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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 5, 2016                                Nokia Bell Labs
                                                        M. Bagnulo Braun
                                        Universidad Carlos III de Madrid
                                                            June 3, 2016


   Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
                           Problem Statement
           draft-briscoe-tsvwg-aqm-tcpm-rmcat-l4s-problem-00

Abstract

   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, but the only solution the IETF can offer for ultra-low
   queuing latency is Diffserv, which only offers low latency for some
   packets at the expense of others.  Diffserv has also proved hard to
   deploy widely end-to-end.

   In contrast, a zero-config incrementally deployable solution has been
   demonstrated that keeps average queuing delay under a millisecond for
   _all_ applications even under very heavy load; and it keeps
   congestion loss to zero.  At the same time it solves the long-running
   problem with the scalability of TCP throughput.  Even with a high
   capacity broadband access, the resulting performance under load is
   remarkably and consistently improved for applications such as
   interactive video, conversational video, voice, Web, gaming, instant
   messaging, remote desktop and cloud-based apps.  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.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.




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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 5, 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  The Application Performance Problem . . . . . . . . . . .   3
     1.2.  The Technology Problem  . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  The Standardization Problem . . . . . . . . . . . . . . .   5
   2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Why These Primary Components? . . . . . . . . . . . . . .   7
     2.2.  Why Not Alternative Approaches? . . . . . . . . . . . . .   7
   3.  Opportunities . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     5.1.  Traffic (Non-)Policing  . . . . . . . . . . . . . . . . .  10
     5.2.  'Latency Friendliness'  . . . . . . . . . . . . . . . . .  11
     5.3.  ECN Integrity . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  The "TCP Prague Requirements"  . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16






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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.  Low loss is also
   important because, for interactive applications, losses translate
   into delays.

   It has been demonstrated that, once access network bit rate reaches
   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
   latency.

   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.

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



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   suitable AQM is deployed in the network the performance under load of
   all the above interactive applications can be stunningly improved -
   even in comparison to a state-of-the-art AQM such as
   fq_CoDel [I-D.ietf-aqm-fq-codel] or PIE [I-D.ietf-aqm-pie].

   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 Fig
   {ToDo: ASCII art of slide 9 from
   https://riteproject.files.wordpress.com/2015/10/1604-l4s-bar-
   bof.pdf}):

   1.  The L4S service needs to be isolated from the queuing latency of
       the Classic service.  However, the two must be able to freely
       share a common pool of capacity.  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.  The Dual
       Queue Coupled AQM is an example of such a 'semi-permeable'
       membrane [I-D.briscoe-aqm-dualq-coupled].  Per-flow queuing such
       as in [I-D.ietf-aqm-fq-codel] could be used, but it is rather
       overkill, which brings disadvantages (see Section 2.2).

   2.  An identifier is needed to so that L4S and Classic packets can be
       classified 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.  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




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       control will also need to be transplanted into protocols other
       than TCP (SCTP, RTP/RTCP, RMCAT, etc.)

1.3.  Terminology

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

   Classic service:  The 'Classic' service is intended for all the
      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,
      etc).

   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.
      With Classic congestion controls such as TCP Reno and Cubic, as
      capacity increases enable higher flow rates, the number of round
      trips between signals becomes very large, so control of queuing
      and/or utilization becomes very slack.

   Classic ECN:  The original Explicit Congestion Notification (ECN)
      protocol [RFC3168].

1.4.  The Standardization Problem

   1.  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:





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   1.  The L4S service needs to be isolated from the queuing latency of
       the Classic service.  However, the two must be able to freely
       share a common pool of capacity.  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.  The Dual
       Queue Coupled AQM is an example of such a 'semi-permeable'
       membrane [I-D.briscoe-aqm-dualq-coupled].  Per-flow queuing such
       as in [I-D.ietf-aqm-fq-codel] could be used, but it has
       disadvantages, not least that thousands of queues are not needed
       if two are sufficient.

   2.  Identifier

       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 experimental assignment of this
           codepoint as an ECN nonce [RFC3540] will need 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 Notification (ECN [RFC3168]).
           'Classic' ECN 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.  A separate queue for L4S allows
           the network to support two separate meanings for ECN.  And
           break from this 'same as drop' constraint is an essential
           feature of a scalable congestion control as well.

   3.  Scalable congestion controls

       A.  Data Centre TCP 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, by
           sending packets with the ECT(1) identifier.  The following
           standards track RFCs currently define these protocols: ECN in
           TCP [RFC3168], in SCTP [RFC4960], in RTP [RFC6679], and in
           DCCP [RFC4340].




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       C.  For the case of TCP, the feedback protocol for ECN is too
           tightly coupled to Classic ECN to be usable 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.

2.  Rationale

2.1.  Why These Primary Components?

   {ToDo: /Why/ the various elements are necessary:}

   ECN rather than drop

   Packet identifier (pretty obvious why)

   Scalable congestion notification (host behaviour)

   Semi-permeable membrane (network behaviour)

   {We will probably move some of the text in the bullets under "The
   Technology Problem" to here, e.g. why you need capacity shared across
   the semi-permeable membrane.}

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




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

   State-of-the-art AQMs:  AQMs such as PIE and fq_CoDel give a
      significant reduction in queuing delay relative to no AQM at all.
      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, they
      cannot reduce queuing delay too far without significantly reducing
      link utilization.  The L4S approach resolves this tension by
      ensuring hosts can minimize the sawtoothing.

   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, which only operates at the IP
          layer;

      C.  fq has to take control of the decisions over which flows are
          scheduled when - 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.  ABE 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.





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   If applications can rely on minimal queues in the network, they can
   focus on reducing their own latency by only minimizing the
   application send queue.  Following existing applications will
   immediately experience a better quality of experience in the best
   effort class:

      Gaming

      VoIP

      Video conferencing

      Web browsing

      (Adaptive) Video Streaming

   The lower transport layer latency will also allow more interactive
   application functions offloading to the cloud.  If last-minute
   interactions need to be done locally, more data must be send over the
   link.  When all interactive processing can be done in the cloud, only
   the info to be rendered to the end user can be sent.  It will allow
   applications such as:

      Cloud based interactive video

      Cloud based virtual and augmented reality

   Also lower network layers can finally be further optimized for low
   latency and stable throughput.  Today it is not cost efficient, as
   the largest part of the traffic (classic best effort) needs to allow
   "big" queues anyway (up to several 100s of milliseconds) to make
   classic congestion control work correctly.  While technology is known
   and feasible to support low latency with reliable throughput (even
   mobile), it is today not considered as economically relevant, as best
   effort can absorb any burst, delay or throughput variations without
   end-users experiencing any difference from the normal tay-to-day
   operation due to congestion control limitations.

3.1.  Use Cases

   {ToDo: Just bullets below - text to be added by those interested in
   various use-cases}

   Different types of access network: DSL, cable, mobile

   The challenges and opportunities with radio links: cellular, Wifi





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   Private networks of heterogeneous data centres (DC interconnect,
   multi-tenant cloud, etc)

   Different types of transport/app: elastic (TCP/SCTP); real-time (RTP,
   RMCAT); query (DNS/LDAP).

   Avoiding reliance on middleboxes to enable encryption/privacy
   (because the L4S approach does not look deeper than IP in the
   network).

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 has to use traffic policers
   to limit how much traffic can access each service, otherwise it
   doesn't work, In turn, traffic policers require traffic contracts
   between users and networks and 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 the scalable congestion controll detects a drop it has to
   respond as if it is a Classic congestion control, and there will then
   be no interworking problems.

   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, they could identify such customers using some other
   field (e.g. source address range), and just ignoring the L4S
   identifier for non-paying customers.  This will 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 complex application-specific traffic contracts of Diffserv.






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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.

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

5.3.  ECN Integrity

   {ToDo: Paraphrase discussion from ecn-l4s-id}

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

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

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







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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/
              dctth_preprint.pdf>.

              (Under submission)

   [Hohlfeld14]
              Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A., and P.
              Barford, "A QoE Perspective on Sizing Network Buffers",
              Proc. ACM Internet Measurement Conf (IMC'14) hmm, November
              2014.

   [I-D.briscoe-aqm-dualq-coupled]
              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.

   [I-D.briscoe-tsvwg-ecn-l4s-id]
              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.

   [I-D.ietf-aqm-fq-codel]
              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.

   [I-D.ietf-aqm-pie]
              Pan, R., Natarajan, P., and F. Baker, "PIE: A Lightweight
              Control Scheme To Address the Bufferbloat Problem", draft-
              ietf-aqm-pie-07 (work in progress), April 2016.

   [I-D.ietf-tcpm-accurate-ecn]
              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
              2015.







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   [I-D.ietf-tcpm-cubic]
              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.

   [I-D.ietf-tcpm-dctcp]
              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.

   [I-D.moncaster-tcpm-rcv-cheat]
              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.

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

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

   [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,
              <http://www.rfc-editor.org/info/rfc3246>.

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

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

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






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

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

   [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,
              <http://www.rfc-editor.org/info/rfc7560>.

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

Appendix A.  The "TCP Prague Requirements"

   This list of requirements was produced at an ad hoc meeting during
   IETF-94 in Prague.  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.  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 been split
   into two, with a common column of row index numbers on the left.





















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   +-----+---------------------------+---------------------------------+
   | #   | 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 | TCP ECN Feedback          | [I-D.ietf-tcpm-accurate-ecn]    |
   | 3-4 | Scaling TCP's Congestion  |                                 |
   |     | 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/


   Koen De Schepper
   Nokia Bell Labs
   Antwerp
   Belgium

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











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

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










































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