maintains constant tight control as flow-rate
   scales.  A background paper [DCttH15] gives the full explanation of

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   why the design solves both the latency and the scaling problems, both
   in plain English and in more precise mathematical form.  The
   explanation is summarised without the maths in the L4S architecture
   document [I-D.ietf-tsvwg-l4s-arch].

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].  In this document, these words will appear with that
   interpretation only when in ALL CAPS.  Lower case uses of these words
   are not to be interpreted as carrying RFC-2119 significance.

   Classic Congestion Control:  A congestion control behaviour that can
      co-exist with standard Reno [RFC5681] without causing
      significantly negative impact on its flow rate [RFC5033].  With
      Classic congestion controls, as flow rate scales, the number of
      round trips between congestion signals (losses or ECN marks) rises
      with the flow rate.  So it takes longer and longer to recover
      after each congestion event.  Therefore control of queuing and
      utilization becomes very slack, and the slightest disturbance
      prevents a high rate from being attained [RFC3649].

      For instance, with 1500 byte packets and an end-to-end round trip
      time (RTT) of 36 ms, over the years, as Reno flow rate scales from
      2 to 100 Mb/s the number of round trips taken to recover from a
      congestion event rises proportionately, from 4 round trips to 200.
      Cubic [RFC8312] was developed to be less unscalable, but it is
      approaching its scaling limit; with the same RTT of 36ms, at
      100Mb/s it takes about 106 round trips to recover, and at 800 Mb/s
      its recovery time triples to over 340 round trips, or still more
      than 12 seconds (Reno would take 57 seconds).  Cubic only becomes
      significantly better than Reno at high delay and rate
      combinations, for example at 90 ms RTT and 800 Mb/s a Reno flow
      takes 4000 RTTs or 6 minutes to recover, whereas Cubic 'only'
      needs 188 RTTs, which is still 17 seconds (double its recovery
      time at 100Mb/s).

   Scalable Congestion Control:  A congestion control where the average
      time from one congestion signal to the next (the recovery time)
      remains invariant as the flow rate scales, all other factors being
      equal.  This maintains the same degree of control over queueing
      and utilization whatever the flow rate, as well as ensuring that
      high throughput is robust to disturbances.  For instance, DCTCP
      averages 2 congestion signals per round-trip whatever the flow
      rate, as do other recently developed scalable congestion controls,
      e.g. Relentless TCP [Mathis09], TCP Prague

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      [I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux] and
      the L4S variant of SCREAM for real-time media [RFC8298]).  See
      Section 4.3 for more explanation.

   Classic service:  The Classic service is intended for all the
      congestion control behaviours that co-exist with Reno [RFC5681]
      (e.g. Reno itself, Cubic [RFC8312], Compound
      [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]).  The term 'Classic
      queue' means a queue providing the Classic service.

   Low-Latency, Low-Loss Scalable throughput (L4S) service:  The 'L4S'
      service is intended for traffic from scalable congestion control
      algorithms, such as TCP Prague
      [I-D.briscoe-iccrg-prague-congestion-control].  The L4S service is
      for more general traffic than just TCP Prague--it allows the set
      of congestion controls with similar scaling properties to Prague
      to evolve, such as the examples listed above (Relentless, SCReAM).
      The term 'L4S queue' means a queue providing the L4S service.

      The terms Classic or L4S can also qualify other nouns, such as
      'queue', 'codepoint', 'identifier', 'classification', 'packet',
      'flow'.  For example: an L4S packet means a packet with an L4S
      identifier sent from an L4S congestion control.

      Both Classic and L4S services can cope with a proportion of
      unresponsive or less-responsive traffic as well, as long as it
      does not build a queue (e.g. DNS, VoIP, game sync datagrams, etc).

   Reno-friendly:  The subset of Classic traffic that is friendly to the
      standard Reno congestion control defined for TCP in [RFC5681].
      Reno-friendly is used in place of 'TCP-friendly', given the latter
      has become imprecise, because the TCP protocol is now used with so
      many different congestion control behaviours, and Reno is used in
      non-TCP transports such as QUIC.

   Classic ECN:  The original Explicit Congestion Notification (ECN)
      protocol [RFC3168], which requires ECN signals to be treated the
      same as drops, both when generated in the network and when
      responded to by the sender.  The names used for the four
      codepoints of the 2-bit IP-ECN field are as defined in [RFC3168]:
      Not ECT, ECT(0), ECT(1) and CE, where ECT stands for ECN-Capable
      Transport and CE stands for Congestion Experienced.  A packet
      marked with the CE codepoint is termed 'ECN-marked' or sometimes
      just 'marked' where the context makes ECN obvious.

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1.3.  Scope

   The new L4S identifier defined in this specification is applicable
   for IPv4 and IPv6 packets (as for Classic ECN [RFC3168]).  It is
   applicable for the unicast, multicast and anycast forwarding modes.

   The L4S identifier is an orthogonal packet classification to the
   Differentiated Services Code Point (DSCP) [RFC2474].  Section 5.4
   explains what this means in practice.

   This document is intended for experimental status, so it does not
   update any standards track RFCs.  Therefore it depends on [RFC8311],
   which is a standards track specification that:

   o  updates the ECN proposed standard [RFC3168] to allow experimental
      track RFCs to relax the requirement that an ECN mark must be
      equivalent to a drop (when the network applies markings and/or
      when the sender responds to them).  For instance, in the ABE
      experiment [RFC8511] this permits a sender to respond less to ECN
      marks than to drops;

   o  changes the status of the experimental ECN nonce [RFC3540] to
      historic;

   o  makes consequent updates to the following additional proposed
      standard RFCs to reflect the above two bullets:

      *  ECN for RTP [RFC6679];

      *  the congestion control specifications of various DCCP
         congestion control identifier (CCID) profiles [RFC4341],
         [RFC4342], [RFC5622].

   This document is about identifiers that are used for interoperation
   between hosts and networks.  So the audience is broad, covering
   developers of host transports and network AQMs, as well as covering
   how operators might wish to combine various identifiers, which would
   require flexibility from equipment developers.

2.  Choice of L4S Packet Identifier: Requirements

   This subsection briefly records the process that led to the chosen
   L4S identifier.

   The identifier for packets using the Low Latency, Low Loss, Scalable
   throughput (L4S) service needs to meet the following requirements:

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   o  it SHOULD survive end-to-end between source and destination end-
      points: across the boundary between host and network, between
      interconnected networks, and through middleboxes;

   o  it SHOULD be visible at the IP layer;

   o  it SHOULD be common to IPv4 and IPv6 and transport-agnostic;

   o  it SHOULD be incrementally deployable;

   o  it SHOULD enable an AQM to classify packets encapsulated by outer
      IP or lower-layer headers;

   o  it SHOULD consume minimal extra codepoints;

   o  it SHOULD be consistent on all the packets of a transport layer
      flow, so that some packets of a flow are not served by a different
      queue to others.

   Whether the identifier would be recoverable if the experiment failed
   is a factor that could be taken into account.  However, this has not
   been made a requirement, because that would favour schemes that would
   be easier to fail, rather than those more likely to succeed.

   It is recognised that any choice of identifier is unlikely to satisfy
   all these requirements, particularly given the limited space left in
   the IP header.  Therefore a compromise will always be necessary,
   which is why all the above requirements are expressed with the word
   'SHOULD' not 'MUST'.

   After extensive assessment of alternative schemes, "ECT(1) and CE
   codepoints" was chosen as the best compromise.  Therefore this scheme
   is defined in detail in the following sections, while Appendix B
   records its pros and cons against the above requirements.

3.  L4S Packet Identification

   The L4S treatment is an experimental track alternative packet marking
   treatment [RFC4774] to the Classic ECN treatment in [RFC3168], which
   has been updated by [RFC8311] to allow experiments such as the one
   defined in the present specification.  Like Classic ECN, L4S ECN
   identifies both network and host behaviour: it identifies the marking
   treatment that network nodes are expected to apply to L4S packets,
   and it identifies packets that have been sent from hosts that are
   expected to comply with a broad type of sending behaviour.

   For a packet to receive L4S treatment as it is forwarded, the sender
   sets the ECN field in the IP header to the ECT(1) codepoint.  See

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   Section 4 for full transport layer behaviour requirements, including
   feedback and congestion response.

   A network node that implements the L4S service always classifies
   arriving ECT(1) packets for L4S treatment and by default classifies
   CE packets for L4S treatment unless the heuristics described in
   Section 5.3 are employed.  See Section 5 for full network element
   behaviour requirements, including classification, ECN-marking and
   interaction of the L4S identifier with other identifiers and per-hop
   behaviours.

4.  Transport Layer Behaviour (the 'Prague Requirements')

4.1.  Codepoint Setting

   A sender that wishes a packet to receive L4S treatment as it is
   forwarded, MUST set the ECN field in the IP header (v4 or v6) to the
   ECT(1) codepoint.

4.2.  Prerequisite Transport Feedback

   For a transport protocol to provide scalable congestion control
   (Section 4.3) it MUST provide feedback of the extent of CE marking on
   the forward path.  When ECN was added to TCP [RFC3168], the feedback
   method reported no more than one CE mark per round trip.  Some
   transport protocols derived from TCP mimic this behaviour while
   others report the accurate extent of ECN marking.  This means that
   some transport protocols will need to be updated as a prerequisite
   for scalable congestion control.  The position for a few well-known
   transport protocols is given below.

   TCP:  Support for the accurate ECN feedback requirements [RFC7560]
      (such as that provided by AccECN [I-D.ietf-tcpm-accurate-ecn]) by
      both ends is a prerequisite for scalable congestion control in
      TCP.  Therefore, the presence of ECT(1) in the IP headers even in
      one direction of a TCP connection will imply that both ends must
      be supporting accurate ECN feedback.  However, the converse does
      not apply.  So even if both ends support AccECN, either of the two
      ends can choose not to use a scalable congestion control, whatever
      the other end's choice.

   SCTP:  A suitable ECN feedback mechanism for SCTP could add a chunk
      to report the number of received CE marks
      (e.g. [I-D.stewart-tsvwg-sctpecn]), and update the ECN feedback
      protocol sketched out in Appendix A of the standards track
      specification of SCTP [RFC4960].

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   RTP over UDP:  A prerequisite for scalable congestion control is for
      both (all) ends of one media-level hop to signal ECN support
      [RFC6679] and use the new generic RTCP feedback format of
      [I-D.ietf-avtcore-cc-feedback-message].  The presence of ECT(1)
      implies that both (all) ends of that media-level hop support ECN.
      However, the converse does not apply.  So each end of a media-
      level hop can independently choose not to use a scalable
      congestion control, even if both ends support ECN.

   QUIC:  Support for sufficiently fine-grained ECN feedback is provided
      by the v1 IETF QUIC transport [I-D.ietf-quic-transport].

   DCCP:  The ACK vector in DCCP [RFC4340] is already sufficient to
      report the extent of CE marking as needed by a scalable congestion
      control.

4.3.  Prerequisite Congestion Response

   As a condition for a host to send packets with the L4S identifier
   (ECT(1)), it SHOULD implement a congestion control behaviour that
   ensures that, in steady state, the average duration between induced
   ECN marks does not increase as flow rate scales up, all other factors
   being equal.  This is termed a scalable congestion control.  This
   invariant duration ensures that, as flow rate scales, the average
   period with no feedback information about capacity does not become
   excessive.  It also ensures that queue variations remain small,
   without having to sacrifice utilization.

   With a congestion control that sawtooths to probe capacity, this
   duration is called the recovery time, because each time the sawtooth
   yields, on average it take this time to recover to its previous high
   point.  A scalable congestion control does not have to sawtooth, but
   it has to coexist with scalable congestion controls that do.

   For instance, for DCTCP [RFC8257], TCP Prague
   [I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux] and the
   L4S variant of SCReAM [RFC8298], the average recovery time is always
   half a round trip (or half a reference round trip), whatever the flow
   rate.

   As with all transport behaviours, a detailed specification (probably
   an experimental RFC) is expected for each congestion control,
   following the guidelines for specifying new congestion control
   algorithms in [RFC5033].  In addition it is expected to document
   these L4S-specific matters, specifically the timescale over which the
   proportionality is averaged, and control of burstiness.  The recovery
   time requirement above is worded as a 'SHOULD' rather than a 'MUST'
   to allow reasonable flexibility for such implementations.

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   The condition 'all other factors being equal', allows the recovery
   time to be different for different round trip times, as long as it
   does not increase with flow rate for any particular RTT.

   Saying that the recovery time remains roughly invariant is equivalent
   to saying that the number of ECN CE marks per round trip remains
   invariant as flow rate scales, all other factors being equal.  For
   instance, an average recovery time of half of 1 RTT is equivalent to
   2 ECN marks per round trip.  For those familiar with steady-state
   congestion response functions, it is also equivalent to say that the
   congestion window is inversely proportional to the proportion of
   bytes in packets marked with the CE codepoint (see section 2 of
   [PI2]).

   In order to coexist safely with other Internet traffic, a scalable
   congestion control MUST NOT tag its packets with the ECT(1) codepoint
   unless it complies with the following bulleted requirements:

   o  A scalable congestion control MUST be capable of being replaced by
      a Classic congestion control (by application and/or by
      administrative control).  If a Classic congestion control is
      activated, it will not tag its packets with the ECT(1) codepoint
      (see Appendix A.1.3 for rationale).

   o  As well as responding to ECN markings, a scalable congestion
      control MUST react to packet loss in a way that will coexist
      safely with Classic congestion controls such as standard Reno
      [RFC5681], as required by [RFC5033] (see Appendix A.1.4 for
      rationale).

   o  In uncontrolled environments, monitoring MUST be implemented to
      support detection of problems with an ECN-capable AQM at the path
      bottleneck that appears not to support L4S and might be in a
      shared queue.  Such monitoring SHOULD be applied to live traffic
      that is using Scalable congestion control.  Alternatively,
      monitoring need not be applied to live traffic, if monitoring has
      been arranged to cover the paths that live traffic takes through
      uncontrolled environments.

      The detection function SHOULD be capable of making the congestion
      control adapt its ECN-marking response to coexist safely with
      Classic congestion controls such as standard Reno [RFC5681], as
      required by [RFC5033].  Alternatively, if adaptation is not
      implemented and problems with such an AQM are detected, the
      scalable congestion control MUST be replaced by a Classic
      congestion control.

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      Note that a scalable congestion control is not expected to change
      to setting ECT(0) while it transiently adapts to coexist with
      Classic congestion controls.

      See Appendix A.1.5 and [I-D.ietf-tsvwg-l4sops] for rationale.

   o  In the range between the minimum likely RTT and typical RTTs
      expected in the intended deployment scenario, a scalable
      congestion control MUST converge towards a rate that is as
      independent of RTT as is possible without compromising stability
      or efficiency (see Appendix A.1.6 for rationale).

   o  A scalable congestion control SHOULD remain responsive to
      congestion when typical RTTs over the public Internet are
      significantly smaller because they are no longer inflated by
      queuing delay.  It would be preferable for the minimum window of a
      scalable congestion control to be lower than 1 segment rather than
      use the timeout approach described for TCP in S.6.1.2 of [RFC3168]
      (or an equivalent for other transports).  However, a lower minimum
      is not set as a formal requirement for L4S experiments (see
      Appendix A.1.7 for rationale).

   o  A scalable congestion control's loss detection SHOULD be resilient
      to reordering over an adaptive time interval that scales with
      throughput and adapts to reordering (as in [RFC8985]), as opposed
      to counting only in fixed units of packets (as in the 3 DupACK
      rule of [RFC5681] and [RFC6675], which is not scalable).  As data
      rates increase (e.g., due to new and/or improved technology),
      congestion controls that detect loss by counting in units of
      packets become more likely to incorrectly treat reordering events
      as congestion-caused loss events (see Appendix A.1.8 for further
      rationale).  This requirement does not apply to congestion
      controls that are solely used in controlled environments where the
      network introduces hardly any reordering.

   o  A scalable congestion control is expected to limit the queue
      caused by bursts of packets.  It would not seem necessary to set
      the limit any lower than 10% of the minimum RTT expected in a
      typical deployment (e.g. additional queuing of roughly 250 us for
      the public Internet).  This would be converted to a number of
      packets under the worst-case assumption that the bottleneck link
      capacity equals the current flow rate.  No normative requirement
      to limit bursts is given here and, until there is more industry
      experience from the L4S experiment, it is not even known whether
      one is needed - it seems to be in an L4S sender's self-interest to
      limit bursts.

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   Each sender in a session can use a scalable congestion control
   independently of the congestion control used by the receiver(s) when
   they send data.  Therefore there might be ECT(1) packets in one
   direction and ECT(0) or Not-ECT in the other.

   Later (Section 5.4.1.1) this document discusses the conditions for
   mixing other "'Safe' Unresponsive Traffic" (e.g. DNS, LDAP, NTP,
   voice, game sync packets) with L4S traffic.  To be clear, although
   such traffic can share the same queue as L4S traffic, it is not
   appropriate for the sender to tag it as ECT(1), except in the
   (unlikely) case that it satisfies the above conditions.

4.4.  Filtering or Smoothing of ECN Feedback

   Section 5.2 below specifies that an L4S AQM is expected to signal L4S
   ECN without filtering or smoothing.  This contrasts with a Classic
   AQM, which filters out variations in the queue before signalling ECN
   marking or drop.  In the L4S architecture [I-D.ietf-tsvwg-l4s-arch],
   responsibility for smoothing out these variations shifts to the
   sender's congestion control.

   This shift of responsibility has the advantage that each sender can
   smooth variations over a timescale proportionate to its own RTT.
   Whereas, in the Classic approach, the network doesn't know the RTTs
   of all the flows, so it has to smooth out variations for a worst-case
   RTT to ensure stability.  For all the typical flows with shorter RTT
   than the worst-case, this makes congestion control unnecessarily
   sluggish.

   This also gives an L4S sender the choice not to smooth, depending on
   its context (start-up, congestion avoidance, etc).  Therefore, this
   document places no requirement on an L4S congestion control to smooth
   out variations in any particular way.  Implementers are encouraged to
   openly publish the approach they take to smoothing, and the results
   and experience they gain during the L4S experiment.

5.  Network Node Behaviour

5.1.  Classification and Re-Marking Behaviour

   A network node that implements the L4S service:

   o  MUST classify arriving ECT(1) packets for L4S treatment, unless
      overridden by another classifier (e.g., see Section 5.4.1.2);

   o  MUST classify arriving CE packets for L4S treatment as well,
      unless overridden by a another classifier or unless the exception
      referred to next applies;

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      CE packets might have originated as ECT(1) or ECT(0), but the
      above rule to classify them as if they originated as ECT(1) is the
      safe choice (see Appendix B for rationale).  The exception is
      where some flow-aware in-network mechanism happens to be available
      for distinguishing CE packets that originated as ECT(0), as
      described in Section 5.3, but there is no implication that such a
      mechanism is necessary.

   An L4S AQM treatment follows similar codepoint transition rules to
   those in RFC 3168.  Specifically, the ECT(1) codepoint MUST NOT be
   changed to any other codepoint than CE, and CE MUST NOT be changed to
   any other codepoint.  An ECT(1) packet is classified as ECN-capable
   and, if congestion increases, an L4S AQM algorithm will increasingly
   mark the ECN field as CE, otherwise forwarding packets unchanged as
   ECT(1).  Necessary conditions for an L4S marking treatment are
   defined in Section 5.2.

   Under persistent overload an L4S marking treatment MUST begin
   applying drop to L4S traffic until the overload episode has subsided,
   as recommended for all AQM methods in [RFC7567] (Section 4.2.1),
   which follows the similar advice in RFC 3168 (Section 7).  During
   overload, it MUST apply the same drop probability to L4S traffic as
   it would to Classic traffic.

   Where an L4S AQM is transport-aware, this requirement could be
   satisfied by using drop in only the most overloaded individual per-
   flow AQMs.  In a DualQ with flow-aware queue protection (e.g.
   [I-D.briscoe-docsis-q-protection]), this could be achieved by
   redirecting packets in those flows contributing most to the overload
   out of the L4S queue so that they are subjected to drop in the
   Classic queue.

   For backward compatibility in uncontrolled environments, a network
   node that implements the L4S treatment MUST also implement an AQM
   treatment for the Classic service as defined in Section 1.2.  This
   Classic AQM treatment need not mark ECT(0) packets, but if it does,
   see Section 5.2 for the strengths of the markings relative to drop.
   It MUST classify arriving ECT(0) and Not-ECT packets for treatment by
   this Classic AQM (for the DualQ Coupled AQM, see the extensive
   discussion on classification in Sections 2.3 and 2.5.1.1 of
   [I-D.ietf-tsvwg-aqm-dualq-coupled]).

   In case unforeseen problems arise with the L4S experiment, it MUST be
   possible to configure an L4S implementation to disable the L4S
   treatment.  Once disabled, all packets of all ECN codepoints will
   receive Classic treatment and ECT(1) packets MUST be treated as if
   they were {ToDo: Not-ECT / ECT(0) ?}.

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5.2.  The Strength of L4S CE Marking Relative to Drop

   The relative strengths of L4S CE and drop are irrelevant where AQMs
   are implemented in separate queues per-application-flow, which are
   then explicitly scheduled (e.g. with an FQ scheduler as in
   [RFC8290]).  Nonetheless, the relationship between them needs to be
   defined for the coupling between L4S and Classic congestion signals
   in a DualQ Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled], as below.

   Unless an AQM node schedules application flows explicitly, the
   likelihood that the AQM drops a Not-ECT Classic packet (p_C) MUST be
   roughly proportional to the square of the likelihood that it would
   have marked it if it had been an L4S packet (p_L).  That is

      p_C ~= (p_L / k)^2

   The constant of proportionality (k) does not have to be standardised
   for interoperability, but a value of 2 is RECOMMENDED.  The term
   'likelihood' is used above to allow for marking and dropping to be
   either probabilistic or deterministic.

   This formula ensures that Scalable and Classic flows will converge to
   roughly equal congestion windows, for the worst case of Reno
   congestion control.  This is because the congestion windows of
   Scalable and Classic congestion controls are inversely proportional
   to p_L and sqrt(p_C) respectively.  So squaring p_C in the above
   formula counterbalances the square root that characterizes Reno-
   friendly flows.

   Note that, contrary to RFC 3168, an AQM implementing the L4S and
   Classic treatments does not mark an ECT(1) packet under the same
   conditions that it would have dropped a Not-ECT packet, as allowed by
   [RFC8311], which updates RFC 3168.  However, if it marks ECT(0)
   packets, it does so under the same conditions that it would have
   dropped a Not-ECT packet [RFC3168].

   Also, L4S CE marking needs to be interpreted as an unsmoothed signal,
   in contrast to the Classic approach in which AQMs filter out
   variations before signalling congestion.  An L4S AQM SHOULD NOT
   smooth or filter out variations in the queue before signalling
   congestion.  In the L4S architecture [I-D.ietf-tsvwg-l4s-arch], the
   sender, not the network, is responsible for smoothing out variations.

   This requirement is worded as 'SHOULD NOT' rather than 'MUST NOT' to
   allow for the case where the signals from a Classic smoothed AQM are
   coupled with those from an unsmoothed L4S AQM.  Nonetheless, the
   spirit of the requirement is for all systems to expect that L4S ECN

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   signalling is unsmoothed and unfiltered, which is important for
   interoperability.

5.3.  Exception for L4S Packet Identification by Network Nodes with
      Transport-Layer Awareness

   To implement the L4S treatment, a network node does not need to
   identify transport-layer flows.  Nonetheless, if an implementer is
   willing to identify transport-layer flows at a network node, and if
   the most recent ECT packet in the same flow was ECT(0), the node MAY
   classify CE packets for Classic ECN [RFC3168] treatment.  In all
   other cases, a network node MUST classify all CE packets for L4S
   treatment.  Examples of such other cases are: i) if no ECT packets
   have yet been identified in a flow; ii) if it is not desirable for a
   network node to identify transport-layer flows; or iii) if the most
   recent ECT packet in a flow was ECT(1).

   If an implementer uses flow-awareness to classify CE packets, to
   determine whether the flow is using ECT(0) or ECT(1) it only uses the
   most recent ECT packet of a flow (this advice will need to be
   verified as part of L4S experiments).  This is because a sender might
   switch from sending ECT(1) (L4S) packets to sending ECT(0) (Classic
   ECN) packets, or back again, in the middle of a transport-layer flow
   (e.g. it might manually switch its congestion control module mid-
   connection, or it might be deliberately attempting to confuse the
   network).

5.4.  Interaction of the L4S Identifier with other Identifiers

   The examples in this section concern how additional identifiers might
   complement the L4S identifier to classify packets between class-based
   queues.  Firstly Section 5.4.1 considers two queues, L4S and Classic,
   as in the Coupled DualQ AQM [I-D.ietf-tsvwg-aqm-dualq-coupled],
   either alone (Section 5.4.1.1) or within a larger queuing hierarchy
   (Section 5.4.1.2).  Then Section 5.4.2 considers schemes that might
   combine per-flow 5-tuples with other identifiers.

5.4.1.  DualQ Examples of Other Identifiers Complementing L4S
        Identifiers

5.4.1.1.  Inclusion of Additional Traffic with L4S

   In a typical case for the public Internet a network element that
   implements L4S in a shared queue might want to classify some low-rate
   but unresponsive traffic (e.g. DNS, LDAP, NTP, voice, game sync
   packets) into the low latency queue to mix with L4S traffic.

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   In this case it would not be appropriate to call the queue an L4S
   queue, because it is shared by L4S and non-L4S traffic.  Instead it
   will be called the low latency or L queue.  The L queue then offers
   two different treatments:

   o  The L4S treatment, which is a combination of the L4S AQM treatment
      and a priority scheduling treatment;

   o  The low latency treatment, which is solely the priority scheduling
      treatment, without ECN-marking by the AQM.

   To identify packets for just the scheduling treatment, it would be
   inappropriate to use the L4S ECT(1) identifier, because such traffic
   is unresponsive to ECN marking.  Examples of relevant non-ECN
   identifiers are:

   o  address ranges of specific applications or hosts configured to be,
      or known to be, safe, e.g. hard-coded IoT devices sending low
      intensity traffic;

   o  certain low data-volume applications or protocols (e.g. ARP, DNS);

   o  specific Diffserv codepoints that indicate traffic with limited
      burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-
      Admit [RFC5865] or proposed NQB (Non-Queue-Building
      [I-D.ietf-tsvwg-nqb]) service classes or equivalent local-use
      DSCPs (see [I-D.briscoe-tsvwg-l4s-diffserv]).

   In summary, a network element that implements L4S in a shared queue
   MAY classify additional types of packets into the L queue based on
   identifiers other than the ECN field, but the types SHOULD be 'safe'
   to mix with L4S traffic, where 'safe' is explained in
   Section 5.4.1.1.1.

   A packet that carries one of these non-ECN identifiers to classify it
   into the L queue would not be subject to the L4S ECN marking
   treatment, unless it also carried an ECT(1) or CE codepoint.  The
   specification of an L4S AQM MUST define the behaviour for packets
   with unexpected combinations of codepoints, e.g. a non-ECN-based
   classifier for the L queue, but ECT(0) in the ECN field (for examples
   see section 2.5.1.1 of [I-D.ietf-tsvwg-aqm-dualq-coupled]).

   For clarity, non-ECN identifiers, such as the examples itemized
   above, might be used by some network operators who believe they
   identify non-L4S traffic that would be safe to mix with L4S traffic.
   They are not alternative ways for a host to indicate that it is
   sending L4S packets.  Only the ECT(1) ECN codepoint indicates to a
   network element that a host is sending L4S packets (and CE indicates

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   that it could have originated as ECT(1)).  Specifically ECT(1)
   indicates that the host claims its behaviour satisfies the
   prerequisite transport requirements in Section 4.

   To include additional traffic with L4S, a network element only reads
   identifiers such as those itemized above.  It MUST NOT alter these
   non-ECN identifiers, so that they survive for any potential use later
   on the network path.

5.4.1.1.1.  'Safe' Unresponsive Traffic

   The above section requires unresponsive traffic to be 'safe' to mix
   with L4S traffic.  Ideally this means that the sender never sends any
   sequence of packets at a rate that exceeds the available capacity of
   the bottleneck link.  However, typically an unresponsive transport
   does not even know the bottleneck capacity of the path, let alone its
   available capacity.  Nonetheless, an application can be considered
   safe enough if it paces packets out (not necessarily completely
   regularly) such that its maximum instantaneous rate from packet to
   packet stays well below a typical broadband access rate.

   This is a vague but useful definition, because many low latency
   applications of interest, such as DNS, voice, game sync packets, RPC,
   ACKs, keep-alives, could match this description.

5.4.1.2.  Exclusion of Traffic From L4S Treatment

   To extend the above example, an operator might want to exclude some
   traffic from the L4S treatment for a policy reason, e.g. security
   (traffic from malicious sources) or commercial (e.g. initially the
   operator may wish to confine the benefits of L4S to business
   customers).

   In this exclusion case, the operator MUST classify on the relevant
   locally-used identifiers (e.g. source addresses) before classifying
   the non-matching traffic on the end-to-end L4S ECN identifier.

   The operator MUST NOT alter the end-to-end L4S ECN identifier from
   L4S to Classic, because an operator decision to exclude certain
   traffic from L4S treatment is local-only.  The end-to-end L4S
   identifier then survives for other operators to use, or indeed, they
   can apply their own policy, independently based on their own choice
   of locally-used identifiers.  This approach also allows any operator
   to remove its locally-applied exclusions in future, e.g. if it wishes
   to widen the benefit of the L4S treatment to all its customers.

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   An operator that excludes traffic carrying the L4S identifier from
   L4S treatment MUST NOT treat such traffic as if it carries the ECT(0)
   codepoint, which could confuse the sender.

5.4.1.3.  Generalized Combination of L4S and Other Identifiers

   L4S concerns low latency, which it can provide for all traffic
   without differentiation and without _necessarily_ affecting bandwidth
   allocation.  Diffserv provides for differentiation of both bandwidth
   and low latency, but its control of latency depends on its control of
   bandwidth.  The two can be combined if a network operator wants to
   control bandwidth allocation but it also wants to provide low latency
   - for any amount of traffic within one of these allocations of
   bandwidth (rather than only providing low latency by limiting
   bandwidth) [I-D.briscoe-tsvwg-l4s-diffserv].

   The DualQ examples so far have been framed in the context of
   providing the default Best Efforts Per-Hop Behaviour (PHB) using two
   queues - a Low Latency (L) queue and a Classic (C) Queue.  This
   single DualQ structure is expected to be the most common and useful
   arrangement.  But, more generally, an operator might choose to
   control bandwidth allocation through a hierarchy of Diffserv PHBs at
   a node, and to offer one (or more) of these PHBs with a low latency
   and a Classic variant.

   In the first case, if we assume that a network element provides no
   PHBs except the DualQ, if a packet carries ECT(1) or CE, the network
   element would classify it for the L4S treatment irrespective of its
   DSCP.  And, if a packet carried (say) the EF DSCP, the network
   element could classify it into the L queue irrespective of its ECN
   codepoint.  However, where the DualQ is in a hierarchy of other PHBs,
   the classifier would classify some traffic into other PHBs based on
   DSCP before classifying between the low latency and Classic queues
   (based on ECT(1), CE and perhaps also the EF DSCP or other
   identifiers as in the above example).
   [I-D.briscoe-tsvwg-l4s-diffserv] gives a number of examples of such
   arrangements to address various requirements.

   [I-D.briscoe-tsvwg-l4s-diffserv] describes how an operator might use
   L4S to offer low latency for all L4S traffic as well as using
   Diffserv for bandwidth differentiation.  It identifies two main types
   of approach, which can be combined: the operator might split certain
   Diffserv PHBs between L4S and a corresponding Classic service.  Or it
   might split the L4S and/or the Classic service into multiple Diffserv
   PHBs.  In either of these cases, a packet would have to be classified
   on its Diffserv and ECN codepoints.

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   In summary, there are numerous ways in which the L4S ECN identifier
   (ECT(1) and CE) could be combined with other identifiers to achieve
   particular objectives.  The following categorization articulates
   those that are valid, but it is not necessarily exhaustive.  Those
   tagged 'Recommended-standard-use' could be set by the sending host or
   a network.  Those tagged 'Local-use' would only be set by a network:

   1.  Identifiers Complementing the L4S Identifier

       A.  Including More Traffic in the L Queue
           (Could use Recommended-standard-use or Local-use identifiers)

       B.  Excluding Certain Traffic from the L Queue
           (Local-use only)

   2.  Identifiers to place L4S classification in a PHB Hierarchy
       (Could use Recommended-standard-use or Local-use identifiers)

       A.  PHBs Before L4S ECN Classification

       B.  PHBs After L4S ECN Classification

5.4.2.  Per-Flow Queuing Examples of Other Identifiers Complementing L4S
        Identifiers

   At a node with per-flow queueing (e.g. FQ-CoDel [RFC8290]), the L4S
   identifier could complement the Layer-4 flow ID as a further level of
   flow granularity (i.e.  Not-ECT and ECT(0) queued separately from
   ECT(1) and CE packets).  "Risk of reordering Classic CE packets" in
   Appendix B discusses the resulting ambiguity if packets originally
   marked ECT(0) are marked CE by an upstream AQM before they arrive at
   a node that classifies CE as L4S.  It argues that the risk of
   reordering is vanishingly small and the consequence of such a low
   level of reordering is minimal.

   Alternatively, it could be assumed that it is not in a flow's own
   interest to mix Classic and L4S identifiers.  Then the AQM could use
   the ECN field to switch itself between a Classic and an L4S AQM
   behaviour within one per-flow queue.  For instance, for ECN-capable
   packets, the AQM might consist of a simple marking threshold and an
   L4S ECN identifier might simply select a shallower threshold than a
   Classic ECN identifier would.

5.5.  Limiting Packet Bursts from Links Supporting L4S AQMs

   As well as senders needing to limit packet bursts (Section 4.3),
   links need to limit the degree of burstiness they introduce.  In both
   cases (senders and links) this is a tradeoff, because batch-handling

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   of packets is done for good reason, e.g. processing efficiency or to
   make efficient use of medium acquisition delay.  Some take the
   attitude that there is no point reducing burst delay at the sender
   below that introduced by links (or vice versa).  However, delay
   reduction proceeds by cutting down 'the longest pole in the tent',
   which turns the spotlight on the next longest, and so on.

   This document does not set any quantified requirements for links to
   limit burst delay, primarily because link technologies are outside
   the remit of L4S specifications.  Nonetheless, it would not make
   sense to implement an L4S AQM that feeds into a particular link
   technology without also reviewing opportunities to reduce any form of
   burst delay introduced by that link technology.  This would at least
   limit the bursts that the link would otherwise introduce into the
   onward traffic, which would cause jumpy feedback to the sender as
   well as potential extra queuing delay downstream.  This document does
   not presume to even give guidance on an appropriate target for such
   burst delay until there is more industry experience of L4S.  However,
   as suggested in Section 4.3 it would not seem necessary to limit
   bursts lower than roughly 10% of the minimum base RTT expected in the
   typical deployment scenario (e.g. 250 us burst duration for links
   within the public Internet).

6.  Behaviour of Tunnels and Encapsulations

6.1.  No Change to ECN Tunnels and Encapsulations in General

   The L4S identifier is expected to work through and within any tunnel
   without modification, as long as the tunnel propagates the ECN field
   in any of the ways that have been defined since the first variant in
   the year 2001 [RFC3168].  L4S will also work with (but does not rely
   on) any of the more recent updates to ECN propagation in [RFC4301],
   [RFC6040] or [I-D.ietf-tsvwg-rfc6040update-shim].  However, it is
   likely that some tunnels still do not implement ECN propagation at
   all.  In these cases, L4S will work through such tunnels, but within
   them the outer header of L4S traffic will appear as Classic.

   AQMs are typically implemented where an IP-layer buffer feeds into a
   lower layer, so they are agnostic to link layer encapsulations.
   Where a bottleneck link is not IP-aware, the L4S identifier is still
   expected to work within any lower layer encapsulation without
   modification, as long it propagates the ECN field as defined for the
   link technology, for example for MPLS [RFC5129] or TRILL
   [I-D.ietf-trill-ecn-support].  In some of these cases, e.g. layer-3
   Ethernet switches, the AQM accesses the IP layer header within the
   outer encapsulation, so again the L4S identifier is expected to work
   without modification.  Nonetheless, the programme to define ECN for

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   other lower layers is still in progress
   [I-D.ietf-tsvwg-ecn-encap-guidelines].

6.2.  VPN Behaviour to Avoid Limitations of Anti-Replay

   If a mix of L4S and Classic packets is sent into the same security
   association (SA) of a virtual private network (VPN), and if the VPN
   egress is employing the optional anti-replay feature, it could
   inappropriately discard Classic packets (or discard the records in
   Classic packets) by mistaking their greater queuing delay for a
   replay attack (see [Heist21] for the potential performance impact).
   This known problem is common to both IPsec [RFC4301] and DTLS
   [RFC6347] VPNs, given they use similar anti-replay window mechanisms.
   The specifications of IPsec AH [RFC4302] and ESP [RFC4303] suggest
   that an implementer scales the size of the anti-replay window with
   interface speed, and the current draft of DTLS 1.3 says "The receiver
   SHOULD pick a window large enough to handle any plausible reordering,
   which depends on the data rate."  However, in practice, the size of
   the VPN's anti-replay window is not always scaled appropriately.

   To avoid this limitation, a VPN ingress participating in the L4S
   experiment SHOULD map packets onto two parallel SAs indexed by the
   lowest significant bit of the IP-ECN field.  The mapping of traffic
   onto these two parallel SAs is local to the VPN ingress and not
   negotiated with the VPN egress.  This recommendation applies to both
   transport and tunnel mode SAs.

   The above recommendation to use two parallel SAs applies to IPsec
   VPNs used in the L4S experiment whether using AH or ESP, and it
   applies to DTLS VPNs used in the L4S experiment whether encapsulated
   in UDP or in other protocols, for instance DTLS over DCCP [RFC5328],
   CAPWAP [RFC5415], SCTP [RFC6083] or SRTP [RFC5764].  These RFCs are
   not updated by the present document, which would only become
   appropriate if L4S progressed from the experimental to the standards
   track.  Longer term, if Classic became used rarely, two parallel SAs
   would become unnecessary.

   Where L4S is used over a VPN, and the ingress VPN's classifier cannot
   be modified as above, one of the alternatives listed in order of
   preference below might be possible instead:

   o  If the VPN can be configured to classify packets into different
      SAs based on DSCP, apply a different DSCP to Classic and L4S
      packets (these DSCPs only need to survive as far as the VPN
      ingress);

   o  If the anti-replay window configured at the VPN egress is
      insufficient, increase it; otherwise no further action is needed;

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   o  If none of the above are possible and it is necessary to use L4S,
      either of the following might be appropriate as a last resort:

      *  disable anti-replay protection at the VPN egress, after
         considering the security implications (optional anti-replay is
         mandatory in both IPsec and DTLS);

      *  configure the tunnel ingress not to propagate ECN to the outer,
         which would lose the benefits of L4S and Classic ECN over the
         VPN.

7.  L4S Experiments

   This section describes open questions that L4S Experiments ought to
   focus on.  This section also documents outstanding open issues that
   will need to be investigated as part of L4S experimentation, given
   they could not be fully resolved during the WG phase.  It also lists
   metrics that will need to be monitored during experiments
   (summarizing text elsewhere in L4S documents) and finally lists some
   potential future directions that researchers might wish to
   investigate.

   In addition to this section, [I-D.ietf-tsvwg-aqm-dualq-coupled] sets
   operational and management requirements for experiments with DualQ
   Coupled AQMs; and General operational and management requirements for
   experiments with L4S congestion controls are given in Section 4 and
   Section 5 above, e.g. co-existence and scaling requirements,
   incremental deployment arrangements.

   The specification of each scalable congestion control will need to
   include protocol-specific requirements for configuration and
   monitoring performance during experiments.  Appendix A of [RFC5706]
   provides a helpful checklist.

7.1.  Open Questions

   L4S experiments would be expected to answer the following questions:

   o  Have all the parts of L4S been deployed, and if so, what
      proportion of paths support it?

   o  Does use of L4S over the Internet result in significantly improved
      user experience?

   o  Has L4S enabled novel interactive applications?

   o  Did use of L4S over the Internet result in improvements to the
      following metrics:

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   o

      *  queue delay (mean and 99th percentile) under various loads;

      *  utilization;

      *  starvation / fairness;

      *  scaling range of flow rates and RTTs?

   o  How much does burstiness in the Internet affect L4S performance,
      and how much limitation of bustiness was needed and/or was
      realized - both at senders and at links, especially radio links?

   o  Was per-flow queue protection typically (un)necessary?

      *  How well did overload protection or queue protection work?

   o  How well did L4S flows coexist with Classic flows when sharing a
      bottleneck?

   o

      *  How frequently did problems arise?

      *  What caused any coexistence problems, and were any problems due
         to single-queue Classic ECN AQMs (this assumes single-queue
         Classic ECN AQMs can be distinguished from FQ ones)?

   o  How prevalent were problems with the L4S service due to tunnels /
      encapsulations that do not support ECN decapsulation?

   o  How easy was it to implement a fully compliant L4S congestion
      control, over various different transport protocols (TCP.  QUIC,
      RMCAT, etc)?

   Monitoring for harm to other traffic, specifically bandwidth
   starvation or excess queuing delay, will need to be conducted
   alongside all early L4S experiments.  It is hard, if not impossible,
   for an individual flow to measure its impact on other traffic.  So
   such monitoring will need to be conducted using bespoke monitoring
   across flows and/or across classes of traffic.

7.2.  Open Issues

   o  What is the best way forward to deal with L4S over single-queue
      Classic ECN AQM bottlenecks, given current problems with
      misdetecting L4S AQMs as Classic ECN AQMs?

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   o  Fixing the poor Interaction between current L4S congestion
      controls and CoDel with only Classic ECN support during flow
      startup.

7.3.  Future Potential

   Researchers might find that L4S opens up the following interesting
   areas for investigation:

   o  Potential for faster convergence time and tracking of available
      capacity;

   o  Potential for improvements to particular link technologies, and
      cross-layer interactions with them;

   o  Potential for using virtual queues, e.g. to further reduce latency
      jitter, or to leave headroom for capacity variation in radio
      networks;

   o  Development and specification of reverse path congestion control
      using L4S building bocks (e.g. AccECN, QUIC);

   o  Once queuing delay is cut down, what becomes the 'second longest
      pole in the tent' (other than the speed of light)?

   o  Novel alternatives to the existing set of L4S AQMs;

   o  Novel applications enabled by L4S.

8.  IANA Considerations

   The 01 codepoint of the ECN Field of the IP header is specified by
   the present Experimental RFC.  The process for an experimental RFC to
   assign this codepoint in the IP header (v4 and v6) is documented in
   Proposed Standard [RFC8311], which updates the Proposed Standard
   [RFC3168].

   When the present document is published as an RFC, IANA is asked to
   update the 01 entry in the registry, "ECN Field (Bits 6-7)" to the
   following (see https://www.iana.org/assignments/dscp-registry/dscp-
   registry.xhtml#ecn-field ):

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   +--------+-----------------------------+----------------------------+
   | Binary | Keyword                     | References                 |
   +--------+-----------------------------+----------------------------+
   | 01     | ECT(1) (ECN-Capable         | [RFC8311]                  |
   |        | Transport(1))[1]            | [RFC Errata 5399]          |
   |        |                             | [RFCXXXX]                  |
   +--------+-----------------------------+----------------------------+

   [XXXX is the number that the RFC Editor assigns to the present
   document (this sentence to be removed by the RFC Editor)].

9.  Security Considerations

   Approaches to assure the integrity of signals using the new
   identifier are introduced in Appendix C.1.  See the security
   considerations in the L4S architecture [I-D.ietf-tsvwg-l4s-arch] for
   further discussion of mis-use of the identifier, as well as extensive
   discussion of policing rate and latency in regard to L4S.

   The anti-replay window mechanism in VPNs is known to incorrectly
   discard higher delay packets (e.g.  Classic) when carried alongside
   low delay packets (e.g.  L4S) in the same VPN.  Section 6.2
   recommends that VPNs used in L4S experiments are configured to
   separate Classic and L4S traffic into separate security associations
   (SAs) to prevent this problem.  This is modelled on the multi-SA
   approach used for Diffserv over IPsec, which is intended to avoid the
   same problem (see section 4.1 of [RFC4301]).

   If a user taking part in the L4S experiment sets up a VPN without
   being aware of the above advice, and if the user allows anyone to
   send traffic into their VPN, they would open up a DoS vulnerability
   in which an attacker could induce the VPN's anti-replay mechanism to
   discard enough of the user's Classic (C) traffic (if they are
   receiving any) to cause a significant rate reduction.  While the user
   is actively downloading C traffic, the attacker sends C traffic into
   the VPN to fill the remainder of the bottleneck link, then sends
   intermittent L4S packets to maximize the chance of exceeding the
   VPN's replay window.  The user can prevent this attack by following
   the recommendations in Section 6.2.

   The recommendation to detect loss in time units prevents the ACK-
   splitting attacks described in [Savage-TCP].

10.  Acknowledgements

   Thanks to Richard Scheffenegger, John Leslie, David Taeht, Jonathan
   Morton, Gorry Fairhurst, Michael Welzl, Mikael Abrahamsson and Andrew
   McGregor for the discussions that led to this specification.  Ing-jyh

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   (Inton) Tsang was a contributor to the early drafts of this document.
   And thanks to Mikael Abrahamsson, Lloyd Wood, Nicolas Kuhn, Greg
   White, Tom Henderson, David Black, Gorry Fairhurst, Brian Carpenter,
   Jake Holland, Rod Grimes, Richard Scheffenegger, Sebastian Moeller,
   Neal Cardwell, Praveen Balasubramanian, Reza Marandian Hagh, Stuart
   Cheshire and Vidhi Goel for providing help and reviewing this draft
   and thanks to Ingemar Johansson for reviewing and providing
   substantial text.  Thanks to Sebastian Moeller for identifying the
   interaction with VPN anti-replay and to Jonathan Morton for
   identifying the attack based on this.  Particular thanks to tsvwg
   chairs Gorry Fairhurst, David Black and Wes Eddy for patiently
   helping this and the other L4S drafts through the IETF process.
   Appendix A listing the Prague L4S Requirements is based on text
   authored by Marcelo Bagnulo Braun that was originally an appendix to
   [I-D.ietf-tsvwg-l4s-arch].  That text was in turn based on the
   collective output of the attendees listed in the minutes of a 'bar
   BoF' on DCTCP Evolution during IETF-94 [TCPPrague].

   The authors' contributions were part-funded by the European Community
   under its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).  Bob Briscoe was also
   funded partly by the Research Council of Norway through the TimeIn
   project, partly by CableLabs and partly by the Comcast Innovation
   Fund.  The views expressed here are solely those of the authors.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,
              <https://www.rfc-editor.org/info/rfc4774>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/info/rfc6679>.

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11.2.  Informative References

   [A2DTCP]   Zhang, T., Wang, J., Huang, J., Huang, Y., Chen, J., and
              Y. Pan, "Adaptive-Acceleration Data Center TCP", IEEE
              Transactions on Computers 64(6):1522-1533, June 2015,
              <http://ieeexplore.ieee.org/xpl/
              articleDetails.jsp?arnumber=6871352>.

   [Ahmed19]  Ahmed, A., "Extending TCP for Low Round Trip Delay",
              Masters Thesis, Uni Oslo , August 2019,
              <https://www.duo.uio.no/handle/10852/70966>.

   [Alizadeh-stability]
              Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
              of DCTCP: Stability, Convergence, and Fairness", ACM
              SIGMETRICS 2011 , June 2011.

   [ARED01]   Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
              Algorithm for Increasing the Robustness of RED's Active
              Queue Management", ACIRI Technical Report , August 2001,
              <http://www.icir.org/floyd/red.html>.

   [DCttH15]  De Schepper, K., Bondarenko, O., Briscoe, B., and I.
              Tsang, "'Data Centre to the Home': Ultra-Low Latency for
              All", RITE Project Technical Report , 2015,
              <http://riteproject.eu/publications/>.

   [ecn-fallback]
              Briscoe, B. and A. Ahmed, "TCP Prague Fall-back on
              Detection of a Classic ECN AQM", bobbriscoe.net Technical
              Report TR-BB-2019-002, April 2020,
              <https://arxiv.org/abs/1911.00710>.

   [Heist21]  Heist, P., "Dropped Packets for Tunnels with Replay
              Protection Enabled", github README, May 2021,
              <https://github.com/heistp/l4s-tests/#dropped-packets-for-
              tunnels-with-replay-protection-enabled>.

   [I-D.briscoe-docsis-q-protection]
              Briscoe, B. and G. White, "Queue Protection to Preserve
              Low Latency", draft-briscoe-docsis-q-protection-00 (work
              in progress), July 2019.

   [I-D.briscoe-iccrg-prague-congestion-control]
              Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague
              Congestion Control", draft-briscoe-iccrg-prague-
              congestion-control-00 (work in progress), March 2021.

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   [I-D.briscoe-tsvwg-l4s-diffserv]
              Briscoe, B., "Interactions between Low Latency, Low Loss,
              Scalable Throughput (L4S) and Differentiated Services",
              draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
              November 2018.

   [I-D.ietf-avtcore-cc-feedback-message]
              Sarker, Z., Perkins, C., Singh, V., and M. A. Ramalho,
              "RTP Control Protocol (RTCP) Feedback for Congestion
              Control", draft-ietf-avtcore-cc-feedback-message-09 (work
              in progress), November 2020.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-34 (work
              in progress), January 2021.

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-14 (work in progress), February 2021.

   [I-D.ietf-tcpm-generalized-ecn]
              Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
              Congestion Notification (ECN) to TCP Control Packets",
              draft-ietf-tcpm-generalized-ecn-07 (work in progress),
              February 2021.

   [I-D.ietf-trill-ecn-support]
              Eastlake, D. E. and B. Briscoe, "TRILL (TRansparent
              Interconnection of Lots of Links): ECN (Explicit
              Congestion Notification) Support", draft-ietf-trill-ecn-
              support-07 (work in progress), February 2018.

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-14 (work in
              progress), March 2021.

   [I-D.ietf-tsvwg-ecn-encap-guidelines]
              Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
              Congestion Notification to Protocols that Encapsulate IP",
              draft-ietf-tsvwg-ecn-encap-guidelines-15 (work in
              progress), March 2021.

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   [I-D.ietf-tsvwg-l4s-arch]
              Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White,
              "Low Latency, Low Loss, Scalable Throughput (L4S) Internet
              Service: Architecture", draft-ietf-tsvwg-l4s-arch-08 (work
              in progress), November 2020.

   [I-D.ietf-tsvwg-l4sops]
              White, G., "Operational Guidance for Deployment of L4S in
              the Internet", draft-ietf-tsvwg-l4sops-00 (work in
              progress), May 2021.

   [I-D.ietf-tsvwg-nqb]
              White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
              Behavior (NQB PHB) for Differentiated Services", draft-
              ietf-tsvwg-nqb-05 (work in progress), March 2021.

   [I-D.ietf-tsvwg-rfc6040update-shim]
              Briscoe, B., "Propagating Explicit Congestion Notification
              Across IP Tunnel Headers Separated by a Shim", draft-ietf-
              tsvwg-rfc6040update-shim-13 (work in progress), March
              2021.

   [I-D.sridharan-tcpm-ctcp]
              Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
              "Compound TCP: A New TCP Congestion Control for High-Speed
              and Long Distance Networks", draft-sridharan-tcpm-ctcp-02
              (work in progress), November 2008.

   [I-D.stewart-tsvwg-sctpecn]
              Stewart, R. R., Tuexen, M., and X. Dong, "ECN for Stream
              Control Transmission Protocol (SCTP)", draft-stewart-
              tsvwg-sctpecn-05 (work in progress), January 2014.

   [LinuxPacedChirping]
              Misund, J. and B. Briscoe, "Paced Chirping - Rethinking
              TCP start-up", Proc. Linux Netdev 0x13 , March 2019,
              <https://www.netdevconf.org/0x13/session.html?talk-chirp>.

   [Mathis09]
              Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
              May 2009, <http://www.hpcc.jp/pfldnet2009/
              Program_files/1569198525.pdf>.

   [Paced-Chirping]
              Misund, J., "Rapid Acceleration in TCP Prague", Masters
              Thesis , May 2018,
              <https://riteproject.files.wordpress.com/2018/07/
              misundjoakimmastersthesissubmitted180515.pdf>.

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   [PI2]      De Schepper, K., Bondarenko, O., Tsang, I., and B.
              Briscoe, "PI^2 : A Linearized AQM for both Classic and
              Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
              2016,
              <http://dl.acm.org/citation.cfm?doid=2999572.2999578>.

   [PragueLinux]
              Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
              Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing
              the `TCP Prague' Requirements for Low Latency Low Loss
              Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
              March 2019, <https://www.netdevconf.org/0x13/
              session.html?talk-tcp-prague-l4s>.

   [QV]       Briscoe, B. and P. Hurtig, "Up to Speed with Queue View",
              RITE Technical Report D2.3; Appendix C.2, August 2015,
              <https://riteproject.files.wordpress.com/2015/12/rite-
              deliverable-2-3.pdf>.

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
              <https://www.rfc-editor.org/info/rfc2309>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
              <https://www.rfc-editor.org/info/rfc3246>.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,
              <https://www.rfc-editor.org/info/rfc3540>.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,
              <https://www.rfc-editor.org/info/rfc3649>.

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   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/info/rfc4340>.

   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, DOI 10.17487/RFC4341, March
              2006, <https://www.rfc-editor.org/info/rfc4341>.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              DOI 10.17487/RFC4342, March 2006,
              <https://www.rfc-editor.org/info/rfc4342>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,
              <https://www.rfc-editor.org/info/rfc5033>.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

   [RFC5328]  Adolf, A. and P. MacAvock, "A Uniform Resource Name (URN)
              Namespace for the Digital Video Broadcasting Project
              (DVB)", RFC 5328, DOI 10.17487/RFC5328, September 2008,
              <https://www.rfc-editor.org/info/rfc5328>.

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   [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>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [RFC5562]  Kuzmanovic, A., Mondal, A., Floyd, S., and K.
              Ramakrishnan, "Adding Explicit Congestion Notification
              (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
              DOI 10.17487/RFC5562, June 2009,
              <https://www.rfc-editor.org/info/rfc5562>.

   [RFC5622]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
              Control for Small Packets (TFRC-SP)", RFC 5622,
              DOI 10.17487/RFC5622, August 2009,
              <https://www.rfc-editor.org/info/rfc5622>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <https://www.rfc-editor.org/info/rfc5681>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,
              <https://www.rfc-editor.org/info/rfc5865>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

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

   [RFC6077]  Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
              Briscoe, "Open Research Issues in Internet Congestion
              Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
              <https://www.rfc-editor.org/info/rfc6077>.

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083,
              DOI 10.17487/RFC6083, January 2011,
              <https://www.rfc-editor.org/info/rfc6083>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6660]  Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
              Pre-Congestion Notification (PCN) States in the IP Header
              Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
              DOI 10.17487/RFC6660, July 2012,
              <https://www.rfc-editor.org/info/rfc6660>.

   [RFC6675]  Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
              and Y. Nishida, "A Conservative Loss Recovery Algorithm
              Based on Selective Acknowledgment (SACK) for TCP",
              RFC 6675, DOI 10.17487/RFC6675, August 2012,
              <https://www.rfc-editor.org/info/rfc6675>.

   [RFC7560]  Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
              "Problem Statement and Requirements for Increased Accuracy
              in Explicit Congestion Notification (ECN) Feedback",
              RFC 7560, DOI 10.17487/RFC7560, August 2015,
              <https://www.rfc-editor.org/info/rfc7560>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,
              <https://www.rfc-editor.org/info/rfc7713>.

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   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
              <https://www.rfc-editor.org/info/rfc8033>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [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>.

   [RFC8298]  Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
              for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
              2017, <https://www.rfc-editor.org/info/rfc8298>.

   [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>.

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,
              <https://www.rfc-editor.org/info/rfc8312>.

   [RFC8511]  Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", RFC 8511,
              DOI 10.17487/RFC8511, December 2018,
              <https://www.rfc-editor.org/info/rfc8511>.

   [RFC8985]  Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
              RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
              DOI 10.17487/RFC8985, February 2021,
              <https://www.rfc-editor.org/info/rfc8985>.

   [Savage-TCP]
              Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
              "TCP Congestion Control with a Misbehaving Receiver", ACM
              SIGCOMM Computer Communication Review 29(5):71--78,
              October 1999.

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   [sub-mss-prob]
              Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
              Window for Small Round Trip Times", BT Technical Report
              TR-TUB8-2015-002, May 2015,
              <https://arxiv.org/abs/1904.07598>.

   [TCP-CA]   Jacobson, V. and M. Karels, "Congestion Avoidance and
              Control", Laurence Berkeley Labs Technical Report ,
              November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>.

   [TCPPrague]
              Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul
              2015, 17:40, Prague", tcpprague mailing list archive ,
              July 2015, <https://www.ietf.org/mail-
              archive/web/tcpprague/current/msg00001.html>.

   [VCP]      Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
              "One more bit is enough", Proc. SIGCOMM'05, ACM CCR
              35(4)37--48, 2005,
              <http://doi.acm.org/10.1145/1080091.1080098>.

Appendix A.  The 'Prague L4S Requirements'

   This appendix is informative, not normative.  It gives a list of
   modifications to current scalable congestion controls so that they
   can be deployed over the public Internet and coexist safely with
   existing traffic.  The list complements the normative requirements in
   Section 4 that a sender has to comply with before it can set the L4S
   identifier in packets it sends into the Internet.  As well as
   necessary safety improvements (requirements) this appendix also
   includes preferable performance improvements (optimizations).

   These recommendations have become know as the Prague L4S
   Requirements, because they were originally identified at an ad hoc
   meeting during IETF-94 in Prague [TCPPrague].  They were originally
   called the 'TCP Prague Requirements', but they are not solely
   applicable to TCP, so the name and wording has been generalized for
   all transport protocols, and the name 'TCP Prague' is now used for a
   specific implementation of the requirements.

   At the time of writing, DCTCP [RFC8257] is the most widely used
   scalable transport protocol.  In its current form, DCTCP is specified
   to be deployable only in controlled environments.  Deploying it in
   the public Internet would lead to a number of issues, both from the
   safety and the performance perspective.  The modifications and
   additional mechanisms listed in this section will be necessary for
   its deployment over the global Internet.  Where an example is needed,
   DCTCP is used as a base, but it is likely that most of these

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   requirements equally apply to other scalable congestion controls,
   covering adaptive real-time media, etc., not just capacity-seeking
   behaviours.

A.1.  Requirements for Scalable Transport Protocols

A.1.1.  Use of L4S Packet Identifier

   Description: A scalable congestion control needs to distinguish the
   packets it sends from those sent by Classic congestion controls (see
   the precise normative requirement wording in Section 4.1).

   Motivation: It needs to be possible for a network node to classify
   L4S packets without flow state into a queue that applies an L4S ECN
   marking behaviour and isolates L4S packets from the queuing delay of
   Classic packets.

A.1.2.  Accurate ECN Feedback

   Description: The transport protocol for a scalable congestion control
   needs to provide timely, accurate feedback about the extent of ECN
   marking experienced by all packets (see the precise normative
   requirement wording in Section 4.2).

   Motivation: Classic congestion controls only need feedback about the
   existence of a congestion episode within a round trip, not precisely
   how many packets were marked with ECN or dropped.  Therefore, in
   2001, when ECN feedback was added to TCP [RFC3168], it could not
   inform the sender of more than one ECN mark per RTT.  Since then,
   requirements for more accurate ECN feedback in TCP have been defined
   in [RFC7560] and [I-D.ietf-tcpm-accurate-ecn] specifies a change to
   the TCP protocol to satisfy these requirements.  Most other transport
   protocols already satisfy this requirement (see Section 4.2).

A.1.3.  Capable of Replacement by Classic Congestion Control

   Description: It needs to be possible to replace the implementation of
   a scalable congestion control with a Classic control (see the precise
   normative requirement wording in Section 4.3).

   Motivation: L4S is an experimental protocol, therefore it seems
   prudent to be able to disable it at source in case of insurmountable
   problems, perhaps due to some unexpected interaction on a particular
   sender; over a particular path or network; with a particular receiver
   or even ultimately an insurmountable problem with the experiment as a
   whole.

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A.1.4.  Fall back to Classic Congestion Control on Packet Loss

   Description: As well as responding to ECN markings in a scalable way,
   a scalable congestion control needs to react to packet loss in a way
   that will coexist safely with a Reno congestion control [RFC5681]
   (see the precise normative requirement wording in Section 4.3).

   Motivation: Part of the safety conditions for deploying a scalable
   congestion control on the public Internet is to make sure that it
   behaves properly when it builds a queue at a network bottleneck that
   has not been upgraded to support L4S.  Packet loss can have many
   causes, but it usually has to be conservatively assumed that it is a
   sign of congestion.  Therefore, on detecting packet loss, a scalable
   congestion control will need to fall back to Classic congestion
   control behaviour.  If it does not comply with this requirement it
   could starve Classic traffic.

   A scalable congestion control can be used for different types of
   transport, e.g. for real-time media or for reliable transport like
   TCP.  Therefore, the particular Classic congestion control behaviour
   to fall back on will need to be dependent on the specific congestion
   control implementation.  In the particular case of DCTCP, the DCTCP
   specification [RFC8257] states that "It is RECOMMENDED that an
   implementation deal with loss episodes in the same way as
   conventional TCP."  For safe deployment of a scalable congestion
   control in the public Internet, the above requirement would need to
   be defined as a "MUST".

   Even though a bottleneck is L4S capable, it might still become
   overloaded and have to drop packets.  In this case, the sender may
   receive a high proportion of packets marked with the CE bit set and
   also experience loss.  Current DCTCP implementations each react
   differently to this situation.  At least one implementation reacts
   only to the drop signal (e.g. by halving the CWND) and at least
   another DCTCP implementation reacts to both signals (e.g. by halving
   the CWND due to the drop and also further reducing the CWND based on
   the proportion of marked packet).  A third approach for the public
   Internet has been proposed that adjusts the loss response to result
   in a halving when combined with the ECN response.  We believe that
   further experimentation is needed to understand what is the best
   behaviour for the public Internet, which may or not be one of these
   existing approaches.

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A.1.5.  Coexistence with Classic Congestion Control at Classic ECN
        bottlenecks

   Description: Monitoring has to be in place so that a non-L4S but ECN-
   capable AQM can be detected at path bottlenecks.  This is in case
   such an AQM has been implemented in a shared queue, in which case any
   long-running scalable flow would predominate over any simultaneous
   long-running Classic flow sharing the queue.  The requirement is
   written so that such a problem could either be resolved in real-time,
   or via administrative intervention (see the precise normative
   requirement wording in Section 4.3).

   Motivation: Similarly to the requirement in Appendix A.1.4, this
   requirement is a safety condition to ensure an L4S congestion control
   coexists well with Classic flows when it builds a queue at a shared
   network bottleneck that has not been upgraded to support L4S.
   Nonetheless, if necessary, it is considered reasonable to resolve
   such problems over management timescales (possibly involving human
   intervention) because:

   o  although a Classic flow can considerably reduce its throughput in
      the face of a competing scalable flow, it still makes progress and
      does not starve;

   o  implementations of a Classic ECN AQM in a queue that is intended
      to be shared are believed to be rare;

   o  detection of such AQMs is not always clear-cut; so focused out-of-
      band testing (or even contacting the relevant network operator)
      would improve certainty.

   Therefore, the relevant normative requirement (Section 4.3) is
   divided into three stages: monitoring, detection and action:

   Monitoring:  Monitoring involves collection of the measurement data
      to be analysed.  Monitoring is expressed as a 'MUST' for
      uncontrolled environments, although the placement of the
      monitoring function is left open.  Whether monitoring has to be
      applied in real-time is expressed as a 'SHOULD'.  This allows for
      the possibility that the operator of an L4S sender (e.g. a CDN)
      might prefer to test out-of-band for signs of Classic ECN AQMs,
      perhaps to avoid continually consuming resources to monitor live
      traffic.

   Detection:  Detection involves analysis of the monitored data to
      detect the likelihood of a Classic ECN AQM.  The requirements
      recommend that detection occurs live in real-time.  However,

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      detection is allowed to be deferred (e.g. it might involve further
      testing targeted at candidate AQMs);

   Action:  This involves the act of switching the sender to a Classic
      congestion control.  This might occur in real-time within the
      congestion control for the subsequent duration of a flow, or it
      might involve administrative action to switch to Classic
      congestion control for a specific interface or for a certain set
      of destination addresses.

      Instead of the sender taking action itself, the operator of the
      sender (e.g. a CDN) might prefer to ask the network operator to
      modify the Classic AQM's treatment of L4S packets; or to ensure
      L4S packets bypass the AQM; or to upgrade the AQM to support L4S.
      Once L4S flows no longer shared the Classic ECN AQM they would
      obviously no longer detect it, and the requirement to act on it
      would no longer apply.

   The whole set of normative requirements concerning Classic ECN AQMs
   does not apply in controlled environments, such as private networks
   or data centre networks.  CDN servers placed within an access ISP's
   network can be considered as a single controlled environment, but any
   onward networks served by the access network, including all the
   attached customer networks, would be unlikely to fall under the same
   degree of coordinated control.  Monitoring is expressed as a 'MUST'
   for these uncontrolled segments of paths (e.g.  beyond the access ISP
   in a home network), because there is a possibility that there might
   be a shared queue Classic ECN AQM in that segment.  Nonetheless, the
   intent is to only require occasional monitoring of these uncontrolled
   regions, and not to burden CDN operators if monitoring never uncovers
   any potential problems, given it is anyway in the CDN's own interests
   not to degrade the service of its own customers.

   More detailed discussion of all the above options and alternatives
   can be found in [I-D.ietf-tsvwg-l4sops].

   Having said all the above, the approach recommended in the
   requirements is to monitor, detect and act in real-time on live
   traffic.  A passive monitoring algorithm to detect a Classic ECN AQM
   at the bottleneck and fall back to Classic congestion control is
   described in an extensive technical report [ecn-fallback], which also
   provides a link to Linux source code, and a large online
   visualization of its evaluation results.  Very briefly, the algorithm
   primarily monitors RTT variation using the same algorithm that
   maintains the mean deviation of TCP's smoothed RTT, but it smooths
   over a duration of the order of a Classic sawtooth.  The outcome is
   also conditioned on other metrics such as the presence of CE marking
   and congestion avoidance phase having stabilized.  The report also

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   identifies further work to improve the approach, for instance
   improvements with low capacity links and combining the measurements
   with a cache of what had been learned about a path in previous
   connections.  The report also suggests alternative approaches.

   Although using passive measurements within live traffic (as above)
   can detect a Classic ECN AQM, it is much harder (perhaps impossible)
   to determine whether or not the AQM is in a shared queue.
   Nonetheless, this is much easier using active test traffic out-of-
   band, because two flows can be used.  Section 4 of the same report
   [ecn-fallback] describes a simple technique to detect a Classic ECN
   AQM and determine whether it is in a shared queue, summarized here.

   An L4S-enabled test server could be set up so that, when a test
   client accesses it, it serves a script that gets the client to open
   two parallel long-running flows.  It could serve one with a Classic
   congestion control (C, that sets ECT(0)) and one with a scaleable CC
   (L, that sets ECT(1)).If neither flow induces any ECN marks, it can
   be presumed the path does not contain a Classic ECN AQM.  If either
   flow induces some ECN marks, the server could measure the relative
   flow rates and round trip times of the two flows.  Table 1 shows the
   AQM that can be inferred for various cases.

                +--------+-------+------------------------+
                | Rate   | RTT   | Inferred AQM           |
                +--------+-------+------------------------+
                | L > C  | L = C | Classic ECN AQM (FIFO) |
                | L = C  | L = C | Classic ECN AQM (FQ)   |
                | L = C  | L < C | FQ-L4S AQM             |
                | L ~= C | L < C | Coupled DualQ AQM      |
                +--------+-------+------------------------+

      Table 1: Out-of-band testing with two parallel flows.  L:=L4S,
                                C:=Classic.

   Finally, we motivate the recommendation in Section 4.3 that a
   scalable congestion control is not expected to change to setting
   ECT(0) while it adapts its behaviour to coexist with Classic flows.
   This is because the sender needs to continue to check whether it made
   the right decision - and switch back if it was wrong, or if a
   different link becomes the bottleneck:

   o  If, as recommended, the sender changes only its behaviour but not
      its codepoint to Classic, its codepoint will still be compatible
      with either an L4S or a Classic AQM.  If the bottleneck does
      actually support both, it will still classify ECT(1) into the same
      L4S queue, where the sender can measure that switching to Classic
      behaviour was wrong, so that it can switch back.

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   o  In contrast, if the sender changes both its behaviour and its
      codepoint to Classic, even if the bottleneck supports both, it
      will classify ECT(0) into the Classic queue, reinforcing the
      sender's incorrect decision so that it never switches back.

   o  Also, not changing codepoint avoids the risk of being flipped to a
      different path by a load balancer or multipath routing that hashes
      on the whole of the ex-ToS byte (unfortunately still a common
      pathology).

   Note that if a flow is configured to _only_ use a Classic congestion
   control, it is then entirely appropriate not to use ECT(1).

A.1.6.  Reduce RTT dependence

   Description: A scalable congestion control needs to reduce RTT bias
   as much as possible at least over the low to typical range of RTTs
   that will interact in the intended deployment scenario (see the
   precise normative requirement wording in Section 4.3).

   Motivation: The throughput of Classic congestion controls is known to
   be inversely proportional to RTT, so one would expect flows over very
   low RTT paths to nearly starve flows over larger RTTs.  However,
   Classic congestion controls have never allowed a very low RTT path to
   exist because they induce a large queue.  For instance, consider two
   paths with base RTT 1ms and 100ms.  If a Classic congestion control
   induces a 100ms queue, it turns these RTTs into 101ms and 200ms
   leading to a throughput ratio of about 2:1.  Whereas if a scalable
   congestion control induces only a 1ms queue, the ratio is 2:101,
   leading to a throughput ratio of about 50:1.

   Therefore, with very small queues, long RTT flows will essentially
   starve, unless scalable congestion controls comply with this
   requirement.

   The RTT bias in current Classic congestion controls works
   satisfactorily when the RTT is higher than typical, and L4S does not
   change that.  So, there is no additional requirement for high RTT L4S
   flows to remove RTT bias - they can but they don't have to.

A.1.7.  Scaling down to fractional congestion windows

   Description: A scalable congestion control needs to remain responsive
   to congestion when typical RTTs over the public Internet are
   significantly smaller because they are no longer inflated by queuing
   delay (see the precise normative requirement wording in Section 4.3).

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   Motivation: As currently specified, the minimum congestion window of
   ECN-capable TCP (and its derivatives) is expected to be 2 sender
   maximum segment sizes (SMSS), or 1 SMSS after a retransmission
   timeout.  Once the congestion window reaches this minimum, if there
   is further ECN-marking, TCP is meant to wait for a retransmission
   timeout before sending another segment (see section 6.1.2 of
   [RFC3168]).  In practice, most known window-based congestion control
   algorithms become unresponsive to congestion signals at this point.
   No matter how much drop or ECN marking, the congestion window no
   longer reduces.  Instead, the sender's lack of any further congestion
   response forces the queue to grow, overriding any AQM and increasing
   queuing delay (making the window large enough to become responsive
   again).

   Most congestion controls for other transport protocols have a similar
   minimum, albeit when measured in bytes for those that use smaller
   packets.

   L4S mechanisms significantly reduce queueing delay so, over the same
   path, the RTT becomes lower.  Then this problem becomes surprisingly
   common [sub-mss-prob].  This is because, for the same link capacity,
   smaller RTT implies a smaller window.  For instance, consider a
   residential setting with an upstream broadband Internet access of 8
   Mb/s, assuming a max segment size of 1500 B.  Two upstream flows will
   each have the minimum window of 2 SMSS if the RTT is 6ms or less,
   which is quite common when accessing a nearby data centre.  So, any
   more than two such parallel TCP flows will become unresponsive and
   increase queuing delay.

   Unless scalable congestion controls address this requirement from the
   start, they will frequently become unresponsive, negating the low
   latency benefit of L4S, for themselves and for others.

   That would seem to imply that scalable congestion controllers ought
   to be required to be able work with a congestion window less than 1
   SMSS.  For instance, if an ECN-capable TCP gets an ECN-mark when it
   is already sitting at a window of 1 SMSS, RFC 3168 requires it to
   defer sending for a retransmission timeout.  A less drastic but more
   complex mechanism can maintain a congestion window less than 1 SMSS
   (significantly less if necessary), as described in [Ahmed19].  Other
   approaches are likely to be feasible.

   However, the requirement in Section 4.3 is worded as a "SHOULD"
   because the existence of a minimum window is not all bad.  When
   competing with an unresponsive flow, a minimum window naturally
   protects the flow from starvation by at least keeping some data
   flowing.

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   By stating the requirement to go lower than 1 SMSS as a "SHOULD",
   while the requirement in RFC 3168 still stands as well, we shall be
   able to watch the choices of minimum window evolve in different
   scalable congestion controllers.

A.1.8.  Measuring Reordering Tolerance in Time Units

   Description: When detecting loss, a scalable congestion control needs
   to be tolerant to reordering over an adaptive time interval, which
   scales with throughput, rather than counting only in fixed units of
   packets, which does not scale (see the precise normative requirement
   wording in Section 4.3).

   Motivation: A primary purpose of L4S is scalable throughput (it's in
   the name).  Scalability in all dimensions is, of course, also a goal
   of all IETF technology.  The inverse linear congestion response in
   Section 4.3 is necessary, but not sufficient, to solve the congestion
   control scalability problem identified in [RFC3649].  As well as
   maintaining frequent ECN signals as rate scales, it is also important
   to ensure that a potentially false perception of loss does not limit
   throughput scaling.

   End-systems cannot know whether a missing packet is due to loss or
   reordering, except in hindsight - if it appears later.  So they can
   only deem that there has been a loss if a gap in the sequence space
   has not been filled, either after a certain number of subsequent
   packets has arrived (e.g. the 3 DupACK rule of standard TCP
   congestion control [RFC5681]) or after a certain amount of time
   (e.g. the RACK approach [RFC8985]).

   As we attempt to scale packet rate over the years:

   o  Even if only _some_ sending hosts still deem that loss has
      occurred by counting reordered packets, _all_ networks will have
      to keep reducing the time over which they keep packets in order.
      If some link technologies keep the time within which reordering
      occurs roughly unchanged, then loss over these links, as perceived
      by these hosts, will appear to continually rise over the years.

   o  In contrast, if all senders detect loss in units of time, the time
      over which the network has to keep packets in order stays roughly
      invariant.

   Therefore hosts have an incentive to detect loss in time units (so as
   not to fool themselves too often into detecting losses when there are
   none).  And for hosts that are changing their congestion control
   implementation to L4S, there is no downside to including time-based
   loss detection code in the change (loss recovery implemented in

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   hardware is an exception, covered later).  Therefore requiring L4S
   hosts to detect loss in time-based units would not be a burden.

   If this requirement is not placed on L4S hosts, even though it would
   be no burden on them to do so, all networks will face unnecessary
   uncertainty over whether some L4S hosts might be detecting loss by
   counting packets.  Then _all_ link technologies will have to
   unnecessarily keep reducing the time within which reordering occurs.
   That is not a problem for some link technologies, but it becomes
   increasingly challenging for other link technologies to continue to
   scale, particularly those relying on channel bonding for scaling,
   such as LTE, 5G and DOCSIS.

   Given Internet paths traverse many link technologies, any scaling
   limit for these more challenging access link technologies would
   become a scaling limit for the Internet as a whole.

   It might be asked how it helps to place this loss detection
   requirement only on L4S hosts, because networks will still face
   uncertainty over whether non-L4S flows are detecting loss by counting
   DupACKs.  The answer is that those link technologies for which it is
   challenging to keep squeezing the reordering time will only need to
   do so for non-L4S traffic (which they can do because the L4S
   identifier is visible at the IP layer).  Therefore, they can focus
   their processing and memory resources into scaling non-L4S (Classic)
   traffic.  Then, the higher the proportion of L4S traffic, the less of
   a scaling challenge they will have.

   To summarize, there is no reason for L4S hosts not to be part of the
   solution instead of part of the problem.

   Requirement ("MUST") or recommendation ("SHOULD")?  As explained
   above, this is a subtle interoperability issue between hosts and
   networks, which seems to need a "MUST".  Unless networks can be
   certain that all L4S hosts follow the time-based approach, they still
   have to cater for the worst case - continually squeeze reordering
   into a smaller and smaller duration - just for hosts that might be
   using the counting approach.  However, it was decided to express this
   as a recommendation, using "SHOULD".  The main justification was that
   networks can still be fairly certain that L4S hosts will follow this
   recommendation, because following it offers only gain and no pain.

   Details:

   The speed of loss recovery is much more significant for short flows
   than long, therefore a good compromise is to adapt the reordering
   window; from a small fraction of the RTT at the start of a flow, to a

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   larger fraction of the RTT for flows that continue for many round
   trips.

   This is broadly the approach adopted by TCP RACK (Recent
   ACKnowledgements) [RFC8985].  However, RACK starts with the 3 DupACK
   approach, because the RTT estimate is not necessarily stable.  As
   long as the initial window is paced, such initial use of 3 DupACK
   counting would amount to time-based loss detection and therefore
   would satisfy the time-based loss detection recommendation of
   Section 4.3.  This is because pacing of the initial window would
   ensure that 3 DupACKs early in the connection would be spread over a
   small fraction of the round trip.

   As mentioned above, hardware implementations of loss recovery using
   DupACK counting exist (e.g. some implementations of RoCEv2 for RDMA).
   For low latency, these implementations can change their congestion
   control to implement L4S, because the congestion control (as distinct
   from loss recovery) is implemented in software.  But they cannot
   easily satisfy this loss recovery requirement.  However, it is
   believed they do not need to, because such implementations are
   believed to solely exist in controlled environments, where the
   network technology keeps reordering extremely low anyway.  This is
   why controlled environments with hardly any reordering are excluded
   from the scope of the normative recommendation in Section 4.3.

   Detecting loss in time units also prevents the ACK-splitting attacks
   described in [Savage-TCP].

A.2.  Scalable Transport Protocol Optimizations

A.2.1.  Setting ECT in Control Packets and Retransmissions

   Description: This item concerns TCP and its derivatives (e.g. SCTP)
   as well as RTP/RTCP [RFC6679].  The original specification of ECN for
   TCP precluded the use of ECN on control packets and retransmissions.
   To improve performance, scalable transport protocols ought to enable
   ECN at the IP layer in TCP control packets (SYN, SYN-ACK, pure ACKs,
   etc.) and in retransmitted packets.  The same is true for derivatives
   of TCP, e.g. SCTP.  Similarly [RFC6679] precludes the use of ECT on
   RTCP datagrams, in case the path changes after it has been checked
   for ECN traversal.

   Motivation (TCP): RFC 3168 prohibits the use of ECN on these types of
   TCP packet, based on a number of arguments.  This means these packets
   are not protected from congestion loss by ECN, which considerably
   harms performance, particularly for short flows.
   [I-D.ietf-tcpm-generalized-ecn] proposes experimental use of ECN on
   all types of TCP packet as long as AccECN feedback

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   [I-D.ietf-tcpm-accurate-ecn] is available (which itself satisfies the
   accurate feedback requirement in Section 4.2 for using a scalable
   congestion control).

   Motivation (RTCP): L4S experiments in general will need to observe
   the rule in [RFC6679] that precludes ECT on RTCP datagrams.
   Nonetheless, as ECN usage becomes more widespread, it would be useful
   to conduct specific experiments with ECN-capable RTCP to gather data
   on whether such caution is necessary.

A.2.2.  Faster than Additive Increase

   Description: It would improve performance if scalable congestion
   controls did not limit their congestion window increase to the
   standard additive increase of 1 SMSS per round trip [RFC5681] during
   congestion avoidance.  The same is true for derivatives of TCP
   congestion control, including similar approaches used for real-time
   media.

   Motivation: As currently defined [RFC8257], DCTCP uses the
   traditional Reno additive increase in congestion avoidance phase.
   When the available capacity suddenly increases (e.g. when another
   flow finishes, or if radio capacity increases) it can take very many
   round trips to take advantage of the new capacity.  TCP Cubic was
   designed to solve this problem, but as flow rates have continued to
   increase, the delay accelerating into available capacity has become
   prohibitive.  See, for instance, the examples in Section 1.2.  Even
   when out of its Reno-compatibility mode, every 8x scaling of Cubic's
   flow rate leads to 2x more acceleration delay.

   In the steady state, DCTCP induces about 2 ECN marks per round trip,
   so it is possible to quickly detect when these signals have
   disappeared and seek available capacity more rapidly, while
   minimizing the impact on other flows (Classic and scalable)
   [LinuxPacedChirping].  Alternatively, approaches such as Adaptive
   Acceleration (A2DTCP [A2DTCP]) have been proposed to address this
   problem in data centres, which might be deployable over the public
   Internet.

A.2.3.  Faster Convergence at Flow Start

   Description: It would improve performance if scalable congestion
   controls converged (reached their steady-state share of the capacity)
   faster than Classic congestion controls or at least no slower.  This
   affects the flow start behaviour of any L4S congestion control
   derived from a Classic transport that uses TCP slow start, including
   those for real-time media.

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   Motivation: As an example, a new DCTCP flow takes longer than a
   Classic congestion control to obtain its share of the capacity of the
   bottleneck when there are already ongoing flows using the bottleneck
   capacity.  In a data centre environment DCTCP takes about a factor of
   1.5 to 2 longer to converge due to the much higher typical level of
   ECN marking that DCTCP background traffic induces, which causes new
   flows to exit slow start early [Alizadeh-stability].  In testing for
   use over the public Internet the convergence time of DCTCP relative
   to a regular loss-based TCP slow start is even less favourable
   [Paced-Chirping] due to the shallow ECN marking threshold needed for
   L4S.  It is exacerbated by the typically greater mismatch between the
   link rate of the sending host and typical Internet access
   bottlenecks.  This problem is detrimental in general, but would
   particularly harm the performance of short flows relative to Classic
   congestion controls.

Appendix B.  Compromises in the Choice of L4S Identifier

   This appendix is informative, not normative.  As explained in
   Section 2, there is insufficient space in the IP header (v4 or v6) to
   fully accommodate every requirement.  So the choice of L4S identifier
   involves tradeoffs.  This appendix records the pros and cons of the
   choice that was made.

   Non-normative recap of the chosen codepoint scheme:

      Packets with ECT(1) and conditionally packets with CE signify L4S
      semantics as an alternative to the semantics of Classic ECN
      [RFC3168], specifically:

      *  The ECT(1) codepoint signifies that the packet was sent by an
         L4S-capable sender.

      *  Given shortage of codepoints, both L4S and Classic ECN sides of
         an AQM have to use the same CE codepoint to indicate that a
         packet has experienced congestion.  If a packet that had
         already been marked CE in an upstream buffer arrived at a
         subsequent AQM, this AQM would then have to guess whether to
         classify CE packets as L4S or Classic ECN.  Choosing the L4S
         treatment is a safer choice, because then a few Classic packets
         might arrive early, rather than a few L4S packets arriving
         late.

      *  Additional information might be available if the classifier
         were transport-aware.  Then it could classify a CE packet for
         Classic ECN treatment if the most recent ECT packet in the same
         flow had been marked ECT(0).  However, the L4S service ought
         not to need tranport-layer awareness.

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   Cons:

   Consumes the last ECN codepoint:  The L4S service could potentially
      supersede the service provided by Classic ECN, therefore using
      ECT(1) to identify L4S packets could ultimately mean that the
      ECT(0) codepoint was 'wasted' purely to distinguish one form of
      ECN from its successor.

   ECN hard in some lower layers:  It is not always possible to support
      the equivalent of an IP-ECN field in an AQM acting in a buffer
      below the IP layer [I-D.ietf-tsvwg-ecn-encap-guidelines].  Then,
      depending on the lower layer scheme, the L4S service might have to
      drop rather than mark frames even though they might encapsulate an
      ECN-capable packet.

   Risk of reordering Classic CE packets within a flow:  Classifying all
      CE packets into the L4S queue risks any CE packets that were
      originally ECT(0) being incorrectly classified as L4S.  If there
      were delay in the Classic queue, these incorrectly classified CE
      packets would arrive early, which is a form of reordering.
      Reordering within a microflow can cause TCP senders (and senders
      of similar transports) to retransmit spuriously.  However, the
      risk of spurious retransmissions would be extremely low for the
      following reasons:

      1.  It is quite unusual to experience queuing at more than one
          bottleneck on the same path (the available capacities have to
          be identical).

      2.  In only a subset of these unusual cases would the first
          bottleneck support Classic ECN marking while the second
          supported L4S ECN marking, which would be the only scenario
          where some ECT(0) packets could be CE marked by an AQM
          supporting Classic ECN then the remainder experienced further
          delay through the Classic side of a subsequent L4S DualQ AQM.

      3.  Even then, when a few packets are delivered early, it takes
          very unusual conditions to cause a spurious retransmission, in
          contrast to when some packets are delivered late.  The first
          bottleneck has to apply CE-marks to at least N contiguous
          packets and the second bottleneck has to inject an
          uninterrupted sequence of at least N of these packets between
          two packets earlier in the stream (where N is the reordering
          window that the transport protocol allows before it considers
          a packet is lost).

             For example consider N=3, and consider the sequence of
             packets 100, 101, 102, 103,... and imagine that packets

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             150,151,152 from later in the flow are injected as follows:
             100, 150, 151, 101, 152, 102, 103... If this were late
             reordering, even one packet arriving out of sequence would
             trigger a spurious retransmission, but there is no spurious
             retransmission here with early reordering, because packet
             101 moves the cumulative ACK counter forward before 3
             packets have arrived out of order.  Later, when packets
             148, 149, 153... arrive, even though there is a 3-packet
             hole, there will be no problem, because the packets to fill
             the hole are already in the receive buffer.

      4.  Even with the current TCP recommendation of N=3 [RFC5681]
          spurious retransmissions will be unlikely for all the above
          reasons.  As RACK [RFC8985] is becoming widely deployed, it
          tends to adapt its reordering window to a larger value of N,
          which will make the chance of a contiguous sequence of N early
          arrivals vanishingly small.

      5.  Even a run of 2 CE marks within a Classic ECN flow is
          unlikely, given FQ-CoDel is the only known widely deployed AQM
          that supports Classic ECN marking and it takes great care to
          separate out flows and to space any markings evenly along each
          flow.

      It is extremely unlikely that the above set of 5 eventualities
      that are each unusual in themselves would all happen
      simultaneously.  But, even if they did, the consequences would
      hardly be dire: the odd spurious fast retransmission.  Whenever
      the traffic source (a Classic congestion control) mistakes the
      reordering of a string of CE marks for a loss, one might think
      that it will reduce its congestion window as well as emitting a
      spurious retransmission.  However, it would have already reduced
      its congestion window when the CE markings arrived early.  If it
      is using ABE [RFC8511], it might reduce cwnd a little more for a
      loss than for a CE mark.  But it will revert that reduction once
      it detects that the retransmission was spurious.

      In conclusion, the impact of early reordering on spurious
      retransmissions due to CE being ambiguous will generally be
      vanishingly small.

   Pre-existing VPNs do not separate SAs by ECN:  If delay is only
      reduced for a subset of the flows within a VPN, the anti-replay
      feature of VPNs is known to potentially mistake the difference in
      delay for a replay attack.  In IPsec [RFC4301], it is recommended
      to classify traffic on its DSCP in order to split flows with
      different delay into separate security associations (SA), and
      therefore separate anti-replay windows.  Section 6.2 recommends a

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      similar approach for the L4S experiment, by using two parallel
      security associations (SAs) indexed by the least significant bit
      of the IP-ECN field.  However, classification on the ECN field is
      unlikely to be supported by existing VPN software.  Section 6.2
      suggests alternative work-rounds, but end-users using L4S over a
      VPN will need to be able to recognize the symptoms of this
      problem, in order to seek out these work-rounds.

   Hard to distinguish Classic ECN AQM:  With this scheme, when a source
      receives ECN feedback, it is not explicitly clear which type of
      AQM generated the CE markings.  This is not a problem for Classic
      ECN sources that send ECT(0) packets, because an L4S AQM will
      recognize the ECT(0) packets as Classic and apply the appropriate
      Classic ECN marking behaviour.

      However, in the absence of explicit disambiguation of the CE
      markings, an L4S source needs to use heuristic techniques to work
      out which type of congestion response to apply (see
      Appendix A.1.5).  Otherwise, if long-running Classic flow(s) are
      sharing a Classic ECN AQM bottleneck with long-running L4S
      flow(s), which then apply an L4S response to Classic CE signals,
      the L4S flows would outcompete the Classic flow(s).  Experiments
      have shown that L4S flows can take about 20 times more capacity
      share than equivalent Classic flows.  Nonetheless, as link
      capacity reduces (e.g. to 4 Mb/s), the inequality reduces.  So
      Classic flows always make progress and are not starved.

      When L4S was first proposed (in 2015, 14 years after [RFC3168] was
      published), it was believed that Classic ECN AQMs had failed to be
      deployed, because research measurements had found little or no
      evidence of CE marking.  In subsequent years Classic ECN was
      included in per-flow-queuing (FQ) deployments, however an FQ
      scheduler stops an L4S flow outcompeting Classic, because it
      enforces equality between flow rates.  It is not known whether
      there have been any non-FQ deployments of Classic ECN AQMs in the
      subsequent years, or whether there will be in future.

      An algorithm for detecting a Classic ECN AQM as soon as a flow
      stabilizes after start-up has been proposed [ecn-fallback] (see
      Appendix A.1.5 for a brief summary).  Testbed evaluations of v2 of
      the algorithm have shown detection is reasonably good for Classic
      ECN AQMs, in a wide range of circumstances.  However, although it
      can correctly detect an L4S ECN AQM in many circumstances, its is
      often incorrect at low link capacities and/or high RTTs.  Although
      this is the safe way round, there is a danger that it will
      discourage use of the algorithm.

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   Non-L4S service for control packets:  Solely for the case of TCP, the
      Classic ECN RFCs [RFC3168] and [RFC5562] require a sender to clear
      the ECN field to Not-ECT on retransmissions and on certain control
      packets specifically pure ACKs, window probes and SYNs.  When L4S
      packets are classified by the ECN field, these TCP control packets
      would not be classified into an L4S queue, and could therefore be
      delayed relative to the other packets in the flow.  This would not
      cause reordering (because retransmissions are already out of
      order, and these control packets typically carry no data).
      However, it would make critical TCP control packets more
      vulnerable to loss and delay.  To address this problem,
      [I-D.ietf-tcpm-generalized-ecn] proposes an experiment in which
      all TCP control packets and retransmissions are ECN-capable as
      long as appropriate ECN feedback is available in each case.

   Pros:

   Should work e2e:  The ECN field generally propagates end-to-end
      across the Internet without being wiped or mangled, at least over
      fixed networks.  Unlike the DSCP, the setting of the ECN field is
      at least meant to be forwarded unchanged by networks that do not
      support ECN.

   Should work in tunnels:  The L4S identifiers work across and within
      any tunnel that propagates the ECN field in any of the variant
      ways it has been defined since ECN-tunneling was first specified
      in the year 2001 [RFC3168].  However, it is likely that some
      tunnels still do not implement ECN propagation at all.

   Should work for many link technologies:  At most, but not all, path
      bottlenecks there is IP-awareness, so that L4S AQMs can be located
      where the IP-ECN field can be manipulated.  Bottlenecks at lower
      layer nodes without IP-awareness either have to use drop to signal
      congestion or a specific congestion notification facility has to
      be defined for that link technology, including propagation to and
      from IP-ECN.  The programme to define these is progressing and in
      each case so far the scheme already defined for ECN inherently
      supports L4S as well (see Section 6.1).

   Could migrate to one codepoint:  If all Classic ECN senders
      eventually evolve to use the L4S service, the ECT(0) codepoint
      could be reused for some future purpose, but only once use of
      ECT(0) packets had reduced to zero, or near-zero, which might
      never happen.

   L4 not required:  Being based on the ECN field, this scheme does not
      need the network to access transport layer flow identifiers.
      Nonetheless, it does not preclude solutions that do.

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Appendix C.  Potential Competing Uses for the ECT(1) Codepoint

   The ECT(1) codepoint of the ECN field has already been assigned once
   for the ECN nonce [RFC3540], which has now been categorized as
   historic [RFC8311].  ECN is probably the only remaining field in the
   Internet Protocol that is common to IPv4 and IPv6 and still has
   potential to work end-to-end, with tunnels and with lower layers.
   Therefore, ECT(1) should not be reassigned to a different
   experimental use (L4S) without carefully assessing competing
   potential uses.  These fall into the following categories:

C.1.  Integrity of Congestion Feedback

   Receiving hosts can fool a sender into downloading faster by
   suppressing feedback of ECN marks (or of losses if retransmissions
   are not necessary or available otherwise).

   The historic ECN nonce protocol [RFC3540] proposed that a TCP sender
   could set either of ECT(0) or ECT(1) in each packet of a flow and
   remember the sequence it had set.  If any packet was lost or
   congestion marked, the receiver would miss that bit of the sequence.
   An ECN Nonce receiver had to feed back the least significant bit of
   the sum, so it could not suppress feedback of a loss or mark without
   a 50-50 chance of guessing the sum incorrectly.

   It is highly unlikely that ECT(1) will be needed for integrity
   protection in future.  The ECN Nonce RFC [RFC3540] as been
   reclassified as historic, partly because other ways have been
   developed to protect feedback integrity of TCP and other transports
   [RFC8311] that do not consume a codepoint in the IP header.  For
   instance:

   o  the sender can test the integrity of the receiver's feedback by
      occasionally setting the IP-ECN field to a value normally only set
      by the network.  Then it can test whether the receiver's feedback
      faithfully reports what it expects (see para 2 of Section 20.2 of
      [RFC3168].  This works for loss and it will work for the accurate
      ECN feedback [RFC7560] intended for L4S.

   o  A network can enforce a congestion response to its ECN markings
      (or packet losses) by auditing congestion exposure (ConEx)
      [RFC7713].  Whether the receiver or a downstream network is
      suppressing congestion feedback or the sender is unresponsive to
      the feedback, or both, ConEx audit can neutralise any advantage
      that any of these three parties would otherwise gain.

   o  The TCP authentication option (TCP-AO [RFC5925]) can be used to
      detect any tampering with TCP congestion feedback (whether

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      malicious or accidental).  TCP's congestion feedback fields are
      immutable end-to-end, so they are amenable to TCP-AO protection,
      which covers the main TCP header and TCP options by default.
      However, TCP-AO is often too brittle to use on many end-to-end
      paths, where middleboxes can make verification fail in their
      attempts to improve performance or security, e.g. by
      resegmentation or shifting the sequence space.

C.2.  Notification of Less Severe Congestion than CE

   Various researchers have proposed to use ECT(1) as a less severe
   congestion notification than CE, particularly to enable flows to fill
   available capacity more quickly after an idle period, when another
   flow departs or when a flow starts, e.g. VCP [VCP], Queue View (QV)
   [QV].

   Before assigning ECT(1) as an identifier for L4S, we must carefully
   consider whether it might be better to hold ECT(1) in reserve for
   future standardisation of rapid flow acceleration, which is an
   important and enduring problem [RFC6077].

   Pre-Congestion Notification (PCN) is another scheme that assigns
   alternative semantics to the ECN field.  It uses ECT(1) to signify a
   less severe level of pre-congestion notification than CE [RFC6660].
   However, the ECN field only takes on the PCN semantics if packets
   carry a Diffserv codepoint defined to indicate PCN marking within a
   controlled environment.  PCN is required to be applied solely to the
   outer header of a tunnel across the controlled region in order not to
   interfere with any end-to-end use of the ECN field.  Therefore a PCN
   region on the path would not interfere with the L4S service
   identifier defined in Section 3.

Authors' Addresses

   Koen De Schepper
   Nokia Bell Labs
   Antwerp
   Belgium

   Email: koen.de_schepper@nokia.com
   URI:   https://www.bell-labs.com/usr/koen.de_schepper

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   Bob Briscoe (editor)
   Independent
   UK

   Email: ietf@bobbriscoe.net
   URI:   http://bobbriscoe.net/

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