Active Queue Management (aqm)                             K. De Schepper
Internet-Draft                                                 Bell Labs
Intended status: Standards Track                         B. Briscoe, Ed.
Expires: February 8, 2016                                    Independent
                                                           O. Bondarenko
                                                     Simula Research Lab
                                                                I. Tsang
                                                               Bell Labs
                                                         August 07, 2015

  DualQ Coupled AQM for Low Latency, Low Loss and Scalable Throughput


   Data Centre TCP (DCTCP) was designed to provide predictably low
   queuing latency, near-zero loss, and throughput scalability using
   explicit congestion notification (ECN) and an extremely simple
   marking behaviour on switches.  However, DCTCP does not co-exist with
   existing TCP traffic---throughput starves.  So, until now, DCTCP
   could only be deployed where a clean-slate environment could be
   arranged, such as in private data centres.  This specification
   defines `DualQ Coupled Active Queue Management (AQM)' to allow
   scalable congestion controls like DCTCP to safely co-exist with
   classic Internet traffic.  The Coupled AQM ensures that a flow runs
   at about the same rate whether it uses DCTCP or TCP Reno/Cubic, but
   without inspecting transport layer flow identifiers.  When tested in
   a residential broadband setting, DCTCP achieved sub-millisecond
   average queuing delay and zero congestion loss under a wide range of
   mixes of DCTCP and `Classic' broadband Internet traffic, without
   compromising the performance of the Classic traffic.  The solution
   also reduces network complexity and eliminates network configuration.

Status of This Memo

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   This Internet-Draft will expire on February 8, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Problem and Scope . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  DualQ Coupled AQM Algorithm . . . . . . . . . . . . . . . . .   6
     2.1.  Coupled AQM . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Dual Queue  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Traffic Classification  . . . . . . . . . . . . . . . . .   7
     2.4.  Normative Requirements  . . . . . . . . . . . . . . . . .   9
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     4.1.  Overload Handling . . . . . . . . . . . . . . . . . . . .  10
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Example DualQ Coupled Algorithm  . . . . . . . . . .  14
   Appendix B.  Guidance on Controlling Throughput Equivalence . . .  20
   Appendix C.  DCTCP Safety Enhancements  . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

1.1.  Problem and Scope

   Latency is becoming the critical performance factor for many (most?)
   applications on the public Internet, e.g.  Web, voice, conversational
   video, gaming and finance apps.  In the developed world, further
   increases in access network bit-rate offer diminishing returns,

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   whereas latency is still a multi-faceted problem.  In the last decade
   or so, much has been done to reduce propagation time by placing
   caches or servers closer to users.  However, queuing remains a major
   component of latency.

   The Diffserv architecture provides Expedited Forwarding [RFC3246], so
   that low latency traffic can jump the queue of other traffic.
   However, on access links dedicated to individual sites (homes, small
   enterprises or mobile devices), often all traffic at any one time
   will be latency-sensitive.  Then Diffserv is of little use.  Instead,
   we need to remove the causes of any unnecessary delay.

   The bufferbloat project has shown that excessively-large buffering
   (`bufferbloat') has been introducing significantly more delay than
   the underlying propagation time.  These delays appear only
   intermittently--only when a capacity-seeking (e.g.  TCP) flow is long
   enough for the queue to fill the buffer, making every packet in other
   flows sharing the buffer sit through the queue.

   Active queue management (AQM) was originally developed to solve this
   problem (and others).  Unlike Diffserv, AQM controls latency for
   _all_ traffic in a class.  In general, AQMs introduce an increasing
   level of discard from the buffer the longer the queue persists above
   a shallow threshold.  This gives sufficient signals to capacity-
   seeking (aka. greedy) flows to keep the buffer empty for its intended
   purpose: absorbing bursts.  However, RED [RFC2309] and other
   algorithms from the 1990s were sensitive to their configuration and
   hard to set correctly.  So, AQM was not widely deployed.  More recent
   state-of-the-art AQMs, e.g.  fq_CoDel [I-D.ietf-aqm-fq-codel],
   PIE [I-D.ietf-aqm-pie], Adaptive RED [ARED01], define the threshold
   in time not bytes, so it is invariant for different link rates.

   It seems that further changes to the network alone will now yield
   diminishing returns.  Data Centre TCP
   (DCTCP [I-D.bensley-tcpm-dctcp]) teaches us that a small but radical
   change to TCP is needed to cut two major outstanding causes of
   queuing delay variability:

   1.  the `sawtooth' varying rate of TCP itself;

   2.  the smoothing delay deliberately introduced into AQMs to permit
       bursts without triggering losses.

   The former causes a flow's round trip time (RTT) to vary from about 1
   to 2 times the base RTT between the machines in question.  The latter
   delays the system's response to change by a worst-case
   (transcontinental) RTT, which could be hundreds of times the actual
   RTT of typical traffic from localized CDNs.

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   Latency is not our only concern:

   3.  It was known when TCP was first developed that it would not scale
       to high bandwidth-delay products.

   Given regular broadband bit-rates over WAN distances are
   already [RFC3649] beyond the scaling range of `classic' TCP Reno,
   `less unscalable' Cubic [I-D.zimmermann-tcpm-cubic] and
   Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been
   successfully deployed.  However, these are now approaching their
   scaling limits.  Unfortunately, fully scalable TCPs such as DCTCP
   cause `classic' TCP to starve itself, which is why they have been
   confined to private data centres or research testbeds (until now).

   This document specifies a `DualQ Coupled AQM' that solves the problem
   of coexistence between DCTCP and classic flows, without having to
   inspect flow identifiers.  The AQM is not like flow-queuing
   approaches [I-D.ietf-aqm-fq-codel] that classify packets by flow
   identifier into numerous separate queues in order to isolate sparse
   flows from the higher latency in the queues assigned to heavier flow.
   In contrast, the AQM exploits the behaviour of scalable congestion
   controls like DCTCP so that every packet in every flow sharing the
   queue for DCTCP-like traffic can be served with very low latency.

   The AQM needs fewer operations per packet than RED uses.  Also, no
   network configuration is needed for a wide range of scenarios where
   the range of RTTs is typical for the public Internet.  Therefore it
   is believed the Coupled AQM would be applicable and easy to deploy in
   all types of buffers; buffers in cost-reduced mass-market residential
   equipment; buffers in end-system stacks; buffers in carrier-scale
   equipment including remote access servers, routers, firewalls and
   Ethernet switches; buffers in network interface cards, buffers in
   virtualized network appliances, hypervisors, and so on.

   The supporting paper [DCttH15] gives the full rationale for the AQM's
   design, both discursively and in more precise mathematical form.

1.2.  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.

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   The DualQ Coupled AQM uses two queues for two services.  Each of the
   following terms identifies both the service and the queue that
   provides the service:

   Classic (denoted by subscript C):  The `Classic' service is intended
      for all the behaviours that currently co-exist with TCP Reno (TCP
      Cubic, Compound, SCTP, etc).

   Low-Latency, Low-Loss and Scalable (L4S, denoted by subscript L):
      The `L4S' service is intended for DCTCP traffic 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.

   Either service can cope with a proportion of unresponsive or less-
   responsive traffic as well (e.g.  DNS, VoIP, etc).

1.3.  Features

   The AQM couples marking and/or dropping across the two queues such
   that a flow will get roughly the same throughput whichever it uses.
   Therefore both queues can feed into the full capacity of a link and
   no rates need to be configured for the queues.  The L4S queue enables
   scalable congestion controls like DCTCP to give stunningly low and
   predictably low latency, without compromising the performance of
   competing 'Classic' Internet traffic.  Thousands of tests have been
   conducted in a typical fixed residential broadband setting.  Typical
   experiments used a base round trip delay of 7ms between the data
   centre and home network, and large amounts of background traffic in
   both queues.  For every L4S packet, the AQM kept the 99th percentile
   of queuing delay to about 1ms, and no losses at all were introduced
   by the AQM.  Details of the extensive experiments will be made
   available [DCttH15].

   Subjective testing was also conducted using a demanding panoramic
   interactive video application run over a stack with DCTCP enabled and
   deployed on the testbed.  Each user could pan or zoom their own high
   definition (HD) sub-window of a larger video scene from a football
   match.  Even though the user was also downloading large amounts of
   L4S and Classic data, latency was so low that the picture appeared to
   stick to their finger on the touchpad (all the L4S data achieved the
   same ultra-low latency).  With an alternative AQM, the video
   noticeably lagged behind the finger gestures.

   Unlike Diffserv Expedited Forwarding, the L4S queue does not have to
   be limited to a small proportion of the link capacity in order to
   achieve low delay.  The L4S queue can be filled with a heavy load of
   capacity-seeking flows like DCTCP and still achieve low delay.  The

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   L4S queue does not rely on the presence of other traffic in the
   Classic queue that can be 'overtaken'.  It gives low latency to L4S
   traffic whether or not there is Classic traffic, and the latency of
   Classic traffic does not suffer when a proportion of the traffic is
   L4S.  The two queues are only necessary because DCTCP-like flows
   cannot keep latency predictably low and keep utilization high if they
   are mixed with legacy TCP flows,

   The experiments used the Linux implementation of DCTCP that is
   deployed in private data centres, without any modification despite
   its known deficiencies.  Nonetheless, certain modifications will be
   necessary before DCTCP is safe to use on the Internet, which are
   recorded for now in Appendix C.  However, the focus of this
   specification is to get the network service in place.  Then, without
   any management intervention, applications can exploit it by migrating
   to scalable controls like DCTCP, which can then evolve _while_ their
   benefits are being enjoyed by everyone on the Internet.

2.  DualQ Coupled AQM Algorithm

   There are two main aspects to the algorithm:

   o  the Coupled AQM that addresses throughput equivalence between
      Classic (e.g.  Reno, Cubic) flows and L4S (e.g.  DCTCP) flows

   o  the Dual Queue structure that provides latency separation for L4S
      flows to isolate them from the typically large Classic queue.

2.1.  Coupled AQM

   In the 1990s, the `TCP formula' was derived for the relationship
   between TCP's congestion window, cwnd, and its drop probability, p.
   To a first order approximation, cwnd of TCP Reno is inversely
   proportional to the square root of p.  TCP Cubic implements a Reno-
   compatibility mode, which is the only relevant mode for typical RTTs
   under 20ms, while the throughput of a single flow is less than about
   500Mb/s.  Therefore we can assume that Cubic traffic behaves similar
   to Reno (but with a slightly different constant of proportionality),
   and we shall use the term 'Classic' for the collection of Reno and
   Cubic in Reno mode.

   In our supporting paper [DCttH15], we derive the equivalent rate
   equation for DCTCP, for which cwnd is inversely proportional to p
   (not the square root), where in this case p is the ECN marking
   probability.  DCTCP is not the only congestion control that behaves
   like this, so we use the term 'L4S' traffic for all similar

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   In order to make a DCTCP flow run at roughly the same rate as a Reno
   TCP flow (all other factors being equal), we make the drop
   probability for Classic traffic, p_C distinct from the marking
   probability for L4S traffic, p_L (in contrast to RFC3168 which
   requires them to be the same).  We make the Classic drop probability
   p_C proportional to the square of the L4S marking probability p_L.
   This is because we need to make the Reno flow rate equal the DCTCP
   flow rate, so we have to square the square root of p_C in the Reno
   rate equation to make it the same as the straight p_L in the DCTCP
   rate equation.

   There is a really simple way to implement the square of a probability
   - by testing the queue against two random numbers not one.  This is
   the approach adopted in Appendix A.

   Stating this as a formula, the relation between Classic drop
   probability, p_C, and L4S marking probability, p_L needs to take the

       p_C = ( p_L / 2^k )^2                  (1)

   where 2^k is the constant of proportionality, which is expressed as a
   power of 2 so that implementations can avoid costly division by
   shifting p_L by k bits to the right.

2.2.  Dual Queue

   Classic traffic builds a large queue, so a separate queue is provided
   for L4S traffic, and it is scheduled with strict priority.
   Nonetheless, coupled marking ensures that giving priority to L4S
   traffic still leaves the right amount of spare scheduling time for
   Classic flows to each get equivalent throughput to DCTCP flows (all
   other factors such as RTT being equal).  The algorithm achieves this
   without having to inspect flow identifiers.

2.3.  Traffic Classification

   Both the Coupled AQM and DualQ mechanisms need an identifier to
   distinguish L4S and C packets, which will need to be standardized.
   In our tests we used a cleared ECN field to indicate C packets and
   L4S otherwise.  The ECN specification [RFC3168] currently defines a
   mark as equivalent to a drop.  However, it says

      "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."

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   and [RFC4774]} gives valid ways to alter ECN's semantics without
   harming interoperability.

   Since publication in 2001,deployment of RFC3168 ECN has been dogged
   by bugs and misunderstandings.  In recent years RFC3168 ECN has been
   deployed quite successfully on servers [ECN_Deploy], and until
   recently it was deployed but not enabled on a fair proportion of user
   machines.  Recently one major developer of client devices has
   configured ECN on-by-default in its beta releases.  However although
   some network equipment vendors and developers have implemented ECN,
   there is little evidence that any public network operator is
   considering or has deployed ECN-capable AQMs on network equipment

   A number of private data centre operators have deployed ECN, but not
   RFC3168 ECN.  Instead, they are using DCTCP to get predictable ultra-
   low latency, and they are either ensuring that there is no non-DCTCP
   traffic [I-D.bensley-tcpm-dctcp], or they are segregating such
   traffic from DCTCP using Diffserv [DCTCP_Pitfalls].  The RFC3168
   approach merely prevents drop, whereas the DCTCP approach provides
   scalable throughput and ultra-low latency as well as avoiding drop.
   Consequently it has been questioned whether the RFC3168 approach
   offers enough performance improvement for an operator to countenance
   the cost and risk of deployment.  There has been some discussions at
   the IETF on changing the meaning of an ECN mark to move towards the
   DCTCP approach.The performance results from our experiments with
   DCTCP for broadband residential users are certainly significant
   enough to warrant interest from operators.

   For those who have managed to get classic ECN widely deployed on end-
   systems, moving the goalposts at this stage would be harsh.  If the
   meaning of ECN cannot be changed from "equivalent to drop", it would
   be possible to identify the L4S service in another way, e.g. a
   combination of ECN and Diffserv, or using the ECT(1) codepoint.  The
   Diffserv codepoint is not ideal, because L4S is an end-to-end service
   and a DSCP is not preserved end-to-end.  However, combining ECN and
   Diffserv may be sufficient for initial deployment, while confined to
   controlled sets of networks, during which time any users of classic
   ECN can upgrade to L4S.  The ECT(1) codepoint is perhaps less ideal,
   because two separate uses of ECN really need two codepoints each, and
   anyway it could be argued that the last ECN codepoint should not be
   burned when the current one is not being used.

   This draft does not currently recommend an approach for identifying
   for the L4S service, which is initially left open for discussion
   within the IETF.

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2.4.  Normative Requirements

   In the Dual Queue, L4S packets MUST be given priority over Classic,
   although strict priority MAY not be appropriate.

   All L4S traffic MUST be ECN-capable, although some Classic traffic
   MAY also be ECN-capable.

   Whatever identifier is used for L4S traffic, it will still be
   necessary to agree on the meaning of an ECN marking on L4S traffic,
   relative to a drop of Classic traffic.  In order to prevent
   starvation of Classic traffic by scalable L4S traffic (e.g.  DCTCP)
   the drop probability of Classic traffic MUST be proportional to the
   square of the marking probability of L4S traffic, In other words, the
   power to which p_L is raised in Eqn. (1) MUST be 2.

   The constant of proportionality, k, in Eqn (1) determines the
   relative flow rates of Classic and L4S flows when the AQM concerned
   is the bottleneck (all other factors being equal). k does not have to
   be standardized because differences do not prevent interoperability.
   However, k has to take some value, and each operator can make that

   A value of k=0 is RECOMMENDED as the default for public Internet
   access networks, assuming the DCTCP algorithm remains similar to that
   in [I-D.bensley-tcpm-dctcp].  Nonetheless choice of k is a matter of
   operator policy, and operators MAY choose a different value using
   Table 1 and the guidelines in Appendix B.

   Typically, access network operators isolate customers from each other
   with some form of layer-2 multiplexing (TDM in DOCSIS, CDMA in 3G) or
   L3 scheduling (WRR in broadband), rather than relying on TCP to share
   capacity between customers [RFC0970].  In such cases, the choice of k
   will solely affect relative flow rates within the customer's access
   capacity, not between customers.  Also, k would not affect rates of
   small flows, nor long flows at any times when they are all Classic or
   all L4S.

   An example DualQ Coupled AQM algorithm is given in Appendix A.
   Marking and dropping in each queue is based on an AQM called Curvy
   RED, which is intended to improve on RED, PIE and CoDel.  We have
   found that Curvy RED offers good performance, requires less
   operations per packet than RED and is insensitive to configuration.
   Nonetheless, it would be possible to control each queue with an
   alternative AQM, as long as the above normative requirements (those
   expressed in capitals) are observed, which are intended to be
   independent of the specific AQM.

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   {ToDo: Add management and monitoring requirements}

3.  IANA Considerations

   This specification contains no IANA considerations.

4.  Security Considerations

4.1.  Overload Handling

   Where the interests of users or flows might conflict, it could be
   necessary to police traffic to isolate any harm to performance.  This
   is a policy issue that needs to be separable from a basic AQM, but
   the scheme does need to handle overload.  A trade-off needs to be
   made between complexity and the risk of either class harming the
   other.  It is an operator policy to define what must happen if the
   service time of the classic queue becomes too great.  In the
   following subsections three optional non-exclusive overload
   protections are defined.  Their objective is for the overload
   behaviour of the DualQ AQM to be similar to a single queue AQM.
   Other overload protections can be envisaged:

   Minimum throughput service:   By replacing the priority scheduler
      with a weighted round robin scheduler, a minimum throughput
      service can be guaranteed for Classic traffic.  Typically the
      scheduling weight of the Classic queue will be small (e.g. 5%) to
      avoid interference with the coupling but big enough to avoid
      complete starvation of Classic traffic.

   Drop on overload:  On severe overload, e.g. due to non responsive
      traffic, queues will typically overflow and packet drop will be
      unavoidable when the queues reach their limits.  The drop-limit of
      each queue should be configured by specifying the maximum
      supported load and determining the expected maximum size of each
      queue when that load is separately applied to each queue.  The
      Classic queue limit will typically be larger than the L4S queue
      limit.  Overflow of one traffic type will automatically result in
      drop in its respective queue.  Both traffic types will get a high
      congestion signal, due to the coupled marking, which will result
      in similar starvation of responsive traffic in both queues.  Thus,
      the behaviour will be like a single queue AQM.  To further improve
      the arrival fairness of a single queue an extra overall AQM limit
      can be applied, which is a limit to the sum of both queues.  To be
      effective, it should be configured to be less than the sum of the
      limits of both queues, but greater than the maximum individual
      queue limit.  It ensures that the drop probability of unresponsive
      traffic will be independent of its traffic type.

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   Delay on overload:  To control milder overload of responsive traffic,
      particularly when close to the maximum congestion signal, delay
      can be used as an alternative congestion control mechanism.  The
      Dual Queue Coupled AQM can be made to behave like a single FIFO
      queue with differentiated service times by replacing the priority
      scheduler with a very simple "biased longest sojourn time first
      scheduler".  The bias is defined as a maximum sojourn time
      difference (T_m) between the Classic and L4S packets.  The
      scheduler adds T_m to the sojourn time of the next L4S packet,
      before comparing it with the timestamp of the next Classic packet,
      then it selects the packet with the greater adjusted sojourn time.
      This time shifted FIFO queue behaves just like a single FIFO queue
      under moderate and high overload.

5.  Acknowledgements

   Thanks to Anil Agarwal for detailed review comments and suggestions
   on how to make our explanation clearer.

   The authors' contributions are part-funded by the European Community
   under its Seventh Framework Programme through the Reducing Internet
   Transport Latency (RITE) project (ICT-317700).  The views expressed
   here are solely those of the authors.

6.  References

6.1.  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,

6.2.  Informative References

   [ARED01]   Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
              Algorithm for Increasing the Robustness of RED's Active
              Queue Management", ACIRI Technical Report , August 2001,

   [CoDel]    Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              ACM Queue 10(5), May 2012,

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              Briscoe, B., "Insights from Curvy RED (Random Early
              Detection)", BT Technical Report TR-TUB8-2015-003, July

              Judd, G., "Attaining the Promise and Avoiding the Pitfalls
              of TCP in the Datacenter", 12th USENIX Symposium on
              Networked Systems Design and Implementation (NSDI 15)
              145--157, May 2015,

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

              (Under submission)

              Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
              Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
              Wide Deployment of Explicit Congestion Notification", Proc
              Passive & Active Measurement (PAM'15) Conference , 2015,

              Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,
              and G. Judd, "Microsoft's Datacenter TCP (DCTCP): TCP
              Congestion Control for Datacenters", draft-bensley-tcpm-
              dctcp-05 (work in progress), July 2015.

              Hoeiland-Joergensen, T., McKenney, P.,
    , d., Gettys, J., and E. Dumazet,
              "FlowQueue-Codel", draft-ietf-aqm-fq-codel-01 (work in
              progress), July 2015.

              Pan, R., Natarajan, P., Baker, F., and G. White, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", draft-ietf-aqm-pie-01 (work in progress), March

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              Kuehlewind, M., Scheffenegger, R., and B. Briscoe,
              "Problem Statement and Requirements for a More Accurate
              ECN Feedback", draft-ietf-tcpm-accecn-reqs-08 (work in
              progress), March 2015.

              Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
              "Compound TCP: A New TCP Congestion Control for High-Speed
              and Long Distance Networks", draft-sridharan-tcpm-ctcp-02
              (work in progress), November 2008.

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

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

   [RFC0970]  Nagle, J., "On Packet Switches With Infinite Storage",
              RFC 970, DOI 10.17487/RFC0970, December 1985,

   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,

   [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,

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

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

              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,

Appendix A.  Example DualQ Coupled Algorithm

   As a concrete example, the pseudocode below gives the DualQ Coupled
   AQM algorithm we used in testing.  Although we designed the AQM to be
   efficient in integer arithmetic, to aid understanding it is first
   given using real-number arithmetic.  Then, one possible optimization
   for integer arithmetic is given, also in pseudocode.  To aid
   comparison, the line numbers are kept in step between the two by
   using letter suffixes where the longer code needs extra lines.

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   1:  dualq_dequeue(lq, cq) {  % Couples L4S & Classic queues, lq & cq
   2:     if ( lq.dequeue(pkt) ) {
   3a:       p_L = cq.sec() / 2^S_L
   3b:       if ( lq.byt() > T )
   3c:           mark(pkt)
   3d:        elif ( p_L > maxrand(U) )
   4:           mark(pkt)
   5:        return(pkt)              % return the packet and stop here
   6:     }
   7:     while ( cq.dequeue(pkt) ) {
   8a:       alpha = 2^(-f_C)
   8b:       Q_C = alpha * pkt.sec() + (1-alpha)* Q_C % Classic Q EWMA
   9a:       sqrt_p_C = Q_C / 2^S_C
   9b:       if ( sqrt_p_C > maxrand(2*U) )
   10:          drop(pkt)                     % Squared drop, redo loop
   11:       else
   12:          return(pkt)           % return the packet and stop here
   13:    }
   14:    return(NULL)                           % no packet to dequeue
   15: }

   16: maxrand(u) {                % return the max of u random numbers
   17:     maxr=0
   18:     while (u-- > 0)
   19:         maxr = max(maxr, rand())               % 0 <= rand() < 1
   20:     return(maxr)
   21: }

        Figure 1: Example Dequeue Pseudocode for Coupled DualQ AQM

   Packet classification code is not shown, as it is no different from
   regular packet classification.  Potential classification schemes are
   discussed in Section 2.  Overload protection code will be included in
   a future draft {ToDo}.

   At the outer level, the structure of dualq_dequeue() implements
   strict priority scheduling.  The code is written assuming the AQM is
   applied on dequeue (Note 1) . Every time dualq_dequeue() is called,
   the if-block in lines 2-6 determines whether there is an L4S packet
   to dequeue by calling lq.dequeue(pkt), and otherwise the while-block
   in lines 7-13 determines whether there is a Classic packet to
   dequeue, by calling cq.dequeue(pkt).  (Note 2)

   In the lower priority Classic queue, a while loop is used so that, if
   the AQM determines that a classic packet should be dropped, it
   continues to test for classic packets deciding whether to drop each
   until it actually forwards one.  Thus, every call to dualq_dequeue()

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   returns one packet if at least one is present in either queue,
   otherwise it returns NULL at line 14.  (Note 3)

   Within each queue, the decision whether to drop or mark is taken as
   follows (to simplify the explanation, it is assumed that U=1):

   L4S:  If the test at line 2 determines there is an L4S packet to
      dequeue, the tests at lines 3a and 3c determine whether to mark
      it.  The first is a simple test of whether the L4S queue (lq.byt()
      in bytes) is greater than a step threshold T in bytes (Note 4).
      The second test is similar to the random ECN marking in RED, but
      with the following differences: i) the marking function does not
      start with a plateau of zero marking until a minimum threshold,
      rather the marking probability starts to increase as soon as the
      queue is positive; ii) marking depends on queuing time, not bytes,
      in order to scale for any link rate without being reconfigured;
      iii) marking of the L4S queue does not depend on itself, it
      depends on the queuing time of the _other_ (Classic) queue, where
      cq.sec() is the queuing time of the packet at the head of the
      Classic queue (zero if empty); iv) marking depends on the
      instantaneous queuing time (of the other queue), not a smoothed
      average; v) the queue is compared with the maximum of U random
      numbers (but if U=1, this is the same as the single random number
      used in RED).

      Specifically, in line 3a the marking probability p_L is set to the
      Classic queueing time qc.sec() in seconds divided by the L4S
      scaling parameter 2^S_L, which represents the queuing time (in
      seconds) at which marking probability would hit 100%. Then in line
      3d (if U=1) the result is compared with a uniformly distributed
      random number between 0 and 1, which ensures that marking
      probability will linearly increase with queueing time.  The
      scaling parameter is expressed as a power of 2 so that division
      can be implemented as a right bit-shift (>>) in line 3 of the
      integer variant of the pseudocode (Figure 2).

   Classic:  If the test at line 7 determines that there is at least one
      Classic packet to dequeue, the test at line 9b determines whether
      to drop it.  But before that, line 8b updates Q_C, which is an
      exponentially weighted moving average (Note 5) of the queuing time
      in the Classic queue, where pkt.sec() is the instantaneous
      queueing time of the current Classic packet and alpha is the EWMA
      constant for the classic queue.  In line 8a, alpha is represented
      as an integer power of 2, so that in line 8 of the integer code
      the division needed to weight the moving average can be
      implemented by a right bit-shift (>> f_C).

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      Lines 9a and 9b implement the drop function.  In line 9a the
      averaged queuing time Q_C is divided by the Classic scaling
      parameter 2^S_C, in the same way that queuing time was scaled for
      L4S marking.  This scaled queuing time is given the variable name
      sqrt_p_C because it will be squared to compute Classic drop
      probability, so before it is squared it is effectively the square
      root of the drop probability.  The squaring is done by comparing
      it with the maximum out of two random numbers (assuming U=1).
      Comparing it with the maximum out of two is the same as the
      logical `AND' of two tests, which ensures drop probability rises
      with the square of queuing time (Note 6).  Again, the scaling
      parameter is expressed as a power of 2 so that division can be
      implemented as a right bit-shift in line 9 of the integer

   The marking/dropping functions in each queue (lines 3 & 9) are two
   cases of a new generalization of RED called Curvy RED, motivated as
   follows.  When we compared the performance of our AQM with fq_CoDel
   and PIE, we came to the conclusion that their goal of holding queuing
   delay to a fixed target is misguided [CRED_Insights].  As the number
   of flows increases, if the AQM does not allow TCP to increase queuing
   delay, it has to introduce abnormally high levels of loss.  Then loss
   rather than queuing becomes the dominant cause of delay for short
   flows, due to timeouts and tail losses.

   Curvy RED constrains delay with a softened target that allows some
   increase in delay as load increases.  This is achieved by increasing
   drop probability on a convex curve relative to queue growth (the
   square curve in the Classic queue, if U=1).  Like RED, the curve hugs
   the zero axis while the queue is shallow.  Then, as load increases,
   it introduces a growing barrier to higher delay.  But, unlike RED, it
   requires only one parameter, the scaling, not three.

   There follows a summary listing of the two parameters used for each
   of the two queues:


      S_C :   The scaling factor of the dropping function scales Classic
         queuing times in the range [0, 2^(S_C)] seconds into a dropping
         probability in the range [0,1].  To make division efficient, it
         is constrained to be an integer power of two;

      f_C :  To smooth the queuing time of the Classic queue and make
         multiplication efficient, we use a negative integer power of
         two for the dimensionless EWMA constant, which we define as

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   L4S :

      S_L (and k):   As for the Classic queue, the scaling factor of the
         L4S marking function scales Classic queueing times in the range
         [0, 2^(S_L)] seconds into a probability in the range [0,1].
         Note that S_L = S_C + k, where k is the coupling between the
         queues (Section 2.1).  So S_L and k count as only one

      T :  The queue size in bytes at which step threshold marking
         starts in the L4S queue.

   {ToDo: These are the raw parameters used within the algorithm.  A
   configuration front-end could accept more meaningful parameters and
   convert them into these raw parameters.}

   From our experiments so far, recommended values for these parameters
   are: S_C = -1; f_C = 5; T = 5 * MTU for the range of base RTTs
   typical on the public Internet.  [CRED_Insights] explains why these
   parameters are applicable whatever rate link this AQM implementation
   is deployed on and how the parameters would need to be adjusted for a
   scenario with a different range of RTTs (e.g. a data centre) {ToDo
   incorporate a summary of that report into this draft}. The setting of
   k depends on policy (see Section 2.4 and Appendix B respectively for
   its recommended setting and guidance on alternatives).

   There is also a cUrviness parameter, U, which is a small positive
   integer.  It is likely to take the same hard-coded value for all
   implementations, once experiments have determined a good value.  We
   have solely used U=1 in our experiments so far, but results might be
   even better with U=2 or higher.

   Note that the dropping function at line 9 calls maxrand(2*U), which
   gives twice as much curviness as the call to maxrand(U) in the
   marking function at line 3.  This is the trick that implements the
   square rule in equation (1) (Section 2.1).  This is based on the fact
   that, given a number X from 1 to 6, the probability that two dice
   throws will both be less than X is the square of the probability that
   one throw will be less than X.  So, when U=1, the L4S marking
   function is linear and the Classic dropping function is squared.  If
   U=2, L4S would be a square function and Classic would be quartic.
   And so on.

   The maxrand(u) function in lines 16-21 simply generates u random
   numbers and returns the maximum (Note 7).  Typically, maxrand(u)
   could be run in parallel out of band.  For instance, if U=1, the
   Classic queue would require the maximum of two random numbers.  So,
   instead of calling maxrand(2*U) in-band, the maximum of every pair of

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   values from a pseudorandom number generator could be generated out-
   of-band, and held in a buffer ready for the Classic queue to consume.

   1:  dualq_dequeue(lq, cq) {  % Couples L4S & Classic queues, lq & cq
   2:     if ( lq.dequeue(pkt) ) {
   3:        if ((lq.byt() > T) || ((cq.ns() >> (S_L-2)) > maxrand(U)))
   4:           mark(pkt)
   5:        return(pkt)              % return the packet and stop here
   6:     }
   7:     while ( cq.dequeue(pkt) ) {
   8:         Q_C += (pkt.ns() - Q_C) >> f_C           % Classic Q EWMA
   9:        if ( (Q_C >> (S_C-2) ) > maxrand(2*U) )
   10:          drop(pkt)                     % Squared drop, redo loop
   11:       else
   12:          return(pkt)           % return the packet and stop here
   13:    }
   14:    return(NULL)                           % no packet to dequeue
   15: }

   Figure 2: Optimised Example Dequeue Pseudocode for Coupled DualQ AQM
                         using Integer Arithmetic


   1.  The drain rate of the queue can vary if it is scheduled relative
       to other queues, or to cater for fluctuations in a wireless
       medium.  To auto-adjust to changes in drain rate, the queue must
       be measured in time, not bytes or packets [CoDel].  In our Linux
       implementation, it was easiest to measure queuing time at
       dequeue.  Queuing time can be estimated when a packet is enqueued
       by measuring the queue length in bytes and dividing by the recent
       drain rate.

   2.  An implementation has to use priority queueing, but it need not
       implement strict priority.

   3.  If packets can be enqueued while processing dequeue code, an
       implementer might prefer to place the while loop around both
       queues so that it goes back to test again whether any L4S packets
       arrived while it was dropping a Classic packet.

   4.  In order not to change too many factors at once, for now, we keep
       the marking function for DCTCP-only traffic as similar as
       possible to DCTCP.  However, unlike DCTCP, all processing is at
       dequeue, so we determine whether to mark a packet at the head of
       the queue by the byte-length of the queue _behind_ it.  We plan
       to test whether using queuing time will work in all
       circumstances, and if we find that the step can cause

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       oscillations, we will investigate replacing it with a steep
       random marking curve.

   5.  An EWMA is only one possible way to filter bursts; other more
       adaptive smoothing methods could be valid and it might be
       appropriate to decrease the EWMA faster than it increases.

   6.  In practice at line 10 the Classic queue would probably test for
       ECN capability on the packet to determine whether to drop or mark
       the packet.  However, for brevity such detail is omitted.  All
       packets classified into the L4S queue have to be ECN-capable, so
       no dropping logic is necessary at line 3.  Nonetheless, L4S
       packets could be dropped by overload code (see Section 4.1).

   7.  In the integer variant of the pseudocode (Figure 2) real numbers
       are all represented as integers scaled up by 2^32.  In lines 3 &
       9 the function maxrand() is arranged to return an integer in the
       range 0 <= maxrand() < 2^32.  Queuing times are also scaled up by
       2^32, but in two stages: i) In lines 3 and 8 queuing times
       cq.ns() and pkt.ns() are returned in integer nanoseconds, making
       the values about 2^30 times larger than when the units were
       seconds, ii) then in lines 3 and 9 an adjustment of -2 to the
       right bit-shift multiplies the result by 2^2, to complete the
       scaling by 2^32.

Appendix B.  Guidance on Controlling Throughput Equivalence

                     | RTT_C / RTT_L | Reno | Cubic |
                     |             1 | k=1  | k=0   |
                     |             2 | k=2  | k=1   |
                     |             3 | k=2  | k=2   |
                     |             4 | k=3  | k=2   |
                     |             5 | k=3  | k=3   |

   Table 1: Value of k for which DCTCP throughput is roughly the same as
                Reno or Cubic, for some example RTT ratios

   To determine the appropriate policy, the operator first has to judge
   whether it wants DCTCP flows to have roughly equal throughput with
   Reno or with Cubic (because, even in its Reno-compatibility mode,
   Cubic is about 1.4 times more aggressive than Reno).  Then the
   operator needs to decide at what ratio of RTTs it wants DCTCP and
   Classic flows to have roughly equal throughput.  For example choosing
   the recommended value of k=0 will make DCTCP throughput roughly the
   same as Cubic, _if their RTTs are the same_.

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   However, even if the base RTTs are the same, the actual RTTs are
   unlikely to be the same, because Classic (Cubic or Reno) traffic
   needs a large queue to avoid under-utilization and excess drop,
   whereas L4S (DCTCP) does not.  The operator might still choose this
   policy if it judges that DCTCP throughput should be rewarded for
   keeping its own queue short.

   On the other hand, the operator will choose one of the higher values
   for k, if it wants to slow DCTCP down to roughly the same throughput
   as Classic flows, to compensate for Classic flows slowing themselves
   down by causing themselves extra queuing delay.

   The values for k in the table are derived from the formulae, which
   was developed in [DCttH15]:

       2^k = 1.64 (RTT_reno / RTT_dc)                  (2)
       2^k = 1.19 (RTT_cubic / RTT_dc )                (3)

   For localized traffic from a particular ISP's data centre, we used
   the measured RTTs to calculate that a value of k=3 would achieve
   throughput equivalence, and our experiments verified the formula very

Appendix C.  DCTCP Safety Enhancements

   This Appendix is informational not normative.  It records changes
   needed to DCTCP implementations so they can co-exist safely alongside
   other traffic sources.  They are recorded here until a more
   appropriate draft is available to hold them.

   Proposed changes are listed in rough order of criticality.  Therefore
   those later in the list may not be necessary:

   o  Negotiate its altered feedback semantics, which conveys the extent
      of ECN marking, not just its existence, and this feedback needs to
      be robust to loss [I-D.ietf-tcpm-accecn-reqs];

   o  fall back to Reno or Cubic behaviour on loss;

   o  use a packet identifier associated with the L4S service;

   o  average ECN feedback over its own RTT, not the hard-coded RTT
      suitable only for data-centres, perhaps like Relentless
      TCP [Mathis09];

   o  handle a window of less than 2 when the RTT is low, rather than
      increase the queue [TCP-sub-mss-w].

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   o  test heuristically whether ECN marking is emanating from an
      RFC3168 AQM.

   Other, non-essential enhancements to DCTCP can be envisaged.

Authors' Addresses

   Koen De Schepper
   Bell Labs


   Bob Briscoe (editor)


   Olga Bondarenko
   Simula Research Lab


   Ing-jyh Tsang
   Bell Labs


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