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Internet Engineering Task Force (IETF)                        N. Khademi
Request for Comments: 8511                                      M. Welzl
Category: Experimental                                University of Oslo
ISSN: 2070-1721                                              G. Armitage
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                           December 2018

                 TCP Alternative Backoff with ECN (ABE)


   Active Queue Management (AQM) mechanisms allow for burst tolerance
   while enforcing short queues to minimise the time that packets spend
   enqueued at a bottleneck.  This can cause noticeable performance
   degradation for TCP connections traversing such a bottleneck,
   especially if there are only a few flows or their bandwidth-delay
   product (BDP) is large.  The reception of a Congestion Experienced
   (CE) Explicit Congestion Notification (ECN) mark indicates that an
   AQM mechanism is used at the bottleneck, and the bottleneck network
   queue is therefore likely to be short.  Feedback of this signal
   allows the TCP sender-side ECN reaction in congestion avoidance to
   reduce the Congestion Window (cwnd) by a smaller amount than the
   congestion control algorithm's reaction to inferred packet loss.
   Therefore, this specification defines an experimental change to the
   TCP reaction specified in RFC 3168, as permitted by RFC 8311.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2018 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
   Provisions Relating to IETF Documents
   (https://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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Choice of ABE Multiplier  . . . . . . . . . . . . . . . .   4
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Rationale for Using ECN to Vary the Degree of Backoff . .   6
     4.2.  An RTT-Based Response to Indicated Congestion . . . . . .   7
   5.  ABE Deployment Requirements . . . . . . . . . . . . . . . . .   7
   6.  ABE Experiment Goals  . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] makes it possible
   for an Active Queue Management (AQM) mechanism to signal the presence
   of incipient congestion without necessarily incurring packet loss.
   This lets the network deliver some packets to an application that
   would have been dropped if the application or transport did not
   support ECN.  This packet loss reduction is the most obvious benefit
   of ECN, but it is often relatively modest.  Other benefits of
   deploying ECN have been documented in [RFC8087].

   The rules for ECN were originally written to be very conservative,
   and they required the congestion control algorithms of ECN-Capable
   Transport (ECT) protocols to treat indications of congestion
   signalled by ECN exactly the same as they would treat an inferred
   packet loss [RFC3168].  Research has demonstrated the benefits of
   reducing network delays that are caused by interaction of loss-based
   TCP congestion control and excessive buffering [BUFFERBLOAT].  This
   has led to the creation of AQM mechanisms like Proportional Integral
   Controller Enhanced (PIE) [RFC8033] and Controlling Queue Delay
   (CoDel) [RFC8289], which prevent bloated queues that are common with
   unmanaged and excessively large buffers deployed across the Internet

   The AQM mechanisms mentioned above aim to keep a sustained queue
   short while tolerating transient (short-term) packet bursts.
   However, currently used loss-based congestion control mechanisms are
   not always able to effectively utilise a bottleneck link where there
   are short queues.  For example, a TCP sender using the Reno
   congestion control needs to be able to store at least an end-to-end
   bandwidth-delay product (BDP) worth of data at the bottleneck buffer
   if it is to maintain full path utilisation in the face of loss-
   induced reduction of the congestion window (cwnd) [RFC5681].  This
   amount of buffering effectively doubles the amount of data that can
   be in flight and the maximum round-trip time (RTT) experienced by the
   TCP sender.

   Modern AQM mechanisms can use ECN to signal the early signs of
   impending queue buildup long before a tail-drop queue would be forced
   to resort to dropping packets.  It is therefore appropriate for the
   transport protocol congestion control algorithm to have a more
   measured response when it receives an indication with an early
   warning of congestion after the remote endpoint receives an ECN
   CE-marked packet.  Recognizing these changes in modern AQM practices,
   the strict requirement that ECN CE signals be treated identically to
   inferred packet loss has been relaxed [RFC8311].  This document
   therefore defines a new sender-side-only congestion control response

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   called "ABE" (Alternative Backoff with ECN).  ABE improves TCP's
   average throughput when routers use AQM-controlled buffers that allow
   only for short queues.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Specification

   This specification changes the congestion control algorithm of an
   ECN-Capable TCP transport protocol by changing the TCP-sender
   response to feedback from the TCP receiver that indicates the
   reception of a CE-marked packet, i.e., receipt of a packet with the
   ECN-Echo flag (defined in [RFC3168]) set, following the process
   defined in [RFC8311].

   The TCP-sender response is currently specified in Section 6.1.2 of
   the ECN specification [RFC3168] and has been slightly updated by
   Section 4.1 of [RFC8311] to read as:

      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", unless otherwise specified by an
      Experimental RFC in the IETF document stream.

   As permitted by RFC 8311, this document specifies a sender-side
   change to TCP where receipt of a packet with the ECN-Echo flag SHOULD
   trigger the TCP source to set the slow start threshold (ssthresh) to
   0.8 times the FlightSize, with a lower bound of 2 * SMSS applied to
   the result (where SMSS stands for Sender Maximum Segment Size)).  As
   in [RFC5681], the TCP sender also reduces the cwnd value to no more
   than the new ssthresh value.  Section 6.1.2 of RFC 3168 provides
   guidance on setting a cwnd less than 2 * SMSS.

3.1.  Choice of ABE Multiplier

   ABE decouples the reaction of a TCP sender to inferred packet loss
   from the indication of ECN-signalled congestion in the congestion
   avoidance phase.  To achieve this, ABE uses a different scaling
   factor for Equation 4 in Section 3.1 of [RFC5681].  The description
   respectively uses beta_{loss} and beta_{ecn} to refer to the
   multiplicative decrease factors applied in response to inferred

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   packet loss, and in response to a receiver indicating ECN-signalled
   congestion.  For non-ECN-enabled TCP connections, only beta_{loss}

   In other words, in response to inferred packet loss:

      ssthresh = max (FlightSize * beta_{loss}, 2 * SMSS)

   and in response to an indication of an ECN-signalled congestion:

      ssthresh = max (FlightSize * beta_{ecn}, 2 * SMSS)


      cwnd = ssthresh

      (If ssthresh == 2 * SMSS, Section 6.1.2 of RFC 3168 provides
      guidance on setting a cwnd lower than 2 * SMSS.)

   where FlightSize is the amount of outstanding data in the network,
   upper-bounded by the smaller of the sender's cwnd and the receiver's
   advertised window (rwnd) [RFC5681].  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 the
   underutilisation of 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 standard value is 0.5
   [RFC5681], and the Linux implementation of CUBIC [RFC8312] has used a
   multiplier of 0.7 since kernel version 2.6.25 released in 2008.  ABE
   does not change the value of beta_{loss} used by current TCP

   The recommendation in this document specifies a value of
   beta_{ecn}=0.8.  This recommended beta_{ecn} value is only applicable
   for the standard TCP congestion control [RFC5681].  The selection of
   beta_{ecn} enables tuning the response of a TCP connection to shallow
   AQM-marking thresholds.  beta_{loss} characterizes the response of a
   congestion control algorithm to packet loss, i.e., exhaustion of
   buffers (of unknown depth).  Different values for beta_{loss} have
   been suggested for TCP congestion control algorithms.  Consequently,
   beta_{ecn} is likely to be an algorithm-specific parameter rather
   than a constant multiple of the algorithm's existing beta_{loss}.

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   A range of tests (Section IV of [ABE2017]) with NewReno and CUBIC
   over CoDel and PIE in lightly multiplexed scenarios have explored
   this choice of parameter.  The results of these tests 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.  Discussion

   Much of the technical background for ABE can be found in [ABE2017],
   which uses a mix of experiments, theory, and simulations with NewReno
   [RFC5681] and CUBIC [RFC8312] to evaluate its performance.  ABE was
   shown to present significant performance gains in lightly-multiplexed
   (few concurrent flows) scenarios, without losing the delay-reduction
   benefits of deploying CoDel or PIE.  The performance improvement is
   achieved when reacting to ECN-Echo in congestion avoidance (when
   ssthresh > cwnd) by multiplying cwnd and ssthresh with a value in the
   range [0.7,0.85].  Applying ABE when cwnd is smaller than or equal to
   ssthresh is not currently recommended, but its use in that scenario
   may benefit from additional attention, experimentation, and

4.1.  Rationale for Using ECN to Vary the Degree of Backoff

   AQM mechanisms such as CoDel [RFC8289] and PIE [RFC8033] set a delay
   target in routers and 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
   bottleneck with a short queue (Section II of [ABE2017]) while also
   allowing short traffic bursts into the queue.  This provides
   acceptable performance for TCP connections over a path with a low
   BDP, or in highly multiplexed scenarios (many concurrent transport
   flows).  However, in a lightly multiplexed case over a path with a
   large BDP, conventional TCP backoff leads to gaps in packet
   transmission and underutilisation of the path.

   Instead of discarding packets, an AQM mechanism is allowed to mark
   ECN-Capable packets with an ECN CE mark.  The reception of CE-mark
   feedback not only indicates congestion on the network path, it also
   indicates that an AQM mechanism exists at the bottleneck along the
   path.  Therefore, the CE mark likely came from a bottleneck with a
   controlled short queue.  Reacting differently to an ECN-signalled
   congestion than to an inferred packet loss can then yield the benefit
   of a reduced backoff when queues are short.  Using ECN can also be
   advantageous for several other reasons [RFC8087].

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   The idea of reacting differently to inferred packet loss and
   detection of an ECN-signalled congestion predates this specification,
   e.g., previous research proposed using ECN CE-marked feedback to
   modify TCP congestion control behaviour via a larger multiplicative
   decrease factor in conjunction with a smaller additive increase
   factor [ICC2002].  The goal of this former work was to operate across
   AQM bottlenecks (using Random Early Detection (RED)) that were not
   necessarily configured to emulate a short queue.  (The current usage
   of RED as an Internet AQM method is limited [RFC7567].)

4.2.  An RTT-Based Response to Indicated Congestion

   This specification applies to the use of ECN feedback as defined in
   [RFC3168], which specifies a response to indicated congestion that is
   no more frequent than once per path round-trip time.  Since ABE
   responds to indicated congestion once per RTT, it does not respond to
   any further loss within the same RTT because an ABE sender has
   already reduced the congestion window.  If congestion persists after
   such reduction, ABE continues to reduce the congestion window in each
   consecutive RTT.  This consecutive reduction can protect the network
   against long-standing unfairness in the case of AQM algorithms that
   do not keep a small average queue length.  The mechanism does not
   rely on Accurate ECN [ACC-ECN-FEEDBACK].

   In contrast, transport protocol mechanisms can also be designed to
   utilise more frequent and detailed ECN feedback (e.g., Accurate ECN
   [ACC-ECN-FEEDBACK]), which then permit a congestion control response
   that adjusts the sending rate more frequently.  Data Center TCP
   (DCTCP) [RFC8257] is an example of this approach.

5.  ABE Deployment Requirements

   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.  It does not require any ABE-
   specific changes in routers or the use of Accurate ECN feedback
   [ACC-ECN-FEEDBACK] by a receiver.

   If the method is only deployed by some senders, and not by others,
   the senders using it can gain some advantage, possibly at the expense
   of other flows that do not use this updated method.  Because this
   advantage applies only to ECN-marked packets and not to packet-loss
   indications, an ECN-Capable bottleneck will still fall back to
   dropping packets if a TCP sender using ABE is too aggressive.  The
   result is no different than if the TCP sender were using traditional
   loss-based congestion control.

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   When used with bottlenecks that do not support ECN marking, the
   specification does not modify the transport protocol.

6.  ABE Experiment Goals

   [RFC3168] states that the congestion control response following an
   indication of ECN-signalled congestion is the same as the response to
   a dropped packet.  [RFC8311] updates this specification to allow
   systems to provide a different behaviour when they experience ECN-
   signalled congestion rather than packet loss.  The present
   specification defines such an experiment and is an Experimental RFC.
   We expect to propose it as a Standards-Track document in the future.

   The purpose of the Internet experiment is to collect experience with
   the deployment of ABE and confirm acceptable safety in deployed
   networks that use this update to TCP congestion control.  To evaluate
   ABE, this experiment requires support in AQM routers for the ECN-
   marking of packets carrying the ECN-Capable Transport codepoint
   ECT(0) [RFC3168].

   The result of this Internet experiment ought to include an
   investigation of the implications of experiencing an ECN-CE mark
   followed by loss within the same RTT.  At the end of the experiment,
   this will be reported to the TCPM Working Group or the IESG.

   ABE is implemented as a patch for Linux and FreeBSD.  This is meant
   for research and experimentation and is available for download at
   <https://heim.ifi.uio.no/michawe/research/abe/>.  This code was used
   to produce the test results that are reported in [ABE2017].  The
   FreeBSD code was committed to the mainline kernel on March 19, 2018

7.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

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

   This is 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.  This could result in some flows receiving more
   than their fair share of capacity.  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

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   (e.g., CUBIC [RFC8312]).  Unfairness may also be a result of other
   factors, including the round-trip time experienced by a flow.  ABE
   applies only when ECN-marked packets are received, not when packets
   are lost.  Therefore, use of ABE cannot lead to congestion collapse.

9.  References

9.1.  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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

9.2.  Informative References

              Stewart, L., "ABE patch review in FreeBSD",
              Revision 331214, March 2018, <https://svnweb.freebsd.org/

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RFC 8511                           ABE                     December 2018

   [ABE2017]  Khademi, N., Armitage, G., Welzl, M., Zander, S.,
              Fairhurst, G., and D. Ros, "Alternative backoff: Achieving
              low latency and high throughput with ECN and AQM", IFIP
              Networking Conference and Workshops Stockholm, Sweden,
              DOI 10.23919/IFIPNetworking.2017.8264863, June 2017.

              Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
              Accurate ECN Feedback in TCP", Work in Progress,
              draft-ietf-tcpm-accurate-ecn-07, July 2018.

              Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
              the Internet", ACM Queue, Volume 9, Issue 11,
              DOI 10.1145/2063166.2071893, November 2011,

   [ICC2002]  Kwon, M. and S. Fahmy, "TCP increase/decrease behavior
              with explicit congestion notification (ECN)", 2002 IEEE
              International Conference on Communications Conference
              Proceedings, ICC 2002, Cat. No.02CH37333,
              DOI 10.1109/ICC.2002.997262, May 2002,

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,

   [RFC8087]  Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,

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   Authors N. Khademi, M. Welzl, and G. Fairhurst were partly 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.

   Author G. Armitage performed most of his work on this document while
   employed by Swinburne University of Technology, Melbourne, Australia.

   The authors would like to thank Stuart Cheshire for many suggestions
   when revising this document.  They would also like to thank the
   following people for their contributions to [ABE2017]: Chamil
   Kulatunga, David Ros, Stein Gjessing, and Sebastian Zander.  Thanks
   also to (in alphabetical order) David Black, Roland Bless, Bob
   Briscoe, Markku Kojo, John Leslie, Lawrence Stewart, and the TCPM
   Working Group for providing valuable feedback on this document.

   Finally, the authors would like to thank everyone who provided
   feedback on the congestion control behaviour specified in this
   document that was received from the IRTF Internet Congestion Control
   Research Group (ICCRG).

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Authors' Addresses

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

   Email: naeemk@ifi.uio.no

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

   Email: michawe@ifi.uio.no

   Grenville Armitage
   Netflix Inc.

   Email: garmitage@netflix.com

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

   Email: gorry@erg.abdn.ac.uk

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