draft-ietf-quic-recovery-24.txt		draft-ietf-quic-recovery-latest.txt
	QUIC Working Group J. Iyengar, Ed.		QUIC Working Group J. Iyengar, Ed.
	Internet-Draft Fastly		Internet-Draft Fastly
	Intended status: Standards Track I. Swett, Ed.		Intended status: Standards Track I. Swett, Ed.
	Expires: May 7, 2020 Google		Expires: June 6, 2020 Google
	November 4, 2019		December 4, 2019
	QUIC Loss Detection and Congestion Control		QUIC Loss Detection and Congestion Control
	draft-ietf-quic-recovery-24		draft-ietf-quic-recovery-latest
	Abstract		Abstract
	This document describes loss detection and congestion control		This document describes loss detection and congestion control
	mechanisms for QUIC.		mechanisms for QUIC.
	Note to Readers		Note to Readers
	Discussion of this draft takes place on the QUIC working group		Discussion of this draft takes place on the QUIC working group
	mailing list (quic@ietf.org), which is archived at		mailing list (quic@ietf.org), which is archived at
	skipping to change at page 1, line 42 ¶		skipping to change at page 1, line 42 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on May 7, 2020.		This Internet-Draft will expire on June 6, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2019 IETF Trust and the persons identified as the		Copyright (c) 2019 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 40 ¶		skipping to change at page 2, line 40 ¶
	4.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 8		4.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 8
	5. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9		5. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9
	5.1. Acknowledgement-based Detection . . . . . . . . . . . . . 10		5.1. Acknowledgement-based Detection . . . . . . . . . . . . . 10
	5.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10		5.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10
	5.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10		5.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10
	5.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 11		5.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 11
	5.2.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 11		5.2.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 11
	5.3. Handshakes and New Paths . . . . . . . . . . . . . . . . 12		5.3. Handshakes and New Paths . . . . . . . . . . . . . . . . 12
	5.3.1. Sending Probe Packets . . . . . . . . . . . . . . . . 13		5.3.1. Sending Probe Packets . . . . . . . . . . . . . . . . 13
	5.3.2. Loss Detection . . . . . . . . . . . . . . . . . . . 14		5.3.2. Loss Detection . . . . . . . . . . . . . . . . . . . 14
	5.4. Handling Retry Packets . . . . . . . . . . . . . . . . . 14		5.4. Handling Retry Packets . . . . . . . . . . . . . . . . . 15
	5.5. Discarding Keys and Packet State . . . . . . . . . . . . 14		5.5. Discarding Keys and Packet State . . . . . . . . . . . . 15
	6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15		6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15
	6.1. Explicit Congestion Notification . . . . . . . . . . . . 15		6.1. Explicit Congestion Notification . . . . . . . . . . . . 16
	6.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 16		6.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 16
	6.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 16		6.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 16
	6.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16		6.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16
	6.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 16		6.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 17
	6.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 16		6.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 17
	6.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 17		6.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 17
	6.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 18		6.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 18
	6.9. Under-utilizing the Congestion Window . . . . . . . . . . 18		6.9. Under-utilizing the Congestion Window . . . . . . . . . . 19
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 19		7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
	7.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 19		7.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 19
	7.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 19		7.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 19
	7.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 19		7.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 20
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
	9.1. Normative References . . . . . . . . . . . . . . . . . . 20		9.1. Normative References . . . . . . . . . . . . . . . . . . 20
	9.2. Informative References . . . . . . . . . . . . . . . . . 20		9.2. Informative References . . . . . . . . . . . . . . . . . 21
	9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 22		9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 22
	Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 22		Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 22
	A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 22		A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 23
	A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 22		A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 23
	A.2. Constants of interest . . . . . . . . . . . . . . . . . . 23		A.2. Constants of interest . . . . . . . . . . . . . . . . . . 23
	A.3. Variables of interest . . . . . . . . . . . . . . . . . . 23		A.3. Variables of interest . . . . . . . . . . . . . . . . . . 24
	A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 24		A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 25
	A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 24		A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 25
	A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 25		A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 26
	A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 26		A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 27
	A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 27		A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 27
	A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 29		A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 29
	A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 29		A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 29
	Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 30		Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 30
	B.1. Constants of interest . . . . . . . . . . . . . . . . . . 30		B.1. Constants of interest . . . . . . . . . . . . . . . . . . 30
	B.2. Variables of interest . . . . . . . . . . . . . . . . . . 31		B.2. Variables of interest . . . . . . . . . . . . . . . . . . 31
	B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 32		B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 32
	B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 32		B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 32
	B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 32		B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 32
	B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 33		B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 33
	skipping to change at page 4, line 20 ¶		skipping to change at page 4, line 20 ¶
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 40		Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 40
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
	1. Introduction		1. Introduction
	QUIC is a new multiplexed and secure transport atop UDP. QUIC builds		QUIC is a new multiplexed and secure transport atop UDP. QUIC builds
	on decades of transport and security experience, and implements		on decades of transport and security experience, and implements
	mechanisms that make it attractive as a modern general-purpose		mechanisms that make it attractive as a modern general-purpose
	transport. The QUIC protocol is described in [QUIC-TRANSPORT].		transport. The QUIC protocol is described in [QUIC-TRANSPORT].
	QUIC implements the spirit of existing TCP loss recovery mechanisms,		QUIC implements the spirit of existing TCP congestion control and
	described in RFCs, various Internet-drafts, and also those prevalent		loss recovery mechanisms, described in RFCs, various Internet-drafts,
	in the Linux TCP implementation. This document describes QUIC		and also those prevalent in the Linux TCP implementation. This
	congestion control and loss recovery, and where applicable,		document describes QUIC congestion control and loss recovery, and
	attributes the TCP equivalent in RFCs, Internet-drafts, academic		where applicable, attributes the TCP equivalent in RFCs, Internet-
	papers, and/or TCP implementations.		drafts, academic papers, and/or TCP implementations.
	2. Conventions and Definitions		2. Conventions and Definitions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all		14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	Definitions of terms that are used in this document:		Definitions of terms that are used in this document:
	skipping to change at page 4, line 50 ¶		skipping to change at page 4, line 50 ¶
	and are not ACK-only, and they are not acknowledged, declared		and are not ACK-only, and they are not acknowledged, declared
	lost, or abandoned along with old keys.		lost, or abandoned along with old keys.
	Ack-eliciting Frames: All frames other than ACK, PADDING, and		Ack-eliciting Frames: All frames other than ACK, PADDING, and
	CONNECTION_CLOSE are considered ack-eliciting.		CONNECTION_CLOSE are considered ack-eliciting.
	Ack-eliciting Packets: Packets that contain ack-eliciting frames		Ack-eliciting Packets: Packets that contain ack-eliciting frames
	elicit an ACK from the receiver within the maximum ack delay and		elicit an ACK from the receiver within the maximum ack delay and
	are called ack-eliciting packets.		are called ack-eliciting packets.
	Crypto Packets: Packets containing CRYPTO data sent in Initial or
	Handshake packets.
	Out-of-order Packets: Packets that do not increase the largest		Out-of-order Packets: Packets that do not increase the largest
	received packet number for its packet number space by exactly one.		received packet number for its packet number space by exactly one.
	Packets arrive out of order when earlier packets are lost or		Packets arrive out of order when earlier packets are lost or
	delayed.		delayed.
	3. Design of the QUIC Transmission Machinery		3. Design of the QUIC Transmission Machinery
	All transmissions in QUIC are sent with a packet-level header, which		All transmissions in QUIC are sent with a packet-level header, which
	indicates the encryption level and includes a packet sequence number		indicates the encryption level and includes a packet sequence number
	(referred to below as a packet number). The encryption level		(referred to below as a packet number). The encryption level
	indicates the packet number space, as described in [QUIC-TRANSPORT].		indicates the packet number space, as described in [QUIC-TRANSPORT].
	Packet numbers never repeat within a packet number space for the		Packet numbers never repeat within a packet number space for the
	lifetime of a connection. Packet numbers monotonically increase		lifetime of a connection. Packet numbers are sent in monotonically
	within a space, preventing ambiguity.		increasing order within a space, preventing ambiguity.
	This design obviates the need for disambiguating between		This design obviates the need for disambiguating between
	transmissions and retransmissions and eliminates significant		transmissions and retransmissions and eliminates significant
	complexity from QUIC's interpretation of TCP loss detection		complexity from QUIC's interpretation of TCP loss detection
	mechanisms.		mechanisms.
	QUIC packets can contain multiple frames of different types. The		QUIC packets can contain multiple frames of different types. The
	recovery mechanisms ensure that data and frames that need reliable		recovery mechanisms ensure that data and frames that need reliable
	delivery are acknowledged or declared lost and sent in new packets as		delivery are acknowledged or declared lost and sent in new packets as
	necessary. The types of frames contained in a packet affect recovery		necessary. The types of frames contained in a packet affect recovery
	skipping to change at page 6, line 43 ¶		skipping to change at page 6, line 43 ¶
	retransmissions are trivially detected, and mechanisms such as Fast		retransmissions are trivially detected, and mechanisms such as Fast
	Retransmit can be applied universally, based only on packet number.		Retransmit can be applied universally, based only on packet number.
	This design point significantly simplifies loss detection mechanisms		This design point significantly simplifies loss detection mechanisms
	for QUIC. Most TCP mechanisms implicitly attempt to infer		for QUIC. Most TCP mechanisms implicitly attempt to infer
	transmission ordering based on TCP sequence numbers - a non-trivial		transmission ordering based on TCP sequence numbers - a non-trivial
	task, especially when TCP timestamps are not available.		task, especially when TCP timestamps are not available.
	3.1.3. Clearer Loss Epoch		3.1.3. Clearer Loss Epoch
	QUIC ends a loss epoch when a packet sent after loss is declared is		QUIC starts a loss epoch when a packet is lost and ends one when any
	acknowledged. TCP waits for the gap in the sequence number space to		packet sent after the epoch starts is acknowledged. TCP waits for
	be filled, and so if a segment is lost multiple times in a row, the		the gap in the sequence number space to be filled, and so if a
	loss epoch may not end for several round trips. Because both should		segment is lost multiple times in a row, the loss epoch may not end
	reduce their congestion windows only once per epoch, QUIC will do it		for several round trips. Because both should reduce their congestion
	correctly once for every round trip that experiences loss, while TCP		windows only once per epoch, QUIC will do it once for every round
	may only do it once across multiple round trips.		trip that experiences loss, while TCP may only do it once across
			multiple round trips.
	3.1.4. No Reneging		3.1.4. No Reneging
	QUIC ACKs contain information that is similar to TCP SACK, but QUIC		QUIC ACKs contain information that is similar to TCP SACK, but QUIC
	does not allow any acked packet to be reneged, greatly simplifying		does not allow any acked packet to be reneged, greatly simplifying
	implementations on both sides and reducing memory pressure on the		implementations on both sides and reducing memory pressure on the
	sender.		sender.
	3.1.5. More ACK Ranges		3.1.5. More ACK Ranges
	skipping to change at page 7, line 30 ¶		skipping to change at page 7, line 30 ¶
	QUIC endpoints measure the delay incurred between when a packet is		QUIC endpoints measure the delay incurred between when a packet is
	received and when the corresponding acknowledgment is sent, allowing		received and when the corresponding acknowledgment is sent, allowing
	a peer to maintain a more accurate round-trip time estimate (see		a peer to maintain a more accurate round-trip time estimate (see
	Section 13.2 of [QUIC-TRANSPORT]).		Section 13.2 of [QUIC-TRANSPORT]).
	4. Estimating the Round-Trip Time		4. Estimating the Round-Trip Time
	At a high level, an endpoint measures the time from when a packet was		At a high level, an endpoint measures the time from when a packet was
	sent to when it is acknowledged as a round-trip time (RTT) sample.		sent to when it is acknowledged as a round-trip time (RTT) sample.
	The endpoint uses RTT samples and peer-reported host delays (see		The endpoint uses RTT samples and peer-reported ACK delays (see
	Section 13.2 of [QUIC-TRANSPORT]) to generate a statistical		Section 13.2 of [QUIC-TRANSPORT]) to generate a statistical
	description of the connection's RTT. An endpoint computes the		description of the connection's RTT. An endpoint computes the
	following three values: the minimum value observed over the lifetime		following three values: the minimum value observed over the lifetime
	of the connection (min_rtt), an exponentially-weighted moving average		of the connection (min_rtt), an exponentially-weighted moving average
	(smoothed_rtt), and the variance in the observed RTT samples		(smoothed_rtt), and the variance in the observed RTT samples
	(rttvar).		(rttvar).
	4.1. Generating RTT samples		4.1. Generating RTT samples
	An endpoint generates an RTT sample on receiving an ACK frame that		An endpoint generates an RTT sample on receiving an ACK frame that
	skipping to change at page 8, line 5 ¶		skipping to change at page 8, line 5 ¶
	o the largest acknowledged packet number is newly acknowledged, and		o the largest acknowledged packet number is newly acknowledged, and
	o at least one of the newly acknowledged packets was ack-eliciting.		o at least one of the newly acknowledged packets was ack-eliciting.
	The RTT sample, latest_rtt, is generated as the time elapsed since		The RTT sample, latest_rtt, is generated as the time elapsed since
	the largest acknowledged packet was sent:		the largest acknowledged packet was sent:
	latest_rtt = ack_time - send_time_of_largest_acked		latest_rtt = ack_time - send_time_of_largest_acked
	An RTT sample is generated using only the largest acknowledged packet		An RTT sample is generated using only the largest acknowledged packet
	in the received ACK frame. This is because a peer reports host		in the received ACK frame. This is because a peer reports ACK delays
	delays for only the largest acknowledged packet in an ACK frame.		for only the largest acknowledged packet in an ACK frame. While the
	While the reported host delay is not used by the RTT sample		reported ACK delay is not used by the RTT sample measurement, it is
	measurement, it is used to adjust the RTT sample in subsequent		used to adjust the RTT sample in subsequent computations of
	computations of smoothed_rtt and rttvar Section 4.3.		smoothed_rtt and rttvar Section 4.3.
	To avoid generating multiple RTT samples using the same packet, an		To avoid generating multiple RTT samples for a single packet, an ACK
	ACK frame SHOULD NOT be used to update RTT estimates if it does not		frame SHOULD NOT be used to update RTT estimates if it does not newly
	newly acknowledge the largest acknowledged packet.		acknowledge the largest acknowledged packet.
	An RTT sample MUST NOT be generated on receiving an ACK frame that		An RTT sample MUST NOT be generated on receiving an ACK frame that
	does not newly acknowledge at least one ack-eliciting packet. A peer		does not newly acknowledge at least one ack-eliciting packet. A peer
	does not send an ACK frame on receiving only non-ack-eliciting		does not send an ACK frame on receiving only non-ack-eliciting
	packets, so an ACK frame that is subsequently sent can include an		packets, so an ACK frame that is subsequently sent can include an
	arbitrarily large Ack Delay field. Ignoring such ACK frames avoids		arbitrarily large Ack Delay field. Ignoring such ACK frames avoids
	complications in subsequent smoothed_rtt and rttvar computations.		complications in subsequent smoothed_rtt and rttvar computations.
	A sender might generate multiple RTT samples per RTT when multiple		A sender might generate multiple RTT samples per RTT when multiple
	ACK frames are received within an RTT. As suggested in [RFC6298],		ACK frames are received within an RTT. As suggested in [RFC6298],
	skipping to change at page 8, line 36 ¶		skipping to change at page 8, line 36 ¶
	open research question.		open research question.
	4.2. Estimating min_rtt		4.2. Estimating min_rtt
	min_rtt is the minimum RTT observed over the lifetime of the		min_rtt is the minimum RTT observed over the lifetime of the
	connection. min_rtt is set to the latest_rtt on the first sample in a		connection. min_rtt is set to the latest_rtt on the first sample in a
	connection, and to the lesser of min_rtt and latest_rtt on subsequent		connection, and to the lesser of min_rtt and latest_rtt on subsequent
	samples.		samples.
	An endpoint uses only locally observed times in computing the min_rtt		An endpoint uses only locally observed times in computing the min_rtt
	and does not adjust for host delays reported by the peer. Doing so		and does not adjust for ACK delays reported by the peer. Doing so
	allows the endpoint to set a lower bound for the smoothed_rtt based		allows the endpoint to set a lower bound for the smoothed_rtt based
	entirely on what it observes (see Section 4.3), and limits potential		entirely on what it observes (see Section 4.3), and limits potential
	underestimation due to erroneously-reported delays by the peer.		underestimation due to erroneously-reported delays by the peer.
	4.3. Estimating smoothed_rtt and rttvar		4.3. Estimating smoothed_rtt and rttvar
	smoothed_rtt is an exponentially-weighted moving average of an		smoothed_rtt is an exponentially-weighted moving average of an
	endpoint's RTT samples, and rttvar is the endpoint's estimated		endpoint's RTT samples, and rttvar is the endpoint's estimated
	variance in the RTT samples.		variance in the RTT samples.
	The calculation of smoothed_rtt uses path latency after adjusting RTT		The calculation of smoothed_rtt uses path latency after adjusting RTT
	samples for host delays. For packets sent in the ApplicationData		samples for ACK delays. For packets sent in the ApplicationData
	packet number space, a peer limits any delay in sending an		packet number space, a peer limits any delay in sending an
	acknowledgement for an ack-eliciting packet to no greater than the		acknowledgement for an ack-eliciting packet to no greater than the
	value it advertised in the max_ack_delay transport parameter.		value it advertised in the max_ack_delay transport parameter.
	Consequently, when a peer reports an Ack Delay that is greater than		Consequently, when a peer reports an Ack Delay that is greater than
	its max_ack_delay, the delay is attributed to reasons out of the		its max_ack_delay, the delay is attributed to reasons out of the
	peer's control, such as scheduler latency at the peer or loss of		peer's control, such as scheduler latency at the peer or loss of
	previous ACK frames. Any delays beyond the peer's max_ack_delay are		previous ACK frames. Any delays beyond the peer's max_ack_delay are
	therefore considered effectively part of path delay and incorporated		therefore considered effectively part of path delay and incorporated
	into the smoothed_rtt estimate.		into the smoothed_rtt estimate.
	skipping to change at page 9, line 46 ¶		skipping to change at page 9, line 46 ¶
	ack_delay = min(Ack Delay in ACK Frame, max_ack_delay)		ack_delay = min(Ack Delay in ACK Frame, max_ack_delay)
	adjusted_rtt = latest_rtt		adjusted_rtt = latest_rtt
	if (min_rtt + ack_delay < latest_rtt):		if (min_rtt + ack_delay < latest_rtt):
	adjusted_rtt = latest_rtt - ack_delay		adjusted_rtt = latest_rtt - ack_delay
	smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt		smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
	rttvar_sample = abs(smoothed_rtt - adjusted_rtt)		rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
	rttvar = 3/4 * rttvar + 1/4 * rttvar_sample		rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
	5. Loss Detection		5. Loss Detection
	QUIC senders use both ack information and timeouts to detect lost		QUIC senders use acknowledgements to detect lost packets, and a probe
	packets, and this section provides a description of these algorithms.		time out Section 5.2 to ensure acknowledgements are received. This
			section provides a description of these algorithms.
	If a packet is lost, the QUIC transport needs to recover from that		If a packet is lost, the QUIC transport needs to recover from that
	loss, such as by retransmitting the data, sending an updated frame,		loss, such as by retransmitting the data, sending an updated frame,
	or abandoning the frame. For more information, see Section 13.3 of		or abandoning the frame. For more information, see Section 13.3 of
	[QUIC-TRANSPORT].		[QUIC-TRANSPORT].
	5.1. Acknowledgement-based Detection		5.1. Acknowledgement-based Detection
	Acknowledgement-based loss detection implements the spirit of TCP's		Acknowledgement-based loss detection implements the spirit of TCP's
	Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK [FACK],		Fast Retransmit [RFC5681], Early Retransmit [RFC5827], FACK [FACK],
	skipping to change at page 10, line 22 ¶		skipping to change at page 10, line 24 ¶
	A packet is declared lost if it meets all the following conditions:		A packet is declared lost if it meets all the following conditions:
	o The packet is unacknowledged, in-flight, and was sent prior to an		o The packet is unacknowledged, in-flight, and was sent prior to an
	acknowledged packet.		acknowledged packet.
	o Either its packet number is kPacketThreshold smaller than an		o Either its packet number is kPacketThreshold smaller than an
	acknowledged packet (Section 5.1.1), or it was sent long enough in		acknowledged packet (Section 5.1.1), or it was sent long enough in
	the past (Section 5.1.2).		the past (Section 5.1.2).
	The acknowledgement indicates that a packet sent later was delivered,		The acknowledgement indicates that a packet sent later was delivered,
	while the packet and time thresholds provide some tolerance for		and the packet and time thresholds provide some tolerance for packet
	packet reordering.		reordering.
	Spuriously declaring packets as lost leads to unnecessary		Spuriously declaring packets as lost leads to unnecessary
	retransmissions and may result in degraded performance due to the		retransmissions and may result in degraded performance due to the
	actions of the congestion controller upon detecting loss.		actions of the congestion controller upon detecting loss.
	Implementations that detect spurious retransmissions and increase the		Implementations that detect spurious retransmissions and increase the
	reordering threshold in packets or time MAY choose to start with		reordering threshold in packets or time MAY choose to start with
	smaller initial reordering thresholds to minimize recovery latency.		smaller initial reordering thresholds to minimize recovery latency.
	5.1.1. Packet Threshold		5.1.1. Packet Threshold
	skipping to change at page 10, line 45 ¶		skipping to change at page 10, line 47 ¶
	(kPacketThreshold) is 3, based on best practices for TCP loss		(kPacketThreshold) is 3, based on best practices for TCP loss
	detection [RFC5681] [RFC6675].		detection [RFC5681] [RFC6675].
	Some networks may exhibit higher degrees of reordering, causing a		Some networks may exhibit higher degrees of reordering, causing a
	sender to detect spurious losses. Implementers MAY use algorithms		sender to detect spurious losses. Implementers MAY use algorithms
	developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's		developed for TCP, such as TCP-NCR [RFC4653], to improve QUIC's
	reordering resilience.		reordering resilience.
	5.1.2. Time Threshold		5.1.2. Time Threshold
	Once a later packet packet within the same packet number space has		Once a later packet within the same packet number space has been
	been acknowledged, an endpoint SHOULD declare an earlier packet lost		acknowledged, an endpoint SHOULD declare an earlier packet lost if it
	if it was sent a threshold amount of time in the past. To avoid		was sent a threshold amount of time in the past. To avoid declaring
	declaring packets as lost too early, this time threshold MUST be set		packets as lost too early, this time threshold MUST be set to at
	to at least kGranularity. The time threshold is:		least kGranularity. The time threshold is:
			max(kTimeThreshold * max(smoothed_rtt, latest_rtt), kGranularity)
	kTimeThreshold * max(smoothed_rtt, latest_rtt, kGranularity)
	If packets sent prior to the largest acknowledged packet cannot yet		If packets sent prior to the largest acknowledged packet cannot yet
	be declared lost, then a timer SHOULD be set for the remaining time.		be declared lost, then a timer SHOULD be set for the remaining time.
	Using max(smoothed_rtt, latest_rtt) protects from the two following		Using max(smoothed_rtt, latest_rtt) protects from the two following
	cases:		cases:
	o the latest RTT sample is lower than the smoothed RTT, perhaps due		o the latest RTT sample is lower than the smoothed RTT, perhaps due
	to reordering where the acknowledgement encountered a shorter		to reordering where the acknowledgement encountered a shorter
	path;		path;
	skipping to change at page 11, line 33 ¶		skipping to change at page 11, line 36 ¶
	variance. Smaller thresholds reduce reordering resilience and		variance. Smaller thresholds reduce reordering resilience and
	increase spurious retransmissions, and larger thresholds increase		increase spurious retransmissions, and larger thresholds increase
	loss detection delay.		loss detection delay.
	5.2. Probe Timeout		5.2. Probe Timeout
	A Probe Timeout (PTO) triggers sending one or two probe datagrams		A Probe Timeout (PTO) triggers sending one or two probe datagrams
	when ack-eliciting packets are not acknowledged within the expected		when ack-eliciting packets are not acknowledged within the expected
	period of time or the handshake has not been completed. A PTO		period of time or the handshake has not been completed. A PTO
	enables a connection to recover from loss of tail packets or		enables a connection to recover from loss of tail packets or
	acknowledgements. The PTO algorithm used in QUIC implements the		acknowledgements.
	reliability functions of Tail Loss Probe [RACK], RTO [RFC5681] and
	F-RTO algorithms for TCP [RFC5682], and the timeout computation is		As with loss detection, the probe timeout is per packet number space.
	based on TCP's retransmission timeout period [RFC6298].		The PTO algorithm used in QUIC implements the reliability functions
			of Tail Loss Probe [RACK], RTO [RFC5681], and F-RTO algorithms for
			TCP [RFC5682]. The timeout computation is based on TCP's
			retransmission timeout period [RFC6298].
	5.2.1. Computing PTO		5.2.1. Computing PTO
	When an ack-eliciting packet is transmitted, the sender schedules a		When an ack-eliciting packet is transmitted, the sender schedules a
	timer for the PTO period as follows:		timer for the PTO period as follows:
	PTO = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay		PTO = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay
	kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in		kGranularity, smoothed_rtt, rttvar, and max_ack_delay are defined in
	Appendix A.2 and Appendix A.3.		Appendix A.2 and Appendix A.3.
	The PTO period is the amount of time that a sender ought to wait for		The PTO period is the amount of time that a sender ought to wait for
	an acknowledgement of a sent packet. This time period includes the		an acknowledgement of a sent packet. This time period includes the
	estimated network roundtrip-time (smoothed_rtt), the variance in the		estimated network roundtrip-time (smoothed_rtt), the variance in the
	estimate (4*rttvar), and max_ack_delay, to account for the maximum		estimate (4*rttvar), and max_ack_delay, to account for the maximum
	time by which a receiver might delay sending an acknowledgement.		time by which a receiver might delay sending an acknowledgement.
			When the PTO is armed for Initial or Handshake packet number spaces,
			the max_ack_delay is 0, as specified in 13.2.5 of [QUIC-TRANSPORT].
	The PTO value MUST be set to at least kGranularity, to avoid the		The PTO value MUST be set to at least kGranularity, to avoid the
	timer expiring immediately.		timer expiring immediately.
			A sender computes its PTO timer every time an ack-eliciting packet is
			sent. When ack-eliciting packets are in-flight in multiple packet
			number spaces, the timer MUST be set for the packet number space with
			the earliest timeout, except for ApplicationData, which MUST be
			ignored until the handshake completes; see Section 4.1.1 of
			[QUIC-TLS]. Not arming the PTO for ApplicationData prioritizes
			completing the handshake and prevents the server from sending a 1-RTT
			packet on a PTO before before it has the keys to process a 1-RTT
			packet.
	When a PTO timer expires, the PTO period MUST be set to twice its		When a PTO timer expires, the PTO period MUST be set to twice its
	current value. This exponential reduction in the sender's rate is		current value. This exponential reduction in the sender's rate is
	important because the PTOs might be caused by loss of packets or		important because consecutive PTOs might be caused by loss of packets
	acknowledgements due to severe congestion. The life of a connection		or acknowledgements due to severe congestion. Even when there are
	that is experiencing consecutive PTOs is limited by the endpoint's		ack-eliciting packets in-flight in multiple packet number spaces, the
	idle timeout.		exponential increase in probe timeout occurs across all spaces to
			prevent excess load on the network. For example, a timeout in the
			Initial packet number space doubles the length of the timeout in the
			Handshake packet number space.
	A sender computes its PTO timer every time an ack-eliciting packet is		The life of a connection that is experiencing consecutive PTOs is
	sent. A sender might choose to optimize this by setting the timer		limited by the endpoint's idle timeout.
	fewer times if it knows that more ack-eliciting packets will be sent
	within a short period of time.
	The probe timer is not set if the time threshold Section 5.1.2 loss		The probe timer is not set if the time threshold Section 5.1.2 loss
	detection timer is set. The time threshold loss detection timer is		detection timer is set. The time threshold loss detection timer is
	expected to both expire earlier than the PTO and be less likely to		expected to both expire earlier than the PTO and be less likely to
	spuriously retransmit data.		spuriously retransmit data.
	5.3. Handshakes and New Paths		5.3. Handshakes and New Paths
	The initial probe timeout for a new connection or new path SHOULD be		The initial probe timeout for a new connection or new path SHOULD be
	set to twice the initial RTT. Resumed connections over the same		set to twice the initial RTT. Resumed connections over the same
	network SHOULD use the previous connection's final smoothed RTT value		network SHOULD use the previous connection's final smoothed RTT value
	as the resumed connection's initial RTT. If no previous RTT is		as the resumed connection's initial RTT. If no previous RTT is
	available, the initial RTT SHOULD be set to 500ms, resulting in a 1		available, the initial RTT SHOULD be set to 500ms, resulting in a 1
	second initial timeout as recommended in [RFC6298].		second initial timeout as recommended in [RFC6298].
	A connection MAY use the delay between sending a PATH_CHALLENGE and		A connection MAY use the delay between sending a PATH_CHALLENGE and
	receiving a PATH_RESPONSE to seed initial_rtt for a new path, but the		receiving a PATH_RESPONSE to set the initial RTT (see kInitialRtt in
	delay SHOULD NOT be considered an RTT sample.		Appendix A.2) for a new path, but the delay SHOULD NOT be considered
			an RTT sample.
	Until the server has validated the client's address on the path, the		Until the server has validated the client's address on the path, the
	amount of data it can send is limited to three times the amount of		amount of data it can send is limited to three times the amount of
	data received, as specified in Section 8.1 of [QUIC-TRANSPORT]. If		data received, as specified in Section 8.1 of [QUIC-TRANSPORT]. If
	no data can be sent, then the PTO alarm MUST NOT be armed.		no data can be sent, then the PTO alarm MUST NOT be armed until
			datagrams have been received from the client.
	Since the server could be blocked until more packets are received		Since the server could be blocked until more packets are received
	from the client, it is the client's responsibility to send packets to		from the client, it is the client's responsibility to send packets to
	unblock the server until it is certain that the server has finished		unblock the server until it is certain that the server has finished
	its address validation (see Section 8 of [QUIC-TRANSPORT]). That is,		its address validation (see Section 8 of [QUIC-TRANSPORT]). That is,
	the client MUST set the probe timer if the client has not received an		the client MUST set the probe timer if the client has not received an
	acknowledgement for one of its Handshake or 1-RTT packets.		acknowledgement for one of its Handshake or 1-RTT packets.
	Prior to handshake completion, when few to none RTT samples have been		Prior to handshake completion, when few to none RTT samples have been
	generated, it is possible that the probe timer expiration is due to		generated, it is possible that the probe timer expiration is due to
	an incorrect RTT estimate at the client. To allow the client to		an incorrect RTT estimate at the client. To allow the client to
	improve its RTT estimate, the new packet that it sends MUST be ack-		improve its RTT estimate, the new packet that it sends MUST be ack-
	eliciting. If Handshake keys are available to the client, it MUST		eliciting. If Handshake keys are available to the client, it MUST
	send a Handshake packet, and otherwise it MUST send an Initial packet		send a Handshake packet, and otherwise it MUST send an Initial packet
	in a UDP datagram of at least 1200 bytes.		in a UDP datagram of at least 1200 bytes.
	Initial packets and Handshake packets may never be acknowledged, but		Initial packets and Handshake packets could be never acknowledged,
	they are removed from bytes in flight when the Initial and Handshake		but they are removed from bytes in flight when the Initial and
	keys are discarded.		Handshake keys are discarded.
	5.3.1. Sending Probe Packets		5.3.1. Sending Probe Packets
	When a PTO timer expires, a sender MUST send at least one ack-		When a PTO timer expires, a sender MUST send at least one ack-
	eliciting packet as a probe, unless there is no data available to		eliciting packet in the packet number space as a probe, unless there
	send. An endpoint MAY send up to two full-sized datagrams containing		is no data available to send. An endpoint MAY send up to two full-
	ack-eliciting packets, to avoid an expensive consecutive PTO		sized datagrams containing ack-eliciting packets, to avoid an
	expiration due to a single lost datagram.		expensive consecutive PTO expiration due to a single lost datagram or
			transmit data from multiple packet number spaces.
			In addition to sending data in the packet number space for which the
			timer expired, the sender SHOULD send ack-eliciting packets from
			other packet number spaces with in-flight data, coalescing packets if
			possible.
	When the PTO timer expires, and there is new or previously sent		When the PTO timer expires, and there is new or previously sent
	unacknowledged data, it MUST be sent. Data that was previously sent		unacknowledged data, it MUST be sent.
	with Initial encryption MUST be sent before Handshake data and data
	previously sent at Handshake encryption MUST be sent before any
	ApplicationData data.
	It is possible the sender has no new or previously-sent data to send.		It is possible the sender has no new or previously-sent data to send.
	As an example, consider the following sequence of events: new		As an example, consider the following sequence of events: new
	application data is sent in a STREAM frame, deemed lost, then		application data is sent in a STREAM frame, deemed lost, then
	retransmitted in a new packet, and then the original transmission is		retransmitted in a new packet, and then the original transmission is
	acknowledged. When there is no data to send, the sender SHOULD send		acknowledged. When there is no data to send, the sender SHOULD send
	a PING or other ack-eliciting frame in a single packet, re-arming the		a PING or other ack-eliciting frame in a single packet, re-arming the
	PTO timer.		PTO timer.
	Alternatively, instead of sending an ack-eliciting packet, the sender		Alternatively, instead of sending an ack-eliciting packet, the sender
	skipping to change at page 15, line 43 ¶		skipping to change at page 16, line 19 ¶
	congestion window, unless the packet is a probe packet sent after a		congestion window, unless the packet is a probe packet sent after a
	PTO timer expires, as described in Section 5.2.		PTO timer expires, as described in Section 5.2.
	Implementations MAY use other congestion control algorithms, such as		Implementations MAY use other congestion control algorithms, such as
	Cubic [RFC8312], and endpoints MAY use different algorithms from one		Cubic [RFC8312], and endpoints MAY use different algorithms from one
	another. The signals QUIC provides for congestion control are		another. The signals QUIC provides for congestion control are
	generic and are designed to support different algorithms.		generic and are designed to support different algorithms.
	6.1. Explicit Congestion Notification		6.1. Explicit Congestion Notification
	If a path has been verified to support ECN, QUIC treats a Congestion		If a path has been verified to support ECN [RFC3168] [RFC8311], QUIC
	Experienced codepoint in the IP header as a signal of congestion.		treats a Congestion Experienced(CE) codepoint in the IP header as a
	This document specifies an endpoint's response when its peer receives		signal of congestion. This document specifies an endpoint's response
	packets with the Congestion Experienced codepoint. As discussed in		when its peer receives packets with the Congestion Experienced
	[RFC8311], endpoints are permitted to experiment with other response		codepoint.
	functions.
	6.2. Slow Start		6.2. Slow Start
	QUIC begins every connection in slow start and exits slow start upon		QUIC begins every connection in slow start and exits slow start upon
	loss or upon increase in the ECN-CE counter. QUIC re-enters slow		loss or upon increase in the ECN-CE counter. QUIC re-enters slow
	start anytime the congestion window is less than ssthresh, which only		start any time the congestion window is less than ssthresh, which
	occurs after persistent congestion is declared. While in slow start,		only occurs after persistent congestion is declared. While in slow
	QUIC increases the congestion window by the number of bytes		start, QUIC increases the congestion window by the number of bytes
	acknowledged when each acknowledgment is processed.		acknowledged when each acknowledgment is processed.
	6.3. Congestion Avoidance		6.3. Congestion Avoidance
	Slow start exits to congestion avoidance. Congestion avoidance in		Slow start exits to congestion avoidance. Congestion avoidance in
	NewReno uses an additive increase multiplicative decrease (AIMD)		NewReno uses an additive increase multiplicative decrease (AIMD)
	approach that increases the congestion window by one maximum packet		approach that increases the congestion window by one maximum packet
	size per congestion window acknowledged. When a loss is detected,		size per congestion window acknowledged. When a loss is detected,
	NewReno halves the congestion window and sets the slow start		NewReno halves the congestion window and sets the slow start
	threshold to the new congestion window.		threshold to the new congestion window.
	skipping to change at page 17, line 40 ¶		skipping to change at page 18, line 17 ¶
	+-----+------------------------+		+-----+------------------------+
	| t=1 | Send Pkt #2 (PTO 1) |		| t=1 | Send Pkt #2 (PTO 1) |
	| | |		| | |
	| t=3 | Send Pkt #3 (PTO 2) |		| t=3 | Send Pkt #3 (PTO 2) |
	| | |		| | |
	| t=7 | Send Pkt #4 (PTO 3) |		| t=7 | Send Pkt #4 (PTO 3) |
	| | |		| | |
	| t=8 | Recv ACK of Pkt #4 |		| t=8 | Recv ACK of Pkt #4 |
	+-----+------------------------+		+-----+------------------------+
	The first three packets are determined to be lost when the ACK of		The first three packets are determined to be lost when the
	packet 4 is received at t=8. The congestion period is calculated as		acknowlegement of packet 4 is received at t=8. The congestion period
	the time between the oldest and newest lost packets: (3 - 0) = 3.		is calculated as the time between the oldest and newest lost packets:
	The duration for persistent congestion is equal to: (1 *		(3 - 0) = 3. The duration for persistent congestion is equal to: (1
	kPersistentCongestionThreshold) = 3. Because the threshold was		* kPersistentCongestionThreshold) = 3. Because the threshold was
	reached and because none of the packets between the oldest and the		reached and because none of the packets between the oldest and the
	newest packets are acknowledged, the network is considered to have		newest packets are acknowledged, the network is considered to have
	experienced persistent congestion.		experienced persistent congestion.
	When persistent congestion is established, the sender's congestion		When persistent congestion is established, the sender's congestion
	window MUST be reduced to the minimum congestion window		window MUST be reduced to the minimum congestion window
	(kMinimumWindow). This response of collapsing the congestion window		(kMinimumWindow). This response of collapsing the congestion window
	on persistent congestion is functionally similar to a sender's		on persistent congestion is functionally similar to a sender's
	response on a Retransmission Timeout (RTO) in TCP [RFC5681] after		response on a Retransmission Timeout (RTO) in TCP [RFC5681] after
	Tail Loss Probes (TLP) [RACK].		Tail Loss Probes (TLP) [RACK].
	skipping to change at page 19, line 37 ¶		skipping to change at page 20, line 13 ¶
	frames to reduce leaked information.		frames to reduce leaked information.
	7.3. Misreporting ECN Markings		7.3. Misreporting ECN Markings
	A receiver can misreport ECN markings to alter the congestion		A receiver can misreport ECN markings to alter the congestion
	response of a sender. Suppressing reports of ECN-CE markings could		response of a sender. Suppressing reports of ECN-CE markings could
	cause a sender to increase their send rate. This increase could		cause a sender to increase their send rate. This increase could
	result in congestion and loss.		result in congestion and loss.
	A sender MAY attempt to detect suppression of reports by marking		A sender MAY attempt to detect suppression of reports by marking
	occasional packets that they send with ECN-CE. If a packet marked		occasional packets that they send with ECN-CE. If a packet sent with
	with ECN-CE is not reported as having been marked when the packet is		ECN-CE is not reported as having been CE marked when the packet is
	acknowledged, the sender SHOULD then disable ECN for that path.		acknowledged, then the sender SHOULD disable ECN for that path.
	Reporting additional ECN-CE markings will cause a sender to reduce		Reporting additional ECN-CE markings will cause a sender to reduce
	their sending rate, which is similar in effect to advertising reduced		their sending rate, which is similar in effect to advertising reduced
	connection flow control limits and so no advantage is gained by doing		connection flow control limits and so no advantage is gained by doing
	so.		so.
	Endpoints choose the congestion controller that they use. Though		Endpoints choose the congestion controller that they use. Though
	congestion controllers generally treat reports of ECN-CE markings as		congestion controllers generally treat reports of ECN-CE markings as
	equivalent to loss [RFC8311], the exact response for each controller		equivalent to loss [RFC8311], the exact response for each controller
	could be different. Failure to correctly respond to information		could be different. Failure to correctly respond to information
	skipping to change at page 20, line 15 ¶		skipping to change at page 20, line 38 ¶
	8. IANA Considerations		8. IANA Considerations
	This document has no IANA actions. Yet.		This document has no IANA actions. Yet.
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[QUIC-TLS]		[QUIC-TLS]
	Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure		Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
	QUIC", draft-ietf-quic-tls-24 (work in progress).		QUIC", draft-ietf-quic-tls-latest (work in progress).
	[QUIC-TRANSPORT]		[QUIC-TRANSPORT]
	Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based		Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
	Multiplexed and Secure Transport", draft-ietf-quic-		Multiplexed and Secure Transport", draft-ietf-quic-
	transport-24 (work in progress).		transport-latest (work in progress).
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
	Notification (ECN) Experimentation", RFC 8311,
	DOI 10.17487/RFC8311, January 2018,
	<https://www.rfc-editor.org/info/rfc8311>.
	9.2. Informative References		9.2. Informative References
	[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:		[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
	Refining TCP Congestion Control", ACM SIGCOMM , August		Refining TCP Congestion Control", ACM SIGCOMM , August
	1996.		1996.
	[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:		[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
	a time-based fast loss detection algorithm for TCP",		a time-based fast loss detection algorithm for TCP",
	draft-ietf-tcpm-rack-06 (work in progress), November 2019.		draft-ietf-tcpm-rack-06 (work in progress), November 2019.
			[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
			of Explicit Congestion Notification (ECN) to IP",
			RFC 3168, DOI 10.17487/RFC3168, September 2001,
			<https://www.rfc-editor.org/info/rfc3168>.
	[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte		[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
	Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February		Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
	2003, <https://www.rfc-editor.org/info/rfc3465>.		2003, <https://www.rfc-editor.org/info/rfc3465>.
	[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,		[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
	"Improving the Robustness of TCP to Non-Congestion		"Improving the Robustness of TCP to Non-Congestion
	Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,		Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,
	<https://www.rfc-editor.org/info/rfc4653>.		<https://www.rfc-editor.org/info/rfc4653>.
	[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion		[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
	skipping to change at page 22, line 5 ¶		skipping to change at page 22, line 26 ¶
	[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,		[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
	"Increasing TCP's Initial Window", RFC 6928,		"Increasing TCP's Initial Window", RFC 6928,
	DOI 10.17487/RFC6928, April 2013,		DOI 10.17487/RFC6928, April 2013,
	<https://www.rfc-editor.org/info/rfc6928>.		<https://www.rfc-editor.org/info/rfc6928>.
	[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating		[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
	TCP to Support Rate-Limited Traffic", RFC 7661,		TCP to Support Rate-Limited Traffic", RFC 7661,
	DOI 10.17487/RFC7661, October 2015,		DOI 10.17487/RFC7661, October 2015,
	<https://www.rfc-editor.org/info/rfc7661>.		<https://www.rfc-editor.org/info/rfc7661>.
			[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
			Notification (ECN) Experimentation", RFC 8311,
			DOI 10.17487/RFC8311, January 2018,
			<https://www.rfc-editor.org/info/rfc8311>.
	[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and		[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
	R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",		R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
	RFC 8312, DOI 10.17487/RFC8312, February 2018,		RFC 8312, DOI 10.17487/RFC8312, February 2018,
	<https://www.rfc-editor.org/info/rfc8312>.		<https://www.rfc-editor.org/info/rfc8312>.
	9.3. URIs		9.3. URIs
	[1] https://mailarchive.ietf.org/arch/search/?email_list=quic		[1] https://mailarchive.ietf.org/arch/search/?email_list=quic
	[2] https://github.com/quicwg		[2] https://github.com/quicwg
	skipping to change at page 24, line 6 ¶		skipping to change at page 24, line 39 ¶
	described in [RFC6298]		described in [RFC6298]
	rttvar: The RTT variance, computed as described in [RFC6298]		rttvar: The RTT variance, computed as described in [RFC6298]
	min_rtt: The minimum RTT seen in the connection, ignoring ack delay.		min_rtt: The minimum RTT seen in the connection, ignoring ack delay.
	max_ack_delay: The maximum amount of time by which the receiver		max_ack_delay: The maximum amount of time by which the receiver
	intends to delay acknowledgments for packets in the		intends to delay acknowledgments for packets in the
	ApplicationData packet number space. The actual ack_delay in a		ApplicationData packet number space. The actual ack_delay in a
	received ACK frame may be larger due to late timers, reordering,		received ACK frame may be larger due to late timers, reordering,
	or lost ACKs.		or lost ACK frames.
	loss_detection_timer: Multi-modal timer used for loss detection.		loss_detection_timer: Multi-modal timer used for loss detection.
	pto_count: The number of times a PTO has been sent without receiving		pto_count: The number of times a PTO has been sent without receiving
	an ack.		an ack.
	time_of_last_sent_ack_eliciting_packet: The time the most recent		time_of_last_sent_ack_eliciting_packet[kPacketNumberSpace]: The time
	ack-eliciting packet was sent.		the most recent ack-eliciting packet was sent.
	largest_acked_packet[kPacketNumberSpace]: The largest packet number		largest_acked_packet[kPacketNumberSpace]: The largest packet number
	acknowledged in the packet number space so far.		acknowledged in the packet number space so far.
	loss_time[kPacketNumberSpace]: The time at which the next packet in		loss_time[kPacketNumberSpace]: The time at which the next packet in
	that packet number space will be considered lost based on		that packet number space will be considered lost based on
	exceeding the reordering window in time.		exceeding the reordering window in time.
	sent_packets[kPacketNumberSpace]: An association of packet numbers		sent_packets[kPacketNumberSpace]: An association of packet numbers
	in a packet number space to information about them. Described in		in a packet number space to information about them. Described in
	skipping to change at page 24, line 39 ¶		skipping to change at page 25, line 25 ¶
	At the beginning of the connection, initialize the loss detection		At the beginning of the connection, initialize the loss detection
	variables as follows:		variables as follows:
	loss_detection_timer.reset()		loss_detection_timer.reset()
	pto_count = 0		pto_count = 0
	latest_rtt = 0		latest_rtt = 0
	smoothed_rtt = 0		smoothed_rtt = 0
	rttvar = 0		rttvar = 0
	min_rtt = 0		min_rtt = 0
	max_ack_delay = 0		max_ack_delay = 0
	time_of_last_sent_ack_eliciting_packet = 0
	for pn_space in [ Initial, Handshake, ApplicationData ]:		for pn_space in [ Initial, Handshake, ApplicationData ]:
	largest_acked_packet[pn_space] = infinite		largest_acked_packet[pn_space] = infinite
			time_of_last_sent_ack_eliciting_packet[pn_space] = 0
	loss_time[pn_space] = 0		loss_time[pn_space] = 0
	A.5. On Sending a Packet		A.5. On Sending a Packet
	After a packet is sent, information about the packet is stored. The		After a packet is sent, information about the packet is stored. The
	parameters to OnPacketSent are described in detail above in		parameters to OnPacketSent are described in detail above in
	Appendix A.1.1.		Appendix A.1.1.
	Pseudocode for OnPacketSent follows:		Pseudocode for OnPacketSent follows:
	OnPacketSent(packet_number, pn_space, ack_eliciting,		OnPacketSent(packet_number, pn_space, ack_eliciting,
	in_flight, sent_bytes):		in_flight, sent_bytes):
	sent_packets[pn_space][packet_number].packet_number =		sent_packets[pn_space][packet_number].packet_number =
	packet_number		packet_number
	sent_packets[pn_space][packet_number].time_sent = now		sent_packets[pn_space][packet_number].time_sent = now
	sent_packets[pn_space][packet_number].ack_eliciting =		sent_packets[pn_space][packet_number].ack_eliciting =
	ack_eliciting		ack_eliciting
	sent_packets[pn_space][packet_number].in_flight = in_flight		sent_packets[pn_space][packet_number].in_flight = in_flight
	if (in_flight):		if (in_flight):
	if (ack_eliciting):		if (ack_eliciting):
	time_of_last_sent_ack_eliciting_packet = now		time_of_last_sent_ack_eliciting_packet[pn_space] = now
	OnPacketSentCC(sent_bytes)		OnPacketSentCC(sent_bytes)
	sent_packets[pn_space][packet_number].size = sent_bytes		sent_packets[pn_space][packet_number].size = sent_bytes
	SetLossDetectionTimer()		SetLossDetectionTimer()
	A.6. On Receiving an Acknowledgment		A.6. On Receiving an Acknowledgment
	When an ACK frame is received, it may newly acknowledge any number of		When an ACK frame is received, it may newly acknowledge any number of
	packets.		packets.
	Pseudocode for OnAckReceived and UpdateRtt follow:		Pseudocode for OnAckReceived and UpdateRtt follow:
	skipping to change at page 28, line 5 ¶		skipping to change at page 28, line 5 ¶
	which is set in the packet and timer events further below. The		which is set in the packet and timer events further below. The
	function SetLossDetectionTimer defined below shows how the single		function SetLossDetectionTimer defined below shows how the single
	timer is set.		timer is set.
	This algorithm may result in the timer being set in the past,		This algorithm may result in the timer being set in the past,
	particularly if timers wake up late. Timers set in the past SHOULD		particularly if timers wake up late. Timers set in the past SHOULD
	fire immediately.		fire immediately.
	Pseudocode for SetLossDetectionTimer follows:		Pseudocode for SetLossDetectionTimer follows:
	// Returns the earliest loss_time and the packet number		GetEarliestTimeAndSpace(times):
	// space it's from. Returns 0 if all times are 0.		time = times[Initial]
	GetEarliestLossTime():
	time = loss_time[Initial]
	space = Initial		space = Initial
	for pn_space in [ Handshake, ApplicationData ]:		for pn_space in [ Handshake, ApplicationData ]:
	if (loss_time[pn_space] != 0 &&		if (times[pn_space] != 0 &&
	(time == 0 || loss_time[pn_space] < time)):		(time == 0 || times[pn_space] < time) &&
	time = loss_time[pn_space];		# Skip ApplicationData until handshake completion.
			(pn_space != ApplicationData ||
			IsHandshakeComplete()):
			time = times[pn_space];
	space = pn_space		space = pn_space
	return time, space		return time, space
	PeerNotAwaitingAddressValidation():		PeerNotAwaitingAddressValidation():
	# Assume clients validate the server's address implicitly.		# Assume clients validate the server's address implicitly.
	if (endpoint is server):		if (endpoint is server):
	return true		return true
	# Servers complete address validation when a		# Servers complete address validation when a
	# protected packet is received.		# protected packet is received.
	return has received Handshake ACK ||		return has received Handshake ACK ||
	has received 1-RTT ACK		has received 1-RTT ACK
	SetLossDetectionTimer():		SetLossDetectionTimer():
	loss_time, _ = GetEarliestLossTime()		earliest_loss_time, _ = GetEarliestTimeAndSpace(loss_time)
	if (loss_time != 0):		if (earliest_loss_time != 0):
	// Time threshold loss detection.		// Time threshold loss detection.
	loss_detection_timer.update(loss_time)		loss_detection_timer.update(earliest_loss_time)
	return		return
	if (no ack-eliciting packets in flight &&		if (no ack-eliciting packets in flight &&
	PeerNotAwaitingAddressValidation()):		PeerNotAwaitingAddressValidation()):
	loss_detection_timer.cancel()		loss_detection_timer.cancel()
	return		return
	// Use a default timeout if there are no RTT measurements		// Use a default timeout if there are no RTT measurements
	if (smoothed_rtt == 0):		if (smoothed_rtt == 0):
	timeout = 2 * kInitialRtt		timeout = 2 * kInitialRtt
	else:		else:
	// Calculate PTO duration		// Calculate PTO duration
	timeout = smoothed_rtt + max(4 * rttvar, kGranularity) +		timeout = smoothed_rtt + max(4 * rttvar, kGranularity) +
	max_ack_delay		max_ack_delay
	timeout = timeout * (2 ^ pto_count)		timeout = timeout * (2 ^ pto_count)
	loss_detection_timer.update(		sent_time, _ = GetEarliestTimeAndSpace(
	time_of_last_sent_ack_eliciting_packet + timeout)		time_of_last_sent_ack_eliciting_packet)
			loss_detection_timer.update(sent_time + timeout)
	A.9. On Timeout		A.9. On Timeout
	When the loss detection timer expires, the timer's mode determines		When the loss detection timer expires, the timer's mode determines
	the action to be performed.		the action to be performed.
	Pseudocode for OnLossDetectionTimeout follows:		Pseudocode for OnLossDetectionTimeout follows:
	OnLossDetectionTimeout():		OnLossDetectionTimeout():
	loss_time, pn_space = GetEarliestLossTime()		earliest_loss_time, pn_space =
	if (loss_time != 0):		GetEarliestTimeAndSpace(loss_time)
			if (earliest_loss_time != 0):
	// Time threshold loss Detection		// Time threshold loss Detection
	DetectLostPackets(pn_space)		DetectLostPackets(pn_space)
	SetLossDetectionTimer()		SetLossDetectionTimer()
	return		return
	if (endpoint is client without 1-RTT keys):		if (endpoint is client without 1-RTT keys):
	// Client sends an anti-deadlock packet: Initial is padded		// Client sends an anti-deadlock packet: Initial is padded
	// to earn more anti-amplification credit,		// to earn more anti-amplification credit,
	// a Handshake packet proves address ownership.		// a Handshake packet proves address ownership.
	if (has Handshake keys):		if (has Handshake keys):
	SendOneAckElicitingHandshakePacket()		SendOneAckElicitingHandshakePacket()
	else:		else:
	SendOneAckElicitingPaddedInitialPacket()		SendOneAckElicitingPaddedInitialPacket()
	else:		else:
	// PTO. Send new data if available, else retransmit old data.		// PTO. Send new data if available, else retransmit old data.
	// If neither is available, send a single PING frame.		// If neither is available, send a single PING frame.
	SendOneOrTwoAckElicitingPackets()		_, pn_space = GetEarliestTimeAndSpace(
			time_of_last_sent_ack_eliciting_packet)
			SendOneOrTwoAckElicitingPackets(pn_space)
	pto_count++		pto_count++
	SetLossDetectionTimer()		SetLossDetectionTimer()
	A.10. Detecting Lost Packets		A.10. Detecting Lost Packets
	DetectLostPackets is called every time an ACK is received and		DetectLostPackets is called every time an ACK is received and
	operates on the sent_packets for that packet number space.		operates on the sent_packets for that packet number space.
	Pseudocode for DetectLostPackets follows:		Pseudocode for DetectLostPackets follows:
	skipping to change at page 33, line 14 ¶		skipping to change at page 33, line 14 ¶
	InCongestionRecovery(sent_time):		InCongestionRecovery(sent_time):
	return sent_time <= congestion_recovery_start_time		return sent_time <= congestion_recovery_start_time
	OnPacketAckedCC(acked_packet):		OnPacketAckedCC(acked_packet):
	// Remove from bytes_in_flight.		// Remove from bytes_in_flight.
	bytes_in_flight -= acked_packet.size		bytes_in_flight -= acked_packet.size
	if (InCongestionRecovery(acked_packet.time_sent)):		if (InCongestionRecovery(acked_packet.time_sent)):
	// Do not increase congestion window in recovery period.		// Do not increase congestion window in recovery period.
	return		return
	if (IsAppLimited()):		if (IsAppOrFlowControlLimited()):
	// Do not increase congestion_window if application		// Do not increase congestion_window if application
	// limited.		// limited or flow control limited.
	return		return
	if (congestion_window < ssthresh):		if (congestion_window < ssthresh):
	// Slow start.		// Slow start.
	congestion_window += acked_packet.size		congestion_window += acked_packet.size
	else:		else:
	// Congestion avoidance.		// Congestion avoidance.
	congestion_window += max_datagram_size * acked_packet.size		congestion_window += max_datagram_size * acked_packet.size
	/ congestion_window		/ congestion_window
	B.6. On New Congestion Event		B.6. On New Congestion Event
	skipping to change at page 34, line 50 ¶		skipping to change at page 35, line 5 ¶
	o PTO MUST send data if possible (#3056, #3057)		o PTO MUST send data if possible (#3056, #3057)
	o Connection Close is not ack-eliciting (#3097, #3098)		o Connection Close is not ack-eliciting (#3097, #3098)
	o MUST limit bursts to the initial congestion window (#3160)		o MUST limit bursts to the initial congestion window (#3160)
	o Define the current max_datagram_size for congestion control		o Define the current max_datagram_size for congestion control
	(#3041, #3167)		(#3041, #3167)
	o Separate PTO by packet number space (#3067, #3074, #3066)
	C.2. Since draft-ietf-quic-recovery-22		C.2. Since draft-ietf-quic-recovery-22
	o PTO should always send an ack-eliciting packet (#2895)		o PTO should always send an ack-eliciting packet (#2895)
	o Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)		o Unify the Handshake Timer with the PTO timer (#2648, #2658, #2886)
	o Move ACK generation text to transport draft (#1860, #2916)		o Move ACK generation text to transport draft (#1860, #2916)
	C.3. Since draft-ietf-quic-recovery-21		C.3. Since draft-ietf-quic-recovery-21
End of changes. 59 change blocks.
	132 lines changed or deleted		161 lines changed or added
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