draft-ietf-quic-recovery-18.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: July 27, 2019 Google		Expires: August 24, 2019 Google
	January 23, 2019		February 20, 2019
	QUIC Loss Detection and Congestion Control		QUIC Loss Detection and Congestion Control
	draft-ietf-quic-recovery-18		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 July 27, 2019.		This Internet-Draft will expire on August 24, 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.
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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
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	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 . . . . . . . . . . 4		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 . . . . . . . . . . . . 5		3.1.1. Separate Packet Number Spaces . . . . . . . . . . . . 5
	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. No Reneging . . . . . . . . . . . . . . . . . . . . . 6
	3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6		3.1.4. More ACK Ranges . . . . . . . . . . . . . . . . . . . 6
	3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 6		3.1.5. Explicit Correction For Delayed ACKs . . . . . . . . 7
	4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7		4. Generating Acknowledgements . . . . . . . . . . . . . . . . . 7
	4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7		4.1. Crypto Handshake Data . . . . . . . . . . . . . . . . . . 7
	4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 7		4.2. ACK Ranges . . . . . . . . . . . . . . . . . . . . . . . 8
	4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8		4.3. Receiver Tracking of ACK Frames . . . . . . . . . . . . . 8
	5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8		5. Computing the RTT estimate . . . . . . . . . . . . . . . . . 8
	6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9		6. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 9
	6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9		6.1. Acknowledgement-based Detection . . . . . . . . . . . . . 9
	6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10		6.1.1. Packet Threshold . . . . . . . . . . . . . . . . . . 10
	6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10		6.1.2. Time Threshold . . . . . . . . . . . . . . . . . . . 10
	6.2. Timeout Loss Detection . . . . . . . . . . . . . . . . . 10		6.2. Crypto Retransmission Timeout . . . . . . . . . . . . . . 11
	6.2.1. Crypto Retransmission Timeout . . . . . . . . . . . . 11		6.2.1. Retry and Version Negotiation . . . . . . . . . . . . 11
	6.2.2. Probe Timeout . . . . . . . . . . . . . . . . . . . . 12		6.2.2. Discarding Keys and Packet State . . . . . . . . . . 12
	6.3. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 14		6.3. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 12
	6.3.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 14		6.3.1. Computing PTO . . . . . . . . . . . . . . . . . . . . 13
	6.4. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 15		6.3.2. Sending Probe Packets . . . . . . . . . . . . . . . . 13
	6.4.1. Constants of interest . . . . . . . . . . . . . . . . 15		6.3.3. Loss Detection . . . . . . . . . . . . . . . . . . . 14
	6.4.2. Variables of interest . . . . . . . . . . . . . . . . 15		6.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14
	6.4.3. Initialization . . . . . . . . . . . . . . . . . . . 16		7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 15
	6.4.4. On Sending a Packet . . . . . . . . . . . . . . . . . 17		7.1. Explicit Congestion Notification . . . . . . . . . . . . 15
	6.4.5. On Receiving an Acknowledgment . . . . . . . . . . . 17		7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15
	6.4.6. On Packet Acknowledgment . . . . . . . . . . . . . . 19		7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 15
	6.4.7. Setting the Loss Detection Timer . . . . . . . . . . 19		7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 16
	6.4.8. On Timeout . . . . . . . . . . . . . . . . . . . . . 20		7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 16
	6.4.9. Detecting Lost Packets . . . . . . . . . . . . . . . 21		7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 16
	6.5. Discussion . . . . . . . . . . . . . . . . . . . . . . . 22		7.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 16
	7. Congestion Control . . . . . . . . . . . . . . . . . . . . . 23		7.8. Sending data after an idle period . . . . . . . . . . . . 17
	7.1. Explicit Congestion Notification . . . . . . . . . . . . 23		7.9. Application Limited Sending . . . . . . . . . . . . . . . 17
	7.2. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 23
	7.3. Congestion Avoidance . . . . . . . . . . . . . . . . . . 23		8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
	7.4. Recovery Period . . . . . . . . . . . . . . . . . . . . . 24		8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 18
	7.5. Ignoring Loss of Undecryptable Packets . . . . . . . . . 24		8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 18
	7.6. Probe Timeout . . . . . . . . . . . . . . . . . . . . . . 24		8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 18
	7.7. Pacing . . . . . . . . . . . . . . . . . . . . . . . . . 24		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
	7.8. Sending data after an idle period . . . . . . . . . . . . 25		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
	7.9. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 25		10.1. Normative References . . . . . . . . . . . . . . . . . . 19
	7.9.1. Constants of interest . . . . . . . . . . . . . . . . 25		10.2. Informative References . . . . . . . . . . . . . . . . . 19
	7.9.2. Variables of interest . . . . . . . . . . . . . . . . 26		10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
	7.9.3. Initialization . . . . . . . . . . . . . . . . . . . 27		Appendix A. Loss Recovery Pseudocode . . . . . . . . . . . . . . 21
	7.9.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 27		A.1. Tracking Sent Packets . . . . . . . . . . . . . . . . . . 21
	7.9.5. On Packet Acknowledgement . . . . . . . . . . . . . . 27		A.1.1. Sent Packet Fields . . . . . . . . . . . . . . . . . 21
	7.9.6. On New Congestion Event . . . . . . . . . . . . . . . 27		A.2. Constants of interest . . . . . . . . . . . . . . . . . . 22
	7.9.7. Process ECN Information . . . . . . . . . . . . . . . 28		A.3. Variables of interest . . . . . . . . . . . . . . . . . . 22
	7.9.8. On Packets Lost . . . . . . . . . . . . . . . . . . . 28		A.4. Initialization . . . . . . . . . . . . . . . . . . . . . 23
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 28		A.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 24
	8.1. Congestion Signals . . . . . . . . . . . . . . . . . . . 29		A.6. On Receiving an Acknowledgment . . . . . . . . . . . . . 24
	8.2. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 29		A.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 26
	8.3. Misreporting ECN Markings . . . . . . . . . . . . . . . . 29		A.8. Setting the Loss Detection Timer . . . . . . . . . . . . 26
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29		A.9. On Timeout . . . . . . . . . . . . . . . . . . . . . . . 27
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29		A.10. Detecting Lost Packets . . . . . . . . . . . . . . . . . 28
	10.1. Normative References . . . . . . . . . . . . . . . . . . 30		Appendix B. Congestion Control Pseudocode . . . . . . . . . . . 29
	10.2. Informative References . . . . . . . . . . . . . . . . . 30		B.1. Constants of interest . . . . . . . . . . . . . . . . . . 29
	10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 32		B.2. Variables of interest . . . . . . . . . . . . . . . . . . 30
	Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 32		B.3. Initialization . . . . . . . . . . . . . . . . . . . . . 31
	A.1. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 32		B.4. On Packet Sent . . . . . . . . . . . . . . . . . . . . . 31
	A.2. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 32		B.5. On Packet Acknowledgement . . . . . . . . . . . . . . . . 31
	A.3. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 33		B.6. On New Congestion Event . . . . . . . . . . . . . . . . . 32
	A.4. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 33		B.7. Process ECN Information . . . . . . . . . . . . . . . . . 32
	A.5. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 34		B.8. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 32
	A.6. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 34		Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 33
	A.7. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 34		C.1. Since draft-ietf-quic-recovery-17 . . . . . . . . . . . . 33
	A.8. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 34		C.2. Since draft-ietf-quic-recovery-16 . . . . . . . . . . . . 33
	A.9. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 34		C.3. Since draft-ietf-quic-recovery-14 . . . . . . . . . . . . 34
	A.10. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 34		C.4. Since draft-ietf-quic-recovery-13 . . . . . . . . . . . . 34
	A.11. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 35		C.5. Since draft-ietf-quic-recovery-12 . . . . . . . . . . . . 34
	A.12. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 35		C.6. Since draft-ietf-quic-recovery-11 . . . . . . . . . . . . 35
	A.13. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 35		C.7. Since draft-ietf-quic-recovery-10 . . . . . . . . . . . . 35
	A.14. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 35		C.8. Since draft-ietf-quic-recovery-09 . . . . . . . . . . . . 35
	A.15. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 35		C.9. Since draft-ietf-quic-recovery-08 . . . . . . . . . . . . 35
	A.16. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 35		C.10. Since draft-ietf-quic-recovery-07 . . . . . . . . . . . . 35
	A.17. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 35		C.11. Since draft-ietf-quic-recovery-06 . . . . . . . . . . . . 35
	A.18. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 35		C.12. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 35
			C.13. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 36
			C.14. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 36
			C.15. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 36
			C.16. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 36
			C.17. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 36
			C.18. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 36
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36		Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
	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 known 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
	in the Linux TCP implementation. This document describes QUIC		in the Linux TCP implementation. This document describes QUIC
	congestion control and loss recovery, and where applicable,		congestion control and loss recovery, and where applicable,
	attributes the TCP equivalent in RFCs, Internet-drafts, academic		attributes the TCP equivalent in RFCs, Internet-drafts, academic
	papers, and/or TCP implementations.		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
	skipping to change at page 4, line 45 ¶		skipping to change at page 4, line 48 ¶
	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.
			Out-of-order Packets: Packets that do not increase the largest
			received packet number for its packet number space by exactly one.
			Packets arrive out of order when earlier packets are lost or
			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 monotonically increase
	within a space, preventing ambiguity.		within a space, preventing ambiguity.
	skipping to change at page 5, line 27 ¶		skipping to change at page 5, line 35 ¶
	o All packets are acknowledged, though packets that contain no ack-		o All packets are acknowledged, though packets that contain no ack-
	eliciting frames are only acknowledged along with ack-eliciting		eliciting frames are only acknowledged along with ack-eliciting
	packets.		packets.
	o Long header packets that contain CRYPTO frames are critical to the		o Long header packets that contain CRYPTO frames are critical to the
	performance of the QUIC handshake and use shorter timers for		performance of the QUIC handshake and use shorter timers for
	acknowledgement and retransmission.		acknowledgement and retransmission.
	o Packets that contain only ACK frames do not count toward		o Packets that contain only ACK frames do not count toward
	congestion control limits and are not considered in-flight. Note		congestion control limits and are not considered in-flight.
	that this means PADDING frames cause packets to contribute toward
	bytes in flight without directly causing an acknowledgment to be		o PADDING frames cause packets to contribute toward bytes in flight
	sent.		without directly causing an acknowledgment to be sent.
	3.1. Relevant Differences Between QUIC and TCP		3.1. Relevant Differences Between QUIC and TCP
	Readers familiar with TCP's loss detection and congestion control		Readers familiar with TCP's loss detection and congestion control
	will find algorithms here that parallel well-known TCP ones.		will find algorithms here that parallel well-known TCP ones.
	Protocol differences between QUIC and TCP however contribute to		Protocol differences between QUIC and TCP however contribute to
	algorithmic differences. We briefly describe these protocol		algorithmic differences. We briefly describe these protocol
	differences below.		differences below.
	3.1.1. Separate Packet Number Spaces		3.1.1. Separate Packet Number Spaces
	QUIC uses separate packet number spaces for each encryption level,		QUIC uses separate packet number spaces for each encryption level,
	except 0-RTT and all generations of 1-RTT keys use the same packet		except 0-RTT and all generations of 1-RTT keys use the same packet
	number space. Separate packet number spaces ensures acknowledgement		number space. Separate packet number spaces ensures acknowledgement
	of packets sent with one level of encryption will not cause spurious		of packets sent with one level of encryption will not cause spurious
	retransmission of packets sent with a different encryption level.		retransmission of packets sent with a different encryption level.
	Congestion control and RTT measurement are unified across packet		Congestion control and round-trip time (RTT) measurement are unified
	number spaces.		across packet number spaces.
	3.1.2. Monotonically Increasing Packet Numbers		3.1.2. Monotonically Increasing Packet Numbers
	TCP conflates transmission order at the sender with delivery order at		TCP conflates transmission order at the sender with delivery order at
	the receiver, which results in retransmissions of the same data		the receiver, which results in retransmissions of the same data
	carrying the same sequence number, and consequently leads to		carrying the same sequence number, and consequently leads to
	"retransmission ambiguity". QUIC separates the two: QUIC uses a		"retransmission ambiguity". QUIC separates the two: QUIC uses a
	packet number to indicate transmission order, and any application		packet number to indicate transmission order, and any application
	data is sent in one or more streams, with delivery order determined		data is sent in one or more streams, with delivery order determined
	by stream offsets encoded within STREAM frames.		by stream offsets encoded within STREAM frames.
	skipping to change at page 7, line 17 ¶		skipping to change at page 7, line 26 ¶
	4. Generating Acknowledgements		4. Generating Acknowledgements
	QUIC SHOULD delay sending acknowledgements in response to packets,		QUIC SHOULD delay sending acknowledgements in response to packets,
	but MUST NOT excessively delay acknowledgements of ack-eliciting		but MUST NOT excessively delay acknowledgements of ack-eliciting
	packets. Specifically, implementations MUST attempt to enforce a		packets. Specifically, implementations MUST attempt to enforce a
	maximum ack delay to avoid causing the peer spurious timeouts. The		maximum ack delay to avoid causing the peer spurious timeouts. The
	maximum ack delay is communicated in the "max_ack_delay" transport		maximum ack delay is communicated in the "max_ack_delay" transport
	parameter and the default value is 25ms.		parameter and the default value is 25ms.
	An acknowledgement SHOULD be sent immediately upon receipt of a		An acknowledgement SHOULD be sent immediately upon receipt of a
	second packet but the delay SHOULD NOT exceed the maximum ack delay.		second ack-eliciting packet. QUIC recovery algorithms do not assume
	QUIC recovery algorithms do not assume the peer generates an		the peer sends an ACK immediately when receiving a second ack-
	acknowledgement immediately when receiving a second full-packet.		eliciting packet.
	Out-of-order packets SHOULD be acknowledged more quickly, in order to		In order to accelerate loss recovery and reduce timeouts, the
	accelerate loss recovery. The receiver SHOULD send an immediate ACK		receiver SHOULD send an immediate ACK after it receives an out-of-
	when it receives a new packet which is not one greater than the		order packet. It could send immediate ACKs for in-order packets for
	largest received packet number.		a period of time that SHOULD NOT exceed 1/8 RTT unless more out-of-
			order packets arrive. If every packet arrives out-of- order, then an
			immediate ACK SHOULD be sent for every received packet.
	Similarly, packets marked with the ECN Congestion Experienced (CE)		Similarly, packets marked with the ECN Congestion Experienced (CE)
	codepoint in the IP header SHOULD be acknowledged immediately, to		codepoint in the IP header SHOULD be acknowledged immediately, to
	reduce the peer's response time to congestion events.		reduce the peer's response time to congestion events.
	As an optimization, a receiver MAY process multiple packets before		As an optimization, a receiver MAY process multiple packets before
	sending any ACK frames in response. In this case they can determine		sending any ACK frames in response. In this case the receiver can
	whether an immediate or delayed acknowledgement should be generated		determine whether an immediate or delayed acknowledgement should be
	after processing incoming packets.		generated after processing incoming packets.
	4.1. Crypto Handshake Data		4.1. Crypto Handshake Data
	In order to quickly complete the handshake and avoid spurious		In order to quickly complete the handshake and avoid spurious
	retransmissions due to crypto retransmission timeouts, crypto packets		retransmissions due to crypto retransmission timeouts, crypto packets
	SHOULD use a very short ack delay, such as 1ms. ACK frames MAY be		SHOULD use a very short ack delay, such as the local timer
	sent immediately when the crypto stack indicates all data for that		granularity. ACK frames MAY be sent immediately when the crypto
	packet number space has been received.		stack indicates all data for that packet number space has been
			received.
	4.2. ACK Ranges		4.2. ACK Ranges
	When an ACK frame is sent, one or more ranges of acknowledged packets		When an ACK frame is sent, one or more ranges of acknowledged packets
	are included. Including older packets reduces the chance of spurious		are included. Including older packets reduces the chance of spurious
	retransmits caused by losing previously sent ACK frames, at the cost		retransmits caused by losing previously sent ACK frames, at the cost
	of larger ACK frames.		of larger ACK frames.
	ACK frames SHOULD always acknowledge the most recently received		ACK frames SHOULD always acknowledge the most recently received
	packets, and the more out-of-order the packets are, the more		packets, and the more out-of-order the packets are, the more
	skipping to change at page 8, line 30 ¶		skipping to change at page 8, line 42 ¶
	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.
	5. Computing the RTT estimate		5. Computing the RTT estimate
	RTT is calculated when an ACK frame arrives by computing the		Round-trip time (RTT) is calculated when an ACK frame arrives by
	difference between the current time and the time the largest acked		computing the difference between the current time and the time the
	packet was sent. An RTT sample MUST NOT be taken for a packet that		largest acked packet was sent. An RTT sample MUST NOT be taken for a
	is not newly acknowledged or not ack-eliciting.		packet that is not newly acknowledged or not ack-eliciting.
	When RTT is calculated, the ack delay field from the ACK frame SHOULD		When RTT is calculated, the ack delay field from the ACK frame SHOULD
	be limited to the max_ack_delay specified by the peer. Limiting		be limited to the max_ack_delay specified by the peer. Limiting
	ack_delay to max_ack_delay ensures a peer specifying an extremely		ack_delay to max_ack_delay ensures a peer specifying an extremely
	small max_ack_delay doesn't cause more spurious timeouts than a peer		small max_ack_delay doesn't cause more spurious timeouts than a peer
	that correctly specifies max_ack_delay. It SHOULD be subtracted from		that correctly specifies max_ack_delay. It SHOULD be subtracted from
	the RTT as long as the result is larger than the min_rtt. If the		the RTT as long as the result is larger than the min_rtt. If the
	result is smaller than the min_rtt, the RTT should be used, but the		result is smaller than the min_rtt, the RTT should be used, but the
	ack delay field should be ignored.		ack delay field should be ignored.
	A sender calculates both smoothed RTT and RTT variance similar to		A sender calculates both smoothed RTT (SRTT) and RTT variance
	those specified in [RFC6298], see Section 6.4.5.		(RTTVAR) similar to those specified in [RFC6298], see Appendix A.6.
	A sender takes an RTT sample when an ACK frame is received that		A sender takes an RTT sample when an ACK frame is received that
	acknowledges a larger packet number than before (see Section 6.4.5).		acknowledges a larger packet number than before (see Appendix A.6).
	A sender will take multiple RTT samples per RTT when multiple such		A sender will take multiple RTT samples per RTT when multiple such
	ACK frames are received within an RTT. When multiple samples are		ACK frames are received within an RTT. When multiple samples are
	generated within an RTT, the smoothed RTT and RTT variance could		generated within an RTT, the smoothed RTT and RTT variance could
	retain inadequate history, as suggested in [RFC6298]. Changing these		retain inadequate history, as suggested in [RFC6298]. Changing these
	computations is currently an open research question.		computations is currently an open research question.
	min_rtt is the minimum RTT measured over the connection, prior to		min_rtt is the minimum RTT measured over the connection, prior to
	adjusting by ack delay. Ignoring ack delay for min RTT prevents		adjusting by ack delay. Ignoring ack delay for min RTT prevents
	intentional or unintentional underestimation of min RTT, which in		intentional or unintentional underestimation of min RTT, which in
	turn prevents underestimating smoothed RTT.		turn prevents underestimating smoothed RTT.
	6. Loss Detection		6. Loss Detection
	QUIC senders use both ack information and timeouts to detect lost		QUIC senders use both ack information and timeouts to detect lost
	packets, and this section provides a description of these algorithms.		packets, and this section provides a description of these algorithms.
	Estimating the network round-trip time (RTT) is critical to these
	algorithms and is described first.
	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 13.2 of		or abandoning the frame. For more information, see Section 13.2 of
	[QUIC-TRANSPORT].		[QUIC-TRANSPORT].
	6.1. Acknowledgement-based Detection		6.1. 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 31 ¶		skipping to change at page 10, line 42 ¶
	max(SRTT, latest_RTT). If packets sent prior to the largest		max(SRTT, latest_RTT). If packets sent prior to the largest
	acknowledged packet cannot yet be declared lost, then a timer SHOULD		acknowledged packet cannot yet be declared lost, then a timer SHOULD
	be set for the remaining time.		be set for the remaining time.
	The RECOMMENDED time threshold (kTimeThreshold), expressed as a		The RECOMMENDED time threshold (kTimeThreshold), expressed as a
	round-trip time multiplier, is 9/8.		round-trip time multiplier, is 9/8.
	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 packet whose ack triggered the Early Retransmit		reordering where the acknowledgement encountered a shorter path;
	process 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.
	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. Timeout Loss Detection		6.2. Crypto Retransmission Timeout
	Timeout loss detection recovers from losses that cannot be handled by
	acknowledgement-based loss detection. It uses a single timer which
	switches between a crypto retransmission timer and a probe timer.
	6.2.1. 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
	negotiation, so a more aggressive timeout is used to retransmit it.		negotiation, so a more aggressive timeout is used to retransmit it.
	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 100ms. When an		changes, the initial RTT SHOULD be set to 100ms. When an
	acknowledgement is received, a new RTT is computed and the timer		acknowledgement is received, a new RTT is computed and the timer
	SHOULD be set for twice the newly computed smoothed RTT.		SHOULD be set for twice the newly computed smoothed RTT.
	When crypto packets are sent, the sender MUST set a timer for the		When a crypto packet is sent, the sender MUST set a timer for the
	crypto timeout period. Upon timeout, the sender MUST retransmit all		crypto timeout period. This timer MUST be updated when a new crypto
			packet is sent. Upon timeout, the sender MUST retransmit all
	unacknowledged CRYPTO data if possible.		unacknowledged CRYPTO data if possible.
	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		amount of data it can send is limited, as specified in
	[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,
	skipping to change at page 11, line 44 ¶		skipping to change at page 11, line 45 ¶
	is no unacknowledged CRYPTO data. If the timer expires and the		is no unacknowledged CRYPTO data. If the timer expires and the
	client has no CRYPTO data to retransmit and does not have Handshake		client has no CRYPTO data to retransmit and does not have Handshake
	keys, it SHOULD send an Initial packet in a UDP datagram of at least		keys, it SHOULD send an Initial packet in a UDP datagram of at least
	1200 bytes. If the client has Handshake keys, it SHOULD send a		1200 bytes. If the client has Handshake keys, it SHOULD send a
	Handshake packet.		Handshake packet.
	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 SHOULD double the		an acknowledgement for a new packet, the sender SHOULD double the
	crypto retransmission timeout and set a timer for this period.		crypto retransmission timeout and set a timer for this period.
	When crypto packets are in flight, the probe timer (Section 6.2.2) is		When crypto packets are in flight, the probe timer (Section 6.3) is
	not active.		not active.
	6.2.1.1. Retry and Version Negotiation		6.2.1. Retry and Version Negotiation
	A Retry or Version Negotiation packet causes a client to send another		A Retry or Version Negotiation packet causes a client to send another
	Initial packet, effectively restarting the connection process and		Initial packet, effectively restarting the connection process and
	resetting congestion control and loss recovery state, including		resetting congestion control and loss recovery state, including
	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.
	6.2.1.2. Discarding Keys and Packet State		6.2.2. Discarding Keys and Packet State
	When packet protection keys are discarded (see Section 4.9 of		When packet protection keys are discarded (see Section 4.9 of
	[QUIC-TLS]), all packets that were sent with those keys can no longer		[QUIC-TLS]), all packets that were sent with those keys can no longer
	be acknowledged because their acknowledgements cannot be processed		be acknowledged because their acknowledgements cannot be processed
	anymore. The sender considers them no longer in flight. That is,		anymore. The sender MUST discard all recovery state associated with
	the sender SHOULD discard all recovery state associated with those		those packets and MUST remove them from the count of bytes in flight.
	packets and MUST remove them from the count of bytes in flight.
	Endpoints stop sending and receiving Initial packets once they start		Endpoints stop sending and receiving Initial packets once they start
	exchanging Handshake packets (see Section 17.2.2.1 of		exchanging Handshake packets (see Section 17.2.2.1 of
	[QUIC-TRANSPORT]). At this point, recovery state for all in-flight		[QUIC-TRANSPORT]). At this point, recovery state for all in-flight
	Initial packets is discarded.		Initial packets is discarded.
	When 0-RTT is rejected, recovery state for all in-flight 0-RTT		When 0-RTT is rejected, recovery state for all in-flight 0-RTT
	packets is discarded.		packets is discarded.
	If a server accepts 0-RTT, but does not buffer 0-RTT packets that		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		arrive before Initial packets, early 0-RTT packets will be declared
	lost, but that is expected to be infrequent.		lost, but that is expected to be infrequent.
	It is expected that keys are discarded after packets encrypted with		It is expected that keys are discarded after packets encrypted with
	them would be acknowledged or declared lost. Initial secrets however		them would be acknowledged or declared lost. Initial secrets however
	might be destroyed sooner, as soon as handshake keys are available		might be destroyed sooner, as soon as handshake keys are available
	(see Section 4.10 of [QUIC-TLS]).		(see Section 4.10 of [QUIC-TLS]).
	6.2.2. 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].
	6.2.2.1. Computing PTO		6.3.1. 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 = max(smoothed_rtt + 4*rttvar + max_ack_delay, kGranularity)		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
	Section 6.4.1 and Section 6.4.2.		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.
	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.
	When a PTO timer expires, the PTO period MUST be set to twice its		When a PTO timer expires, the sender probes the network as described
	current value. This exponential reduction in the sender's rate is		in the next section. The PTO period MUST be set to twice its current
	important because the PTOs might be caused by loss of packets or		value. This exponential reduction in the sender's rate is important
			because the PTOs might be caused by loss of packets or
	acknowledgements due to severe congestion.		acknowledgements due to severe congestion.
	A sender computes its PTO timer every time an ack-eliciting packet is		A sender computes its PTO timer every time an ack-eliciting packet is
	sent. A sender might choose to optimize this by setting the timer		sent. A sender might choose to optimize this by setting the timer
	fewer times if it knows that more ack-eliciting packets will be sent		fewer times if it knows that more ack-eliciting packets will be sent
	within a short period of time.		within a short period of time.
	6.2.2.2. Sending Probe Packets		6.3.2. Sending Probe Packets
	When a PTO timer expires, the sender MUST send one ack-eliciting		When a PTO timer expires, the sender MUST send one ack-eliciting
	packet as a probe. A sender MAY send up to two ack-eliciting		packet as a probe. A sender MAY send up to two ack-eliciting
	packets, to avoid an expensive consecutive PTO expiration due to a		packets, to avoid an expensive consecutive PTO expiration due to a
	single packet loss.		single packet loss.
	Consecutive PTO periods increase exponentially, and as a result,		Consecutive PTO periods increase exponentially, and as a result,
	connection recovery latency increases exponentially as packets		connection recovery latency increases exponentially as packets
	continue to be dropped in the network. Sending two packets on PTO		continue to be dropped in the network. Sending two packets on PTO
	expiration increases resilience to packet drops, thus reducing the		expiration increases resilience to packet drops, thus reducing the
	skipping to change at page 14, line 7 ¶		skipping to change at page 14, line 8 ¶
	Probe packets sent on a PTO MUST be ack-eliciting. A probe packet		Probe packets sent on a PTO MUST be ack-eliciting. A probe packet
	SHOULD carry new data when possible. A probe packet MAY carry		SHOULD carry new data when possible. A probe packet MAY carry
	retransmitted unacknowledged data when new data is unavailable, when		retransmitted unacknowledged data when new data is unavailable, when
	flow control does not permit new data to be sent, or to		flow control does not permit new data to be sent, or to
	opportunistically reduce loss recovery delay. Implementations MAY		opportunistically reduce loss recovery delay. Implementations MAY
	use alternate strategies for determining the content of probe		use alternate strategies for determining the content of probe
	packets, including sending new or retransmitted data based on the		packets, including sending new or retransmitted data based on the
	application's priorities.		application's priorities.
			When the PTO timer expires multiple times and new data cannot be
			sent, implementations must choose between sending the same payload
			every time or sending different payloads. Sending the same payload
			may be simpler and ensures the highest priority frames arrive first.
			Sending different payloads each time reduces the chances of spurious
			retransmission.
	When a PTO timer expires, new or previously-sent data may not be		When a PTO timer expires, new or previously-sent data may not be
	available to send and packets may still be in flight. A sender can		available to send and packets may still be in flight. A sender can
	be blocked from sending new data in the future if packets are left in		be blocked from sending new data in the future if packets are left in
	flight. Under these conditions, a sender SHOULD mark any packets		flight. Under these conditions, a sender SHOULD mark any packets
	still in flight as lost. If a sender wishes to establish delivery of		still in flight as lost. If a sender wishes to establish delivery of
	packets still in flight, it MAY send an ack-eliciting packet and re-		packets still in flight, it MAY send an ack-eliciting packet and re-
	arm the PTO timer instead.		arm the PTO timer instead.
	6.2.2.3. 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. After a		NOT cause prior unacknowledged packets to be marked as lost. When an
	PTO timer has expired, an endpoint uses the following rules to mark		acknowledgement is received that newly acknowledges packets, loss
	packets as lost when an acknowledgement is received that newly		detection proceeds as dictated by packet and time threshold
	acknowledges packets.
	When an acknowledgement is received that newly acknowledges packets,
	loss detection proceeds as dictated by packet and time threshold
	mechanisms, see Section 6.1.		mechanisms, see Section 6.1.
	6.3. Tracking Sent Packets		6.4. Discussion
			The majority of constants were derived from best common practices
			among widely deployed TCP implementations on the internet.
			Exceptions follow.
			A shorter delayed ack time of 25ms was chosen because longer delayed
			acks can delay loss recovery and for the small number of connections
			where less than packet per 25ms is delivered, acking every packet is
			beneficial to congestion control and loss recovery.
			The default initial RTT of 100ms was chosen because it is slightly
			higher than both the median and mean min_rtt typically observed on
			the public internet.
			7. Congestion Control
			QUIC's congestion control is based on TCP NewReno [RFC6582]. NewReno
			is a congestion window based congestion control. QUIC specifies the
			congestion window in bytes rather than packets due to finer control
			and the ease of appropriate byte counting [RFC3465].
			QUIC hosts MUST NOT send packets if they would increase
			bytes_in_flight (defined in Appendix B.2) beyond the available
			congestion window, unless the packet is a probe packet sent after a
			PTO timer expires, as described in Section 6.3.
			Implementations MAY use other congestion control algorithms, such as
			Cubic [RFC8312], and endpoints MAY use different algorithms from one
			another. The signals QUIC provides for congestion control are
			generic and are designed to support different algorithms.
			7.1. Explicit Congestion Notification
			If a path has been verified to support ECN, QUIC treats a Congestion
			Experienced codepoint in the IP header as a signal of congestion.
			This document specifies an endpoint's response when its peer receives
			packets with the Congestion Experienced codepoint. As discussed in
			[RFC8311], endpoints are permitted to experiment with other response
			functions.
			7.2. Slow Start
			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
			start anytime the congestion window is less than ssthresh, which
			typically only occurs after an PTO. While in slow start, QUIC
			increases the congestion window by the number of bytes acknowledged
			when each acknowledgment is processed.
			7.3. Congestion Avoidance
			Slow start exits to congestion avoidance. Congestion avoidance in
			NewReno uses an additive increase multiplicative decrease (AIMD)
			approach that increases the congestion window by one maximum packet
			size per congestion window acknowledged. When a loss is detected,
			NewReno halves the congestion window and sets the slow start
			threshold to the new congestion window.
			7.4. Recovery Period
			Recovery is a period of time beginning with detection of a lost
			packet or an increase in the ECN-CE counter. Because QUIC does not
			retransmit packets, it defines the end of recovery as a packet sent
			after the start of recovery being acknowledged. This is slightly
			different from TCP's definition of recovery, which ends when the lost
			packet that started recovery is acknowledged.
			The recovery period limits congestion window reduction to once per
			round trip. During recovery, the congestion window remains unchanged
			irrespective of new losses or increases in the ECN-CE counter.
			7.5. Ignoring Loss of Undecryptable Packets
			During the handshake, some packet protection keys might not be
			available when a packet arrives. In particular, Handshake and 0-RTT
			packets cannot be processed until the Initial packets arrive, and
			1-RTT packets cannot be processed until the handshake completes.
			Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets
			that might arrive before the peer has packet protection keys to
			process those packets.
			7.6. Probe Timeout
			Probe packets MUST NOT be blocked by the congestion controller. A
			sender MUST however count these packets as being additionally in
			flight, since these packets add network load without establishing
			packet loss. Note that sending probe packets might cause the
			sender's bytes in flight to exceed the congestion window until an
			acknowledgement is received that establishes loss or delivery of
			packets.
			When an ACK frame is received that establishes loss of all in-flight
			packets sent prior to a threshold number of consecutive PTOs
			(pto_count is more than kPersistentCongestionThreshold, see
			Appendix B.1), the network is considered to be experiencing
			persistent congestion, and the sender's congestion window MUST be
			reduced to the minimum congestion window (kMinimumWindow). This
			response of collapsing the congestion window on persistent congestion
			is functionally similar to a sender's response on a Retransmission
			Timeout (RTO) in TCP [RFC5681].
			7.7. Pacing
			This document does not specify a pacer, but it is RECOMMENDED that a
			sender pace sending of all in-flight packets based on input from the
			congestion controller. For example, a pacer might distribute the
			congestion window over the SRTT when used with a window-based
			controller, and a pacer might use the rate estimate of a rate-based
			controller.
			An implementation should take care to architect its congestion
			controller to work well with a pacer. For instance, a pacer might
			wrap the congestion controller and control the availability of the
			congestion window, or a pacer might pace out packets handed to it by
			the congestion controller. Timely delivery of ACK frames is
			important for efficient loss recovery. Packets containing only ACK
			frames should therefore not be paced, to avoid delaying their
			delivery to the peer.
			As an example of a well-known and publicly available implementation
			of a flow pacer, implementers are referred to the Fair Queue packet
			scheduler (fq qdisc) in Linux (3.11 onwards).
			7.8. Sending data after an idle period
			A sender becomes idle if it ceases to send data and has no bytes in
			flight. A sender's congestion window MUST NOT increase while it is
			idle.
			When sending data after becoming idle, a sender MUST reset its
			congestion window to the initial congestion window (see Section 4.1
			of [RFC5681]), unless it paces the sending of packets. A sender MAY
			retain its congestion window if it paces the sending of any packets
			in excess of the initial congestion window.
			A sender MAY implement alternate mechanisms to update its congestion
			window after idle periods, such as those proposed for TCP in
			[RFC7661].
			7.9. Application Limited Sending
			The congestion window should not be increased in slow start or
			congestion avoidance when it is not fully utilized. The congestion
			window could be under-utilized due to insufficient application data
			or flow control credit.
			A sender that paces packets (see Section 7.7) might delay sending
			packets and not fully utilize the congestion window due to this
			delay. A sender should not consider itself application limited if it
			would have fully utilized the congestion window without pacing delay.
			8. Security Considerations
			8.1. Congestion Signals
			Congestion control fundamentally involves the consumption of signals
			- both loss and ECN codepoints - from unauthenticated entities. On-
			path attackers can spoof or alter these signals. An attacker can
			cause endpoints to reduce their sending rate by dropping packets, or
			alter send rate by changing ECN codepoints.
			8.2. Traffic Analysis
			Packets that carry only ACK frames can be heuristically identified by
			observing packet size. Acknowledgement patterns may expose
			information about link characteristics or application behavior.
			Endpoints can use PADDING frames or bundle acknowledgments with other
			frames to reduce leaked information.
			8.3. Misreporting ECN Markings
			A receiver can misreport ECN markings to alter the congestion
			response of a sender. Suppressing reports of ECN-CE markings could
			cause a sender to increase their send rate. This increase could
			result in congestion and loss.
			A sender MAY attempt to detect suppression of reports by marking
			occasional packets that they send with ECN-CE. If a packet marked
			with ECN-CE is not reported as having been marked when the packet is
			acknowledged, the sender SHOULD then disable ECN for that path.
			Reporting additional ECN-CE markings will cause a sender to reduce
			their sending rate, which is similar in effect to advertising reduced
			connection flow control limits and so no advantage is gained by doing
			so.
			Endpoints choose the congestion controller that they use. Though
			congestion controllers generally treat reports of ECN-CE markings as
			equivalent to loss [RFC8311], the exact response for each controller
			could be different. Failure to correctly respond to information
			about ECN markings is therefore difficult to detect.
			9. IANA Considerations
			This document has no IANA actions. Yet.
			10. References
			10.1. Normative References
			[QUIC-TLS]
			Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
			QUIC", draft-ietf-quic-tls-latest (work in progress).
			[QUIC-TRANSPORT]
			Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
			Multiplexed and Secure Transport", draft-ietf-quic-
			transport-latest (work in progress).
			[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
			Requirement Levels", BCP 14, RFC 2119,
			DOI 10.17487/RFC2119, March 1997,
			<https://www.rfc-editor.org/info/rfc2119>.
			[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
			2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
			May 2017, <https://www.rfc-editor.org/info/rfc8174>.
			[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>.
			10.2. Informative References
			[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
			Refining TCP Congestion Control", ACM SIGCOMM , August
			1996.
			[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
			a time-based fast loss detection algorithm for TCP",
			draft-ietf-tcpm-rack-04 (work in progress), July 2018.
			[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
			Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
			2003, <https://www.rfc-editor.org/info/rfc3465>.
			[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
			"Improving the Robustness of TCP to Non-Congestion
			Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,
			<https://www.rfc-editor.org/info/rfc4653>.
			[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
			Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
			<https://www.rfc-editor.org/info/rfc5681>.
			[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
			"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
			Spurious Retransmission Timeouts with TCP", RFC 5682,
			DOI 10.17487/RFC5682, September 2009,
			<https://www.rfc-editor.org/info/rfc5682>.
			[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
			P. Hurtig, "Early Retransmit for TCP and Stream Control
			Transmission Protocol (SCTP)", RFC 5827,
			DOI 10.17487/RFC5827, May 2010,
			<https://www.rfc-editor.org/info/rfc5827>.
			[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
			"Computing TCP's Retransmission Timer", RFC 6298,
			DOI 10.17487/RFC6298, June 2011,
			<https://www.rfc-editor.org/info/rfc6298>.
			[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
			NewReno Modification to TCP's Fast Recovery Algorithm",
			RFC 6582, DOI 10.17487/RFC6582, April 2012,
			<https://www.rfc-editor.org/info/rfc6582>.
			[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
			and Y. Nishida, "A Conservative Loss Recovery Algorithm
			Based on Selective Acknowledgment (SACK) for TCP",
			RFC 6675, DOI 10.17487/RFC6675, August 2012,
			<https://www.rfc-editor.org/info/rfc6675>.
			[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
			"Increasing TCP's Initial Window", RFC 6928,
			DOI 10.17487/RFC6928, April 2013,
			<https://www.rfc-editor.org/info/rfc6928>.
			[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
			TCP to Support Rate-Limited Traffic", RFC 7661,
			DOI 10.17487/RFC7661, October 2015,
			<https://www.rfc-editor.org/info/rfc7661>.
			[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
			R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
			RFC 8312, DOI 10.17487/RFC8312, February 2018,
			<https://www.rfc-editor.org/info/rfc8312>.
			[TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
			"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
			Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
			in progress), February 2013.
			10.3. URIs
			[1] https://mailarchive.ietf.org/arch/search/?email_list=quic
			[2] https://github.com/quicwg
			[3] https://github.com/quicwg/base-drafts/labels/-recovery
			Appendix A. Loss Recovery Pseudocode
			We now describe an example implementation of the loss detection
			mechanisms described in Section 6.
			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
	(Section 6.3.1) 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, it SHOULD be tracked for an amount
	of time comparable to the maximum expected packet reordering, such as		of time comparable to the maximum expected packet reordering, such as
	1 RTT. This allows for detection of spurious retransmissions.		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.
	6.3.1. 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
	be received, though the peer could delay sending the ACK frame		be received, though the peer could delay sending the ACK frame
	containing it by up to the MaxAckDelay.		containing it by up to the MaxAckDelay.
	in_flight: A boolean that indicates whether the packet counts		in_flight: A boolean that indicates whether the packet counts
	towards bytes in flight.		towards bytes in flight.
	skipping to change at page 15, line 21 ¶		skipping to change at page 22, line 12 ¶
	contains cryptographic handshake messages critical to the		contains cryptographic handshake messages critical to the
	completion of the QUIC handshake. In this version of QUIC, this		completion of the QUIC handshake. In this version of QUIC, this
	includes any packet with the long header that includes a CRYPTO		includes any packet with the long header that includes a CRYPTO
	frame.		frame.
	sent_bytes: The number of bytes sent in the packet, not including		sent_bytes: The number of bytes sent in the packet, not including
	UDP or IP overhead, but including QUIC framing overhead.		UDP or IP overhead, but including QUIC framing overhead.
	time_sent: The time the packet was sent.		time_sent: The time the packet was sent.
	6.4. Pseudocode		A.2. Constants of interest
	6.4.1. Constants of interest
	Constants used in loss recovery are based on a combination of RFCs,		Constants used in loss recovery are based on a combination of RFCs,
	papers, and common practice. Some may need to be changed or		papers, and common practice. Some may need to be changed or
	negotiated in order to better suit a variety of environments.		negotiated in order to better suit a variety of environments.
	kPacketThreshold: Maximum reordering in packets before packet		kPacketThreshold: Maximum reordering in packets before packet
	threshold loss detection considers a packet lost. The RECOMMENDED		threshold loss detection considers a packet lost. The RECOMMENDED
	value is 3.		value is 3.
	kTimeThreshold: Maximum reordering in time before time threshold		kTimeThreshold: Maximum reordering in time before time threshold
	loss detection considers a packet lost. Specified as an RTT		loss detection considers a packet lost. Specified as an RTT
	multiplier. The RECOMMENDED value is 9/8.		multiplier. The RECOMMENDED value is 9/8.
	kGranularity: Timer granularity. This is a system-dependent value.		kGranularity: Timer granularity. This is a system-dependent value.
	However, implementations SHOULD use a value no smaller than 1ms.		However, implementations SHOULD use a value no smaller than 1ms.
	kInitialRtt: The RTT used before an RTT sample is taken. The		kInitialRtt: The RTT used before an RTT sample is taken. The
	RECOMMENDED value is 100ms.		RECOMMENDED value is 100ms.
	6.4.2. Variables of interest		kPacketNumberSpace: An enum to enumerate the three packet number
			spaces. ~~~ enum kPacketNumberSpace { Initial, Handshake,
			ApplicationData, } ~~~
			A.3. Variables of interest
	Variables required to implement the congestion control mechanisms are		Variables required to implement the congestion control mechanisms are
	described in this section.		described in this section.
	loss_detection_timer: Multi-modal timer used for loss detection.		loss_detection_timer: Multi-modal timer used for loss detection.
	crypto_count: The number of times all unacknowledged CRYPTO data has		crypto_count: The number of times all unacknowledged CRYPTO data has
	been retransmitted without receiving an ack.		been retransmitted without receiving an ack.
	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: The time the most recent
	ack-eliciting packet was sent.		ack-eliciting packet was sent.
	time_of_last_sent_crypto_packet: The time the most recent crypto		time_of_last_sent_crypto_packet: The time the most recent crypto
	packet was sent.		packet was sent.
	largest_sent_packet: The packet number of the most recently sent		largest_acked_packet[kPacketNumberSpace]: The largest packet number
	packet.		acknowledged in the packet number space so far.
	largest_acked_packet: The largest packet number 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, in milliseconds. The actual
	ack_delay in a received ACK frame may be larger due to late		ack_delay in a received ACK frame may be larger due to late
	timers, reordering, or lost ACKs.		timers, reordering, or lost ACKs.
	loss_time: The time at which the next packet will be considered lost		loss_time: The time at which the next packet will be considered lost
	based on early transmit or exceeding the reordering window in		based on exceeding the reordering window in time. Only applies to
	time.		the ApplicationData packet number space.
	sent_packets: An association of packet numbers to information about		sent_packets[kPacketNumberSpace]: An association of packet numbers
	them. Described in detail above in Section 6.3.		in a packet number space to information about them. Described in
			detail above in Appendix A.1.
	6.4.3. Initialization		A.4. Initialization
	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
	loss_time = 0		loss_time = 0
	smoothed_rtt = 0		smoothed_rtt = 0
	rttvar = 0		rttvar = 0
	min_rtt = infinite		min_rtt = infinite
	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
	largest_sent_packet = 0		for pn_space in [ Initial, Handshake, ApplicatonData ]:
	largest_acked_packet = 0		largest_acked_packet[pn_space] = 0
	6.4.4. 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
	Section 6.3.1.		Appendix A.1.1.
	Pseudocode for OnPacketSent follows:		Pseudocode for OnPacketSent follows:
	OnPacketSent(packet_number, ack_eliciting, in_flight,		OnPacketSent(packet_number, pn_space, ack_eliciting,
	is_crypto_packet, sent_bytes):		in_flight, is_crypto_packet, sent_bytes):
	largest_sent_packet = packet_number		sent_packets[pn_space][packet_number].packet_number =
	sent_packets[packet_number].packet_number = packet_number		packet_number
	sent_packets[packet_number].time_sent = now		sent_packets[pn_space][packet_number].time_sent = now
	sent_packets[packet_number].ack_eliciting = ack_eliciting		sent_packets[pn_space][packet_number].ack_eliciting =
	sent_packets[packet_number].in_flight = in_flight		ack_eliciting
			sent_packets[pn_space][packet_number].in_flight = in_flight
	if (in_flight):		if (in_flight):
	if (is_crypto_packet):		if (is_crypto_packet):
	time_of_last_sent_crypto_packet = now		time_of_last_sent_crypto_packet = now
	if (ack_eliciting):		if (ack_eliciting):
	time_of_last_sent_ack_eliciting_packet = now		time_of_last_sent_ack_eliciting_packet = now
	OnPacketSentCC(sent_bytes)		OnPacketSentCC(sent_bytes)
	sent_packets[packet_number].size = sent_bytes		sent_packets[pn_space][packet_number].size = sent_bytes
	SetLossDetectionTimer()		SetLossDetectionTimer()
	6.4.5. 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):		OnAckReceived(ack, pn_space):
	largest_acked_packet = max(largest_acked_packet,		largest_acked_packet[pn_space] = max(largest_acked_packet,
	ack.largest_acked)		ack.largest_acked)
	// If the largest acknowledged is newly acked and		// If the largest acknowledged is newly acked and
	// ack-eliciting, update the RTT.		// ack-eliciting, update the RTT.
	if (sent_packets[ack.largest_acked] &&		if (sent_packets[pn_space][ack.largest_acked] &&
	sent_packets[ack.largest_acked].ack_eliciting):		sent_packets[pn_space][ack.largest_acked].ack_eliciting):
	latest_rtt =		latest_rtt =
	now - sent_packets[ack.largest_acked].time_sent		now - sent_packets[pn_space][ack.largest_acked].time_sent
	UpdateRtt(latest_rtt, ack.ack_delay)		UpdateRtt(latest_rtt, ack.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)
	// Find all newly acked packets in this ACK frame		// Find all newly acked packets in this ACK frame
	newly_acked_packets = DetermineNewlyAckedPackets(ack)		newly_acked_packets = DetermineNewlyAckedPackets(ack, pn_space)
	if (newly_acked_packets.empty()):		if (newly_acked_packets.empty()):
	return		return
	for acked_packet in newly_acked_packets:		for acked_packet in newly_acked_packets:
	OnPacketAcked(acked_packet.packet_number)		OnPacketAcked(acked_packet.packet_number, pn_space)
	DetectLostPackets()		DetectLostPackets(pn_space)
	crypto_count = 0		crypto_count = 0
	pto_count = 0		pto_count = 0
	SetLossDetectionTimer()		SetLossDetectionTimer()
	UpdateRtt(latest_rtt, ack_delay):		UpdateRtt(latest_rtt, ack_delay):
	// min_rtt ignores ack delay.		// min_rtt ignores ack delay.
	min_rtt = min(min_rtt, latest_rtt)		min_rtt = min(min_rtt, latest_rtt)
	// Limit ack_delay by max_ack_delay		// Limit ack_delay by max_ack_delay
	ack_delay = min(ack_delay, max_ack_delay)		ack_delay = min(ack_delay, max_ack_delay)
	// Adjust for ack delay if it's plausible.		// Adjust for ack delay if it's plausible.
	if (latest_rtt - min_rtt > ack_delay):		if (latest_rtt - min_rtt > ack_delay):
	latest_rtt -= ack_delay		latest_rtt -= ack_delay
	// Based on {{RFC6298}}.		// Based on {{RFC6298}}.
	if (smoothed_rtt == 0):		if (smoothed_rtt == 0):
	smoothed_rtt = latest_rtt		smoothed_rtt = latest_rtt
	rttvar = latest_rtt / 2		rttvar = latest_rtt / 2
	else:		else:
	rttvar_sample = abs(smoothed_rtt - latest_rtt)		rttvar_sample = abs(smoothed_rtt - latest_rtt)
	rttvar = 3/4 * rttvar + 1/4 * rttvar_sample		rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
	smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt		smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
	6.4.6. On Packet Acknowledgment		A.7. On Packet Acknowledgment
	When a packet is acknowledged for the first time, the following		When a packet is acknowledged for the first time, the following
	OnPacketAcked function is called. Note that a single ACK frame may		OnPacketAcked function is called. Note that a single ACK frame may
	newly acknowledge several packets. OnPacketAcked must be called once		newly acknowledge several packets. OnPacketAcked must be called once
	for each of these newly acknowledged packets.		for each of these newly acknowledged packets.
	OnPacketAcked takes one parameter, acked_packet, which is the struct		OnPacketAcked takes two parameters: acked_packet, which is the struct
	detailed in Section 6.3.1.		detailed in Appendix A.1.1, and the packet number space that this ACK
			frame was sent for.
	Pseudocode for OnPacketAcked follows:		Pseudocode for OnPacketAcked follows:
	OnPacketAcked(acked_packet):		OnPacketAcked(acked_packet, pn_space):
	if (acked_packet.ack_eliciting):		if (acked_packet.ack_eliciting):
	OnPacketAckedCC(acked_packet)		OnPacketAckedCC(acked_packet)
	sent_packets.remove(acked_packet.packet_number)		sent_packets[pn_space].remove(acked_packet.packet_number)
	6.4.7. Setting the Loss Detection Timer		A.8. Setting the Loss Detection Timer
	QUIC loss detection uses a single timer for all timeout loss		QUIC loss detection uses a single timer for all timeout loss
	detection. The duration of the timer is based on the timer's mode,		detection. The duration of the timer is based on the timer's mode,
	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:
	SetLossDetectionTimer():		SetLossDetectionTimer():
	// Don't arm timer if there are no ack-eliciting packets		// Don't arm timer if there are no ack-eliciting packets
	// in flight.		// in flight.
	if (no ack-eliciting packets in flight):		if (no ack-eliciting packets in flight):
	loss_detection_timer.cancel()		loss_detection_timer.cancel()
	return		return
	if (crypto packets are in flight):
	// Crypto retransmission timer.
	if (smoothed_rtt == 0):
	timeout = 2 * kInitialRtt
	else:
	timeout = 2 * smoothed_rtt
	timeout = max(timeout, kGranularity)
	timeout = timeout * (2 ^ crypto_count)
	loss_detection_timer.set(
	time_of_last_sent_crypto_packet + timeout)
	return
	if (loss_time != 0):
	// Time threshold loss detection.
	loss_detection_timer.set(loss_time)
	return
	// Calculate PTO duration		if (crypto packets are in flight):
	timeout =		// Crypto retransmission timer.
	smoothed_rtt + 4 * rttvar + max_ack_delay		if (smoothed_rtt == 0):
			timeout = 2 * kInitialRtt
			else:
			timeout = 2 * smoothed_rtt
	timeout = max(timeout, kGranularity)		timeout = max(timeout, kGranularity)
	timeout = timeout * (2 ^ pto_count)		timeout = timeout * (2 ^ crypto_count)
			loss_detection_timer.update(
			time_of_last_sent_crypto_packet + timeout)
			return
	loss_detection_timer.set(		if (loss_time != 0):
	time_of_last_sent_ack_eliciting_packet + timeout)		// Time threshold loss detection.
			// Only applies to the ApplicationData packet number space.
			loss_detection_timer.update(loss_time)
			return
	6.4.8. On Timeout		// Calculate PTO duration
			timeout =
			smoothed_rtt + 4 * rttvar + max_ack_delay
			timeout = max(timeout, kGranularity)
			timeout = timeout * (2 ^ pto_count)
			loss_detection_timer.update(
			time_of_last_sent_ack_eliciting_packet + 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():
	if (crypto packets are in flight):		if (crypto packets are in flight):
	// Crypto retransmission timeout.		// Crypto retransmission timeout.
	RetransmitUnackedCryptoData()		RetransmitUnackedCryptoData()
	crypto_count++		crypto_count++
	else if (loss_time != 0):		else if (loss_time != 0):
	// Time threshold loss Detection		// Time threshold loss Detection
	DetectLostPackets()		// Only applies to the ApplicationData packet number space.
	else:		DetectLostPackets(ApplicationData)
	// PTO		else:
	SendTwoPackets()		// PTO
	pto_count++		SendOneOrTwoPackets()
			pto_count++
	SetLossDetectionTimer()		SetLossDetectionTimer()
	6.4.9. 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. If the		operates on the sent_packets for that packet number space. If the
	loss detection timer expires and the loss_time is set, the previous		loss detection timer expires and the loss_time is set, the previous
	largest acknowledged packet is supplied.		largest acknowledged packet is supplied.
	Pseudocode for DetectLostPackets follows:		Pseudocode for DetectLostPackets follows:
	DetectLostPackets():		DetectLostPackets(pn_space):
	loss_time = 0		if (pn_space == ApplicationData):
			loss_time = 0
	lost_packets = {}		lost_packets = {}
	loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)		loss_delay = kTimeThreshold * max(latest_rtt, smoothed_rtt)
	// 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.		// Packets with packet numbers before this are deemed lost.
	lost_pn = largest_acked_packet - kPacketThreshold		lost_pn = largest_acked_packet - kPacketThreshold
	foreach unacked in sent_packets:		foreach unacked in sent_packets:
	if (unacked.packet_number > largest_acked_packet):		if (unacked.packet_number > largest_acked_packet):
	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):		unacked.packet_number <= lost_pn):
	sent_packets.remove(unacked.packet_number)		sent_packets.remove(unacked.packet_number)
	if (unacked.in_flight):		if (unacked.in_flight):
	lost_packets.insert(unacked)		lost_packets.insert(unacked)
	else if (loss_time == 0):		else if (pn_space == ApplicationData):
	loss_time = unacked.time_sent + loss_delay		if (loss_time == 0):
	else:		loss_time = unacked.time_sent + loss_delay
	loss_time = min(loss_time, unacked.time_sent + loss_delay)		else:
			loss_time = min(loss_time, unacked.time_sent +
			loss_delay)
	// Inform the congestion controller of lost packets and		// Inform the congestion controller of lost packets and
	// let it decide whether to retransmit immediately.		// let it decide whether to retransmit immediately.
	if (!lost_packets.empty()):		if (!lost_packets.empty()):
	OnPacketsLost(lost_packets)		OnPacketsLost(lost_packets)
	6.5. Discussion		Appendix B. Congestion Control Pseudocode
	The majority of constants were derived from best common practices
	among widely deployed TCP implementations on the internet.
	Exceptions follow.
	A shorter delayed ack time of 25ms was chosen because longer delayed
	acks can delay loss recovery and for the small number of connections
	where less than packet per 25ms is delivered, acking every packet is
	beneficial to congestion control and loss recovery.
	The default initial RTT of 100ms was chosen because it is slightly
	higher than both the median and mean min_rtt typically observed on
	the public internet.
	7. Congestion Control
	QUIC's congestion control is based on TCP NewReno [RFC6582]. NewReno
	is a congestion window based congestion control. QUIC specifies the
	congestion window in bytes rather than packets due to finer control
	and the ease of appropriate byte counting [RFC3465].
	QUIC hosts MUST NOT send packets if they would increase
	bytes_in_flight (defined in Section 7.9.2) beyond the available
	congestion window, unless the packet is a probe packet sent after a
	PTO timer expires, as described in Section 6.2.2.
	Implementations MAY use other congestion control algorithms, such as
	Cubic [RFC8312], and endpoints MAY use different algorithms from one
	another. The signals QUIC provides for congestion control are
	generic and are designed to support different algorithms.
	7.1. Explicit Congestion Notification
	If a path has been verified to support ECN, QUIC treats a Congestion
	Experienced codepoint in the IP header as a signal of congestion.
	This document specifies an endpoint's response when its peer receives
	packets with the Congestion Experienced codepoint. As discussed in
	[RFC8311], endpoints are permitted to experiment with other response
	functions.
	7.2. Slow Start
	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
	start anytime the congestion window is less than ssthresh, which
	typically only occurs after an PTO. While in slow start, QUIC
	increases the congestion window by the number of bytes acknowledged
	when each acknowledgment is processed.
	7.3. Congestion Avoidance
	Slow start exits to congestion avoidance. Congestion avoidance in
	NewReno uses an additive increase multiplicative decrease (AIMD)
	approach that increases the congestion window by one maximum packet
	size per congestion window acknowledged. When a loss is detected,
	NewReno halves the congestion window and sets the slow start
	threshold to the new congestion window.
	7.4. Recovery Period
	Recovery is a period of time beginning with detection of a lost
	packet or an increase in the ECN-CE counter. Because QUIC does not
	retransmit packets, it defines the end of recovery as a packet sent
	after the start of recovery being acknowledged. This is slightly
	different from TCP's definition of recovery, which ends when the lost
	packet that started recovery is acknowledged.
	The recovery period limits congestion window reduction to once per
	round trip. During recovery, the congestion window remains unchanged
	irrespective of new losses or increases in the ECN-CE counter.
	7.5. Ignoring Loss of Undecryptable Packets
	During the handshake, some packet protection keys might not be
	available when a packet arrives. In particular, Handshake and 0-RTT
	packets cannot be processed until the Initial packets arrive, and
	1-RTT packets cannot be processed until the handshake completes.
	Endpoints MAY ignore the loss of Handshake, 0-RTT, and 1-RTT packets
	that might arrive before the peer has packet protection keys to
	process those packets.
	7.6. Probe Timeout
	Probe packets MUST NOT be blocked by the congestion controller. A
	sender MUST however count these packets as being additionally in
	flight, since these packets adds network load without establishing
	packet loss. Note that sending probe packets might cause the
	sender's bytes in flight to exceed the congestion window until an
	acknowledgement is received that establishes loss or delivery of
	packets.
	When an ACK frame is received that establishes loss of all in-flight
	packets sent prior to a threshold number of consecutive PTOs
	(pto_count is more than kPersistentCongestionThreshold, see
	Section 7.9.1), the network is considered to be experiencing
	persistent congestion, and the sender's congestion window MUST be
	reduced to the minimum congestion window (kMinimumWindow). This
	response of collapsing the congestion window on persistent congestion
	is functionally similar to a sender's response on a Retransmission
	Timeout (RTO) in TCP [RFC5681].
	7.7. Pacing
	This document does not specify a pacer, but it is RECOMMENDED that a
	sender pace sending of all in-flight packets based on input from the
	congestion controller. For example, a pacer might distribute the
	congestion window over the SRTT when used with a window-based
	controller, and a pacer might use the rate estimate of a rate-based
	controller.
	An implementation should take care to architect its congestion
	controller to work well with a pacer. For instance, a pacer might
	wrap the congestion controller and control the availability of the
	congestion window, or a pacer might pace out packets handed to it by
	the congestion controller. Timely delivery of ACK frames is
	important for efficient loss recovery. Packets containing only ACK
	frames should therefore not be paced, to avoid delaying their
	delivery to the peer.
	As an example of a well-known and publicly available implementation
	of a flow pacer, implementers are referred to the Fair Queue packet
	scheduler (fq qdisc) in Linux (3.11 onwards).
	7.8. Sending data after an idle period
	A sender becomes idle if it ceases to send data and has no bytes in
	flight. A sender's congestion window MUST NOT increase while it is
	idle.
	When sending data after becoming idle, a sender MUST reset its
	congestion window to the initial congestion window (see Section 4.1
	of [RFC5681]), unless it paces the sending of packets. A sender MAY
	retain its congestion window if it paces the sending of any packets
	in excess of the initial congestion window.
	A sender MAY implement alternate mechanisms to update its congestion
	window after idle periods, such as those proposed for TCP in
	[RFC7661].
	7.9. Pseudocode		We now describe an example implementation of the congestion
			controller described in Section 7.
	7.9.1. Constants of interest		B.1. Constants of interest
	Constants used in congestion control are based on a combination of		Constants used in congestion control are based on a combination of
	RFCs, papers, and common practice. Some may need to be changed or		RFCs, papers, and common practice. Some may need to be changed or
	negotiated in order to better suit a variety of environments.		negotiated in order to better suit a variety of environments.
	kMaxDatagramSize: The sender's maximum payload size. Does not		kMaxDatagramSize: The sender's maximum payload size. Does not
	include UDP or IP overhead. The max packet size is used for		include UDP or IP overhead. The max packet size is used for
	calculating initial and minimum congestion windows. The		calculating initial and minimum congestion windows. The
	RECOMMENDED value is 1200 bytes.		RECOMMENDED value is 1200 bytes.
	skipping to change at page 26, line 22 ¶		skipping to change at page 30, line 27 ¶
	kPersistentCongestionThreshold: Number of consecutive PTOs required		kPersistentCongestionThreshold: Number of consecutive PTOs required
	for persistent congestion to be established. The rationale for		for persistent congestion to be established. The rationale for
	this threshold is to enable a sender to use initial PTOs for		this threshold is to enable a sender to use initial PTOs for
	aggressive probing, as TCP does with Tail Loss Probe (TLP) [TLP]		aggressive probing, as TCP does with Tail Loss Probe (TLP) [TLP]
	[RACK], before establishing persistent congestion, as TCP does		[RACK], before establishing persistent congestion, as TCP does
	with a Retransmission Timeout (RTO) [RFC5681]. The RECOMMENDED		with a Retransmission Timeout (RTO) [RFC5681]. The RECOMMENDED
	value for kPersistentCongestionThreshold is 2, which is equivalent		value for kPersistentCongestionThreshold is 2, which is equivalent
	to having two TLPs before an RTO in TCP.		to having two TLPs before an RTO in TCP.
	7.9.2. Variables of interest		B.2. Variables of interest
	Variables required to implement the congestion control mechanisms are		Variables required to implement the congestion control mechanisms are
	described in this section.		described in this section.
	ecn_ce_counter: The highest value reported for the ECN-CE counter by		ecn_ce_counter: The highest value reported for the ECN-CE counter by
	the peer in an ACK frame. This variable is used to detect		the peer in an ACK frame. This variable is used to detect
	increases in the reported ECN-CE counter.		increases in the reported ECN-CE counter.
	bytes_in_flight: The sum of the size in bytes of all sent packets		bytes_in_flight: The sum of the size in bytes of all sent packets
	that contain at least one ack-eliciting or PADDING frame, and have		that contain at least one ack-eliciting or PADDING frame, and have
	skipping to change at page 27, line 5 ¶		skipping to change at page 31, line 9 ¶
	sent.		sent.
	recovery_start_time: The time when QUIC first detects a loss,		recovery_start_time: The time when QUIC first detects a loss,
	causing it to enter recovery. When a packet sent after this time		causing it to enter recovery. When a packet sent after this time
	is acknowledged, QUIC exits recovery.		is acknowledged, QUIC exits recovery.
	ssthresh: Slow start threshold in bytes. When the congestion window		ssthresh: Slow start threshold in bytes. When the congestion window
	is below ssthresh, the mode is slow start and the window grows by		is below ssthresh, the mode is slow start and the window grows by
	the number of bytes acknowledged.		the number of bytes acknowledged.
	7.9.3. Initialization		B.3. Initialization
	At the beginning of the connection, initialize the congestion control		At the beginning of the connection, initialize the congestion control
	variables as follows:		variables as follows:
	congestion_window = kInitialWindow		congestion_window = kInitialWindow
	bytes_in_flight = 0		bytes_in_flight = 0
	recovery_start_time = 0		recovery_start_time = 0
	ssthresh = infinite		ssthresh = infinite
	ecn_ce_counter = 0		ecn_ce_counter = 0
	7.9.4. On Packet Sent		B.4. On Packet Sent
	Whenever a packet is sent, and it contains non-ACK frames, the packet		Whenever a packet is sent, and it contains non-ACK frames, the packet
	increases bytes_in_flight.		increases bytes_in_flight.
	OnPacketSentCC(bytes_sent):		OnPacketSentCC(bytes_sent):
	bytes_in_flight += bytes_sent		bytes_in_flight += bytes_sent
	7.9.5. On Packet Acknowledgement		B.5. On Packet Acknowledgement
	Invoked from loss detection's OnPacketAcked and is supplied with the		Invoked from loss detection's OnPacketAcked and is supplied with the
	acked_packet from sent_packets.		acked_packet from sent_packets.
	InRecovery(sent_time):		InRecovery(sent_time):
	return sent_time <= recovery_start_time		return sent_time <= 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 (InRecovery(acked_packet.time_sent)):		if (InRecovery(acked_packet.time_sent)):
	// Do not increase congestion window in recovery period.		// Do not increase congestion window in recovery period.
	return		return
			if (IsAppLimited())
			// Do not increase congestion_window if application
			// limited.
			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 += kMaxDatagramSize * acked_packet.size		congestion_window += kMaxDatagramSize * acked_packet.size
	/ congestion_window		/ congestion_window
	7.9.6. On New Congestion Event		B.6. On New Congestion Event
	Invoked from ProcessECN and OnPacketsLost when a new congestion event		Invoked from ProcessECN and OnPacketsLost when a new congestion event
	is detected. May start a new recovery period and reduces the		is detected. May start a new recovery period and reduces the
	congestion window.		congestion window.
	CongestionEvent(sent_time):		CongestionEvent(sent_time):
	// Start a new congestion event if the sent time is larger		// Start a new congestion event if the sent time is larger
	// than the start time of the previous recovery epoch.		// than the start time of the previous recovery epoch.
	if (!InRecovery(sent_time)):		if (!InRecovery(sent_time)):
	recovery_start_time = Now()		recovery_start_time = Now()
	congestion_window *= kLossReductionFactor		congestion_window *= kLossReductionFactor
	congestion_window = max(congestion_window, kMinimumWindow)		congestion_window = max(congestion_window, kMinimumWindow)
	ssthresh = congestion_window		ssthresh = congestion_window
	// Collapse congestion window if persistent congestion		// Collapse congestion window if persistent congestion
	if (pto_count > kPersistentCongestionThreshold):		if (pto_count > kPersistentCongestionThreshold):
	congestion_window = kMinimumWindow		congestion_window = kMinimumWindow
	7.9.7. Process ECN Information		B.7. Process ECN Information
	Invoked when an ACK frame with an ECN section is received from the		Invoked when an ACK frame with an ECN section is received from the
	peer.		peer.
	ProcessECN(ack):		ProcessECN(ack):
	// If the ECN-CE counter reported by the peer has increased,		// If the ECN-CE counter reported by the peer has increased,
	// this could be a new congestion event.		// this could be a new congestion event.
	if (ack.ce_counter > ecn_ce_counter):		if (ack.ce_counter > ecn_ce_counter):
	ecn_ce_counter = ack.ce_counter		ecn_ce_counter = ack.ce_counter
	// Start a new congestion event if the last acknowledged		// Start a new congestion event if the last acknowledged
	// packet was sent after the start of the previous		// packet was sent after the start of the previous
	// recovery epoch.		// recovery epoch.
	CongestionEvent(sent_packets[ack.largest_acked].time_sent)		CongestionEvent(sent_packets[ack.largest_acked].time_sent)
	7.9.8. On Packets Lost		B.8. On Packets Lost
	Invoked by loss detection from DetectLostPackets when new packets are		Invoked by loss detection from DetectLostPackets when new packets are
	detected lost.		detected lost.
	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()
	// Start a new congestion epoch if the last lost packet		// Start a new congestion epoch if the last lost packet
	// is past the end of the previous recovery epoch.		// is past the end of the previous recovery epoch.
	CongestionEvent(largest_lost_packet.time_sent)		CongestionEvent(largest_lost_packet.time_sent)
	8. Security Considerations		Appendix C. Change Log
	8.1. Congestion Signals
	Congestion control fundamentally involves the consumption of signals
	- both loss and ECN codepoints - from unauthenticated entities. On-
	path attackers can spoof or alter these signals. An attacker can
	cause endpoints to reduce their sending rate by dropping packets, or
	alter send rate by changing ECN codepoints.
	8.2. Traffic Analysis
	Packets that carry only ACK frames can be heuristically identified by
	observing packet size. Acknowledgement patterns may expose
	information about link characteristics or application behavior.
	Endpoints can use PADDING frames or bundle acknowledgments with other
	frames to reduce leaked information.
	8.3. Misreporting ECN Markings
	A receiver can misreport ECN markings to alter the congestion
	response of a sender. Suppressing reports of ECN-CE markings could
	cause a sender to increase their send rate. This increase could
	result in congestion and loss.
	A sender MAY attempt to detect suppression of reports by marking
	occasional packets that they send with ECN-CE. If a packet marked
	with ECN-CE is not reported as having been marked when the packet is
	acknowledged, the sender SHOULD then disable ECN for that path.
	Reporting additional ECN-CE markings will cause a sender to reduce
	their sending rate, which is similar in effect to advertising reduced
	connection flow control limits and so no advantage is gained by doing
	so.
	Endpoints choose the congestion controller that they use. Though
	congestion controllers generally treat reports of ECN-CE markings as
	equivalent to loss [RFC8311], the exact response for each controller
	could be different. Failure to correctly respond to information
	about ECN markings is therefore difficult to detect.
	9. IANA Considerations
	This document has no IANA actions. Yet.
	10. References
	10.1. Normative References
	[QUIC-TLS]
	Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
	QUIC", draft-ietf-quic-tls-18 (work in progress).
	[QUIC-TRANSPORT]
	Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
	Multiplexed and Secure Transport", draft-ietf-quic-
	transport-18 (work in progress).
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[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>.
	10.2. Informative References
	[FACK] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
	Refining TCP Congestion Control", ACM SIGCOMM , August
	1996.
	[RACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
	a time-based fast loss detection algorithm for TCP",
	draft-ietf-tcpm-rack-04 (work in progress), July 2018.
	[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
	Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
	2003, <https://www.rfc-editor.org/info/rfc3465>.
	[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
	"Improving the Robustness of TCP to Non-Congestion
	Events", RFC 4653, DOI 10.17487/RFC4653, August 2006,
	<https://www.rfc-editor.org/info/rfc4653>.
	[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
	Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
	<https://www.rfc-editor.org/info/rfc5681>.
	[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
	"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
	Spurious Retransmission Timeouts with TCP", RFC 5682,
	DOI 10.17487/RFC5682, September 2009,
	<https://www.rfc-editor.org/info/rfc5682>.
	[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
	P. Hurtig, "Early Retransmit for TCP and Stream Control
	Transmission Protocol (SCTP)", RFC 5827,
	DOI 10.17487/RFC5827, May 2010,
	<https://www.rfc-editor.org/info/rfc5827>.
	[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
	"Computing TCP's Retransmission Timer", RFC 6298,
	DOI 10.17487/RFC6298, June 2011,
	<https://www.rfc-editor.org/info/rfc6298>.
	[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
	NewReno Modification to TCP's Fast Recovery Algorithm",
	RFC 6582, DOI 10.17487/RFC6582, April 2012,
	<https://www.rfc-editor.org/info/rfc6582>.
	[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
	and Y. Nishida, "A Conservative Loss Recovery Algorithm
	Based on Selective Acknowledgment (SACK) for TCP",
	RFC 6675, DOI 10.17487/RFC6675, August 2012,
	<https://www.rfc-editor.org/info/rfc6675>.
	[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
	"Increasing TCP's Initial Window", RFC 6928,
	DOI 10.17487/RFC6928, April 2013,
	<https://www.rfc-editor.org/info/rfc6928>.
	[RFC7661] Fairhurst, G., Sathiaseelan, A., and R. Secchi, "Updating
	TCP to Support Rate-Limited Traffic", RFC 7661,
	DOI 10.17487/RFC7661, October 2015,
	<https://www.rfc-editor.org/info/rfc7661>.
	[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
	R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
	RFC 8312, DOI 10.17487/RFC8312, February 2018,
	<https://www.rfc-editor.org/info/rfc8312>.
	[TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
	"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
	Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
	in progress), February 2013.
	10.3. URIs
	[1] https://mailarchive.ietf.org/arch/search/?email_list=quic
	[2] https://github.com/quicwg
	[3] https://github.com/quicwg/base-drafts/labels/-recovery
	Appendix A. 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.
	A.1. Since draft-ietf-quic-recovery-17		C.1. Since draft-ietf-quic-recovery-17
	o After Probe Timeout discard in-flight packets or send another		o After Probe Timeout discard in-flight packets or send another
	(#2212, #1965)		(#2212, #1965)
	o Endpoints discard initial keys as soon as handshake keys are		o Endpoints discard initial keys as soon as handshake keys are
	available (#1951, #2045)		available (#1951, #2045)
	o 0-RTT state is discarded when 0-RTT is rejected (#2300)		o 0-RTT state is discarded when 0-RTT is rejected (#2300)
	o Loss detection timer is cancelled when ack-eliciting frames are in		o Loss detection timer is cancelled when ack-eliciting frames are in
	skipping to change at page 32, line 44 ¶		skipping to change at page 33, line 36 ¶
	o After becoming idle, either pace packets or reset the congestion		o After becoming idle, either pace packets or reset the congestion
	controller (#2138, 2187)		controller (#2138, 2187)
	o Process ECN counts before marking packets lost (#2142)		o Process ECN counts before marking packets lost (#2142)
	o Mark packets lost before resetting crypto_count and pto_count		o Mark packets lost before resetting crypto_count and pto_count
	(#2208, #2209)		(#2208, #2209)
	o Congestion and loss recovery state are discarded when keys are		o Congestion and loss recovery state are discarded when keys are
	discarded (#2237)		discarded (#2327)
	A.2. Since draft-ietf-quic-recovery-16		C.2. Since draft-ietf-quic-recovery-16
	o Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP		o Unify TLP and RTO into a single PTO; eliminate min RTO, min TLP
	and min crypto timeouts; eliminate timeout validation (#2114,		and min crypto timeouts; eliminate timeout validation (#2114,
	#2166, #2168, #1017)		#2166, #2168, #1017)
	o Redefine how congestion avoidance in terms of when the period		o Redefine how congestion avoidance in terms of when the period
	starts (#1928, #1930)		starts (#1928, #1930)
	o Document what needs to be tracked for packets that are in flight		o Document what needs to be tracked for packets that are in flight
	(#765, #1724, #1939)		(#765, #1724, #1939)
	skipping to change at page 33, line 30 ¶		skipping to change at page 34, line 21 ¶
	o Limit ack_delay by max_ack_delay (#2060, #2099)		o Limit ack_delay by max_ack_delay (#2060, #2099)
	o Initial keys are discarded once Handshake are avaialble (#1951,		o Initial keys are discarded once Handshake are avaialble (#1951,
	#2045)		#2045)
	o Reorder ECN and loss detection in pseudocode (#2142)		o Reorder ECN and loss detection in pseudocode (#2142)
	o Only cancel loss detection timer if ack-eliciting packets are in		o Only cancel loss detection timer if ack-eliciting packets are in
	flight (#2093, #2117)		flight (#2093, #2117)
	A.3. Since draft-ietf-quic-recovery-14		C.3. Since draft-ietf-quic-recovery-14
	o Used max_ack_delay from transport params (#1796, #1782)		o Used max_ack_delay from transport params (#1796, #1782)
	o Merge ACK and ACK_ECN (#1783)		o Merge ACK and ACK_ECN (#1783)
	A.4. Since draft-ietf-quic-recovery-13		C.4. Since draft-ietf-quic-recovery-13
	o Corrected the lack of ssthresh reduction in CongestionEvent		o Corrected the lack of ssthresh reduction in CongestionEvent
	pseudocode (#1598)		pseudocode (#1598)
	o Considerations for ECN spoofing (#1426, #1626)		o Considerations for ECN spoofing (#1426, #1626)
	o Clarifications for PADDING and congestion control (#837, #838,		o Clarifications for PADDING and congestion control (#837, #838,
	#1517, #1531, #1540)		#1517, #1531, #1540)
	o Reduce early retransmission timer to RTT/8 (#945, #1581)		o Reduce early retransmission timer to RTT/8 (#945, #1581)
	o Packets are declared lost after an RTO is verified (#935, #1582)		o Packets are declared lost after an RTO is verified (#935, #1582)
	A.5. Since draft-ietf-quic-recovery-12		C.5. Since draft-ietf-quic-recovery-12
	o Changes to manage separate packet number spaces and encryption		o Changes to manage separate packet number spaces and encryption
	levels (#1190, #1242, #1413, #1450)		levels (#1190, #1242, #1413, #1450)
	o Added ECN feedback mechanisms and handling; new ACK_ECN frame		o Added ECN feedback mechanisms and handling; new ACK_ECN frame
	(#804, #805, #1372)		(#804, #805, #1372)
	A.6. Since draft-ietf-quic-recovery-11		C.6. Since draft-ietf-quic-recovery-11
	No significant changes.		No significant changes.
	A.7. Since draft-ietf-quic-recovery-10		C.7. Since draft-ietf-quic-recovery-10
	o Improved text on ack generation (#1139, #1159)		o Improved text on ack generation (#1139, #1159)
	o Make references to TCP recovery mechanisms informational (#1195)		o Make references to TCP recovery mechanisms informational (#1195)
	o Define time_of_last_sent_handshake_packet (#1171)		o Define time_of_last_sent_handshake_packet (#1171)
	o Added signal from TLS the data it includes needs to be sent in a		o Added signal from TLS the data it includes needs to be sent in a
	Retry packet (#1061, #1199)		Retry packet (#1061, #1199)
	o Minimum RTT (min_rtt) is initialized with an infinite value		o Minimum RTT (min_rtt) is initialized with an infinite value
	(#1169)		(#1169)
	A.8. Since draft-ietf-quic-recovery-09		C.8. Since draft-ietf-quic-recovery-09
	No significant changes.		No significant changes.
	A.9. Since draft-ietf-quic-recovery-08		C.9. Since draft-ietf-quic-recovery-08
	o Clarified pacing and RTO (#967, #977)		o Clarified pacing and RTO (#967, #977)
	A.10. Since draft-ietf-quic-recovery-07		C.10. Since draft-ietf-quic-recovery-07
	o Include Ack Delay in RTO(and TLP) computations (#981)		o Include Ack Delay in RTO(and TLP) computations (#981)
	o Ack Delay in SRTT computation (#961)		o Ack Delay in SRTT computation (#961)
	o Default RTT and Slow Start (#590)		o Default RTT and Slow Start (#590)
	o Many editorial fixes.		o Many editorial fixes.
	A.11. Since draft-ietf-quic-recovery-06		C.11. Since draft-ietf-quic-recovery-06
	No significant changes.		No significant changes.
	A.12. Since draft-ietf-quic-recovery-05		C.12. Since draft-ietf-quic-recovery-05
	o Add more congestion control text (#776)		o Add more congestion control text (#776)
	A.13. Since draft-ietf-quic-recovery-04		C.13. Since draft-ietf-quic-recovery-04
	No significant changes.		No significant changes.
	A.14. Since draft-ietf-quic-recovery-03		C.14. Since draft-ietf-quic-recovery-03
	No significant changes.		No significant changes.
	A.15. Since draft-ietf-quic-recovery-02		C.15. Since draft-ietf-quic-recovery-02
	o Integrate F-RTO (#544, #409)		o Integrate F-RTO (#544, #409)
	o Add congestion control (#545, #395)		o Add congestion control (#545, #395)
	o Require connection abort if a skipped packet was acknowledged		o Require connection abort if a skipped packet was acknowledged
	(#415)		(#415)
	o Simplify RTO calculations (#142, #417)		o Simplify RTO calculations (#142, #417)
	A.16. Since draft-ietf-quic-recovery-01		C.16. Since draft-ietf-quic-recovery-01
	o Overview added to loss detection		o Overview added to loss detection
	o Changes initial default RTT to 100ms		o Changes initial default RTT to 100ms
	o Added time-based loss detection and fixes early retransmit		o Added time-based loss detection and fixes early retransmit
	o Clarified loss recovery for handshake packets		o Clarified loss recovery for handshake packets
	o Fixed references and made TCP references informative		o Fixed references and made TCP references informative
	A.17. Since draft-ietf-quic-recovery-00		C.17. Since draft-ietf-quic-recovery-00
	o Improved description of constants and ACK behavior		o Improved description of constants and ACK behavior
	A.18. Since draft-iyengar-quic-loss-recovery-01		C.18. Since draft-iyengar-quic-loss-recovery-01
	o Adopted as base for draft-ietf-quic-recovery		o Adopted as base for draft-ietf-quic-recovery
	o Updated authors/editors list		o Updated authors/editors list
	o Added table of contents		o Added table of contents
	Acknowledgments		Acknowledgments
	Authors' Addresses		Authors' Addresses
	Jana Iyengar (editor)		Jana Iyengar (editor)
	Fastly		Fastly
	Email: jri.ietf@gmail.com		Email: jri.ietf@gmail.com
	Ian Swett (editor)		Ian Swett (editor)
	Google		Google
End of changes. 108 change blocks.
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