Network Working Group N. Khademi
Internet-Draft M. Welzl
Intended status: Experimental University of Oslo
Expires: August 18, 2018 G. Armitage
Swinburne University of Technology
G. Fairhurst
University of Aberdeen
February 14, 2018
TCP Alternative Backoff with ECN (ABE)
draft-ietf-tcpm-alternativebackoff-ecn-06
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 is large. An Explicit Congestion Notification (ECN) signal
indicates that an AQM mechanism is used at the bottleneck, and
therefore the bottleneck network queue is likely to be short. This
document therefore proposes an update to RFC3168, which changes 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.
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 https://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 August 18, 2018.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Why Use ECN to Vary the Degree of Backoff? . . . . . . . 4
4.2. Focus on ECN as Defined in RFC3168 . . . . . . . . . . . 5
4.3. Choice of ABE Multiplier . . . . . . . . . . . . . . . . 5
5. ABE Deployment Requirements . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Revision Information . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 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 [RFC8087].
The rules for ECN were originally written to be very conservative,
and required the congestion control algorithms of ECN-Capable
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transport protocols to treat ECN congestion signals 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 new AQM mechanisms like PIE [RFC8033] and CoDel
[CODEL2012][I-D.CoDel], 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
cannot always utilise a bottleneck link well where there are short
queues. For example, a TCP sender must 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 cwnd [RFC5681], which effectively
doubles the amount of data that can be in flight, the maximum round-
trip time (RTT) experience, and the path's effective RTT using the
network path.
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 an early-warning signal of congestion is
received in the form of an ECN CE-marked packet. Recognizing these
changes in modern AQM practices, more recent rules have relaxed the
strict requirement that ECN signals be treated identically to
inferred packet loss [I-D.ECN-exp]. Following these newer, more
flexible rules, this document defines a new sender-side-only
congestion control response, called "ABE" (Alternative Backoff with
ECN). ABE improves TCP's average throughput when routers use AQM
controlled buffers that allow for short queues only.
2. Definitions
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 RFC 2119 [RFC2119].
3. Specification
This specification updates 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 reception
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of a CE-marked packet, i.e., receipt of a packet with the ECN-Echo
flag (defined in [RFC3168]) set.
It updates the following text in section 6.1.2 of the ECN
specification [RFC3168] :
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".
Replacing this with:
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.
As in [RFC5681], the TCP sender also reduces the cwnd value to no
more than the new ssthresh value.
4. Discussion
Much of the technical background to ABE can be found in a research
paper [ABE2017]. This paper used a mix of experiments, theory and
simulations with NewReno [RFC5681] and CUBIC [I-D.CUBIC] to evaluate
the technique. The technique 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 by multiplying cwnd and ssthresh
with a value in the range [0.7,0.85].
4.1. Why Use ECN to Vary the Degree of Backoff?
The classic rule-of-thumb dictates that a network path needs to
provide a BDP of bottleneck buffering if a TCP connection wishes to
optimise path utilisation. A single TCP bulk transfer running
through such a bottleneck will have increased its congestion window
(cwnd) up to 2*BDP by the time that packet loss occurs. According to
[RFC5681], when a TCP sender detects segment loss using the
retransmission timer and the given segment has not yet been resent by
way of the retransmission timer, the value of ssthresh must be set to
no more of the maximum of half of the FlightSize and 2*SMSS. The
same equation is also used during Fast Retransmit/Fast Recovery
[RFC5681]. As a result, the TCP congestion control only allows one
BDP of packets in flight. This is just sufficient to maintain 100%
utilisation of the bottleneck on the network path.
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AQM mechanisms such as CoDel [I-D.CoDel] 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, [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 under-utilisation 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 a 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, and hence 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 back-off when queues are short. Using ECN can also be
advantageous for several other reasons [RFC8087].
The idea of reacting differently to inferred packet loss and
detection of an ECN-signalled congestion pre-dates this document.
For example, 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. Focus on ECN as Defined in RFC3168
Some transport protocol mechanisms rely on ECN semantics that differ
from the original ECN definition [RFC3168]. For instance, Accurate
ECN [I-D.ietf-tcpm-accurate-ecn] permits more frequent and detailed
feedback. Use of mechanisms (such as Accurate ECN, Datacenter TCP
(DCTCP) [RFC8257], or Congestion Exposure (ConEx) [RFC7713]) is out
of scope for this document. This specification focuses on ECN as
defined in [RFC3168].
4.3. Choice of ABE Multiplier
ABE decouples the reaction of a TCP sender to inferred packet loss
and ECN-signalled congestion in the congestion avoidance phase. To
achieve this, ABE uses different the scaling factor in Equation 4 in
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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 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
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 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 standard value is 0.5
[RFC5681], and the Linux implementation of CUBIC [I-D.CUBIC] has used
a multiplier of 0.7 since kernel version 2.6.25 released in 2008.
ABE proposes no change to beta_{loss} used by current TCP
implementations.
The recommendation in Section 3 in this document corresponds to 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, [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).
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
[I-D.ietf-tcpm-accurate-ecn] by a receiver.
RFC3168 states that the congestion control response to an ECN-
signalled congestion is the same as the response to a dropped packet
[RFC3168]. [I-D.ECN-exp] 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 has thus been assigned an Experimental
status before being proposed as a Standards-Track update.
The purpose of the Internet experiment is to collect experience with
deployment of ABE, and confirm the safety in deployed networks using
this update to TCP congestion control.
When used with bottlenecks that do not support ECN-marking the
specification does not modify the transport protocol.
To evaluate the benefit, this experiment therefore requires support
in AQM routers for ECN-marking of packets carrying the ECN-Capable
Transport, ECT(0), codepoint [RFC3168].
If the method is only deployed by some senders, and not by others,
the senders that use this method 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 an TCP sender using ABE is too
aggressive, and the result is no different than if the TCP sender was
using traditional loss-based congestion control.
A TCP sender reacts to loss or ECN marks only once per round-trip
time. Hence, if a sender would first be notified of an ECN mark and
then learn about loss in the same round-trip, it would only react to
the first notification (ECN) but not to the second (loss). RFC3168
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specified a reaction to ECN that was equal to the reaction to loss
[RFC3168].
ABE also responds to congestion once per RTT, and therefore it does
not respond to further loss within the same RTT, since ABE 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 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 WG (or IESG).
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 Stuart Cheshire for many suggestions
when revising the draft, and the following people for their
contributions to [ABE2017]: Chamil Kulatunga, David Ros, Stein
Gjessing, Sebastian Zander. Thanks also to (in alphabetical order)
Roland Bless, Bob Briscoe, David Black, Markku Kojo, John Leslie,
Lawrence Stewart, Dave Taht and the TCPM working group for providing
valuable feedback on this document.
The authors would finally like to thank everyone who provided
feedback on the congestion control behaviour specified in this update
received from the IRTF Internet Congestion Control Research Group
(ICCRG).
7. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This document includes no request to IANA.
8. Implementation Status
ABE is implemented as a patch for Linux and FreeBSD. It is meant for
research and available for download from
http://heim.ifi.uio.no/naeemk/research/ABE/. This code was used to
produce the test results that are reported in [ABE2017]. An evolved
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version of the patch for FreeBSD is currently under review for
potential inclusion in the mainline kernel [ABE-FreeBSD].
9. Security Considerations
The described method is a sender-side only transport change, and does
not change the protocol messages exchanged. The security
considerations for ECN [RFC3168] therefore 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
(e.g., CUBIC [I-D.CUBIC]). 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, hence use of ABE cannot lead to congestion collapse.
10. Revision Information
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
-06. Addressed Michael Scharf's comments.
-05. Refined the description of the experiment based on feedback at
IETF-100. Incorporated comments from David Black.
-04. Incorporates review comments from Lawrence Stewart and the
remaining comments from Roland Bless. References are updated.
-03. Several review comments from Roland Bless are addressed.
Consistent terminology and equations. Clarification on the scope of
recommended beta_{ecn} value.
-02. Corrected the equations in Section 4.3. Updated the
affiliations. Lower bound for cwnd is defined. A recommendation for
window-based transport protocols is changed to cover all transport
protocols that implement a congestion control reduction to an ECN
congestion signal. Added text about ABE's FreeBSD mainline kernel
status including a reference to the FreeBSD code review page.
References are updated.
-01. Text improved, mainly incorporating comments from Stuart
Cheshire. The reference to a technical report has been updated to a
published version of the tests [ABE2017]. Used "AQM Mechanism"
throughout in place of other alternatives, and more consistent use of
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technical language and clarification on the intended purpose of the
experiments required by EXP status. There was no change to the
technical content.
-00. draft-ietf-tcpm-alternativebackoff-ecn-00 replaces draft-
khademi-tcpm-alternativebackoff-ecn-01. Text describing the nature
of the experiment was added.
Individual draft -01. This I-D now refers to draft-black-tsvwg-ecn-
experimentation-02, which replaces draft-khademi-tsvwg-ecn-
response-00 to make a broader update to RFC3168 for the sake of
allowing experiments. As a result, some of the motivating and
discussing text that was moved from draft-khademi-alternativebackoff-
ecn-03 to draft-khademi-tsvwg-ecn-response-00 has now been re-
inserted here.
Individual draft -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.
11. References
11.1. Normative References
[I-D.ECN-exp]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", Internet-draft, IETF work-in-progress
draft-ietf-tsvwg-ecn-experimentation-08, November 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7567>.
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[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>.
11.2. Informative References
[ABE-FreeBSD]
"ABE patch review in FreeBSD",
<https://reviews.freebsd.org/D11616>.
[ABE2017] Khademi, N., Armitage, G., Welzl, M., Fairhurst, G.,
Zander, S., and D. Ros, "Alternative Backoff: Achieving
Low Latency and High Throughput with ECN and AQM", IFIP
NETWORKING 2017, Stockholm, Sweden, June 2017.
[BUFFERBLOAT]
Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
the Internet", November 2011.
[CODEL2012]
Nichols, K. and V. Jacobson, "Controlling Queue Delay",
July 2012, <http://queue.acm.org/detail.cfm?id=2209336>.
[I-D.CoDel]
Nichols, K., Jacobson, V., McGregor, V., and J. Iyengar,
"Controlled Delay Active Queue Management", Internet-
draft, IETF work-in-progress draft-ietf-aqm-codel-10,
October 2017.
[I-D.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-07, November 2017.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-03 (work in progress), May 2017.
[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>.
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[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<https://www.rfc-editor.org/info/rfc7713>.
[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,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>.
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
Internet For Things (I4T) Research Group
Swinburne University of Technology
PO Box 218
John Street, Hawthorn
Victoria 3122
Australia
Email: garmitage@swin.edu.au
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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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