Transport Area Working Group                             B. Briscoe, Ed.
Internet-Draft                                               Independent
Intended status: Informational                            K. De Schepper
Expires: September 10, 2020                              Nokia Bell Labs
                                                        M. Bagnulo Braun
                                        Universidad Carlos III de Madrid
                                                                G. White
                                                           March 9, 2020

   Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:


   This document describes the L4S architecture, which enables Internet
   applications to achieve Low Latency, Low Loss, and Scalable
   throughput (L4S).  The insight on which L4S is based is that the root
   cause of queuing delay is in the congestion controllers of senders,
   not in the queue itself.  The L4S architecture is intended to enable
   *all* Internet applications to transition away from congestion
   control algorithms that cause queuing delay, to a new class of
   congestion controls that utilize explicit congestion signaling
   provided by the network.  This new class of congestion control can
   provide low latency for capacity-seeking flows, so applications can
   achieve both high bandwidth and low latency.

   The architecture primarily concerns incremental deployment.  It
   defines mechanisms that allow both classes of congestion control to
   coexist in a shared network.  These mechanisms aim to ensure that the
   latency and throughput performance using an L4S-compliant congestion
   controller is usually much better (and never worse) than the
   performance would have been using a 'Classic' congestion controller,
   and that competing flows continuing to use 'Classic' controllers are
   typically not impacted by the presence of L4S.  These characteristics
   are important to encourage adoption of L4S congestion control
   algorithms and L4S compliant network elements.

   The L4S architecture consists of three components: network support to
   isolate L4S traffic from classic traffic and to provide appropriate
   congestion signaling to both types; protocol features that allow
   network elements to identify L4S traffic and allow for communication
   of congestion signaling; and host support for immediate congestion
   signaling with an appropriate congestion response that enables
   scalable performance.

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Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  L4S Architecture Overview . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  L4S Architecture Components . . . . . . . . . . . . . . . . .   8
   5.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Why These Primary Components? . . . . . . . . . . . . . .  11
     5.2.  Why Not Alternative Approaches? . . . . . . . . . . . . .  13
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Applications  . . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.3.  Deployment Considerations . . . . . . . . . . . . . . . .  18
       6.3.1.  Deployment Topology . . . . . . . . . . . . . . . . .  19
       6.3.2.  Deployment Sequences  . . . . . . . . . . . . . . . .  20
       6.3.3.  L4S Flow but Non-L4S Bottleneck . . . . . . . . . . .  22

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       6.3.4.  Other Potential Deployment Issues . . . . . . . . . .  23
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
     8.1.  Traffic (Non-)Policing  . . . . . . . . . . . . . . . . .  23
     8.2.  'Latency Friendliness'  . . . . . . . . . . . . . . . . .  24
     8.3.  Interaction between Rate Policing and L4S . . . . . . . .  25
     8.4.  ECN Integrity . . . . . . . . . . . . . . . . . . . . . .  26
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     10.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Standardization items  . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   It is increasingly common for _all_ of a user's applications at any
   one time to require low delay: interactive Web, Web services, voice,
   conversational video, interactive video, interactive remote presence,
   instant messaging, online gaming, remote desktop, cloud-based
   applications and video-assisted remote control of machinery and
   industrial processes.  In the last decade or so, much has been done
   to reduce propagation delay by placing caches or servers closer to
   users.  However, queuing remains a major, albeit intermittent,
   component of latency.  For instance spikes of hundreds of
   milliseconds are common.  During a long-running flow, even with
   state-of-the-art active queue management (AQM), the base speed-of-
   light path delay roughly doubles.  Low loss is also important
   because, for interactive applications, losses translate into even
   longer retransmission delays.

   It has been demonstrated that, once access network bit rates reach
   levels now common in the developed world, increasing capacity offers
   diminishing returns if latency (delay) is not addressed.
   Differentiated services (Diffserv) offers Expedited Forwarding (EF
   [RFC3246]) for some packets at the expense of others, but this is not
   sufficient when all (or most) of a user's applications require low

   Therefore, the goal is an Internet service with ultra-Low queueing
   Latency, ultra-Low Loss and Scalable throughput (L4S).  Ultra-low
   queuing latency means less than 1 millisecond (ms) on average and
   less than about 2 ms at the 99th percentile.  L4S is potentially for
   _all_ traffic - a service for all traffic needs none of the
   configuration or management baggage (traffic policing, traffic
   contracts) associated with favouring some traffic over others.  This
   document describes the L4S architecture for achieving these goals.

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   It must be said that queuing delay only degrades performance
   infrequently [Hohlfeld14].  It only occurs when a large enough
   capacity-seeking (e.g.  TCP) flow is running alongside the user's
   traffic in the bottleneck link, which is typically in the access
   network.  Or when the low latency application is itself a large
   capacity-seeking flow (e.g. interactive video).  At these times, the
   performance improvement from L4S must be sufficient that network
   operators will be motivated to deploy it.

   Active Queue Management (AQM) is part of the solution to queuing
   under load.  AQM improves performance for all traffic, but there is a
   limit to how much queuing delay can be reduced by solely changing the
   network; without addressing the root of the problem.

   The root of the problem is the presence of standard TCP congestion
   control (Reno [RFC5681]) or compatible variants (e.g.  TCP Cubic
   [RFC8312]).  We shall use the term 'Classic' for these Reno-friendly
   congestion controls.  It has been demonstrated that if the sending
   host replaces a Classic congestion control with a 'Scalable'
   alternative, when a suitable AQM is deployed in the network the
   performance under load of all the above interactive applications can
   be significantly improved.  For instance, queuing delay under heavy
   load with the example DCTCP/DualQ solution cited below is roughly 1
   millisecond (1 to 2 ms) at the 99th percentile without losing link
   utilization.  This compares with 5 to 20 ms on _average_ with a
   Classic congestion control and current state-of-the-art AQMs such as
   fq_CoDel [RFC8290] or PIE [RFC8033] and about 20-30 ms at the 99th
   percentile.  Also, with a Classic congestion control, reducing
   queueing to even 5 ms is typically only possible by losing some

   It has been demonstrated [DCttH15] that it is possible to deploy such
   an L4S service alongside the existing best efforts service so that
   all of a user's applications can shift to it when their stack is
   updated.  Access networks are typically designed with one link as the
   bottleneck for each site (which might be a home, small enterprise or
   mobile device), so deployment at a single network node should give
   nearly all the benefit.  The L4S approach also requires component
   mechanisms at the endpoints to fulfill its goal.  This document
   presents the L4S architecture, by describing the different components
   and how they interact to provide the scalable low-latency, low-loss,
   Internet service.

2.  L4S Architecture Overview

   There are three main components to the L4S architecture (illustrated
   in Figure 1):

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   1) Network:  L4S traffic needs to be isolated from the queuing
      latency of Classic traffic.  One queue per application flow (FQ)
      is one way to achieve this, e.g.  [RFC8290].  However, just two
      queues is sufficient and does not require inspection of transport
      layer headers in the network, which is not always possible (see
      Section 5.2).  With just two queues, it might seem impossible to
      know how much capacity to schedule for each queue without
      inspecting how many flows at any one time are using each.  And
      capacity in access networks is too costly to arbitrarily partition
      into two.  The Dual Queue Coupled AQM was developed as a minimal
      complexity solution to this problem.  It acts like a 'semi-
      permeable' membrane that partitions latency but not bandwidth.
      Note that there is no bandwidth priority between the two queues
      because they are for transition from Classic to L4S behaviour, not
      prioritization.  Section 4 gives a high level explanation of how
      FQ and DualQ solutions work, and
      [I-D.ietf-tsvwg-aqm-dualq-coupled] gives a full explanation of the
      DualQ Coupled AQM framework.

   2) Protocol:  A host needs to distinguish L4S and Classic packets
      with an identifier so that the network can classify them into
      their separate treatments.  [I-D.ietf-tsvwg-ecn-l4s-id] considers
      various alternative identifiers, and concludes that all
      alternatives involve compromises, but the ECT(1) and CE codepoints
      of the ECN field represent a workable solution.

   3) Host:  Scalable congestion controls already exist.  They solve the
      scaling problem with Reno congestion control that was explained in
      [RFC3649].  The one used most widely (in controlled environments)
      is Data Center TCP (DCTCP [RFC8257]), which has been implemented
      and deployed in Windows Server Editions (since 2012), in Linux and
      in FreeBSD.  Although DCTCP as-is 'works' well over the public
      Internet, most implementations lack certain safety features that
      will be necessary once it is used outside controlled environments
      like data centres (see Section 6.3.3 and Appendix A).  A similar
      scalable congestion control will also need to be transplanted into
      protocols other than TCP (QUIC, SCTP, RTP/RTCP, RMCAT, etc.)
      Indeed, between the present document being drafted and published,
      the following scalable congestion controls were implemented: TCP
      Prague [PragueLinux], QUIC Prague, an L4S variant of the RMCAT
      SCReAM controller [RFC8298] and the L4S ECN part of BBRv2
      [I-D.cardwell-iccrg-bbr-congestion-control] intended for TCP and

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                        (2)                     (1)
                 .-------^------. .--------------^-------------------.
    ,-(3)-----.                                  ______
   ; ________  :            L4S   --------.     |      |
   :|Scalable| :               _\        ||___\_| mark |
   :| sender | :  __________  / /        ||   / |______|\   _________
   :|________|\; |          |/    --------'         ^    \1|condit'nl|
    `---------'\_|  IP-ECN  |              Coupling :     \|priority |_\
     ________  / |Classifier|                       :     /|scheduler| /
    |Classic |/  |__________|\    --------.      ___:__  / |_________|
    | sender |                \_\  || | |||___\_| mark/|/
    |________|                  /  || | |||   / | drop |
                         Classic  --------'     |______|

     Figure 1: Components of an L4S Solution: 1) Isolation in separate
    network queues; 2) Packet Identification Protocol; and 3) Scalable
                               Sending Host

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.  COMMENT: Since this
   will be an information document, This should be removed.

   Classic Congestion Control:  A congestion control behaviour that can
      co-exist with standard TCP 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 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).

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   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.  See Section 4.3 of [I-D.ietf-tsvwg-ecn-l4s-id] for more

   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 Data Center TCP [RFC8257].  The L4S service is
      for more general traffic than just DCTCP--it allows the set of
      congestion controls with similar scaling properties to DCTCP to
      evolve (e.g.  Relentless TCP [Mathis09], TCP Prague [PragueLinux]
      and the L4S variant of SCREAM for real-time media [RFC8298]).  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,

   Reno-friendly:  The subset of Classic traffic that excludes
      unresponsive traffic and excludes experimental congestion controls
      intended to coexist with Reno but without always being strictly
      friendly to it (as allowed by [RFC5033]).  Reno-friendly is used
      in place of 'TCP-friendly', given that the TCP protocol is used
      with many different congestion control behaviours.

   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.

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      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

   Site:  A home, mobile device, small enterprise or campus, where the
      network bottleneck is typically the access link to the site.  Not
      all network arrangements fit this model but it is a useful, widely
      applicable generalization.

4.  L4S Architecture Components

   The L4S architecture is composed of the following elements.

   Protocols:The L4S architecture encompasses the two identifier changes
   (an unassignment and an assignment) and optional further identifiers:

   a.  An essential aspect of a scalable congestion control is the use
       of explicit congestion signals rather than losses, because the
       signals need to be sent immediately and frequently.  'Classic'
       ECN [RFC3168] requires an ECN signal to be treated the same as a
       drop, both when it is generated in the network and when it is
       responded to by hosts.  L4S needs networks and hosts to support a
       different meaning for ECN:

       *  much more frequent signals--too often to use drops;

       *  immediately tracking every fluctuation of the queue--too soon
          to commit to dropping packets.

       So the standards track [RFC3168] has had to be updated to allow
       L4S packets to depart from the 'same as drop' constraint.
       [RFC8311] is a standards track update to relax specific
       requirements in RFC 3168 (and certain other standards track
       RFCs), which clears the way for the experimental changes proposed
       for L4S.  [RFC8311] also reclassifies the original experimental
       assignment of the ECT(1) codepoint as an ECN nonce [RFC3540] as

   b.  [I-D.ietf-tsvwg-ecn-l4s-id] recommends ECT(1) is used as the
       identifier to classify L4S packets into a separate treatment from
       Classic packets.  This satisfies the requirements for identifying
       an alternative ECN treatment in [RFC4774].

       The CE codepoint is used to indicate Congestion Experienced by
       both L4S and Classic treatments.  This raises the concern that a
       Classic AQM earlier on the path might have marked some ECT(0)
       packets as CE.  Then these packets will be erroneously classified

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       into the L4S queue.  [I-D.ietf-tsvwg-ecn-l4s-id] explains why 5
       unlikely eventualities all have to coincide for this to have any
       detrimental effect, which even then would only involve a
       vanishingly small likelihood of a spurious retransmission.

   c.  A network operator might wish to include certain unresponsive,
       non-L4S traffic in the L4S queue if it is deemed to be smoothly
       enough paced and low enough rate not to build a queue.  For
       instance, VoIP, low rate datagrams to sync online games,
       relatively low rate application-limited traffic, DNS, LDAP, etc.
       This traffic would need to be tagged with specific identifiers,
       e.g. a low latency Diffserv Codepoint such as Expedited
       Forwarding (EF [RFC3246]), Non-Queue-Building (NQB
       [I-D.white-tsvwg-nqb]), or operator-specific identifiers.

   Network components: The L4S architecture encompasses either dual-
   queue or per-flow queue solutions:

   a.  The Coupled Dual Queue AQM achieves the 'semi-permeable' membrane
       property mentioned earlier as follows.  The obvious part is that
       using two separate queues isolates the queuing delay of one from
       the other.  The less obvious part is how the two queues act as if
       they are a single pool of bandwidth without the scheduler needing
       to decide between them.  This is achieved by having the Classic
       AQM provide a congestion signal to both queues in a manner that
       ensures a consistent response from the two types of congestion
       control.  In other words, the Classic AQM generates a drop/mark
       probability based on congestion in the Classic queue, uses this
       probability to drop/mark packets in that queue, and also uses
       this probability to affect the marking probability in the L4S
       queue.  This coupling of the congestion signaling between the two
       queues makes the L4S flows slow down to leave the right amount of
       capacity for the Classic traffic (as they would if they were the
       same type of traffic sharing the same queue).  Then the scheduler
       can serve the L4S queue with priority, because the L4S traffic
       isn't offering up enough traffic to use all the priority that it
       is given.  Therefore, on short time-scales (sub-round-trip) the
       prioritization of the L4S queue protects its low latency by
       allowing bursts to dissipate quickly; but on longer time-scales
       (round-trip and longer) the Classic queue creates an equal and
       opposite pressure against the L4S traffic to ensure that neither
       has priority when it comes to bandwidth.  The tension between
       prioritizing L4S and coupling marking from Classic results in
       per-flow fairness.  To protect against unresponsive traffic in
       the L4S queue taking advantage of the prioritization and starving
       the Classic queue, it is advisable not to use strict priority,
       but instead to use a weighted scheduler.

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       When there is no Classic traffic, the L4S queue's AQM comes into
       play, and it sets an appropriate marking rate to maintain ultra-
       low queuing delay.

       The Coupled Dual Queue AQM has been specified as generically as
       possible [I-D.ietf-tsvwg-aqm-dualq-coupled] without specifying
       the particular AQMs to use in the two queues so that designers
       are free to implement diverse ideas.  Informational appendices in
       that draft give pseudocode examples of two different specific AQM
       approaches: a variant of PIE called DualPI2 (pronounced Dual PI
       Squared) [DualPI2Linux], and a zero-config variant of RED called
       Curvy RED.  A DualQ Coupled AQM variant based on PIE has also
       been specified and implemented for Low Latency DOCSIS

   b.  A scheduler with per-flow queues can be used for L4S.  It is
       simple to modify an existing design such as FQ-CoDel or FQ-PIE.
       For instance within each queue of an FQ_CoDel system, as well as
       a CoDel AQM, immediate (unsmoothed) shallow threshold ECN marking
       has been added.  Then the Classic AQM such as CoDel or PIE is
       applied to non-ECN or ECT(0) packets, while the shallow threshold
       is applied to ECT(1) packets, to give sub-millisecond average
       queue delay.

   Host mechanisms: The L4S architecture includes a number of mechanisms
   in the end host that we enumerate next:

   a.  Data Center TCP is the most widely used example of a scalable
       congestion control.  It has been documented as an informational
       record of the protocol currently in use [RFC8257].  It will be
       necessary to define a number of safety features for a variant
       usable on the public Internet.  A draft list of these, known as
       the Prague L4S requirements, has been drawn up (see Appendix A of
       [I-D.ietf-tsvwg-ecn-l4s-id]).  The list also includes some
       optional performance improvements.

   b.  Transport protocols other than TCP use various congestion
       controls designed to be friendly with Reno.  Before they can use
       the L4S service, it will be necessary to implement scalable
       variants of each of these congestion control behaviours.  The
       following standards track RFCs currently define these protocols:
       ECN in TCP [RFC3168], in SCTP [RFC4960], in RTP [RFC6679], and in
       DCCP [RFC4340].  Not all are in widespread use, but those that
       are will eventually need to be updated to allow a different
       congestion response, which they will have to indicate by using
       the ECT(1) codepoint.  Scalable variants are under consideration
       for some new transport protocols that are themselves under
       development, e.g.  QUIC [I-D.ietf-quic-transport] and certain

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       real-time media congestion avoidance techniques (RMCAT)

   c.  ECN feedback is sufficient for L4S in some transport protocols
       (RTCP, DCCP) but not others:

       *  For the case of TCP, the feedback protocol for ECN embeds the
          assumption from Classic ECN that an ECN mark is the same as a
          drop, making it unusable for a scalable TCP.  Therefore, the
          implementation of TCP receivers will have to be upgraded
          [RFC7560].  Work to standardize and implement more accurate
          ECN feedback for TCP (AccECN) is in progress
          [I-D.ietf-tcpm-accurate-ecn], [PragueLinux].

       *  ECN feedback is only roughly sketched in an appendix of the
          SCTP specification.  A fuller specification has been proposed
          [I-D.stewart-tsvwg-sctpecn], which would need to be
          implemented and deployed before SCTCP could support L4S.

5.  Rationale

5.1.  Why These Primary Components?

   Explicit congestion signalling (protocol):  Explicit congestion
      signalling is a key part of the L4S approach.  In contrast, use of
      drop as a congestion signal creates a tension because drop is both
      a useful signal (more would reduce delay) and an impairment (less
      would reduce delay):

      *  Explicit congestion signals can be used many times per round
         trip, to keep tight control, without any impairment.  Under
         heavy load, even more explicit signals can be applied so the
         queue can be kept short whatever the load.  Whereas state-of-
         the-art AQMs have to introduce very high packet drop at high
         load to keep the queue short.  Further, when using ECN, the
         congestion control's sawtooth reduction can be smaller and
         therefore return to the operating point more often, without
         worrying that this causes more signals (one at the top of each
         smaller sawtooth).  The consequent smaller amplitude sawteeth
         fit between a very shallow marking threshold and an empty
         queue, so delay variation can be very low, without risk of

      *  Explicit congestion signals can be sent immediately to track
         fluctuations of the queue.  L4S shifts smoothing from the
         network (which doesn't know the round trip times of all the
         flows) to the host (which knows its own round trip time).

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         Previously, the network had to smooth to keep a worst-case
         round trip stable, delaying congestion signals by 100-200ms.

      All the above makes it clear that explicit congestion signalling
      is only advantageous for latency if it does not have to be
      considered 'the same as' drop (as was required with Classic ECN
      [RFC3168]).  Therefore, in a DualQ AQM, the L4S queue uses a new
      L4S variant of ECN that is not equivalent to drop
      [I-D.ietf-tsvwg-ecn-l4s-id], while the Classic queue uses either
      classic ECN [RFC3168] or drop, which are equivalent.

      Before Classic ECN was standardized, there were various proposals
      to give an ECN mark a different meaning from drop.  However, there
      was no particular reason to agree on any one of the alternative
      meanings, so 'the same as drop' was the only compromise that could
      be reached.  RFC 3168 contains a statement that:

         "An environment where all end nodes were ECN-Capable could
         allow new criteria to be developed for setting the CE
         codepoint, and new congestion control mechanisms for end-node
         reaction to CE packets.  However, this is a research issue, and
         as such is not addressed in this document."

   Latency isolation with coupled congestion notification (network):
      Using just two queues is not essential to L4S (more would be
      possible), but it is the simplest way to isolate all the L4S
      traffic that keeps latency low from all the legacy Classic traffic
      that does not.

      Similarly, coupling the congestion notification between the queues
      is not necessarily essential, but it is a clever and simple way to
      allow senders to determine their rate, packet-by-packet, rather
      than be overridden by a network scheduler.  Because otherwise a
      network scheduler would have to inspect at least transport layer
      headers, and it would have to continually assign a rate to each
      flow without any easy way to understand application intent.

   L4S packet identifier (protocol):  Once there are at least two
      separate treatments in the network, hosts need an identifier at
      the IP layer to distinguish which treatment they intend to use.

   Scalable congestion notification (host):  A scalable congestion
      control keeps the signalling frequency high so that rate
      variations can be small when signalling is stable, and rate can
      track variations in available capacity as rapidly as possible

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   Low loss:  Latency is not the only concern of L4S.  The 'Low Loss"
      part of the name denotes that L4S generally achieves zero
      congestion loss due to its use of ECN.  Otherwise, loss would
      itself cause delay, particularly for short flows, due to
      retransmission delay [RFC2884].

   Scalable throughput:  The "Scalable throughput" part of the name
      denotes that the per-flow throughput of scalable congestion
      controls should scale indefinitely, avoiding the imminent scaling
      problems with Reno-friendly congestion control algorithms
      [RFC3649].  It was known when TCP congestion avoidance was first
      developed that it would not scale to high bandwidth-delay products
      (see footnote 6 in [TCP-CA]).  Today, regular broadband bit-rates
      over WAN distances are already beyond the scaling range of Classic
      Reno congestion control.  So `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.  For instance, at 800Mb/s with a 20ms round trip,
      Cubic induces a congestion signal only every 500 round trips or 10
      seconds, which makes its dynamic control very sloppy.  In contrast
      on average a scalable congestion control like DCTCP or TCP Prague
      induces 2 congestion signals per round trip, which remains
      invariant for any flow rate, keeping dynamic control very tight.

5.2.  Why Not Alternative Approaches?

   All the following approaches address some part of the same problem
   space as L4S.  In each case, it is shown that L4S complements them or
   improves on them, rather than being a mutually exclusive alternative:

   Diffserv:  Diffserv addresses the problem of bandwidth apportionment
      for important traffic as well as queuing latency for delay-
      sensitive traffic.  L4S solely addresses the problem of queuing
      latency (as well as loss and throughput scaling).  Diffserv will
      still be necessary where important traffic requires priority (e.g.
      for commercial reasons, or for protection of critical
      infrastructure traffic) - see [I-D.briscoe-tsvwg-l4s-diffserv].
      Nonetheless, if there are Diffserv classes for important traffic,
      the L4S approach can provide low latency for _all_ traffic within
      each Diffserv class (including the case where there is only one
      Diffserv class).

      Also, as already explained, Diffserv only works for a small subset
      of the traffic on a link.  It is not applicable when all the
      applications in use at one time at a single site (home, small
      business or mobile device) require low latency.  Also, because L4S
      is for all traffic, it needs none of the management baggage
      (traffic policing, traffic contracts) associated with favouring

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      some packets over others.  This baggage has held Diffserv back
      from widespread end-to-end deployment.

   State-of-the-art AQMs:  AQMs such as PIE and fq_CoDel give a
      significant reduction in queuing delay relative to no AQM at all.
      L4S is intended to complement these AQMs, and should not distract
      from the need to deploy them as widely as possible.  Nonetheless,
      without addressing the large saw-toothing rate variations of
      Classic congestion controls, AQMs alone cannot reduce queuing
      delay too far without significantly reducing link utilization.
      The L4S approach resolves this tension by ensuring hosts can
      minimize the size of their sawteeth without appearing so
      aggressive to legacy flows that they starve them.

   Per-flow queuing:  Similarly, per-flow queuing is not incompatible
      with the L4S approach.  However, one queue for every flow can be
      thought of as overkill compared to the minimum of two queues for
      all traffic needed for the L4S approach.  The overkill of per-flow
      queuing has side-effects:

      A.  fq makes high performance networking equipment costly
          (processing and memory) - in contrast dual queue code can be
          very simple;

      B.  fq requires packet inspection into the end-to-end transport
          layer, which doesn't sit well alongside encryption for privacy
          - in contrast the use of ECN as the classifier for L4S
          requires no deeper inspection than the IP layer;

      C.  fq isolates the queuing of each flow from the others but not
          from itself so existing FQ implementations still need to have
          support for scalable congestion control added.

          It might seem that self-inflicted queuing delay should not
          count, because if the delay wasn't in the network it would
          just shift to the sender.  However, modern adaptive
          applications, e.g.  HTTP/2 [RFC7540] or the interactive media
          applications described in Section 6, can keep low latency
          objects at the front of their local send queue by shuffling
          priorities of other objects dependent on the progress of other
          transfers.  They cannot shuffle packets once they have
          released them into the network.

      D.  fq prevents any one flow from consuming more than 1/N of the
          capacity at any instant, where N is the number of flows.  This
          is fine if all flows are elastic, but it does not sit well
          with a variable bit rate real-time multimedia flow, which

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          requires wriggle room to sometimes take more and other times
          less than a 1/N share.

          It might seem that an fq scheduler offers the benefit that it
          prevents individual flows from hogging all the bandwidth.
          However, L4S has been deliberately designed so that policing
          of individual flows can be added as a policy choice, rather
          than requiring one specific policy choice as the mechanism
          itself.  A scheduler (like fq) has to decide packet-by-packet
          which flow to schedule without knowing application intent.
          Whereas a separate policing function can be configured less
          strictly, so that senders can still control the instantaneous
          rate of each flow dependent on the needs of each application
          (e.g. variable rate video), giving more wriggle-room before a
          flow is deemed non-compliant.  Also policing of queuing and of
          flow-rates can be applied independently.

   Alternative Back-off ECN (ABE):  Here again, L4S is not an
      alternative to ABE but a complement that introduces much lower
      queuing delay.  ABE [RFC8511] alters the host behaviour in
      response to ECN marking to utilize a link better and give ECN
      flows faster throughput.  It uses ECT(0) and assumes the network
      still treats ECN and drop the same.  Therefore ABE exploits any
      lower queuing delay that AQMs can provide.  But as explained
      above, AQMs still cannot reduce queuing delay too far without
      losing link utilization (to allow for other, non-ABE, flows).

   BBRv1:  v1 of Bottleneck Bandwidth and Round-trip propagation time
      (BBR [I-D.cardwell-iccrg-bbr-congestion-control]) controls queuing
      delay end-to-end without needing any special logic in the network,
      such as an AQM - so it works pretty-much on any path.  Setting
      some problems with capacity sharing aside, queuing delay is good
      with BBRv1, but perhaps not quite as low as with state-of-the-art
      AQMs such as PIE or fq_CoDel, and certainly nowhere near as low as
      with L4S.  Queuing delay is also not consistently low, due to its
      regular bandwidth probes and the aggressive flow start-up phase.

      L4S is a complement to BBRv1.  Indeed BBRv2 uses L4S ECN and a
      scalable L4S congestion control behaviour in response to any ECN
      signalling from the path.

6.  Applicability

6.1.  Applications

   A transport layer that solves the current latency issues will provide
   new service, product and application opportunities.

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   With the L4S approach, the following existing applications will
   experience significantly better quality of experience under load:

   o  Gaming, including cloud based gaming;

   o  VoIP;

   o  Video conferencing;

   o  Web browsing;

   o  (Adaptive) video streaming;

   o  Instant messaging.

   The significantly lower queuing latency also enables some interactive
   application functions to be offloaded to the cloud that would hardly
   even be usable today:

   o  Cloud based interactive video;

   o  Cloud based virtual and augmented reality.

   The above two applications have been successfully demonstrated with
   L4S, both running together over a 40 Mb/s broadband access link
   loaded up with the numerous other latency sensitive applications in
   the previous list as well as numerous downloads - all sharing the
   same bottleneck queue simultaneously [L4Sdemo16].  For the former, a
   panoramic video of a football stadium could be swiped and pinched so
   that, on the fly, a proxy in the cloud could generate a sub-window of
   the match video under the finger-gesture control of each user.  For
   the latter, a virtual reality headset displayed a viewport taken from
   a 360 degree camera in a racing car.  The user's head movements
   controlled the viewport extracted by a cloud-based proxy.  In both
   cases, with 7 ms end-to-end base delay, the additional queuing delay
   of roughly 1 ms was so low that it seemed the video was generated

   Using a swiping finger gesture or head movement to pan a video are
   extremely latency-demanding actions--far more demanding than VoIP.
   Because human vision can detect extremely low delays of the order of
   single milliseconds when delay is translated into a visual lag
   between a video and a reference point (the finger or the orientation
   of the head sensed by the balance system in the inner ear --- the
   vestibular system).

   Without the low queuing delay of L4S, cloud-based applications like
   these would not be credible without significantly more access

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   bandwidth (to deliver all possible video that might be viewed) and
   more local processing, which would increase the weight and power
   consumption of head-mounted displays.  When all interactive
   processing can be done in the cloud, only the data to be rendered for
   the end user needs to be sent.

   Other low latency high bandwidth applications such as:

   o  Interactive remote presence;

   o  Video-assisted remote control of machinery or industrial

   are not credible at all without very low queuing delay.  No amount of
   extra access bandwidth or local processing can make up for lost time.

6.2.  Use Cases

   The following use-cases for L4S are being considered by various
   interested parties:

   o  Where the bottleneck is one of various types of access network:
      DSL, cable, mobile, satellite

      *  Radio links (cellular, WiFi, satellite) that are distant from
         the source are particularly challenging.  The radio link
         capacity can vary rapidly by orders of magnitude, so it is
         often desirable to hold a buffer to utilize sudden increases of

      *  cellular networks are further complicated by a perceived need
         to buffer in order to make hand-overs imperceptible;

      *  Satellite networks generally have a very large base RTT, so
         even with minimal queuing, overall delay can never be extremely

      *  Nonetheless, it is certainly desirable not to hold a buffer
         purely because of the sawteeth of Classic congestion controls,
         when it is more than is needed for all the above reasons.

   o  Private networks of heterogeneous data centres, where there is no
      single administrator that can arrange for all the simultaneous
      changes to senders, receivers and network needed to deploy DCTCP:

      *  a set of private data centres interconnected over a wide area
         with separate administrations, but within the same company

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      *  a set of data centres operated by separate companies
         interconnected by a community of interest network (e.g. for the
         finance sector)

      *  multi-tenant (cloud) data centres where tenants choose their
         operating system stack (Infrastructure as a Service - IaaS)

   o  Different types of transport (or application) congestion control:

      *  elastic (TCP/SCTP);

      *  real-time (RTP, RMCAT);

      *  query (DNS/LDAP).

   o  Where low delay quality of service is required, but without
      inspecting or intervening above the IP layer

      *  mobile and other networks have tended to inspect higher layers
         in order to guess application QoS requirements.  However, with
         growing demand for support of privacy and encryption, L4S
         offers an alternative.  There is no need to select which
         traffic to favour for queuing, when L4S gives favourable
         queuing to all traffic.

   o  If queuing delay is minimized, applications with a fixed delay
      budget can communicate over longer distances, or via a longer
      chain of service functions [RFC7665] or onion routers.

6.3.  Deployment Considerations

   The DualQ is, in itself, an incremental deployment framework for L4S
   AQMs so that L4S traffic can coexist with existing Classic (Reno-
   friendly) traffic.  Section 6.3.1 explains why only deploying a DualQ
   AQM [I-D.ietf-tsvwg-aqm-dualq-coupled] in one node at each end of the
   access link will realize nearly all the benefit of L4S.

   L4S involves both end systems and the network, so Section 6.3.2
   suggests some typical sequences to deploy each part, and why there
   will be an immediate and significant benefit after deploying just one

   If an ECN-enabled DualQ AQM has not been deployed at a bottleneck, an
   L4S flow is required to include a fall-back strategy to Classic
   behaviour.  Section 6.3.3 describes how an L4S flow detects this, and
   how to minimize the effect of false negative detection.

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6.3.1.  Deployment Topology

   DualQ AQMs will not have to be deployed throughout the Internet
   before L4S will work for anyone.  Operators of public Internet access
   networks typically design their networks so that the bottleneck will
   nearly always occur at one known (logical) link.  This confines the
   cost of queue management technology to one place.

   The case of mesh networks is different and will be discussed later in
   this section.  But the known bottleneck case is generally true for
   Internet access to all sorts of different 'sites', where the word
   'site' includes home networks, small-to-medium sized campus or
   enterprise networks and even cellular devices (Figure 2).  Also, this
   known-bottleneck case tends to be applicable whatever the access link
   technology; whether xDSL, cable, cellular, line-of-sight wireless or

   Therefore, the full benefit of the L4S service should be available in
   the downstream direction when the DualQ AQM is deployed at the
   ingress to this bottleneck link (or links for multihomed sites).  And
   similarly, the full upstream service will be available once the DualQ
   is deployed at the upstream ingress.

                                           (      )
                         __          __  (          )
                        |DQ\________/DQ|( enterprise )
                    ___ |__/        \__| ( /campus  )
                   (   )                   (______)
                 (      )                           ___||_
   +----+      (          )  __                 __ /      \
   | DC |-----(    Core    )|DQ\_______________/DQ|| home |
   +----+      (          ) |__/               \__||______|
                  (_____) __
                         |DQ\__/\        __ ,===.
                         |__/    \  ____/DQ||| ||mobile
                                  \/    \__|||_||device
                                            | o |

   Figure 2: Likely location of DualQ (DQ) Deployments in common access

   Deployment in mesh topologies depends on how over-booked the core is.
   If the core is non-blocking, or at least generously provisioned so
   that the edges are nearly always the bottlenecks, it would only be
   necessary to deploy the DualQ AQM at the edge bottlenecks.  For

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   example, some data-centre networks are designed with the bottleneck
   in the hypervisor or host NICs, while others bottleneck at the top-
   of-rack switch (both the output ports facing hosts and those facing
   the core).

   The DualQ would eventually also need to be deployed at any other
   persistent bottlenecks such as network interconnections, e.g. some
   public Internet exchange points and the ingress and egress to WAN
   links interconnecting data-centres.

6.3.2.  Deployment Sequences

   For any one L4S flow to work, it requires 3 parts to have been
   deployed.  This was the same deployment problem that ECN faced
   [RFC8170] so we have learned from this.

   Firstly, L4S deployment exploits the fact that DCTCP already exists
   on many Internet hosts (Windows, FreeBSD and Linux); both servers and
   clients.  Therefore, just deploying DualQ AQM at a network bottleneck
   immediately gives a working deployment of all the L4S parts.  DCTCP
   needs some safety concerns to be fixed for general use over the
   public Internet (see Section 2.3 of [I-D.ietf-tsvwg-ecn-l4s-id]), but
   DCTCP is not on by default, so these issues can be managed within
   controlled deployments or controlled trials.

   Secondly, the performance improvement with L4S is so significant that
   it enables new interactive services and products that were not
   previously possible.  It is much easier for companies to initiate new
   work on deployment if there is budget for a new product trial.  If,
   in contrast, there were only an incremental performance improvement
   (as with Classic ECN), spending on deployment tends to be much harder
   to justify.

   Thirdly, the L4S identifier is defined so that initially network
   operators can enable L4S exclusively for certain customers or certain
   applications.  But this is carefully defined so that it does not
   compromise future evolution towards L4S as an Internet-wide service.
   This is because the L4S identifier is defined not only as the end-to-
   end ECN field, but it can also optionally be combined with any other
   packet header or some status of a customer or their access link
   [I-D.ietf-tsvwg-ecn-l4s-id].  Operators could do this anyway, even if
   it were not blessed by the IETF.  However, it is best for the IETF to
   specify that they must use their own local identifier in combination
   with the IETF's identifier.  Then, if an operator enables the
   optional local-use approach, they only have to remove this extra rule
   to make the service work Internet-wide - it will already traverse
   middleboxes, peerings, etc.

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   | | Servers or proxies |      Access link     |             Clients |
   |1| DCTCP (existing)   |                      |    DCTCP (existing) |
   | |                    | DualQ AQM downstream |                     |
   |2| TCP Prague         |                      |  AccECN (already in |
   | |                    |                      | progress:DCTCP/BBR) |
   | |                 FULLY     WORKS     DOWNSTREAM                  |
   |3|                    |  DualQ AQM upstream  |          TCP Prague |
   | |                    |                      |                     |
   | |              FULLY WORKS UPSTREAM AND DOWNSTREAM                |

                Figure 3: Example L4S Deployment Sequences

   Figure 3 illustrates some example sequences in which the parts of L4S
   might be deployed.  It consists of the following stages:

   1.  Here, the immediate benefit of a single AQM deployment can be
       seen, but limited to a controlled trial or controlled deployment.
       In this example downstream deployment is first, but in other
       scenarios the upstream might be deployed first.  If no AQM at all
       was previously deployed for the downstream access, the DualQ AQM
       greatly improves the Classic service (as well as adding the L4S
       service).  If an AQM was already deployed, the Classic service
       will be unchanged (and L4S will add an improvement on top).

   2.  In this stage, the name 'TCP Prague' is used to represent a
       variant of DCTCP that is safe to use in a production environment.
       If the application is primarily unidirectional, 'TCP Prague' at
       one end will provide all the benefit needed.  Accurate ECN
       feedback (AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the
       other end, but it is a generic ECN feedback facility that is
       already planned to be deployed for other purposes, e.g.  DCTCP,
       BBR [I-D.cardwell-iccrg-bbr-congestion-control].  The two ends
       can be deployed in either order, because, in TCP, an L4S
       congestion control only enables itself if it has negotiated the
       use of AccECN feedback with the other end during the connection
       handshake.  Thus, deployment of TCP Prague on a server enables
       L4S trials to move to a production service in one direction,
       wherever AccECN is deployed at the other end.  This stage might
       be further motivated by the performance improvements of TCP
       Prague relative to DCTCP (see Appendix A.2 of

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   3.  This is a two-move stage to enable L4S upstream.  The DualQ or
       TCP Prague can be deployed in either order as already explained.
       To motivate the first of two independent moves, the deferred
       benefit of enabling new services after the second move has to be
       worth it to cover the first mover's investment risk.  As
       explained already, the potential for new interactive services
       provides this motivation.  The DualQ AQM also greatly improves
       the upstream Classic service, assuming no other AQM has already
       been deployed.

   Note that other deployment sequences might occur.  For instance: the
   upstream might be deployed first; a non-TCP protocol might be used
   end-to-end, e.g.  QUIC, RMCAT; a body such as the 3GPP might require
   L4S to be implemented in 5G user equipment, or other random acts of

6.3.3.  L4S Flow but Non-L4S Bottleneck

   If L4S is enabled between two hosts but there is no L4S AQM at the
   bottleneck, any drop from the bottleneck will trigger the L4S sender
   to fall back to a classic ('Reno-friendly') behaviour (see
   Appendix A.1.3 of [I-D.ietf-tsvwg-ecn-l4s-id]).

   Unfortunately, as well as protecting legacy traffic, this rule
   degrades the L4S service whenever there is a loss, even if the loss
   was not from a non-DualQ bottleneck (false negative).  And
   unfortunately, prevalent drop can be due to other causes, e.g.:

   o  congestion loss at other transient bottlenecks, e.g. due to bursts
      in shallower queues;

   o  transmission errors, e.g. due to electrical interference;

   o  rate policing.

   Three complementary approaches are in progress to address this issue,
   but they are all currently research:

   o  In Prague congestion control, ignore certain losses deemed
      unlikely to be due to congestion (using some ideas from BBR
      [I-D.cardwell-iccrg-bbr-congestion-control] but with no need to
      ignore nearly all losses).  This could mask any of the above types
      of loss (requires consensus on how to safely interoperate with
      drop-based congestion controls).

   o  A combination of RACK, reconfigured link retransmission and L4S
      could address transmission errors [UnorderedLTE],

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   o  Hybrid ECN/drop rate policers (see Section 8.3).

   L4S deployment scenarios that minimize these issues (e.g. over
   wireline networks) can proceed in parallel to this research, in the
   expectation that research success could continually widen L4S

   Classic ECN support is starting to materialize on the Internet as an
   increased level of CE marking.  Given some of this Classic ECN might
   be due to single-queue ECN deployment, an L4S sender will have to
   fall back to a classic ('Reno-friendly') behaviour if it detects that
   ECN marking is accompanied by greater queuing delay or greater delay
   variation than would be expected with L4S (see Appendix A.1.4 of
   [I-D.ietf-tsvwg-ecn-l4s-id]).  It is hard to detect whether this is
   all due to the addition of support for ECN in the Linux
   implementation of FQ-CoDel, which would not require fall-back to
   Classic behaviour, because FQ inherently forces the throughput of
   each flow to be equal irrespective of its aggressiveness.

6.3.4.  Other Potential Deployment Issues

   An L4S AQM uses the ECN field to signal congestion.  So, in common
   with Classic ECN, if the AQM is within a tunnel or at a lower layer,
   correct functioning of ECN signalling requires correct propagation of
   the ECN field up the layers [RFC6040],

7.  IANA Considerations

   This specification contains no IANA considerations.

8.  Security Considerations

8.1.  Traffic (Non-)Policing

   Because the L4S service can serve all traffic that is using the
   capacity of a link, it should not be necessary to police access to
   the L4S service.  In contrast, Diffserv only works if some packets
   get less favourable treatment than others.  So Diffserv has to use
   traffic rate policers to limit how much traffic can be favoured.  In
   turn, traffic policers require traffic contracts between users and
   networks as well as pairwise between networks.  Because L4S will lack
   all this management complexity, it is more likely to work end-to-end.

   During early deployment (and perhaps always), some networks will not
   offer the L4S service.  These networks do not need to police or re-
   mark L4S traffic - they just forward it unchanged as best efforts
   traffic, as they already forward traffic with ECT(1) today.  At a

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   bottleneck, such networks will introduce some queuing and dropping.
   When a scalable congestion control detects a drop it will have to
   respond as if it is a Classic congestion control (as required in
   Section 2.3 of [I-D.ietf-tsvwg-ecn-l4s-id]).  This will ensure safe
   interworking with other traffic at the 'legacy' bottleneck, but it
   will degrade the L4S service to no better (but never worse) than
   classic best efforts, whenever a legacy (non-L4S) bottleneck is
   encountered on a path.

   Certain network operators might choose to restrict access to the L4S
   class, perhaps only to selected premium customers as a value-added
   service.  Their packet classifier (item 2 in Figure 1) could identify
   such customers against some other field (e.g. source address range)
   as well as ECN.  If only the ECN L4S identifier matched, but not the
   source address (say), the classifier could direct these packets (from
   non-premium customers) into the Classic queue.  Clearly explaining
   how operators can use an additional local classifiers (see
   [I-D.ietf-tsvwg-ecn-l4s-id]) is intended to remove any tendency to
   bleach the L4S identifier.  Then at least the L4S ECN identifier will
   be more likely to survive end-to-end even though the service may not
   be supported at every hop.  Such arrangements would only require
   simple registered/not-registered packet classification, rather than
   the managed, application-specific traffic policing against customer-
   specific traffic contracts that Diffserv uses.

8.2.  'Latency Friendliness'

   Like the Classic service, the L4S service relies on self-constraint -
   limiting rate in response to congestion.  In addition, the L4S
   service requires self-constraint in terms of limiting latency
   (burstiness).  It is hoped that self-interest and standardization of
   dynamic behaviour (especially flow start-up) will be sufficient to
   prevent transports from sending excessive bursts of L4S traffic,
   given the application's own latency will suffer most from such

   Whether burst policing becomes necessary remains to be seen.  Without
   it, there will be potential for attacks on the low latency of the L4S
   service.  However it may only be necessary to apply such policing
   reactively, e.g. punitively targeted at any deployments of new bursty

   A per-flow (5-tuple) queue protection function
   [I-D.briscoe-docsis-q-protection] has been developed for the low
   latency queue in DOCSIS, which has adopted the DualQ L4S
   architecture.  It protects the low latency service from any queue-
   building flows that accidentally or maliciously classify themselves
   into the low latency queue.  It is designed to score flows based

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   solely on their contribution to queuing (not flow rate in itself).
   Then, if the shared low latency queue is at risk of exceeding a
   threshold, the function redirects enough packets of the highest
   scoring flow(s) into the Classic queue to preserve low latency.

   Such a queue protection function is not considered a necessary part
   of the L4S architecture, which works without it (in a similar way to
   how the Internet works without per-flow rate policing).  Indeed,
   under normal circumstances, DOCSIS queue protection does not
   intervene, and if operators find it is not necessary they can disable
   it.  Part of the L4S experiment will be to see whether such a
   function is necessary.

8.3.  Interaction between Rate Policing and L4S

   As mentioned in Section 5.2, L4S should remove the need for low
   latency Diffserv classes.  However, those Diffserv classes that give
   certain applications or users priority over capacity, would still be
   applicable in certain scenarios (e.g.  corporate networks).  Then,
   within such Diffserv classes, L4S would often be applicable to give
   traffic low latency and low loss as well.  Within such a Diffserv
   class, the bandwidth available to a user or application is often
   limited by a rate policer.  Similarly, in the default Diffserv class,
   rate policers are used to partition shared capacity.

   A classic rate policer drops any packets exceeding a set rate,
   usually also giving a burst allowance (variants exist where the
   policer re-marks non-compliant traffic to a discard-eligible Diffserv
   codepoint, so they may be dropped elsewhere during contention).
   Whenever L4S traffic encounters one of these rate policers, it will
   experience drops and the source has to fall back to a Classic
   congestion control, thus losing the benefits of L4S.  So, in networks
   that already use rate policers and plan to deploy L4S, it will be
   preferable to redesign these rate policers to be more friendly to the
   L4S service.

   L4S-friendly rate policing is currently a research area (note that
   this is not the same as latency policing).  It might be achieved by
   setting a threshold where ECN marking is introduced, such that it is
   just under the policed rate or just under the burst allowance where
   drop is introduced.  This could be applied to various types of rate
   policer, e.g.  [RFC2697], [RFC2698] or the 'local' (non-ConEx)
   variant of the ConEx congestion policer [I-D.briscoe-conex-policing].
   It might also be possible to design scalable congestion controls to
   respond less catastrophically to loss that has not been preceded by a
   period of increasing delay.

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   The design of L4S-friendly rate policers will require a separate
   dedicated document.  For further discussion of the interaction
   between L4S and Diffserv, see [I-D.briscoe-tsvwg-l4s-diffserv].

8.4.  ECN Integrity

   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).  Various ways to protect
   transport feedback integrity have been developed.  For instance:

   o  The sender can test the integrity of the receiver's feedback by
      occasionally setting the IP-ECN field to the congestion
      experienced (CE) codepoint, which is normally only set by a
      congested link.  Then the sender can test whether the receiver's
      feedback faithfully reports what it expects (see 2nd para of
      Section 20.2 of [RFC3168]).

   o  A network can enforce a congestion response to its ECN markings
      (or packet losses) by auditing congestion exposure (ConEx)

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

   o  The ECN Nonce [RFC3540] was proposed to detect tampering with
      congestion feedback, but it has been reclassified as historic

   Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] gives more details of
   these techniques including their applicability and pros and cons.

9.  Acknowledgements

   Thanks to Richard Scheffenegger, Wes Eddy, Karen Nielsen, David Black
   and Jake Holland for their useful review comments.

   Bob Briscoe and Koen De Schepper 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 part-funded 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

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10.  References

10.1.  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,

10.2.  Informative References

   [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,

              CableLabs, "MAC and Upper Layer Protocols Interface
              (MULPI) Specification, CM-SP-MULPIv3.1", Data-Over-Cable
              Service Interface Specifications DOCSIS(R) 3.1 Version i17
              or later, January 2019, <https://specification-

              Albisser, O., De Schepper, K., Briscoe, B., Tilmans, O.,
              and H. Steen, "DUALPI2 - Low Latency, Low Loss and
              Scalable (L4S) AQM", Proc. Linux Netdev 0x13 , March 2019,

              Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A., and P.
              Barford, "A QoE Perspective on Sizing Network Buffers",
              Proc. ACM Internet Measurement Conf (IMC'14) hmm, November

              Briscoe, B., "Network Performance Isolation using
              Congestion Policing", draft-briscoe-conex-policing-01
              (work in progress), February 2014.

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

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              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.

              Cardwell, N., Cheng, Y., Yeganeh, S., and V. Jacobson,
              "BBR Congestion Control", draft-cardwell-iccrg-bbr-
              congestion-control-00 (work in progress), July 2017.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-27 (work
              in progress), February 2020.

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-11 (work in progress), March 2020.

              Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
              Congestion Notification (ECN) to TCP Control Packets",
              draft-ietf-tcpm-generalized-ecn-05 (work in progress),
              November 2019.

              Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-10 (work in
              progress), July 2019.

              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
              encap-guidelines-13 (work in progress), May 2019.

              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
              id-09 (work in progress), February 2020.

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              Smith, K., "Network management of encrypted traffic",
              draft-smith-encrypted-traffic-management-05 (work in
              progress), May 2016.

              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.

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

              White, G. and T. Fossati, "Identifying and Handling Non
              Queue Building Flows in a Bottleneck Link", draft-white-
              tsvwg-nqb-02 (work in progress), June 2019.

              Bondarenko, O., De Schepper, K., Tsang, I., and B.
              Briscoe, "orderedUltra-Low Delay for All: Live Experience,
              Live Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
              (videos of demos:

              Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
              May 2009, <

              Eggert, L., "Experimental Specification of New Congestion
              Control Algorithms", IETF Operational Note ion-tsv-alt-cc,
              July 2007.

              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, <

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   [RFC2697]  Heinanen, J. and R. Guerin, "A Single Rate Three Color
              Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,

   [RFC2698]  Heinanen, J. and R. Guerin, "A Two Rate Three Color
              Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999,

   [RFC2884]  Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
              Explicit Congestion Notification (ECN) in IP Networks",
              RFC 2884, DOI 10.17487/RFC2884, July 2000,

   [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,

   [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,

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, DOI 10.17487/RFC3540, June 2003,

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,

   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, DOI 10.17487/RFC4774, November 2006,

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

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   [RFC5033]  Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,

   [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,

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <>.

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

   [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, <>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [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,

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

   [RFC7713]  Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
              Concepts, Abstract Mechanism, and Requirements", RFC 7713,
              DOI 10.17487/RFC7713, December 2015,

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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,

   [RFC8170]  Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <>.

   [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, <>.

   [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,

   [RFC8298]  Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
              for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
              2017, <>.

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

   [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,

   [RFC8511]  Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", RFC 8511,
              DOI 10.17487/RFC8511, December 2018,

   [TCP-CA]   Jacobson, V. and M. Karels, "Congestion Avoidance and
              Control", Laurence Berkeley Labs Technical Report ,
              November 1988, <>.

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              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,

              Austrheim, M., "Implementing immediate forwarding for 4G
              in a network simulator", Masters Thesis, Uni Oslo , June

Appendix A.  Standardization items

   The following table includes all the items that will need to be
   standardized to provide a full L4S architecture.

   The table is too wide for the ASCII draft format, so it has been
   split into two, with a common column of row index numbers on the

   The columns in the second part of the table have the following

   WG:  The IETF WG most relevant to this requirement.  The "tcpm/iccrg"
      combination refers to the procedure typically used for congestion
      control changes, where tcpm owns the approval decision, but uses
      the iccrg for expert review [NewCC_Proc];

   TCP:  Applicable to all forms of TCP congestion control;

   DCTCP:  Applicable to Data Center TCP as currently used (in
      controlled environments);

   DCTCP bis:  Applicable to an future Data Center TCP congestion
      control intended for controlled environments;

   XXX Prague:  Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT)
      congestion control.

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   | Req | Requirement            | Reference                          |
   | #   |                        |                                    |
   | 0   | ARCHITECTURE           |                                    |
   | 1   | L4S IDENTIFIER         | [I-D.ietf-tsvwg-ecn-l4s-id]        |
   | 2   | DUAL QUEUE AQM         | [I-D.ietf-tsvwg-aqm-dualq-coupled] |
   | 3   | Suitable ECN Feedback  | [I-D.ietf-tcpm-accurate-ecn],      |
   |     |                        | [I-D.stewart-tsvwg-sctpecn].       |
   |     |                        |                                    |
   |     | SCALABLE TRANSPORT -   |                                    |
   |     | SAFETY ADDITIONS       |                                    |
   | 4-1 | Fall back to           | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3, |
   |     | Reno/Cubic on loss     | [RFC8257]                          |
   | 4-2 | Fall back to           | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3  |
   |     | Reno/Cubic if classic  |                                    |
   |     | ECN bottleneck         |                                    |
   |     | detected               |                                    |
   |     |                        |                                    |
   | 4-3 | Reduce RTT-dependence  | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3  |
   |     |                        |                                    |
   | 4-4 | Scaling TCP's          | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3, |
   |     | Congestion Window for  | [TCP-sub-mss-w]                    |
   |     | Small Round Trip Times |                                    |
   |     | SCALABLE TRANSPORT -   |                                    |
   |     | PERFORMANCE            |                                    |
   |     | ENHANCEMENTS           |                                    |
   | 5-1 | Setting ECT in TCP     | [I-D.ietf-tcpm-generalized-ecn]    |
   |     | Control Packets and    |                                    |
   |     | Retransmissions        |                                    |
   | 5-2 | Faster-than-additive   | [I-D.ietf-tsvwg-ecn-l4s-id] (Appx  |
   |     | increase               | A.2.2)                             |
   | 5-3 | Faster Convergence at  | [I-D.ietf-tsvwg-ecn-l4s-id] (Appx  |
   |     | Flow Start             | A.2.2)                             |

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   | #   | WG     | TCP | DCTCP | DCTCP-bis | TCP    | SCTP   | RMCAT  |
   |     |        |     |       |           | Prague | Prague | Prague |
   | 0   | tsvwg  | Y   | Y     | Y         | Y      | Y      | Y      |
   | 1   | tsvwg  |     |       | Y         | Y      | Y      | Y      |
   | 2   | tsvwg  | n/a | n/a   | n/a       | n/a    | n/a    | n/a    |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 3   | tcpm   | Y   | Y     | Y         | Y      | n/a    | n/a    |
   |     |        |     |       |           |        |        |        |
   | 4-1 | tcpm   |     | Y     | Y         | Y      | Y      | Y      |
   |     |        |     |       |           |        |        |        |
   | 4-2 | tcpm/  |     |       |           | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 4-3 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   | 4-4 | tcpm   | Y   | Y     | Y         | Y      | Y      | ?      |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 5-1 | tcpm   | Y   | Y     | Y         | Y      | n/a    | n/a    |
   |     |        |     |       |           |        |        |        |
   | 5-2 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   | 5-3 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |

Authors' Addresses

   Bob Briscoe (editor)


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   Koen De Schepper
   Nokia Bell Labs


   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid 28911

   Phone: 34 91 6249500

   Greg White


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