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Versions: (draft-khademi-alternativebackoff-ecn) 00 01 draft-ietf-tcpm-alternativebackoff-ecn

Network Working Group                                         N. Khademi
Internet-Draft                                                  M. Welzl
Intended status: Experimental                         University of Oslo
Expires: December 2, 2016                                    G. Armitage
                                                 Swinburne University of
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                            May 31, 2016

                 TCP Alternative Backoff with ECN (ABE)


   This memo updates the TCP sender-side reaction to a congestion
   notification received via Explicit Congestion Notification (ECN).
   The updated method reduces FlightSize in Congestion Avoidance by a
   smaller amount than the TCP 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 also describe a corresponding method for SCTP.

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/.

   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 2, 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Discussion: Choice of ABE Multiplier  . . . . . . . . . . . . . 3
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Status of the Update  . . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   9.  Revision Information  . . . . . . . . . . . . . . . . . . . . . 6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     10.1.  Normative References . . . . . . . . . . . . . . . . . . . 6
     10.2.  Informative References . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Introduction

   Complementing [I-D.AQM-ECN-benefits], [I-D.ECN-response] encourages
   wider ECN deployment by relaxing a rule that prohibited certain
   experiments.  This rule, from [RFC3168], required the congestion
   control response to an ECN CE-mark to be similar to the response to
   packet loss.  Without this rule, it is possible to define a new
   sender reaction to being notified of a CE-mark that differs from the
   reaction to the detection of loss.  [I-D.ECN-response] provides the
   rationale for such a different behaviour; in brief, a CE-mark is
   likely to indicate a shorter queue than packet loss, and the standard
   TCP backoff behaviour defined in [RFC5681] entails reduced link
   utilisation in situations with short queues and low statistical
   multiplexing.  This memo proposes a concrete sender-side-only
   congestion control response that remedies this problem.

   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 as
   a reaction to the receiver reporting 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" [I-D.CoDel]
   [I-D.PIE].  This is achieved when reacting to ECN-Echo in Congestion
   Avoidance by multiplying cwnd and sstthresh with a value in the range

3.  Discussion: Choice of ABE Multiplier

   Alternative Backoff with ECN (ABE) decouples a TCP sender's reaction
   to loss and ECN CE-marks in Congestion Avoidance.  The description
   respectively uses beta_{loss} and beta_{ecn} to refer to the
   multiplicative decrease factors applied in response to packet loss,
   and also in response to a receiver indicating that an ECN CE-mark was
   received on an ECN-enabled TCP connection (based on the terms used in
   [ABE2015]).  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:

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      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_{loss} and beta_{ecn}, 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 the response of a TCP connection to shallow
   AQM marking thresholds is optimised. 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 (cf. beta_{loss} = 0.5).

4.  Specification

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

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5.  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.

6.  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, Markku Kojo, 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).

7.  IANA Considerations


   This memo includes no request to IANA.

8.  Security Considerations

   The described method is a sender-side only transport change, and does
   not change the protocol messages exchanged.  The security
   considerations of [RFC3168] therefore still apply.

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   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
   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 [I-D.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.

9.  Revision Information


   -00. draft-khademi-tsvwg-ecn-response-00 and
   draft-khademi-tcpm-alternativebackoff-ecn-00 replace
   draft-khademi-alternativebackoff-ecn-03, following discussion in the
   TSVWG and TCPM working groups.

10.  References

10.1.  Normative References

              Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "Updating the ECN Congestion Control Response", Internet-
              draft, IETF
              work-in-progress draft-khademi-tsvwg-ecn-response-00,
              May 2016.

   [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",

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              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,

10.2.  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,

              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.

              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-01, January 2016.

              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.

Authors' Addresses

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

   Email: naeemk@ifi.uio.no

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

   Email: michawe@ifi.uio.no

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

   Email: garmitage@swin.edu.au

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

   Email: gorry@erg.abdn.ac.uk

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