draft-ietf-tcpm-alternativebackoff-ecn-03.txt		draft-ietf-tcpm-alternativebackoff-ecn-04.txt
	Network Working Group N. Khademi		Network Working Group N. Khademi
	Internet-Draft M. Welzl		Internet-Draft M. Welzl
	Intended status: Experimental University of Oslo		Intended status: Experimental University of Oslo
	Expires: May 3, 2018 G. Armitage		Expires: May 19, 2018 G. Armitage
	Swinburne University of Technology		Swinburne University of Technology
	G. Fairhurst		G. Fairhurst
	University of Aberdeen		University of Aberdeen
	October 30, 2017		November 15, 2017
	TCP Alternative Backoff with ECN (ABE)		TCP Alternative Backoff with ECN (ABE)
	draft-ietf-tcpm-alternativebackoff-ecn-03		draft-ietf-tcpm-alternativebackoff-ecn-04
	Abstract		Abstract
	Recent Active Queue Management (AQM) mechanisms allow for burst		Recent Active Queue Management (AQM) mechanisms allow for burst
	tolerance while enforcing short queues to minimise the time that		tolerance while enforcing short queues to minimise the time that
	packets spend enqueued at a bottleneck. This can cause noticeable		packets spend enqueued at a bottleneck. This can cause noticeable
	performance degradation for TCP connections traversing such a		performance degradation for TCP connections traversing such a
	bottleneck, especially if they are only a few or their bandwidth-		bottleneck, especially if they are only a few or their bandwidth-
	delay-product is large. An Explicit Congestion Notification (ECN)		delay-product is large. An Explicit Congestion Notification (ECN)
	signal indicates that an AQM mechanism is used at the bottleneck, and		signal indicates that an AQM mechanism is used at the bottleneck, and
	therefore the bottleneck network queue is likely to be short. This		therefore the bottleneck network queue is likely to be short. This
	document therefore proposes an update to the TCP sender-side ECN		document therefore proposes an update to the TCP sender-side ECN
	reaction in congestion avoidance to reduce the Congestion Window		reaction in congestion avoidance to reduce the Congestion Window
	(cwnd) by a smaller amount than the congestion control algorithm's		(cwnd) by a smaller amount than the congestion control algorithm's
	reaction to loss.		reaction to inferred packet loss.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on May 3, 2018.		This Internet-Draft will expire on May 19, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 28 ¶		skipping to change at page 2, line 28 ¶
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
	4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4		4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4
	4.1. Why Use ECN to Vary the Degree of Backoff? . . . . . . . 4		4.1. Why Use ECN to Vary the Degree of Backoff? . . . . . . . 4
	4.2. Focus on ECN as Defined in RFC3168 . . . . . . . . . . . 5		4.2. Focus on ECN as Defined in RFC3168 . . . . . . . . . . . 5
	4.3. Choice of ABE Multiplier . . . . . . . . . . . . . . . . 6		4.3. Choice of ABE Multiplier . . . . . . . . . . . . . . . . 5
	5. ABE Requirements . . . . . . . . . . . . . . . . . . . . . . 7		5. ABE Requirements . . . . . . . . . . . . . . . . . . . . . . 7
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	8. Implementation Status . . . . . . . . . . . . . . . . . . . . 8		8. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 8		9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	10. Revision Information . . . . . . . . . . . . . . . . . . . . 9		10. Revision Information . . . . . . . . . . . . . . . . . . . . 9
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
	11.1. Normative References . . . . . . . . . . . . . . . . . . 10		11.1. Normative References . . . . . . . . . . . . . . . . . . 10
	11.2. Informative References . . . . . . . . . . . . . . . . . 10		11.2. Informative References . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
	1. Definitions		1. Definitions
	skipping to change at page 3, line 12 ¶		skipping to change at page 3, line 12 ¶
	network deliver some packets to an application that would have been		network deliver some packets to an application that would have been
	dropped if the application or transport did not support ECN. This		dropped if the application or transport did not support ECN. This
	packet loss reduction is the most obvious benefit of ECN, but it is		packet loss reduction is the most obvious benefit of ECN, but it is
	often relatively modest. There are also significant other benefits		often relatively modest. There are also significant other benefits
	from deploying ECN [RFC8087], including reduced end-to-end network		from deploying ECN [RFC8087], including reduced end-to-end network
	latency.		latency.
	The rules for ECN were originally written to be very conservative,		The rules for ECN were originally written to be very conservative,
	and required the congestion control algorithms of ECN-capable		and required the congestion control algorithms of ECN-capable
	transport protocols to treat ECN congestion signals exactly the same		transport protocols to treat ECN congestion signals exactly the same
	as they would treat a packet loss [RFC3168].		as they would treat an inferred packet loss [RFC3168].
	Research has demonstrated the benefits of reducing network delays		Research has demonstrated the benefits of reducing network delays
	that are caused by interaction of loss-based TCP congestion control		that are caused by interaction of loss-based TCP congestion control
	and excessive buffering [BUFFERBLOAT]. There are two main approaches		and excessive buffering [BUFFERBLOAT]. This has led to the creation
	to reduce such delay: (1) to use a congestion control mechanism that		of new AQM mechanisms like PIE [RFC8033] and CoDel
	reacts to the changes in end-to-end delay (i.e. delay-based) instead		[CODEL2012][I-D.CoDel], which prevent bloated queues that are common
	of or in addition to loss or ECN signal (2) or to use AQM mechanisms		with unmanaged and excessively large buffers deployed across the
	like PIE [RFC8033] and CoDel [CODEL2012] [I-D.CoDel], which avoid		Internet [BUFFERBLOAT].
	causing bloated queues that are common with a simple tail-drop
	behaviour (also known as a First-In First-Out, FIFO, queue). The
	delay-based approach suffers from poor performance when competing
	with flows using loss-based TCP congestion control mechanisms and is
	out of scope of this document.
	The AQM mechanisms mentioned above aim to keep a sustained queue		The AQM mechanisms mentioned above aim to keep a sustained queue
	short while tolerating transient (short-term) packet bursts.		short while tolerating transient (short-term) packet bursts.
	However, currently used loss-based congestion control mechanisms		However, currently used loss-based congestion control mechanisms
	cannot always utilise a bottleneck link well where there are short		cannot always utilise a bottleneck link well where there are short
	queues. For example, to allow a single TCP connection to fully		queues. For example, a TCP sender must be able to store at least an
	utilise a network path, the queue at the bottleneck link must be able		end-to-end bandwidth-delay product (BDP) worth of data at the
	to compensate for TCP reducing the "cwnd" and "ssthresh" variables in		bottleneck buffer if it is to maintain full path utilisation in the
	response to a lost packet [RFC5681]. This requires the bottleneck		face of loss-induced reduction of cwnd [RFC5681], which effectively
	buffer to be able to store at least an end-to-end bandwidth-delay		doubles the amount of data that can be in flight, the maximum round-
	product (BDP) of data, which effectively doubles both the amount of		trip time (RTT) experience, and the path's effective RTT using the
	data that can be in flight and the maximum round-trip time (RTT)		network path.
	experience using the network path.
	Modern AQM mechanisms can use ECN to signal the early signs of		Modern AQM mechanisms can use ECN to signal the early signs of
	impending queue buildup long before a tail-drop queue would be forced		impending queue buildup long before a tail-drop queue would be forced
	to resort to dropping packets. It is therefore appropriate for the		to resort to dropping packets. It is therefore appropriate for the
	transport protocol congestion control algorithm to have a more		transport protocol congestion control algorithm to have a more
	measured response when an early-warning signal of congestion is		measured response when an early-warning signal of congestion is
	received in the form of an ECN CE-marked packet. Recognizing these		received in the form of an ECN CE-marked packet. Recognizing these
	changes in modern AQM practices, more recent rules have relaxed the		changes in modern AQM practices, more recent rules have relaxed the
	strict requirement that ECN signals be treated identically to packet		strict requirement that ECN signals be treated identically to
	loss [I-D.ECN-exp]. Following these newer, more flexible rules, this		inferred packet loss [I-D.ECN-exp]. Following these newer, more
	document defines a new sender-side-only congestion control response,		flexible rules, this document defines a new sender-side-only
	called "ABE" (Alternative Backoff with ECN). ABE improves the		congestion control response, called "ABE" (Alternative Backoff with
	performance when routers use AQM controlled buffers that allow for		ECN). ABE improves TCP's average throughput when routers use AQM
	short queues only.		controlled buffers that allow for short queues only.
	3. Specification		3. Specification
	This specification describes an update to the congestion control		This specification describes an update to the congestion control
	algorithm of an ECN-capable TCP transport protocol. It allows a TCP		algorithm of an ECN-capable TCP transport protocol. It allows a TCP
	stack to update the TCP sender response when it receives feedback		stack to update the TCP sender response when it receives feedback
	indicating reception of a CE-marked packet. It RECOMMENDS that a TCP		indicating reception of a CE-marked packet (ECN-signalled congestion
			hereafter) per Equation 4 of [RFC5681]. It RECOMMENDS that a TCP
	sender multiplies the slow start threshold (ssthresh) by 0.8 times of		sender multiplies the slow start threshold (ssthresh) by 0.8 times of
	the FlightSize (with its minimum value set to 2 * SMSS) and reduces		the FlightSize (with its minimum value set to 2 * SMSS) and reduces
	the cwnd in congestion avoidance following reception of a TCP segment		the cwnd in congestion avoidance following reception of a TCP segment
	that sets the ECN-Echo flag (defined in [RFC3168]). While this		that sets the ECN-Echo flag (defined in [RFC3168]). While this
	specification concerns TCP, other transports also support a per-RTT		specification concerns TCP, other transports also support a per-RTT
	response to ECN. The method defined in this document is also		response to ECN. The method defined in this document is also
	applicable for such transports.		applicable for such transports.
	4. Discussion		4. Discussion
	Much of the technical background to this congestion control response		Much of the technical background to ABE can be found in a research
	can be found in a research paper [ABE2017]. This paper used a mix of		paper [ABE2017]. This paper used a mix of experiments, theory and
	experiments, theory and simulations with standard NewReno and CUBIC		simulations with standard NewReno and CUBIC to evaluate the
	to evaluate the technique. It examined the impact of enabling ECN		technique. The technique was shown to present "...significant
	and letting individual TCP senders back off by a reduced amount in		performance gains in lightly-multiplexed [few concurrent flows]
	reaction to the receiver that reports ECN CE-marks from AQM-enabled
	bottlenecks. The technique was shown to present "...significant
	performance gains in lightly-multiplexed [few concurrent connections]
	scenarios, without losing the delay-reduction benefits of deploying		scenarios, without losing the delay-reduction benefits of deploying
	CoDel or PIE". The performance improvement is achieved when reacting		CoDel or PIE". The performance improvement is achieved when reacting
	to ECN-Echo in congestion avoidance by multiplying cwnd and ssthresh		to ECN-Echo in congestion avoidance by multiplying cwnd and ssthresh
	with a value in the range [0.7..0.85].		with a value in the range [0.7,0.85].
	4.1. Why Use ECN to Vary the Degree of Backoff?		4.1. Why Use ECN to Vary the Degree of Backoff?
	The classic rule-of-thumb dictates that a network path needs to		The classic rule-of-thumb dictates that a network path needs to
	provide a BDP of bottleneck buffering if a TCP connection wishes to		provide a BDP of bottleneck buffering if a TCP connection wishes to
	optimise path utilisation. A single TCP bulk transfer running		optimise path utilisation. A single TCP bulk transfer running
	through such a bottleneck will have increased its congestion window		through such a bottleneck will have increased its congestion window
	(cwnd) up to 2*BDP by the time that packet loss occurs. When packet		(cwnd) up to 2*BDP by the time that packet loss occurs. When packet
	loss is detected using the retransmission timer and the given packet		loss is inferred using the retransmission timer and the given packet
	has not yet been resent by way of the retransmission timer (regarded		has not yet been resent by way of the retransmission timer (regarded
	as a notification of congestion), Standard TCP sets the ssthresh to		as a notification of congestion), Standard TCP sets the ssthresh to
	the maximum of half of the FlightSize and 2*SMSS [RFC5681], which		the maximum of half of the FlightSize and 2*SMSS [RFC5681], which
	causes the TCP congestion control to go back to allowing only a BDP		causes the TCP congestion control to go back to allowing only a BDP
	of packets in flight -- just sufficient to maintain 100% utilisation		of packets in flight -- just sufficient to maintain 100% utilisation
	of the bottleneck on the network path.		of the bottleneck on the network path.
	AQM mechanisms such as CoDel [I-D.CoDel] and PIE [RFC8033] set a		AQM mechanisms such as CoDel [I-D.CoDel] and PIE [RFC8033] set a
	delay target in routers and use congestion notifications to constrain		delay target in routers and use congestion notifications to constrain
	the queuing delays experienced by packets, rather than in response to		the queuing delays experienced by packets, rather than in response to
	impending or actual bottleneck buffer exhaustion. With current		impending or actual bottleneck buffer exhaustion. With current
	default delay targets, CoDel and PIE both effectively emulate a		default delay targets, CoDel and PIE both effectively emulate a
	bottleneck with a short queue (section II, [ABE2017]) while also		bottleneck with a short queue (section II, [ABE2017]) while also
	allowing short traffic bursts into the queue. This provides		allowing short traffic bursts into the queue. This provides
	acceptable performance for TCP connections over a path with a low		acceptable performance for TCP connections over a path with a low
	BDP, or in highly multiplexed scenarios (many concurrent transport		BDP, or in highly multiplexed scenarios (many concurrent transport
	connections). However, in a lightly-multiplexed case over a path		flows). However, in a lightly-multiplexed case over a path with a
	with a large BDP, conventional TCP backoff leads to gaps in packet		large BDP, conventional TCP backoff leads to gaps in packet
	transmission and under-utilisation of the path.		transmission and under-utilisation of the path.
	Instead of discarding packets, an AQM mechanism is allowed to mark		Instead of discarding packets, an AQM mechanism is allowed to mark
	ECN-capable packets with an ECN CE-mark. The reception of a CE-mark		ECN-capable packets with an ECN CE-mark. The reception of a CE-mark
	not only indicates congestion on the network path, it also indicates		feedback not only indicates congestion on the network path, it also
	that an AQM mechanism exists at the bottleneck along the path, and		indicates that an AQM mechanism exists at the bottleneck along the
	hence the CE-mark likely came from a bottleneck with a controlled		path, and hence the CE-mark likely came from a bottleneck with a
	short queue. Reacting differently to an ECN CE-mark than to packet		controlled short queue. Reacting differently to an ECN-signalled
	loss can then yield the benefit of a reduced back-off, as with CUBIC		congestion than to an inferred packet loss can then yield the benefit
	[I-D.CUBIC], when queues are short, yet it can avoid generating		of a reduced back-off when queues are short. Using ECN can also be
	excessive delay when queues are long. Using ECN can also be
	advantageous for several other reasons [RFC8087].		advantageous for several other reasons [RFC8087].
	The idea of reacting differently to loss and detection of an ECN CE-		The idea of reacting differently to inferred packet loss and
	mark pre-dates this document. For example, previous research		detection of an ECN-signalled congestion pre-dates this document.
	proposed using ECN CE-marks to modify TCP congestion control		For example, previous research proposed using ECN CE-marked feedback
	behaviour via a larger multiplicative decrease factor in conjunction		to modify TCP congestion control behaviour via a larger
	with a smaller additive increase factor [ICC2002]. The goal of this		multiplicative decrease factor in conjunction with a smaller additive
	former work was to operate across AQM bottlenecks using Random Early		increase factor [ICC2002]. The goal of this former work was to
	Detection (RED) that were not necessarily configured to emulate a		operate across AQM bottlenecks using Random Early Detection (RED)
	short queue ([RFC7567] notes the current status of RED as an AQM		that were not necessarily configured to emulate a short queue
	method.)		([RFC7567] notes the current status of RED as an AQM method.)
	4.2. Focus on ECN as Defined in RFC3168		4.2. Focus on ECN as Defined in RFC3168
	Some transport protocol mechanisms rely on ECN semantics that differ		Some transport protocol mechanisms rely on ECN semantics that differ
	from the original ECN definition [RFC3168] -- for example, Congestion		from the original ECN definition [RFC3168] -- for example, Congestion
	Exposure (ConEx) [RFC7713] and Datacenter TCP (DCTCP)		Exposure (ConEx) [RFC7713] and Datacenter TCP (DCTCP)
	[I-D.ietf-tcpm-dctcp] need more accurate ECN information than that		[I-D.ietf-tcpm-dctcp] need more accurate ECN information than that
	offered by the original feedback method. Other mechanisms (e.g.,		offered by the original feedback method. Other mechanisms (e.g.,
	[I-D.ietf-tcpm-accurate-ecn]) allow the sender to adjust the rate		[I-D.ietf-tcpm-accurate-ecn]) allow the sender to adjust the rate
	more frequently than once each path RTT. Use of these mechanisms is		more frequently than once each path RTT. Use of these mechanisms is
	out of the scope of the current document.		out of scope for this document.
	4.3. Choice of ABE Multiplier		4.3. Choice of ABE Multiplier
	ABE decouples the reaction of a TCP sender to loss and ECN CE-marks		ABE decouples the reaction of a TCP sender to inferred packet loss
	when in the congestion avoidance phase by differentiating the scaling		and ECN-signalled congestion when in the congestion avoidance phase
	factor used in Equation 4 in Section 3.1 of [RFC5681]. The		by differentiating the scaling factor used in Equation 4 in
	description respectively uses beta_{loss} and beta_{ecn} to refer to		Section 3.1 of [RFC5681]. The description respectively uses
	the multiplicative decrease factors applied in response to packet		beta_{loss} and beta_{ecn} to refer to the multiplicative decrease
	loss, and in response to a receiver indicating that an ECN CE-mark		factors applied in response to inferred packet loss, and in response
	was received on an ECN-enabled TCP connection. For non-ECN-enabled		to a receiver indicating ECN-signalled congestion. For non-ECN-
	TCP connections, no ECN CE-marks are received and only beta_{loss}		enabled TCP connections, only beta_{loss} applies.
	applies.
	In other words, in response to detected loss:		In other words, in response to inferred packet loss:
	ssthresh = max (FlightSize * beta_{loss}, 2 * SMSS)		ssthresh = max (FlightSize * beta_{loss}, 2 * SMSS)
	and in response to an indication of a received ECN CE-mark:		and in response to an indication of an ECN-signalled congestion:
	ssthresh = max (FlightSize * beta_{ecn}, 2 * SMSS)		ssthresh = max (FlightSize * beta_{ecn}, 2 * SMSS)
	and		and
	cwnd = ssthresh		cwnd = ssthresh
	where FlightSize is the amount of outstanding data in the network,		where FlightSize is the amount of outstanding data in the network,
	upper-bounded by the sender's cwnd and the receiver's advertised		upper-bounded by the smaller of the sender's cwnd and the receiver's
	window (rwnd) [RFC5681]. The higher the values of beta_{loss} and		advertised window (rwnd) [RFC5681]. The higher the values of
	beta_{ecn}, the less aggressive the response of any individual		beta_{loss} and beta_{ecn}, the less aggressive the response of any
	backoff event.		individual backoff event.
	The appropriate choice for beta_{loss} and beta_{ecn} values is a		The appropriate choice for beta_{loss} and beta_{ecn} values is a
	balancing act between path utilisation and draining the bottleneck		balancing act between path utilisation and draining the bottleneck
	queue. More aggressive backoff (smaller beta_*) risks underutilising		queue. More aggressive backoff (smaller beta_*) risks underutilising
	the path, while less aggressive backoff (larger beta_*) can result in		the path, while less aggressive backoff (larger beta_*) can result in
	slower draining of the bottleneck queue.		slower draining of the bottleneck queue.
	The Internet has already been running with at least two different		The Internet has already been running with at least two different
	beta_{loss} values for several years: the standard value is 0.5		beta_{loss} values for several years: the standard value is 0.5
	[RFC5681], and the Linux implementation of CUBIC [I-D.CUBIC] has used		[RFC5681], and the Linux implementation of CUBIC [I-D.CUBIC] has used
	skipping to change at page 7, line 24 ¶		skipping to change at page 7, line 13 ¶
	0.7 to 0.85 (cf. beta_{loss} = 0.5).		0.7 to 0.85 (cf. beta_{loss} = 0.5).
	5. ABE Requirements		5. ABE Requirements
	This update is a sender-side only change. Like other changes to		This update is a sender-side only change. Like other changes to
	congestion control algorithms, it does not require any change to the		congestion control algorithms, it does not require any change to the
	TCP receiver or to network devices. It does not require any ABE-		TCP receiver or to network devices. It does not require any ABE-
	specific changes in routers or the use of Accurate ECN feedback		specific changes in routers or the use of Accurate ECN feedback
	[I-D.ietf-tcpm-accurate-ecn] by a receiver.		[I-D.ietf-tcpm-accurate-ecn] by a receiver.
	The currently published ECN specification requires that the		RFC 3168 states that the congestion control response to an ECN-
	congestion control response to a CE-marked packet is the same as the		signalled congestion is the same as the response to a dropped packet
	response to a dropped packet [RFC3168]. The specification is		[RFC3168]. [I-D.ECN-exp] updates this specification to allow systems
	currently being updated to allow for specifications that do not		to provide a different behaviour when they experience ECN-signalled
	follow this rule [I-D.ECN-exp]. The present specification defines		congestion rather than packet loss. The present specification
	such an experiment and has thus been assigned an Experimental status		defines such an experiment and has thus been assigned an Experimental
	before being proposed as a Standards-Track update.		status before being proposed as a Standards-Track update.
	The purpose of the Internet experiment is to collect experience with		The purpose of the Internet experiment is to collect experience with
	deployment of ABE, and confirm the safety in deployed networks using		deployment of ABE, and confirm the safety in deployed networks using
	this update to TCP congestion control.		this update to TCP congestion control.
	When used with bottlenecks that do not support ECN-marking the		When used with bottlenecks that do not support ECN-marking the
	specification does not modify the transport protocol.		specification does not modify the transport protocol.
	To evaluate the benefit, this experiment therefore requires support		To evaluate the benefit, this experiment therefore requires support
	in AQM routers (except to enable an ECN-marking mechanism [RFC3168]		in AQM routers (except to enable an ECN-marking mechanism [RFC3168]
	[RFC7567]) for ECN-marking of packets carrying the ECN Capable		[RFC7567]) for ECN-marking of packets carrying the ECN Capable
	Transport, ECT(0), codepoint [RFC3168].		Transport, ECT(0), codepoint [RFC3168].
	If the method is only deployed by some senders, and not by others,		If the method is only deployed by some senders, and not by others,
	the senders that use this method can gain some advantage, possibly at		the senders that use this method can gain some advantage, possibly at
	the expense of other flows that do not use this updated method.		the expense of other flows that do not use this updated method.
	Because this advantage applies only to ECN-marked packets and not to		Because this advantage applies only to ECN-marked packets and not to
	loss indications, the new method cannot lead to congestion collapse.		packet loss indications, in the worst-case (e.g., an ABE-compliant
			TCP sender using beta_{ecn} = 1.0) the ECN-capable bottleneck will
			still fall back to dropping packets, and the result is no different
			than if the TCP sender was using traditional loss-based congestion
			control.
	The result of this Internet experiment will be reported by		The result of this Internet experiment will be reported by
	presentation to the TCPM WG (or IESG) or an implementation report at		presentation to the TCPM WG (or IESG) or an implementation report at
	the end of the experiment.		the end of the experiment.
	6. Acknowledgements		6. Acknowledgements
	Authors N. Khademi, M. Welzl and G. Fairhurst were part-funded by		Authors N. Khademi, M. Welzl and G. Fairhurst were part-funded by
	the European Community under its Seventh Framework Programme through		the European Community under its Seventh Framework Programme through
	the Reducing Internet Transport Latency (RITE) project (ICT-317700).		the Reducing Internet Transport Latency (RITE) project (ICT-317700).
	skipping to change at page 9, line 15 ¶		skipping to change at page 9, line 9 ¶
	mechanisms that have been in use in the Internet for many years		mechanisms that have been in use in the Internet for many years
	(e.g., CUBIC [I-D.CUBIC]). Unfairness may also be a result of other		(e.g., CUBIC [I-D.CUBIC]). Unfairness may also be a result of other
	factors, including the round trip time experienced by a flow. ABE		factors, including the round trip time experienced by a flow. ABE
	applies only when ECN-marked packets are received, not when packets		applies only when ECN-marked packets are received, not when packets
	are lost, hence use of ABE cannot lead to congestion collapse.		are lost, hence use of ABE cannot lead to congestion collapse.
	10. Revision Information		10. Revision Information
	XX RFC ED - PLEASE REMOVE THIS SECTION XXX		XX RFC ED - PLEASE REMOVE THIS SECTION XXX
			-04. Incorporates review comments from Larence Stewart and the
			remaining comments from Roland Bless. References are updated.
	-03. Several review comments from Roland Bless are addressed.		-03. Several review comments from Roland Bless are addressed.
	Consistent terminology and equations. Clarification on the scope of		Consistent terminology and equations. Clarification on the scope of
	recommended beta_{ecn} value.		recommended beta_{ecn} value.
	-02. Corrected the equations in Section 4.3. Updated the		-02. Corrected the equations in Section 4.3. Updated the
	affiliations. Lower bound for cwnd is defined. A recommendation for		affiliations. Lower bound for cwnd is defined. A recommendation for
	window-based transport protocols is changed to cover all transport		window-based transport protocols is changed to cover all transport
	protocols that implement a congestion control reduction to an ECN		protocols that implement a congestion control reduction to an ECN
	congestion signal. Added text about ABE's FreeBSD mainline kernel		congestion signal. Added text about ABE's FreeBSD mainline kernel
	status including a reference to the FreeBSD code review page.		status including a reference to the FreeBSD code review page.
	skipping to change at page 10, line 12 ¶		skipping to change at page 10, line 12 ¶
	alternativebackoff-ecn-03, following discussion in the TSVWG and TCPM		alternativebackoff-ecn-03, following discussion in the TSVWG and TCPM
	working groups.		working groups.
	11. References		11. References
	11.1. Normative References		11.1. Normative References
	[I-D.ECN-exp]		[I-D.ECN-exp]
	Black, D., "Explicit Congestion Notification (ECN)		Black, D., "Explicit Congestion Notification (ECN)
	Experimentation", Internet-draft, IETF work-in-progress		Experimentation", Internet-draft, IETF work-in-progress
	draft-ietf-tsvwg-ecn-experimentation-06, September 2017.		draft-ietf-tsvwg-ecn-experimentation-08, November 2017.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition		[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
	of Explicit Congestion Notification (ECN) to IP",		of Explicit Congestion Notification (ECN) to IP",
	RFC 3168, DOI 10.17487/RFC3168, September 2001,		RFC 3168, DOI 10.17487/RFC3168, September 2001,
	<https://www.rfc-editor.org/info/rfc3168>.		<https://www.rfc-editor.org/info/rfc3168>.
	skipping to change at page 10, line 45 ¶		skipping to change at page 10, line 45 ¶
	[ABE-FreeBSD]		[ABE-FreeBSD]
	"ABE patch review in FreeBSD",		"ABE patch review in FreeBSD",
	<https://reviews.freebsd.org/D11616>.		<https://reviews.freebsd.org/D11616>.
	[ABE2017] Khademi, N., Armitage, G., Welzl, M., Fairhurst, G.,		[ABE2017] Khademi, N., Armitage, G., Welzl, M., Fairhurst, G.,
	Zander, S., and D. Ros, "Alternative Backoff: Achieving		Zander, S., and D. Ros, "Alternative Backoff: Achieving
	Low Latency and High Throughput with ECN and AQM", IFIP		Low Latency and High Throughput with ECN and AQM", IFIP
	NETWORKING 2017, Stockholm, Sweden, June 2017.		NETWORKING 2017, Stockholm, Sweden, June 2017.
	[BUFFERBLOAT]		[BUFFERBLOAT]
	"Bufferbloat project",		Gettys, J. and K. Nichols, "Bufferbloat: Dark Buffers in
	<https://www.bufferbloat.net/projects/bloat/wiki/		the Internet", November 2011.
	Introduction/>.
	[CODEL2012]		[CODEL2012]
	Nichols, K. and V. Jacobson, "Controlling Queue Delay",		Nichols, K. and V. Jacobson, "Controlling Queue Delay",
	July 2012, <http://queue.acm.org/detail.cfm?id=2209336>.		July 2012, <http://queue.acm.org/detail.cfm?id=2209336>.
	[I-D.CoDel]		[I-D.CoDel]
	Nichols, K., Jacobson, V., McGregor, V., and J. Iyengar,		Nichols, K., Jacobson, V., McGregor, V., and J. Iyengar,
	"Controlled Delay Active Queue Management", Internet-		"Controlled Delay Active Queue Management", Internet-
	draft, IETF work-in-progress draft-ietf-aqm-codel-09,		draft, IETF work-in-progress draft-ietf-aqm-codel-10,
	September 2017.		October 2017.
	[I-D.CUBIC]		[I-D.CUBIC]
	Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and		Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
	R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",		R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
	Internet-draft, IETF work-in-progress draft-ietf-tcpm-		Internet-draft, IETF work-in-progress draft-ietf-tcpm-
	cubic-06, September 2017.		cubic-07, November 2017.
	[I-D.ietf-tcpm-accurate-ecn]		[I-D.ietf-tcpm-accurate-ecn]
	Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More		Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
	Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-		Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
	ecn-03 (work in progress), May 2017.		ecn-03 (work in progress), May 2017.
	[I-D.ietf-tcpm-dctcp]		[I-D.ietf-tcpm-dctcp]
	Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,		Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
	and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion		and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
	Control for Datacenters", draft-ietf-tcpm-dctcp-10 (work		Control for Datacenters", draft-ietf-tcpm-dctcp-10 (work
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