Network Working Group N. Khademi
Internet-Draft M. Welzl
Intended status: Experimental University of Oslo
Expires: April 23, 2018 G. Armitage
Swinburne University of Technology
G. Fairhurst
University of Aberdeen
October 20, 2017
TCP Alternative Backoff with ECN (ABE)
draft-ietf-tcpm-alternativebackoff-ecn-02
Abstract
Recent Active Queue Management (AQM) mechanisms instantiate shallow
buffers with burst tolerance to minimise the time that packets spend
enqueued at a bottleneck. However, shallow buffering can cause
noticeable performance degradation when TCP is used over a network
path with a large bandwidth-delay-product. Traditional methods rely
on detecting network congestion through reported loss of transport
packets. Explicit Congestion Notification (ECN) instead allows a
router to directly signal incipient congestion. A sending endpoint
can distinguish when congestion is signalled via ECN, rather than by
packet loss. An ECN signal indicates that an AQM mechanism has done
its job, and therefore the bottleneck network queue is likely to be
shallow. This document therefore proposes an update to 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 loss. This document also recommends
this approach to be adopted by any other transport protocol that
implements a congestion control reduction to an ECN congestion
signal.
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-
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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."
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This Internet-Draft will expire on April 23, 2018.
Copyright Notice
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
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. Discussion: Choice of ABE Multiplier . . . . . . . . . . 5
5. Status of the Update . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Revision Information . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. 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].
2. 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
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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. There are also significant other benefits
from deploying ECN [RFC8087], including reduced end-to-end network
latency.
The rules for ECN were originally written to be very conservative,
and required the congestion control algorithms of ECN-capable
transport protocols to treat ECN congestion signals exactly the same
as they would treat a packet loss [RFC3168].
Research has demonstrated the benefits of reducing network delays due
to excessive buffering [BUFFERBLOAT]; this has led to the creation of
new AQM mechanisms like PIE [RFC8033] and CoDel [CODEL2012]
[I-D.CoDel], which avoid causing bloated queues that are common with
a simple tail-drop behaviour (also known as a First-In First-Out,
FIFO, queue).
These AQM mechanisms instantiate short queues that are designed to
tolerate packet bursts. However, congestion control mechanisms
cannot always utilise a bottleneck link well where there are short
queues. For example, to allow a single TCP connection to fully
utilise a network path, the queue at the bottleneck link must be able
to compensate for TCP halving the "cwnd" and "ssthresh" variables in
response to a lost packet [RFC5681]. This requires the bottleneck
queue to be able to store at least an end-to-end bandwidth-delay
product (BDP) of data, which effectively doubles both the amount of
data that can be in flight and the round-trip time (RTT) experience
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 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 the
performance when routers use shallow buffered AQM mechanisms.
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3. Specification
This specification describes an update to the congestion control
algorithm of an ECN-capable TCP transport protocol. It allows a TCP
stack to update the TCP sender response when it receives feedback
indicating reception of a CE-marked packet. It RECOMMENDS that a TCP
sender multiplies the cwnd by 0.8 and reduces the slow start
threshold (ssthresh) in congestion avoidance following reception of a
TCP segment that sets the ECN-Echo flag (defined in [RFC3168]).
While this specification concerns TCP, other transports also support
a per-RTT response to ECN. The method defined in this document is
also applicable for such transports.
4. Discussion
Much of the technical background to this congestion control response
can be found in a research paper [ABE2017]. This paper used a mix of
experiments, theory and simulations with standard NewReno and CUBIC
to evaluate the technique. It examined the impact of enabling ECN
and letting individual TCP senders back off by a reduced amount in
reaction to the receiver that reports ECN CE-marks from AQM-enabled
bottlenecks. The technique was shown to present "...significant
performance gains in lightly-multiplexed 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. When packet
loss is detected (regarded as a notification of congestion), Standard
TCP halves the cwnd and ssthresh [RFC5681], which causes the TCP
congestion control to go back to allowing only a BDP of packets in
flight -- just sufficient to maintain 100% utilisation of the
bottleneck on the network path.
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
shallow buffered bottleneck (section II, [ABE2017]) while also
allowing short traffic bursts into the queue. This provides
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acceptable performance for TCP connections over a path with a low
BDP, or in highly multiplexed scenarios (many concurrent transport
connections). However, it interacts badly for a lightly-multiplexed
case (few concurrent connections) over a path with a large BDP.
Conventional TCP backoff in such cases 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
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 shallow queue.
Reacting differently to an ECN CE-mark than to packet loss can then
yield the benefit of a reduced back-off, as with CUBIC [I-D.CUBIC],
when queues are short, yet it can avoid generating excessive delay
when queues are long. Using ECN can also be advantageous for several
other reasons [RFC8087].
The idea of reacting differently to loss and detection of an ECN CE-
mark pre-dates this document. For example, previous research
proposed using ECN CE-marks 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
shallow queue ([RFC7567] notes the current status of RED as an AQM
method.)
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 example, Congestion
Exposure (ConEx) [RFC7713] and Datacenter TCP (DCTCP)
[I-D.ietf-tcpm-dctcp] need more accurate ECN information than that
offered by the original feedback method. Other mechanisms (e.g.,
[I-D.ietf-tcpm-accurate-ecn]) allow the sender to adjust the rate
more frequently than once each path RTT. Use of these mechanisms is
out of the scope of the current document.
4.3. Discussion: Choice of ABE Multiplier
ABE decouples the reaction of a TCP sender to loss and ECN CE-marks
when in the congestion avoidance phase by differentiating the scaling
factor used in 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 packet
loss, and in response to a receiver indicating that an ECN CE-mark
was received on an ECN-enabled TCP connection. For non-ECN-enabled
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TCP connections, no ECN CE-marks are received and only beta_{loss}
applies.
In other words, in response to detected loss:
ssthresh_(t+1) = max (FlightSize_t * beta_{loss}, 2 * SMSS)
and in response to an indication of a received ECN CE-mark:
ssthresh_(t+1) = max (FlightSize_t * beta_{ecn}, 2 * SMSS)
and
cwnd_(t+1) = ssthresh_(t+1)
where FlightSize is the amount of outstanding data in the network,
upper-bounded by 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.
beta_{ecn} depends on how the response of a TCP connection to shallow
AQM marking thresholds is optimised. beta_{loss} reflects the
preferred response of each congestion control algorithm when faced
with exhaustion of buffers (of unknown depth) signalled by packet
loss. Consequently, for any given TCP congestion control 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 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).
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5. Status of the Update
This update is a sender-side only change. Like other changes to
congestion-control algorithms, it does not require any change to the
TCP receiver or to network devices. 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.
The currently published ECN specification requires that the
congestion control response to a CE-marked packet is the same as the
response to a dropped packet [RFC3168]. The specification is
currently being updated to allow for specifications that do not
follow this rule [I-D.ECN-exp]. 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 (except to enable an ECN-marking mechanism [RFC3168]
[RFC7567]) 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
loss indications, the new method cannot lead to congestion collapse.
The result of this Internet experiment will be reported by
presentation to the TCPM WG (or IESG) or an implementation report at
the end of the experiment.
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
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Gjessing, Sebastian Zander. Thanks also to (in alphabetical order)
Bob Briscoe, Markku Kojo, John Leslie, 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
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
-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
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protocols that implements 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
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-06, September 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>.
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[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>.
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]
"Bufferbloat project",
<https://www.bufferbloat.net/projects/bloat/wiki/
Introduction/>.
[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-09,
September 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-06, September 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.
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[I-D.ietf-tcpm-dctcp]
Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
Control for Datacenters", draft-ietf-tcpm-dctcp-10 (work
in progress), August 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>.
[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
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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
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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