draft-ietf-tcpm-alternativebackoff-ecn-00.txt		draft-ietf-tcpm-alternativebackoff-ecn-01.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: August 6, 2017 G. Armitage		Expires: November 5, 2017 G. Armitage
	Swinburne University of Technology		Swinburne University of Technology
	G. Fairhurst		G. Fairhurst
	University of Aberdeen		University of Aberdeen
	February 2, 2017		May 4, 2017
	TCP Alternative Backoff with ECN (ABE)		TCP Alternative Backoff with ECN (ABE)
	draft-ietf-tcpm-alternativebackoff-ecn-00		draft-ietf-tcpm-alternativebackoff-ecn-01
	Abstract		Abstract
	This memo updates the TCP sender-side reaction to a congestion		Recent Active Queue Management (AQM) mechanisms instantiate shallow
	notification received via Explicit Congestion Notification (ECN).		buffers with burst tolerance to minimise the time that packets spend
	The updated method reduces FlightSize in Congestion Avoidance by a		enqueued at a bottleneck. However, shallow buffering can cause
	smaller amount than the TCP reaction to loss. The intention is to		noticeable performance degradation when TCP is used over a network
	achieve good throughput when the queue at the bottleneck is smaller		path with a large bandwidth-delay-product. Traditional methods rely
	than the bandwidth-delay-product of the connection. This is more		on detecting network congestion through reported loss of transport
	likely when an Active Queue Management (AQM) mechanism has used ECN		packets. Explicit Congestion Notification (ECN) instead allows a
	to CE-mark a packet, than when a packet was lost. Future versions of		router to directly signal incipient congestion. A sending endpoint
	this document will also describe a corresponding method for SCTP.		can distinguish when congestion is signalled via ECN, rather than by
			packet loss. An ECN signal indicates that an AQM mechanism has done
			its job, and therefore the bottleneck network queue is likely to be
			shallow. This document therefore proposes an update to the TCP
			sender-side ECN reaction in congestion avoidance to reduce the
			FlightSize by a smaller amount than the congestion control
			algorithm's reaction to loss. Future versions of this document will
			also describe a corresponding method for the Stream Control
			Transmission Protocol (SCTP).
	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.
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			This Internet-Draft will expire on November 5, 2017.
	This Internet-Draft will expire on August 6, 2017.
	Copyright Notice		Copyright Notice
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	Table of Contents		Table of Contents
	1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
	3.1. Why Use ECN to Vary the Degree of Backoff? . . . . . . . 4		4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4
	3.2. Focus on ECN as Defined in RFC3168 . . . . . . . . . . . 4		4.1. Why Use ECN to Vary the Degree of Backoff? . . . . . . . 4
	3.3. Discussion: Choice of ABE Multiplier . . . . . . . . . . 4		4.2. Focus on ECN as Defined in RFC3168 . . . . . . . . . . . 5
	4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 6		4.3. Discussion: Choice of ABE Multiplier . . . . . . . . . . 5
	5. Status of the Update . . . . . . . . . . . . . . . . . . . . 6		5. Status of the Update . . . . . . . . . . . . . . . . . . . . 6
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	8. Implementation Status . . . . . . . . . . . . . . . . . . . . 7		8. Implementation Status . . . . . . . . . . . . . . . . . . . . 8
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 7		9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	10. Revision Information . . . . . . . . . . . . . . . . . . . . 7		10. Revision Information . . . . . . . . . . . . . . . . . . . . 8
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
	11.1. Normative References . . . . . . . . . . . . . . . . . . 8		11.1. Normative References . . . . . . . . . . . . . . . . . . 9
	11.2. Informative References . . . . . . . . . . . . . . . . . 8		11.2. Informative References . . . . . . . . . . . . . . . . . 9
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Definitions		1. Definitions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in RFC 2119 [RFC2119].		document are to be interpreted as described in RFC 2119 [RFC2119].
	2. Introduction		2. Introduction
	Complementing [I-D.AQM-ECN-benefits], [I-D.ECN-exp] enables wider ECN		Explicit Congestion Notification (ECN) [RFC3168] makes it possible
	deployment by updating rules in [RFC3168] that prohibited certain		for an Active Queue Management (AQM) mechanism to signal the presence
	experiments. Specifically, [I-D.ECN-exp] allows for experiments to		of incipient congestion without incurring packet loss. This lets the
	specify a congestion control response to a CE-marked packet that		network deliver some packets to an application that would have been
	differs from the response to a dropped packet. This memo defines		dropped if the application or transport did not support ECN. This
	such a different congestion control response, called "ABE"		packet loss reduction is the most obvious benefit of ECN, but it is
	(Alternative Backoff with ECN). ABE is thus an Experiment in		often relatively modest. There are also significant other benefits
	accordance with [I-D.ECN-exp].		from deploying ECN [RFC8087], including reduced end-to-end network
			latency.
	[RFC5681] stipulates that TCP congestion control sets "ssthresh" to		The rules for ECN were originally written to be very conservative,
	max(FlightSize / 2, 2*SMSS) in response to packet loss. This		and required the congestion control algorithms of ECN-capable
	corresponds to a backoff multiplier of 0.5 (halving cwnd and		transport protocols to treat ECN congestion signals exactly the same
	sshthresh after packet loss). Consequently, a standard TCP flow		as they would treat a packet loss [RFC3168].
	using this reaction needs significant network queue space: it can
	only fully utilise a bottleneck when the length of the link queue (or
	the AQM dropping threshold) is at least the bandwidth-delay product
	(BDP) of the flow.
	A backoff multiplier of 0.5 is not the only available strategy. As		Research has demonstrated the benefits of reducing network delays due
	defined in [I-D.CUBIC], CUBIC multiplies the current cwnd by 0.7 in		to excessive buffering [BUFFERBLOAT]; this has led to the creation of
	response to loss (the Linux implementation of CUBIC has used a		new AQM mechanisms like PIE [RFC8033] and CoDel [CODEL2012]
	multiplier of 0.7 since kernel version 2.6.25 released in 2008).		[I-D.CoDel], which avoid causing the bloated queues that are common
	Consequently, CUBIC utilises paths well even when the bottleneck		with a simple tail-drop behaviour (also known as a First-In First-
	queue is shorter than the bandwidth-delay product of the flow.		Out, FIFO, queue).
	However, in the case of a DropTail (FIFO) queue without AQM, such
	less-aggressive backoff increases the risk of creating a standing
	queue [CODEL2012].
	The standard TCP backoff behaviour defined in [RFC5681] entails		These AQM mechanisms instantiate short queues that are designed to
	reduced link utilisation in situations with short queues and low		tolerate packet bursts. However, congestion control mechanisms
	statistical multiplexing. This memo proposes a concrete sender-side-		cannot always utilise a bottleneck link well where there are short
	only congestion control response that remedies this problem.		queues. For example, to allow a single TCP connection to fully
			utilise a network path, the queue at the bottleneck link must be able
			to compensate for TCP halving the "FlightSize" and "ssthresh"
			variables in response to a lost packet [RFC5681]. This requires the
			bottleneck queue to be able to store at least an end-to-end
			bandwidth-delay product (BDP) of data, which effectively doubles both
			the amount of data that can be in flight and the round-trip time
			(RTT) experience using the network path.
	Devices implementing AQM are likely to be the dominant (and possibly		Modern AQM mechanisms can use ECN to signal the early signs of
	only) source of ECN CE-marking for packets from ECN-capable senders.		impending queue buildup long before a tail-drop queue would be forced
	AQM mechanisms typically strive to maintain a small average queue		to resort to dropping packets. It is therefore appropriate for the
	length, regardless of the bandwidth-delay product of flows passing		transport protocol congestion control algorithm to have a more
	through them. Receipt of an ECN CE-mark might therefore reasonably		measured response when an early-warning signal of congestion is
	be taken to indicate that a small bottleneck queue exists in the		received in the form of an ECN CE-marked packet. Recognizing these
	path, and hence the TCP flow would benefit from using a less		changes in modern AQM practices, more recent rules have relaxed the
	aggressive backoff multiplier.		strict requirement that ECN signals be treated identically to packet
			loss [I-D.ECN-exp]. Following these newer, more flexible rules, this
			document defines a new sender-side-only congestion control response,
			called "ABE" (Alternative Backoff with ECN). ABE improves the
			performance when routers use shallow buffered AQM mechanisms.
	Much of the background to this proposal can be found in [ABE2015].		3. Specification
	Using a mix of experiments, theory and simulations with standard
	NewReno and CUBIC, [ABE2015] recommends enabling ECN and letting		This specification describes an update to the congestion control
	individual TCP senders use a larger multiplicative decrease factor as		algorithm of an ECN-capable TCP transport protocol. It allows a TCP
	a reaction to the receiver reporting ECN CE-marks from AQM-enabled		stack to update the TCP sender response when it receives feedback
	bottlenecks. Such a change is noted to result in "...significant		indicating reception of a CE-marked packet. It RECOMMENDS that a TCP
			sender multiplies the FlightSize by 0.8 and reduces the slow start
			threshold (ssthresh) in congestion avoidance following reception of a
			TCP segment that sets the ECN-Echo flag (defined in [RFC3168]).
			4. Discussion
			Much of the technical background to this congestion control response
			can be found in a research paper [ABE2017]. This paper used a mix of
			experiments, theory and simulations with standard NewReno and CUBIC
			to evaluate the technique. It examined the impact of enabling ECN
			and letting individual TCP senders back off by a reduced amount in
			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 scenarios, without losing		performance gains in lightly-multiplexed scenarios, without losing
	the delay-reduction benefits of deploying CoDel or PIE" [I-D.CoDel]		the delay-reduction benefits of deploying CoDel or PIE". The
	[I-D.PIE]. This is achieved when reacting to ECN-Echo in Congestion		performance improvement is achieved when reacting to ECN-Echo in
	Avoidance by multiplying cwnd and sstthresh with a value in the range		congestion avoidance by multiplying FlightSize and sstthresh with a
	[0.7..0.85].		value in the range [0.7..0.85].
	3. Discussion		4.1. Why Use ECN to Vary the Degree of Backoff?
	3.1. Why Use ECN to Vary the Degree of Backoff?		The classic rule-of-thumb dictates that a network path needs to
			provide a BDP of bottleneck buffering if a TCP connection wishes to
			optimise path utilisation. A single TCP bulk transfer running
			through such a bottleneck will have increased its congestion window
			(cwnd) up to 2*BDP by the time that packet loss occurs. When packet
			loss is detected (regarded as a notification of congestion), Standard
			TCP halves the FlightSize and ssthresh [RFC5681], which causes the
			TCP congestion control to go back to allowing only a BDP of packets
			in flight -- just sufficient to maintain 100% utilisation of the
			bottleneck on the network path.
	The classic rule-of-thumb dictates that a transport provides a BDP of		AQM mechanisms such as CoDel [I-D.CoDel] and PIE [RFC8033] set a
	bottleneck buffering if a TCP connection wishes to optimise path		delay target in routers and use congestion notifications to constrain
	utilisation. A single TCP connection running through such a		the queuing delays experienced by packets, rather than in response to
	bottleneck will have opened cwnd up to 2*BDP by the time packet loss		impending or actual bottleneck buffer exhaustion. With current
	occurs. [RFC5681]'s halving of cwnd and ssthresh pushes the TCP		default delay targets, CoDel and PIE both effectively emulate a
	connection back to allowing only a BDP of packets in flight -- just		shallow buffered bottleneck (section II, [ABE2017]) while also
	sufficient to maintain 100% utilisation of the network path.		allowing short traffic bursts into the queue. This provides
			acceptable performance for TCP connections over a path with a low
			BDP, or in highly multiplexed scenarios (many concurrent transport
			connections). However, it interacts badly for a lightly-multiplexed
			case (few concurrent connections) over a path with a large BDP.
			Conventional TCP backoff in such cases leads to gaps in packet
			transmission and under-utilisation of the path.
	AQM schemes like CoDel [I-D.CoDel] and PIE [I-D.PIE] use congestion		Instead of discarding packets, an AQM mechanism is allowed to mark
	notifications to constrain the queuing delays experienced by packets,		ECN-capable packets with an ECN CE-mark. The reception of a CE-mark
	rather than in response to impending or actual bottleneck buffer		not only indicates congestion on the network path, it also indicates
	exhaustion. With current default delay targets, CoDel and PIE both		that an AQM mechanism exists at the bottleneck along the path, and
	effectively emulate a shallow buffered bottleneck (section II,		hence the CE-mark likely came from a bottleneck with a shallow queue.
	[ABE2015]) while allowing short traffic bursts into the queue. This		Reacting differently to an ECN CE-mark than to packet loss can then
	interacts acceptably for TCP connections over low BDP paths, or		yield the benefit of a reduced back-off, as with CUBIC [I-D.CUBIC],
	highly multiplexed scenarios (many concurrent TCP connections).		when queues are short, yet it can avoid generating excessive delay
	However, it interacts badly with lightly-multiplexed cases (few		when queues are long. Using ECN can also be advantageous for several
	concurrent connections) over a high BDP path. Conventional TCP		other reasons [RFC8087].
	backoff in such cases leads to gaps in packet transmission and under-
	utilisation of the path.
	The idea to react differently to loss upon detecting an ECN CE-mark		The idea of reacting differently to loss and detection of an ECN CE-
	pre-dates [ABE2015]. [ICC2002] also proposed using ECN CE-marks to		mark pre-dates this document. For example, previous research
	modify TCP congestion control behaviour, using a larger		proposed using ECN CE-marks to modify TCP congestion control
	multiplicative decrease factor in conjunction with a smaller additive		behaviour via a larger multiplicative decrease factor in conjunction
	increase factor to work with RED-based bottlenecks that were not		with a smaller additive increase factor [ICC2002]. The goal of this
	necessarily configured to emulate a shallow queue.		former work was to operate across AQM bottlenecks using Random Early
			Detection (RED) that were not necessarily configured to emulate a
			shallow queue ([RFC7567] notes the current status of RED as an AQM
			method.)
	3.2. Focus on ECN as Defined in RFC3168		4.2. Focus on ECN as Defined in RFC3168
	Some mechanisms rely on ECN semantics that differ from the		Some transport protocol mechanisms rely on ECN semantics that differ
	definitions in [RFC3168] -- for example, Congestion Exposure (ConEx)		from the original ECN definition [RFC3168] -- for example, Congestion
	[RFC7713] and DCTCP [I-D.ietf-tcpm-dctcp] need more accurate ECN		Exposure (ConEx) [RFC7713] and Datacenter TCP (DCTCP)
	information than the feedback mechanism in [RFC3168] offers (defined		[I-D.ietf-tcpm-dctcp] need more accurate ECN information than that
	in [I-D.ietf-tcpm-accurate-ecn]). Such mechanisms allow a sending		offered by the original feedback method. Other mechanisms (e.g.,
	rate adjustment more frequent than each RTT. These mechanisms are		[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
	out of the scope of the current document.		out of the scope of the current document.
	3.3. Discussion: Choice of ABE Multiplier		4.3. Discussion: Choice of ABE Multiplier
	Alternative Backoff with ECN (ABE) decouples a TCP sender's reaction		ABE decouples the reaction of a TCP sender to loss and ECN CE-marks
	to loss and ECN CE-marks in Congestion Avoidance. The description		when in the congestion avoidance phase. The description respectively
	respectively uses beta_{loss} and beta_{ecn} to refer to the		uses beta_{loss} and beta_{ecn} to refer to the multiplicative
	multiplicative decrease factors applied in response to packet loss,		decrease factors applied in response to packet loss, and in response
	and also in response to a receiver indicating that an ECN CE-mark was		to a receiver indicating that an ECN CE-mark was received on an ECN-
	received on an ECN-enabled TCP connection (based on the terms used in		enabled TCP connection. For non-ECN-enabled TCP connections, no ECN
	[ABE2015]). For non-ECN-enabled TCP connections, no ECN CE-marks are		CE-marks are received and only beta_{loss} applies.
	received and only beta_{loss} applies.
	In other words, in response to detected loss:		In other words, in response to detected loss:
	FlightSize_(n+1) = FlightSize_n * beta_{loss}		FlightSize_(n+1) = FlightSize_n * beta_{loss}
	and in response to an indication of a received ECN CE-mark:		and in response to an indication of a received ECN CE-mark:
	FlightSize_(n+1) = FlightSize_n * beta_{ecn}		FlightSize_(n+1) = FlightSize_n * beta_{ecn}
	where, as in [RFC5681], FlightSize is the amount of outstanding data		where FlightSize is the amount of outstanding data in the network,
	in the network, upper-bounded by the sender's congestion window		upper-bounded by the sender's cwnd and the receiver's advertised
	(cwnd) and the receiver's advertised window (rwnd). The higher the		window (rwnd) [RFC5681]. The higher the values of beta_{loss} and
	values of beta_{loss} and beta_{ecn}, the less aggressive the		beta_{ecn}, the less aggressive the response of any individual
	response of any individual backoff event.		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 value in [RFC5681] is 0.5,		beta_{loss} values for several years: the standard value is 0.5
	and Linux CUBIC uses 0.7. ABE proposes no change to beta_{loss} used		[RFC5681], and the Linux implementation of CUBIC [I-D.CUBIC] has used
	by any current TCP implementations.		a multiplier of 0.7 since kernel version 2.6.25 released in 2008.
			ABE proposes no change to beta_{loss} used by current TCP
			implementations.
	beta_{ecn} depends on how the response of a TCP connection to shallow		beta_{ecn} depends on how the response of a TCP connection to shallow
	AQM marking thresholds is optimised. beta_{loss} reflects the		AQM marking thresholds is optimised. beta_{loss} reflects the
	preferred response of each TCP algorithm when faced with exhaustion		preferred response of each congestion control algorithm when faced
	of buffers (of unknown depth) signalled by packet loss.		with exhaustion of buffers (of unknown depth) signalled by packet
	Consequently, for any given TCP algorithm the choice of beta_{ecn} is		loss. Consequently, for any given TCP congestion control algorithm
	likely to be algorithm-specific, rather than a constant multiple of		the choice of beta_{ecn} is likely to be algorithm-specific, rather
	the algorithm's existing beta_{loss}.		than a constant multiple of the algorithm's existing beta_{loss}.
	A range of experiments (section IV, [ABE2015]) with NewReno and CUBIC		A range of tests (section IV, [ABE2017]) with NewReno and CUBIC over
	over CoDel and PIE in lightly-multiplexed scenarios have explored		CoDel and PIE in lightly-multiplexed scenarios have explored this
	this choice of parameter. These experiments indicate that CUBIC		choice of parameter. The results of these tests indicate that CUBIC
	connections benefit from beta_{ecn} of 0.85 (cf. beta_{loss} = 0.7),		connections benefit from beta_{ecn} of 0.85 (cf. beta_{loss} = 0.7),
	and NewReno connections see improvements with beta_{ecn} in the range		and NewReno connections see improvements with beta_{ecn} in the range
	0.7 to 0.85 (cf. beta_{loss} = 0.5).		0.7 to 0.85 (cf. beta_{loss} = 0.5).
	4. Specification
	This document RECOMMENDS that experimental deployments multiply the
	FlightSize by 0.8 and reduce the slow start threshold 'ssthresh' in
	Congestion Avoidance in response to reception of a TCP segment that
	sets the ECN-Echo flag.
	5. Status of the Update		5. Status of the Update
	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 (except to enable an ECN-marking		TCP receiver or to network devices. It does not require any ABE-
	algorithm [RFC3168] [RFC7567]). If the method is only deployed by		specific changes in routers or the use of Accurate ECN feedback
	some TCP senders, and not by others, the senders that use this method		[I-D.ietf-tcpm-accurate-ecn] by a receiver.
	can gain advantage, possibly at the expense of other flows that do
	not use this updated method. This advantage applies only to ECN-
	marked packets and not to loss indications. Hence, the new method
	can not lead to congestion collapse.
	The present specification has been assigned an Experimental status,		The currently published ECN specification requires that the
	to provide Internet deployment experience before being proposed as a		congestion control response to a CE-marked packet is the same as the
	Standards-Track update.		response to a dropped packet [RFC3168]. The specification is
			currently being updated to allow for specifications that do not
			follow this rule [I-D.ECN-exp]. The present specification defines
			such an experiment and has thus been assigned an Experimental status
			before being proposed as a Standards-Track update.
	This experiment will evaluate the impact of ABE on the Internet. The		The purpose of the Internet experiment is to collect experience with
	result will be reported by presentation to the TCPM WG (or IESG) or		deployment of ABE, and confirm the safety in deployed networks using
	an implementation report at the end of the experiment. Progressing		this update to TCP congestion control.
	the experiment requires support of ECN-marking packets carrying the
	ECT(0) codepoint by routers [I-D.ECN-exp], but does not require any		When used with bottlenecks that do not support ECN-marking the
	ABE-specific changes in routers or Accurate ECN feedback		specification does not modify the transport protocol.
	[I-D.ietf-tcpm-accurate-ecn] from receivers.
			To evaluate the benefit, this experiment therefore requires support
			in AQM routers (except to enable an ECN-marking mechanism [RFC3168]
			[RFC7567]) for ECN-marking of packets carrying the ECN Capable
			Transport, ECT(0), codepoint [RFC3168].
			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 expense of other flows that do not use this updated method.
			Because this advantage applies only to ECN-marked packets and not to
			loss indications, the new method cannot lead to congestion collapse.
			The result of this Internet experiment will be reported by
			presentation to the TCPM WG (or IESG) or an implementation report at
			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).
	The views expressed are solely those of the authors.		The views expressed are solely those of the authors.
	The authors would like to thank the following people for their		The authors would like to thank Stuart Cheshire for many suggestions
	contributions to [ABE2015]: Chamil Kulatunga, David Ros, Stein		when revising the draft, and the following people for their
	Gjessing, Sebastian Zander. Thanks to (in alphabetical order) Bob		contributions to [ABE2017]: Chamil Kulatunga, David Ros, Stein
	Briscoe, Markku Kojo, John Leslie, Dave Taht and the TCPM WG for		Gjessing, Sebastian Zander. Thanks also to (in alphabetical order)
	providing valuable feedback on this document.		Bob Briscoe, Markku Kojo, John Leslie, Dave Taht and the TCPM working
			group for providing valuable feedback on this document.
	The authors would like to thank feedback on the congestion control		The authors would finally like to thank everyone who provided
	behaviour specified in this update received from the IRTF Internet		feedback on the congestion control behaviour specified in this update
	Congestion Control Research Group (ICCRG).		received from the IRTF Internet Congestion Control Research Group
			(ICCRG).
	7. IANA Considerations		7. IANA Considerations
	XX RFC ED - PLEASE REMOVE THIS SECTION XXX		XX RFC ED - PLEASE REMOVE THIS SECTION XXX
	This memo includes no request to IANA.		This document includes no request to IANA.
	8. Implementation Status		8. Implementation Status
	ABE is implemented as a patch for Linux and FreeBSD. It is meant for		ABE is implemented as a patch for Linux and FreeBSD. It is meant for
	research and available for download from		research and available for download from
	http://heim.ifi.uio.no/naeemk/research/ABE/ This code was used to		http://heim.ifi.uio.no/naeemk/research/ABE/ This code was used to
	produce the test results that are reported in [ABE2015].		produce the test results that are reported in [ABE2017].
	9. Security Considerations		9. Security Considerations
	The described method is a sender-side only transport change, and does		The described method is a sender-side only transport change, and does
	not change the protocol messages exchanged. The security		not change the protocol messages exchanged. The security
	considerations of [RFC3168] therefore still apply.		considerations for ECN [RFC3168] therefore still apply.
	This document describes a change to TCP congestion control with ECN		This is a change to TCP congestion control with ECN that will
	that will typically lead to a change in the capacity achieved when		typically lead to a change in the capacity achieved when flows share
	flows share a network bottleneck. Similar unfairness in the way that		a network bottleneck. This could result in some flows receiving more
	capacity is shared is also exhibited by other congestion control		than their fair share of capacity. Similar unfairness in the way
			that capacity is shared is also exhibited by other congestion control
	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. This		factors, including the round trip time experienced by a flow. ABE
	advantage applies only to ECN-marked packets and not to loss		applies only when ECN-marked packets are received, not when packets
	indications, and will therefore not 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
			-01. Text improved, mainly incorporating comments from Stuart
			Cheshire. The reference to a technical report has been updated to a
			published version of the tests [ABE2017]. Used "AQM Mechanism"
			throughout in place of other alternatives, and more consistent use of
			technical language and clarification on the intended purpose of the
			experiments required by EXP status. There was no change to the
			technical content.
	-00. draft-ietf-tcpm-alternativebackoff-ecn-00 replaces draft-		-00. draft-ietf-tcpm-alternativebackoff-ecn-00 replaces draft-
	khademi-tcpm-alternativebackoff-ecn-01. Text describing the nature		khademi-tcpm-alternativebackoff-ecn-01. Text describing the nature
	of the experiment was added.		of the experiment was added.
	-01. This I-D now refers to draft-black-tsvwg-ecn-experimentation-		Individual draft -01. This I-D now refers to draft-black-tsvwg-ecn-
	02, which replaces draft-khademi-tsvwg-ecn-response-00 to make a		experimentation-02, which replaces draft-khademi-tsvwg-ecn-
	broader update to RFC3168 for the sake of allowing experiments. As a		response-00 to make a broader update to RFC3168 for the sake of
	result, some of the motivating and discussing text that was moved		allowing experiments. As a result, some of the motivating and
	from draft-khademi-alternativebackoff-ecn-03 to draft-khademi-tsvwg-		discussing text that was moved from draft-khademi-alternativebackoff-
	ecn-response-00 has now been re-inserted here.		ecn-03 to draft-khademi-tsvwg-ecn-response-00 has now been re-
			inserted here.
	-00. draft-khademi-tsvwg-ecn-response-00 and draft-khademi-tcpm-		Individual draft -00. draft-khademi-tsvwg-ecn-response-00 and draft-
	alternativebackoff-ecn-00 replace draft-khademi-alternativebackoff-		khademi-tcpm-alternativebackoff-ecn-00 replace draft-khademi-
	ecn-03, following discussion in the TSVWG and TCPM working groups.		alternativebackoff-ecn-03, following discussion in the TSVWG and TCPM
			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-black-tsvwg-ecn-experimentation-02, October 2016.		draft-ietf-tsvwg-ecn-experimentation-02, April 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,
	<http://www.rfc-editor.org/info/rfc2119>.		<http://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,
	<http://www.rfc-editor.org/info/rfc3168>.		<http://www.rfc-editor.org/info/rfc3168>.
	skipping to change at page 8, line 35 ¶		skipping to change at page 9, line 45 ¶
	Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,		Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
	<http://www.rfc-editor.org/info/rfc5681>.		<http://www.rfc-editor.org/info/rfc5681>.
	[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF		[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
	Recommendations Regarding Active Queue Management",		Recommendations Regarding Active Queue Management",
	BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,		BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
	<http://www.rfc-editor.org/info/rfc7567>.		<http://www.rfc-editor.org/info/rfc7567>.
	11.2. Informative References		11.2. Informative References
	[ABE2015] Khademi, N., Welzl, M., Armitage, G., Kulatunga, C., Ros,		[ABE2017] Khademi, N., Armitage, G., Welzl, M., Fairhurst, G.,
	D., Fairhurst, G., Gjessing, S., and S. Zander,		Zander, S., and D. Ros, "Alternative Backoff: Achieving
	"Alternative Backoff: Achieving Low Latency and High		Low Latency and High Throughput with ECN and AQM", IFIP
	Throughput with ECN and AQM", CAIA Technical Report CAIA-		NETWORKING 2017, Stockholm, Sweden, June 2017.
	TR-150710A, Swinburne University of Technology, July 2015,
	<http://caia.swin.edu.au/reports/150710A/		[BUFFERBLOAT]
	CAIA-TR-150710A.pdf>.		"Bufferbloat project",
			<https://www.bufferbloat.net/projects/bloat/wiki/
			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.AQM-ECN-benefits]
	Fairhurst, G. and M. Welzl, "The Benefits of using
	Explicit Congestion Notification (ECN)", Internet-draft,
	IETF work-in-progress draft-ietf-aqm-ecn-benefits-08,
	November 2015.
	[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-04, June		draft, IETF work-in-progress draft-ietf-aqm-codel-07,
	2016.		March 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-02, August 2016.		cubic-04, February 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-01 (work in progress), June 2016.		ecn-01 (work in progress), June 2016.
	[I-D.ietf-tcpm-dctcp]		[I-D.ietf-tcpm-dctcp]
	Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,		Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P.,
	and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion		and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
	Control for Datacenters", draft-ietf-tcpm-dctcp-02 (work		Control for Datacenters", draft-ietf-tcpm-dctcp-02 (work
	in progress), July 2016.		in progress), July 2016.
	[I-D.PIE] Pan, R., Natarajan, P., Baker, F., and G. White, "PIE: A
	Lightweight Control Scheme To Address the Bufferbloat
	Problem", Internet-draft, IETF work-in-progress draft-
	ietf-aqm-pie-10, September 2016.
	[ICC2002] Kwon, M. and S. Fahmy, "TCP Increase/Decrease Behavior		[ICC2002] Kwon, M. and S. Fahmy, "TCP Increase/Decrease Behavior
	with Explicit Congestion Notification (ECN)", IEEE		with Explicit Congestion Notification (ECN)", IEEE
	ICC 2002, New York, New York, USA, May 2002,		ICC 2002, New York, New York, USA, May 2002,
	<http://dx.doi.org/10.1109/ICC.2002.997262>.		<http://dx.doi.org/10.1109/ICC.2002.997262>.
	[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)		[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
	Concepts, Abstract Mechanism, and Requirements", RFC 7713,		Concepts, Abstract Mechanism, and Requirements", RFC 7713,
	DOI 10.17487/RFC7713, December 2015,		DOI 10.17487/RFC7713, December 2015,
	<http://www.rfc-editor.org/info/rfc7713>.		<http://www.rfc-editor.org/info/rfc7713>.
			[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
			"Proportional Integral Controller Enhanced (PIE): A
			Lightweight Control Scheme to Address the Bufferbloat
			Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
			<http://www.rfc-editor.org/info/rfc8033>.
			[RFC8087] Fairhurst, G. and M. Welzl, "The Benefits of Using
			Explicit Congestion Notification (ECN)", RFC 8087,
			DOI 10.17487/RFC8087, March 2017,
			<http://www.rfc-editor.org/info/rfc8087>.
	Authors' Addresses		Authors' Addresses
	Naeem Khademi		Naeem Khademi
	University of Oslo		University of Oslo
	PO Box 1080 Blindern		PO Box 1080 Blindern
	Oslo N-0316		Oslo N-0316
	Norway		Norway
	Email: naeemk@ifi.uio.no		Email: naeemk@ifi.uio.no
	Michael Welzl		Michael Welzl
	University of Oslo		University of Oslo
	PO Box 1080 Blindern		PO Box 1080 Blindern
	Oslo N-0316		Oslo N-0316
	Norway		Norway
	Email: michawe@ifi.uio.no		Email: michawe@ifi.uio.no
	Grenville Armitage		Grenville Armitage
	Centre for Advanced Internet Architectures		Centre for Advanced Internet Architectures
End of changes. 49 change blocks.
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