Network Working Group N. Khademi
Internet-Draft M. Welzl
Updates: 3168 (if approved) University of Oslo
Intended status: Experimental G. Armitage
Expires: March 24, 2016 Swinburne University of Technology
G. Fairhurst
University of Aberdeen
September 21, 2015
TCP Alternative Backoff with ECN (ABE)
draft-khademi-alternativebackoff-ecn-01
Abstract
This memo provides an experimental update to RFC3168. It updates the
TCP sender-side reaction to a congestion notification received via
Explicit Congestion Notification (ECN). ECN-marking can allow a
network device to signal congestion at a point before a transport
experiences congestion loss or additional queueing delay. The
updated method is less conservative than the TCP reaction in response
to loss. The intention is to achieve good throughput when the queue
at the bottleneck is smaller than the bandwidth-delay-product of the
connection. This is more likely when an Active Queue Management
(AQM) mechanism has used ECN to CE-mark a packet, than when a packet
was lost. Future versions of this document will discuss SCTP as well
as other transports using ECN.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 March 24, 2016.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Why use ECN to vary the degree of backoff? . . . . . . . 4
2.2. Choice of ABE multiplier . . . . . . . . . . . . . . . . 5
3. Updating the Sender-side ECN Reaction . . . . . . . . . . . . 6
3.1. RFC 2119 . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Update to RFC 3168 . . . . . . . . . . . . . . . . . . . 6
3.3. Status of the Update . . . . . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Explicit Congestion Notification (ECN) is specified in [RFC3168]. It
allows a network device that uses Active Queue Management (AQM) to
set the congestion experienced, CE, codepoint in the ECN field of the
IP packet header, rather than to drop ECN-capable packets when
incipient congestion is detected. When an ECN-capable transport is
used over a path that supports ECN, it provides the opportunity for
flows to improve their performance in the presence of incipient
congestion [I-D.AQM-ECN-benefits].
[RFC3168] not only specifies the router use of the ECN field, it also
specifies a TCP procedure for using ECN. This states that a TCP
sender should treat the ECN indication of congestion in the same way
as that of a non-ECN-Capable TCP flow experiencing loss, by halving
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the congestion window "cwnd" and by reducing the slow start threshold
"ssthresh". [RFC5681] stipulates that TCP congestion control sets
"ssthresh" to max(FlightSize / 2, 2*SMSS) in response to packet loss.
Consequently, a non-ECN enabled standard TCP flow using this reaction
needs significant network queue space: it can only fully utilize a
bottleneck when the length of the link queue (or the AQM dropping
threshold) is at least the bandwidth-delay product (BDP) of the flow.
A backoff multipler of 0.5 (halving cwnd and sshthresh after packet
loss) is not the only available strategy. As defined in [ID.CUBIC],
CUBIC multiplies the current cwnd by 0.8 in response to loss
(although the Linux implementation of CUBIC has used a multiplier of
0.7 since kernel version 2.6.25 released in 2008). Consequently,
CUBIC flows more fully utilize paths even when the bottleneck queue
is slightly shorter than the bandwidth-delay product of the flow.
However, in the case of a DropTail (FIFO) queue without AQM, such
less-aggressive backoff increases the risk of creating a standing
queue [CODEL2012].
Devices implementing AQM are likely to be the dominant (and possibly
only) source of ECN CE-marking for packets from ECN-capable senders.
AQM mechanisms typically strive to maintain a small queue length,
regardless of the bandwidth-delay product of flows passing through
them. Receipt of an ECN CE-mark might therefore reasonably be taken
to indicate that a small bottleneck queue exists in the path, and
hence the TCP flow would benefit from using a less aggressive backoff
multiplier.
Results reported in [ABE2015] show significant benefits (improved
throughput, resulting in reduced completion times for short flows)
when reacting to ECN-Echo by multiplying cwnd and sstthresh with a
value in the range [0.7..0.85]. Section 2 describes the rationale
for this change. Section 3 specifies a change to the TCP sender
backoff behaviour in response to an indication that CE-marks have
been received by the receiver.
2. Discussion
Much of the background to this proposal can be found in [ABE2015].
Using a mix of experiments, theory and simulations with standard
NewReno and CUBIC, [ABE2015] recommends enabling ECN and "...letting
individual TCP senders use a larger multiplicative decrease factor in
reaction to ECN CE-marks from AQM-enabled bottlenecks." Such a
change is noted to result in "...significant performance gains in
lightly-multiplexed scenarios, without losing the delay-reduction
benefits of deploying CoDel or PIE."
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2.1. Why use ECN to vary the degree of backoff?
The classic rule-of-thumb dictates a BDP of bottleneck buffering if a
TCP connection wishes to optimise path utilisation. A single TCP
connection running through such a bottleneck will have opened cwnd up
to 2*BDP by the time packet loss occurs. [RFC5681]'s halving of cwnd
and ssthresh pushes the TCP connection back to allowing only a BDP of
packets in flight -- just enough to maintain 100% utilisation of the
network path.
AQM schemes like CoDel and PIE use congestion notifications to
constrain the queuing delays experienced by packets, rather than in
response to impending or actual bottleneck buffer exhaustion. With
current default delay targets, CoDel and PIE both effectively emulate
a shallow buffered bottleneck (section II, [ABE2015]). This
interacts acceptably for TCP connections over low BDP paths, or
highly multiplexed scenarios (lmany concurrent TCP connections).
However, it interacts badly with lightly-multiplexed cases (few
concurrent connections) over high BDP paths. Conventional TCP
backoff in such cases leads to gaps in packet transmission and
underutilisation of the path.
In an ideal world, the TCP sender would adapt its backoff strategy to
match the effective depth at which a bottleneck begins indicating
congestion. In the practical world, [ABE2015] proposes using the
existence of ECN CE-marks to infer whether a path's bottleneck is
AQM-enabled (shallow queue) or classic DropTail (deep queue), and
adjust backoff accordingly. This results in a change to the
requirements of [RFC3168], which required TCP senders to respond the
same following indication of a received ECN CE-mark and a packet
loss, making these equivalent signals of congestion. (The idea to
change this behaviour pre-dates ABE. [ICC2002] also proposed using
ECN CE-marks to modify TCP congestion control behaviour, using a
larger multiplicative decrease factor in conjunction with a smaller
additive increase factor to deal with RED-based bottlenecks that were
not necessarily configured to emulate a shallow queue.)
[RFC7567] states that "deployed AQM algorithms SHOULD support
Explicit Congestion Notification (ECN) as well as loss to signal
congestion to endpoints" and [I-D.AQM-ECN-benefits] encourages this
deployment. Apple recently announced their intention to enable ECN
in iOS 9 and OS X 10.11 devices [WWDC2015]. By 2014, server-side ECN
negotiation was observed to be provided by the majority of the top
million web servers [PAM2015], and only 0.5% of websites incurred
additional connection setup latency using RFC3168-compliant ECN-
fallback mechanisms.
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2.2. Choice of ABE multiplier
ABE decouples a TCP sender's reaction to loss and ECN CE-marks. The
description respectively uses beta_{loss} and beta_{ecn} to refer to
the multiplicative decrease factors applied in response to packet
loss and in response to an indication of a received CN CE-mark on an
ECN-enabled TCP connection (based on the terms used in [ABE2015]).
For non-ECN-enabled TCP connections, no ECN CE-marks are received and
only beta_{loss} applies.
In other words, in response to detected loss:
cwnd_(n+1) = cwnd_n * beta_{loss}
and in response to an indication of a received ECN CE-mark:
cwnd_(n+1) = cwnd_n * beta_{ecn}
The higher the values of beta_*, the less aggressive the response of
any individual backoff event.
The appropriate choice for beta_{loss} and beta_{ecn} values is a
balancing act between path utilisation and draining the bottleneck
queue. More aggressive backoff (smaller beta_*) risks underutilising
the path, while less aggressive backoff (larger beta_*) can result in
slower draining of the bottleneck queue.
The Internet is already running with at least two different
beta_{loss} values, [RFC5681]'s 0.5, and Linux CUBIC's 0.7. ABE
proposes no change to beta_{loss} used by any current TCP
implementations.
beta_{ecn} depends on how we want to optimise the reponse of a TCP
connection to shallow AQM marking thresholds. beta_{loss} reflects
the preferred response of each TCP algorithm when faced with
exhaustion of buffers (of unknown depth) signalled by packet loss.
Consequently, for any given TCP algorithm the choice of beta_{ecn} is
likely to be algorithm-specific, rather than a constant multiple of
the algorithm's existing beta_{loss}.
A range of experiments (section IV, [ABE2015]) with NewReno and CUBIC
over CoDel and PIE in lightly multiplexed scenarios have explored
this choice of parameter. These experiments indicate that CUBIC
connections benefit from beta_{ecn} of 0.85 (cf. beta_{loss} = 0.7),
and NewReno connections see improvements with beta_{ecn} in the range
0.7 to 0.85 (c.f., beta_{loss} = 0.5).
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3. Updating the Sender-side ECN Reaction
This section specifies an experimental update to [RFC3168].
3.1. RFC 2119
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3.2. Update to RFC 3168
This document specifies an update to the TCP sender reaction that
follows when the TCP receiver signals that ECN CE-marked packets have
been received.
The first paragraph of Section 6.1.2, "The TCP Sender", in [RFC3168]
contains the following text:
"If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK
packet with the ECN-Echo flag set in the TCP header), then the sender
knows that congestion was encountered in the network on the path from
the sender to the receiver. The indication of congestion should be
treated just as a congestion loss in non-ECN-Capable TCP. That is,
the TCP source halves the congestion window "cwnd" and reduces the
slow start threshold "ssthresh"."
This memo updates this by replacing this with the following text:
"If the sender receives an ECN-Echo (ECE) ACK packet (that is, an ACK
packet with the ECN-Echo flag set in the TCP header), then the sender
knows that congestion was encountered in the network on the path from
the sender to the receiver. The indication of congestion SHOULD
induce a less conservative reaction than loss: the TCP source
multiplies the congestion window 'cwnd' with 0.8 and reduces the slow
start threshold 'ssthresh'."
3.3. Status of the Update
XXX Author's note: Once ICCRG evalaution has been completed an
appropriate outcome may be inserted here XXX
The congestion control behaviour specified in this update will be
evaluated by the IRTF Internet Congestion Control Research Group
(ICCRG), to determine whether it is thought safe for deployment in
the general Internet.
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XXX Author's note: If this is adopted for publication as an
Experimental RFC we need to explain why this is not PS XXX
The present specification has been assigned an Experimental status,
because this is common practice for first introduction of changes to
the TCP protocol specification, where deployment experience is
usually required prior to publishing a Standards-Track document.
This update is a sender-side only change. Like other changes to
congestion-control algorithms it does not require any change to the
TCP receiver or to network devices (except to enable an ECN-marking
algorithm [RFC3168] [RFC7567]). If the method is only deployed by
some TCP senders, and not by others, the senders that use this method
can gain advantage, possibly at the expense of other flows that do
not use this updated method. This advantage applies only to ECN-
marked packets and not to loss indications. Hence, the new method
can not lead to congestion collapse.
4. Acknowledgements
Authors N. Khademi, M. Welzl and G. Fairhurst were part-funded by
the European Community under its Seventh Framework Programme through
the Reducing Internet Transport Latency (RITE) project (ICT-317700).
The views expressed are solely those of the authors.
The authors would like to thank the following people for their
contributions to [ABE2015]: Chamil Kulatunga, David Ros, Stein
Gjessing, Sebastian Zander.
5. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
6. Security Considerations
The described method is a sender-side only transport change, and does
not change the protocol messages exchanged. The security
considerations of RFC 3819 therefore still apply.
This document describes a change to TCP congestion control that can
make TCP senders more aggressive than flows using TCP as specified in
RFC 3819. This could lead to a change in the capacity achieved by
flows sharing a network bottleneck. If some flows use this method
and share capacity with other flows using previous methods this could
reduce fairness in the capacity allocation. Similar unfairness is
also exhibited by other congestion control mechanisms that have been
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in use in the Internet for many years (e.g., CUBIC [ID.CUBIC]).
Unfairness may also be a result of other factors, including the round
trip time experienced by a flow. This advantage applies only to ECN-
marked packets and not to loss indications, and will therefore can
not lead to congestion collapse.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<http://www.rfc-editor.org/info/rfc7567>.
7.2. Informative References
[ABE2015] Khademi, N., Welzl, M., Armitage, G., Kulatunga, C., Ros,
D., Fairhurst, G., Gjessing, S., and S. Zander,
"Alternative Backoff: Achieving Low Latency and High
Throughput with ECN and AQM", CAIA Technical Report CAIA-
TR-150710A, Swinburne University of Technology, July 2015,
<http://caia.swin.edu.au/reports/150710A/
CAIA-TR-150710A.pdf>.
[CODEL2012]
Nichols, K. and V. Jacobson, "Controlling Queue Delay",
July 2012, <http://queue.acm.org/detail.cfm?id=2209336>.
[I-D.AQM-ECN-benefits]
Fairhurst, G. and M. Welzl, "The Benefits of using
Explicit Congestion Notification (ECN)", Internet-draft,
IETF work-in-progress draft-ietf-aqm-ecn-benefits-05, June
2015.
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[ICC2002] Kwon, M. and S. Fahmy, "TCP Increase/Decrease Behavior
with Explicit Congestion Notification (ECN)", IEEE
ICC 2002, New York, New York, USA, May 2002,
<http://dx.doi.org/10.1109/ICC.2002.997262>.
[ID.CUBIC]
Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
Internet-draft, IETF work-in-progress draft-ietf-tcpm-
cubic-00, June 2015.
[PAM2015] Trammell, B., Kuhlewind, M., Boppart, D., Learmonth, I.,
Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
wide Deployment of Explicit Congestion Notification",
Proceedings of the 2015 Passive and Active Measurement
Conference, New York, March 2015,
<http://ecn.ethz.ch/ecn-pam15.pdf>.
[WWDC2015]
Lakhera, P. and S. Cheshire, "Your App and Next Generation
Networks", Apple Worldwide Developers Conference 2015, San
Francisco, USA, June 2015,
<https://developer.apple.com/videos/wwdc/2015/?id=719>.
Authors' Addresses
Naeem Khademi
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Email: naeemk@ifi.uio.no
Michael Welzl
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Email: michawe@ifi.uio.no
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Grenville Armitage
Centre for Advanced Internet Architectures
Swinburne University of Technology
PO Box 218
John Street, Hawthorn
Victoria 3122
Australia
Email: garmitage@swin.edu.au
Godred Fairhurst
University of Aberdeen
School of Engineering, Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
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