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IETF RFC 8511
Last modified on Tuesday, December 18th, 2018
 Permanent link to RFC 8511
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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
Netflix
G. Fairhurst
University of Aberdeen
December 2018
TCP Alternative Backoff with ECN (ABE)
 Abstract
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
evaluation.
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
https://www.rfc-editor.org/info/RFC 8511.
Khademi, et al. Experimental PAGE 1 
RFC 8511 ABE December 2018
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
Khademi, et al. Experimental PAGE 2
RFC 8511 ABE December 2018
1. Introduction
Explicit Congestion Notification (ECN) [RFC 3168] 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 [RFC 8087].
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 [RFC 3168]. 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) [RFC 8033] and Controlling Queue Delay
(CoDel) [RFC 8289], which prevent bloated queues that are common with
unmanaged and excessively large buffers deployed across the Internet
[BUFFERBLOAT].
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) [RFC 5681]. 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 [RFC 8311]. This document
therefore defines a new sender-side-only congestion control response
Khademi, et al. Experimental PAGE 3
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] 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 [RFC 3168]) set, following the process
defined in [RFC 8311].
The TCP-sender response is currently specified in Section 6.1.2 of
the ECN specification [RFC 3168] and has been slightly updated by
Section 4.1 of [RFC 8311] 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 [RFC 5681], 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 [RFC 5681]. The description
respectively uses beta_{loss} and beta_{ecn} to refer to the
multiplicative decrease factors applied in response to inferred
Khademi, et al. Experimental PAGE 4
RFC 8511 ABE December 2018
packet loss, and in response to a receiver indicating ECN-signalled
congestion. For non-ECN-enabled TCP connections, only beta_{loss}
applies.
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)
and
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) [RFC 5681]. 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
[RFC 5681], and the Linux implementation of CUBIC [RFC 8312] 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
implementations.
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 [RFC 5681]. 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}.
Khademi, et al. Experimental PAGE 5
RFC 8511 ABE December 2018
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
[RFC 5681] and CUBIC [RFC 8312] 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
specification.
4.1. Rationale for Using ECN to Vary the Degree of Backoff
AQM mechanisms such as CoDel [RFC 8289] and PIE [RFC 8033] 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 [RFC 8087].
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RFC 8511 ABE December 2018
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 [RFC 7567].)
4.2. An RTT-Based Response to Indicated Congestion
This specification applies to the use of ECN feedback as defined in
[RFC 3168], 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) [RFC 8257] 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.
Khademi, et al. Experimental PAGE 7
RFC 8511 ABE December 2018
When used with bottlenecks that do not support ECN marking, the
specification does not modify the transport protocol.
6. ABE Experiment Goals
[RFC 3168] states that the congestion control response following an
indication of ECN-signalled congestion is the same as the response to
a dropped packet. [RFC 8311] 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) [RFC 3168].
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
[ABE-REVISION].
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 [RFC 3168] 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
Khademi, et al. Experimental PAGE 8
RFC 8511 ABE December 2018
(e.g., CUBIC [RFC 8312]). 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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC 3168, September 2001,
<https://www.rfc-editor.org/info/RFC 3168>.
[RFC 5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC 5681, September 2009,
<https://www.rfc-editor.org/info/RFC 5681>.
[RFC 7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC 7567, July 2015,
<https://www.rfc-editor.org/info/RFC 7567>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8257] 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/RFC 8257,
October 2017, <https://www.rfc-editor.org/info/RFC 8257>.
[RFC 8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC 8311, January 2018,
<https://www.rfc-editor.org/info/RFC 8311>.
9.2. Informative References
[ABE-REVISION]
Stewart, L., "ABE patch review in FreeBSD",
Revision 331214, March 2018, <https://svnweb.freebsd.org/
base?view=revision&revision=331214>.
Khademi, et al. Experimental PAGE 9
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.
[ACC-ECN-FEEDBACK]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", Work in Progress,
draft-ietf-tcpm-accurate-ecn-07, July 2018.
[BUFFERBLOAT]
Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
the Internet", ACM Queue, Volume 9, Issue 11,
DOI 10.1145/2063166.2071893, November 2011,
<https://queue.acm.org/detail.cfm?id=2071893>.
[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,
<http://dx.doi.org/10.1109/ICC.2002.997262>.
[RFC 8033] 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/RFC 8033, February 2017,
<https://www.rfc-editor.org/info/RFC 8033>.
[RFC 8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC 8087, March 2017,
<https://www.rfc-editor.org/info/RFC 8087>.
[RFC 8289] Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
Iyengar, Ed., "Controlled Delay Active Queue Management",
RFC 8289, DOI 10.17487/RFC 8289, January 2018,
<https://www.rfc-editor.org/info/RFC 8289>.
[RFC 8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC 8312, February 2018,
<https://www.rfc-editor.org/info/RFC 8312>.
Khademi, et al. Experimental PAGE 10
RFC 8511 ABE December 2018
Acknowledgements
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).
Khademi, et al. Experimental PAGE 11
RFC 8511 ABE December 2018
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
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
Khademi, et al. Experimental PAGE 12
RFC TOTAL SIZE: 27408 bytes
PUBLICATION DATE: Tuesday, December 18th, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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