Network Working Group                                         N. Khademi
Internet-Draft                                                  M. Welzl
Updates: 3168 (if approved)                           University of Oslo
Intended status: Experimental                                G. Armitage
Expires: October 5, 2016              Swinburne University of Technology
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                          April 03, 2016

                 TCP Alternative Backoff with ECN (ABE)


   This memo provides an experimental update to RFC3168.  It updates the
   TCP sender-side reaction to a congestion notification received via
   Explicit Congestion Notification (ECN).  The updated method reduces
   cwnd by a smaller amount than TCP does in reaction to loss.  The
   intention is to achieve good throughput when the queue at the
   bottleneck is smaller than the bandwidth-delay-product of the
   connection.  This is more likely when an Active Queue Management
   (AQM) mechanism has used ECN to CE-mark a packet, than when a packet
   was lost.  Future versions of this document will discuss SCTP as well
   as other transports using ECN.

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
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   This Internet-Draft will expire on October 5, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Why use ECN to vary the degree of backoff?  . . . . . . .   3
     2.2.  Choice of ABE multiplier  . . . . . . . . . . . . . . . .   4
   3.  NEW: Updating the Sender-side ECN Reaction  . . . . . . . . .   5
     3.1.  RFC 2119  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Update to RFC 3168  . . . . . . . . . . . . . . . . . . .   6
     3.3.  Status of the Update  . . . . . . . . . . . . . . . . . .   7
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Explicit Congestion Notification (ECN) is specified in [RFC3168].  It
   allows a network device that uses Active Queue Management (AQM) to
   set the congestion experienced, CE, codepoint in the ECN field of the
   IP packet header, rather than drop ECN-capable packets when incipient
   congestion is detected.  When an ECN-capable transport is used over a
   path that supports ECN, it provides the opportunity for flows to
   improve their performance in the presence of incipient congestion

   [RFC3168] not only specifies the router use of the ECN field, it also
   specifies a TCP procedure for using ECN.  This states that a TCP
   sender should treat the ECN indication of congestion in the same way
   as that of a non-ECN-Capable TCP flow experiencing loss, by halving
   the congestion window "cwnd" and by reducing the slow start threshold
   "ssthresh".  [RFC5681] stipulates that TCP congestion control sets
   "ssthresh" to max(FlightSize / 2, 2*SMSS) in response to packet loss.
   Consequently, a standard TCP flow using this reaction needs
   significant network queue space: it can only fully utilise a

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   bottleneck when the length of the link queue (or the AQM dropping
   threshold) is at least the bandwidth-delay product (BDP) of the flow.

   A backoff multipler of 0.5 (halving cwnd and sshthresh after packet
   loss) is not the only available strategy.  As defined in [ID.CUBIC],
   CUBIC multiplies the current cwnd by 0.8 in response to loss
   (although the Linux implementation of CUBIC has used a multiplier of
   0.7 since kernel version 2.6.25 released in 2008).  Consequently,
   CUBIC utilise paths well even when the bottleneck queue is shorter
   than the bandwidth-delay product of the flow.  However, in the case
   of a DropTail (FIFO) queue without AQM, such less-aggressive backoff
   increases the risk of creating a standing queue [CODEL2012].

   Devices implementing AQM are likely to be the dominant (and possibly
   only) source of ECN CE-marking for packets from ECN-capable senders.
   AQM mechanisms typically strive to maintain a small queue length,
   regardless of the bandwidth-delay product of flows passing through
   them.  Receipt of an ECN CE-mark might therefore reasonably be taken
   to indicate that a small bottleneck queue exists in the path, and
   hence the TCP flow would benefit from using a less aggressive backoff

   Results reported in [ABE2015] show significant benefits (improved
   throughput) when reacting to ECN-Echo by multiplying cwnd and
   sstthresh with a value in the range [0.7..0.85].  Section 2 describes
   the rationale for this change.  Section 3 specifies a change to the
   TCP sender backoff behaviour in response to an indication that CE-
   marks have been received by the receiver.

2.  Discussion

   Much of the background to this proposal can be found in [ABE2015].
   Using a mix of experiments, theory and simulations with standard
   NewReno and CUBIC, [ABE2015] recommends enabling ECN and "...letting
   individual TCP senders use a larger multiplicative decrease factor in
   reaction to ECN CE-marks from AQM-enabled bottlenecks."  Such a
   change is noted to result in "...significant performance gains in
   lightly-multiplexed scenarios, without losing the delay-reduction
   benefits of deploying CoDel or PIE."

2.1.  Why use ECN to vary the degree of backoff?

   The classic rule-of-thumb dictates a BDP of bottleneck buffering if a
   TCP connection wishes to optimise path utilisation.  A single TCP
   connection running through such a bottleneck will have opened cwnd up
   to 2*BDP by the time packet loss occurs.  [RFC5681]'s halving of cwnd
   and ssthresh pushes the TCP connection back to allowing only a BDP of

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   packets in flight -- just enough to maintain 100% utilisation of the
   network path.

   AQM schemes like CoDel [I-D.CoDel] and PIE [I-D.PIE] use congestion
   notifications to constrain the queuing delays experienced by packets,
   rather than in response to impending or actual bottleneck buffer
   exhaustion.  With current default delay targets, CoDel and PIE both
   effectively emulate a shallow buffered bottleneck (section II,
   [ABE2015]) while allowing short traffic bursts into the queue.  This
   interacts acceptably for TCP connections over low BDP paths, or
   highly multiplexed scenarios (lmany concurrent TCP connections).
   However, it interacts badly with lightly-multiplexed cases (few
   concurrent connections) over high BDP paths.  Conventional TCP
   backoff in such cases leads to gaps in packet transmission and under-
   utilisation of the path.

   In an ideal world, the TCP sender would adapt its backoff strategy to
   match the effective depth at which a bottleneck begins indicating
   congestion.  In the practical world, [ABE2015] proposes using the
   existence of ECN CE-marks to infer whether a path's bottleneck is
   AQM-enabled (shallow queue) or classic DropTail (deep queue), and
   adjust backoff accordingly.  This results in a change to [RFC3168],
   which recommended that TCP senders respond in the same way following
   indication of a received ECN CE-mark and a packet loss, making these
   equivalent signals of congestion.  (The idea to change this behaviour
   pre-dates ABE.  [ICC2002] also proposed using ECN CE-marks to modify
   TCP congestion control behaviour, using a larger multiplicative
   decrease factor in conjunction with a smaller additive increase
   factor to deal with RED-based bottlenecks that were not necessarily
   configured to emulate a shallow queue.)

   [RFC7567] states that "deployed AQM algorithms SHOULD support
   Explicit Congestion Notification (ECN) as well as loss to signal
   congestion to endpoints" and [I-D.AQM-ECN-benefits] encourages this
   deployment.  Apple recently announced their intention to enable ECN
   in iOS 9 and OS X 10.11 devices [WWDC2015].  By 2014, server-side ECN
   negotiation was observed to be provided by the majority of the top
   million web servers [PAM2015], and only 0.5% of websites incurred
   additional connection setup latency using RFC3168-compliant ECN-
   fallback mechanisms.

2.2.  Choice of ABE multiplier

   ABE decouples a TCP sender's reaction to loss and ECN CE-marks.  The
   description respectively uses beta_{loss} and beta_{ecn} to refer to
   the multiplicative decrease factors applied in response to packet
   loss and in response to an indication of a received CN CE-mark on an
   ECN-enabled TCP connection (based on the terms used in [ABE2015]).

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   For non-ECN-enabled TCP connections, no ECN CE-marks are received and
   only beta_{loss} applies.

   In other words, in response to detected loss:

      FlightSize_(n+1) = FlightSize_n * beta_{loss}

   and in response to an indication of a received ECN CE-mark:

      FlightSize_(n+1) = FlightSize_n * beta_{ecn}

   where, as in [RFC5681], FlightSize is the amount of outstanding data
   in the network, upper-bounded by the sender's congestion window
   (cwnd) and the receiver's advertised window (rwnd).  The higher the
   values of beta_*, the less aggressive the response of any individual
   backoff event.

   The appropriate choice for beta_{loss} and beta_{ecn} values is a
   balancing act between path utilisation and draining the bottleneck
   queue.  More aggressive backoff (smaller beta_*) risks underutilising
   the path, while less aggressive backoff (larger beta_*) can result in
   slower draining of the bottleneck queue.

   The Internet has already been running with at least two different
   beta_{loss} values for several years: the value in [RFC5681] is 0.5,
   and Linux CUBIC uses 0.7.  ABE proposes no change to beta_{loss} used
   by any current TCP implementations.

   beta_{ecn} depends on how we want to optimise the reponse of a TCP
   connection to shallow AQM marking thresholds. beta_{loss} reflects
   the preferred response of each TCP algorithm when faced with
   exhaustion of buffers (of unknown depth) signalled by packet loss.
   Consequently, for any given TCP algorithm the choice of beta_{ecn} is
   likely to be algorithm-specific, rather than a constant multiple of
   the algorithm's existing beta_{loss}.

   A range of experiments (section IV, [ABE2015]) with NewReno and CUBIC
   over CoDel and PIE in lightly multiplexed scenarios have explored
   this choice of parameter.  These experiments indicate that CUBIC
   connections benefit from beta_{ecn} of 0.85 (cf.  beta_{loss} = 0.7),
   and NewReno connections see improvements with beta_{ecn} in the range
   0.7 to 0.85 (c.f., beta_{loss} = 0.5).

3.  NEW: Updating the Sender-side ECN Reaction

   This section specifies an experimental update to [RFC3168].

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3.1.  RFC 2119

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.2.  Update to RFC 3168

   This document specifies an update to the TCP sender reaction that
   follows when the TCP receiver signals that ECN CE-marked packets have
   been received.

   The first paragraph of Section 6.1.2, "The TCP Sender", in [RFC3168]
   contains the following text:

   "If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK
   packet with the ECN-Echo flag set in the TCP header), then the sender
   knows that congestion was encountered in the network on the path from
   the sender to the receiver.  The indication of congestion should be
   treated just as a congestion loss in non-ECN-Capable TCP.  That is,
   the TCP source halves the congestion window "cwnd" and reduces the
   slow start threshold "ssthresh"."

   This memo updates this by replacing it with the following text:

   "If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK
   packet with the ECN-Echo flag set in the TCP header), then the sender
   knows that congestion was encountered in the network on the path from
   the sender to the receiver.  This indication of congestion could be
   treated in the same way as a congestion loss, however reception of
   the ECN-Echo flag SHOULD produce a reduction in FlightSize that is
   less than the reduction had the flow experienced loss.  The reduction
   needs to be sufficient to allow flows sharing a bottleneck to
   increase their share of the capacity.  This reduction MUST be less
   than 0.85 (at least a 15% reduction).

   An ECN-capable network device cannot eliminate the possibility of
   loss, because a drop may occur due to a traffic burst exceeding the
   instantaneous available capacity of a network buffer or as a result
   of the AQM algorithm (overload protection mechanisms, etc [RFC7567]).
   Whatever the cause of loss, detection of a missing packet needs to
   trigger the standard loss-based congestion control response.  This
   explicitly does not update this behaviour.

   In addition, this document RECOMMENDS that experimental deployments
   method multiply the FlightSize by 0.8 and reduce the slow start
   threshold 'ssthresh' in response to reception of a TCP segment that
   sets the ECN-Echo flag."

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3.3.  Status of the Update

   This update is a sender-side only change.  Like other changes to
   congestion-control algorithms it does not require any change to the
   TCP receiver or to network devices (except to enable an ECN-marking
   algorithm [RFC3168] [RFC7567]).  If the method is only deployed by
   some TCP senders, and not by others, the senders that use this method
   can gain advantage, possibly at the expense of other flows that do
   not use this updated method.  This advantage applies only to ECN-
   marked packets and not to loss indications.  Hence, the new method
   can not lead to congestion collapse.

   The present specification has been assigned an Experimental status,
   to provide Internet deployment experience before being proposed as a
   Standards-Track update.

4.  Acknowledgements

   Authors N.  Khademi, M.  Welzl and G.  Fairhurst were part-funded by
   the European Community under its Seventh Framework Programme through
   the Reducing Internet Transport Latency (RITE) project (ICT-317700).
   The views expressed are solely those of the authors.

   The authors would like to thank the following people for their
   contributions to [ABE2015]: Chamil Kulatunga, David Ros, Stein
   Gjessing, Sebastian Zander.  Thanks to (in alphabetical order) Bob
   Briscoe, John Leslie, Dave Taht and the TCPM WG for providing
   valuable feedback on this document.

   The authors would like to thank feedback on the congestion control
   behaviour specified in this update received from the IRTF Internet
   Congestion Control Research Group (ICCRG).

5.  IANA Considerations


   This memo includes no request to IANA.

6.  Security Considerations

   The described method is a sender-side only transport change, and does
   not change the protocol messages exchanged.  The security
   considerations of RFC 3819 therefore still apply.

   This document describes a change to TCP congestion control with ECN
   that will typically lead to a change in the capacity achieved when
   flows share a network bottleneck.  Similar unfairness in the way that

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   capacity is shared is also exhibited by other congestion control
   mechanisms that have been in use in the Internet for many years
   (e.g., CUBIC [ID.CUBIC]).  Unfairness may also be a result of other
   factors, including the round trip time experienced by a flow.  This
   advantage applies only to ECN-marked packets and not to loss
   indications, and will therefore not lead to congestion collapse.

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,

   [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,

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

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,

7.2.  Informative References

   [ABE2015]  Khademi, N., Welzl, M., Armitage, G., Kulatunga, C., Ros,
              D., Fairhurst, G., Gjessing, S., and S. Zander,
              "Alternative Backoff: Achieving Low Latency and High
              Throughput with ECN and AQM", CAIA Technical Report CAIA-
              TR-150710A, Swinburne University of Technology, July 2015,

              Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              July 2012, <>.

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              Fairhurst, G. and M. Welzl, "The Benefits of using
              Explicit Congestion Notification (ECN)", Internet-draft,
              IETF work-in-progress draft-ietf-aqm-ecn-benefits-08,
              November 2015.

              Nichols, K., Jacobson, V., McGregor, V., and J. Iyengar,
              "The Benefits of using Explicit Congestion Notification
              (ECN)", Internet-draft, IETF work-in-progress draft-ietf-
              aqm-codel-02, December 2015.

   [I-D.PIE]  Pan, R., Natarajan, P., Baker, F., White, G., VerSteeg,
              B., Prabhu, M., Piglione, C., and V. Subramanian, "PIE: A
              Lightweight Control Scheme To Address the Bufferbloat
              Problem", Internet-draft, IETF work-in-progress draft-
              ietf-aqm-pie-03, November 2015.

   [ICC2002]  Kwon, M. and S. Fahmy, "TCP Increase/Decrease Behavior
              with Explicit Congestion Notification (ECN)", IEEE
              ICC 2002, New York, New York, USA, May 2002,

              Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              Internet-draft, IETF work-in-progress draft-ietf-tcpm-
              cubic-00, June 2015.

   [PAM2015]  Trammell, B., Kuhlewind, M., Boppart, D., Learmonth, I.,
              Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
              wide Deployment of Explicit Congestion Notification",
              Proceedings of the 2015 Passive and Active Measurement
              Conference, New York, March 2015,

              Lakhera, P. and S. Cheshire, "Your App and Next Generation
              Networks", Apple Worldwide Developers Conference 2015, San
              Francisco, USA, June 2015,

Authors' Addresses

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   Naeem Khademi
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316


   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316


   Grenville Armitage
   Centre for Advanced Internet Architectures
   Swinburne University of Technology
   PO Box 218
   John Street, Hawthorn
   Victoria  3122


   Godred Fairhurst
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen  AB24 3UE


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