Transport Area Working Group                             B. Briscoe, Ed.
Internet-Draft                                       Simula Research Lab
Intended status: Informational                            K. De Schepper
Expires: October 1, 2017                                 Nokia Bell Labs
                                                        M. Bagnulo Braun
                                        Universidad Carlos III de Madrid
                                                          March 30, 2017

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


   This document describes the L4S architecture for the provision of a
   new service that the Internet could provide to 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  L4S architecture overview . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  L4S architecture components . . . . . . . . . . . . . . . . .   7
   5.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Why These Primary Components? . . . . . . . . . . . . . .   9
     5.2.  Why Not Alternative Approaches? . . . . . . . . . . . . .  10
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Deployment Considerations . . . . . . . . . . . . . . . .  14
       6.2.1.  Deployment Topology . . . . . . . . . . . . . . . . .  15
       6.2.2.  Deployment Sequences  . . . . . . . . . . . . . . . .  16
       6.2.3.  L4S Flow but Non-L4S Bottleneck . . . . . . . . . . .  18
       6.2.4.  Other Potential Deployment Issues . . . . . . . . . .  19

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   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Traffic (Non-)Policing  . . . . . . . . . . . . . . . . .  19
     8.2.  'Latency Friendliness'  . . . . . . . . . . . . . . . . .  20
     8.3.  Policing Prioritized L4S Bandwidth  . . . . . . . . . . .  20
     8.4.  ECN Integrity . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Required features for scalable transport protocols
                to be safely deployable in the Internet (a.k.a. TCP
                Prague requirements) . . . . . . . . . . . . . . . .  26
   Appendix B.  Standardization items  . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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, instant messaging, online
   gaming, remote desktop and cloud-based applications.  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.  When present it
   typically doubles the path delay from that due to the base speed-of-
   light.  Low loss is also important because, for interactive
   applications, losses translate into even longer retransmission

   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
   [RFC3246] for some packets at the expense of others, but this is not
   applicable 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) - 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
   capacity-seeking (e.g.  TCP) flow is running alongside the user's

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   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 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
   [I-D.ietf-tcpm-cubic]).  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 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 [I-D.ietf-aqm-fq-codel] or PIE [RFC8033].
   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 node
   should give nearly all the benefit.  The L4S approach requires a
   number of mechanisms in different parts of the Internet 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:  The L4S service traffic needs to be isolated from the
      queuing latency of the Classic service 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 service and capacity in access networks is too

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      scarce to partition into two.  So a 'semi-permeable' membrane is
      needed that partitions latency but not bandwidth.  The Dual Queue
      Coupled AQM [I-D.briscoe-aqm-dualq-coupled] is an example of such
      a semi-permeable membrane.

      Per-flow queuing such as in [I-D.ietf-aqm-fq-codel] could be used,
      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 thousands 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.briscoe-tsvwg-ecn-l4s-id]
      considers various alternative identifiers, and concludes that all
      alternatives involve compromises, but the ECT(1) codepoint of the
      ECN field is a workable solution.

   3) Host:  Scalable congestion controls already exist.  They solve the
      scaling problem with TCP first pointed out in [RFC3649].  The one
      used most widely (in controlled environments) is Data Centre TCP
      (DCTCP [I-D.ietf-tcpm-dctcp]), 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 (SCTP, RTP/RTCP, RMCAT, etc.)

                        (2)                     (1)
                 .-------^------. .--------------^-------------------.
    ,-(3)-----.                                  ______
   ; ________  :            L4S   --------.     |      |
   :|Scalable| :               _\        ||___\_| mark |
   :| sender | :  __________  / /        ||   / |______|\   _________
   :|________|\; |          |/    --------'         ^    \1|         |
    `---------'\_|  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

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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 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
      Centre 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,

   Scalable Congestion Control:  A congestion control where flow rate is
      inversely proportional to the level 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, DCTCP averages 2 congestion signals per round-trip
      whatever the flow rate.

   Classic Congestion Control:  A congestion control with a flow rate
      compatible with standard TCP Reno [RFC5681].  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 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

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      Reno-compatible up to 13 Mb/s.  All examples assume 1,500 B

   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 by the following elements.

   Protocols:The L4S architecture encompass the two protocol changes
   that we describe next:

   a.  [I-D.briscoe-tsvwg-ecn-l4s-id] recommends ECT(1) is used as the
       identifier to classify L4S and Classic packets into their
       separate treatments, as required by [RFC4774].

   b.  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--too often to
       use drops.  '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 allows
       networks and hosts to support two separate meanings for ECN.  So
       the standards track [RFC3168] will need to be updated to allow
       ECT(1) packets to depart from the 'same as drop' constraint.

       [I-D.ietf-tsvwg-ecn-experimentation] has been prepared as a
       standards track update to relax specific requirements in RFC 3168
       (and certain other standards track RFCs), which clears the way
       for the above experimental changes proposed for L4S.
       [I-D.ietf-tsvwg-ecn-experimentation] also obsoletes the original
       experimental assignment of the ECT(1) codepoint as an ECN nonce
       [RFC3540] (it was never deployed, and it offers no security
       benefit now that deployment is optional).

   Network components:The Dual Queue Coupled AQM has been specified as
   generically as possible [I-D.briscoe-aqm-dualq-coupled] as a 'semi-
   permeable' membrane without specifying the particular AQMs to use in
   the two queues.  An informational appendix of the draft is provided
   for pseudocode examples of different possible AQM approaches.

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   Initially a zero-config variant of RED called Curvy RED was
   implemented, tested and documented.  The aim is for designers to be
   free to implement diverse ideas.  So the brief normative body of the
   draft only specifies the minimum constraints an AQM needs to comply
   with to ensure that the L4S and Classic services will coexist.  For
   instance, a variant of PIE called Dual PI Squared [PI2] has been
   implemented and found to perform better over a wide range of
   conditions, so it has been documented in a second appendix of

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

   a.  Data Centre TCP is the most widely used example of a scalable
       congestion control.  It is being documented in the TCPM WG as an
       informational record of the protocol currently in use
       [I-D.ietf-tcpm-dctcp].  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).  The list also includes some
       optional performance improvements.

   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 transport 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.johansson-quic-ecn] and certain
       real-time media congestion avoidandance 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 it is the same as drop,
          making it unusable for a scalable TCP.  Therefore, the
          implementation of TCP receivers will have to be upgraded
          [RFC7560].  Work to standardize more accurate ECN feedback for
          TCP (AccECN [I-D.ietf-tcpm-accurate-ecn]) is already in

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       *  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, TCP'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 under-utilization.

      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 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.briscoe-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):

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      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

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

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   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.

   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 a dual queue only operates at the IP layer;

      C.  fq isolates the queuing of each flow from the others and it
          prevents any one flow from consuming more than 1/N of the
          capacity.  In contrast, all L4S flows are expected to keep the
          queue shallow, and policing of individual flows to enforce
          this may be applied separately, as a policy choice.

          An fq scheduler 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 [I-D.khademi-tcpm-alternativebackoff-ecn] alters the host
      behaviour in response to ECN marking to utilize a link better and
      give ECN flows a faster throughput, but it assumes the network
      still treats ECN and drop the same.  Therefore ABE exploits any
      lower queuing delay that AQMs can provide.  But as explained

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      above, AQMs still cannot reduce queuing delay too far without
      losing link utilization (for other non-ABE flows).

6.  Applicability

   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 in the best effort class:

   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.  A panoramic video
   of a football stadium can be swiped and pinched so that on the fly a
   proxy in the cloud generates a sub-window of the match video under
   the finger-gesture control of each user.  At the same time, a virtual
   reality headset fed from a 360 degree camera in a racing car has been
   demonstrated, where the user's head movements control the scene
   generated in the cloud.  In both cases, with 7 ms end-to-end base
   delay, the additional queuing delay of roughly 1 ms is so low that it
   seems the video is generated locally.  See
   dctth/ for videos of these demonstrations.

   Using a swiping finger gesture or head movement to pan a video are
   extremely demanding applications--far more demanding than VoIP.

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   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).

   If low network delay is not available, all fine interaction has to be
   done locally and therefore much more redundant data has to be
   downloaded.  When all interactive processing can be done in the
   cloud, only the data to be rendered for the end user needs to be
   sent.  Whereas, once applications can rely on minimal queues in the
   network, they can focus on reducing their own latency by only
   minimizing the application send queue.

6.1.  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) 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

      *  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)

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      *  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.

6.2.  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 6.2.1 explains why only deploying AQM in
   one node at each end of the access link will realize nearly all the

   L4S involves both end systems and the network, so Section 6.2.2
   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 6.2.3 describes how an L4S flow detects this, and
   how to minimize the effect of false negative detection.

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6.2.1.  Deployment Topology

   Nonetheless, 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 true 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.

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

      Figure 2: Likely location of DualQ 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
   example, some datacentre networks are designed with the bottleneck in

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   the hypervisor or host NICs, while others bottleneck at the top-of-
   rack switch (both the output ports facing hosts and those facing the

   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 datacentres.

6.2.2.  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
   [I-D.iab-protocol-transitions] 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 Appendix A), 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 intially 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.
   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 go first.  The DualQ AQM also
       greatly improves the downstream Classic service, assuming no
       other AQM has already been deployed.

   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' is
       only needed at one end.  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 [BBR].  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 on both ends (and in some cases only one) enables L4S
       trials to move to a production service, in one direction.  This
       stage might be further motivated by performance improvements
       between DCTCP and TCP Prague Appendix A.

   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

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       worth it to cover the first mover's investment risk.  As
       explained already, the potential for new 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

6.2.3.  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 'TCP-Friendly' behaviour (Requirement #4.1 in
   Appendix A).

   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, but they are all
   currently research:

   o  In TCP Prague, use a similar approach to BBR [BBR] to ignore
      selected 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 (no reference yet);

   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 will continually widen L4S

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   In recent studies there has been no evidence of Classic ECN support
   in AQMs on the Internet.  If Classic ECN support does materialize, a
   way to satisfy Requirement #4.2 in Appendix A will have to be added
   to TCP Prague.

6.2.4.  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 [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 treatement than others.  So it 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 would 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 (see item 3-1 in
   Appendix A).  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 restict access to the L4S
   class, perhaps only to customers who have paid a premium.  Their
   packet classifer (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-paying
   customers) into the Classic queue.  Allowing operators to use an

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   additional local classifier is intended to remove any incentive 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 requires.

8.2.  'Latency Friendliness'

   The L4S service does rely on self-constraint - not in terms of
   limiting capacity usage, but in terms of limiting burstiness.  It is
   hoped that standardisation of dynamic behaviour (cf.  TCP slow-start)
   and self-interest 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

8.3.  Policing Prioritized L4S Bandwidth

   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.  Then, within such Diffserv classes, L4S would often be
   applicable to give traffic low latency and low loss.  WIthin such a
   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 (variant exist where the
   policer re-marks non-compliant traffic to a discard-eligible Diffserv
   codepoint, so they may be dropped elsewhere during contention).  In
   networks that deploy L4S and use rate policers, it will be preferable
   to deploy a policer designed 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].  Otherwise, whenever
   L4S traffic encounters a rate policer, it will experience drops and

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   the source will fall back to a Classic congestion control, thus
   losing all the benefits of L4S.

   Further discussion of the applicability of L4S to the various
   Diffserv classes, and the design of suitable L4S rate policers.

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).  [RFC3540] proposes that a
   TCP sender could pseudorandomly set either of ECT(0) or ECT(1) in
   each packet of a flow and remember the sequence it had set, termed
   the ECN nonce.  If the receiver supports the nonce, it can prove that
   it is not suppressing feedback by reflecting its knowledge of the
   sequence back to the sender.  The nonce was proposed on the
   assumption that receivers might be more likely to cheat congestion
   control than senders (although senders also have a motive to cheat).

   If L4S uses the ECT(1) codepoint of ECN for packet classification, it
   will have to obsolete the experimental nonce.  As far as is known,
   the ECN Nonce has never been deployed, and it was only implemented
   for a couple of testbed evaluations.  It would be nearly impossible
   to deploy now, because any misbehaving receiver can simply opt-out,
   which would be unremarkable given all receivers currently opt-out.

   Other ways to protect TCP feedback integrity have since been
   developed.  For instance:

   o  the sender can test the integrity of the receiver's feedback by
      occasionally setting the IP-ECN field to a value normally only set
      by the network.  Then it can test whether the receiver's feedback
      faithfully reports what it expects [I-D.moncaster-tcpm-rcv-cheat].
      This method consumes no extra codepoints.  It works for loss and
      it will work for ECN feedback in any transport protocol suitable
      for L4S.  However, it shares the same assumption as the nonce;
      that the sender is not cheating and it is motivated to prevent the
      receiver cheating;

   o  A network can enforce a congestion response to its ECN markings
      (or packet losses) by auditing congestion exposure (ConEx)
      [RFC7713].  Whether the receiver or a downstream network is
      suppressing congestion feedback or the sender is unresponsive to
      the feedback, or both, ConEx audit can neutralise any advantage
      that any of these three parties would otherwise gain.  ConEx is
      only currently defined for IPv6 and consumes a destination option
      header.  It has been implemented, but not deployed as far as is

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9.  Acknowledgements

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

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

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

   [BBR]      Cardwell, N., Cheng, Y., Gunn, C., Yeganeh, S., and V.
              Jacobson, "BBR: Congestion-Based Congestion Control;
              Measuring bottleneck bandwidth and round-trip propagation
              time", ACM Queue (14)5, December 2016.

   [DCttH15]  De Schepper, K., Bondarenko, O., Tsang, I., and B.
              Briscoe, "'Data Centre to the Home': Ultra-Low Latency for
              All", 2015, <

              (Under submission)

              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

              Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
              "DualQ Coupled AQM for Low Latency, Low Loss and Scalable
              Throughput", draft-briscoe-aqm-dualq-coupled-01 (work in
              progress), March 2016.

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

              Schepper, K., Briscoe, B., and I. Tsang, "Identifying
              Modified Explicit Congestion Notification (ECN) Semantics
              for Ultra-Low Queuing Delay", draft-briscoe-tsvwg-ecn-l4s-
              id-02 (work in progress), October 2016.

              Thaler, D., "Planning for Protocol Adoption and Subsequent
              Transitions", draft-iab-protocol-transitions-08 (work in
              progress), March 2017.

              Hoeiland-Joergensen, T., McKenney, P.,
    , d., Gettys, J., and E. Dumazet, "The
              FlowQueue-CoDel Packet Scheduler and Active Queue
              Management Algorithm", draft-ietf-aqm-fq-codel-06 (work in
              progress), March 2016.

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
              ecn-02 (work in progress), October 2016.

              Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              draft-ietf-tcpm-cubic-04 (work in progress), February

              Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-ietf-tcpm-dctcp-05 (work
              in progress), March 2017.

              Briscoe, B., Kaippallimalil, J., and P. Thaler,
              "Guidelines for Adding Congestion Notification to
              Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
              encap-guidelines-08 (work in progress), March 2017.

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              Black, D., "Explicit Congestion Notification (ECN)
              Experimentation", draft-ietf-tsvwg-ecn-experimentation-01
              (work in progress), March 2017.

              Johansson, I., "ECN support in QUIC", draft-johansson-
              quic-ecn-01 (work in progress), February 2017.

              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", draft-khademi-
              tcpm-alternativebackoff-ecn-01 (work in progress), October

              Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
              Allow Senders to Identify Receiver Non-Compliance", draft-
              moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.

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

              You, J. and C. Xiong, "The Effect of Encrypted Traffic on
              the QoS Mechanisms in Cellular Networks", draft-you-
              encrypted-traffic-management-00 (work in progress),
              October 2015.

              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.

   [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

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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,

   [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,

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

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   [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, <>.

   [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,

   [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,

              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,

              Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul
              2015, 17:40, Prague", tcpprague mailing list archive ,
              July 2015.

Appendix A.  Required features for scalable transport protocols to be
             safely deployable in the Internet (a.k.a.  TCP Prague

   This list contains a list of features, mechanisms and modifications
   from currently defined behaviour for scalable Transport protocols so
   that they can be safely deployed over the public Internet.  This list
   of requirements was produced at an ad hoc meeting during IETF-94 in
   Prague [TCPPrague].

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   One of such scalable transport protocols is DCTCP, currently
   specified in [I-D.ietf-tcpm-dctcp].  In its current form, DCTCP is
   specified to be deployable in controlled environments and deploying
   it in the public Internet would lead to a number of issues, both from
   the safety and the performance perspective.  In this section, we
   describe the modifications and additional mechanisms that are
   required for its deployment over the global Internet.  We use DCTCP
   as a base, but it is likely that most of these requirements equally
   apply to other scalable transport protocols.

   We next provide a brief description of each required feature.

   Requirement #4.1: Fall back to Reno/Cubic congestion control on
   packet loss.

   Description: In case of packet loss, the scalable transport MUST
   react as classic TCP (whatever the classic version of TCP is running
   in the host, e.g.  Reno, Cubic).

   Motivation: As part of the safety conditions for deploying a scalable
   transport over the public Internet is to make sure that it behaves
   properly when some or all the network devices connecting the two
   endpoints that implement the scalable transport have not been
   upgraded.  In particular, it may be the case that some of the
   switches along the path between the two endpoints may only react to
   congestion by dropping packets (i.e. no ECN marking).  It is
   important that in these cases, the scalable transport react to the
   congestion signal in the form of a packet drop similarly to classic

   In the particular case of DCTCP, the current DCTCP specification
   states that "It is RECOMMENDED that an implementation deal with loss
   episodes in the same way as conventional TCP."  For safe deployment
   in the public Internet of a scalable transport, the above requirement
   needs to be defined as a MUST.

   Packet loss, while rare, may also occur in the case that the
   bottleneck is L4S capable.  In this case, the sender may receive a
   high number of packets marked with the CE bit set and also experience
   a loss.  Current DCTCP implementations react differently to this
   situation.  At least one implementation reacts only to the drop
   signal (e.g. by halving the CWND) and at least another DCTCP
   implementation reacts to both signals (e.g. by halving the CWND due
   to the drop and also further reducing the CWND based on the
   proportion of marked packet).  We believe that further
   experimentation is needed to understand what is the best behaviour
   for the public Internet, which may or not be one of the existent

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   Requirement #4.2: Fall back to Reno/Cubic congestion control on
   classic ECN bottlenecks.

   Description: The scalable transport protocol SHOULD/MAY? behave as
   classic TCP with classic ECN if the path contains a legacy bottleneck
   which marks both ect(0) and ect(1) in the same way as drop (non L4S,
   but ECN capable bottleneck).

   Motivation: Similarly to Requirement #3.1, this requirement is a
   safety condition in case L4S-capable endpoints are communicating over
   a path that contains one or more non-L4S but ECN capable switches and
   one of them happens to be the bottleneck.  In this case, the scalable
   transport will attempt to fill in the buffer of the bottleneck switch
   up to the marking threshold and produce a small sawtooth around that
   operation point.  The result is that the switch will set its
   operation point with the buffer full and all other non-scalable
   transports will be starved (as they will react reducing their CWND
   more aggressively than the scalable transport).

   Scalable transports then MUST be able to detect the presence of a
   classic ECN bottleneck and fall back to classic TCP/classic ECN
   behaviour in this case.

   Discussion: It is not clear at this point if it is possible to design
   a mechanism that always detect the aforementioned cases.  One
   possibility is to base the detection on an increase on top of a
   minimum RTT, but it is not yet clear which value should trigger this.
   Having a delay based fall back response on L4S may as well be
   beneficial for preserving low latency without legacy network nodes.
   Even if it possible to design such a mechanism, it may well be that
   it would encompass additional complexity that implementers may
   consider unnecessary.  The need for this mechanism depends on the
   extent of classic ECN deployment.

   Requirement #4.3: Reduce RTT dependence

   Description: Scalable transport congestion control algorithms MUST
   reduce or eliminate the RTT bias within the range of RTTs available.

   Motivation: Classic TCP's throughput is known to be inversely
   proportional to RTT.  One would expect flows over very low RTT paths
   to nearly starve flows over larger RTTs.  However, because Classic
   TCP induces a large queue, it has never allowed a very low RTT path
   to exist, so far.  For instance, consider two paths with base RTT 1ms
   and 100ms.  If Classic TCP induces a 20ms queue, it turns these RTTs
   into 21ms and 120ms leading to a throughput ratio of about 1:6.
   Whereas if a Scalable TCP induces only a 1ms queue, the ratio is

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   2:101.  Therefore, with small queues, long RTT flows will essentially

   Scalable transport protocol MUST then accommodate flows across the
   range of RTTs enabled by the deployment of L4S service over the
   public Internet.

   Requirement #4.4: Scaling down the congestion window.

   Description: Scalable transports MUST be responsive to congestion
   when RTTs are significantly smaller than in the current public

   Motivation: As currently specified, the minimum CWND of TCP (and the
   scalable extensions such as DCTCP), is set to 2 MSS.  Once this
   minimum CWND is reached, the transport protocol ceases to react to
   congestion signals (the CWND is not further reduced beyond this
   minimum size).

   L4S mechanisms reduce significantly the queueing delay, achieving
   smaller RTTs over the Internet.  For the same CWND, smaller RTTs
   imply higher transmission rates.  The result is that when scalable
   transport are used and small RTTs are achieved, the minimum value of
   the CWND currently defined in 2 MSS may still result in a high
   transmission rate for a large number of common scenarios.  For
   example, as described in [TCP-sub-mss-w], consider a residential
   setting with an broadband Internet access of 40Mbps.  Suppose now a
   number of equal TCP flows running in parallel with the Internet
   access link being the bottleneck.  Suppose that for these flows, the
   RTT is 6ms and the MSS is 1500B.  The minimum transmission rate
   supported by TCP in this scenario is when CWND is set to 2 MSS, which
   results in 4Mbps for each flow.  This means that in this scenario, if
   the number of flows is higher than 10, the congestion control ceases
   to be responsive and starts to build up a queue in the network.

   In order to address this issue, the congestion control mechanism for
   scalable transports MUST be responsive for the new range of RTT
   resulting from the decrease of the queueing delay.

   There are several ways how this can be achieved.  One possible sub-
   MSS window mechanism is described in [TCP-sub-mss-w].

   In addition to the safety requirements described before, there are
   some optimizations that while not required for the safe deployment of
   scalable transports over the public Internet, would results in an
   optimized performance.  We describe them next.

   Optimization #5.1: Setting ECT in SYN, SYN/ACK and pure ACK packets.

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   Description: Scalable transport SHOULD set the ECT bit in SYN, SYN/
   ACK and pure ACK packets.

   Motivation: Failing to set the ECT bit in SYN, SYN/ACK or ACK packets
   results in these packets being more likely dropped during congestion
   events.  Dropping SYN and SYN/ACK packets is particularly bad for
   performance as the retransmission timers for these packets are large.
   [RFC3168] prevents from marking these packets due to security
   reasons.  The arguments provided should be revisited in the the
   context of L4S and evaluate if avoiding marking these packets is
   still the best approach.

   Optimization #5.2: Faster than additive increase.

   Description: Scalable transport MAY support faster than additive
   increase in the congestion avoidance phase.

   Motivation: As currently defined, DCTCP supports additive increase in
   congestion avoidance phase.  It would be beneficial for performance
   to update the congestion control algorithm to increase the CWND more
   than 1 MSS per RTT during the congestion avoidance phase.  In the
   context of L4S such mechanism, must also provide fairness with other
   classes of traffic, including classic TCP and possibly scalable TCP
   that uses additive increase.

   Optimization #5.3: Faster convergence to fairness.

   Description: Scalable transport SHOULD converge to a fair share
   allocation of the available capacity as fast as classic TCP or

   Motivation: The time required for a new flow to obtain its fair share
   of the capacity of the bottleneck when the there are already ongoing
   flows using up all the bottleneck capacity is higher in the case of
   DCTCP than in the case of classic TCP (about a factor of 1,5 and 2
   larger according to [Alizadeh-stability]).  This is detrimental in
   general, but it is very harmful for short flows, which performance
   can be worse than the one obtained with classic TCP. for this reason
   it is desirable that scalable transport provide convergence times no
   larger than classic TCP.

Appendix B.  Standardization items

   The following table includes all the itmes that should be
   standardized to provide a full L4S architecture.

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   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 Centre TCP as currently used (in
      controlled environments);

   DCTCP bis:  Applicable to an future Data Centre 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.briscoe-tsvwg-ecn-l4s-id]      |
   | 2   | DUAL QUEUE AQM        | [I-D.briscoe-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-tcpm-dctcp]               |
   |     | Reno/Cubic on loss    |                                     |
   | 4-2 | Fall back to          |                                     |
   |     | Reno/Cubic if classic |                                     |
   |     | ECN bottleneck        |                                     |
   |     | detected              |                                     |
   |     |                       |                                     |
   | 4-3 | Reduce RTT-dependence |                                     |
   |     |                       |                                     |
   | 4-4 | Scaling TCP's         | [TCP-sub-mss-w]                     |
   |     | Congestion Window for |                                     |
   |     | Small Round Trip      |                                     |
   |     | Times                 |                                     |
   |     | SCALABLE TRANSPORT -  |                                     |
   |     | PERFORMANCE           |                                     |
   |     | ENHANCEMENTS          |                                     |
   | 5-1 | Setting ECT in SYN,   | draft-bagnulo-tsvwg-generalized-ECN |
   |     | SYN/ACK and pure ACK  |                                     |
   |     | packets               |                                     |
   | 5-2 | Faster-than-additive  |                                     |
   |     | increase              |                                     |
   | 5-3 | Less drastic exit     |                                     |
   |     | from slow-start       |                                     |

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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   | aqm?   | 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 | tsvwg  | 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)
   Simula Research Lab


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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

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