Network Working Group N. Khademi
Internet-Draft M. Welzl
Updates: 3168 (if approved) University of Oslo
Intended status: Experimental G. Armitage
Expires: January 1, 2016 Swinburne University of
Technology
G. Fairhurst
University of Aberdeen
June 30, 2015
TCP Alternative Backoff with ECN (ABE)
draft-khademi-alternativebackoff-ecn-00
Abstract
This memo updates the TCP sender-side reaction to a congestion
notification received via Explicit Congestion Notification (ECN).
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 ECN-marked 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
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Updating the Sender-side ECN Reaction . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
Explicit Congestion Notification (ECN) is specified in [RFC3168].
This allows a network device that uses Active Queue Management (AQM)
to CE-mark rather than 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 improves 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
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 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 of the flow. CUBIC can fully
utilize a link with a smaller queue because it multiplies the current
cwnd with 0.8 in response to packet loss [ID.CUBIC] (since kernel
version 2.6.25 (2008), the Linux implementation uses a value of 0.7).
In case of a DropTail (FIFO) queue without AQM, this increases the
risk of creating a standing queue [CODEL2012].
Devices implementing AQM should be the only source of marking for
packets from ECN-capable senders. AQM mechanisms typically strive to
maintain a small queue length, regardless of the bandwidth-delay
product of a flows passing through them. Receipt of a CE-mark
therefore indicates that reacting less conservatively would be
appropriate.
Results reported in [ABE-paper] 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] rather than 0.5.
2. Updating the Sender-side ECN Reaction
This document specifies an updated TCP reaction to the receipt of CE-
marked packets.
The first paragraph of Section 6.1.2, "The TCP Sender", in [RFC3168]
contains the following text:
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"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 text to:
"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. Acknowledgements
The authors 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 [ABE-paper]: Chamil Kulatunga, David Ros, Stein
Gjessing, Sebastian Zander.
4. IANA Considerations
XX RFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
5. Security Considerations
This document describes a change to TCP congestion control that can
make a TCP sender more aggressive than flows using RFC 3819. This
could lead to an unfair allocation in rates at a bottleneck. Similar
unfairness is also exhibited by other congestion control mechanisms
that have been in use in the Internet for many years (e.g. Cubic
[ID.CUBIC]).
XXX This section to be completed XXX.
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6. References
6.1. Normative References
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
6.2. Informative References
[ABE-paper]
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, July 2015 http://caia.swin.edu.au/reports/
150710A/CAIA-TR-150710A.pdf, NOTE: this document may not
be available at draft submission date. We will
do our best to make it available as soon as possible, and
definitely before IETF-93.
[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.
[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.
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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
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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