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Versions: (draft-briscoe-tsvwg-l4s-arch) 00 01 02 03 04

Transport Area Working Group                             B. Briscoe, Ed.
Internet-Draft                                                 CableLabs
Intended status: Informational                            K. De Schepper
Expires: January 9, 2020                                 Nokia Bell Labs
                                                        M. Bagnulo Braun
                                        Universidad Carlos III de Madrid
                                                                G. White
                                                               CableLabs
                                                            July 8, 2019


   Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
                              Architecture
                      draft-ietf-tsvwg-l4s-arch-04

Abstract

   This document describes the L4S architecture for the provision of a
   new Internet service that could eventually replace best efforts for
   all traffic: Low Latency, Low Loss, Scalable throughput (L4S).  It is
   becoming common for _all_ (or most) applications being run by a user
   at any one time to require low latency.  However, the only solution
   the IETF can offer for ultra-low queuing delay is Diffserv, which
   only favours a minority of packets at the expense of others.  In
   extensive testing the new L4S service keeps average queuing delay
   under a millisecond for _all_ applications even under very heavy
   load, without sacrificing utilization; and it keeps congestion loss
   to zero.  It is becoming widely recognized that adding more access
   capacity gives diminishing returns, because latency is becoming the
   critical problem.  Even with a high capacity broadband access, the
   reduced latency of L4S remarkably and consistently improves
   performance under load for applications such as interactive video,
   conversational video, voice, Web, gaming, instant messaging, remote
   desktop and cloud-based apps (even when all being used at once over
   the same access link).  The insight is that the root cause of queuing
   delay is in TCP, not in the queue.  By fixing the sending TCP (and
   other transports) queuing latency becomes so much better than today
   that operators will want to deploy the network part of L4S to enable
   new products and services.  Further, the network part is simple to
   deploy - incrementally with zero-config.  Both parts, sender and
   network, ensure coexistence with other legacy traffic.  At the same
   time L4S solves the long-recognized problem with the future
   scalability of TCP throughput.

   This document describes the L4S architecture, briefly describing the
   different components and how the work together to provide the
   aforementioned enhanced Internet service.




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Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  L4S Architecture Overview . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  L4S Architecture Components . . . . . . . . . . . . . . . . .   7
   5.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Why These Primary Components? . . . . . . . . . . . . . .  10
     5.2.  Why Not Alternative Approaches? . . . . . . . . . . . . .  12
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Applications  . . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Deployment Considerations . . . . . . . . . . . . . . . .  17
       6.3.1.  Deployment Topology . . . . . . . . . . . . . . . . .  18
       6.3.2.  Deployment Sequences  . . . . . . . . . . . . . . . .  19
       6.3.3.  L4S Flow but Non-L4S Bottleneck . . . . . . . . . . .  21



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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 . . . . . . . .  24
     8.4.  ECN Integrity . . . . . . . . . . . . . . . . . . . . . .  25
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     10.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  Standardization items  . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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
   latency.

   Therefore, the goal is an Internet service with ultra-Low queueing
   Latency, ultra-Low Loss and Scalable throughput (L4S) - for _all_
   traffic.  A service for all traffic will need none of the
   configuration or management baggage (traffic policing, traffic
   contracts) associated with favouring some packets over others.  This
   document describes the L4S architecture for achieving that goal.

   It must be said that queuing delay only degrades performance
   infrequently [Hohlfeld14].  It only occurs when a large enough



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   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 so remarkable 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 call this family of congestion controls
   'Classic' TCP.  It has been demonstrated that if the sending host
   replaces Classic TCP 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 stunningly 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 TCP 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 TCP, 5 ms of queuing is
   usually only possible by losing some utilization.

   It has been convincingly 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):

   1) Network:  L4S traffic needs to be isolated from the queuing
      latency of Classic traffic.  However, the two should be able to
      freely share a common pool of capacity.  This is because there is
      no way to predict how many flows at any one time might use each



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      service and capacity in access networks is too scarce to partition
      into two.  The Dual Queue Coupled AQM
      [I-D.ietf-tsvwg-aqm-dualq-coupled] was developed as a minimal
      complexity solution to this problem.  The two queues appear to be
      separated by a 'semi-permeable' membrane that partitions latency
      but not bandwidth (explained later).

      Per-flow queuing such as in [RFC8290] could be used (see
      Section 4), but it partitions both latency and bandwidth between
      every end-to-end flow.  So it is rather overkill, which brings
      disadvantages (see Section 5.2), not least that large number of
      queues are needed when two are sufficient.

   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 TCP that was first pointed out 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 later).  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 and an L4S variant of the RMCAT SCReAM
      controller [RFC8298].
















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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",
   "SHOULD", "SHOULD 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.  COMMENT: Since this
   will be an information document, This should be removed.

   Classic service:  The 'Classic' service is intended for all the
      congestion control behaviours that currently co-exist with TCP
      Reno (e.g.  TCP Cubic, Compound, SCTP, etc).

   Low-Latency, Low-Loss and Scalable (L4S) service:  The 'L4S' service
      is intended for traffic from scalable TCP algorithms such as Data
      Center TCP.  But it is also more general--it will allow a set of
      congestion controls with similar scaling properties to DCTCP (e.g.
      Relentless [Mathis09]) to evolve.

      Both Classic and L4S services can cope with a proportion of
      unresponsive or less-responsive traffic as well (e.g.  DNS, VoIP,
      etc).

   Scalable Congestion Control:  A congestion control where the packet
      flow rate per round trip (the window) is inversely proportional to
      the level (probability) of congestion signals.  Then, as flow rate
      scales, the number of congestion signals per round trip remains
      invariant, maintaining the same degree of control.  For instance,



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      DCTCP averages 2 congestion signals per round-trip whatever the
      flow rate.

   Classic Congestion Control:  A congestion control with a flow rate
      that can co-exist with standard TCP Reno [RFC5681] without
      starvation.  With Classic congestion controls, as capacity
      increases enabling higher flow rates, the number of round trips
      between congestion signals (losses or ECN marks) rises in
      proportion to the flow rate.  So control of queuing and/or
      utilization becomes very slack.  For instance, with 1500 B packets
      and an RTT of 18 ms, as TCP Reno flow rate increases from 2 to 100
      Mb/s the number of round trips between congestion signals rises
      proportionately, from 2 to 100.

      The default congestion control in Linux (TCP Cubic) is Reno-
      compatible for most Internet access scenarios expected for some
      years.  For instance, with a typical domestic round-trip time
      (RTT) of 18ms, TCP Cubic only switches out of Reno-compatibility
      mode once the flow rate approaches 1 Gb/s.  For a typical data
      centre RTT of 1 ms, the switch-over point is theoretically 1.3 Tb/
      s.  However, with a less common transcontinental RTT of 100 ms, it
      only remains Reno-compatible up to 13 Mb/s.  All examples assume
      1,500 B packets.

   Classic ECN:  The original proposed standard 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.

   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 generalisation.

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:



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       *  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
       historic.

   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
       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 Dual Queue Coupled AQM has been specified as generically as
       possible [I-D.ietf-tsvwg-aqm-dualq-coupled] as a 'semi-permeable'
       membrane without specifying the particular AQMs to use in the two
       queues.  Informational appendices of the draft are provided for
       pseudocode examples of different possible AQM approaches.  The
       aim is for designers to be free to implement diverse ideas.  So
       the brief normative body of the draft only specifies the minimum



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       constraints an AQM needs to comply with to ensure that the L4S
       and Classic services will coexist.  The core idea is the tension
       between the scheduler's prioritization of L4S over Classic and
       the coupling from the Classic to the L4S AQM.  The L4S AQM
       derives its level of ECN marking from the maximum of the
       congestion levels in both queues.  So L4S flows leave enough
       space between their packets for Classic flows, as if they were
       all the same type of TCP, all sharing one FIFO queue.

       Initially a zero-config variant of RED called Curvy RED was
       implemented, tested and documented.  Then, a variant of PIE
       called DualPI2 (pronounced Dual PI Squared) [PI2] was implemented
       and found to perform better than Curvy RED over a wide range of
       conditions, so it was documented in another appendix of
       [I-D.ietf-tsvwg-aqm-dualq-coupled].

   b.  A scheduler with per-flow queues can be used for L4S.  It would
       be simple to modify an existing design such as FQ-CoDel or FQ-
       PIE, although this has not been implemented and evaluated because
       the goal of the original proponents of L4S was to avoid per-flow
       scheduling.

       The idea would be to implement two AQMs (Classic and Scalable)
       and switch each per-flow queue to use an instance of the
       appropriate AQM for the flow, based on the ECN codepoints of the
       packets.  Flows of non-ECN or ECT(0) packets would use a Classic
       AQM such as CoDel or PIE, while flows of ECT(1) packets without
       any ECT(0) packets would use a simple shallow threshold AQM with
       immediate (unsmoothed) marking.  The FQ scheduler might work as
       is, because it is likely that L4S flows would be continually
       categorized as 'new' flows.  However, this presumption has not
       been tested under a wide range of conditions.  A variant of FQ-
       CoDel already exists that adapts to a shallower threshold AQM for
       ECN-capable packets.

   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 TCP Prague requirements, has been drawn up (see Appendix A of
       [I-D.ietf-tsvwg-ecn-l4s-id]).  The list also includes some
       optional performance improvements.





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   b.  Transport protocols other than TCP use various congestion
       controls designed to be friendly with Classic TCP.  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 real-time media congestion
       avoidance techniques (RMCAT) protocols.

   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, TCP's
         sawtooth reduction can be smaller and therefore return to the



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         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 under-utilization.

      *  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).
         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.





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   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
      otherwise.

   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 TCP-Friendly congestion control algorithms
      [RFC3649].  It was known when TCP 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' TCP
      Reno.  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,



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      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
      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.
      The L4S work is intended to complement these AQMs, and we
      definitely do not want to 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 needs 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



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          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
          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):  Yet 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 (not released at the
      time of writing) is likely to use L4S ECN and a TCP-Prague-like



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      behaviour if it discovers a compatible path.  Otherwise it will
      use an evolution of BBRv1.

6.  Applicability

6.1.  Applications

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

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

   o  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
   locally.



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   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
   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
      processes.

   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 utilise sudden increases of
         capacity;

      *  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
         low;





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      *  Nonetheless, it is certainly desirable not to hold a buffer
         purely because of the sawteeth of Classic TCP, 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

      *  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
      [I-D.smith-encrypted-traffic-management]:

      *  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 "TCP-
   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.



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   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
   part.

   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.

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.
   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 satellite.

   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.




















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                                            ______
                                           (      )
                         __          __  (          )
                        |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
                                topologies

   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
   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




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   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.

   +-+--------------------+----------------------+---------------------+
   | | Servers or proxies |      Access link     |             Clients |
   +-+--------------------+----------------------+---------------------+
   |1| DCTCP (existing)   |                      |    DCTCP (existing) |
   | |                    | DualQ AQM downstream |                     |
   | |       WORKS DOWNSTREAM FOR CONTROLLED DEPLOYMENTS/TRIALS        |
   +-+--------------------+----------------------+---------------------+
   |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:




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   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 still be added).

   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 TCP Prague 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 [I-D.ietf-tsvwg-ecn-l4s-id]).

   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
   kindness.

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 ('TCP-Friendly') behaviour (see
   Appendix A.1.3 of [I-D.ietf-tsvwg-ecn-l4s-id]).



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   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 TCP Prague, 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],
      [I-D.ietf-tsvwg-ecn-l4s-id];

   o  Hybrid ECN/drop 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
   applicability.

   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 ('TCP-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.







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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],
   [I-D.ietf-tsvwg-ecn-encap-guidelines].

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 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
   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



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   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'

   The L4S service does rely on self-constraint - not in terms of
   limiting rate, but in terms of limiting latency (burstiness).  It is
   hoped that self-interest and standardisation of dynamic behaviour
   (cf.  TCP slow-start) will be sufficient to prevent transports from
   sending excessive bursts of L4S traffic, given the application's own
   latency will suffer most from such behaviour.

   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
   malware.

   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
   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



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   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.

   This is currently a research area.  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 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.

   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
   TCP 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)
      [RFC7713].





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   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
      [RFC8311].

   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 and David
   Black 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.  The views expressed here are solely those of the
   authors.

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,
              <https://www.rfc-editor.org/info/rfc2119>.

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,
              <http://riteproject.eu/publications/>.

   [Hohlfeld14]
              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
              2014.

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



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

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

   [I-D.ietf-tcpm-accurate-ecn]
              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-08 (work in progress), March 2019.

   [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-03 (work in progress),
              October 2018.

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
              AQMs for Low Latency, Low Loss and Scalable Throughput
              (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-09 (work in
              progress), July 2019.

   [I-D.ietf-tsvwg-ecn-encap-guidelines]
              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.








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   [I-D.ietf-tsvwg-ecn-l4s-id]
              Schepper, K. and B. Briscoe, "Identifying Modified
              Explicit Congestion Notification (ECN) Semantics for
              Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
              id-06 (work in progress), March 2019.

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

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

   [I-D.white-tsvwg-nqb]
              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.

   [L4Sdemo16]
              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,
              <http://dl.acm.org/citation.cfm?doid=2910017.2910633
              (videos of demos: https://riteproject.eu/
              dctth/#1511dispatchwg )>.

   [Mathis09]
              Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
              May 2009, <https://www.gdt.id.au/~gdt/
              presentations/2010-07-06-questnet-tcp/reference-
              materials/papers/
              mathis-relentless-congestion-control.pdf>.

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





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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>.

   [RFC2697]  Heinanen, J. and R. Guerin, "A Single Rate Three Color
              Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,
              <https://www.rfc-editor.org/info/rfc2697>.

   [RFC2698]  Heinanen, J. and R. Guerin, "A Two Rate Three Color
              Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999,
              <https://www.rfc-editor.org/info/rfc2698>.

   [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,
              <https://www.rfc-editor.org/info/rfc2884>.

   [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>.

   [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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   [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>.

   [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>.

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

   [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>.

   [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>.

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

   [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>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.





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   [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>.

   [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>.

   [RFC8170]  Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <https://www.rfc-editor.org/info/rfc8170>.

   [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>.

   [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>.



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   [TCP-sub-mss-w]
              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,
              <http://www.bobbriscoe.net/projects/latency/
              sub-mss-w.pdf>.

   [UnorderedLTE]
              Austrheim, M., "Implementing immediate forwarding for 4G
              in a network simulator", Masters Thesis, Uni Oslo , June
              2019.

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
   left.

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

   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)
   CableLabs
   UK

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










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Internet-Draft              L4S Architecture                   July 2019


   Koen De Schepper
   Nokia Bell Labs
   Antwerp
   Belgium

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


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

   Phone: 34 91 6249500
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es


   Greg White
   CableLabs
   US

   Email: G.White@CableLabs.com


























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