TCP Alternative Backoff with ECN (ABE)
draft-khademi-alternativebackoff-ecn-01

Network Working Group                                         N. Khademi
Internet-Draft                                                  M. Welzl
Updates: 3168 (if approved)                           University of Oslo
Intended status: Experimental                                G. Armitage
Expires: March 24, 2016               Swinburne University of Technology
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                      September 21, 2015

                 TCP Alternative Backoff with ECN (ABE)
                draft-khademi-alternativebackoff-ecn-01

Abstract

   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).  ECN-marking can allow a
   network device to signal congestion at a point before a transport
   experiences congestion loss or additional queueing delay.  The
   updated method is less conservative than the TCP reaction in response
   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 March 24, 2016.

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Copyright Notice

   Copyright (c) 2015 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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   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?  . . . . . . .   4
     2.2.  Choice of ABE multiplier  . . . . . . . . . . . . . . . .   5
   3.  Updating the Sender-side ECN Reaction . . . . . . . . . . . .   6
     3.1.  RFC 2119  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Update to RFC 3168  . . . . . . . . . . . . . . . . . . .   6
     3.3.  Status of the Update  . . . . . . . . . . . . . . . . . .   6
   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 to 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 [I-D.AQM-ECN-benefits].

   [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

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   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 non-ECN enabled standard TCP flow using this reaction
   needs significant network queue space: it can only fully utilize a
   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 flows more fully utilize paths even when the bottleneck queue
   is slightly 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
   multiplier.

   Results reported in [ABE2015] show significant benefits (improved
   throughput, resulting in reduced completion times for short flows)
   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."

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

   AQM schemes like CoDel and 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]).  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
   underutilisation 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 the
   requirements of [RFC3168], which required TCP senders to respond the
   same 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.

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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]).
   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:

      cwnd_(n+1) = cwnd_n * beta_{loss}

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

      cwnd_(n+1) = cwnd_n * beta_{ecn}

   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 is already running with at least two different
   beta_{loss} values, [RFC5681]'s 0.5, and Linux CUBIC's 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).

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3.  Updating the Sender-side ECN Reaction

   This section specifies an experimental update to [RFC3168].

3.1.  RFC 2119

   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].

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 this 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.  The indication of congestion SHOULD
   induce a less conservative reaction than loss: the TCP source
   multiplies the congestion window 'cwnd' with 0.8 and reduces the slow
   start threshold 'ssthresh'."

3.3.  Status of the Update

   XXX Author's note: Once ICCRG evalaution has been completed an
   appropriate outcome may be inserted here XXX

   The congestion control behaviour specified in this update will be
   evaluated by the IRTF Internet Congestion Control Research Group
   (ICCRG), to determine whether it is thought safe for deployment in
   the general Internet.

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   XXX Author's note: If this is adopted for publication as an
   Experimental RFC we need to explain why this is not PS XXX

   The present specification has been assigned an Experimental status,
   because this is common practice for first introduction of changes to
   the TCP protocol specification, where deployment experience is
   usually required prior to publishing a Standards-Track document.

   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.

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.

5.  IANA Considerations

   XX RFC ED - PLEASE REMOVE THIS SECTION XXX

   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 that can
   make TCP senders more aggressive than flows using TCP as specified in
   RFC 3819.  This could lead to a change in the capacity achieved by
   flows sharing a network bottleneck.  If some flows use this method
   and share capacity with other flows using previous methods this could
   reduce fairness in the capacity allocation.  Similar unfairness is
   also exhibited by other congestion control mechanisms that have been

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   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 can
   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,
              <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>.

   [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>.

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <http://www.rfc-editor.org/info/rfc7567>.

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,
              <http://caia.swin.edu.au/reports/150710A/
              CAIA-TR-150710A.pdf>.

   [CODEL2012]
              Nichols, K. and V. Jacobson, "Controlling Queue Delay",
              July 2012, <http://queue.acm.org/detail.cfm?id=2209336>.

   [I-D.AQM-ECN-benefits]
              Fairhurst, G. and M. Welzl, "The Benefits of using
              Explicit Congestion Notification (ECN)", Internet-draft,
              IETF work-in-progress draft-ietf-aqm-ecn-benefits-05, June
              2015.

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   [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,
              <http://dx.doi.org/10.1109/ICC.2002.997262>.

   [ID.CUBIC]
              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,
              <http://ecn.ethz.ch/ecn-pam15.pdf>.

   [WWDC2015]
              Lakhera, P. and S. Cheshire, "Your App and Next Generation
              Networks", Apple Worldwide Developers Conference 2015, San
              Francisco, USA, June 2015,
              <https://developer.apple.com/videos/wwdc/2015/?id=719>.

Authors' Addresses

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

   Email: naeemk@ifi.uio.no

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

   Email: michawe@ifi.uio.no

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

   Email: garmitage@swin.edu.au

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

   Email: gorry@erg.abdn.ac.uk

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