draft-ietf-tcpm-proportional-rate-reduction-00.txt		draft-ietf-tcpm-proportional-rate-reduction-01.txt
	TCP Maintenance Working Group M. Mathis		TCP Maintenance Working Group M. Mathis
	Internet-Draft N. Dukkipati		Internet-Draft N. Dukkipati
	Intended status: Experimental Y. Cheng		Intended status: Experimental Y. Cheng
	Expires: April 26, 2012 Google, Inc		Expires: August 27, 2012 Google, Inc
	October 24, 2011		February 24, 2012
	Proportional Rate Reduction for TCP		Proportional Rate Reduction for TCP
	draft-ietf-tcpm-proportional-rate-reduction-00.txt		draft-ietf-tcpm-proportional-rate-reduction-01.txt
	Abstract		Abstract
	This document describes an experimental algorithm, Proportional Rate		This document describes an experimental algorithm, Proportional Rate
	Reduction (PPR) to improve the accuracy of the amount of data sent by		Reduction (PPR) to improve the accuracy of the amount of data sent by
	TCP during loss recovery. Standard Congestion Control requires that		TCP during loss recovery. Standard Congestion Control requires that
	TCP and other protocols reduce their congestion window in response to		TCP and other protocols reduce their congestion window in response to
	losses. This window reduction naturally occurs in the same round		losses. This window reduction naturally occurs in the same round
	trip as the data retransmissions to repair the losses, and is		trip as the data retransmissions to repair the losses, and is
	implemented by choosing not to transmit any data in response to some		implemented by choosing not to transmit any data in response to some
	ACKs arriving from the receiver. Two widely deployed algorithms are		ACKs arriving from the receiver. Two widely deployed algorithms are
	used to implement this window reduction: Fast Recovery and Rate		used to implement this window reduction: Fast Recovery and Rate
	Halving. Both algorithms are needlessly fragile under a number of		Halving. Both algorithms are needlessly fragile under a number of
	conditions, particularly when there is a burst of losses that such		conditions, particularly when there is a burst of losses that such
	that the number of ACKs returning to the sender is small.		that the number of ACKs returning to the sender is small.
	Proportional Rate Reduction avoids these excess window reductions		Proportional Rate Reduction minimizes these excess window reductions
	such that at the end of recovery the actual window size will be as		such that at the end of recovery the actual window size will be as
	close as possible to ssthresh, the window size determined by the		close as possible to ssthresh, the window size determined by the
	congestion control algorithm. It is patterned after Rate Halving,		congestion control algorithm. It is patterned after Rate Halving,
	but using the fraction that is appropriate for target window chosen		but using the fraction that is appropriate for target window chosen
	by the congestion control algorithm.		by the congestion control algorithm.
	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.
	skipping to change at page 1, line 48		skipping to change at page 1, line 48
	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 April 26, 2012.		This Internet-Draft will expire on August 27, 2012.
	Copyright Notice		Copyright Notice
	Copyright (c) 2011 IETF Trust and the persons identified as the		Copyright (c) 2012 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
	skipping to change at page 3, line 28		skipping to change at page 3, line 28
	It is fragile because it can not compensate for the implicit window		It is fragile because it can not compensate for the implicit window
	reduction caused by the losses themselves, and is exposed to		reduction caused by the losses themselves, and is exposed to
	timeouts. For example if half of the data or ACKs are lost, Fast		timeouts. For example if half of the data or ACKs are lost, Fast
	Recovery's expected behavior would be wait for half window of ACKs to		Recovery's expected behavior would be wait for half window of ACKs to
	pass and then not receive any ACKs for the recovery and suffer a		pass and then not receive any ACKs for the recovery and suffer a
	timeout.		timeout.
	The rate-halving algorithm improves this situation by sending data on		The rate-halving algorithm improves this situation by sending data on
	alternate ACKs during recovery, such that after one RTT the window		alternate ACKs during recovery, such that after one RTT the window
	has been halved. Rate-having is implemented in Linux after only		has been halved. Rate-having is implemented in Linux after only
	being informally published [RHweb], including from an uncompleted		being informally published [RHweb], including an uncompleted
	Internet-Draft [RHID]. Rate-halving does not adequately compensate		Internet-Draft [RHID]. Rate-halving does not adequately compensate
	for the implicit window reduction caused by the losses and also		for the implicit window reduction caused by the losses and assumes a
	assumes a 50% window reduction, which was completely standard at the		50% window reduction, which was completely standard at the time it
	time it was written, but not appropriate for modern congestion		was written, but not appropriate for modern congestion control
	control algorithms such as Cubic [CUBIC], which can reduce the window		algorithms such as Cubic [CUBIC], which can reduce the window by less
	by less than 50%. As a consequence rate-halving often allows the		than 50%. As a consequence rate-halving often allows the window to
	window to fall further than necessary, reducing performance and		fall further than necessary, reducing performance and increasing the
	increasing the risk of timeouts if there are additional losses.		risk of timeouts if there are additional losses.
	Proportional Rate Reduction (PPR) avoids these excess window		Proportional Rate Reduction (PPR) avoids these excess window
	reductions such that at the end of recovery the actual window size		reductions such that at the end of recovery the actual window size
	will be as close as possible to, ssthresh, the window size determined		will be as close as possible to, ssthresh, the window size determined
	by the congestion control algorithm. It is patterned after Rate		by the congestion control algorithm. It is patterned after Rate
	Halving, but using the fraction that is appropriate for target window		Halving, but using the fraction that is appropriate for target window
	chosen by the congestion control algorithm. During PRR one of two		chosen by the congestion control algorithm. During PRR one of two
	additional reduction bound algorithms limits the total window		additional reduction bound algorithms limits the total window
	reduction due to all mechanisms, including application stalls and the		reduction due to all mechanisms, including application stalls and the
	losses themselves.		losses themselves.
	We describe two slightly different reduction bound algorithms:		We describe two slightly different reduction bound algorithms:
	conservative reduction bound (CRB), which is strictly packet		conservative reduction bound (CRB), which is strictly packet
	conserving; and a slow start reduction bound (SSRB), which is more		conserving; and a slow start reduction bound (SSRB), which is more
	aggressive than CRB by at most one segment per ACK. PRR-CRB meets		aggressive than CRB by at most one segment per ACK. PRR-CRB meets
	the strong conservative bound described in Appendix A, however in		the strong conservative bound described in Appendix A, however in
	real networks it does not perform as well as the algorithms described		real networks it does not perform as well as the algorithms described
	in RFC 3517, which prove to be non-conservative in a statistically		in RFC 3517, which prove to be non-conservative in a significant
	significant number of cases. SSRB offers a compromise by allowing		number of cases. SSRB offers a compromise by allowing TCP to send
	TCP to send one additional segment per ACK relative to CRB in some		one additional segment per ACK relative to CRB in some situations.
	situations. Although SSRB is less aggressive than RFC 3517		Although SSRB is less aggressive than RFC 3517 (transmitting fewer
	(transmitting fewer segments or taking more time to transmit them) it		segments or taking more time to transmit them) it outperforms it, due
	outperforms it, due to the lower probability of additional losses		to the lower probability of additional losses during recovery.
	during recovery.
	PRR and both reduction bounds are based on common design principles,		PRR and both reduction bounds are based on common design principles,
	derived from Van Jacobson's packet conservation principle: segments		derived from Van Jacobson's packet conservation principle: segments
	delivered to the receiver are used as the clock to trigger sending		delivered to the receiver are used as the clock to trigger sending
	the same number of segments back into the network. As much as		the same number of segments back into the network. As much as
	possible Proportional Rate Reduction and the reduction bound rely on		possible Proportional Rate Reduction and the reduction bound rely on
	this self clock process, and are only slightly affected by the		this self clock process, and are only slightly affected by the
	accuracy of other estimators, such as pipe [RFC3517] and cwnd. This		accuracy of other estimators, such as pipe [RFC3517] and cwnd. This
	is what gives the algorithms their precision in the presence of		is what gives the algorithms their precision in the presence of
	events that cause uncertainty in other estimators.		events that cause uncertainty in other estimators.
	skipping to change at page 4, line 45		skipping to change at page 4, line 44
	The following terms, parameters and state variables are used as they		The following terms, parameters and state variables are used as they
	are defined in earlier documents:		are defined in earlier documents:
	RFC 3517: covered (as in "covered sequence numbers")		RFC 3517: covered (as in "covered sequence numbers")
	RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size		RFC 5681: duplicate ACK, FlightSize, Sender Maximum Segment Size
	(SMSS)		(SMSS)
	Voluntary window reductions: choosing not to send data in response to		Voluntary window reductions: choosing not to send data in response to
	some ACKs, for the purpose of reducing the sending window size or		some ACKs, for the purpose of reducing the sending window size and
	data rate.		data rate.
	We define some additional variables:		We define some additional variables:
	SACKd: The total number of bytes that the scoreboard indicates have		SACKd: The total number of bytes that the scoreboard indicates have
	been delivered to the receiver. This can be computed by scanning the		been delivered to the receiver. This can be computed by scanning the
	scoreboard and counting the total number of bytes covered by all sack		scoreboard and counting the total number of bytes covered by all sack
	blocks. If SACK is not in use, SACKd is not defined.		blocks. If SACK is not in use, SACKd is not defined.
	DeliveredData: The total number of bytes that the current ACK		DeliveredData: The total number of bytes that the current ACK
	skipping to change at page 6, line 16		skipping to change at page 6, line 16
	prr_delivered += DeliveredData		prr_delivered += DeliveredData
	pipe = (RFC 3517 pipe algorithm)		pipe = (RFC 3517 pipe algorithm)
	if (pipe > ssthresh) {		if (pipe > ssthresh) {
	// Proportional Rate Reduction		// Proportional Rate Reduction
	sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out		sndcnt = CEIL(prr_delivered * ssthresh / RecoverFS) - prr_out
	} else {		} else {
	// Two version of the reduction bound		// Two version of the reduction bound
	if (conservative) { // PRR+CRB		if (conservative) { // PRR+CRB
	limit = prr_delivered - prr_out		limit = prr_delivered - prr_out
	} else { // PRR+SSRB		} else { // PRR+SSRB
	limit = MAX(prr_delivered - prr_out, DeliveredData) + 1		limit = MAX(prr_delivered - prr_out, DeliveredData) + MSS
	}		}
	// Attempt to catch up, as permitted by limit		// Attempt to catch up, as permitted by limit
	sndcnt = MIN(ssthresh - pipe, limit)		sndcnt = MIN(ssthresh - pipe, limit)
	}		}
	On any data transmission or retransmission:		On any data transmission or retransmission:
	prr_out += (data sent) // strictly less than or equal to sndcnt		prr_out += (data sent) // strictly less than or equal to sndcnt
	The following examples will make these algorithms clearer.
	3.1. Examples		3.1. Examples
	We illustrate these algorithms by showing their different behaviors		We illustrate these algorithms by showing their different behaviors
	for two scenarios: TCP experiencing either a single loss or a burst		for two scenarios: TCP experiencing either a single loss or a burst
	of 15 consecutive losses. In all cases we assume bulk data, standard		of 15 consecutive losses. In all cases we assume bulk data, standard
	AIMD congestion control (the ssthresh is set to FlightSize/2) and		AIMD congestion control and cwnd = FlightSize = pipe = 20 segments,
	cwnd = FlightSize = pipe = 20 segments, so ssthresh will be set to 10		so ssthresh will be set to 10 at the beginning of recovery. We also
	at the beginning of recovery. We also assume standard Fast		assume standard Fast Retransmit and Limited Transmit, so we send two
	Retransmit and Limited Transmit, so we send two new segments followed		new segments followed by one retransmit on the first 3 duplicate ACKs
	by one retransmit on the first 3 duplicate ACKs after the losses.		after the losses.
	Each of the diagrams below shows the per ACK response to the first		Each of the diagrams below shows the per ACK response to the first
	round trip for the various recovery algorithms when the zeroth		round trip for the various recovery algorithms when the zeroth
	segment is lost. The top line indicates the transmitted segment		segment is lost. The top line indicates the transmitted segment
	number triggering the ACKs, with an X for the lost segment. "cwnd"		number triggering the ACKs, with an X for the lost segment. "cwnd"
	and "pipe" indicate the values of these algorithms after processing		and "pipe" indicate the values of these algorithms after processing
	each returning ACK. "Sent" indicates how much 'N'ew or		each returning ACK. "Sent" indicates how much 'N'ew or
	'R'etransmitted data would be sent. Note that the algorithms for		'R'etransmitted data would be sent. Note that the algorithms for
	deciding which data to send are out of scope of this document.		deciding which data to send are out of scope of this document.
	skipping to change at page 9, line 22		skipping to change at page 9, line 22
	window reduction causes it to take excessively long to recover the		window reduction causes it to take excessively long to recover the
	losses and exposes it to additional timeouts.		losses and exposes it to additional timeouts.
	PRR-SSRB increases the window by exactly 1 segment per ACK until pipe		PRR-SSRB increases the window by exactly 1 segment per ACK until pipe
	rises to sshthresh during recovery. This is accomplished by setting		rises to sshthresh during recovery. This is accomplished by setting
	limit to one greater than the data reported to have been delivered to		limit to one greater than the data reported to have been delivered to
	the receiver on this ACK, implementing slowstart during recovery, and		the receiver on this ACK, implementing slowstart during recovery, and
	indicated by RB:d tagging in the figure. Although increasing the		indicated by RB:d tagging in the figure. Although increasing the
	window during recovery seems to be ill advised, it is important to		window during recovery seems to be ill advised, it is important to
	remember that this actually less aggressive than permitted by RFC		remember that this actually less aggressive than permitted by RFC
	5681, which sends the same quantity of extra data as a single burst		5681, which sends the same quantity of additional data as a single
	in response to the ACK that triggered Fast Retransmit		burst in response to the ACK that triggered Fast Retransmit
	Under less extreme conditions, when the total losses are smaller than		For less extreme events, where the total losses are smaller than the
	the difference between Flight Size and ssthresh, PRR-CRB and PRR-SSRB		difference between Flight Size and ssthresh, PRR-CRB and PRR-SSRB
	have identical behaviours.		have identical behaviours.
	4. Properties		4. Properties
	The following properties are common to both PRR-CRB and PRR-SSRB:		The following properties are common to both PRR-CRB and PRR-SSRB
			except as noted:
			Proportional Rate Reduction maintains TCPs ACK clocking across most
			recovery events, including burst losses. RFC 3517 can send large
			unclocked bursts following burst losses.
	Normally Proportional Rate Reduction will spread voluntary window		Normally Proportional Rate Reduction will spread voluntary window
	reductions out evenly across a full RTT. This has the potential to		reductions out evenly across a full RTT. This has the potential to
	generally reduce the burstiness of Internet traffic, and could be		generally reduce the burstiness of Internet traffic, and could be
	considered to be a type of soft pacing. Theoretically any pacing		considered to be a type of soft pacing. Hypothetically, any pacing
	increases the probability that different flows are interleaved,		increases the probability that different flows are interleaved,
	reducing the opportunity for ACK compression and other phenomena that		reducing the opportunity for ACK compression and other phenomena that
	increase traffic burstiness. However these effects have not been		increase traffic burstiness. However these effects have not been
	quantified.		quantified.
	If there are minimal losses, Proportional Rate Reduction will		If there are minimal losses, Proportional Rate Reduction will
	converge to exactly the target window chosen by the congestion		converge to exactly the target window chosen by the congestion
	control algorithm. Note that as TCP approaches the end of recovery		control algorithm. Note that as TCP approaches the end of recovery
	prr_delivered will approach RecoverFS and sndcnt will be computed		prr_delivered will approach RecoverFS and sndcnt will be computed
	such that prr_out approaches ssthresh.		such that prr_out approaches ssthresh.
	skipping to change at page 10, line 11		skipping to change at page 10, line 16
	recovery cause later voluntary reductions to be skipped. For small		recovery cause later voluntary reductions to be skipped. For small
	numbers of losses the window size ends at exactly the window chosen		numbers of losses the window size ends at exactly the window chosen
	by the congestion control algorithm.		by the congestion control algorithm.
	For burst losses, earlier voluntary window reductions can be undone		For burst losses, earlier voluntary window reductions can be undone
	by sending extra segments in response to ACKs arriving later during		by sending extra segments in response to ACKs arriving later during
	recovery. Note that as long as some voluntary window reductions are		recovery. Note that as long as some voluntary window reductions are
	not undone, the final value for pipe will be the same as ssthresh,		not undone, the final value for pipe will be the same as ssthresh,
	the target cwnd value chosen by the congestion control algorithm.		the target cwnd value chosen by the congestion control algorithm.
	Proportional Rate Reduction with either reduction round improves the		Proportional Rate Reduction with either reduction bound improves the
	situation when there are application stalls (e.g. when the sending		situation when there are application stalls (e.g. when the sending
	application does not queue data for transmission quickly enough or		application does not queue data for transmission quickly enough or
	the receiver stops advancing rwnd). When there is an application		the receiver stops advancing rwnd). When there is an application
	stall early during recovery prr_out will fall behind the sum of the		stall early during recovery prr_out will fall behind the sum of the
	transmissions permitted by sndcnt. The missed opportunities to send		transmissions permitted by sndcnt. The missed opportunities to send
	due to stalls are treated like banked voluntary window reductions:		due to stalls are treated like banked voluntary window reductions:
	specifically they cause prr_delivered-prr_out to be significantly		specifically they cause prr_delivered-prr_out to be significantly
	positive. If the application catches up while TCP is still in		positive. If the application catches up while TCP is still in
	recovery, TCP will send a partial window burst to catch up to exactly		recovery, TCP will send a partial window burst to catch up to exactly
	where it would have been, had the application never stalled.		where it would have been, had the application never stalled.
	skipping to change at page 10, line 50		skipping to change at page 11, line 6
	Since short term errors in pipe are smoothed out across multiple ACKs		Since short term errors in pipe are smoothed out across multiple ACKs
	and both Proportional Rate Reduction and the reduction bound converge		and both Proportional Rate Reduction and the reduction bound converge
	to the same final window, errors in the pipe estimator have less		to the same final window, errors in the pipe estimator have less
	impact on the final outcome.		impact on the final outcome.
	Under all conditions and sequences of events during recovery, PRR-CRB		Under all conditions and sequences of events during recovery, PRR-CRB
	strictly bounds the data transmitted to be equal to or less than the		strictly bounds the data transmitted to be equal to or less than the
	amount of data delivered to the receiver. We claim that this packet		amount of data delivered to the receiver. We claim that this packet
	conservation bound is the most aggressive algorithm that does not		conservation bound is the most aggressive algorithm that does not
	lead to additional forced losses in some environments. It has the		lead to additional forced losses in some environments. It has the
	property that if there is a standing queue at a bottleneck that is		property that if there is a standing queue at a bottleneck with no
	carrying no other traffic, the queue will maintain exactly constant		cross traffic, the queue will maintain exactly constant length for
	length for the duration of the recovery (except for +1/-1 fluctuation		the duration of the recovery, except for +1/-1 fluctuation due to
	due to differences in packet arrival and exit times) . See		differences in packet arrival and exit times. See Appendix A for a
	Appendix A for a detailed discussion of this property.		detailed discussion of this property.
	Although the packet Packet Conserving Bound in very appealing for a		Although the packet Packet Conserving Bound in very appealing for a
	number of reasons, our measurements summarized in Section 5		number of reasons, our measurements summarized in Section 5
	demonstrate that it is less aggressive and does not perform as well		demonstrate that it is less aggressive and does not perform as well
	as RFC3517, which permits large bursts of data when there are bursts		as RFC3517, which permits large bursts of data when there are bursts
	of losses. PRR-SSRB is a compromise that permits TCP to send one		of losses. PRR-SSRB is a compromise that permits TCP to send one
	extra segment per ACK as compared to the packet conserving bound.		extra segment per ACK as compared to the packet conserving bound.
	From the perspective of the packet conserving bound, PRR-SSRB does		From the perspective of the packet conserving bound, PRR-SSRB does
	indeed open the window during recovery, however it is significantly		indeed open the window during recovery, however it is significantly
	less aggressive than RFC3517 in the presence of burst losses.		less aggressive than RFC3517 in the presence of burst losses.
	5. Measurements		5. Measurements
	In a companion IMC11 paper [IMC11] we describe some measurements		In a companion IMC11 paper [IMC11] we describe some measurements
	comparing the various strategies for reducing the window during		comparing the various strategies for reducing the window during
	recovery. The results are summarized here.		recovery. The results are summarized here.
	The various window reduction algorithms and extensive instrumentation		The various window reduction algorithms and extensive instrumentation
	were all implemented in Linux 2.6. We used the uniform set of		were all implemented in Linux 2.6. We used the uniform set of
	algorithms present in the base Linux implementation, including CUBIC		algorithms present in the base Linux implementation, including CUBIC
	[CUBIC], limited transmit [LT], threshold transmit from [FACK] and		[CUBIC], limited transmit [RFC3742], threshold transmit from [FACK]
	lost retransmission detection algorithms. We confirmed that the		and lost retransmission detection algorithms. We confirmed that the
	behaviors of Rate Halving (the Linux default), RFC 3517 and PRR were		behaviors of Rate Halving (the Linux default), RFC 3517 and PRR were
	authentic to their respective specifications and that performance and		authentic to their respective specifications and that performance and
	features were comparable to the kernels in production use. The		features were comparable to the kernels in production use. The
	different window reduction algorithms were all present in the same		different window reduction algorithms were all present in the same
	kernel and could be selected with a sysctl, such that we had an		kernel and could be selected with a sysctl, such that we had an
	absolutely uniform baseline for comparing them.		absolutely uniform baseline for comparing them.
	Our experiments included an additional algorithm, PRR with an		Our experiments included an additional algorithm, PRR with an
	unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe		unlimited bound (PRR-UB), which sends ssthresh-pipe bursts when pipe
	falls below ssthresh. This behavior parallels RFC 3517.		falls below ssthresh. This behavior parallels RFC 3517.
	skipping to change at page 12, line 39		skipping to change at page 12, line 44
	bursts of data when there are large bursts of losses. PRR-SSRB is a		bursts of data when there are large bursts of losses. PRR-SSRB is a
	compromise that permits TCP to send one extra segment per ACK as		compromise that permits TCP to send one extra segment per ACK as
	relative to the packet conserving bound. From the perspective of the		relative to the packet conserving bound. From the perspective of the
	packet conserving bound, PRR-SSRB does indeed open the window during		packet conserving bound, PRR-SSRB does indeed open the window during
	recovery, however it is significantly less aggressive than RFC3517 in		recovery, however it is significantly less aggressive than RFC3517 in
	the presence of burst losses. Even so, it often out performs		the presence of burst losses. Even so, it often out performs
	RFC3517, because it avoids some of the self inflicted losses caused		RFC3517, because it avoids some of the self inflicted losses caused
	by bursts from RFC3517.		by bursts from RFC3517.
	At this time we see no reason not to test and deploy PRR-SSRB on a		At this time we see no reason not to test and deploy PRR-SSRB on a
	large scale, indeed it already is. Implementers worried about any		large scale. Implementers worried about any potential impact of
	potential impact of raising the window during recovery may want to		raising the window during recovery may want to optionally support
	optionally support PRR-CRB (which is actually simpler to implement)		PRR-CRB (which is actually simpler to implement) for comparison
	for comparison studies.		studies.
	One final comment about terminology: we expect that common usage will		One final comment about terminology: we expect that common usage will
	drop "slow start reduction bound" from the algorithm name. This		drop "slow start reduction bound" from the algorithm name. This
	document needs to be pedantic about having distinct name for		document needed to be pedantic about having distinct names for
	proportional rate reduction and every variant of the reduction bound.		proportional rate reduction and every variant of the reduction bound.
	However, once paired they become one.		However, once paired they become one.
	7. Acknowledgements		7. Acknowledgements
	This draft is based in part on previous incomplete work by Matt		This draft is based in part on previous incomplete work by Matt
	Mathis, Jeff Semke and Jamshid Mahdavi [RHID] and influenced by		Mathis, Jeff Semke and Jamshid Mahdavi [RHID] and influenced by
	several discussion with John Heffner.		several discussion with John Heffner.
	Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the		Monia Ghobadi and Sivasankar Radhakrishnan helped analyze the
	experiments.		experiments.
			Ilpo Jarvinen for reviewing the code.
	8. Security Considerations		8. Security Considerations
	Proportional Rate Reduction does not change the risk profile for TCP.		Proportional Rate Reduction does not change the risk profile for TCP.
	Implementers that change PRR from counting bytes to segments have to		Implementers that change PRR from counting bytes to segments have to
	be cautious about the effects of ACK splitting attacks[SPLIT], where		be cautious about the effects of ACK splitting attacks [Savage99],
	the receiver acknowledges partial segments for the purpose of		where the receiver acknowledges partial segments for the purpose of
	confusing the sender's congestion accounting.		confusing the sender's congestion accounting.
	9. IANA Considerations		9. IANA Considerations
	This document makes no request of IANA.		This document makes no request of IANA.
	Note to RFC Editor: this section may be removed on publication as an		Note to RFC Editor: this section may be removed on publication as an
	RFC.		RFC.
	10. References		10. 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, October 1996.
	[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A		[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
	Conservative Selective Acknowledgment (SACK)-based Loss		Conservative Selective Acknowledgment (SACK)-based Loss
	Recovery Algorithm for TCP", RFC 3517, April 2003.		Recovery Algorithm for TCP", RFC 3517, April 2003.
			[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
			Congestion Windows", RFC 3742, March 2004.
	[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, September 2009.
	[IMC11] Dukkipati, N., Mathis, M., and Y. Cheng, "Proportional		[IMC11] Dukkipati, N., Mathis, M., and Y. Cheng, "Proportional
	Rate Reduction for TCP", ACM Internet Measurement		Rate Reduction for TCP", ACM Internet Measurement
	Conference IMC11, December 2011.		Conference IMC11, December 2011.
	[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:		[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgment:
	Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,		Refining TCP Congestion Control", ACM SIGCOMM SIGCOMM96,
	August 1996.		August 1996.
	skipping to change at page 14, line 15		skipping to change at page 14, line 23
	[RHID] Mathis, M., Semke, J., Mahdavi, J., and K. Lahey, "The		[RHID] Mathis, M., Semke, J., Mahdavi, J., and K. Lahey, "The
	Rate-Halving Algorithm for TCP Congestion Control", draft-		Rate-Halving Algorithm for TCP Congestion Control", draft-
	ratehalving (work in progress), June 1999.		ratehalving (work in progress), June 1999.
	[RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding		[RHweb] Mathis, M. and J. Mahdavi, "TCP Rate-Halving with Bounding
	Parameters", Web publication , December 1997.		Parameters", Web publication , December 1997.
	[CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed		[CUBIC] Rhee, I. and L. Xu, "CUBIC: A new TCP-friendly high-speed
	TCP variant", PFLDnet 2005, Feb 2005.		TCP variant", PFLDnet 2005, Feb 2005.
			[Savage99]
			Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
			"TCP congestion control with a misbehaving receiver",
			SIGCOMM Comput. Commun. Rev. 29(5), October 1999.
	Appendix A. Packet Conservation Bound		Appendix A. Packet Conservation Bound
	PRR-CRB meets a conservative, philosophically pure and aesthetically		PRR-CRB meets a conservative, philosophically pure and aesthetically
	appealing notion of correct, described here. However, in real		appealing notion of correct, described here. However, in real
	networks it does not perform as well as the algorithms described in		networks it does not perform as well as the algorithms described in
	RFC 3517, which prove to be non-conservative in a statistically		RFC 3517, which proves to be non-conservative in a significant number
	significant number of cases.		of cases.
	Under all conditions and sequences of events during recovery, PRR-CRB		Under all conditions and sequences of events during recovery, PRR-CRB
	strictly bounds the data transmitted to be equal to or less than the		strictly bounds the data transmitted to be equal to or less than the
	amount of data delivered to the receiver. We claim that this packet		amount of data delivered to the receiver. We claim that this packet
	conservation bound is the most aggressive algorithm that does not		conservation bound is the most aggressive algorithm that does not
	lead to additional forced losses in some environments. It has the		lead to additional forced losses in some environments. It has the
	property that if there is a standing queue at a bottleneck that is		property that if there is a standing queue at a bottleneck that is
	carrying no other traffic, the queue will maintain exactly constant		carrying no other traffic, the queue will maintain exactly constant
	length for the entire recovery duration (except for +1/-1 fluctuation		length for the entire duration of the recovery, except for +1/-1
	due to differences in packet arrival and exit times) . Any less		fluctuation due to differences in packet arrival and exit times. Any
	aggressive algorithm will result in a declining queue at the		less aggressive algorithm will result in a declining queue at the
	bottleneck. Any more aggressive algorithm will result in an		bottleneck. Any more aggressive algorithm will result in an
	increasing queue or additional losses at the bottleneck.		increasing queue or additional losses if it is a full drop tail
			queue.
	We demonstrate this property with a little thought experiment:		We demonstrate this property with a little thought experiment:
	Imagine a network path that has insignificant delays in both		Imagine a network path that has insignificant delays in both
	directions, except the processing time and queue at a single		directions, except for the processing time and queue at a single
	bottleneck in the forward path. By insignificant delay, I mean when		bottleneck in the forward path. By insignificant delay, I mean when
	a packet is "served" at the head of the bottleneck queue, the		a packet is "served" at the head of the bottleneck queue, the
	following events happen in much less than one bottleneck packet time:		following events happen in much less than one bottleneck packet time:
	the packet arrives at the receiver; the receiver sends an ACK; which		the packet arrives at the receiver; the receiver sends an ACK; which
	arrives at the sender; the sender processes the ACK and sends some		arrives at the sender; the sender processes the ACK and sends some
	data; the data is queued at the bottleneck.		data; the data is queued at the bottleneck.
	If sndcnt is set to DeliveredData and nothing else is inhibiting		If sndcnt is set to DeliveredData and nothing else is inhibiting
	sending data, then clearly the data arriving at the bottleneck queue		sending data, then clearly the data arriving at the bottleneck queue
	will exactly replace the data that was served at the head of the		will exactly replace the data that was served at the head of the
	skipping to change at page 15, line 14		skipping to change at page 15, line 29
	reordering on the ACK path only cause wider fluctuations in the queue		reordering on the ACK path only cause wider fluctuations in the queue
	size, but do not raise the peak size, independent of whether the data		size, but do not raise the peak size, independent of whether the data
	is in order or out-of-order (including loss recovery from an earlier		is in order or out-of-order (including loss recovery from an earlier
	RTT). Any more aggressive algorithm which sends additional data will		RTT). Any more aggressive algorithm which sends additional data will
	cause a queue overflow and loss. Any less aggressive algorithm will		cause a queue overflow and loss. Any less aggressive algorithm will
	under fill the queue. Therefore setting sndcnt to DeliveredData is		under fill the queue. Therefore setting sndcnt to DeliveredData is
	the most aggressive algorithm that does not cause forced losses in		the most aggressive algorithm that does not cause forced losses in
	this simple network. Relaxing the assumptions (e.g. making delays		this simple network. Relaxing the assumptions (e.g. making delays
	more authentic and adding more flows, delayed ACKs, etc) may		more authentic and adding more flows, delayed ACKs, etc) may
	increases the fine grained fluctuations in queue size but does not		increases the fine grained fluctuations in queue size but does not
	change it's basic behavior.		change its basic behavior.
	Note that the congestion control algorithm implements a broader		Note that the congestion control algorithm implements a broader
	notion of optimal that includes appropriately sharing of the network.		notion of optimal that includes appropriately sharing of the network.
	Typical congestion control algorithms are likely to reduce the data		Typical congestion control algorithms are likely to reduce the data
	sent relative to the packet conserving bound implemented by PRR		sent relative to the packet conserving bound implemented by PRR
	bringing TCP's actual window down to ssthresh.		bringing TCP's actual window down to ssthresh.
	Authors' Addresses		Authors' Addresses
	Matt Mathis		Matt Mathis
End of changes. 30 change blocks.
	59 lines changed or deleted		72 lines changed or added
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