< draft-floyd-tcp-slowstart-01.txt		draft-floyd-tcp-slowstart-02.txt >
	Internet Engineering Task Force Sally Floyd		Internet Engineering Task Force Sally Floyd
	INTERNET DRAFT ICSI		INTERNET DRAFT ICSI
	draft-floyd-tcp-slowstart-01.txt August, 2002		draft-floyd-tcp-slowstart-02.txt June, 2003
	Limited Slow-Start for TCP with Large Congestion Windows		Limited Slow-Start for TCP with Large Congestion Windows
	Status of this Memo		Status of this Memo
	This document is an Internet-Draft and is in full conformance with		This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC2026.		all provisions of Section 10 of RFC2026.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	skipping to change at page 1, line 32 ¶		skipping to change at page 1, line 32 ¶
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	The list of current Internet-Drafts can be accessed at		The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/1id-abstracts.txt		http://www.ietf.org/ietf/1id-abstracts.txt
	The list of Internet-Draft Shadow Directories can be accessed at		The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.		http://www.ietf.org/shadow.html.
	Abstract		Abstract
	This note proposes a modification for TCP's slow-start for use with		This note describes an optional modification for TCP's slow-start for
	TCP connections with large congestion windows. For TCP connections		use with TCP connections with large congestion windows. For TCP
	that are able to use congestion windows of thousands (or tens of		connections that are able to use congestion windows of thousands (or
	thousands) of MSS-sized segments (for MSS the sender's MAXIMUM		tens of thousands) of MSS-sized segments (for MSS the sender's
	SEGMENT SIZE), the current slow-start procedure can result in		MAXIMUM SEGMENT SIZE), the current slow-start procedure can result in
	increasing the congestion window by thousands of segments in a single		increasing the congestion window by thousands of segments in a single
	round-trip time. Such an increase can easily result in thousands of		round-trip time. Such an increase can easily result in thousands of
	packets being dropped in one round-trip time. This is often counter-		packets being dropped in one round-trip time. This is often counter-
	productive for the TCP flow itself, and is also hard on the rest of		productive for the TCP flow itself, and is also hard on the rest of
	the traffic sharing the congested link. This note proposes Limited		the traffic sharing the congested link. This note describes Limited
	Slow-Start as an optional mechanism for limiting the number of		Slow-Start as an optional mechanism for limiting the number of
	segments by which the congestion window is increased for one window		segments by which the congestion window is increased for one window
	of data during slow-start, in order to improve performance for TCP		of data during slow-start, in order to improve performance for TCP
	connections with large congestion windows.		connections with large congestion windows.
			Changes from draft-floyd-tcp-slowstart-01.txt:
			* Minor changes in language to submit for an Experimental RFC.
	Changes from draft-floyd-tcp-slowstart-00.txt:		Changes from draft-floyd-tcp-slowstart-00.txt:
	* Small changes in presentation.		* Small changes in presentation.
	* The addition of a section of Experiments.		* The addition of a section of Experiments.
	* More citations to related work.		* More citations to related work.
	1. Introduction		1. Introduction
	This note proposes a modification for TCP's slow-start for use with		This note describes an optional modification for TCP's slow-start for
	TCP connections with large congestion windows. For TCP connections		use with TCP connections with large congestion windows. For TCP
	that are able to use congestion windows of thousands (or tens of		connections that are able to use congestion windows of thousands (or
	thousands) of MSS-sized segments (for MSS the sender's MAXIMUM		tens of thousands) of MSS-sized segments (for MSS the sender's
	SEGMENT SIZE), the current slow-start procedure can result in		MAXIMUM SEGMENT SIZE), the current slow-start procedure can result in
	increasing the congestion window by thousands of segments in a single		increasing the congestion window by thousands of segments in a single
	round-trip time. Such an increase can easily result in thousands of		round-trip time. Such an increase can easily result in thousands of
	packets being dropped in one round-trip time. This is often counter-		packets being dropped in one round-trip time. This is often counter-
	productive for the TCP flow itself, and is also hard on the rest of		productive for the TCP flow itself, and is also hard on the rest of
	the traffic sharing the congested link. This note proposes Limited		the traffic sharing the congested link. This note describes Limited
	Slow-Start, limiting the number of segments by which the congestion		Slow-Start, limiting the number of segments by which the congestion
	window is increased for one window of data during slow-start, in		window is increased for one window of data during slow-start, in
	order to improve performance for TCP connections with large		order to improve performance for TCP connections with large
	congestion windows.		congestion windows.
	When slow-start results in a large increase in the congestion window		When slow-start results in a large increase in the congestion window
	in one round-trip time, a large number of packets might be dropped in		in one round-trip time, a large number of packets might be dropped in
	the network (even with carefully-tuned active queue management		the network (even with carefully-tuned active queue management
	mechanisms in the routers). This drop of a large number of packets		mechanisms in the routers). This drop of a large number of packets
	in the network can result in unnecessary retransmit timeouts for the		in the network can result in unnecessary retransmit timeouts for the
	skipping to change at page 3, line 28 ¶		skipping to change at page 3, line 31 ¶
	each arriving ACK, for K = int(cwnd/(0.5 max_ssthresh)), instead of		each arriving ACK, for K = int(cwnd/(0.5 max_ssthresh)), instead of
	by 1 MSS as in the standard slow-start [RFC2581].		by 1 MSS as in the standard slow-start [RFC2581].
	When		When
	ssthresh < cwnd,		ssthresh < cwnd,
	slow-start is exited, and the sender is in the Congestion Avoidance		slow-start is exited, and the sender is in the Congestion Avoidance
	phase.		phase.
	Our current recommendation would be for max_ssthresh to be set to 100		Our recommendation would be for max_ssthresh to be set to 100 MSS.
	MSS. (This is illustrated in the NS simulator, for snapshots after		(This is illustrated in the NS simulator, for snapshots after May 1,
	May 1, 2002, in the tests "./test-all-tcpHighspeed tcp1A" and		2002, in the tests "./test-all-tcpHighspeed tcp1A" and "./test-all-
	"./test-all-tcpHighspeed tcpHighspeed1" in the subdirectory		tcpHighspeed tcpHighspeed1" in the subdirectory "tcl/lib". Setting
	"tcl/lib".) However, max_ssthresh is an optional, configurable		max_ssthresh to Infinity causes the TCP connection in NS not to use
	parameter. TCP implementations that do not wish to use Limited Slow-		Limited Slow-Start.)
	Start would set max_ssthresh to Infinity.
	With Limited Slow-Start, when the congestion window is greater than		With Limited Slow-Start, when the congestion window is greater than
	max_ssthresh the window is increased by at most 1/2 MSS for each		max_ssthresh the window is increased by at most 1/2 MSS for each
	arriving ACK, when the congestion window is greater than 1.5		arriving ACK, when the congestion window is greater than 1.5
	max_ssthresh the window is increased by at most 1/3 MSS for each		max_ssthresh the window is increased by at most 1/3 MSS for each
	arriving ACK, and so on.		arriving ACK, and so on.
	With Limited Slow-Start it takes:		With Limited Slow-Start it takes:
	log(max_ssthresh)		log(max_ssthresh)
	skipping to change at page 4, line 23 ¶		skipping to change at page 4, line 27 ¶
	By limiting the maximum increase in the congestion window in a round-		By limiting the maximum increase in the congestion window in a round-
	trip time, Limited Slow-Start can reduce the number of drops during		trip time, Limited Slow-Start can reduce the number of drops during
	slow-start, and improve the performance of TCP connections with large		slow-start, and improve the performance of TCP connections with large
	congestion windows.		congestion windows.
	3. Experimental Results		3. Experimental Results
	Tom Dunigan has added Limited Slow-Start to the Linux 2.4.16 Web100		Tom Dunigan has added Limited Slow-Start to the Linux 2.4.16 Web100
	kernel, and performed experiments comparing TCP with and without		kernel, and performed experiments comparing TCP with and without
	Limited Slow-Start [D02]. Preliminary results show improved		Limited Slow-Start [D02]. Results so far show improved performance
	performance for TCPs using Limited Slow-Start. There are also		for TCPs using Limited Slow-Start. There are also several
	several experiments comparing different values for max_ssthresh.		experiments comparing different values for max_ssthresh.
	More experiments will be needed to more fully evaluate Limited Slow-
	Start.
	4. Related Proposals		4. Related Proposals
	There has been considerable research on mechanisms for the TCP sender		There has been considerable research on mechanisms for the TCP sender
	to learn about the limitations of the available bandwidth, and to		to learn about the limitations of the available bandwidth, and to
	exit slow-start before receiving a congestion indication from the		exit slow-start before receiving a congestion indication from the
	network [VEGAS,H96]. Other proposals set TCP's slow-start parameter		network [VEGAS,H96]. Other proposals set TCP's slow-start parameter
	ssthresh based on information from previous TCP connections to the		ssthresh based on information from previous TCP connections to the
	same destination [WS95,G00]. This draft proposes a simple limitation		same destination [WS95,G00]. This draft proposes a simple limitation
	on slow-start that can be effective in some cases even in the absence		on slow-start that can be effective in some cases even in the absence
	of such mechanisms. The max_ssthresh parameter does not replace		of such mechanisms. The max_ssthresh parameter does not replace
	ssthresh, but is an additional parameter. Thus, Limited Slow-Start		ssthresh, but is an additional parameter. Thus, Limited Slow-Start
	could be used in addition to mechanisms for setting ssthresh.		could be used in addition to mechanisms for setting ssthresh.
	Rate-based pacing has also been proposed to improve the performance		Rate-based pacing has also been proposed to improve the performance
	of TCP during slow-start [VH97,AD98,KCRP99,ASA00]. We believe that		of TCP during slow-start [VH97,AD98,KCRP99,ASA00]. We believe that
	rate-based pacing could be of significant benefit, and could be used		rate-based pacing could be of significant benefit, and could be used
	in addition to the Limited Slow-Start in this proposal.		in addition to the Limited Slow-Start in this proposal.
	Appropriate Byte Counting [A01] proposes that TCP increase its		Appropriate Byte Counting [RFC3465] proposes that TCP increase its
	congestion window as a function of the number of bytes acknowledged,		congestion window as a function of the number of bytes acknowledged,
	rather than as a function of the number of ACKs received.		rather than as a function of the number of ACKs received.
	Appropriate Byte Counting is largely orthogonal to this proposal for		Appropriate Byte Counting is largely orthogonal to this proposal for
	Limited Slow-Start.		Limited Slow-Start.
	Limited Slow-Start is also orthogonal to other proposals to change		Limited Slow-Start is also orthogonal to other proposals to change
	mechanisms for exiting slow-start. For example, FACK TCP includes an		mechanisms for exiting slow-start. For example, FACK TCP includes an
	overdamping mechanism to decrease the congestion window somewhat more		overdamping mechanism to decrease the congestion window somewhat more
	aggressively when a loss occurs during slow-start [MM96]. It is also		aggressively when a loss occurs during slow-start [MM96]. It is also
	true that larger values for the MSS would reduce the size of the		true that larger values for the MSS would reduce the size of the
	skipping to change at page 5, line 31 ¶		skipping to change at page 5, line 31 ¶
	with Tom Kelly, who has used a similar modified slow-start in his own		with Tom Kelly, who has used a similar modified slow-start in his own
	research with congestion control for high-bandwidth connections. We		research with congestion control for high-bandwidth connections. We
	also thank Tom Dunigan for his experiments with Limited Slow-Start.		also thank Tom Dunigan for his experiments with Limited Slow-Start.
	We thank Andrei Gurtov, Reiner Ludwig, members of the End-to-End		We thank Andrei Gurtov, Reiner Ludwig, members of the End-to-End
	Research Group, and members of the Transport Area Working Group, for		Research Group, and members of the Transport Area Working Group, for
	feedback on this document.		feedback on this document.
	6. Normative References		6. Normative References
	[A01] Mark Allman, "TCP Congestion Control with Appropriate Byte
	Counting", draft-allman-tcp-abc-02.txt, internet-draft, work-in-
	progress, November 2001.
	[RFC2581] M. Allman and V. Paxson, "TCP Congestion Control", RFC		[RFC2581] M. Allman and V. Paxson, "TCP Congestion Control", RFC
	2581, April 1999.		2581, April 1999.
			[RFC3465] Mark Allman, "TCP Congestion Control with Appropriate Byte
			Counting", RFC 3465, February 2003.
	7. Informative References		7. Informative References
	[AD98] Mohit Aron and Peter Druschel, "TCP: Improving Start-up		[AD98] Mohit Aron and Peter Druschel, "TCP: Improving Start-up
	Dynamics by Adaptive Timers and Congestion Control"", TR98-318, Rice		Dynamics by Adaptive Timers and Congestion Control"", TR98-318, Rice
	University, 1998. URL "http://cs-		University, 1998. URL "http://cs-
	tr.cs.rice.edu/Dienst/UI/2.0/Describe/ncstrl.rice_cs/TR98-318/".		tr.cs.rice.edu/Dienst/UI/2.0/Describe/ncstrl.rice_cs/TR98-318/".
	[ASA00] A. Aggarwal, S. Savage, and T. Anderson, "Understanding the		[ASA00] A. Aggarwal, S. Savage, and T. Anderson, "Understanding the
	Performance of TCP Pacing", Proceedings of the 2000 IEEE Infocom		Performance of TCP Pacing", Proceedings of the 2000 IEEE Infocom
	Conference, Tel-Aviv, Israel, March, 2000. URL		Conference, Tel-Aviv, Israel, March, 2000. URL
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