draft-ietf-tcpm-cubic-00.txt		draft-ietf-tcpm-cubic-01.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: December 20, 2015 UNL		Expires: July 21, 2016 UNL
	S. Ha		S. Ha
	NCSU		Colorado
	A. Zimmermann		A. Zimmermann
	L. Eggert		L. Eggert
	R. Scheffenegger		R. Scheffenegger
	NetApp		NetApp
	June 18, 2015		January 18, 2016
	CUBIC for Fast Long-Distance Networks		CUBIC for Fast Long-Distance Networks
	draft-ietf-tcpm-cubic-00		draft-ietf-tcpm-cubic-01
	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 of the current TCP		cubic function instead of a linear window increase of the current TCP
	standards to improve scalability and stability under fast and long		standards to improve scalability and stability under fast and long
	distance networks. BIC-TCP, a predecessor of CUBIC, has been a		distance networks. BIC-TCP, a predecessor of CUBIC, has been a
	default TCP adopted by Linux since year 2005 and has already been		default TCP adopted by Linux since year 2005 and has already been
	skipping to change at page 2, line 7		skipping to change at page 2, line 7
	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 http://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 December 20, 2015.		This Internet-Draft will expire on July 21, 2016.
	Copyright Notice		Copyright Notice
	Copyright (c) 2015 IETF Trust and the persons identified as the		Copyright (c) 2016 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		(http://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
	2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5		2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
	3. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5		3. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5
	3.1. Window growth function . . . . . . . . . . . . . . . . . 5		3.1. Window growth function . . . . . . . . . . . . . . . . . 5
	3.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 6		3.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 6
	3.3. Concave region . . . . . . . . . . . . . . . . . . . . . 6		3.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7
	3.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7		3.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7
	3.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 7		3.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 7
	3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 7		3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 7
	4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8		4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8
	4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8		4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8
	4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10		4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10
	4.3. Difficult Environments . . . . . . . . . . . . . . . . . 10		4.3. Difficult Environments . . . . . . . . . . . . . . . . . 11
	4.4. Investigating a Range of Environments . . . . . . . . . . 11		4.4. Investigating a Range of Environments . . . . . . . . . . 11
	4.5. Protection against Congestion Collapse . . . . . . . . . 11		4.5. Protection against Congestion Collapse . . . . . . . . . 11
	4.6. Fairness within the Alternative Congestion Control		4.6. Fairness within the Alternative Congestion Control
	Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11		Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11
	4.7. Performance with Misbehaving Nodes and Outside Attackers 11		4.7. Performance with Misbehaving Nodes and Outside Attackers 11
	4.8. Responses to Sudden or Transient Events . . . . . . . . . 11		4.8. Responses to Sudden or Transient Events . . . . . . . . . 11
	4.9. Incremental Deployment . . . . . . . . . . . . . . . . . 11		4.9. Incremental Deployment . . . . . . . . . . . . . . . . . 11
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 11		5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
	7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12		7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
	8.1. Normative References . . . . . . . . . . . . . . . . . . 12		8.1. Normative References . . . . . . . . . . . . . . . . . . 12
	8.2. Informative References . . . . . . . . . . . . . . . . . 12		8.2. Informative References . . . . . . . . . . . . . . . . . 13
	Appendix A. ToDo List . . . . . . . . . . . . . . . . . . . . . 13		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
	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
	increase of congestion window following a congestion event in a		increase of congestion window following a congestion event in a
	network with a large bandwidth delay product (BDP). Our experience		network with a large bandwidth delay product (BDP). Our experience
	[HKLRX06] indicates that this problem is frequently observed even in		[HKLRX06] indicates that this problem is frequently observed even in
	the range of congestion window sizes over several hundreds of packets		the range of congestion window sizes over several hundreds of packets
	(each packet is sized around 1000 bytes) especially under a network		(each packet is sized around 1000 bytes) especially under a network
	skipping to change at page 4, line 13		skipping to change at page 4, line 13
	synchronizations.		synchronizations.
	Another notable feature of CUBIC is that its window increase rate is		Another notable feature of CUBIC is that its window increase rate is
	mostly independent of RTT, and follows a (cubic) function of the		mostly independent of RTT, and follows a (cubic) function of the
	elapsed time since the last loss event. This feature promotes per-		elapsed time since the last loss event. This feature promotes per-
	flow fairness to Standard TCP as well as RTT-fairness. Note that		flow fairness to Standard TCP as well as RTT-fairness. Note that
	Standard TCP performs well under short RTT and small bandwidth (or		Standard TCP performs well under short RTT and small bandwidth (or
	small BDP) networks. Only in a large long RTT and large bandwidth		small BDP) networks. Only in a large long RTT and large bandwidth
	(or large BDP) networks, it has the scalability problem. An		(or large BDP) networks, it has the scalability problem. An
	alternative protocol to Standard TCP designed to be friendly to		alternative protocol to Standard TCP designed to be friendly to
	Standard TCP at a per-flow basis must operate must increase its		Standard TCP at a per-flow basis must operate to increase its window
	window much less aggressively in small BDP networks than in large BDP		much less aggressively in small BDP networks than in large BDP
	networks. In CUBIC, its window growth rate is slowest around the		networks. In CUBIC, its window growth rate is slowest around the
	inflection point of the cubic function and this function does not		inflection point of the cubic function and this function does not
	depend on RTT. In a smaller BDP network where Standard TCP flows are		depend on RTT. In a smaller BDP network where Standard TCP flows are
	working well, the absolute amount of the window decrease at a loss		working well, the absolute amount of the window decrease at a loss
	event is always smaller because of the multiplicative decrease.		event is always smaller because of the multiplicative decrease.
	Therefore, in CUBIC, the starting window size after a loss event from		Therefore, in CUBIC, the starting window size after a loss event from
	which the window starts to increase, is smaller in a smaller BDP		which the window starts to increase, is smaller in a smaller BDP
	network, thus falling nearer to the plateau of the cubic function		network, thus falling nearer to the plateau of the cubic function
	where the growth rate is slowest. By setting appropriate values of		where the growth rate is slowest. By setting appropriate values of
	the cubic function parameters, CUBIC sets its growth rate always no		the cubic function parameters, CUBIC sets its growth rate always no
	skipping to change at page 4, line 50		skipping to change at page 4, line 50
	multiplexing environments, the bandwidth share ratio of Standard TCP		multiplexing environments, the bandwidth share ratio of Standard TCP
	flows with different RTTs is squarely proportional to the inverse of		flows with different RTTs is squarely proportional to the inverse of
	their RTT ratio [XHR04]. CUBIC always ensures the linear ratio		their RTT ratio [XHR04]. CUBIC always ensures the linear ratio
	independent of the levels of statistical multiplexing. This is an		independent of the levels of statistical multiplexing. This is an
	improvement over Standard TCP. While there is no consensus on a		improvement over Standard TCP. While there is no consensus on a
	particular bandwidth share ratios of different RTT flows, we believe		particular bandwidth share ratios of different RTT flows, we believe
	that under wired Internet, use of the linear share notion seems more		that under wired Internet, use of the linear share notion seems more
	reasonable than equal share or a higher order shares. HTCP [LS08]		reasonable than equal share or a higher order shares. HTCP [LS08]
	currently uses the equal share.		currently uses the equal share.
	CUBIC sets the multiplicative window decrease factor to 0.2 while		CUBIC sets the multiplicative window decrease factor to 0.7 while
	Standard TCP uses 0.5. While this improves the scalability of the		Standard TCP uses 0.5. While this improves the scalability of the
	protocol, a side effect of this decision is slower convergence		protocol, a side effect of this decision is slower convergence
	especially under low statistical multiplexing environments. This		especially under low statistical multiplexing environments. This
	design choice is following the observation that the author of HSTCP		design choice is following the observation that the author of HSTCP
	[RFC3649] has made along with other researchers (e.g., [GV02]): the		[RFC3649] has made along with other researchers (e.g., [GV02]): the
	current Internet becomes more asynchronous with less frequent loss		current Internet becomes more asynchronous with less frequent loss
	synchronizations with high statistical multiplexing. Under this		synchronizations with high statistical multiplexing. Under this
	environment, even strict MIMD can converge. CUBIC flows with the		environment, even strict MIMD can converge. CUBIC flows with the
	same RTT always converge to the same share of bandwidth independent		same RTT always converge to the same share of bandwidth independent
	of statistical multiplexing, thus achieving intra-protocol fairness.		of statistical multiplexing, thus achieving intra-protocol fairness.
	skipping to change at page 5, line 27		skipping to change at page 5, line 27
	specified in [RFC5033].		specified in [RFC5033].
	2. Conventions		2. Conventions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	3. CUBIC Congestion Control		3. CUBIC Congestion Control
			The unit of all window sizes in this document is segments of the
			maximum segment size (MSS), and the unit of all times is seconds.
	3.1. Window growth function		3.1. Window growth function
	CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by		CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by
	increasing congestion window only at the reception of ACK. The		increasing congestion window only at the reception of ACK. The
	protocol does not make any change to the fast recovery and retransmit		protocol does not make any change to the fast recovery and retransmit
	of TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During congestion		of TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During congestion
	avoidance after fast recovery, CUBIC changes the window update		avoidance after fast recovery, CUBIC changes the window update
	algorithm of Standard TCP. Suppose that W_max is the window size		algorithm of Standard TCP. Suppose that W_max is the window size
	before the window is reduced in the last fast retransmit and		before the window is reduced in the last fast retransmit and
	recovery.		recovery.
	The window growth function of CUBIC uses the following function:		The window growth function of CUBIC uses the following function:
	W(t) = C*(t-K)^3 + W_max (Eq. 1)		W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
	where C is a constant fixed to determine the aggressiveness of window		where C is a constant fixed to determine the aggressiveness of window
	growth in high BDP networks, t is the elapsed time from the last		growth in high BDP networks, t is the elapsed time from the last
	window reduction,and K is the time period that the above function		window reduction,and K is the time period that the above function
	takes to increase W to W_max when there is no further loss event and		takes to increase the current window size to W_max when there is no
	is calculated by using the following equation:		further loss event and is calculated by using the following equation:
	K = cubic_root(W_max*beta/C) (Eq. 2)		K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2)
	where beta is the multiplication decrease factor. We discuss how we		where beta_cubic is the CUBIC multiplication decrease factor, that
	set C in the next Section in more details.		is, when a packet loss occurs, CUBIC reduces its current window cwnd
			to cwnd*beta_cubic. We discuss how we set C in the next Section in
			more details.
	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(t+RTT) as the candidate target value of congestion window. Suppose		W_cubic(t+RTT) as the candidate target value of congestion window.
	that the current window size is cwnd. Depending on the value of
	cwnd, CUBIC runs in three different modes. First, if cwnd is less		Depending on the value of the current window size cwnd, CUBIC runs in
	than the window size that Standard TCP would reach at time t after		three different modes. First, if cwnd is less than the window size
	the last loss event, then CUBIC is in the TCP friendly region (we		that Standard TCP would reach at time t after the last loss event,
	describe below how to determine this window size of Standard TCP in		then CUBIC is in the TCP friendly region (we describe below how to
	term of time t). Otherwise, if cwnd is less than W_max, then CUBIC		determine this window size of Standard TCP in term of time t).
	is the concave region, and if cwnd is larger than W_max, CUBIC is in		Otherwise, if cwnd is less than W_max, then CUBIC is the concave
	the convex region. Below, we describe the exact actions taken by		region, and if cwnd is larger than W_max, CUBIC is in the convex
	CUBIC in each region.		region. Below, we describe the exact actions taken by CUBIC in each
			region.
	3.2. TCP-friendly region		3.2. TCP-friendly region
	When receiving an ACK in congestion avoidance, we first check whether		When receiving an ACK in congestion avoidance, we first check whether
	the protocol is in the TCP region or not. This is done as follows.		the protocol is in the TCP region or not. This is done as follows.
	We can analyze the window size of Standard TCP in terms of the		We can analyze the window size of a TCP-friendly AIMD in terms of the
	elapsed time t. Using a simple analysis in [FHP00], we can analyze		elapsed time t. Using a simple analysis in [FHP00], we can analyze
	the average window size of additive increase and multiplicative		the average window size of additive increase and multiplicative
	decrease (AIMD) with an additive factor alpha and a multiplicative		decrease (AIMD) with an additive factor alpha_aimd and a
	factor beta to be the following function:		multiplicative factor beta_aimd with the following function:
	(alpha/2 * (2-beta)/beta * 1/p)^0.5 (Eq. 3)		AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
			(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
	By the same analysis, the average window size of Standard TCP with		By the same analysis, the average window size of Standard TCP is
	alpha 1 and beta 0.5 is (3/2 *1/p)^0.5. Thus, for Eq. 3 to be the		(1.5/p)^0.5, as the Standard TCP is a special case of AIMD with
	same as that of Standard TCP, alpha must be equal to 3*beta/(2-beta).		alpha_aimd=1 and beta_aimd=0.5. Thus, for Eq. 3 to be the same as
	As Standard TCP increases its window by alpha per RTT, we can get the		that of Standard TCP, alpha_aimd must be equal to
	window size of Standard TCP in terms of the elapsed time t as		3*(1-beta_aimd)/(1+beta_aimd). As AIMD increases its window by
	follows:		alpha_aimd per RTT, we can get the window size of AIMD in terms of
			the elapsed time t as follows:
	W_tcp(t) = W_max*(1-beta) + 3*beta/(2-beta)* t/RTT (Eq. 4)		W_aimd(t) = W_max*beta_aimd +
			[3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4)
	If cwnd is less than W_tcp(t), then the protocol is in the TCP		If W_cubic(t) is less than W_aimd(t), then the protocol is in the TCP
	friendly region and cwnd SHOULD be set to W_tcp(t) at each reception		friendly region and cwnd SHOULD be set to W_aimd(t) at each reception
	of ACK.		of ACK.
	3.3. Concave region		3.3. 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(t+RTT) - cwnd)/cwnd.		incremented by (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK.
	3.4. Convex region		3.4. Convex region
	When the window size of CUBIC is larger than W_max, it passes the		When the current window size of CUBIC is larger than W_max, it passes
	plateau of the cubic function after which CUBIC follows the convex		the plateau of the cubic function after which CUBIC follows the
	profile of the cubic function. Since cwnd is larger than the		convex profile of the cubic function. Since cwnd is larger than the
	previous saturation point W_max, this indicates that the network		previous saturation point W_max, this indicates that the network
	conditions might have been perturbed since the last loss event,		conditions might have been perturbed since the last loss event,
	possibly implying more available bandwidth after some flow		possibly implying more available bandwidth after some flow
	departures. Since the Internet is highly asynchronous, some amount		departures. Since the Internet is highly asynchronous, some amount
	of perturbation is always possible without causing a major change in		of perturbation is always possible without causing a major change in
	available bandwidth. In this phase, CUBIC is being very careful by		available bandwidth. In this phase, CUBIC is being very careful by
	very slowly increasing its window size. The convex profile ensures		very slowly increasing its window size. The convex profile ensures
	that the window increases very slowly at the beginning and gradually		that the window increases very slowly at the beginning and gradually
	increases its growth rate. We also call this phase as the maximum		increases its growth rate. We also call this phase as the maximum
	probing phase since CUBIC is searching for a new W_max. In this		probing phase since CUBIC is searching for a new W_max. In this
	region, cwnd MUST be incremented by (W(t+RTT) - cwnd)/cwnd for each		region, cwnd MUST be incremented by (W_cubic(t+RTT) - cwnd)/cwnd for
	received ACK.		each received ACK.
	3.5. Multiplicative decrease		3.5. Multiplicative decrease
	When a packet loss occurs, CUBIC reduces its window size by a factor		When a packet loss occurs, CUBIC reduces its window size by a factor
	of beta. Parameter beta SHOULD be set to 0.2.		of beta. Parameter beta_cubic SHOULD be set to 0.7.
	W_max = cwnd; // save window size before reduction		W_max = cwnd; // save window size before reduction
	cwnd = cwnd * (1-beta); // window reduction		cwnd = cwnd * beta_cubic; // window reduction
	A side effect of setting beta to a smaller value than 0.5 is slower		A side effect of setting beta_cubic to a bigger value than 0.5 is
	convergence. We believe that while a more adaptive setting of beta		slower convergence. We believe that while a more adaptive setting of
	could result in faster convergence, it will make the analysis of the		beta_cubic could result in faster convergence, it will make the
	protocol much harder. This adaptive adjustment of beta is an item		analysis of the protocol much harder. This adaptive adjustment of
	for the next version of CUBIC.		beta_cubic is an item for the next version of CUBIC.
	3.6. Fast convergence		3.6. Fast convergence
	To improve the convergence speed of CUBIC, we add a heuristic in the		To improve the convergence speed of CUBIC, we add a heuristic in the
	protocol. When a new flow joins the network, existing flows in the		protocol. When a new flow joins the network, existing flows in the
	network need to give up their bandwidth shares to allow the flow some		network need to give up their bandwidth shares to allow the flow some
	room for growth if the existing flows have been using all the		room for growth if the existing flows have been using all the
	bandwidth of the network. To increase this release of bandwidth by		bandwidth of the network. To increase this release of bandwidth by
	existing flows, the following mechanism called fast convergence		existing flows, the following mechanism called fast convergence
	SHOULD be implemented.		SHOULD be implemented.
	With fast convergence, when a loss event occurs, before a window		With fast convergence, when a loss event occurs, before a window
	reduction of congestion window, a flow remembers the last value of		reduction of congestion window, a flow remembers the last value of
	W_max before it updates W_max for the current loss event. Let us		W_max before it updates W_max for the current loss event. Let us
	call the last value of W_max to be W_last_max.		call the last value of W_max to be W_last_max.
	if (W_max < W_last_max){ // check downward trend		if (W_max < W_last_max){ // check downward trend
	W_last_max = W_max; // remember the last W_max		W_last_max = W_max; // remember the last W_max
	W_max = W_max*(2-beta)/2; // further reduce W_max		W_max = W_max*(1+beta_cubic)/2; // further reduce W_max
	} else { // check upward trend		} else { // check upward trend
	W_last_max = W_max // remember the last W_max		W_last_max = W_max // remember the last W_max
	}		}
	This allows W_max to be slightly less than the original W_max. Since		This allows W_max to be slightly less than the original W_max. Since
	flows spend most of time around their W_max, flows with larger		flows spend most of time around their W_max, flows with larger
	bandwidth shares tend to spend more time around the plateau allowing		bandwidth shares tend to spend more time around the plateau allowing
	more time for flows with smaller shares to increase their windows.		more time for flows with smaller shares to increase their windows.
	4. Discussion		4. Discussion
	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:
	(C*(4-beta)/4/beta)^0.25 * RTT^0.75 / p^0.75 (Eq. 5)		AVG_W_cubic = [C*(3+beta_cubic)/(4*(1-beta_cubic))]^0.25 *
			(RTT^0.75) / (p^0.75) (Eq. 5)
	With beta set to 0.2, the above formula is reduced to:		With beta_cubic set to 0.7, the above formula is reduced to:
	(C*3.8/0.8)^0.25 * RTT^0.75 / p^0.75 (Eq. 6)		AVG_W_cubic = (C*3.7/1.2)^0.25 * (RTT^0.75) / (p^0.75) (Eq. 6)
	We will determine the value of C in the following subsection using		We will determine the value of C in the following subsection using
	Eq. 6.		Eq. 6.
	4.1. Fairness to standard TCP		4.1. Fairness to standard TCP
	In environments where standard TCP is able to make reasonable use of		In environments where standard TCP is able to make reasonable use of
	the available bandwidth, CUBIC does not significantly change this		the available bandwidth, CUBIC does not significantly change this
	state.		state.
	skipping to change at page 9, line 10		skipping to change at page 9, line 17
	average window size of standard TCP, HSTCP, and CUBIC. The average		average window size of standard TCP, HSTCP, and CUBIC. The average
	window size of standard TCP and HSTCP is from [RFC3649]. The average		window size of standard TCP and HSTCP is from [RFC3649]. The average
	window size of CUBIC is calculated by using Eq. 6 and CUBIC TCP		window size of CUBIC is calculated by using Eq. 6 and CUBIC TCP
	friendly mode for three different values of C.		friendly mode for three different values of C.
	+----------+-------+--------+-------------+-------------+-----------+		+----------+-------+--------+-------------+-------------+-----------+
	| Loss | TCP | HSTCP | CUBIC | CUBIC | CUBIC |		| Loss | TCP | HSTCP | CUBIC | CUBIC | CUBIC |
	| Rate P | | | (C=0.04) | (C=0.4) | (C=4) |		| Rate P | | | (C=0.04) | (C=0.4) | (C=4) |
	+----------+-------+--------+-------------+-------------+-----------+		+----------+-------+--------+-------------+-------------+-----------+
	| 10^-2 | 12 | 12 | 12 | 12 | 12 |		| 10^-2 | 12 | 12 | 12 | 12 | 12 |
	| 10^-3 | 38 | 38 | 38 | 38 | 66 |		| 10^-3 | 38 | 38 | 38 | 38 | 59 |
	| 10^-4 | 120 | 263 | 120 | 209 | 371 |		| 10^-4 | 120 | 263 | 120 | 187 | 333 |
	| 10^-5 | 379 | 1795 | 660 | 1174 | 2087 |		| 10^-5 | 379 | 1795 | 593 | 1054 | 1874 |
	| 10^-6 | 1200 | 12279 | 3713 | 6602 | 11740 |		| 10^-6 | 1200 | 12279 | 3332 | 5926 | 10538 |
	| 10^-7 | 3795 | 83981 | 20878 | 37126 | 66022 |		| 10^-7 | 3795 | 83981 | 18740 | 33325 | 59261 |
	| 10^-8 | 12000 | 574356 | 117405 | 208780 | 371269 |		| 10^-8 | 12000 | 574356 | 105383 | 187400 | 333250 |
	+----------+-------+--------+-------------+-------------+-----------+		+----------+-------+--------+-------------+-------------+-----------+
	Response function of standard TCP, HSTCP, and CUBIC in networks with		Response function of standard TCP, HSTCP, and CUBIC in networks with
	RTT = 100ms. The average window size W is in MSS-sized segments.		RTT = 0.1 seconds. The average window size is in MSS-sized segments.
	Table 1		Table 1
	+--------+-----------+-----------+------------+-----------+---------+		+--------+-----------+-----------+------------+-----------+---------+
	| Loss | Average | Average | CUBIC | CUBIC | CUBIC |		| Loss | Average | Average | CUBIC | CUBIC | CUBIC |
	| Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) |		| Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) |
	+--------+-----------+-----------+------------+-----------+---------+		+--------+-----------+-----------+------------+-----------+---------+
	| 10^-2 | 12 | 12 | 12 | 12 | 12 |		| 10^-2 | 12 | 12 | 12 | 12 | 12 |
	| 10^-3 | 38 | 38 | 38 | 38 | 38 |		| 10^-3 | 38 | 38 | 38 | 38 | 38 |
	| 10^-4 | 120 | 263 | 120 | 120 | 120 |		| 10^-4 | 120 | 263 | 120 | 120 | 120 |
	| 10^-5 | 379 | 1795 | 379 | 379 | 379 |		| 10^-5 | 379 | 1795 | 379 | 379 | 379 |
	| 10^-6 | 1200 | 12279 | 1200 | 1200 | 2087 |		| 10^-6 | 1200 | 12279 | 1200 | 1200 | 1874 |
	| 10^-7 | 3795 | 83981 | 3795 | 6603 | 11740 |		| 10^-7 | 3795 | 83981 | 3795 | 5926 | 10538 |
	| 10^-8 | 12000 | 574356 | 20878 | 37126 | 66022 |		| 10^-8 | 12000 | 574356 | 18740 | 33325 | 59261 |
	+--------+-----------+-----------+------------+-----------+---------+		+--------+-----------+-----------+------------+-----------+---------+
	Response function of standard TCP, HSTCP, and CUBIC in networks with		Response function of standard TCP, HSTCP, and CUBIC in networks with
	RTT = 10ms. The average window size W is in MSS-sized segments.		RTT = 0.01 seconds. The average window size is in MSS-sized
			segments.
	Table 2		Table 2
	Both tables show that CUBIC with any of these three C values is more		Both tables show that CUBIC with any of these three C values is more
	friendly to TCP than HSTCP, especially in networks with a short RTT		friendly to TCP than HSTCP, especially in networks with a short RTT
	where TCP performs reasonably well. For example, in a network with		where TCP performs reasonably well. For example, in a network with
	RTT = 10ms and p=10^-6, TCP has an average window of 1200 packets.		RTT = 0.01 seconds and p=10^-6, TCP has an average window of 1200
	If the packet size is 1500 bytes, then TCP can achieve an average		packets. If the packet size is 1500 bytes, then TCP can achieve an
	rate of 1.44 Gbps. In this case, CUBIC with C=0.04 or C=0.4 achieves		average rate of 1.44 Gbps. In this case, CUBIC with C=0.04 or C=0.4
	exactly the same rate as Standard TCP, whereas HSTCP is about ten		achieves exactly the same rate as Standard TCP, whereas HSTCP is
	times more aggressive than Standard TCP.		about ten times more aggressive than Standard TCP.
	We can see that C determines the aggressiveness of CUBIC in competing		We can see that C determines the aggressiveness of CUBIC in competing
	with other protocols for the bandwidth. CUBIC is more friendly to		with other protocols for the bandwidth. CUBIC is more friendly to
	the Standard TCP, if the value of C is lower. However, we do not		the Standard TCP, if the value of C is lower. However, we do not
	recommend to set C to a very low value like 0.04, since CUBIC with a		recommend to set C to a very low value like 0.04, since CUBIC with a
	low C cannot efficiently use the bandwidth in long RTT and high		low C cannot efficiently use the bandwidth in long RTT and high
	bandwidth networks. Based on these observations, we find C=0.4 gives		bandwidth networks. Based on these observations, we find C=0.4 gives
	a good balance between TCP-friendliness and aggressiveness of window		a good balance between TCP-friendliness and aggressiveness of window
	growth. Therefore, C SHOULD be set to 0.4. With C set to 0.4, Eq. 6		growth. Therefore, C SHOULD be set to 0.4. With C set to 0.4, Eq. 6
	is reduced to:		is reduced to:
	1.17 * RTT^0.75 / p^0.75 (Eq. 7)		AVG_W_cubic = 1.054 * (RTT^0.75) / (p^0.75) (Eq. 7)
	Eq. 7 is then used in the next subsection to show the scalability of		Eq. 7 is then used in the next subsection to show the scalability of
	CUBIC.		CUBIC.
	4.2. Using Spare Capacity		4.2. Using Spare Capacity
	CUBIC uses a more aggressive window growth function than Standard TCP		CUBIC uses a more aggressive window growth function than Standard TCP
	under long RTT and high bandwidth networks.		under long RTT and high bandwidth networks.
	The following table shows that to achieve 10Gbps rate, standard TCP		The following table shows that to achieve 10Gbps rate, standard TCP
	requires a packet loss rate of 2.0e-10, while CUBIC requires a packet		requires a packet loss rate of 2.0e-10, while CUBIC requires a packet
	loss rate of 3.4e-8.		loss rate of 2.9e-8.
	+------------------+-----------+---------+---------+---------+		+------------------+-----------+---------+---------+---------+
	| Throughput(Mbps) | Average W | TCP P | HSTCP P | CUBIC P |		| Throughput(Mbps) | Average W | TCP P | HSTCP P | CUBIC P |
	+------------------+-----------+---------+---------+---------+		+------------------+-----------+---------+---------+---------+
	| 1 | 8.3 | 2.0e-2 | 2.0e-2 | 2.0e-2 |		| 1 | 8.3 | 2.0e-2 | 2.0e-2 | 2.0e-2 |
	| 10 | 83.3 | 2.0e-4 | 3.9e-4 | 3.3e-4 |		| 10 | 83.3 | 2.0e-4 | 3.9e-4 | 2.9e-4 |
	| 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.6e-5 |		| 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.4e-5 |
	| 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 7.3e-7 |		| 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 6.3e-7 |
	| 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 3.4e-8 |		| 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 2.9e-8 |
	+------------------+-----------+---------+---------+---------+		+------------------+-----------+---------+---------+---------+
	Required packet loss rate for Standard TCP, HSTCP, and CUBIC to		Required packet loss rate for Standard TCP, HSTCP, and CUBIC to
	achieve a certain throughput. We use 1500-byte packets and an RTT of		achieve a certain throughput. We use 1500-byte packets and an RTT of
	0.1 seconds.		0.1 seconds.
	Table 3		Table 3
	Our test results in [HKLRX06] indicate that CUBIC uses the spare		Our test results in [HKLRX06] indicate that CUBIC uses the spare
	bandwidth left unused by existing Standard TCP flows in the same		bandwidth left unused by existing Standard TCP flows in the same
	skipping to change at page 12, line 19		skipping to change at page 12, line 27
	2014-2018 under grant agreement No. 644866 (SSICLOPS). This document		2014-2018 under grant agreement No. 644866 (SSICLOPS). This document
	reflects only the authors' views and the European Commission is not		reflects only the authors' views and the European Commission is not
	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.
	8. References		8. References
	8.1. Normative References		8.1. Normative References
	[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP		[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
	Selective Acknowledgment Options", RFC 2018, October 1996.		Selective Acknowledgment Options", RFC 2018,
			DOI 10.17487/RFC2018, October 1996,
			<http://www.rfc-editor.org/info/rfc2018>.
	[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, March 1997.		Requirement Levels", BCP 14, RFC 2119,
			DOI 10.17487/RFC2119, March 1997,
			<http://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, December 2003.		RFC 3649, DOI 10.17487/RFC3649, December 2003,
			<http://www.rfc-editor.org/info/rfc3649>.
	[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC		[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
	4960, September 2007.		RFC 4960, DOI 10.17487/RFC4960, September 2007,
			<http://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, August 2007.		Control Algorithms", BCP 133, RFC 5033,
			DOI 10.17487/RFC5033, August 2007,
			<http://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", RFC		Friendly Rate Control (TFRC): Protocol Specification",
	5348, September 2008.		RFC 5348, DOI 10.17487/RFC5348, September 2008,
			<http://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, September 2009.		Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
			<http://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, April 2012.		RFC 6582, DOI 10.17487/RFC6582, April 2012,
			<http://www.rfc-editor.org/info/rfc6582>.
	8.2. Informative References		8.2. 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.
	skipping to change at page 13, line 31		skipping to change at page 14, line 5
	Communication Review , April 2003.		Communication Review , April 2003.
	[LS08] Leith, D. and R. Shorten, "H-TCP: TCP Congestion Control		[LS08] Leith, D. and R. Shorten, "H-TCP: TCP Congestion Control
	for High Bandwidth-Delay Product Paths", Internet-draft		for High Bandwidth-Delay Product Paths", Internet-draft
	draft-leith-tcp-htcp-06 , April 2008.		draft-leith-tcp-htcp-06 , April 2008.
	[XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase		[XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase
	Congestion Control for Fast, Long Distance Networks", In		Congestion Control for Fast, Long Distance Networks", In
	Proceedings of IEEE INFOCOM , March 2004.		Proceedings of IEEE INFOCOM , March 2004.
	Appendix A. ToDo List
	o Incorporate ICCRG's feedback (see
	http://trac.tools.ietf.org/group/irtf/trac/wiki/ICCRG_cubic)
	o ncorporate feedback from Neal Cardwell (see http://www.ietf.org/
	mail-archive/web/tcpm/current/msg09508.html)
	Authors' Addresses		Authors' Addresses
	Injong Rhee		Injong Rhee
	North Carolina State University		North Carolina State University
	Department of Computer Science		Department of Computer Science
	Raleigh, NC 27695-7534		Raleigh, NC 27695-7534
	US		US
	Email: rhee@ncsu.edu		Email: rhee@ncsu.edu
	Lisong Xu		Lisong Xu
	University of Nebraska-Lincoln		University of Nebraska-Lincoln
	Department of Computer Science and Engineering		Department of Computer Science and Engineering
	Lincoln, NE 68588-01150		Lincoln, NE 68588-01150
	US		US
	Email: xu@unl.edu		Email: xu@unl.edu
	Sangtae Ha		Sangtae Ha
	University of Colorado at Boulder		University of Colorado at Boulder
End of changes. 53 change blocks.
	106 lines changed or deleted		120 lines changed or added
This html diff was produced by rfcdiff 1.42. The latest version is available from http://tools.ietf.org/tools/rfcdiff/