draft-ietf-tcpm-cubic-05.txt		draft-ietf-tcpm-cubic-06.txt
	TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee		TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee
	Internet-Draft NCSU		Internet-Draft NCSU
	Intended status: Informational L. Xu		Intended status: Informational L. Xu
	Expires: January 18, 2018 UNL		Expires: March 21, 2018 UNL
	S. Ha		S. Ha
	Colorado		Colorado
	A. Zimmermann		A. Zimmermann
	L. Eggert		L. Eggert
	NetApp		NetApp
	R. Scheffenegger		R. Scheffenegger
	July 17, 2017		September 17, 2017
	CUBIC for Fast Long-Distance Networks		CUBIC for Fast Long-Distance Networks
	draft-ietf-tcpm-cubic-05		draft-ietf-tcpm-cubic-06
	Abstract		Abstract
	CUBIC is an extension to the current TCP standards. The protocol		CUBIC is an extension to the current TCP standards. The protocol
	differs from the current TCP standards only in the congestion window		differs from the current TCP standards only in the congestion window
	adjustment function in the sender side. In particular, it uses a		adjustment function in the sender side. In particular, it uses a
	cubic function instead of a linear window increase function of the		cubic function instead of a linear window increase function of the
	current TCP standards to improve scalability and stability under fast		current TCP standards to improve scalability and stability under fast
	and long distance networks. CUBIC and its predecessor algorithm have		and long distance networks. CUBIC and its predecessor algorithm have
	been adopted as default by Linux and have been used for many years.		been adopted as default by Linux and have been used for many years.
	skipping to change at page 1, line 40 ¶		skipping to change at page 1, line 40 ¶
	experimentation on the performance of CUBIC.		experimentation on the performance of CUBIC.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 18, 2018.		This Internet-Draft will expire on March 21, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	skipping to change at page 2, line 36 ¶		skipping to change at page 2, line 36 ¶
	4.1. Window growth function . . . . . . . . . . . . . . . . . 6		4.1. Window growth function . . . . . . . . . . . . . . . . . 6
	4.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 7		4.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 7
	4.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7		4.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7
	4.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7		4.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7
	4.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 8		4.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 8
	4.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 8		4.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 8
	4.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 9		4.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 9
	4.8. Slowstart . . . . . . . . . . . . . . . . . . . . . . . . 9		4.8. Slowstart . . . . . . . . . . . . . . . . . . . . . . . . 9
	5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 9		5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 9
	5.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 10		5.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 10
	5.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 11		5.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 12
	5.3. Difficult Environments . . . . . . . . . . . . . . . . . 12		5.3. Difficult Environments . . . . . . . . . . . . . . . . . 12
	5.4. Investigating a Range of Environments . . . . . . . . . . 12		5.4. Investigating a Range of Environments . . . . . . . . . . 13
	5.5. Protection against Congestion Collapse . . . . . . . . . 13		5.5. Protection against Congestion Collapse . . . . . . . . . 13
	5.6. Fairness within the Alternative Congestion Control		5.6. Fairness within the Alternative Congestion Control
	Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 13		Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 13
	5.7. Performance with Misbehaving Nodes and Outside Attackers 13		5.7. Performance with Misbehaving Nodes and Outside Attackers 13
	5.8. Behavior for Application-Limited Flows . . . . . . . . . 13		5.8. Behavior for Application-Limited Flows . . . . . . . . . 13
	5.9. Responses to Sudden or Transient Events . . . . . . . . . 13		5.9. Responses to Sudden or Transient Events . . . . . . . . . 14
	5.10. Incremental Deployment . . . . . . . . . . . . . . . . . 13		5.10. Incremental Deployment . . . . . . . . . . . . . . . . . 14
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 13		6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
	8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14		8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
	9.1. Normative References . . . . . . . . . . . . . . . . . . 14		9.1. Normative References . . . . . . . . . . . . . . . . . . 14
	9.2. Informative References . . . . . . . . . . . . . . . . . 15		9.2. Informative References . . . . . . . . . . . . . . . . . 15
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
	1. Introduction		1. Introduction
	The low utilization problem of TCP in fast long-distance networks is		The low utilization problem of TCP in fast long-distance networks is
	well documented in [K03][RFC3649]. This problem arises from a slow		well documented in [K03][RFC3649]. This problem arises from a slow
	skipping to change at page 6, line 43 ¶		skipping to change at page 6, line 43 ¶
	congestion detected by ECN-Echo ACKs occurs, CUBIC reduces its		congestion detected by ECN-Echo ACKs occurs, CUBIC reduces its
	current window cwnd to W_cubic(0)=W_max*beta_cubic. We discuss how		current window cwnd to W_cubic(0)=W_max*beta_cubic. We discuss how
	we set beta_cubic in Section 4.5 and how we set C in Section 5.		we set beta_cubic in Section 4.5 and how we set C in Section 5.
	Upon receiving an ACK during congestion avoidance, CUBIC computes the		Upon receiving an ACK during congestion avoidance, CUBIC computes the
	window growth rate during the next RTT period using Eq. 1. It sets		window growth rate during the next RTT period using Eq. 1. It sets
	W_cubic(t+RTT) as the candidate target value of congestion window,		W_cubic(t+RTT) as the candidate target value of congestion window,
	where RTT is the weithed average RTT calculated by the standard TCP.		where RTT is the weithed average RTT calculated by the standard TCP.
	Depending on the value of the current window size cwnd, CUBIC runs in		Depending on the value of the current window size cwnd, CUBIC runs in
	three different modes. First, if cwnd is less than the window size		three different modes.
	that Standard TCP would reach at time t after the last loss event,
	then CUBIC is in the TCP friendly region (we describe below how to		1) The TCP-friendly region, which ensures that CUBIC achieves at
	determine this window size of Standard TCP in term of time t).		least the same throughput as the standard TCP.
	Otherwise, if cwnd is less than W_max, then CUBIC is the concave
	region, and if cwnd is larger than W_max, CUBIC is in the convex		2) The concave region, if CUBIC is not in the TCP-friendly region
	region. Below, we describe the exact actions taken by CUBIC in each		and cwnd is less than W_max.
	region.
			3) The convex region, if CUBIC is not in the TCP-friendly region
			and cwnd is greater than W_max.
			Below, we describe the exact actions taken by CUBIC in each region.
	4.2. TCP-friendly region		4.2. TCP-friendly region
	Standard TCP performs well in certain types of networks, for example,		Standard TCP performs well in certain types of networks, for example,
	under short RTT and small bandwidth (or small BDP) networks. In		under short RTT and small bandwidth (or small BDP) networks. In
	these networks, we use the TCP-friendly region to ensure that CUBIC		these networks, we use the TCP-friendly region to ensure that CUBIC
	achieves at least the same throughput as the standard TCP.		achieves at least the same throughput as the standard TCP.
	When receiving an ACK in congestion avoidance, we first check whether		The TCP-friendly region is designed according to the analysis
	the protocol is in the TCP region or not. This is done by estimating		described in [FHP00]. The analysis studies the performance of an
	the average rate of the Standard TCP using a simple analysis		Additive Increase and Multiplicative Decrease (AIMD) algorithm with
	described in [FHP00]. It considers the Standard TCP as a special		an additive factor of alpha_aimd (segment per RTT) and a
	case of an Additive Increase and Multiplicative Decrease algorithm		multiplicative factor of beta_aimd, denoted by AIMD(alpha_aimd,
	(AIMD), which has an additive factor alpha_aimd and a multiplicative		beta_aimd). Specifically, the average window size of
	factor beta_aimd with the following function:		AIMD(alpha_aimd, beta_aimd) can be calculated using Eq. 3. The
			analysis shows that AIMD(alpha_aimd, beta_aimd) with
			alpha_aimd=3*(1-beta_aimd)/(1+beta_aimd) achieves the same average
			window size as the standard TCP that uses AIMD(1, 0.5).
	AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /		AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
	(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)		(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
	By the same analysis, the average window size of Standard TCP is		Based on the above analysis, CUBIC uses Eq. 4 to estimate the window
	(1.5/p)^0.5, as the Standard TCP is a special case of AIMD with		size W_est of AIMD(alpha_aimd, beta_aimd) with
	alpha_aimd=1 and beta_aimd=0.5. Thus, for Eq. 3 to be the same as		alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic,
	that of Standard TCP, alpha_aimd must be equal to		which achieves the same average window size as the standard TCP.
	3*(1-beta_aimd)/(1+beta_aimd). As AIMD increases its window by		When receiving an ACK in congestion avoidance (cwnd could be greater
	alpha_aimd per RTT, we can get the window size of AIMD in terms of		than or less than W_max), CUBIC checks whether W_cubic(t) is less
	the elapsed time t as follows:		than W_est(t). If so, CUBIC is in the TCP-friendly region and cwnd
			SHOULD be set to W_est(t) at each reception of ACK.
	W_aimd(t) = W_max*beta_aimd +
	[3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4)
	If W_cubic(t) is less than W_aimd(t) (it does not matter whether cwnd		W_est(t) = W_max*beta_cubic +
	is greater than or less than W_max), then the protocol is in the TCP		[3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4)
	friendly region and cwnd SHOULD be set to W_aimd(t) at each reception
	of ACK.
	4.3. Concave region		4.3. Concave region
	When receiving an ACK in congestion avoidance, if the protocol is not		When receiving an ACK in congestion avoidance, if the protocol is not
	in the TCP-friendly region and cwnd is less than W_max, then the		in the TCP-friendly region and cwnd is less than W_max, then the
	protocol is in the concave region. In this region, cwnd MUST be		protocol is in the concave region. In this region, cwnd MUST be
	incremented by (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK,		incremented by (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK,
	where W_cubic(t+RTT) is calculated using Eq. 1.		where W_cubic(t+RTT) is calculated using Eq. 1.
	4.4. Convex region		4.4. Convex region
	skipping to change at page 9, line 35 ¶		skipping to change at page 9, line 35 ¶
	4.7. Timeout		4.7. Timeout
	In case of timeout, CUBIC follows the standard TCP to reduce cwnd,		In case of timeout, CUBIC follows the standard TCP to reduce cwnd,
	but sets ssthresh using beta_cubic (same as in Section 4.5).		but sets ssthresh using beta_cubic (same as in Section 4.5).
	4.8. Slowstart		4.8. Slowstart
	CUBIC MUST employ a slow start algorithm, when the cwnd is no more		CUBIC MUST employ a slow start algorithm, when the cwnd is no more
	than ssthresh. Among the slow start algorithms, CUBIC MAY choose the		than ssthresh. Among the slow start algorithms, CUBIC MAY choose the
	standard TCP slow start[RFC5681] in general networks, or the limited		standard TCP slow start [RFC5681] in general networks, or the limited
	slow start [RFC3742] or hybrid slow start [HR08] for high-bandwidth		slow start [RFC3742] or hybrid slow start [HR08] for high-bandwidth
	and long-distance networks.		and long-distance networks.
			In the case when CUBIC runs the hybrid slow start [HR08], it may exit
			the first slow start without incurring any packet loss and thus W_max
			is undefined. In this special case, CUBIC switches to congestion
			avoidance and increases its congestion window size using Eq. 1 where
			K is set to 0 and W_max is set to the window size when CUBIC just
			exits the slow start.
	5. Discussion		5. Discussion
	In this section, we further discuss the safety features of CUBIC		In this section, we further discuss the safety features of CUBIC
	following the guidelines specified in [RFC5033].		following the guidelines specified in [RFC5033].
	With a deterministic loss model where the number of packets between		With a deterministic loss model where the number of packets between
	two successive lost events is always 1/p, CUBIC always operates with		two successive lost events is always 1/p, CUBIC always operates with
	the concave window profile which greatly simplifies the performance		the concave window profile which greatly simplifies the performance
	analysis of CUBIC. The average window size of CUBIC can be obtained		analysis of CUBIC. The average window size of CUBIC can be obtained
	by the following function:		by the following function:
	skipping to change at page 13, line 29 ¶		skipping to change at page 13, line 44 ¶
	link.		link.
	5.7. Performance with Misbehaving Nodes and Outside Attackers		5.7. Performance with Misbehaving Nodes and Outside Attackers
	This is not considered in the current CUBIC.		This is not considered in the current CUBIC.
	5.8. Behavior for Application-Limited Flows		5.8. Behavior for Application-Limited Flows
	CUBIC does not raise its congestion window size if the flow is		CUBIC does not raise its congestion window size if the flow is
	currently limited by the application instead of the congestion		currently limited by the application instead of the congestion
	window. In case of long periods when cwnd is not updated due to the		window. In case of long periods when cwnd has not been updated due
	application rate limit, such as idle periods, t in Eq. 1 MUST NOT		to the application rate limit, such as idle periods, t in Eq. 1 MUST
	include these periods; otherwise, W_cubic(t) might be very high after		NOT include these periods; otherwise, W_cubic(t) might be very high
	restarting from these periods.		after restarting from these periods.
	5.9. Responses to Sudden or Transient Events		5.9. Responses to Sudden or Transient Events
	In case that there is a sudden congestion, a routing change, or a		In case that there is a sudden congestion, a routing change, or a
	mobility event, CUBIC behaves the same as Standard TCP.		mobility event, CUBIC behaves the same as Standard TCP.
	5.10. Incremental Deployment		5.10. Incremental Deployment
	CUBIC requires only the change of TCP senders, and does not require		CUBIC requires only the change of TCP senders, and does not require
	any assistant of routers.		any assistant of routers.
	skipping to change at page 14, line 21 ¶		skipping to change at page 14, line 39 ¶
	responsible for any use that may be made of the information it		responsible for any use that may be made of the information it
	contains.		contains.
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<http://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",		[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
	RFC 3649, DOI 10.17487/RFC3649, December 2003,		RFC 3649, DOI 10.17487/RFC3649, December 2003,
	<http://www.rfc-editor.org/info/rfc3649>.		<https://www.rfc-editor.org/info/rfc3649>.
	[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large		[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
	Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March		Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March
	2004, <http://www.rfc-editor.org/info/rfc3742>.		2004, <https://www.rfc-editor.org/info/rfc3742>.
	[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",		[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
	RFC 4960, DOI 10.17487/RFC4960, September 2007,		RFC 4960, DOI 10.17487/RFC4960, September 2007,
	<http://www.rfc-editor.org/info/rfc4960>.		<https://www.rfc-editor.org/info/rfc4960>.
	[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion		[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
	Control Algorithms", BCP 133, RFC 5033,		Control Algorithms", BCP 133, RFC 5033,
	DOI 10.17487/RFC5033, August 2007,		DOI 10.17487/RFC5033, August 2007,
	<http://www.rfc-editor.org/info/rfc5033>.		<https://www.rfc-editor.org/info/rfc5033>.
	[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP		[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
	Friendly Rate Control (TFRC): Protocol Specification",		Friendly Rate Control (TFRC): Protocol Specification",
	RFC 5348, DOI 10.17487/RFC5348, September 2008,		RFC 5348, DOI 10.17487/RFC5348, September 2008,
	<http://www.rfc-editor.org/info/rfc5348>.		<https://www.rfc-editor.org/info/rfc5348>.
	[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion		[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
	Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,		Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
	<http://www.rfc-editor.org/info/rfc5681>.		<https://www.rfc-editor.org/info/rfc5681>.
	[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The		[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
	NewReno Modification to TCP's Fast Recovery Algorithm",		NewReno Modification to TCP's Fast Recovery Algorithm",
	RFC 6582, DOI 10.17487/RFC6582, April 2012,		RFC 6582, DOI 10.17487/RFC6582, April 2012,
	<http://www.rfc-editor.org/info/rfc6582>.		<https://www.rfc-editor.org/info/rfc6582>.
	[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,		[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
	and Y. Nishida, "A Conservative Loss Recovery Algorithm		and Y. Nishida, "A Conservative Loss Recovery Algorithm
	Based on Selective Acknowledgment (SACK) for TCP",		Based on Selective Acknowledgment (SACK) for TCP",
	RFC 6675, DOI 10.17487/RFC6675, August 2012,		RFC 6675, DOI 10.17487/RFC6675, August 2012,
	<http://www.rfc-editor.org/info/rfc6675>.		<https://www.rfc-editor.org/info/rfc6675>.
	[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF		[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
	Recommendations Regarding Active Queue Management",		Recommendations Regarding Active Queue Management",
	BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,		BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
	<http://www.rfc-editor.org/info/rfc7567>.		<https://www.rfc-editor.org/info/rfc7567>.
	9.2. Informative References		9.2. Informative References
	[CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic		[CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic
	Ordering for Internet Congestion Control and its		Ordering for Internet Congestion Control and its
	Applications", In Proceedings of IEEE INFOCOM , May 2007.		Applications", In Proceedings of IEEE INFOCOM , May 2007.
	[FHP00] Floyd, S., Handley, M., and J. Padhye, "A Comparison of		[FHP00] Floyd, S., Handley, M., and J. Padhye, "A Comparison of
	Equation-Based and AIMD Congestion Control", May 2000.		Equation-Based and AIMD Congestion Control", May 2000.
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