draft-ietf-quic-recovery-20.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: October 25, 2019 Google		Expires: December 19, 2019 Google
	April 23, 2019		June 17, 2019
	QUIC Loss Detection and Congestion Control		QUIC Loss Detection and Congestion Control
	draft-ietf-quic-recovery-20		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 October 25, 2019.		This Internet-Draft will expire on December 19, 2019.
	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 23 ¶		skipping to change at page 2, line 23 ¶
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
	2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4		2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
	3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5		3. Design of the QUIC Transmission Machinery . . . . . . . . . . 5
	3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5		3.1. Relevant Differences Between QUIC and TCP . . . . . . . . 5
	3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 6		3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 6
	3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6		3.1.2. Monotonically Increasing Packet Numbers . . . . . . . 6
	3.1.3. No Reneging . . . . . . . . . . . . . . . . . . . . . 6		3.1.3. Clearer Loss Epoch . . . . . . . . . . . . . . . . . 6
	3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 7		3.1.4. No Reneging . . . . . . . . . . . . . . . . . . . . . 7
	3.1.5. Explicit Correction For Delayed Acknowledgements . . 7		3.1.5. More ACK Ranges . . . . . . . . . . . . . . . . . . . 7
			3.1.6. Explicit Correction For Delayed Acknowledgements . . 7
	4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7		4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7
	4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7		4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 8
	4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8		4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8
	4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8		4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
	4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 8		4.4. Measuring and Reporting Host Delay . . . . . . . . . . . 8
	5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9		5. Estimating the Round-Trip Time . . . . . . . . . . . . . . . 9
	5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9		5.1. Generating RTT samples . . . . . . . . . . . . . . . . . 9
	5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10		5.2. Estimating min_rtt . . . . . . . . . . . . . . . . . . . 10
	5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10		5.3. Estimating smoothed_rtt and rttvar . . . . . . . . . . . 10
	6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11		6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 11
	6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 11		6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 11
	6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12		6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 12
	6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12		6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 12
	6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 13		6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 13
	6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 14		6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 14
	6.2.2. Discarding Keys and Packet State . . . . . . . . . . 14
	6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 15		6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 15
	6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 15		6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 15
	6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 16		6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 15
	6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 17		6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 16
	6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17		6.4. Discarding Keys and Packet State . . . . . . . . . . . . 17
			6.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 17
	7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17		7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 17
	7.1. Explicit Congestion Notification . . . . . . . . . . . . 17		7.1. Explicit Congestion Notification . . . . . . . . . . . . 18
	7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 18		7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 18
	7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18		7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 18
	7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18		7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 18
	7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 18		7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 19
	7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 18		7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 19
	7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 19		7.7. Persistent Congestion . . . . . . . . . . . . . . . . . . 19
	7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 20		7.8. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 20
	7.9. Under-utilizing the Congestion Window . . . . . . . . . . 20		7.9. Under-utilizing the Congestion Window . . . . . . . . . . 21
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 21		8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
	8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 21		8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 21
	8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21		8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 21
	8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 21		8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 22
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
	10.1. Normative References . . . . . . . . . . . . . . . . . . 22		10.1. Normative References . . . . . . . . . . . . . . . . . . 22
	10.2. Informative References . . . . . . . . . . . . . . . . . 22		10.2. Informative References . . . . . . . . . . . . . . . . . 23
	10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24		10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24
	Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24		Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 24
	A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 24		A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 25
	A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 24		A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 25
	A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25		A.2. Constants of interest . . . . . . . . . . . . . . . . . . 25
	A.3. Variables of interest . . . . . . . . . . . . . . . . . . 25		A.3. Variables of interest . . . . . . . . . . . . . . . . . . 26
	A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 26		A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 27
	A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 27		A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 27
	A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 27		A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 28
	A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 29		A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 29
	A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 29		A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 30
	A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 31		A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 32
	A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 31		A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 32
	Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 32		Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 33
	B.1. Constants of interest . . . . . . . . . . . . . . . . . . 32		B.1. Constants of interest . . . . . . . . . . . . . . . . . . 33
	B.2. Variables of interest . . . . . . . . . . . . . . . . . . 33		B.2. Variables of interest . . . . . . . . . . . . . . . . . . 34
	B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 34		B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 35
	B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 34		B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 35
	B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 34		B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 35
	B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 35		B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 36
	B.7. Process ECN Information . . . . . . . . . . . . . . . . . 35		B.7. Process ECN Information . . . . . . . . . . . . . . . . . 36
	B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 36		B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 37
	Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 36		Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 37
	C.1. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 36		C.1. Since draft-ietf-quic-recovery-19 . . . . . . . . . . . . 37
	C.2. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 37		C.2. Since draft-ietf-quic-recovery-18 . . . . . . . . . . . . 38
	C.3. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 37		C.3. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 38
	C.4. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 38		C.4. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 39
	C.5. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 38		C.5. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 39
	C.6. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 38		C.6. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 39
	C.7. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 39		C.7. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 40
	C.8. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 39		C.8. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 40
	C.9. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 39		C.9. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 40
	C.10. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 39		C.10. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 40
	C.11. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 39		C.11. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 40
	C.12. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 39		C.12. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 40
	C.13. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 40		C.13. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 41
	C.14. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 40		C.14. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 41
	C.15. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 40		C.15. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 41
	C.16. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 40		C.16. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 41
	C.17. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 40		C.17. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 41
	C.18. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 40		C.18. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 41
	C.19. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 40		C.19. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 41
	C.20. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 40		C.20. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 42
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41		Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
	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 loss recovery mechanisms,
	described in RFCs, various Internet-drafts, and also those prevalent		described in RFCs, various Internet-drafts, and also those prevalent
	skipping to change at page 4, line 41 ¶		skipping to change at page 4, line 42 ¶
	"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:
	ACK-only: Any packet containing only one or more ACK frame(s).		ACK-only: Any packet containing only one or more ACK frame(s).
	In-flight: Packets are considered in-flight when they have been sent		In-flight: Packets are considered in-flight when they have been sent
	and neither acknowledged nor declared lost, and they are not ACK-		and are not ACK-only, and they are not acknowledged, declared
	only.		lost, or abandoned along with old keys.
	Ack-eliciting Frames: All frames besides ACK or PADDING are		Ack-eliciting Frames: All frames besides ACK or PADDING are
	considered ack-eliciting.		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		Crypto Packets: Packets containing CRYPTO data sent in Initial or
	Handshake packets.		Handshake packets.
	skipping to change at page 6, line 43 ¶		skipping to change at page 6, line 43 ¶
	ambiguity about which packet is acknowledged when an ACK is received.		ambiguity about which packet is acknowledged when an ACK is received.
	Consequently, more accurate RTT measurements can be made, spurious		Consequently, more accurate RTT measurements can be made, spurious
	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. No Reneging		3.1.3. Clearer Loss Epoch
			QUIC ends a loss epoch when a packet sent after loss is declared is
			acknowledged. TCP waits for the gap in the sequence number space to
			be filled, and so if a segment is lost multiple times in a row, the
			loss epoch may not end for several round trips. Because both should
			reduce their congestion windows only once per epoch, QUIC will do it
			correctly once for every round trip that experiences loss, while TCP
			may only do it once across multiple round trips.
			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.
	3.1.4. More ACK Ranges		3.1.5. More ACK Ranges
	QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In		QUIC supports many ACK ranges, opposed to TCP's 3 SACK ranges. In
	high loss environments, this speeds recovery, reduces spurious		high loss environments, this speeds recovery, reduces spurious
	retransmits, and ensures forward progress without relying on		retransmits, and ensures forward progress without relying on
	timeouts.		timeouts.
	3.1.5. Explicit Correction For Delayed Acknowledgements		3.1.6. Explicit Correction For Delayed Acknowledgements
	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 4.4).		Section 4.4).
	4. Generating Acknowledgements		4. Generating Acknowledgements
	An acknowledgement SHOULD be sent immediately upon receipt of a		An acknowledgement SHOULD be sent immediately upon receipt of a
	second ack-eliciting packet. QUIC recovery algorithms do not assume		second ack-eliciting packet. QUIC recovery algorithms do not assume
	skipping to change at page 8, line 40 ¶		skipping to change at page 8, line 49 ¶
	of 1 RTT of reordering. In cases with ACK frame loss and reordering,		of 1 RTT of reordering. In cases with ACK frame loss and reordering,
	this approach does not guarantee that every acknowledgement is seen		this approach does not guarantee that every acknowledgement is seen
	by the sender before it is no longer included in the ACK frame.		by the sender before it is no longer included in the ACK frame.
	Packets could be received out of order and all subsequent ACK frames		Packets could be received out of order and all subsequent ACK frames
	containing them could be lost. In this case, the loss recovery		containing them could be lost. In this case, the loss recovery
	algorithm may cause spurious retransmits, but the sender will		algorithm may cause spurious retransmits, but the sender will
	continue making forward progress.		continue making forward progress.
	4.4. Measuring and Reporting Host Delay		4.4. Measuring and Reporting Host Delay
	An endpoint measures the delay incurred between when a packet is		An endpoint measures the delays intentionally introduced between when
	received and when the corresponding acknowledgment is sent. The		an ACK-eliciting packet is received and the corresponding
	endpoint encodes this host delay for the largest acknowledged packet		acknowledgment is sent. The endpoint encodes this delay for the
	in the Ack Delay field of an ACK frame (see Section 19.3 of		largest acknowledged packet in the Ack Delay field of an ACK frame
	[QUIC-TRANSPORT]). This allows the receiver of the ACK to adjust for		(see Section 19.3 of [QUIC-TRANSPORT]). This allows the receiver of
	any host delays, which is important for delayed acknowledgements,		the ACK to adjust for any intentional delays, which is important for
	when estimating the path RTT. In certain deployments, a packet might		delayed acknowledgements, when estimating the path RTT. A packet
	be held in the OS kernel or elsewhere on the host before being		might be held in the OS kernel or elsewhere on the host before being
	processed by the QUIC stack. Where possible, an endpoint MAY include		processed. An endpoint SHOULD NOT include these unintentional delays
	these delays when populating the Ack Delay field in an ACK frame.		when populating the Ack Delay field in an ACK frame.
	An endpoint MUST NOT excessively delay acknowledgements of ack-		An endpoint MUST NOT excessively delay acknowledgements of ack-
	eliciting packets. The maximum ack delay is communicated in the		eliciting packets. The maximum ack delay is communicated in the
	max_ack_delay transport parameter, see Section 18.1 of		max_ack_delay transport parameter; see Section 18.1 of
	[QUIC-TRANSPORT]. max_ack_delay implies an explicit contract: an		[QUIC-TRANSPORT]. max_ack_delay implies an explicit contract: an
	endpoint promises to never delay acknowledgments of an ack-eliciting		endpoint promises to never delay acknowledgments of an ack-eliciting
	packet by more than the indicated value. If it does, any excess		packet by more than the indicated value. If it does, any excess
	accrues to the RTT estimate and could result in spurious		accrues to the RTT estimate and could result in spurious
	retransmissions from the peer.		retransmissions from the peer. For Initial and Handshake packets, a
			max_ack_delay of 0 is used.
	5. Estimating the Round-Trip Time		5. 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		The endpoint uses RTT samples and peer-reported host delays
	(Section 4.4) to generate a statistical description of the		(Section 4.4) to generate a statistical description of the
	connection's RTT. An endpoint computes the following three values:		connection's RTT. An endpoint computes the following three values:
	the minimum value observed over the lifetime of the connection		the minimum value observed over the lifetime of the connection
	(min_rtt), an exponentially-weighted moving average (smoothed_rtt),		(min_rtt), an exponentially-weighted moving average (smoothed_rtt),
	skipping to change at page 10, line 33 ¶		skipping to change at page 10, line 42 ¶
	the smoothed_rtt based entirely on what it observes (see		the smoothed_rtt based entirely on what it observes (see
	Section 5.3), and limits potential underestimation due to		Section 5.3), and limits potential underestimation due to
	erroneously-reported delays by the peer.		erroneously-reported delays by the peer.
	5.3. Estimating smoothed_rtt and rttvar		5.3. 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.
	smoothed_rtt uses path latency after adjusting RTT samples for peer-		The calculation of smoothed_rtt uses path latency after adjusting RTT
	reported host delays (Section 4.4). A peer limits any delay in		samples for host delays (Section 4.4). For packets sent in the
			ApplicationData packet number space, a peer limits any delay in
	sending an acknowledgement for an ack-eliciting packet to no greater		sending an acknowledgement for an ack-eliciting packet to no greater
	than the advertised max_ack_delay transport parameter. Consequently,		than the value it advertised in the max_ack_delay transport
	when a peer reports an Ack Delay that is greater than its		parameter. Consequently, when a peer reports an Ack Delay that is
	max_ack_delay, the delay is attributed to reasons out of the peer's		greater than its max_ack_delay, the delay is attributed to reasons
	control, such as scheduler latency at the peer or loss of previous		out of the peer's control, such as scheduler latency at the peer or
	ACK frames. Any delays beyond the peer's max_ack_delay are therefore		loss of previous ACK frames. Any delays beyond the peer's
	considered effectively part of path delay and incorporated into the		max_ack_delay are therefore considered effectively part of path delay
	smoothed_rtt estimate.		and incorporated into the smoothed_rtt estimate.
	When adjusting an RTT sample using peer-reported acknowledgement		When adjusting an RTT sample using peer-reported acknowledgement
	delays, an endpoint:		delays, an endpoint:
			o MUST ignore the Ack Delay field of the ACK frame for packets sent
			in the Initial and Handshake packet number space.
	o MUST use the lesser of the value reported in Ack Delay field of		o MUST use the lesser of the value reported in Ack Delay field of
	the ACK frame and the peer's max_ack_delay transport parameter		the ACK frame and the peer's max_ack_delay transport parameter
	(Section 4.4).		(Section 4.4).
	o MUST NOT apply the adjustment if the resulting RTT sample is		o MUST NOT apply the adjustment if the resulting RTT sample is
	smaller than the min_rtt. This limits the underestimation that a		smaller than the min_rtt. This limits the underestimation that a
	misreporting peer can cause to the smoothed_rtt.		misreporting peer can cause to the smoothed_rtt.
	On the first RTT sample in a connection, the smoothed_rtt is set to		On the first RTT sample in a connection, the smoothed_rtt is set to
	the latest_rtt.		the latest_rtt.
	skipping to change at page 12, line 29 ¶		skipping to change at page 12, line 40 ¶
	(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.
	6.1.2. Time Threshold		6.1.2. Time Threshold
	Once a later packet has been acknowledged, an endpoint SHOULD declare		Once a later packet packet within the same packet number space has
	an earlier packet lost if it was sent a threshold amount of time in		been acknowledged, an endpoint SHOULD declare an earlier packet lost
	the past. The time threshold is computed as kTimeThreshold *		if it was sent a threshold amount of time in the past. To avoid
	max(SRTT, latest_RTT). If packets sent prior to the largest		declaring packets as lost too early, this time threshold MUST be set
	acknowledged packet cannot yet be declared lost, then a timer SHOULD		to at least kGranularity. The time threshold is:
	be set for the remaining time.
	The RECOMMENDED time threshold (kTimeThreshold), expressed as a		kTimeThreshold * max(SRTT, latest_RTT, kGranularity)
	round-trip time multiplier, is 9/8.
			If packets sent prior to the largest acknowledged packet cannot yet
			be declared lost, then a timer SHOULD be set for the remaining time.
	Using max(SRTT, latest_RTT) protects from the two following cases:		Using max(SRTT, latest_RTT) protects from the two following cases:
	o the latest RTT sample is lower than the SRTT, perhaps due to		o the latest RTT sample is lower than the SRTT, perhaps due to
	reordering where the acknowledgement encountered a shorter path;		reordering where the acknowledgement encountered a shorter path;
	o the latest RTT sample is higher than the SRTT, perhaps due to a		o the latest RTT sample is higher than the SRTT, perhaps due to a
	sustained increase in the actual RTT, but the smoothed SRTT has		sustained increase in the actual RTT, but the smoothed SRTT has
	not yet caught up.		not yet caught up.
	An endpoint might consistently record RTT samples as 0 in extremely		The RECOMMENDED time threshold (kTimeThreshold), expressed as a
	low latency networks, leading to a smoothed_rtt of 0. Consequently,		round-trip time multiplier, is 9/8.
	the endpoint could declare all earlier packets as lost immediately
	upon receiving an acknowledgement for a later packet. That is, the
	endpoint would not provide any reordering tolerance. To avoid
	declaring packets as lost too early, the time threshold MUST be set
	to at least kGranularity (defined in Appendix A.2).
	Implementations MAY experiment with absolute thresholds, thresholds		Implementations MAY experiment with absolute thresholds, thresholds
	from previous connections, adaptive thresholds, or including RTT		from previous connections, adaptive thresholds, or including RTT
	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.
	6.2. Crypto Retransmission Timeout		6.2. Crypto Retransmission Timeout
	Data in CRYPTO frames is critical to QUIC transport and crypto		Data in CRYPTO frames is critical to QUIC transport and crypto
	skipping to change at page 13, line 28 ¶		skipping to change at page 13, line 36 ¶
	The initial crypto retransmission timeout SHOULD be set to twice the		The initial crypto retransmission timeout SHOULD be set to twice the
	initial RTT.		initial RTT.
	At the beginning, there are no prior RTT samples within a connection.		At the beginning, there are no prior RTT samples within a connection.
	Resumed connections over the same network SHOULD use the previous		Resumed connections over the same network SHOULD use the previous
	connection's final smoothed RTT value as the resumed connection's		connection's final smoothed RTT value as the resumed connection's
	initial RTT. If no previous RTT is available, or if the network		initial RTT. If no previous RTT is available, or if the network
	changes, the initial RTT SHOULD be set to 500ms, resulting in a 1		changes, the initial RTT SHOULD be set to 500ms, resulting in a 1
	second initial handshake timeout as recommended in [RFC6298].		second initial handshake timeout as recommended in [RFC6298].
			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
			delay SHOULD NOT be considered an RTT sample.
	When a crypto packet is sent, the sender MUST set a timer for twice		When a crypto packet is sent, the sender MUST set a timer for twice
	the smoothed RTT. This timer MUST be updated when a new crypto		the smoothed RTT. This timer MUST be updated when a new crypto
	packet is sent and when an acknowledgement is received which computes		packet is sent and when an acknowledgement is received which computes
	a new RTT sample. Upon timeout, the sender MUST retransmit all		a new RTT sample. Upon timeout, the sender MUST retransmit all
	unacknowledged CRYPTO data if possible. The sender MUST NOT declare		unacknowledged CRYPTO data if possible. The sender MUST NOT declare
	in-flight crypto packets as lost when the crypto timer expires.		in-flight crypto packets as lost when the crypto timer expires.
	On each consecutive expiration of the crypto timer without receiving		On each consecutive expiration of the crypto timer without receiving
	an acknowledgement for a new packet, the sender MUST double the		an acknowledgement for a new packet, the sender MUST double the
	crypto retransmission timeout and set a timer for this period.		crypto retransmission timeout and set a timer for this period.
	skipping to change at page 13, line 41 ¶		skipping to change at page 14, line 4 ¶
	a new RTT sample. Upon timeout, the sender MUST retransmit all		a new RTT sample. Upon timeout, the sender MUST retransmit all
	unacknowledged CRYPTO data if possible. The sender MUST NOT declare		unacknowledged CRYPTO data if possible. The sender MUST NOT declare
	in-flight crypto packets as lost when the crypto timer expires.		in-flight crypto packets as lost when the crypto timer expires.
	On each consecutive expiration of the crypto timer without receiving		On each consecutive expiration of the crypto timer without receiving
	an acknowledgement for a new packet, the sender MUST double the		an acknowledgement for a new packet, the sender MUST double the
	crypto retransmission timeout and set a timer for this period.		crypto retransmission timeout and set a timer for this period.
	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, as specified in Section 8.1 of		amount of data it can send is limited, as specified in Section 8.1 of
	[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,		[QUIC-TRANSPORT]. If not all unacknowledged CRYPTO data can be sent,
	then all unacknowledged CRYPTO data sent in Initial packets should be		then all unacknowledged CRYPTO data sent in Initial packets should be
	retransmitted. If no data can be sent, then no alarm should be armed		retransmitted. If no data can be sent, then no alarm should be armed
	until data has been received from the client.		until data has been received from the client.
	Because the server could be blocked until more packets are received,		Because the server could be blocked until more packets are received,
	the client MUST ensure that the crypto retransmission timer is set if		the client MUST ensure that the crypto retransmission timer is set if
	there is unacknowledged crypto data or if the client does not yet		there is unacknowledged crypto data or if the client does not yet
	have 1-RTT keys. If the crypto retransmission timer expires before		have 1-RTT keys. If the crypto retransmission timer expires before
	the client has 1-RTT keys, it is possible that the client may not		the client has 1-RTT keys, it is possible that the client may not
	have any crypto data to retransmit. However, the client MUST send a		have any crypto data to retransmit. However, the client MUST send a
	new packet, containing only PING or PADDING frames if necessary, to		new packet, containing only PADDING frames if necessary, to allow the
	allow the server to continue sending data. If Handshake keys are		server to continue sending data. If Handshake keys are available to
	available to the client, it MUST send a Handshake packet, and		the client, it MUST send a Handshake packet, and otherwise it MUST
	otherwise it MUST send an Initial packet in a UDP datagram of at		send an Initial packet in a UDP datagram of at least 1200 bytes.
	least 1200 bytes.
			Because packets only containing PADDING do not elicit an
			acknowledgement, they may never be acknowledged, but they are removed
			from bytes in flight when the client gets Handshake keys and the
			Initial keys are discarded.
	The crypto retransmission timer is not set if the time threshold		The crypto retransmission timer is not set if the time threshold
	Section 6.1.2 loss detection timer is set. The time threshold loss		Section 6.1.2 loss detection timer is set. The time threshold loss
	detection timer is expected to both expire earlier than the crypto		detection timer is expected to both expire earlier than the crypto
	retransmission timeout and be less likely to spuriously retransmit		retransmission timeout and be less likely to spuriously retransmit
	data. The Initial and Handshake packet number spaces will typically		data. The Initial and Handshake packet number spaces will typically
	contain a small number of packets, so losses are less likely to be		contain a small number of packets, so losses are less likely to be
	detected using packet-threshold loss detection.		detected using packet-threshold loss detection.
	When the crypto retransmission timer is active, the probe timer		When the crypto retransmission timer is active, the probe timer
	skipping to change at page 14, line 36 ¶		skipping to change at page 15, line 5 ¶
	resetting any pending timers. Either packet indicates that the		resetting any pending timers. Either packet indicates that the
	Initial was received but not processed. Neither packet can be		Initial was received but not processed. Neither packet can be
	treated as an acknowledgment for the Initial.		treated as an acknowledgment for the Initial.
	The client MAY however compute an RTT estimate to the server as the		The client MAY however compute an RTT estimate to the server as the
	time period from when the first Initial was sent to when a Retry or a		time period from when the first Initial was sent to when a Retry or a
	Version Negotiation packet is received. The client MAY use this		Version Negotiation packet is received. The client MAY use this
	value to seed the RTT estimator for a subsequent connection attempt		value to seed the RTT estimator for a subsequent connection attempt
	to the server.		to the server.
	6.2.2. Discarding Keys and Packet State
	When packet protection keys are discarded (see Section 4.9 of
	[QUIC-TLS]), all packets that were sent with those keys can no longer
	be acknowledged because their acknowledgements cannot be processed
	anymore. The sender MUST discard all recovery state associated with
	those packets and MUST remove them from the count of bytes in flight.
	Endpoints stop sending and receiving Initial packets once they start
	exchanging Handshake packets (see Section 17.2.2.1 of
	[QUIC-TRANSPORT]). At this point, recovery state for all in-flight
	Initial packets is discarded.
	When 0-RTT is rejected, recovery state for all in-flight 0-RTT
	packets is discarded.
	If a server accepts 0-RTT, but does not buffer 0-RTT packets that
	arrive before Initial packets, early 0-RTT packets will be declared
	lost, but that is expected to be infrequent.
	It is expected that keys are discarded after packets encrypted with
	them would be acknowledged or declared lost. Initial secrets however
	might be destroyed sooner, as soon as handshake keys are available
	(see Section 4.10 of [QUIC-TLS]).
	6.3. Probe Timeout		6.3. Probe Timeout
	A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data		A Probe Timeout (PTO) triggers a probe packet when ack-eliciting data
	is in flight but an acknowledgement is not received within the		is in flight but an acknowledgement is not received within the
	expected period of time. A PTO enables a connection to recover from		expected period of time. A PTO enables a connection to recover from
	loss of tail packets or acks. The PTO algorithm used in QUIC		loss of tail packets or acks. The PTO algorithm used in QUIC
	implements the reliability functions of Tail Loss Probe [TLP] [RACK],		implements the reliability functions of Tail Loss Probe [TLP] [RACK],
	RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout		RTO [RFC5681] and F-RTO algorithms for TCP [RFC5682], and the timeout
	computation is based on TCP's retransmission timeout period		computation is based on TCP's retransmission timeout period
	[RFC6298].		[RFC6298].
	skipping to change at page 17, line 14 ¶		skipping to change at page 17, line 5 ¶
	6.3.3. Loss Detection		6.3.3. Loss Detection
	Delivery or loss of packets in flight is established when an ACK		Delivery or loss of packets in flight is established when an ACK
	frame is received that newly acknowledges one or more packets.		frame is received that newly acknowledges one or more packets.
	A PTO timer expiration event does not indicate packet loss and MUST		A PTO timer expiration event does not indicate packet loss and MUST
	NOT cause prior unacknowledged packets to be marked as lost. When an		NOT cause prior unacknowledged packets to be marked as lost. When an
	acknowledgement is received that newly acknowledges packets, loss		acknowledgement is received that newly acknowledges packets, loss
	detection proceeds as dictated by packet and time threshold		detection proceeds as dictated by packet and time threshold
	mechanisms, see Section 6.1.		mechanisms; see Section 6.1.
	6.4. Discussion		6.4. Discarding Keys and Packet State
			When packet protection keys are discarded (see Section 4.9 of
			[QUIC-TLS]), all packets that were sent with those keys can no longer
			be acknowledged because their acknowledgements cannot be processed
			anymore. The sender MUST discard all recovery state associated with
			those packets and MUST remove them from the count of bytes in flight.
			Endpoints stop sending and receiving Initial packets once they start
			exchanging Handshake packets (see Section 17.2.2.1 of
			[QUIC-TRANSPORT]). At this point, recovery state for all in-flight
			Initial packets is discarded.
			When 0-RTT is rejected, recovery state for all in-flight 0-RTT
			packets is discarded.
			If a server accepts 0-RTT, but does not buffer 0-RTT packets that
			arrive before Initial packets, early 0-RTT packets will be declared
			lost, but that is expected to be infrequent.
			It is expected that keys are discarded after packets encrypted with
			them would be acknowledged or declared lost. Initial secrets however
			might be destroyed sooner, as soon as handshake keys are available
			(see Section 4.10 of [QUIC-TLS]).
			6.5. Discussion
	The majority of constants were derived from best common practices		The majority of constants were derived from best common practices
	among widely deployed TCP implementations on the internet.		among widely deployed TCP implementations on the internet.
	Exceptions follow.		Exceptions follow.
	A shorter delayed ack time of 25ms was chosen because longer delayed		A shorter delayed ack time of 25ms was chosen because longer delayed
	acks can delay loss recovery and for the small number of connections		acks can delay loss recovery and for the small number of connections
	where less than packet per 25ms is delivered, acking every packet is		where less than packet per 25ms is delivered, acking every packet is
	beneficial to congestion control and loss recovery.		beneficial to congestion control and loss recovery.
	skipping to change at page 18, line 9 ¶		skipping to change at page 18, line 25 ¶
	Experienced codepoint in the IP header as a signal of congestion.		Experienced codepoint in the IP header as a signal of congestion.
	This document specifies an endpoint's response when its peer receives		This document specifies an endpoint's response when its peer receives
	packets with the Congestion Experienced codepoint. As discussed in		packets with the Congestion Experienced codepoint. As discussed in
	[RFC8311], endpoints are permitted to experiment with other response		[RFC8311], endpoints are permitted to experiment with other response
	functions.		functions.
	7.2. Slow Start		7.2. 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		start anytime the congestion window is less than ssthresh, which only
	typically only occurs after an PTO. While in slow start, QUIC		occurs after persistent congestion is declared. While in slow start,
	increases the congestion window by the number of bytes acknowledged		QUIC increases the congestion window by the number of bytes
	when each acknowledgment is processed.		acknowledged when each acknowledgment is processed.
	7.3. Congestion Avoidance		7.3. 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 22, line 15 ¶		skipping to change at page 22, line 38 ¶
	9. IANA Considerations		9. IANA Considerations
	This document has no IANA actions. Yet.		This document has no IANA actions. Yet.
	10. References		10. References
	10.1. Normative References		10.1. 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-20 (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-20 (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>.
	skipping to change at page 22, line 44 ¶		skipping to change at page 23, line 18 ¶
	<https://www.rfc-editor.org/info/rfc8311>.		<https://www.rfc-editor.org/info/rfc8311>.
	10.2. Informative References		10.2. 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-04 (work in progress), July 2018.		draft-ietf-tcpm-rack-05 (work in progress), April 2019.
	[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>.
	skipping to change at page 24, line 36 ¶		skipping to change at page 25, line 13 ¶
	mechanisms described in Section 6.		mechanisms described in Section 6.
	A.1. Tracking Sent Packets		A.1. Tracking Sent Packets
	To correctly implement congestion control, a QUIC sender tracks every		To correctly implement congestion control, a QUIC sender tracks every
	ack-eliciting packet until the packet is acknowledged or lost. It is		ack-eliciting packet until the packet is acknowledged or lost. It is
	expected that implementations will be able to access this information		expected that implementations will be able to access this information
	by packet number and crypto context and store the per-packet fields		by packet number and crypto context and store the per-packet fields
	(Appendix A.1.1) for loss recovery and congestion control.		(Appendix A.1.1) for loss recovery and congestion control.
	After a packet is declared lost, it SHOULD be tracked for an amount		After a packet is declared lost, the endpoint can track it for an
	of time comparable to the maximum expected packet reordering, such as		amount of time comparable to the maximum expected packet reordering,
	1 RTT. This allows for detection of spurious retransmissions.		such as 1 RTT. This allows for detection of spurious
			retransmissions.
	Sent packets are tracked for each packet number space, and ACK		Sent packets are tracked for each packet number space, and ACK
	processing only applies to a single space.		processing only applies to a single space.
	A.1.1. Sent Packet Fields		A.1.1. Sent Packet Fields
	packet_number: The packet number of the sent packet.		packet_number: The packet number of the sent packet.
	ack_eliciting: A boolean that indicates whether a packet is ack-		ack_eliciting: A boolean that indicates whether a packet is ack-
	eliciting. If true, it is expected that an acknowledgement will		eliciting. If true, it is expected that an acknowledgement will
	skipping to change at page 26, line 29 ¶		skipping to change at page 27, line 4 ¶
	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.
	latest_rtt: The most recent RTT measurement made when receiving an		latest_rtt: The most recent RTT measurement made when receiving an
	ack for a previously unacked packet.		ack for a previously unacked packet.
	smoothed_rtt: The smoothed RTT of the connection, computed as		smoothed_rtt: The smoothed RTT of the connection, computed as
	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, in milliseconds. The actual		intends to delay acknowledgments for packets in the
	ack_delay in a received ACK frame may be larger due to late		ApplicationData packet number space. The actual ack_delay in a
	timers, reordering, or lost ACKs.		received ACK frame may be larger due to late timers, reordering,
			or lost ACKs.
	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
	detail above in Appendix A.1.		detail above in Appendix A.1.
	A.4. Initialization		A.4. Initialization
	skipping to change at page 27, line 12 ¶		skipping to change at page 27, line 32 ¶
	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()
	crypto_count = 0		crypto_count = 0
	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
	time_of_last_sent_ack_eliciting_packet = 0		time_of_last_sent_ack_eliciting_packet = 0
	time_of_last_sent_crypto_packet = 0		time_of_last_sent_crypto_packet = 0
	for pn_space in [ Initial, Handshake, ApplicationData ]:		for pn_space in [ Initial, Handshake, ApplicationData ]:
	largest_acked_packet[pn_space] = 0		largest_acked_packet[pn_space] = infinite
	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:
	skipping to change at page 27, line 51 ¶		skipping to change at page 28, line 30 ¶
	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:
	OnAckReceived(ack, pn_space):		OnAckReceived(ack, pn_space):
	largest_acked_packet[pn_space] =		if (largest_acked_packet[pn_space] == infinite):
	max(largest_acked_packet[pn_space], ack.largest_acked)		largest_acked_packet[pn_space] = ack.largest_acked
			else:
			largest_acked_packet[pn_space] =
			max(largest_acked_packet[pn_space], ack.largest_acked)
	// Nothing to do if there are no newly acked packets.		// Nothing to do if there are no newly acked packets.
	newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)		newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
	if (newly_acked_packets.empty()):		if (newly_acked_packets.empty()):
	return		return
	// If the largest acknowledged is newly acked and		// If the largest acknowledged is newly acked and
	// at least one ack-eliciting was newly acked, update the RTT.		// at least one ack-eliciting was newly acked, update the RTT.
	if (sent_packets[pn_space][ack.largest_acked] &&		if (sent_packets[pn_space][ack.largest_acked] &&
	IncludesAckEliciting(newly_acked_packets))		IncludesAckEliciting(newly_acked_packets))
	latest_rtt =		latest_rtt =
	now - sent_packets[pn_space][ack.largest_acked].time_sent		now - sent_packets[pn_space][ack.largest_acked].time_sent
	UpdateRtt(ack.ack_delay)		ack_delay = 0
			if pn_space == ApplicationData:
			ack_delay = ack.ack_delay
			UpdateRtt(ack_delay)
	// Process ECN information if present.		// Process ECN information if present.
	if (ACK frame contains ECN information):		if (ACK frame contains ECN information):
	ProcessECN(ack)		ProcessECN(ack)
	for acked_packet in newly_acked_packets:		for acked_packet in newly_acked_packets:
	OnPacketAcked(acked_packet.packet_number, pn_space)		OnPacketAcked(acked_packet.packet_number, pn_space)
	DetectLostPackets(pn_space)		DetectLostPackets(pn_space)
	crypto_count = 0		crypto_count = 0
	pto_count = 0		pto_count = 0
	skipping to change at page 32, line 6 ¶		skipping to change at page 33, line 6 ¶
	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:
	DetectLostPackets(pn_space):		DetectLostPackets(pn_space):
			assert(largest_acked_packet[pn_space] != infinite)
	loss_time[pn_space] = 0		loss_time[pn_space] = 0
	lost_packets = {}		lost_packets = {}
	loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)		loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
	// Minimum time of kGranularity before packets are deemed lost.		// Minimum time of kGranularity before packets are deemed lost.
	loss_delay = max(loss_delay, kGranularity)		loss_delay = max(loss_delay, kGranularity)
	// Packets sent before this time are deemed lost.		// Packets sent before this time are deemed lost.
	lost_send_time = now() - loss_delay		lost_send_time = now() - loss_delay
	// Packets with packet numbers before this are deemed lost.
	lost_pn = largest_acked_packet[pn_space] - kPacketThreshold
	foreach unacked in sent_packets[pn_space]:		foreach unacked in sent_packets[pn_space]:
	if (unacked.packet_number > largest_acked_packet[pn_space]):		if (unacked.packet_number > largest_acked_packet[pn_space]):
	continue		continue
	// Mark packet as lost, or set time when it should be marked.		// Mark packet as lost, or set time when it should be marked.
	if (unacked.time_sent <= lost_send_time ||		if (unacked.time_sent <= lost_send_time ||
	unacked.packet_number <= lost_pn):		largest_acked_packet[pn_space] >=
			unacked.packet_number + kPacketThreshold):
	sent_packets[pn_space].remove(unacked.packet_number)		sent_packets[pn_space].remove(unacked.packet_number)
	if (unacked.in_flight):		if (unacked.in_flight):
	lost_packets.insert(unacked)		lost_packets.insert(unacked)
	else:		else:
	if (loss_time[pn_space] == 0):		if (loss_time[pn_space] == 0):
	loss_time[pn_space] = unacked.time_sent + loss_delay		loss_time[pn_space] = unacked.time_sent + loss_delay
	else:		else:
	loss_time[pn_space] = min(loss_time[pn_space],		loss_time[pn_space] = min(loss_time[pn_space],
	unacked.time_sent + loss_delay)		unacked.time_sent + loss_delay)
	skipping to change at page 36, line 13 ¶		skipping to change at page 37, line 13 ¶
	CongestionEvent(sent_packets[ack.largest_acked].time_sent)		CongestionEvent(sent_packets[ack.largest_acked].time_sent)
	B.8. On Packets Lost		B.8. On Packets Lost
	Invoked from DetectLostPackets when packets are deemed lost.		Invoked from DetectLostPackets when packets are deemed lost.
	InPersistentCongestion(largest_lost_packet):		InPersistentCongestion(largest_lost_packet):
	pto = smoothed_rtt + max(4 * rttvar, kGranularity) +		pto = smoothed_rtt + max(4 * rttvar, kGranularity) +
	max_ack_delay		max_ack_delay
	congestion_period = pto * kPersistentCongestionThreshold		congestion_period = pto * kPersistentCongestionThreshold
	// Determine if all packets in the window before the		// Determine if all packets in the time period before the
	// newest lost packet, including the edges, are marked		// newest lost packet, including the edges, are marked
	// lost		// lost
	return IsWindowLost(largest_lost_packet, congestion_period)		return AreAllPacketsLost(largest_lost_packet,
			congestion_period)
	OnPacketsLost(lost_packets):		OnPacketsLost(lost_packets):
	// Remove lost packets from bytes_in_flight.		// Remove lost packets from bytes_in_flight.
	for (lost_packet : lost_packets):		for (lost_packet : lost_packets):
	bytes_in_flight -= lost_packet.size		bytes_in_flight -= lost_packet.size
	largest_lost_packet = lost_packets.last()		largest_lost_packet = lost_packets.last()
	CongestionEvent(largest_lost_packet.time_sent)		CongestionEvent(largest_lost_packet.time_sent)
	// Collapse congestion window if persistent congestion		// Collapse congestion window if persistent congestion
	if (InPersistentCongestion(largest_lost_packet)):		if (InPersistentCongestion(largest_lost_packet)):
	skipping to change at page 36, line 38 ¶		skipping to change at page 37, line 39 ¶
	Appendix C. Change Log		Appendix C. Change Log
	*RFC Editor's Note:* Please remove this section prior to		*RFC Editor's Note:* Please remove this section prior to
	publication of a final version of this document.		publication of a final version of this document.
	Issue and pull request numbers are listed with a leading octothorp.		Issue and pull request numbers are listed with a leading octothorp.
	C.1. Since draft-ietf-quic-recovery-19		C.1. Since draft-ietf-quic-recovery-19
			o Change kPersistentThreshold from an exponent to a multiplier
			(#2557)
	o Send a PING if the PTO timer fires and there's nothing to send		o Send a PING if the PTO timer fires and there's nothing to send
	(#2624)		(#2624)
	o Set loss delay to at least kGranularity (#2617)		o Set loss delay to at least kGranularity (#2617)
	o Merge application limited and sending after idle sections. Always		o Merge application limited and sending after idle sections. Always
	limit burst size instead of requiring resetting CWND to initial		limit burst size instead of requiring resetting CWND to initial
	CWND after idle (#2605)		CWND after idle (#2605)
	o Rewrite RTT estimation, allow RTT samples where a newly acked		o Rewrite RTT estimation, allow RTT samples where a newly acked
End of changes. 53 change blocks.
	155 lines changed or deleted		188 lines changed or added
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