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Versions: 00 01 02 draft-briscoe-tsvwg-l4s-arch

Transport Services (tsv)                                 B. Briscoe, Ed.
Internet-Draft                                       Simula Research Lab
Intended status: Informational                            K. De Schepper
Expires: December 12, 2016                               Nokia Bell Labs
                                                        M. Bagnulo Braun
                                        Universidad Carlos III de Madrid
                                                           June 10, 2016

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


   This document motivates 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 explains the underlying problems that have been
   preventing the Internet from enjoying such performance improvements.
   It then outlines the parts necessary for a solution and the steps
   that will be needed to standardize them.  It points out opportunities
   that will open up, and sets out some likely use-cases, including
   ultra-low latency interaction with cloud processing over the public

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Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on December 12, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  The Application Performance Problem . . . . . . . . . . .   3
     1.2.  The Technology Problem  . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  The Standardization Problem . . . . . . . . . . . . . . .   7
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  Why These Primary Components? . . . . . . . . . . . . . .   9
     2.2.  Why Not Alternative Approaches? . . . . . . . . . . . . .  10
   3.  Opportunities . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     5.1.  Traffic (Non-)Policing  . . . . . . . . . . . . . . . . .  14
     5.2.  'Latency Friendliness'  . . . . . . . . . . . . . . . . .  14

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     5.3.  ECN Integrity . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  The "TCP Prague Requirements"  . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

1.1.  The Application Performance Problem

   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.  Having motivated the goal of 'L4S for all', this document
   enumerates the problems that have to be overcome to reach it.

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

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1.2.  The Technology Problem

   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 [I-D.ietf-aqm-pie].  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.  Although the main incremental
   deployment problem has been solved, and the remaining work seems
   straightforward, there may need to be changes in approach during the
   process of engineering a complete solution.

   There are three main parts to the L4S approach (illustrated in
   Figure 1):

   2) Network:  The L4S service needs to be isolated from the queuing
      latency of the Classic service.  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 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 bandwdith between every e2e

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      flow.  So it is rather overkill, which brings disadvantages (see
      Section 2.2), not least that thousands of queues are needed when
      two are sufficient.

   1) 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.)

                       (1)                      (2)
                .-------^------.  .--------------^-------------------.
  ,-(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

1.3.  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

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   in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

   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
      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.

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   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.

1.4.  The Standardization Problem

   0) Architecture:  The first step will be to articulate the structure
      and interworking requirements of the set of parts that would
      satisfy the overall application performance requirements.

   Then specific interworking aspects of the following three components
   parts will need to be defined:

   1) Protocol:

      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].  The draft also
          points out that the original experimental assignment of this
          codepoint as an ECN nonce [RFC3540] needs to be made obsolete
          (it was never deployed, and it offers no security benefit now
          that deployment is optional).

      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.

   2) Network:  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.  Initially a zero-config variant of RED called Curvy
      RED was implemented, tested and documented.  A variant of PIE has
      been implemented and tested and is about to be 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.

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   3) Host:

      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).

      B.  Transport protocols other than TCP use various congestion
          controls designed to be friendly with Classic TCP.  It will be
          necessary to implement scalable variants of each of these
          transport behaviours before they can use the L4S service.  The
          following standards track RFCs currently define these
          protocols, and they will need to be updated to allow a
          different congestion response, which they will have to
          indicate by using the ECT(1) codepoint: ECN in TCP [RFC3168],
          in SCTP [RFC4960], in RTP [RFC6679], and in DCCP [RFC4340].

      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 progress.

          +  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.

   Currently, the new specification of the ECN protocol
   [I-D.briscoe-tsvwg-ecn-l4s-id] has been written for the experimental
   track.  Perhaps a better approach would be to make this a standards
   track protocol draft that updates the definition of ECT(1) in all the
   above standards track RFCs and obsoletes its experimental use for the
   ECN nonce.  Then experimental specifications of example network (AQM)
   and host (congestion control) algorithms can be written.

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2.  Rationale

2.1.  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]).  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

         "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

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      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

2.2.  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.  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.

   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.

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   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 decides packet-by-packet which flow to schedule without
          knowing application intent.  In contrast, in the L4S approach
          the sender still controls the relative rate of each flow
          dependent on the needs of each application.

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

3.  Opportunities

   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

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   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 https://riteproject.eu/
   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.
   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.

3.1.  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

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      *  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)

      *  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.

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4.  IANA Considerations

   This specification contains no IANA considerations.

5.  Security Considerations

5.1.  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.

   Certain network operators might choose to restict access to the L4S
   class, perhaps only to customers who have paid a premium.  In the
   packet classifer (item 2 in Figure 1), they could identify such
   customers using some other field than ECN (e.g. source address
   range), and just ignore the L4S identifier for non-paying customers.
   This would ensure that the L4S identifier survives end-to-end even
   though the service does not have to 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.

5.2.  '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
   believed 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.

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   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

5.3.  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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6.  Acknowledgements

7.  References

7.1.  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,

7.2.  Informative References

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

              (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.

              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-01 (work in progress), March 2016.

              Hoeiland-Joergensen, T., McKenney, P.,
              dave.taht@gmail.com, 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.

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              Pan, R., Natarajan, P., Baker, F., and G. White, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", draft-ietf-aqm-pie-08 (work in progress), June

              Briscoe, B., K&#258;&#378;hlewind, M., and R.
              Scheffenegger, "More Accurate ECN Feedback in TCP", draft-
              ietf-tcpm-accurate-ecn-00 (work in progress), December

              Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              draft-ietf-tcpm-cubic-01 (work in progress), January 2016.

              Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,
              and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
              Control for Datacenters", draft-ietf-tcpm-dctcp-01 (work
              in progress), November 2015.

              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", draft-khademi-
              tcpm-alternativebackoff-ecn-00 (work in progress), May

              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.

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              Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
              May 2009, <http://www.hpcc.jp/pfldnet2009/

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

   [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, <http://www.rfc-editor.org/info/rfc6679>.

   [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,

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

              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.  The "TCP Prague Requirements"

   This list of requirements was produced at an ad hoc meeting during
   IETF-94 in Prague [TCPPrague].  The list prioritised features that
   would need to be added to DCTCP to make it safe for use on the public
   Internet alongside existing non-DCTCP traffic (up to #3-7).  After
   that, it also includes features to improve the performance of DCTCP
   in the wider range of conditions found on the public Internet.

   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 references should be consulted for why each requirement is
   considered necessary for safety.  There follow brief reasons for
   those that are not self-explanatory and have not yet been written up:

   #3-5 Reduce RTT-dependence:  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,

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

   Smooth ECN feedback over own RTT:  DCTCP currently smooths feedback
      over a hard-coded number of segments, with the value optimized for
      data centres.  For the wider range of round-trip times on the
      public Internet, it needs to smooth over roughly one window of
      packets.  Otherwise it could respond too rapidly (or too
      sluggishly) and become unstable (or unresponsive).

   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] |
   |     |                           |                                 |
   |     | SCALABLE TRANSPORT -      |                                 |
   |     | SAFETY ADDITIONS          |                                 |
   | 3-1 | Fall back to Reno/Cubic   | [I-D.ietf-tcpm-dctcp]           |
   |     | on loss                   |                                 |
   | 3-2 | Suitable ECN Feedback     | [I-D.ietf-tcpm-accurate-ecn],   |
   |     |                           | [I-D.stewart-tsvwg-sctpecn].    |
   | 3-4 | Scaling TCP's Congestion  | [TCP-sub-mss-w]                 |
   |     | Window for Small Round    |                                 |
   |     | Trip Times                |                                 |
   | 3-5 | Reduce RTT-dependence     |                                 |
   |     |                           |                                 |
   | 3-6 | Smooth ECN feedback over  |                                 |
   |     | own RTT                   |                                 |
   | 3-7 | Fall back to Reno/Cubic   |                                 |
   |     | if classic ECN bottleneck |                                 |
   |     | detected                  |                                 |
   |     |                           |                                 |
   |     | SCALABLE TRANSPORT -      |                                 |
   |     | PERFORMANCE ENHANCEMENTS  |                                 |
   | 3-8 | Faster-than-additive      |                                 |
   |     | increase                  |                                 |
   | 3-9 | 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    |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 3-1 | tcpm   |     | Y     | Y         | Y      | Y      | Y      |
   |     |        |     |       |           |        |        |        |
   | 3-2 | tcpm   | Y   | Y     | Y         | Y      | n/a    | n/a    |
   |     |        |     |       |           |        |        |        |
   | 3-4 | tcpm   | Y   | Y     | Y         | Y      | Y      | ?      |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 3-5 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   | 3-6 | tcpm/  |     | ?     | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   | 3-7 | tcpm/  |     |       |           | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   |     |        |     |       |           |        |        |        |
   | 3-8 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |
   | 3-9 | tcpm/  |     |       | Y         | Y      | Y      | ?      |
   |     | iccrg? |     |       |           |        |        |        |

Authors' Addresses

   Bob Briscoe (editor)
   Simula Research Lab

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

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   Koen De Schepper
   Nokia Bell Labs

   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

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

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