< draft-ietf-tsvwg-ecn-l4s-id-28.txt		draft-ietf-tsvwg-ecn-l4s-id-29b.txt >
	Transport Services (tsv) K. De Schepper		Transport Services (tsv) K. De Schepper
	Internet-Draft Nokia Bell Labs		Internet-Draft Nokia Bell Labs
	Intended status: Experimental B. Briscoe, Ed.		Intended status: Experimental B. Briscoe, Ed.
	Expires: 9 February 2023 Independent		Expires: 1 March 2023 Independent
	8 August 2022		28 August 2022
	Explicit Congestion Notification (ECN) Protocol for Very Low Queuing		Explicit Congestion Notification (ECN) Protocol for Very Low Queuing
	Delay (L4S)		Delay (L4S)
	draft-ietf-tsvwg-ecn-l4s-id-28		draft-ietf-tsvwg-ecn-l4s-id-29
	Abstract		Abstract
	This specification defines the protocol to be used for a new network		This specification defines the protocol to be used for a new network
	service called low latency, low loss and scalable throughput (L4S).		service called low latency, low loss and scalable throughput (L4S).
	L4S uses an Explicit Congestion Notification (ECN) scheme at the IP		L4S uses an Explicit Congestion Notification (ECN) scheme at the IP
	layer that is similar to the original (or 'Classic') ECN approach,		layer that is similar to the original (or 'Classic') ECN approach,
	except as specified within. L4S uses 'scalable' congestion control,		except as specified within. L4S uses 'scalable' congestion control,
	which induces much more frequent control signals from the network and		which induces much more frequent control signals from the network and
	it responds to them with much more fine-grained adjustments, so that		it responds to them with much more fine-grained adjustments, so that
	skipping to change at page 2, line 10 ¶		skipping to change at page 2, line 10 ¶
	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 9 February 2023.		This Internet-Draft will expire on 1 March 2023.
	Copyright Notice		Copyright Notice
	Copyright (c) 2022 IETF Trust and the persons identified as the		Copyright (c) 2022 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 (https://trustee.ietf.org/		Provisions Relating to IETF Documents (https://trustee.ietf.org/
	license-info) in effect on the date of publication of this document.		license-info) in effect on the date of publication of this document.
	Please review these documents carefully, as they describe your rights		Please review these documents carefully, as they describe your rights
	skipping to change at page 2, line 50 ¶		skipping to change at page 2, line 50 ¶
	5. Network Node Behaviour . . . . . . . . . . . . . . . . . . . 20		5. Network Node Behaviour . . . . . . . . . . . . . . . . . . . 20
	5.1. Classification and Re-Marking Behaviour . . . . . . . . . 20		5.1. Classification and Re-Marking Behaviour . . . . . . . . . 20
	5.2. The Strength of L4S CE Marking Relative to Drop . . . . . 22		5.2. The Strength of L4S CE Marking Relative to Drop . . . . . 22
	5.3. Exception for L4S Packet Identification by Network Nodes		5.3. Exception for L4S Packet Identification by Network Nodes
	with Transport-Layer Awareness . . . . . . . . . . . . . 23		with Transport-Layer Awareness . . . . . . . . . . . . . 23
	5.4. Interaction of the L4S Identifier with other		5.4. Interaction of the L4S Identifier with other
	Identifiers . . . . . . . . . . . . . . . . . . . . . . . 23		Identifiers . . . . . . . . . . . . . . . . . . . . . . . 23
	5.4.1. DualQ Examples of Other Identifiers Complementing L4S		5.4.1. DualQ Examples of Other Identifiers Complementing L4S
	Identifiers . . . . . . . . . . . . . . . . . . . . . 23		Identifiers . . . . . . . . . . . . . . . . . . . . . 23
	5.4.1.1. Inclusion of Additional Traffic with L4S . . . . 23		5.4.1.1. Inclusion of Additional Traffic with L4S . . . . 23
	5.4.1.2. Exclusion of Traffic From L4S Treatment . . . . . 25		5.4.1.2. Exclusion of Traffic From L4S Treatment . . . . . 26
	5.4.1.3. Generalized Combination of L4S and Other		5.4.1.3. Generalized Combination of L4S and Other
	Identifiers . . . . . . . . . . . . . . . . . . . . 26		Identifiers . . . . . . . . . . . . . . . . . . . . 26
	5.4.2. Per-Flow Queuing Examples of Other Identifiers		5.4.2. Per-Flow Queuing Examples of Other Identifiers
	Complementing L4S Identifiers . . . . . . . . . . . . 28		Complementing L4S Identifiers . . . . . . . . . . . . 28
	5.5. Limiting Packet Bursts from Links . . . . . . . . . . . . 28		5.5. Limiting Packet Bursts from Links . . . . . . . . . . . . 28
	5.5.1. Limiting Packet Bursts from Links Fed by an L4S		5.5.1. Limiting Packet Bursts from Links Fed by an L4S
	AQM . . . . . . . . . . . . . . . . . . . . . . . . . 28		AQM . . . . . . . . . . . . . . . . . . . . . . . . . 29
	5.5.2. Limiting Packet Bursts from Links Upstream of an L4S		5.5.2. Limiting Packet Bursts from Links Upstream of an L4S
	AQM . . . . . . . . . . . . . . . . . . . . . . . . . 29		AQM . . . . . . . . . . . . . . . . . . . . . . . . . 29
	6. Behaviour of Tunnels and Encapsulations . . . . . . . . . . . 29		6. Behaviour of Tunnels and Encapsulations . . . . . . . . . . . 29
	6.1. No Change to ECN Tunnels and Encapsulations in General . 29		6.1. No Change to ECN Tunnels and Encapsulations in General . 29
	6.2. VPN Behaviour to Avoid Limitations of Anti-Replay . . . . 30		6.2. VPN Behaviour to Avoid Limitations of Anti-Replay . . . . 30
	7. L4S Experiments . . . . . . . . . . . . . . . . . . . . . . . 31		7. L4S Experiments . . . . . . . . . . . . . . . . . . . . . . . 31
	7.1. Open Questions . . . . . . . . . . . . . . . . . . . . . 31		7.1. Open Questions . . . . . . . . . . . . . . . . . . . . . 32
	7.2. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 33		7.2. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 33
	7.3. Future Potential . . . . . . . . . . . . . . . . . . . . 33		7.3. Future Potential . . . . . . . . . . . . . . . . . . . . 33
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 34		9. Security Considerations . . . . . . . . . . . . . . . . . . . 35
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
	10.1. Normative References . . . . . . . . . . . . . . . . . . 35		10.1. Normative References . . . . . . . . . . . . . . . . . . 35
	10.2. Informative References . . . . . . . . . . . . . . . . . 36		10.2. Informative References . . . . . . . . . . . . . . . . . 36
	Appendix A. Rationale for the 'Prague L4S Requirements' . . . . 45		Appendix A. Rationale for the 'Prague L4S Requirements' . . . . 46
	A.1. Rationale for the Requirements for Scalable Transport		A.1. Rationale for the Requirements for Scalable Transport
	Protocols . . . . . . . . . . . . . . . . . . . . . . . . 46		Protocols . . . . . . . . . . . . . . . . . . . . . . . . 47
	A.1.1. Use of L4S Packet Identifier . . . . . . . . . . . . 46		A.1.1. Use of L4S Packet Identifier . . . . . . . . . . . . 47
	A.1.2. Accurate ECN Feedback . . . . . . . . . . . . . . . . 46		A.1.2. Accurate ECN Feedback . . . . . . . . . . . . . . . . 47
	A.1.3. Capable of Replacement by Classic Congestion		A.1.3. Capable of Replacement by Classic Congestion
	Control . . . . . . . . . . . . . . . . . . . . . . . 47		Control . . . . . . . . . . . . . . . . . . . . . . . 47
	A.1.4. Fall back to Classic Congestion Control on Packet		A.1.4. Fall back to Classic Congestion Control on Packet
	Loss . . . . . . . . . . . . . . . . . . . . . . . . 47		Loss . . . . . . . . . . . . . . . . . . . . . . . . 48
	A.1.5. Coexistence with Classic Congestion Control at Classic		A.1.5. Coexistence with Classic Congestion Control at Classic
	ECN bottlenecks . . . . . . . . . . . . . . . . . . . 48		ECN bottlenecks . . . . . . . . . . . . . . . . . . . 49
	A.1.6. Reduce RTT dependence . . . . . . . . . . . . . . . . 51		A.1.6. Reduce RTT dependence . . . . . . . . . . . . . . . . 52
	A.1.7. Scaling down to fractional congestion windows . . . . 53		A.1.7. Scaling down to fractional congestion windows . . . . 53
	A.1.8. Measuring Reordering Tolerance in Time Units . . . . 54		A.1.8. Measuring Reordering Tolerance in Time Units . . . . 54
	A.2. Scalable Transport Protocol Optimizations . . . . . . . . 57		A.2. Scalable Transport Protocol Optimizations . . . . . . . . 57
	A.2.1. Setting ECT in Control Packets and Retransmissions . 57		A.2.1. Setting ECT in Control Packets and Retransmissions . 57
	A.2.2. Faster than Additive Increase . . . . . . . . . . . . 57		A.2.2. Faster than Additive Increase . . . . . . . . . . . . 58
	A.2.3. Faster Convergence at Flow Start . . . . . . . . . . 58		A.2.3. Faster Convergence at Flow Start . . . . . . . . . . 58
	Appendix B. Compromises in the Choice of L4S Identifier . . . . 58		Appendix B. Compromises in the Choice of L4S Identifier . . . . 59
	Appendix C. Potential Competing Uses for the ECT(1) Codepoint . 63		Appendix C. Potential Competing Uses for the ECT(1) Codepoint . 64
	C.1. Integrity of Congestion Feedback . . . . . . . . . . . . 63		C.1. Integrity of Congestion Feedback . . . . . . . . . . . . 64
	C.2. Notification of Less Severe Congestion than CE . . . . . 65		C.2. Notification of Less Severe Congestion than CE . . . . . 65
	Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 65		Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 66
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 66		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 66
	1. Introduction		1. Introduction
	This experimental track specification defines the protocol to be used		This experimental track specification defines the protocol to be used
	for a new network service called low latency, low loss and scalable		for a new network service called low latency, low loss and scalable
	throughput (L4S). L4S uses an Explicit Congestion Notification (ECN)		throughput (L4S). L4S uses an Explicit Congestion Notification (ECN)
	scheme at the IP layer with the same set of codepoint transitions as		scheme at the IP layer with the same set of codepoint transitions as
	the original (or 'Classic') Explicit Congestion Notification		the original (or 'Classic') Explicit Congestion Notification
	(ECN [RFC3168]). RFC 3168 required an ECN mark to be equivalent to a		(ECN [RFC3168]). RFC 3168 required an ECN mark to be equivalent to a
	skipping to change at page 6, line 5 ¶		skipping to change at page 6, line 5 ¶
	growth in latency-sensitive applications continues, periods with		growth in latency-sensitive applications continues, periods with
	solely latency-sensitive traffic will become increasingly common on		solely latency-sensitive traffic will become increasingly common on
	links where traffic aggregation is low. During these periods, if all		links where traffic aggregation is low. During these periods, if all
	the traffic were marked for the same treatment, Diffserv would make		the traffic were marked for the same treatment, Diffserv would make
	no difference. The links with low aggregation also tend to become		no difference. The links with low aggregation also tend to become
	the path bottleneck under load, for instance, the access links		the path bottleneck under load, for instance, the access links
	dedicated to individual sites (homes, small enterprises or mobile		dedicated to individual sites (homes, small enterprises or mobile
	devices). So, instead of differentiation, it becomes imperative to		devices). So, instead of differentiation, it becomes imperative to
	remove the underlying causes of any unnecessary delay.		remove the underlying causes of any unnecessary delay.
	The bufferbloat project has shown that excessively-large buffering		The Bufferbloat project has shown that excessively-large buffering
	('bufferbloat') has been introducing significantly more delay than		('bufferbloat') has been introducing significantly more delay than
	the underlying propagation time. These delays appear only		the underlying propagation time [Bufferbloat]. These delays appear
	intermittently -- only when a capacity-seeking (e.g. TCP) flow is		only intermittently -- only when a capacity-seeking (e.g. TCP) flow
	long enough for the queue to fill the buffer, causing every packet in		is long enough for the queue to fill the buffer, causing every packet
	other flows sharing the buffer to have to work its way through the		in other flows sharing the buffer to have to work its way through the
	queue.		queue.
	Active queue management (AQM) was originally developed to solve this		Active queue management (AQM) was originally developed to solve this
	problem (and others). Unlike Diffserv, which gives low latency to		problem (and others). Unlike Diffserv, which gives low latency to
	some traffic at the expense of others, AQM controls latency for _all_		some traffic at the expense of others, AQM controls latency for _all_
	traffic in a class. In general, AQM methods introduce an increasing		traffic in a class. In general, AQM methods introduce an increasing
	level of discard from the buffer the longer the queue persists above		level of discard from the buffer, the longer the queue persists above
	a shallow threshold. This gives sufficient signals to capacity-		a shallow threshold. This gives sufficient signals to capacity-
	seeking (aka. greedy) flows to keep the buffer empty for its intended		seeking (aka. greedy) flows to keep the buffer empty for its intended
	purpose: absorbing bursts. However, RED [RFC2309] and other		purpose: absorbing bursts. However, RED [RFC2309] and other
	algorithms from the 1990s were sensitive to their configuration and		algorithms from the 1990s were sensitive to their configuration and
	hard to set correctly. So, this form of AQM was not widely deployed.		hard to set correctly. So, this form of AQM was not widely deployed.
	More recent state-of-the-art AQM methods, such as FQ-CoDel [RFC8290],		More recent state-of-the-art AQM methods, such as FQ-CoDel [RFC8290],
	PIE [RFC8033] or Adaptive RED [ARED01], are easier to configure,		PIE [RFC8033] or Adaptive RED [ARED01], are easier to configure,
	because they define the queuing threshold in time not bytes, so		because they define the queuing threshold in time not bytes, so
	configuration is invariant whatever the link rate. However, the		configuration is invariant whatever the link rate. However, the
	sawtoothing window of a Classic congestion control creates a dilemma		sawtoothing window of a Classic congestion control creates a dilemma
	for the operator: i) either configure a shallow AQM operating point,		for the operator: i) either configure a shallow AQM operating point,
	so the tips of the sawteeth cause minimal queue delay but the troughs		so the tips of the sawteeth cause minimal queue delay, but the
	underutilize the link, or ii) configure the operating point deeper		troughs underutilize the link; or ii) configure the operating point
	into the buffer, so the troughs utilize the link better but then the		deeper into the buffer, so the troughs utilize the link better, but
	tips cause more delay variation. Even with a perfectly tuned AQM,		then the tips cause more delay variation. Even with a perfectly
	the additional queuing delay at the tips of the sawteeth will be of		tuned AQM, the additional queuing delay at the tips of the sawteeth
	the same order as the underlying speed-of-light delay across the		will be of the same order as the underlying base round trip time
	network, thereby roughly doubling the total round-trip time.		(RTT), thereby roughly doubling the total round-trip time.
	If a sender's own behaviour is introducing queuing delay variation,		If a sender's own behaviour is introducing queuing delay variation,
	no AQM in the network can 'un-vary' the delay without significantly		no AQM in the network can 'un-vary' the delay without significantly
	compromising link utilization. Even flow-queuing (e.g. [RFC8290]),		compromising link utilization. Even flow-queuing (e.g. [RFC8290]),
	which isolates one flow from another, cannot isolate a flow from the		which isolates one flow from another, cannot isolate a flow from the
	delay variations it inflicts on itself. Therefore those applications		delay variations it inflicts on itself. Therefore, those
	that need to seek out high bandwidth but also need low latency will		applications that need to seek out high bandwidth but also need low
	have to migrate to scalable congestion control, which uses much		latency will have to migrate to scalable congestion control, which
	smaller sawtooth variations.		uses much smaller sawtooth variations.
	Altering host behaviour is not enough on its own though. Even if		Altering host behaviour is not enough on its own though. Even if
	hosts adopt low latency scalable congestion controls, they need to be		hosts adopt low latency scalable congestion controls, they need to be
	isolated from the large queue variations induced by existing Classic		isolated from the large queue variations induced by existing Classic
	congestion controls. L4S AQMs provide that latency isolation in the		congestion controls. L4S AQMs provide that latency isolation in the
	network and the L4S identifier enables the AQMs to distinguish the		network and the L4S identifier enables the AQMs to distinguish the
	two types of packet that need to be isolated: L4S and Classic. L4S		two types of packet that need to be isolated: L4S and Classic. L4S
	isolation can be achieved with a queue per flow (e.g. [RFC8290]) but		isolation can be achieved with a queue per flow (e.g. [RFC8290]) but
	a DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] is sufficient, and		a DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] is sufficient, and
	actually gives better tail latency [DCttH19]. Both approaches are		actually gives better tail latency [DCttH19]. Both approaches are
	skipping to change at page 7, line 40 ¶		skipping to change at page 7, line 40 ¶
	It turns out that these scalable congestion control algorithms that		It turns out that these scalable congestion control algorithms that
	solve the latency problem can also solve the scalability problem of		solve the latency problem can also solve the scalability problem of
	Classic congestion controls. The finer sawteeth in the congestion		Classic congestion controls. The finer sawteeth in the congestion
	window have low amplitude, so they cause very little queuing delay		window have low amplitude, so they cause very little queuing delay
	variation and the average time to recover from one congestion signal		variation and the average time to recover from one congestion signal
	to the next (the average duration of each sawtooth) remains		to the next (the average duration of each sawtooth) remains
	invariant, which maintains constant tight control as flow-rate		invariant, which maintains constant tight control as flow-rate
	scales. A background paper [DCttH19] gives the full explanation of		scales. A background paper [DCttH19] gives the full explanation of
	why the design solves both the latency and the scaling problems, both		why the design solves both the latency and the scaling problems, both
	in plain English and in more precise mathematical form. The		in plain English and in more precise mathematical form. The
	explanation is summarised without the mathematics in Section 4 of the		explanation is summarized without the mathematics in Section 4 of the
	L4S architecture [I-D.ietf-tsvwg-l4s-arch].		L4S architecture [I-D.ietf-tsvwg-l4s-arch].
	1.2. Terminology		1.2. Terminology
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in BCP
	[RFC2119]. In this document, these words will appear with that		14 [RFC2119] [RFC8174] when, and only when, they appear in all
	interpretation only when in ALL CAPS. Lower case uses of these words		capitals, as shown here.
	are not to be interpreted as carrying RFC-2119 significance.
	Note: The L4S architecture [I-D.ietf-tsvwg-l4s-arch] repeats the		[Note to the RFC Editor (to be removed before publication as an RFC):
	following definitions, but if there are accidental differences those		The L4S architecture [I-D.ietf-tsvwg-l4s-arch] repeats the following
	below take precedence.		definitions, which should be identical, except in the architecture
			Classic CC and Scalable CC are condensed because they refer to a
			section later in the architecture.]
	Classic Congestion Control: A congestion control behaviour that can		Classic Congestion Control: A congestion control behaviour that can
	co-exist with standard Reno [RFC5681] without causing		co-exist with standard Reno [RFC5681] without causing
	significantly negative impact on its flow rate [RFC5033]. With		significantly negative impact on its flow rate [RFC5033]. With
	Classic congestion controls, such as Reno or Cubic, because flow		Classic congestion controls, such as Reno or Cubic, because flow
	rate has scaled since TCP congestion control was first designed in		rate has scaled since TCP congestion control was first designed in
	1988, it now takes hundreds of round trips (and growing) to		1988, it now takes hundreds of round trips (and growing) to
	recover after a congestion signal (whether a loss or an ECN mark)		recover after a congestion signal (whether a loss or an ECN mark)
	as shown in the examples in section 5.1 of the L4S		as shown in the examples in section 5.1 of the L4S
	architecture [I-D.ietf-tsvwg-l4s-arch] and in [RFC3649].		architecture [I-D.ietf-tsvwg-l4s-arch] and in [RFC3649].
	Therefore control of queuing and utilization becomes very slack,		Therefore, control of queuing and utilization becomes very slack,
	and the slightest disturbances (e.g. from new flows starting)		and the slightest disturbances (e.g. from new flows starting)
	prevent a high rate from being attained.		prevent a high rate from being attained.
	Scalable Congestion Control: A congestion control where the average		Scalable Congestion Control: A congestion control where the average
	time from one congestion signal to the next (the recovery time)		time from one congestion signal to the next (the recovery time)
	remains invariant as the flow rate scales, all other factors being		remains invariant as the flow rate scales, all other factors being
	equal. This maintains the same degree of control over queueing		equal. This maintains the same degree of control over queueing
	and utilization whatever the flow rate, as well as ensuring that		and utilization whatever the flow rate, as well as ensuring that
	high throughput is robust to disturbances. For instance, DCTCP		high throughput is robust to disturbances. For instance, DCTCP
	averages 2 congestion signals per round-trip whatever the flow		averages 2 congestion signals per round-trip whatever the flow
	rate, as do other recently developed scalable congestion controls,		rate, as do other recently developed scalable congestion controls,
	e.g. Relentless TCP [Mathis09], TCP Prague		e.g. Relentless TCP [I-D.mathis-iccrg-relentless-tcp], TCP Prague
	[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux],		[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux],
	BBRv2 [BBRv2], [I-D.cardwell-iccrg-bbr-congestion-control] and the		BBRv2 [BBRv2], [I-D.cardwell-iccrg-bbr-congestion-control] and the
	L4S variant of SCREAM for real-time media [SCReAM-L4S], [RFC8298].		L4S variant of SCREAM for real-time media [SCReAM-L4S], [RFC8298].
	See Section 4.3 for more explanation.		See Section 4.3 for more explanation.
	Classic service: The Classic service is intended for all the		Classic service: The Classic service is intended for all the
	congestion control behaviours that co-exist with Reno [RFC5681]		congestion control behaviours that co-exist with Reno [RFC5681]
	(e.g. Reno itself, Cubic [RFC8312],		(e.g. Reno itself, Cubic [RFC8312],
	Compound [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]). The term		Compound [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]). The term
	'Classic queue' means a queue providing the Classic service.		'Classic queue' means a queue providing the Classic service.
	Low-Latency, Low-Loss Scalable throughput (L4S) service: The 'L4S'		Low-Latency, Low-Loss Scalable throughput (L4S) service: The 'L4S'
	service is intended for traffic from scalable congestion control		service is intended for traffic from scalable congestion control
	algorithms, such as TCP Prague		algorithms, such as the Prague congestion control
	[I-D.briscoe-iccrg-prague-congestion-control], which was derived		[I-D.briscoe-iccrg-prague-congestion-control], which was derived
	from DCTCP [RFC8257]. The L4S service is for more general traffic		from DCTCP [RFC8257]. The L4S service is for more general traffic
	than just TCP Prague -- it allows the set of congestion controls		than just Prague -- it allows the set of congestion controls with
	with similar scaling properties to Prague to evolve, such as the		similar scaling properties to Prague to evolve, such as the
	examples listed above (Relentless, SCReAM, etc.). The term 'L4S		examples listed above (Relentless, SCReAM, etc.). The term 'L4S
	queue' means a queue providing the L4S service.		queue' means a queue providing the L4S service.
	The terms Classic or L4S can also qualify other nouns, such as		The terms Classic or L4S can also qualify other nouns, such as
	'queue', 'codepoint', 'identifier', 'classification', 'packet',		'queue', 'codepoint', 'identifier', 'classification', 'packet',
	'flow'. For example: an L4S packet means a packet with an L4S		'flow'. For example: an L4S packet means a packet with an L4S
	identifier sent from an L4S congestion control.		identifier sent from an L4S congestion control.
	Both Classic and L4S services can cope with a proportion of		Both Classic and L4S services can cope with a proportion of
	unresponsive or less-responsive traffic as well, but in the L4S		unresponsive or less-responsive traffic as well, but in the L4S
	case its rate has to be smooth enough or low enough not to build a		case its rate has to be smooth enough or low enough not to build a
	queue (e.g. DNS, VoIP, game sync datagrams, etc).		queue (e.g. DNS, VoIP, game sync datagrams, etc.).
	Reno-friendly: The subset of Classic traffic that is friendly to the		Reno-friendly: The subset of Classic traffic that is friendly to the
	standard Reno congestion control defined for TCP in [RFC5681].		standard Reno congestion control defined for TCP in [RFC5681].
	The TFRC spec [RFC5348] indirectly implies that 'friendly' is		The TFRC spec [RFC5348] indirectly implies that 'friendly' is
	defined as "generally within a factor of two of the sending rate		defined as "generally within a factor of two of the sending rate
	of a TCP flow under the same conditions". Reno-friendly is used		of a TCP flow under the same conditions". Reno-friendly is used
	here in place of 'TCP-friendly', given the latter has become		here in place of 'TCP-friendly', given the latter has become
	imprecise, because the TCP protocol is now used with so many		imprecise, because the TCP protocol is now used with so many
	different congestion control behaviours, and Reno can be used in		different congestion control behaviours, and Reno can be used in
	non-TCP transports such as QUIC [RFC9000].		non-TCP transports such as QUIC [RFC9000].
	skipping to change at page 10, line 6 ¶		skipping to change at page 10, line 6 ¶
	The new L4S identifier defined in this specification is applicable		The new L4S identifier defined in this specification is applicable
	for IPv4 and IPv6 packets (as for Classic ECN [RFC3168]). It is		for IPv4 and IPv6 packets (as for Classic ECN [RFC3168]). It is
	applicable for the unicast, multicast and anycast forwarding modes.		applicable for the unicast, multicast and anycast forwarding modes.
	The L4S identifier is an orthogonal packet classification to the		The L4S identifier is an orthogonal packet classification to the
	Differentiated Services Code Point (DSCP) [RFC2474]. Section 5.4		Differentiated Services Code Point (DSCP) [RFC2474]. Section 5.4
	explains what this means in practice.		explains what this means in practice.
	This document is intended for experimental status, so it does not		This document is intended for experimental status, so it does not
	update any standards track RFCs. Therefore it depends on [RFC8311],		update any standards track RFCs. Therefore, it depends on [RFC8311],
	which is a standards track specification that:		which is a standards track specification that:
	* updates the ECN proposed standard [RFC3168] to allow experimental		* updates the ECN proposed standard [RFC3168] to allow experimental
	track RFCs to relax the requirement that an ECN mark must be		track RFCs to relax the requirement that an ECN mark must be
	equivalent to a drop (when the network applies markings and/or		equivalent to a drop (when the network applies markings and/or
	when the sender responds to them). For instance, in the ABE		when the sender responds to them). For instance, in the ABE
	experiment [RFC8511] this permits a sender to respond less to ECN		experiment [RFC8511] this permits a sender to respond less to ECN
	marks than to drops;		marks than to drops;
	* changes the status of the experimental ECN nonce [RFC3540] to		* changes the status of the experimental ECN nonce [RFC3540] to
	skipping to change at page 12, line 25 ¶		skipping to change at page 12, line 25 ¶
	* it SHOULD be consistent on all the packets of a transport layer		* it SHOULD be consistent on all the packets of a transport layer
	flow, so that some packets of a flow are not served by a different		flow, so that some packets of a flow are not served by a different
	queue to others.		queue to others.
	Whether the identifier would be recoverable if the experiment failed		Whether the identifier would be recoverable if the experiment failed
	is a factor that could be taken into account. However, this has not		is a factor that could be taken into account. However, this has not
	been made a requirement, because that would favour schemes that would		been made a requirement, because that would favour schemes that would
	be easier to fail, rather than those more likely to succeed.		be easier to fail, rather than those more likely to succeed.
	It is recognised that any choice of identifier is unlikely to satisfy		It is recognized that any choice of identifier is unlikely to satisfy
	all these requirements, particularly given the limited space left in		all these requirements, particularly given the limited space left in
	the IP header. Therefore a compromise will always be necessary,		the IP header. Therefore, a compromise will always be necessary,
	which is why all the above requirements are expressed with the word		which is why all the above requirements are expressed with the word
	'SHOULD' not 'MUST'.		'SHOULD' not 'MUST'.
	After extensive assessment of alternative schemes, "ECT(1) and CE		After extensive assessment of alternative schemes, "ECT(1) and CE
	codepoints" was chosen as the best compromise. Therefore this scheme		codepoints" was chosen as the best compromise. Therefore, this
	is defined in detail in the following sections, while Appendix B		scheme is defined in detail in the following sections, while
	records its pros and cons against the above requirements.		Appendix B records its pros and cons against the above requirements.
	4. Transport Layer Behaviour (the 'Prague Requirements')		4. Transport Layer Behaviour (the 'Prague Requirements')
	This section defines L4S behaviour at the transport layer, also known		This section defines L4S behaviour at the transport layer, also known
	as the Prague L4S Requirements (see Appendix A for the origin of the		as the Prague L4S Requirements (see Appendix A for the origin of the
	name).		name).
	4.1. Codepoint Setting		4.1. Codepoint Setting
	A sender that wishes a packet to receive L4S treatment as it is		A sender that wishes a packet to receive L4S treatment as it is
	skipping to change at page 13, line 28 ¶		skipping to change at page 13, line 28 ¶
	(such as that provided by AccECN [I-D.ietf-tcpm-accurate-ecn]) by		(such as that provided by AccECN [I-D.ietf-tcpm-accurate-ecn]) by
	both ends is a prerequisite for scalable congestion control in		both ends is a prerequisite for scalable congestion control in
	TCP. Therefore, the presence of ECT(1) in the IP headers even in		TCP. Therefore, the presence of ECT(1) in the IP headers even in
	one direction of a TCP connection will imply that both ends		one direction of a TCP connection will imply that both ends
	support accurate ECN feedback. However, the converse does not		support accurate ECN feedback. However, the converse does not
	apply. So even if both ends support AccECN, either of the two		apply. So even if both ends support AccECN, either of the two
	ends can choose not to use a scalable congestion control, whatever		ends can choose not to use a scalable congestion control, whatever
	the other end's choice.		the other end's choice.
	SCTP: A suitable ECN feedback mechanism for SCTP could add a chunk		SCTP: A suitable ECN feedback mechanism for SCTP could add a chunk
	to report the number of received CE marks		to report the number of received CE marks (as described in a long-
	(e.g. [I-D.stewart-tsvwg-sctpecn]), and update the ECN feedback		expired draft [I-D.stewart-tsvwg-sctpecn] or as sketched out in
	protocol sketched out in Appendix A of the original standards		Appendix A of the now obsolete second standards track
	track specification of SCTP [RFC4960].		specification of SCTP [RFC4960]).
	RTP over UDP: A prerequisite for scalable congestion control is for		RTP over UDP: A prerequisite for scalable congestion control is for
	both (all) ends of one media-level hop to signal ECN		both (all) ends of one media-level hop to signal ECN
	support [RFC6679] and use the new generic RTCP feedback format of		support [RFC6679] and use the new generic RTCP feedback format of
	[RFC8888]. The presence of ECT(1) implies that both (all) ends of		[RFC8888]. The presence of ECT(1) implies that both (all) ends of
	that media-level hop support ECN. However, the converse does not		that media-level hop support ECN. However, the converse does not
	apply. So each end of a media-level hop can independently choose		apply. So each end of a media-level hop can independently choose
	not to use a scalable congestion control, even if both ends		not to use a scalable congestion control, even if both ends
	support ECN.		support ECN.
	skipping to change at page 14, line 19 ¶		skipping to change at page 14, line 19 ¶
	ensures that, in steady state, the average duration between induced		ensures that, in steady state, the average duration between induced
	ECN marks does not increase as flow rate scales up, all other factors		ECN marks does not increase as flow rate scales up, all other factors
	being equal. This is termed a scalable congestion control. This		being equal. This is termed a scalable congestion control. This
	invariant duration ensures that, as flow rate scales, the average		invariant duration ensures that, as flow rate scales, the average
	period with no feedback information about capacity does not become		period with no feedback information about capacity does not become
	excessive. It also ensures that queue variations remain small,		excessive. It also ensures that queue variations remain small,
	without having to sacrifice utilization.		without having to sacrifice utilization.
	With a congestion control that sawtooths to probe capacity, this		With a congestion control that sawtooths to probe capacity, this
	duration is called the recovery time, because each time the sawtooth		duration is called the recovery time, because each time the sawtooth
	yields, on average it take this time to recover to its previous high		yields, on average it takes this time to recover to its previous high
	point. A scalable congestion control does not have to sawtooth, but		point. A scalable congestion control does not have to sawtooth, but
	it has to coexist with scalable congestion controls that do.		it has to coexist with scalable congestion controls that do.
	For instance, for DCTCP [RFC8257], TCP Prague		For instance, for DCTCP [RFC8257], TCP Prague
	[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux] and the		[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux] and the
	L4S variant of SCReAM [SCReAM-L4S], [RFC8298], the average recovery		L4S variant of SCReAM [SCReAM-L4S], [RFC8298], the average recovery
	time is always half a round trip (or half a reference round trip),		time is always half a round trip (or half a reference round trip),
	whatever the flow rate.		whatever the flow rate.
	As with all transport behaviours, a detailed specification (probably		As with all transport behaviours, a detailed specification (probably
	an experimental RFC) is expected for each congestion control,		an experimental RFC) is expected for each congestion control,
	following the guidelines for specifying new congestion control		following the guidelines for specifying new congestion control
	algorithms in [RFC5033]. In addition it is expected to document		algorithms in [RFC5033]. In addition, it is expected to document
	these L4S-specific matters, specifically the timescale over which the		these L4S-specific matters, specifically the timescale over which the
	proportionality is averaged, and control of burstiness. The recovery		proportionality is averaged, and control of burstiness. The recovery
	time requirement above is worded as a 'SHOULD' rather than a 'MUST'		time requirement above is worded as a 'SHOULD' rather than a 'MUST'
	to allow reasonable flexibility for such implementations.		to allow reasonable flexibility for such implementations.
	The condition 'all other factors being equal', allows the recovery		The condition 'all other factors being equal', allows the recovery
	time to be different for different round trip times, as long as it		time to be different for different round trip times, as long as it
	does not increase with flow rate for any particular RTT.		does not increase with flow rate for any particular RTT.
	Saying that the recovery time remains roughly invariant is equivalent		Saying that the recovery time remains roughly invariant is equivalent
	to saying that the number of ECN CE marks per round trip remains		to saying that the number of ECN CE marks per round trip remains
	invariant as flow rate scales, all other factors being equal. For		invariant as flow rate scales, all other factors being equal. For
	instance, an average recovery time of half of 1 RTT is equivalent to		instance, an average recovery time of half of 1 RTT is equivalent to
	2 ECN marks per round trip. For those familiar with steady-state		2 ECN marks per round trip. For those familiar with steady-state
	congestion response functions, it is also equivalent to say that the		congestion response functions, it is also equivalent to say that the
	congestion window is inversely proportional to the proportion of		congestion window is inversely proportional to the proportion of
	bytes in packets marked with the CE codepoint (see section 2 of		bytes in packets marked with the CE codepoint (see section 2 of
	[PI2]).		[PI2]).
	In order to coexist safely with other Internet traffic, a scalable		In order to coexist safely with other Internet traffic, a scalable
	congestion control MUST NOT tag its packets with the ECT(1) codepoint		congestion control is not allowed to tag its packets with the ECT(1)
	unless it complies with the following bulleted requirements:		codepoint unless it complies with the following numbered requirements
			and recommendations:
	1. A scalable congestion control MUST be capable of being replaced		1. A scalable congestion control MUST be capable of being replaced
	by a Classic congestion control (by application and/or by		by a Classic congestion control (by application and/or by
	administrative control). If a Classic congestion control is		administrative control). If a Classic congestion control is
	activated, it will not tag its packets with the ECT(1) codepoint		activated, it will not tag its packets with the ECT(1) codepoint
	(see Appendix A.1.3 for rationale).		(see Appendix A.1.3 for rationale).
	2. As well as responding to ECN markings, a scalable congestion		2. As well as responding to ECN markings, a scalable congestion
	control MUST react to packet loss in a way that will coexist		control MUST react to packet loss in a way that will coexist
	safely with Classic congestion controls such as standard		safely with Classic congestion controls such as standard
	Reno [RFC5681], as required by [RFC5033] (see Appendix A.1.4 for		Reno [RFC5681], as required by [RFC5033] (see Appendix A.1.4 for
	rationale).		rationale).
	3. In uncontrolled environments, monitoring MUST be implemented to		3. In uncontrolled environments, monitoring MUST be implemented to
	support detection of problems with an ECN-capable AQM at the path		support detection of problems with an ECN-capable AQM at the path
	bottleneck that appears not to support L4S and might be in a		bottleneck that appears not to support L4S and might be in a
	shared queue. Such monitoring SHOULD be applied to live traffic		shared queue. Such monitoring SHOULD be applied to live traffic
	that is using Scalable congestion control. Alternatively,		that is using Scalable congestion control. Alternatively,
	monitoring need not be applied to live traffic, if monitoring has		monitoring need not be applied to live traffic, if monitoring
	been arranged to cover the paths that live traffic takes through		with test traffic has been arranged to cover the paths that live
	uncontrolled environments.		traffic takes through uncontrolled environments.
	A function to detect the above problems with an ECN-capable AQM		A function to detect the above problems with an ECN-capable AQM
	MUST also be implemented and used. The detection function SHOULD		MUST also be implemented and used. The detection function SHOULD
	be capable of making the congestion control adapt its ECN-marking		be capable of making the congestion control adapt its ECN-marking
	response in real-time to coexist safely with Classic congestion		response in real-time to coexist safely with Classic congestion
	controls such as standard Reno [RFC5681], as required by		controls such as standard Reno [RFC5681], as required by
	[RFC5033]. This could be complemented by more detailed offline		[RFC5033]. This could be complemented by more detailed offline
	detection of potential problems. If only offline detection is		detection of potential problems. If only offline detection is
	used and potential problems with such an AQM are detected on		used and potential problems with such an AQM are detected on
	certain paths, the scalable congestion control MUST be replaced		certain paths, the scalable congestion control MUST be replaced
	by a Classic congestion control, at least for the problem paths.		by a Classic congestion control, at least for the problem paths.
	See Section 4.3.1, Appendix A.1.5 and the L4S operational		See Section 4.3.1, Appendix A.1.5 and the L4S operational
	guidance [I-D.ietf-tsvwg-l4sops] for rationale.		guidance [I-D.ietf-tsvwg-l4sops] for rationale and explanation.
	Note that a scalable congestion control is not expected to change		Note that a scalable congestion control is not expected to change
	to setting ECT(0) while it transiently adapts to coexist with		to setting ECT(0) while it transiently adapts to coexist with
	Classic congestion controls, whereas a replacement congestion		Classic congestion controls, whereas a replacement congestion
	control that solely behaves in the Classic way will set ECT(0).		control that solely behaves in the Classic way will set ECT(0).
	4. In the range between the minimum likely RTT and typical RTTs		4. In the range between the minimum likely RTT and typical RTTs
	expected in the intended deployment scenario, a scalable		expected in the intended deployment scenario, a scalable
	congestion control MUST converge towards a rate that is as		congestion control MUST converge towards a rate that is as
	independent of RTT as is possible without compromising stability		independent of RTT as is possible without compromising stability
	skipping to change at page 16, line 50 ¶		skipping to change at page 16, line 50 ¶
	packets by multiplying by the current average packet rate. Then,		packets by multiplying by the current average packet rate. Then,
	the queue caused by each burst at the bottleneck link would not		the queue caused by each burst at the bottleneck link would not
	exceed 250us (under the worst-case assumption that the flow is		exceed 250us (under the worst-case assumption that the flow is
	filling the capacity). No normative requirement to limit bursts		filling the capacity). No normative requirement to limit bursts
	is given here and, until there is more industry experience from		is given here and, until there is more industry experience from
	the L4S experiment, it is not even known whether one is needed -		the L4S experiment, it is not even known whether one is needed -
	it seems to be in an L4S sender's self-interest to limit bursts.		it seems to be in an L4S sender's self-interest to limit bursts.
	Each sender in a session can use a scalable congestion control		Each sender in a session can use a scalable congestion control
	independently of the congestion control used by the receiver(s) when		independently of the congestion control used by the receiver(s) when
	they send data. Therefore there might be ECT(1) packets in one		they send data. Therefore, there might be ECT(1) packets in one
	direction and ECT(0) or Not-ECT in the other.		direction and ECT(0) or Not-ECT in the other.
	Later (Section 5.4.1.1) this document discusses the conditions for		Later (Section 5.4.1.1) this document discusses the conditions for
	mixing other "'Safe' Unresponsive Traffic" (e.g. DNS, LDAP, NTP,		mixing other "'Safe' Unresponsive Traffic" (e.g. DNS, LDAP, NTP,
	voice, game sync packets) with L4S traffic. To be clear, although		voice, game sync packets) with L4S traffic. To be clear, although
	such traffic can share the same queue as L4S traffic, it is not		such traffic can share the same queue as L4S traffic, it is not
	appropriate for the sender to tag it as ECT(1), except in the		appropriate for the sender to tag it as ECT(1), except in the
	(unlikely) case that it satisfies the above conditions.		(unlikely) case that it satisfies the above conditions.
	4.3.1. Guidance on Congestion Response in the RFC Series		4.3.1. Guidance on Congestion Response in the RFC Series
	skipping to change at page 18, line 24 ¶		skipping to change at page 18, line 24 ¶
	* Classic ECN [RFC3168]: The compromises centre around cases		* Classic ECN [RFC3168]: The compromises centre around cases
	where the bottleneck supports Classic ECN but not L4S. But it		where the bottleneck supports Classic ECN but not L4S. But it
	depends on which sub-case:		depends on which sub-case:
	- Shared Queue with Classic ECN: At the time of writing, the		- Shared Queue with Classic ECN: At the time of writing, the
	members of the Transport Working group are not aware of any		members of the Transport Working group are not aware of any
	current deployments of single-queue Classic ECN bottlenecks		current deployments of single-queue Classic ECN bottlenecks
	in the Internet. Nonetheless, at the scale of the Internet,		in the Internet. Nonetheless, at the scale of the Internet,
	rarity need not imply small numbers, nor that there will be		rarity need not imply small numbers, nor that there will be
	rarity in future.		rarity in the future.
	- Per-Flow-queues with Classic ECN: Most AQMs with per-flow-		- Per-Flow-queues with Classic ECN: Most AQMs with per-flow-
	queuing (FQ) deployed from 2012 onwards had Classic ECN		queuing (FQ) deployed from 2012 onwards had Classic ECN
	enabled by default, specifically FQ-CoDel [RFC8290] and		enabled by default, specifically FQ-CoDel [RFC8290] and
	COBALT [COBALT]. But the compromises only apply to the		COBALT [COBALT]. But the compromises only apply to the
	second of two further sub-cases:		second of two further sub-cases:
	o With per-flow-queuing, co-existence between Classic and		o With per-flow-queuing, co-existence between Classic and
	L4S flows is not normally a problem, because different		L4S flows is not normally a problem, because different
	flows are not meant to be in the same queue		flows are not meant to be in the same queue
	skipping to change at page 20, line 31 ¶		skipping to change at page 20, line 31 ¶
	This shift of responsibility has the advantage that each sender can		This shift of responsibility has the advantage that each sender can
	smooth variations over a timescale proportionate to its own RTT.		smooth variations over a timescale proportionate to its own RTT.
	Whereas, in the Classic approach, the network doesn't know the RTTs		Whereas, in the Classic approach, the network doesn't know the RTTs
	of any of the flows, so it has to smooth out variations for a worst-		of any of the flows, so it has to smooth out variations for a worst-
	case RTT to ensure stability. For all the typical flows with shorter		case RTT to ensure stability. For all the typical flows with shorter
	RTT than the worst-case, this makes congestion control unnecessarily		RTT than the worst-case, this makes congestion control unnecessarily
	sluggish.		sluggish.
	This also gives an L4S sender the choice not to smooth, depending on		This also gives an L4S sender the choice not to smooth, depending on
	its context (start-up, congestion avoidance, etc). Therefore, this		its context (start-up, congestion avoidance, etc.). Therefore, this
	document places no requirement on an L4S congestion control to smooth		document places no requirement on an L4S congestion control to smooth
	out variations in any particular way. Implementers are encouraged to		out variations in any particular way. Implementers are encouraged to
	openly publish the approach they take to smoothing, and the results		openly publish the approach they take to smoothing, and the results
	and experience they gain during the L4S experiment.		and experience they gain during the L4S experiment.
	5. Network Node Behaviour		5. Network Node Behaviour
	5.1. Classification and Re-Marking Behaviour		5.1. Classification and Re-Marking Behaviour
	A network node that implements the L4S service:		A network node that implements the L4S service:
	* MUST classify arriving ECT(1) packets for L4S treatment, unless		* MUST classify arriving ECT(1) packets for L4S treatment, unless
	overridden by another classifier (e.g., see Section 5.4.1.2);		overridden by another classifier (e.g., see Section 5.4.1.2);
	* MUST classify arriving CE packets for L4S treatment as well,		* MUST classify arriving CE packets for L4S treatment as well,
	unless overridden by a another classifier or unless the exception		unless overridden by another classifier or unless the exception
	referred to next applies;		referred to next applies;
	CE packets might have originated as ECT(1) or ECT(0), but the		CE packets might have originated as ECT(1) or ECT(0), but the
	above rule to classify them as if they originated as ECT(1) is the		above rule to classify them as if they originated as ECT(1) is the
	safe choice (see Appendix B for rationale). The exception is		safe choice (see Appendix B for rationale). The exception is
	where some flow-aware in-network mechanism happens to be available		where some flow-aware in-network mechanism happens to be available
	for distinguishing CE packets that originated as ECT(0), as		for distinguishing CE packets that originated as ECT(0), as
	described in Section 5.3, but there is no implication that such a		described in Section 5.3, but there is no implication that such a
	mechanism is necessary.		mechanism is necessary.
	An L4S AQM treatment follows similar codepoint transition rules to		An L4S AQM treatment follows similar codepoint transition rules to
	skipping to change at page 22, line 22 ¶		skipping to change at page 22, line 22 ¶
	signals in a DualQ Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled], as		signals in a DualQ Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled], as
	below.		below.
	Unless an AQM node schedules application flows explicitly, the		Unless an AQM node schedules application flows explicitly, the
	likelihood that the AQM drops a Not-ECT Classic packet (p_C) MUST be		likelihood that the AQM drops a Not-ECT Classic packet (p_C) MUST be
	roughly proportional to the square of the likelihood that it would		roughly proportional to the square of the likelihood that it would
	have marked it if it had been an L4S packet (p_L). That is		have marked it if it had been an L4S packet (p_L). That is
	p_C ~= (p_L / k)^2		p_C ~= (p_L / k)^2
	The constant of proportionality (k) does not have to be standardised		The constant of proportionality (k) does not have to be standardized
	for interoperability, but a value of 2 is RECOMMENDED. The term		for interoperability, but a value of 2 is RECOMMENDED. The term
	'likelihood' is used above to allow for marking and dropping to be		'likelihood' is used above to allow for marking and dropping to be
	either probabilistic or deterministic.		either probabilistic or deterministic.
	This formula ensures that Scalable and Classic flows will converge to		This formula ensures that Scalable and Classic flows will converge to
	roughly equal congestion windows, for the worst case of Reno		roughly equal congestion windows, for the worst case of Reno
	congestion control. This is because the congestion windows of		congestion control. This is because the congestion windows of
	Scalable and Classic congestion controls are inversely proportional		Scalable and Classic congestion controls are inversely proportional
	to p_L and sqrt(p_C) respectively. So squaring p_C in the above		to p_L and sqrt(p_C) respectively. So squaring p_C in the above
	formula counterbalances the square root that characterizes Reno-		formula counterbalances the square root that characterizes Reno-
	skipping to change at page 23, line 49 ¶		skipping to change at page 23, line 49 ¶
	5.4.1. DualQ Examples of Other Identifiers Complementing L4S		5.4.1. DualQ Examples of Other Identifiers Complementing L4S
	Identifiers		Identifiers
	5.4.1.1. Inclusion of Additional Traffic with L4S		5.4.1.1. Inclusion of Additional Traffic with L4S
	In a typical case for the public Internet a network element that		In a typical case for the public Internet a network element that
	implements L4S in a shared queue might want to classify some low-rate		implements L4S in a shared queue might want to classify some low-rate
	but unresponsive traffic (e.g. DNS, LDAP, NTP, voice, game sync		but unresponsive traffic (e.g. DNS, LDAP, NTP, voice, game sync
	packets) into the low latency queue to mix with L4S traffic. In this		packets) into the low latency queue to mix with L4S traffic. In this
	case it would not be appropriate to call the queue an L4S queue,		case it would not be appropriate to call the queue an L4S queue,
	because it is shared by L4S and non-L4S traffic. Instead it will be		because it is shared by L4S and non-L4S traffic. Instead, it will be
	called the low latency or L queue. The L queue then offers two		called the low latency or L queue. The L queue then offers two
	different treatments:		different treatments:
	* The L4S treatment, which is a combination of the L4S AQM treatment		* The L4S treatment, which is a combination of the L4S AQM treatment
	and a priority scheduling treatment;		and a priority scheduling treatment;
	* The low latency treatment, which is solely the priority scheduling		* The low latency treatment, which is solely the priority scheduling
	treatment, without ECN-marking by the AQM.		treatment, without ECN-marking by the AQM.
	To identify packets for just the scheduling treatment, it would be		To identify packets for just the scheduling treatment, it would be
	skipping to change at page 24, line 28 ¶		skipping to change at page 24, line 28 ¶
	intensity traffic;		intensity traffic;
	* certain low data-volume applications or protocols (e.g. ARP, DNS);		* certain low data-volume applications or protocols (e.g. ARP, DNS);
	* specific Diffserv codepoints that indicate traffic with limited		* specific Diffserv codepoints that indicate traffic with limited
	burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-		burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-
	Admit [RFC5865] or proposed NQB (Non-Queue-		Admit [RFC5865] or proposed NQB (Non-Queue-
	Building [I-D.ietf-tsvwg-nqb]) service classes or equivalent		Building [I-D.ietf-tsvwg-nqb]) service classes or equivalent
	local-use DSCPs (see [I-D.briscoe-tsvwg-l4s-diffserv]).		local-use DSCPs (see [I-D.briscoe-tsvwg-l4s-diffserv]).
			To be clear, classifying into the L queue based on application layer
			identification (e.g. DNS) is an example of a local optimization, not
			a recommendation. Applications will not be able to rely on such
			unsolicited optimization. A more reliable approach would be for the
			sender to set an appropriate IP layer identifier, such as one of the
			above Diffserv codepoints.
	In summary, a network element that implements L4S in a shared queue		In summary, a network element that implements L4S in a shared queue
	MAY classify additional types of packets into the L queue based on		MAY classify additional types of packets into the L queue based on
	identifiers other than the ECN field, but the types SHOULD be 'safe'		identifiers other than the ECN field, but the types SHOULD be 'safe'
	to mix with L4S traffic, where 'safe' is explained in		to mix with L4S traffic, where 'safe' is explained in
	Section 5.4.1.1.1.		Section 5.4.1.1.1.
	A packet that carries one of these non-ECN identifiers to classify it		A packet that carries one of these non-ECN identifiers to classify it
	into the L queue would not be subject to the L4S ECN marking		into the L queue would not be subject to the L4S ECN marking
	treatment, unless it also carried an ECT(1) or CE codepoint. The		treatment, unless it also carried an ECT(1) or CE codepoint. The
	specification of an L4S AQM MUST define the behaviour for packets		specification of an L4S AQM MUST define the behaviour for packets
	skipping to change at page 29, line 28 ¶		skipping to change at page 29, line 38 ¶
	redesign work relevant to the most problematic link types. Such		redesign work relevant to the most problematic link types. Such
	knock-on effects of initial L4S deployment would all be part of the		knock-on effects of initial L4S deployment would all be part of the
	learning from the L4S experiment.		learning from the L4S experiment.
	The details of such link changes are beyond the scope of the present		The details of such link changes are beyond the scope of the present
	document. Nonetheless, where L4S technology is being implemented on		document. Nonetheless, where L4S technology is being implemented on
	an outgoing interface of a device, it would make sense to consider		an outgoing interface of a device, it would make sense to consider
	opportunities for reducing bursts arriving at other incoming		opportunities for reducing bursts arriving at other incoming
	interface(s). For instance, where an L4S AQM is implemented to feed		interface(s). For instance, where an L4S AQM is implemented to feed
	into the upstream WAN interface of a home gateway, there would be		into the upstream WAN interface of a home gateway, there would be
	opportunities to alter the WiFi profiles sent out of any WiFi		opportunities to alter the Wi-Fi profiles sent out of any Wi-Fi
	interfaces from the same device, in order to mitigate incoming bursts		interfaces from the same device, in order to mitigate incoming bursts
	of aggregated WiFi frames from other WiFi stations.		of aggregated Wi-Fi frames from other Wi-Fi stations.
	6. Behaviour of Tunnels and Encapsulations		6. Behaviour of Tunnels and Encapsulations
	6.1. No Change to ECN Tunnels and Encapsulations in General		6.1. No Change to ECN Tunnels and Encapsulations in General
	The L4S identifier is expected to work through and within any tunnel		The L4S identifier is expected to work through and within any tunnel
	without modification, as long as the tunnel propagates the ECN field		without modification, as long as the tunnel propagates the ECN field
	in any of the ways that have been defined since the first variant in		in any of the ways that have been defined since the first variant in
	the year 2001 [RFC3168]. L4S will also work with (but does not rely		the year 2001 [RFC3168]. L4S will also work with (but does not rely
	on) any of the more recent updates to ECN propagation in [RFC4301],		on) any of the more recent updates to ECN propagation in [RFC4301],
	skipping to change at page 30, line 21 ¶		skipping to change at page 30, line 30 ¶
	6.2. VPN Behaviour to Avoid Limitations of Anti-Replay		6.2. VPN Behaviour to Avoid Limitations of Anti-Replay
	If a mix of L4S and Classic packets is sent into the same security		If a mix of L4S and Classic packets is sent into the same security
	association (SA) of a virtual private network (VPN), and if the VPN		association (SA) of a virtual private network (VPN), and if the VPN
	egress is employing the optional anti-replay feature, it could		egress is employing the optional anti-replay feature, it could
	inappropriately discard Classic packets (or discard the records in		inappropriately discard Classic packets (or discard the records in
	Classic packets) by mistaking their greater queuing delay for a		Classic packets) by mistaking their greater queuing delay for a
	replay attack (see "Dropped Packets for Tunnels with Replay		replay attack (see "Dropped Packets for Tunnels with Replay
	Protection Enabled" in [Heist21] for the potential performance		Protection Enabled" in [Heist21] for the potential performance
	impact). This known problem is common to both IPsec [RFC4301] and		impact). This known problem is common to both IPsec [RFC4301] and
	DTLS [RFC6347] VPNs, given they use similar anti-replay window		DTLS [RFC9147] VPNs, given they use similar anti-replay window
	mechanisms. The mechanism used can only check for replay within its		mechanisms. The mechanism used can only check for replay within its
	window, so if the window is smaller than the degree of reordering, it		window, so if the window is smaller than the degree of reordering, it
	can only assume there might be a replay attack and discard all the		can only assume there might be a replay attack and discard all the
	packets behind the trailing edge of the window. The specifications		packets behind the trailing edge of the window. The specifications
	of IPsec AH [RFC4302] and ESP [RFC4303] suggest that an implementer		of IPsec AH [RFC4302] and ESP [RFC4303] suggest that an implementer
	scales the size of the anti-replay window with interface speed, and		scales the size of the anti-replay window with interface speed, and
	DTLS 1.3 [I-D.ietf-tls-dtls13] says "The receiver SHOULD pick a		DTLS v1.3 [RFC9147] says "The receiver SHOULD pick a window large
	window large enough to handle any plausible reordering, which depends		enough to handle any plausible reordering, which depends on the data
	on the data rate." However, in practice, the size of a VPN's anti-		rate." However, in practice, the size of a VPN's anti-replay window
	replay window is not always scaled appropriately.		is not always scaled appropriately.
	If a VPN carrying traffic participating in the L4S experiment		If a VPN carrying traffic participating in the L4S experiment
	experiences inappropriate replay detection, the foremost remedy would		experiences inappropriate replay detection, the foremost remedy would
	be to ensure that the egress is configured to comply with the above		be to ensure that the egress is configured to comply with the above
	window-sizing requirements.		window-sizing requirements.
	If an implementation of a VPN egress does not support a sufficiently		If an implementation of a VPN egress does not support a sufficiently
	large anti-replay window, e.g. due to hardware limitations, one of		large anti-replay window, e.g. due to hardware limitations, one of
	the temporary alternatives listed in order of preference below might		the temporary alternatives listed in order of preference below might
	be feasible instead:		be feasible instead:
	skipping to change at page 33, line 4 ¶		skipping to change at page 33, line 13 ¶
	remedies)?		remedies)?
	* Was per-flow queue protection typically (un)necessary?		* Was per-flow queue protection typically (un)necessary?
	- How well did overload protection or queue protection work?		- How well did overload protection or queue protection work?
	* How well did L4S flows coexist with Classic flows when sharing a		* How well did L4S flows coexist with Classic flows when sharing a
	bottleneck?		bottleneck?
	- How frequently did problems arise?		- How frequently did problems arise?
	- What caused any coexistence problems, and were any problems due		- What caused any coexistence problems, and were any problems due
	to single-queue Classic ECN AQMs (this assumes single-queue		to single-queue Classic ECN AQMs (this assumes single-queue
	Classic ECN AQMs can be distinguished from FQ ones)?		Classic ECN AQMs can be distinguished from FQ ones)?
	* How prevalent were problems with the L4S service due to tunnels /		* How prevalent were problems with the L4S service due to tunnels /
	encapsulations that do not support ECN decapsulation?		encapsulations that do not support ECN decapsulation?
	* How easy was it to implement a fully compliant L4S congestion		* How easy was it to implement a fully compliant L4S congestion
	control, over various different transport protocols (TCP, QUIC,		control, over various different transport protocols (TCP, QUIC,
	RMCAT, etc)?		RMCAT, etc.)?
	Monitoring for harm to other traffic, specifically bandwidth		Monitoring for harm to other traffic, specifically bandwidth
	starvation or excess queuing delay, will need to be conducted		starvation or excess queuing delay, will need to be conducted
	alongside all early L4S experiments. It is hard, if not impossible,		alongside all early L4S experiments. It is hard, if not impossible,
	for an individual flow to measure its impact on other traffic. So		for an individual flow to measure its impact on other traffic. So
	such monitoring will need to be conducted using bespoke monitoring		such monitoring will need to be conducted using bespoke monitoring
	across flows and/or across classes of traffic.		across flows and/or across classes of traffic.
	7.2. Open Issues		7.2. Open Issues
	skipping to change at page 34, line 8 ¶		skipping to change at page 34, line 18 ¶
	* Potential for improvements to particular link technologies, and		* Potential for improvements to particular link technologies, and
	cross-layer interactions with them;		cross-layer interactions with them;
	* Potential for using virtual queues, e.g. to further reduce latency		* Potential for using virtual queues, e.g. to further reduce latency
	jitter, or to leave headroom for capacity variation in radio		jitter, or to leave headroom for capacity variation in radio
	networks;		networks;
	* Development and specification of reverse path congestion control		* Development and specification of reverse path congestion control
	using L4S building blocks (e.g. AccECN, QUIC);		using L4S building blocks (e.g. AccECN, QUIC);
	* Once queuing delay is cut down, what becomes the 'second longest		* Once queuing delay is cut down, what becomes the 'second-longest
	pole in the tent' (other than the speed of light)?		pole in the tent' (other than the speed of light)?
	* Novel alternatives to the existing set of L4S AQMs;		* Novel alternatives to the existing set of L4S AQMs;
	* Novel applications enabled by L4S.		* Novel applications enabled by L4S.
	8. IANA Considerations		8. IANA Considerations
	The 01 codepoint of the ECN Field of the IP header is specified by		The 01 codepoint of the ECN Field of the IP header is specified by
	the present Experimental RFC. The process for an experimental RFC to		the present Experimental RFC. The process for an experimental RFC to
	skipping to change at page 36, line 15 ¶		skipping to change at page 36, line 30 ¶
	[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,		[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
	and K. Carlberg, "Explicit Congestion Notification (ECN)		and K. Carlberg, "Explicit Congestion Notification (ECN)
	for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August		for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
	2012, <https://www.rfc-editor.org/info/rfc6679>.		2012, <https://www.rfc-editor.org/info/rfc6679>.
	10.2. Informative References		10.2. Informative References
	[A2DTCP] Zhang, T., Wang, J., Huang, J., Huang, Y., Chen, J., and		[A2DTCP] Zhang, T., Wang, J., Huang, J., Huang, Y., Chen, J., and
	Y. Pan, "Adaptive-Acceleration Data Center TCP", IEEE		Y. Pan, "Adaptive-Acceleration Data Center TCP", IEEE
	Transactions on Computers 64(6):1522-1533, June 2015,		Transactions on Computers 64(6):1522-1533, June 2015,
	<http://ieeexplore.ieee.org/xpl/		<https://ieeexplore.ieee.org/xpl/
	articleDetails.jsp?arnumber=6871352>.		articleDetails.jsp?arnumber=6871352>.
	[Ahmed19] Ahmed, A.S., "Extending TCP for Low Round Trip Delay",		[Ahmed19] Ahmed, A.S., "Extending TCP for Low Round Trip Delay",
	Masters Thesis, Uni Oslo , August 2019,		Master's Thesis, Uni Oslo , August 2019,
	<https://www.duo.uio.no/handle/10852/70966>.		<https://www.duo.uio.no/handle/10852/70966>.
	[Alizadeh-stability]		[Alizadeh-stability]
	Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis		Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
	of DCTCP: Stability, Convergence, and Fairness", ACM		of DCTCP: Stability, Convergence, and Fairness", ACM
	SIGMETRICS 2011 , June 2011,		SIGMETRICS 2011 , June 2011,
	<https://people.csail.mit.edu/alizadeh/papers/		<https://people.csail.mit.edu/alizadeh/papers/
	dctcp_analysis-sigmetrics11.pdf>.		dctcp_analysis-sigmetrics11.pdf>.
	[ARED01] Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An		[ARED01] Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
	Algorithm for Increasing the Robustness of RED's Active		Algorithm for Increasing the Robustness of RED's Active
	Queue Management", ACIRI Technical Report , August 2001,		Queue Management", ACIRI Technical Report , August 2001,
	<http://www.icir.org/floyd/red.html>.		<https://www.icir.org/floyd/red.html>.
	[BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", github		[BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", GitHub
	repository; Linux congestion control module,		repository; Linux congestion control module,
	<https://github.com/google/bbr/blob/v2alpha/README.md>.		<https://github.com/google/bbr/blob/v2alpha/README.md>.
			[Bufferbloat]
			"Bufferbloat", <https://bufferbloat.net/>. (last accessed
			27 Aug 2022)
	[COBALT] Palmei, J., Gupta, S., Imputato, P., Morton, J.,		[COBALT] Palmei, J., Gupta, S., Imputato, P., Morton, J.,
	Tahiliani, M., Avallone, S., and D. Taht, "Design and		Tahiliani, M., Avallone, S., and D. Taht, "Design and
	Evaluation of COBALT Queue Discipline", In Proc. IEEE		Evaluation of COBALT Queue Discipline", In Proc. IEEE
	Int'l Symp. on Local and Metropolitan Area Networks 2019,		Int'l Symp. on Local and Metropolitan Area Networks 2019,
	pp1--6, 2019,		pp1--6, 2019,
	<https://doi.org/10.1109/LANMAN.2019.8847054>.		<https://doi.org/10.1109/LANMAN.2019.8847054>.
	[DCttH19] De Schepper, K., Bondarenko, O., Tilmans, O., and B.		[DCttH19] De Schepper, K., Bondarenko, O., Tilmans, O., and B.
	Briscoe, "`Data Centre to the Home': Ultra-Low Latency for		Briscoe, "`Data Centre to the Home': Ultra-Low Latency for
	All", Updated RITE project Technical Report , July 2019,		All", Updated RITE project Technical Report , July 2019,
	skipping to change at page 37, line 20 ¶		skipping to change at page 37, line 40 ¶
	the Right Metric for Congestion Control", ACM CCR		the Right Metric for Congestion Control", ACM CCR
	36(1):59--62, January 2006,		36(1):59--62, January 2006,
	<https://dl.acm.org/doi/10.1145/1111322.1111336>.		<https://dl.acm.org/doi/10.1145/1111322.1111336>.
	[ecn-fallback]		[ecn-fallback]
	Briscoe, B. and A.S. Ahmed, "TCP Prague Fall-back on		Briscoe, B. and A.S. Ahmed, "TCP Prague Fall-back on
	Detection of a Classic ECN AQM", bobbriscoe.net Technical		Detection of a Classic ECN AQM", bobbriscoe.net Technical
	Report TR-BB-2019-002, April 2020,		Report TR-BB-2019-002, April 2020,
	<https://arxiv.org/abs/1911.00710>.		<https://arxiv.org/abs/1911.00710>.
	[Heist21] Heist, P. and J. Morton, "L4S Tests", github README, May		[Heist21] Heist, P. and J. Morton, "L4S Tests", GitHub README, May
	2021, <https://github.com/heistp/l4s-tests/>.		2021, <https://github.com/heistp/l4s-tests/>.
	[I-D.briscoe-docsis-q-protection]		[I-D.briscoe-docsis-q-protection]
	Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection		Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection
	Algorithm to Preserve Low Latency", Work in Progress,		Algorithm to Preserve Low Latency", Work in Progress,
	Internet-Draft, draft-briscoe-docsis-q-protection-06, 13		Internet-Draft, draft-briscoe-docsis-q-protection-06, 13
	May 2022, <https://www.ietf.org/archive/id/draft-briscoe-		May 2022,
	docsis-q-protection-06.txt>.		<https://datatracker.ietf.org/api/v1/doc/document/draft-
			briscoe-docsis-q-protection/>.
	[I-D.briscoe-iccrg-prague-congestion-control]		[I-D.briscoe-iccrg-prague-congestion-control]
	Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague		Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague
	Congestion Control", Work in Progress, Internet-Draft,		Congestion Control", Work in Progress, Internet-Draft,
	draft-briscoe-iccrg-prague-congestion-control-01, 11 July		draft-briscoe-iccrg-prague-congestion-control-01, 11 July
	2022, <https://www.ietf.org/archive/id/draft-briscoe-		2022, <https://datatracker.ietf.org/api/v1/doc/document/
	iccrg-prague-congestion-control-01.txt>.		draft-briscoe-iccrg-prague-congestion-control/>.
	[I-D.briscoe-tsvwg-l4s-diffserv]		[I-D.briscoe-tsvwg-l4s-diffserv]
	Briscoe, B., "Interactions between Low Latency, Low Loss,		Briscoe, B., "Interactions between Low Latency, Low Loss,
	Scalable Throughput (L4S) and Differentiated Services",		Scalable Throughput (L4S) and Differentiated Services",
	Work in Progress, Internet-Draft, draft-briscoe-tsvwg-l4s-		Work in Progress, Internet-Draft, draft-briscoe-tsvwg-l4s-
	diffserv-02, November 2018,		diffserv-02, 2 July 2018,
	<https://www.ietf.org/archive/id/draft-briscoe-tsvwg-l4s-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	diffserv-02.txt>.		briscoe-tsvwg-l4s-diffserv/>.
	[I-D.cardwell-iccrg-bbr-congestion-control]		[I-D.cardwell-iccrg-bbr-congestion-control]
	Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V.		Cardwell, N., Cheng, Y., Yeganeh, S. H., Swett, I., and V.
	Jacobson, "BBR Congestion Control", Work in Progress,		Jacobson, "BBR Congestion Control", Work in Progress,
	Internet-Draft, draft-cardwell-iccrg-bbr-congestion-		Internet-Draft, draft-cardwell-iccrg-bbr-congestion-
	control-02, March 2022, <https://www.ietf.org/archive/id/		control-02, 7 March 2022,
	draft-cardwell-iccrg-bbr-congestion-control-02.txt>.		<https://datatracker.ietf.org/api/v1/doc/document/draft-
			cardwell-iccrg-bbr-congestion-control/>.
	[I-D.ietf-tcpm-accurate-ecn]		[I-D.ietf-tcpm-accurate-ecn]
	Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More		Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
	Accurate ECN Feedback in TCP", Work in Progress, Internet-		Accurate ECN Feedback in TCP", Work in Progress, Internet-
	Draft, draft-ietf-tcpm-accurate-ecn-20, 25 July 2022,		Draft, draft-ietf-tcpm-accurate-ecn-20, 25 July 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	ecn-20.txt>.		ietf-tcpm-accurate-ecn/>.
	[I-D.ietf-tcpm-generalized-ecn]		[I-D.ietf-tcpm-generalized-ecn]
	Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit		Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
	Congestion Notification (ECN) to TCP Control Packets",		Congestion Notification (ECN) to TCP Control Packets",
	Work in Progress, Internet-Draft, draft-ietf-tcpm-		Work in Progress, Internet-Draft, draft-ietf-tcpm-
	generalized-ecn-10, 27 July 2022,		generalized-ecn-10, 27 July 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tcpm-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	generalized-ecn-10.txt>.		ietf-tcpm-generalized-ecn/>.
	[I-D.ietf-tls-dtls13]
	Rescorla, E., Tschofenig, H., and N. Modadugu, "The
	Datagram Transport Layer Security (DTLS) Protocol Version
	1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
	dtls13-43, 30 April 2021,
	<https://www.ietf.org/archive/id/draft-ietf-tls-
	dtls13-43.txt>.
	[I-D.ietf-trill-ecn-support]		[I-D.ietf-trill-ecn-support]
	Eastlake, D. E. and B. Briscoe, "TRILL (TRansparent		Eastlake, D. E. and B. Briscoe, "TRILL (TRansparent
	Interconnection of Lots of Links): ECN (Explicit		Interconnection of Lots of Links): ECN (Explicit
	Congestion Notification) Support", Work in Progress,		Congestion Notification) Support", Work in Progress,
	Internet-Draft, draft-ietf-trill-ecn-support-07, 25		Internet-Draft, draft-ietf-trill-ecn-support-07, 25
	February 2018, <https://www.ietf.org/archive/id/draft-		February 2018,
	ietf-trill-ecn-support-07.txt>.		<https://datatracker.ietf.org/api/v1/doc/document/draft-
			ietf-trill-ecn-support/>.
	[I-D.ietf-tsvwg-aqm-dualq-coupled]		[I-D.ietf-tsvwg-aqm-dualq-coupled]
	Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled		Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled
	AQMs for Low Latency, Low Loss and Scalable Throughput		AQMs for Low Latency, Low Loss and Scalable Throughput
	(L4S)", Work in Progress, Internet-Draft, draft-ietf-		(L4S)", Work in Progress, Internet-Draft, draft-ietf-
	tsvwg-aqm-dualq-coupled-24, July 2022,		tsvwg-aqm-dualq-coupled-24, 7 July 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tsvwg-aqm-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	dualq-coupled-24.txt>.		ietf-tsvwg-aqm-dualq-coupled/>.
	[I-D.ietf-tsvwg-ecn-encap-guidelines]		[I-D.ietf-tsvwg-ecn-encap-guidelines]
	Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding		Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
	Congestion Notification to Protocols that Encapsulate IP",		Congestion Notification to Protocols that Encapsulate IP",
	Work in Progress, Internet-Draft, draft-ietf-tsvwg-ecn-		Work in Progress, Internet-Draft, draft-ietf-tsvwg-ecn-
	encap-guidelines-17, 11 July 2022,		encap-guidelines-17, 11 July 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tsvwg-ecn-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	encap-guidelines-17.txt>.		ietf-tsvwg-ecn-encap-guidelines/>.
	[I-D.ietf-tsvwg-l4s-arch]		[I-D.ietf-tsvwg-l4s-arch]
	Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White,		Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White,
	"Low Latency, Low Loss, Scalable Throughput (L4S) Internet		"Low Latency, Low Loss, Scalable Throughput (L4S) Internet
	Service: Architecture", Work in Progress, Internet-Draft,		Service: Architecture", Work in Progress, Internet-Draft,
	draft-ietf-tsvwg-l4s-arch-19, 27 July 2022,		draft-ietf-tsvwg-l4s-arch-19, 27 July 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tsvwg-l4s-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	arch-19.txt>.		ietf-tsvwg-l4s-arch/>.
	[I-D.ietf-tsvwg-l4sops]		[I-D.ietf-tsvwg-l4sops]
	White, G., "Operational Guidance for Deployment of L4S in		White, G., "Operational Guidance for Deployment of L4S in
	the Internet", Work in Progress, Internet-Draft, draft-		the Internet", Work in Progress, Internet-Draft, draft-
	ietf-tsvwg-l4sops-03, 28 April 2022,		ietf-tsvwg-l4sops-03, 28 April 2022,
	<https://www.ietf.org/archive/id/draft-ietf-tsvwg-l4sops-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	03.txt>.		ietf-tsvwg-l4sops/>.
	[I-D.ietf-tsvwg-nqb]		[I-D.ietf-tsvwg-nqb]
	White, G. and T. Fossati, "A Non-Queue-Building Per-Hop		White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
	Behavior (NQB PHB) for Differentiated Services", Work in		Behavior (NQB PHB) for Differentiated Services", Work in
	Progress, Internet-Draft, draft-ietf-tsvwg-nqb-10, March		Progress, Internet-Draft, draft-ietf-tsvwg-nqb-10, 4 March
	2022, <https://www.ietf.org/archive/id/draft-ietf-tsvwg-		2022, <https://datatracker.ietf.org/api/v1/doc/document/
	nqb-10.txt>.		draft-ietf-tsvwg-nqb/>.
	[I-D.ietf-tsvwg-rfc6040update-shim]		[I-D.ietf-tsvwg-rfc6040update-shim]
	Briscoe, B., "Propagating Explicit Congestion Notification		Briscoe, B., "Propagating Explicit Congestion Notification
	Across IP Tunnel Headers Separated by a Shim", Work in		Across IP Tunnel Headers Separated by a Shim", Work in
	Progress, Internet-Draft, draft-ietf-tsvwg-rfc6040update-		Progress, Internet-Draft, draft-ietf-tsvwg-rfc6040update-
	shim-15, 11 July 2022, <https://www.ietf.org/archive/id/		shim-15, 11 July 2022,
	draft-ietf-tsvwg-rfc6040update-shim-15.txt>.		<https://datatracker.ietf.org/api/v1/doc/document/draft-
			ietf-tsvwg-rfc6040update-shim/>.
			[I-D.mathis-iccrg-relentless-tcp]
			Mathis, M., "Relentless Congestion Control", Work in
			Progress, Internet-Draft, draft-mathis-iccrg-relentless-
			tcp-00, 4 March 2009, <https://www.ietf.org/archive/id/
			draft-mathis-iccrg-relentless-tcp-00.txt>.
	[I-D.sridharan-tcpm-ctcp]		[I-D.sridharan-tcpm-ctcp]
	Sridharan, M., Tan, K., Bansal, D., and D. Thaler,		Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
	"Compound TCP: A New TCP Congestion Control for High-Speed		"Compound TCP: A New TCP Congestion Control for High-Speed
	and Long Distance Networks", Work in Progress, Internet-		and Long Distance Networks", Work in Progress, Internet-
	Draft, draft-sridharan-tcpm-ctcp-02, 11 November 2008,		Draft, draft-sridharan-tcpm-ctcp-02, 29 October 2007,
	<https://www.ietf.org/archive/id/draft-sridharan-tcpm-		<https://datatracker.ietf.org/api/v1/doc/document/draft-
	ctcp-02.txt>.		sridharan-tcpm-ctcp/>.
	[I-D.stewart-tsvwg-sctpecn]		[I-D.stewart-tsvwg-sctpecn]
	Stewart, R. R., Tuexen, M., and X. Dong, "ECN for Stream		Stewart, R. R., Tuexen, M., and X. Dong, "ECN for Stream
	Control Transmission Protocol (SCTP)", Work in Progress,		Control Transmission Protocol (SCTP)", Work in Progress,
	Internet-Draft, draft-stewart-tsvwg-sctpecn-05, 15 January		Internet-Draft, draft-stewart-tsvwg-sctpecn-05, 15 January
	2014, <https://www.ietf.org/archive/id/draft-stewart-		2014, <https://www.ietf.org/archive/id/draft-stewart-
	tsvwg-sctpecn-05.txt>.		tsvwg-sctpecn-05.txt>.
	[LinuxPacedChirping]		[LinuxPacedChirping]
	Misund, J. and B. Briscoe, "Paced Chirping - Rethinking		Misund, J. and B. Briscoe, "Paced Chirping - Rethinking
	TCP start-up", Proc. Linux Netdev 0x13 , March 2019,		TCP start-up", Proc. Linux Netdev 0x13 , March 2019,
	<https://www.netdevconf.org/0x13/session.html?talk-chirp>.		<https://www.netdevconf.org/0x13/session.html?talk-chirp>.
	[Mathis09] Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
	May 2009, <http://www.hpcc.jp/pfldnet2009/
	Program_files/1569198525.pdf>.
	[Paced-Chirping]		[Paced-Chirping]
	Misund, J., "Rapid Acceleration in TCP Prague", Masters		Misund, J., "Rapid Acceleration in TCP Prague", Master's
	Thesis , May 2018,		Thesis , May 2018,
	<https://riteproject.files.wordpress.com/2018/07/		<https://riteproject.files.wordpress.com/2018/07/
	misundjoakimmastersthesissubmitted180515.pdf>.		misundjoakimmastersthesissubmitted180515.pdf>.
	[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.		[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.
	Briscoe, "PI^2 : A Linearized AQM for both Classic and		Briscoe, "PI^2 : A Linearized AQM for both Classic and
	Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December		Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
	2016,		2016,
	<http://dl.acm.org/citation.cfm?doid=2999572.2999578>.		<https://dl.acm.org/citation.cfm?doid=2999572.2999578>.
	[PragueLinux]		[PragueLinux]
	Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,		Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
	Tilmans, O., Kühlewind, M., and A.S. Ahmed, "Implementing		Tilmans, O., Kühlewind, M., and A.S. Ahmed, "Implementing
	the `TCP Prague' Requirements for Low Latency Low Loss		the `TCP Prague' Requirements for Low Latency Low Loss
	Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,		Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
	March 2019, <https://www.netdevconf.org/0x13/		March 2019, <https://www.netdevconf.org/0x13/
	session.html?talk-tcp-prague-l4s>.		session.html?talk-tcp-prague-l4s>.
	[QV] Briscoe, B. and P. Hurtig, "Up to Speed with Queue View",		[QV] Briscoe, B. and P. Hurtig, "Up to Speed with Queue View",
	skipping to change at page 40, line 48 ¶		skipping to change at page 41, line 24 ¶
	Queue Management and Congestion Avoidance in the		Queue Management and Congestion Avoidance in the
	Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,		Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
	<https://www.rfc-editor.org/info/rfc2309>.		<https://www.rfc-editor.org/info/rfc2309>.
	[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,		[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
	"Definition of the Differentiated Services Field (DS		"Definition of the Differentiated Services Field (DS
	Field) in the IPv4 and IPv6 Headers", RFC 2474,		Field) in the IPv4 and IPv6 Headers", RFC 2474,
	DOI 10.17487/RFC2474, December 1998,		DOI 10.17487/RFC2474, December 1998,
	<https://www.rfc-editor.org/info/rfc2474>.		<https://www.rfc-editor.org/info/rfc2474>.
	[RFC3246] Davie, B., Charny, A., Bennet, J C R., Benson, K., Le		[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
	Boudec, J Y., Courtney, W., Davari, S., Firoiu, V., and D.		Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
	Stiliadis, "An Expedited Forwarding PHB (Per-Hop		Stiliadis, "An Expedited Forwarding PHB (Per-Hop
	Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,		Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
	<https://www.rfc-editor.org/info/rfc3246>.		<https://www.rfc-editor.org/info/rfc3246>.
	[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit		[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
	Congestion Notification (ECN) Signaling with Nonces",		Congestion Notification (ECN) Signaling with Nonces",
	RFC 3540, DOI 10.17487/RFC3540, June 2003,		RFC 3540, DOI 10.17487/RFC3540, June 2003,
	<https://www.rfc-editor.org/info/rfc3540>.		<https://www.rfc-editor.org/info/rfc3540>.
	[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",		[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
	skipping to change at page 43, line 5 ¶		skipping to change at page 43, line 32 ¶
	[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion		[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
	Notification", RFC 6040, DOI 10.17487/RFC6040, November		Notification", RFC 6040, DOI 10.17487/RFC6040, November
	2010, <https://www.rfc-editor.org/info/rfc6040>.		2010, <https://www.rfc-editor.org/info/rfc6040>.
	[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.		[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
	Briscoe, "Open Research Issues in Internet Congestion		Briscoe, "Open Research Issues in Internet Congestion
	Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,		Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
	<https://www.rfc-editor.org/info/rfc6077>.		<https://www.rfc-editor.org/info/rfc6077>.
	[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
	Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
	January 2012, <https://www.rfc-editor.org/info/rfc6347>.
	[RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three		[RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
	Pre-Congestion Notification (PCN) States in the IP Header		Pre-Congestion Notification (PCN) States in the IP Header
	Using a Single Diffserv Codepoint (DSCP)", RFC 6660,		Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
	DOI 10.17487/RFC6660, July 2012,		DOI 10.17487/RFC6660, July 2012,
	<https://www.rfc-editor.org/info/rfc6660>.		<https://www.rfc-editor.org/info/rfc6660>.
	[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,		[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
	and Y. Nishida, "A Conservative Loss Recovery Algorithm		and Y. Nishida, "A Conservative Loss Recovery Algorithm
	Based on Selective Acknowledgment (SACK) for TCP",		Based on Selective Acknowledgment (SACK) for TCP",
	RFC 6675, DOI 10.17487/RFC6675, August 2012,		RFC 6675, DOI 10.17487/RFC6675, August 2012,
	skipping to change at page 44, line 5 ¶		skipping to change at page 44, line 30 ¶
	[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:		[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
	Circuit Breakers for Unicast RTP Sessions", RFC 8083,		Circuit Breakers for Unicast RTP Sessions", RFC 8083,
	DOI 10.17487/RFC8083, March 2017,		DOI 10.17487/RFC8083, March 2017,
	<https://www.rfc-editor.org/info/rfc8083>.		<https://www.rfc-editor.org/info/rfc8083>.
	[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage		[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
	Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,		Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
	March 2017, <https://www.rfc-editor.org/info/rfc8085>.		March 2017, <https://www.rfc-editor.org/info/rfc8085>.
			[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
			2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
			May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,		[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
	and G. Judd, "Data Center TCP (DCTCP): TCP Congestion		and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
	Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,		Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
	October 2017, <https://www.rfc-editor.org/info/rfc8257>.		October 2017, <https://www.rfc-editor.org/info/rfc8257>.
	[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,		[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
	J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler		J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
	and Active Queue Management Algorithm", RFC 8290,		and Active Queue Management Algorithm", RFC 8290,
	DOI 10.17487/RFC8290, January 2018,		DOI 10.17487/RFC8290, January 2018,
	<https://www.rfc-editor.org/info/rfc8290>.		<https://www.rfc-editor.org/info/rfc8290>.
	skipping to change at page 45, line 5 ¶		skipping to change at page 45, line 34 ¶
	[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based		[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
	Multiplexed and Secure Transport", RFC 9000,		Multiplexed and Secure Transport", RFC 9000,
	DOI 10.17487/RFC9000, May 2021,		DOI 10.17487/RFC9000, May 2021,
	<https://www.rfc-editor.org/info/rfc9000>.		<https://www.rfc-editor.org/info/rfc9000>.
	[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure		[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
	QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,		QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
	<https://www.rfc-editor.org/info/rfc9001>.		<https://www.rfc-editor.org/info/rfc9001>.
			[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
			Datagram Transport Layer Security (DTLS) Protocol Version
			1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
			<https://www.rfc-editor.org/info/rfc9147>.
	[Savage-TCP]		[Savage-TCP]
	Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,		Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
	"TCP Congestion Control with a Misbehaving Receiver", ACM		"TCP Congestion Control with a Misbehaving Receiver", ACM
	SIGCOMM Computer Communication Review 29(5):71--78,		SIGCOMM Computer Communication Review 29(5):71--78,
	October 1999.		October 1999.
	[SCReAM-L4S]		[SCReAM-L4S]
	Johansson, I., "SCReAM", github repository; ,		Johansson, I., "SCReAM", GitHub repository; ,
	<https://github.com/EricssonResearch/scream/blob/master/		<https://github.com/EricssonResearch/scream/blob/master/
	README.md>.		README.md>.
	[sub-mss-prob]		[sub-mss-prob]
	Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion		Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
	Window for Small Round Trip Times", BT Technical Report		Window for Small Round Trip Times", BT Technical Report
	TR-TUB8-2015-002, May 2015,		TR-TUB8-2015-002, May 2015,
	<https://arxiv.org/abs/1904.07598>.		<https://arxiv.org/abs/1904.07598>.
	[TCP-CA] Jacobson, V. and M.J. Karels, "Congestion Avoidance and		[TCP-CA] Jacobson, V. and M.J. Karels, "Congestion Avoidance and
	Control", Laurence Berkeley Labs Technical Report ,		Control", Laurence Berkeley Labs Technical Report ,
	November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>.		November 1988, <https://ee.lbl.gov/papers/congavoid.pdf>.
	[TCPPrague]		[TCPPrague]
	Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul		Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul
	2015, 17:40, Prague", tcpprague mailing list archive ,		2015, 17:40, Prague", tcpprague mailing list archive ,
	July 2015, <https://www.ietf.org/mail-		July 2015, <https://www.ietf.org/mail-
	archive/web/tcpprague/current/msg00001.html>.		archive/web/tcpprague/current/msg00001.html>.
	[VCP] Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,		[VCP] Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
	"One more bit is enough", Proc. SIGCOMM'05, ACM CCR		"One more bit is enough", Proc. SIGCOMM'05, ACM CCR
	35(4)37--48, 2005,		35(4)37--48, 2005,
	<http://doi.acm.org/10.1145/1080091.1080098>.		<https://doi.acm.org/10.1145/1080091.1080098>.
	Appendix A. Rationale for the 'Prague L4S Requirements'		Appendix A. Rationale for the 'Prague L4S Requirements'
	This appendix is informative, not normative. It gives a list of		This appendix is informative, not normative. It gives a list of
	modifications to current scalable congestion controls so that they		modifications to current scalable congestion controls so that they
	can be deployed over the public Internet and coexist safely with		can be deployed over the public Internet and coexist safely with
	existing traffic. The list complements the normative requirements in		existing traffic. The list complements the normative requirements in
	Section 4 that a sender has to comply with before it can set the L4S		Section 4 that a sender has to comply with before it can set the L4S
	identifier in packets it sends into the Internet. As well as		identifier in packets it sends into the Internet. As well as
	rationale for safety improvements (the requirements in Section 4)		rationale for safety improvements (the requirements in Section 4)
	this appendix also includes preferable performance improvements		this appendix also includes preferable performance improvements
	(optimizations).		(optimizations).
	The requirements and recommendations in Section 4) have become know		The requirements and recommendations in Section 4) have become known
	as the Prague L4S Requirements, because they were originally		as the Prague L4S Requirements, because they were originally
	identified at an ad hoc meeting during IETF-94 in Prague [TCPPrague].		identified at an ad hoc meeting during IETF-94 in Prague [TCPPrague].
	They were originally called the 'TCP Prague Requirements', but they		They were originally called the 'TCP Prague Requirements', but they
	are not solely applicable to TCP, so the name and wording has been		are not solely applicable to TCP, so the name and wording has been
	generalized for all transport protocols, and the name 'TCP Prague' is		generalized for all transport protocols, and the name 'TCP Prague' is
	now used for a specific implementation of the requirements.		now used for a specific implementation of the requirements.
	At the time of writing, DCTCP [RFC8257] is the most widely used		At the time of writing, DCTCP [RFC8257] is the most widely used
	scalable transport protocol. In its current form, DCTCP is specified		scalable transport protocol. In its current form, DCTCP is specified
	to be deployable only in controlled environments. Deploying it in		to be deployable only in controlled environments. Deploying it in
	skipping to change at page 55, line 9 ¶		skipping to change at page 55, line 35 ¶
	occurred by counting reordered packets, _all_ networks will have		occurred by counting reordered packets, _all_ networks will have
	to keep reducing the time over which they keep packets in order.		to keep reducing the time over which they keep packets in order.
	If some link technologies keep the time within which reordering		If some link technologies keep the time within which reordering
	occurs roughly unchanged, then loss over these links, as perceived		occurs roughly unchanged, then loss over these links, as perceived
	by these hosts, will appear to continually rise over the years.		by these hosts, will appear to continually rise over the years.
	* In contrast, if all senders detect loss in units of time, the time		* In contrast, if all senders detect loss in units of time, the time
	over which the network has to keep packets in order stays roughly		over which the network has to keep packets in order stays roughly
	invariant.		invariant.
	Therefore hosts have an incentive to detect loss in time units (so as		Therefore, hosts have an incentive to detect loss in time units (so
	not to fool themselves too often into detecting losses when there are		as not to fool themselves too often into detecting losses when there
	none). And for hosts that are changing their congestion control		are none). And for hosts that are changing their congestion control
	implementation to L4S, there is no downside to including time-based		implementation to L4S, there is no downside to including time-based
	loss detection code in the change (loss recovery implemented in		loss detection code in the change (loss recovery implemented in
	hardware is an exception, covered later). Therefore requiring L4S		hardware is an exception, covered later). Therefore, requiring L4S
	hosts to detect loss in time-based units would not be a burden.		hosts to detect loss in time-based units would not be a burden.
	If the requirement in Section 4.3 were not placed on L4S hosts, even		If the requirement in Section 4.3 were not placed on L4S hosts, even
	though it would be no burden on hosts to comply, all networks would		though it would be no burden on hosts to comply, all networks would
	face unnecessary uncertainty over whether some L4S hosts might be		face unnecessary uncertainty over whether some L4S hosts might be
	detecting loss by counting packets. Then _all_ link technologies		detecting loss by counting packets. Then _all_ link technologies
	will have to unnecessarily keep reducing the time within which		will have to unnecessarily keep reducing the time within which
	reordering occurs. That is not a problem for some link technologies,		reordering occurs. That is not a problem for some link technologies,
	but it becomes increasingly challenging for other link technologies		but it becomes increasingly challenging for other link technologies
	to continue to scale, particularly those relying on channel bonding		to continue to scale, particularly those relying on channel bonding
	skipping to change at page 57, line 12 ¶		skipping to change at page 57, line 27 ¶
	Detecting loss in time units also prevents the ACK-splitting attacks		Detecting loss in time units also prevents the ACK-splitting attacks
	described in [Savage-TCP].		described in [Savage-TCP].
	A.2. Scalable Transport Protocol Optimizations		A.2. Scalable Transport Protocol Optimizations
	A.2.1. Setting ECT in Control Packets and Retransmissions		A.2.1. Setting ECT in Control Packets and Retransmissions
	Description: This item concerns TCP and its derivatives (e.g. SCTP)		Description: This item concerns TCP and its derivatives (e.g. SCTP)
	as well as RTP/RTCP [RFC6679]. The original specification of ECN for		as well as RTP/RTCP [RFC6679]. The original specification of ECN for
	TCP precluded the use of ECN on control packets and retransmissions.		TCP precluded the use of ECN on control packets and retransmissions.
	Similarly RFC 6679 precludes the use of ECT on RTCP datagrams, in		Similarly, RFC 6679 precludes the use of ECT on RTCP datagrams, in
	case the path changes after it has been checked for ECN traversal.		case the path changes after it has been checked for ECN traversal.
	To improve performance, scalable transport protocols ought to enable		To improve performance, scalable transport protocols ought to enable
	ECN at the IP layer in TCP control packets (SYN, SYN-ACK, pure ACKs,		ECN at the IP layer in TCP control packets (SYN, SYN-ACK, pure ACKs,
	etc.) and in retransmitted packets. The same is true for other		etc.) and in retransmitted packets. The same is true for other
	transports, e.g. SCTP, RTCP.		transports, e.g. SCTP, RTCP.
	Motivation (TCP): RFC 3168 prohibits the use of ECN on these types of		Motivation (TCP): RFC 3168 prohibits the use of ECN on these types of
	TCP packet, based on a number of arguments. This means these packets		TCP packet, based on a number of arguments. This means these packets
	are not protected from congestion loss by ECN, which considerably		are not protected from congestion loss by ECN, which considerably
	harms performance, particularly for short flows.		harms performance, particularly for short flows.
	skipping to change at page 57, line 45 ¶		skipping to change at page 58, line 15 ¶
	A.2.2. Faster than Additive Increase		A.2.2. Faster than Additive Increase
	Description: It would improve performance if scalable congestion		Description: It would improve performance if scalable congestion
	controls did not limit their congestion window increase to the		controls did not limit their congestion window increase to the
	standard additive increase of 1 SMSS per round trip [RFC5681] during		standard additive increase of 1 SMSS per round trip [RFC5681] during
	congestion avoidance. The same is true for derivatives of TCP		congestion avoidance. The same is true for derivatives of TCP
	congestion control, including similar approaches used for real-time		congestion control, including similar approaches used for real-time
	media.		media.
	Motivation: As currently defined [RFC8257], DCTCP uses the		Motivation: As currently defined [RFC8257], DCTCP uses the
	traditional Reno additive increase in congestion avoidance phase.		conventional Reno additive increase in congestion avoidance phase.
	When the available capacity suddenly increases (e.g. when another		When the available capacity suddenly increases (e.g. when another
	flow finishes, or if radio capacity increases) it can take very many		flow finishes, or if radio capacity increases) it can take very many
	round trips to take advantage of the new capacity. TCP		round trips to take advantage of the new capacity. TCP
	Cubic [RFC8312] was designed to solve this problem, but as flow rates		Cubic [RFC8312] was designed to solve this problem, but as flow rates
	have continued to increase, the delay accelerating into available		have continued to increase, the delay accelerating into available
	capacity has become prohibitive. See, for instance, the examples in		capacity has become prohibitive. See, for instance, the examples in
	Section 5.1 of the L4S architecture [I-D.ietf-tsvwg-l4s-arch]. Even		Section 5.1 of the L4S architecture [I-D.ietf-tsvwg-l4s-arch]. Even
	when out of its Reno-compatibility mode, every 8x scaling of Cubic's		when out of its Reno-compatibility mode, every 8x scaling of Cubic's
	flow rate leads to 2x more acceleration delay.		flow rate leads to 2x more acceleration delay.
	skipping to change at page 65, line 8 ¶		skipping to change at page 65, line 25 ¶
	TCP authentication option (TCP-AO [RFC5925]), QUIC's end-to-end		TCP authentication option (TCP-AO [RFC5925]), QUIC's end-to-end
	protection [RFC9001] or end-to-end IPsec integrity protection		protection [RFC9001] or end-to-end IPsec integrity protection
	[RFC4303] can be used to detect any tampering with congestion		[RFC4303] can be used to detect any tampering with congestion
	feedback (whether malicious or accidental). respectively in TCP,		feedback (whether malicious or accidental). respectively in TCP,
	QUIC or any transport. TCP-AO covers the main TCP header and TCP		QUIC or any transport. TCP-AO covers the main TCP header and TCP
	options by default, but it is often too brittle to use on many		options by default, but it is often too brittle to use on many
	end-to-end paths, where middleboxes can make verification fail in		end-to-end paths, where middleboxes can make verification fail in
	their attempts to improve performance or security, e.g. by		their attempts to improve performance or security, e.g. by
	resegmentation or shifting the sequence space.		resegmentation or shifting the sequence space.
			At the time of writing, It is not common to protect the integrity of
			congestion feedback, whether loss or Classic ECN. If this position
			changes during the L4S experiment, one or more of the above
			techniques might need to be developed and deployed.
	C.2. Notification of Less Severe Congestion than CE		C.2. Notification of Less Severe Congestion than CE
	Various researchers have proposed to use ECT(1) as a less severe		Various researchers have proposed to use ECT(1) as a less severe
	congestion notification than CE, particularly to enable flows to fill		congestion notification than CE, particularly to enable flows to fill
	available capacity more quickly after an idle period, when another		available capacity more quickly after an idle period, when another
	flow departs or when a flow starts, e.g. VCP [VCP], Queue View		flow departs or when a flow starts, e.g. VCP [VCP], Queue View
	(QV) [QV].		(QV) [QV].
	Before assigning ECT(1) as an identifier for L4S, we must carefully		Before assigning ECT(1) as an identifier for L4S, we must carefully
	consider whether it might be better to hold ECT(1) in reserve for		consider whether it might be better to hold ECT(1) in reserve for
	future standardisation of rapid flow acceleration, which is an		future standardisation of rapid flow acceleration, which is an
	important and enduring problem [RFC6077].		important and enduring problem [RFC6077].
	Pre-Congestion Notification (PCN) is another scheme that assigns		Pre-Congestion Notification (PCN) is another scheme that assigns
	alternative semantics to the ECN field. It uses ECT(1) to signify a		alternative semantics to the ECN field. It uses ECT(1) to signify a
	less severe level of pre-congestion notification than CE [RFC6660].		less severe level of pre-congestion notification than CE [RFC6660].
	However, the ECN field only takes on the PCN semantics if packets		However, the ECN field only takes on the PCN semantics if packets
	carry a Diffserv codepoint defined to indicate PCN marking within a		carry a Diffserv codepoint defined to indicate PCN marking within a
	controlled environment. PCN is required to be applied solely to the		controlled environment. PCN is required to be applied solely to the
	outer header of a tunnel across the controlled region in order not to		outer header of a tunnel across the controlled region in order not to
	interfere with any end-to-end use of the ECN field. Therefore a PCN		interfere with any end-to-end use of the ECN field. Therefore, a PCN
	region on the path would not interfere with the L4S service		region on the path would not interfere with the L4S service
	identifier defined in Section 2.		identifier defined in Section 2.
	Acknowledgements		Acknowledgements
	Thanks to Richard Scheffenegger, John Leslie, David Taeht, Jonathan		Thanks to Richard Scheffenegger, John Leslie, David Taeht, Jonathan
	Morton, Gorry Fairhurst, Michael Welzl, Mikael Abrahamsson and Andrew		Morton, Gorry Fairhurst, Michael Welzl, Mikael Abrahamsson and Andrew
	McGregor for the discussions that led to this specification. Ing-jyh		McGregor for the discussions that led to this specification. Ing-jyh
	(Inton) Tsang was a contributor to the early drafts of this document.		(Inton) Tsang was a contributor to the early drafts of this document.
	And thanks to Mikael Abrahamsson, Lloyd Wood, Nicolas Kuhn, Greg		And thanks to Mikael Abrahamsson, Lloyd Wood, Nicolas Kuhn, Greg
	White, Tom Henderson, David Black, Gorry Fairhurst, Brian Carpenter,		White, Tom Henderson, David Black, Gorry Fairhurst, Brian Carpenter,
	Jake Holland, Rod Grimes, Richard Scheffenegger, Sebastian Moeller,		Jake Holland, Rod Grimes, Richard Scheffenegger, Sebastian Moeller,
	Neal Cardwell, Praveen Balasubramanian, Reza Marandian Hagh, Pete		Neal Cardwell, Praveen Balasubramanian, Reza Marandian Hagh, Pete
	Heist, Stuart Cheshire, Vidhi Goel, Mirja Kuehlewind, Ermin Sakic and		Heist, Stuart Cheshire, Vidhi Goel, Mirja Kuehlewind, Ermin Sakic and
	Martin Duke for providing help and reviewing this draft and thanks to		Martin Duke for providing help and reviewing this draft, and thanks
	Ingemar Johansson for reviewing and providing substantial text.		to Ingemar Johansson for reviewing and providing substantial text.
	Thanks also to the area reviewers: Valery Smyslov, Ines Robles and		Thanks also to the area reviewers: Valery Smyslov, Maria Ines Robles,
	Bernard Aboba. Thanks to Sebastian Moeller for identifying the		Bernard Aboba, Lars Eggert, Roman Danyliw and Eric Vyncke. Thanks to
	interaction with VPN anti-replay and to Jonathan Morton for		Sebastian Moeller for identifying the interaction with VPN anti-
	identifying the attack based on this. Particular thanks to tsvwg		replay and to Jonathan Morton for identifying the attack based on
	chairs Gorry Fairhurst, David Black and Wes Eddy for patiently		this. Particular thanks to tsvwg chairs Gorry Fairhurst, David Black
	helping this and the other L4S drafts through the IETF process.		and Wes Eddy for patiently helping this and the other L4S drafts
	Appendix A listing the Prague L4S Requirements is based on text		through the IETF process. Appendix A listing the Prague L4S
	authored by Marcelo Bagnulo Braun that was originally an appendix to		Requirements is based on text authored by Marcelo Bagnulo Braun that
	[I-D.ietf-tsvwg-l4s-arch]. That text was in turn based on the		was originally an appendix to [I-D.ietf-tsvwg-l4s-arch]. That text
	collective output of the attendees listed in the minutes of a 'bar		was in turn based on the collective output of the attendees listed in
	BoF' on DCTCP Evolution during IETF-94 [TCPPrague].		the minutes of a 'bar BoF' on DCTCP Evolution during
			IETF-94 [TCPPrague].
	The authors' contributions were part-funded by the European Community		The authors' contributions were part-funded by the European Community
	under its Seventh Framework Programme through the Reducing Internet		under its Seventh Framework Programme through the Reducing Internet
	Transport Latency (RITE) project (ICT-317700). The contribution of		Transport Latency (RITE) project (ICT-317700). The contribution of
	Koen De Schepper was also part-funded by the 5Growth and DAEMON EU		Koen De Schepper was also part-funded by the 5Growth and DAEMON EU
	H2020 projects. Bob Briscoe was also funded partly by the Research		H2020 projects. Bob Briscoe was also funded partly by the Research
	Council of Norway through the TimeIn project, partly by CableLabs and		Council of Norway through the TimeIn project, partly by CableLabs and
	partly by the Comcast Innovation Fund. The views expressed here are		partly by the Comcast Innovation Fund. The views expressed here are
	solely those of the authors.		solely those of the authors.
	skipping to change at page 66, line 19 ¶		skipping to change at page 67, line 4 ¶
	The authors' contributions were part-funded by the European Community		The authors' contributions were part-funded by the European Community
	under its Seventh Framework Programme through the Reducing Internet		under its Seventh Framework Programme through the Reducing Internet
	Transport Latency (RITE) project (ICT-317700). The contribution of		Transport Latency (RITE) project (ICT-317700). The contribution of
	Koen De Schepper was also part-funded by the 5Growth and DAEMON EU		Koen De Schepper was also part-funded by the 5Growth and DAEMON EU
	H2020 projects. Bob Briscoe was also funded partly by the Research		H2020 projects. Bob Briscoe was also funded partly by the Research
	Council of Norway through the TimeIn project, partly by CableLabs and		Council of Norway through the TimeIn project, partly by CableLabs and
	partly by the Comcast Innovation Fund. The views expressed here are		partly by the Comcast Innovation Fund. The views expressed here are
	solely those of the authors.		solely those of the authors.
	Authors' Addresses		Authors' Addresses
	Koen De Schepper		Koen De Schepper
	Nokia Bell Labs		Nokia Bell Labs
	Antwerp		Antwerp
	Belgium		Belgium
	Email: koen.de_schepper@nokia.com		Email: koen.de_schepper@nokia.com
	URI: https://www.bell-labs.com/usr/koen.de_schepper		URI: https://www.bell-labs.com/about/researcher-profiles/
			koende_schepper/
	Bob Briscoe (editor)		Bob Briscoe (editor)
	Independent		Independent
	United Kingdom		United Kingdom
	Email: ietf@bobbriscoe.net		Email: ietf@bobbriscoe.net
	URI: http://bobbriscoe.net/		URI: https://bobbriscoe.net/
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