draft-ietf-quic-recovery-05.txt		draft-ietf-quic-recovery-06.txt
	QUIC J. Iyengar, Ed.		QUIC J. Iyengar, Ed.
	Internet-Draft I. Swett, Ed.		Internet-Draft I. Swett, Ed.
	Intended status: Standards Track Google		Intended status: Standards Track Google
	Expires: February 16, 2018 August 15, 2017		Expires: March 26, 2018 September 22, 2017
	QUIC Loss Detection and Congestion Control		QUIC Loss Detection and Congestion Control
	draft-ietf-quic-recovery-05		draft-ietf-quic-recovery-06
	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
	https://mailarchive.ietf.org/arch/search/?email_list=quic.		https://mailarchive.ietf.org/arch/search/?email_list=quic .
	Working Group information can be found at https://github.com/quicwg;		Working Group information can be found at https://github.com/quicwg ;
	source code and issues list for this draft can be found at		source code and issues list for this draft can be found at
	https://github.com/quicwg/base-drafts/labels/recovery.		https://github.com/quicwg/base-drafts/labels/recovery .
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	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 http://datatracker.ietf.org/drafts/current/.		Drafts is at http://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 February 16, 2018.		This Internet-Draft will expire on March 26, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 19 ¶		skipping to change at page 2, line 19 ¶
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3		1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
	2. Design of the QUIC Transmission Machinery . . . . . . . . . . 3		2. Design of the QUIC Transmission Machinery . . . . . . . . . . 3
	2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 4		2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 4
	2.1.1. Monotonically Increasing Packet Numbers . . . . . . . 4		2.1.1. Monotonically Increasing Packet Numbers . . . . . . . 4
	2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 4		2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 5
	2.1.3. More ACK Ranges . . . . . . . . . . . . . . . . . . . 5		2.1.3. More ACK Ranges . . . . . . . . . . . . . . . . . . . 5
	2.1.4. Explicit Correction For Delayed Acks . . . . . . . . 5		2.1.4. Explicit Correction For Delayed Acks . . . . . . . . 5
	3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 5		3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 5
	3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5		3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
	3.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 6		3.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 6
	3.2.1. Constants of interest . . . . . . . . . . . . . . . . 6		3.2.1. Constants of interest . . . . . . . . . . . . . . . . 6
	3.2.2. Variables of interest . . . . . . . . . . . . . . . . 6		3.2.2. Variables of interest . . . . . . . . . . . . . . . . 7
	3.2.3. Initialization . . . . . . . . . . . . . . . . . . . 8		3.2.3. Initialization . . . . . . . . . . . . . . . . . . . 8
	3.2.4. On Sending a Packet . . . . . . . . . . . . . . . . . 8		3.2.4. On Sending a Packet . . . . . . . . . . . . . . . . . 8
	3.2.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . 9		3.2.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . 9
	3.2.6. On Packet Acknowledgment . . . . . . . . . . . . . . 9		3.2.6. On Packet Acknowledgment . . . . . . . . . . . . . . 9
	3.2.7. Setting the Loss Detection Alarm . . . . . . . . . . 10		3.2.7. Setting the Loss Detection Alarm . . . . . . . . . . 10
	3.2.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 12		3.2.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 12
	3.2.9. Detecting Lost Packets . . . . . . . . . . . . . . . 13		3.2.9. Detecting Lost Packets . . . . . . . . . . . . . . . 13
	3.3. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14		3.3. Discussion . . . . . . . . . . . . . . . . . . . . . . . 14
	4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 14		4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 14
	4.1. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15		4.1. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15
	4.2. Recovery . . . . . . . . . . . . . . . . . . . . . . . . 15		4.2. Congestion Avoidance . . . . . . . . . . . . . . . . . . 15
	4.3. Constants of interest . . . . . . . . . . . . . . . . . . 15		4.3. Recovery Period . . . . . . . . . . . . . . . . . . . . . 15
	4.4. Variables of interest . . . . . . . . . . . . . . . . . . 15		4.4. Tail Loss Probe . . . . . . . . . . . . . . . . . . . . . 15
	4.5. Initialization . . . . . . . . . . . . . . . . . . . . . 16		4.5. Retransmission Timeout . . . . . . . . . . . . . . . . . 15
	4.6. On Packet Acknowledgement . . . . . . . . . . . . . . . . 16		4.6. Pacing Rate . . . . . . . . . . . . . . . . . . . . . . . 16
	4.7. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 16		4.7. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 16
	4.8. On Retransmission Timeout Verified . . . . . . . . . . . 17		4.7.1. Constants of interest . . . . . . . . . . . . . . . . 16
	4.9. Pacing Packets . . . . . . . . . . . . . . . . . . . . . 17		4.7.2. Variables of interest . . . . . . . . . . . . . . . . 16
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17		4.7.3. Initialization . . . . . . . . . . . . . . . . . . . 17
	6. References . . . . . . . . . . . . . . . . . . . . . . . . . 17		4.7.4. On Packet Sent . . . . . . . . . . . . . . . . . . . 17
	6.1. Normative References . . . . . . . . . . . . . . . . . . 17		4.7.5. On Packet Acknowledgement . . . . . . . . . . . . . . 17
	6.2. Informative References . . . . . . . . . . . . . . . . . 17		4.7.6. On Packets Lost . . . . . . . . . . . . . . . . . . . 17
	Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 18		4.7.7. On Retransmission Timeout Verified . . . . . . . . . 18
	Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 18		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
	B.1. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 18		6. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
	B.2. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 18		6.1. Normative References . . . . . . . . . . . . . . . . . . 18
	B.3. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 18		6.2. Informative References . . . . . . . . . . . . . . . . . 18
	B.4. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 19		Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 19
	B.5. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 19		Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 19
	B.6. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 19		B.1. Since draft-ietf-quic-recovery-05 . . . . . . . . . . . . 19
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19		B.2. Since draft-ietf-quic-recovery-04 . . . . . . . . . . . . 19
			B.3. Since draft-ietf-quic-recovery-03 . . . . . . . . . . . . 19
			B.4. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 20
			B.5. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 20
			B.6. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 20
			B.7. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 20
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
	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 known TCP loss recovery mechanisms,
	described in RFCs, various Internet-drafts, and also those prevalent		described in RFCs, various Internet-drafts, and also those prevalent
	skipping to change at page 5, line 46 ¶		skipping to change at page 6, line 4 ¶
	kReorderingThreshold packets are sent after it, the sender cannot		kReorderingThreshold packets are sent after it, the sender cannot
	expect to detect loss based on the previous mechanism. In this		expect to detect loss based on the previous mechanism. In this
	case, a sender uses both ack information and an alarm to detect		case, a sender uses both ack information and an alarm to detect
	loss. Specifically, when the last sent packet is acknowledged,		loss. Specifically, when the last sent packet is acknowledged,
	the sender waits a short period of time to allow for reordering		the sender waits a short period of time to allow for reordering
	and then marks any unacknowledged packets as lost. This mechanism		and then marks any unacknowledged packets as lost. This mechanism
	is based on the Linux implementation of TCP Early Retransmit.		is based on the Linux implementation of TCP Early Retransmit.
	o If a packet is sent at the tail, there are no packets sent after		o If a packet is sent at the tail, there are no packets sent after
	it, and the sender cannot use ack information to detect its loss.		it, and the sender cannot use ack information to detect its loss.
	The sender therefore relies on an alarm to detect such tail		The sender therefore relies on an alarm to detect such tail
	losses. This mechanism is based on TCP's Tail Loss Probe.		losses. This mechanism is based on TCP's Tail Loss Probe.
	o If all else fails, a Retransmission Timeout (RTO) alarm is always		o If all else fails, a Retransmission Timeout (RTO) alarm is always
	set when any retransmittable packet is outstanding. When this		set when any retransmittable packet is outstanding. When this
	alarm fires, all unacknowledged packets are marked as lost.		alarm fires, all unacknowledged packets are marked as lost.
	o Instead of a packet threshold to tolerate reordering, a QUIC		o Instead of a packet threshold to tolerate reordering, a QUIC
	sender may use a time threshold. This allows for senders to be		sender may use a time threshold. This allows for senders to be
	tolerant of short periods of significant reordering. In this		tolerant of short periods of significant reordering. In this
	mechanism, a QUIC sender marks a packet as lost when a packet		mechanism, a QUIC sender marks a packet as lost when a larger
	larger than it is acknowledged and a threshold amount of time has		packet number is acknowledged and a threshold amount of time has
	passed since the packet was sent.		passed since the packet was sent.
	o Handshake packets, which contain STREAM frames for stream 0, are		o Handshake packets, which contain STREAM frames for stream 0, are
	critical to QUIC transport and crypto negotiation, so a separate		critical to QUIC transport and crypto negotiation, so a separate
	alarm period is used for them.		alarm period is used for them.
	3.2. Algorithm Details		3.2. Algorithm Details
	3.2.1. Constants of interest		3.2.1. Constants of interest
	skipping to change at page 8, line 35 ¶		skipping to change at page 8, line 39 ¶
	largest_sent_packet = 0		largest_sent_packet = 0
	3.2.4. On Sending a Packet		3.2.4. On Sending a Packet
	After any packet is sent, be it a new transmission or a rebundled		After any packet is sent, be it a new transmission or a rebundled
	transmission, the following OnPacketSent function is called. The		transmission, the following OnPacketSent function is called. The
	parameters to OnPacketSent are as follows:		parameters to OnPacketSent are as follows:
	o packet_number: The packet number of the sent packet.		o packet_number: The packet number of the sent packet.
	o is_retransmittable: A boolean that indicates whether the packet		o is_ack_only: A boolean that indicates whether a packet only
	contains at least one frame requiring reliable deliver. The		contains an ACK frame. If true, it is still expected an ack will
	retransmittability of various QUIC frames is described in		be received for this packet, but it is not congestion controlled.
	[QUIC-TRANSPORT]. If false, it is still acceptable for an ack to
	be received for this packet. However, a caller MUST NOT set
	is_retransmittable to true if an ack is not expected.
	o sent_bytes: The number of bytes sent in the packet.		o sent_bytes: The number of bytes sent in the packet, not including
			UDP or IP overhead, but including QUIC framing overhead.
	Pseudocode for OnPacketSent follows:		Pseudocode for OnPacketSent follows:
	OnPacketSent(packet_number, is_retransmittable, sent_bytes):		OnPacketSent(packet_number, is_ack_only, sent_bytes):
	time_of_last_sent_packet = now		time_of_last_sent_packet = now
	largest_sent_packet = packet_number		largest_sent_packet = packet_number
	sent_packets[packet_number].packet_number = packet_number		sent_packets[packet_number].packet_number = packet_number
	sent_packets[packet_number].time = now		sent_packets[packet_number].time = now
	if is_retransmittable:		if !is_ack_only:
			OnPacketSentCC(sent_bytes)
	sent_packets[packet_number].bytes = sent_bytes		sent_packets[packet_number].bytes = sent_bytes
	SetLossDetectionAlarm()		SetLossDetectionAlarm()
	3.2.5. On Ack Receipt		3.2.5. On Ack Receipt
	When an ack is received, it may acknowledge 0 or more packets.		When an ack is received, it may acknowledge 0 or more packets.
	Pseudocode for OnAckReceived and UpdateRtt follow:		Pseudocode for OnAckReceived and UpdateRtt follow:
	OnAckReceived(ack):		OnAckReceived(ack):
	skipping to change at page 9, line 41 ¶		skipping to change at page 9, line 42 ¶
	DetectLostPackets(ack.largest_acked_packet)		DetectLostPackets(ack.largest_acked_packet)
	SetLossDetectionAlarm()		SetLossDetectionAlarm()
	UpdateRtt(latest_rtt):		UpdateRtt(latest_rtt):
	// 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 = 3/4 * rttvar + 1/4 * (smoothed_rtt - latest_rtt)		rttvar = 3/4 * rttvar + 1/4 * abs(smoothed_rtt - latest_rtt)
	smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt		smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
	3.2.6. On Packet Acknowledgment		3.2.6. On Packet Acknowledgment
	When a packet is acked for the first time, the following		When a packet is acked 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 acked packets.		for each of these newly acked packets.
	OnPacketAcked takes one parameter, acked_packet, which is the packet		OnPacketAcked takes one parameter, acked_packet, which is the packet
	skipping to change at page 12, line 23 ¶		skipping to change at page 12, line 23 ¶
	alarm_duration = 2 * kDefaultInitialRtt		alarm_duration = 2 * kDefaultInitialRtt
	else:		else:
	alarm_duration = 2 * smoothed_rtt		alarm_duration = 2 * smoothed_rtt
	alarm_duration = max(alarm_duration, kMinTLPTimeout)		alarm_duration = max(alarm_duration, kMinTLPTimeout)
	alarm_duration = alarm_duration * (2 ^ handshake_count)		alarm_duration = alarm_duration * (2 ^ handshake_count)
	else if (loss_time != 0):		else if (loss_time != 0):
	// Early retransmit timer or time loss detection.		// Early retransmit timer or time loss detection.
	alarm_duration = loss_time - now		alarm_duration = loss_time - now
	else if (tlp_count < kMaxTLPs):		else if (tlp_count < kMaxTLPs):
	// Tail Loss Probe		// Tail Loss Probe
	if (retransmittable_packets_outstanding = 1):		if (retransmittable_packets_outstanding == 1):
	alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout		alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout
	else:		else:
	alarm_duration = kMinTLPTimeout		alarm_duration = kMinTLPTimeout
	alarm_duration = max(alarm_duration, 2 * smoothed_rtt)		alarm_duration = max(alarm_duration, 2 * smoothed_rtt)
	else:		else:
	// RTO alarm		// RTO alarm
	alarm_duration = smoothed_rtt + 4 * rttvar		alarm_duration = smoothed_rtt + 4 * rttvar
	alarm_duration = max(alarm_duration, kMinRTOTimeout)		alarm_duration = max(alarm_duration, kMinRTOTimeout)
	alarm_duration = alarm_duration * (2 ^ rto_count)		alarm_duration = alarm_duration * (2 ^ rto_count)
	skipping to change at page 14, line 50 ¶		skipping to change at page 14, line 50 ¶
	where less than packet per 25ms is delivered, acking every packet is		where less than packet per 25ms is delivered, acking every packet is
	beneficial to congestion control and loss recovery.		beneficial to congestion control and loss recovery.
	The default initial RTT of 100ms was chosen because it is slightly		The default initial RTT of 100ms was chosen because it is slightly
	higher than both the median and mean min_rtt typically observed on		higher than both the median and mean min_rtt typically observed on
	the public internet.		the public internet.
	4. Congestion Control		4. Congestion Control
	QUIC's congestion control is based on TCP NewReno[RFC6582] congestion		QUIC's congestion control is based on TCP NewReno[RFC6582] congestion
	control to determine the congestion window and pacing rate.		control to determine the congestion window and pacing rate. QUIC
			congestion control is specified in bytes due to finer control and the
			ease of appropriate byte counting[RFC3465].
	4.1. Slow Start		4.1. Slow Start
	QUIC begins every connection in slow start and exits slow start upon		QUIC begins every connection in slow start and exits slow start upon
	loss. While in slow start, QUIC increases the congestion window by		loss. QUIC re-enters slow start after a retransmission timeout.
	the number of acknowledged bytes when each ack is processed.		While in slow start, QUIC increases the congestion window by the
			number of acknowledged bytes when each ack is processed.
	4.2. Recovery		4.2. 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 MSS of bytes 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.
			4.3. Recovery Period
	Recovery is a period of time beginning with detection of a lost		Recovery is a period of time beginning with detection of a lost
	packet. It ends when all packets outstanding at the time recovery		packet. Because QUIC retransmits frames, not packets, it defines the
	began have been acknowledged or lost. During recovery, the		end of recovery as all packets outstanding at the start of recovery
	congestion window is not increased or decreased.		being acknowledged or lost. This is slightly different from TCP's
			definition of recovery ending when the lost packet that started
			recovery is acknowledged. During recovery, the congestion window is
			not increased or decreased. As such, multiple lost packets only
			decrease the congestion window once as long as they're lost before
			exiting recovery. This causes QUIC to decrease the congestion window
			multiple times if retransmisions are lost, but limits the reduction
			to once per round trip.
	4.3. Constants of interest		4.4. Tail Loss Probe
			If recovery sends a tail loss probe, no change is made to the
			congestion window or pacing rate. Acknowledgement or loss of tail
			loss probes are treated like any other packet.
			4.5. Retransmission Timeout
			When retransmissions are sent due to a retransmission timeout alarm,
			no change is made to the congestion window or pacing rate until the
			next acknowledgement arrives. When an ack arrives, if packets prior
			to the first retransmission timeout are acknowledged, then the
			congestion window remains the same. If no packets prior to the first
			retransmission timeout are acknowledged, the retransmission timeout
			has been validated and the congestion window must be reduced to the
			minimum congestion window and slow start is begun.
			4.6. Pacing Rate
			The pacing rate is a function of the mode, the congestion window, and
			the smoothed rtt. Specifically, the pacing rate is 2 times the
			congestion window divided by the smoothed RTT during slow start and
			1.25 times the congestion window divided by the smoothed RTT during
			slow start. In order to fairly compete with flows that are not
			pacing, it is recommended to not pace the first 10 sent packets when
			exiting quiescence.
			4.7. Pseudocode
			4.7.1. 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.
	kDefaultMss (default 1460 bytes): The default max packet size used		kDefaultMss (default 1460 bytes): The default max packet size used
	for calculating default and minimum congestion windows.		for calculating default and minimum congestion windows.
	kInitialWindow (default 10 * kDefaultMss): Default limit on the		kInitialWindow (default 10 * kDefaultMss): Default limit on the
	amount of outstanding data in bytes.		amount of outstanding data in bytes.
	kMinimumWindow (default 2 * kDefaultMss): Default minimum congestion		kMinimumWindow (default 2 * kDefaultMss): Default minimum congestion
	window.		window.
	kLossReductionFactor (default 0.5): Reduction in congestion window		kLossReductionFactor (default 0.5): Reduction in congestion window
	when a new loss event is detected.		when a new loss event is detected.
	4.4. Variables of interest		4.7.2. 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.
	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 retransmittable or PADDING frame, and		that contain at least one retransmittable or PADDING frame, and
	have not been acked or declared lost. The size does not include		have not been acked or declared lost. The size does not include
	IP or UDP overhead. Ack only frames do not count towards		IP or UDP overhead. Packets only containing ack frames do not
	byte_in_flight.		count towards byte_in_flight to ensure congestion control does not
			impede congestion feedback.
	congestion_window: Maximum number of bytes in flight that may be		congestion_window: Maximum number of bytes in flight that may be
	sent.		sent.
	end_of_recovery: The packet number after which QUIC will no longer		end_of_recovery: The largest packet number sent when QUIC detects a
	be in recovery.		loss. When a larger packet 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, it grows by the number of bytes acknowledged		is below ssthresh, the mode is slow start and the window grows by
	for each ack.		the number of bytes acknowledged.
	4.5. Initialization		4.7.3. Initialization
	At the beginning of the connection, initialize the loss detection		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
	end_of_recovery = 0		end_of_recovery = 0
	ssthresh = infinite		ssthresh = infinite
	4.6. On Packet Acknowledgement		4.7.4. On Packet Sent
			Whenever a packet is sent, and it contains non-ACK frames, the packet
			increases bytes_in_flight.
			OnPacketSentCC(bytes_sent):
			bytes_in_flight += bytes_sent
			4.7.5. On Packet Acknowledgement
	Invoked from loss detection's OnPacketAcked and is supplied with		Invoked from loss detection's OnPacketAcked and is supplied with
	acked_packet from sent_packets.		acked_packet from sent_packets.
	Pseudocode for OnPacketAckedCC follows:
	OnPacketAckedCC(acked_packet):		OnPacketAckedCC(acked_packet):
			// Remove from bytes_in_flight.
			bytes_in_flight -= acked_packet.bytes
	if (acked_packet.packet_number < end_of_recovery):		if (acked_packet.packet_number < end_of_recovery):
			// Do not increase congestion window in recovery period.
	return		return
	if (congestion_window < ssthresh):		if (congestion_window < ssthresh):
	congestion_window += acket_packets.bytes		// Slow start.
			congestion_window += acked_packets.bytes
	else:		else:
			// Congestion avoidance.
	congestion_window +=		congestion_window +=
	acked_packets.bytes / congestion_window		kDefaultMss * acked_packets.bytes / congestion_window
	4.7. On Packets Lost		4.7.6. 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.
			for (lost_packet : lost_packets):
			bytes_in_flight -= lost_packet.bytes
	largest_lost_packet = lost_packets.last()		largest_lost_packet = lost_packets.last()
	// Start a new recovery epoch if the lost packet is larger		// Start a new recovery epoch if the lost packet is larger
	// than the end of the previous recovery epoch.		// than the end of the previous recovery epoch.
	if (end_of_recovery < largest_lost_packet.packet_number):		if (end_of_recovery < largest_lost_packet.packet_number):
	end_of_recovery = largest_sent_packet		end_of_recovery = largest_sent_packet
	congestion_window *= kLossReductionFactor		congestion_window *= kLossReductionFactor
	congestion_window = max(congestion_window, kMinimumWindow)		congestion_window = max(congestion_window, kMinimumWindow)
	ssthresh = congestion_window		ssthresh = congestion_window
	4.8. On Retransmission Timeout Verified		4.7.7. On Retransmission Timeout Verified
	QUIC decreases the congestion window to the minimum value once the		QUIC decreases the congestion window to the minimum value once the
	retransmission timeout has been confirmed to not be spurious when the		retransmission timeout has been verified.
	first post-RTO acknowledgement is processed.
	OnRetransmissionTimeoutVerified()		OnRetransmissionTimeoutVerified()
	congestion_window = kMinimumWindow		congestion_window = kMinimumWindow
	4.9. Pacing Packets
	QUIC sends a packet if there is available congestion window and
	sending the packet does not exceed the pacing rate.
	TimeToSend returns infinite if the congestion controller is
	congestion window limited, a time in the past if the packet can be
	sent immediately, and a time in the future if sending is pacing
	limited.
	TimeToSend(packet_size):
	if (bytes_in_flight + packet_size > congestion_window)
	return infinite
	return time_of_last_sent_packet +
	(packet_size * smoothed_rtt) / congestion_window
	5. IANA Considerations		5. IANA Considerations
	This document has no IANA actions. Yet.		This document has no IANA actions. Yet.
	6. References		6. References
	6.1. Normative References		6.1. Normative References
	[QUIC-TRANSPORT]		[QUIC-TRANSPORT]
	Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based		Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
	Multiplexed and Secure Transport", draft-ietf-quic-		Multiplexed and Secure Transport", draft-ietf-quic-
	transport (work in progress), August 2017.		transport (work in progress), September 2017.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
	<http://www.rfc-editor.org/info/rfc2119>.		editor.org/info/rfc2119>.
	6.2. Informative References		6.2. Informative References
	[LOSS-PROBE]		[LOSS-PROBE]
	Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,		Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
	"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of		"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
	Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work		Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
	in progress), February 2013.		in progress), February 2013.
			[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>.
	[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,		[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
	"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting		"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
	Spurious Retransmission Timeouts with TCP", RFC 5682,		Spurious Retransmission Timeouts with TCP", RFC 5682,
	DOI 10.17487/RFC5682, September 2009,		DOI 10.17487/RFC5682, September 2009, <https://www.rfc-
	<http://www.rfc-editor.org/info/rfc5682>.		editor.org/info/rfc5682>.
	[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and		[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
	P. Hurtig, "Early Retransmit for TCP and Stream Control		P. Hurtig, "Early Retransmit for TCP and Stream Control
	Transmission Protocol (SCTP)", RFC 5827,		Transmission Protocol (SCTP)", RFC 5827,
	DOI 10.17487/RFC5827, May 2010,		DOI 10.17487/RFC5827, May 2010, <https://www.rfc-
	<http://www.rfc-editor.org/info/rfc5827>.		editor.org/info/rfc5827>.
	[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,		[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
	"Computing TCP's Retransmission Timer", RFC 6298,		"Computing TCP's Retransmission Timer", RFC 6298,
	DOI 10.17487/RFC6298, June 2011,		DOI 10.17487/RFC6298, June 2011, <https://www.rfc-
	<http://www.rfc-editor.org/info/rfc6298>.		editor.org/info/rfc6298>.
	[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The		[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
	NewReno Modification to TCP's Fast Recovery Algorithm",		NewReno Modification to TCP's Fast Recovery Algorithm",
	RFC 6582, DOI 10.17487/RFC6582, April 2012,		RFC 6582, DOI 10.17487/RFC6582, April 2012,
	<http://www.rfc-editor.org/info/rfc6582>.		<https://www.rfc-editor.org/info/rfc6582>.
	Appendix A. Acknowledgments		Appendix A. Acknowledgments
	Appendix B. Change Log		Appendix B. 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.
	B.1. Since draft-ietf-quic-recovery-04		B.1. Since draft-ietf-quic-recovery-05
			o Add more congestion control text (#776)
			B.2. Since draft-ietf-quic-recovery-04
	No significant changes.		No significant changes.
	B.2. Since draft-ietf-quic-recovery-03		B.3. Since draft-ietf-quic-recovery-03
	No significant changes.		No significant changes.
	B.3. Since draft-ietf-quic-recovery-02		B.4. 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)
	B.4. Since draft-ietf-quic-recovery-01		B.5. 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
	B.5. Since draft-ietf-quic-recovery-00		B.6. Since draft-ietf-quic-recovery-00
	o Improved description of constants and ACK behavior		o Improved description of constants and ACK behavior
	B.6. Since draft-iyengar-quic-loss-recovery-01		B.7. 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
	Authors' Addresses		Authors' Addresses
	Jana Iyengar (editor)		Jana Iyengar (editor)
End of changes. 54 change blocks.
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