draft-ietf-quic-recovery-01.txt		draft-ietf-quic-recovery-02.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: July 18, 2017 January 14, 2017		Expires: September 14, 2017 March 13, 2017
	QUIC Loss Detection and Congestion Control		QUIC Loss Detection and Congestion Control
	draft-ietf-quic-recovery-01		draft-ietf-quic-recovery-02
	Abstract		Abstract
	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. QUIC implements the spirit of known TCP loss detection		transport. QUIC implements the spirit of known TCP loss detection
	mechanisms, described in RFCs, various Internet-drafts, and also		mechanisms, described in RFCs, various Internet-drafts, and also
	those prevalent in the Linux TCP implementation. This document		those prevalent in the Linux TCP implementation. This document
	describes QUIC loss detection and congestion control, and attributes		describes QUIC loss detection and congestion control, and attributes
	skipping to change at page 1, line 48 ¶		skipping to change at page 1, line 48 ¶
	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 July 18, 2017.		This Internet-Draft will expire on September 14, 2017.
	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 27 ¶		skipping to change at page 2, line 27 ¶
	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 . . . . . . . . . . . . . . . . . . . . . 5		2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 4
	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. Constants of interest . . . . . . . . . . . . . . . . . . 5		3.1. Constants of interest . . . . . . . . . . . . . . . . . . 5
	3.2. Variables of interest . . . . . . . . . . . . . . . . . . 6		3.2. Variables of interest . . . . . . . . . . . . . . . . . . 6
	3.3. Initialization . . . . . . . . . . . . . . . . . . . . . 6		3.3. Initialization . . . . . . . . . . . . . . . . . . . . . 7
	3.4. Setting the Loss Detection Alarm . . . . . . . . . . . . 7		3.4. On Sending a Packet . . . . . . . . . . . . . . . . . . . 7
	3.5. On Sending a Packet . . . . . . . . . . . . . . . . . . . 8		3.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . . . 8
	3.6. On Ack Receipt . . . . . . . . . . . . . . . . . . . . . 8		3.6. On Packet Acknowledgment . . . . . . . . . . . . . . . . 8
	3.7. On Packet Acknowledgment . . . . . . . . . . . . . . . . 9		3.7. Setting the Loss Detection Alarm . . . . . . . . . . . . 9
			3.7.1. Handshake Packets . . . . . . . . . . . . . . . . . . 9
			3.7.2. Tail Loss Probe and Retransmission Timeout . . . . . 9
			3.7.3. Early Retransmit . . . . . . . . . . . . . . . . . . 9
			3.7.4. Pseudocode . . . . . . . . . . . . . . . . . . . . . 10
	3.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . . . 10		3.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . . . 10
	3.9. Detecting Lost Packets . . . . . . . . . . . . . . . . . 10		3.9. Detecting Lost Packets . . . . . . . . . . . . . . . . . 11
	4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 10		3.9.1. Handshake Packets . . . . . . . . . . . . . . . . . . 11
	5. TCP mechanisms in QUIC . . . . . . . . . . . . . . . . . . . 10		3.9.2. Pseudocode . . . . . . . . . . . . . . . . . . . . . 11
	5.1. RFC 6298 (RTO computation) . . . . . . . . . . . . . . . 11		4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 12
	5.2. FACK Loss Recovery (paper) . . . . . . . . . . . . . . . 11		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
	5.3. RFC 3782, RFC 6582 (NewReno Fast Recovery) . . . . . . . 11		6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
	5.4. TLP (draft) . . . . . . . . . . . . . . . . . . . . . . . 11		6.1. Normative References . . . . . . . . . . . . . . . . . . 12
	5.5. RFC 5827 (Early Retransmit) with Delay Timer . . . . . . 11		6.2. Informative References . . . . . . . . . . . . . . . . . 13
	5.6. RFC 5827 (F-RTO) . . . . . . . . . . . . . . . . . . . . 12
	5.7. RFC 6937 (Proportional Rate Reduction) . . . . . . . . . 12
	5.8. TCP Cubic (draft) with optional RFC 5681 (Reno) . . . . . 12
	5.9. Hybrid Slow Start (paper) . . . . . . . . . . . . . . . . 12
	5.10. RACK (draft) . . . . . . . . . . . . . . . . . . . . . . 12
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
	7. Normative References . . . . . . . . . . . . . . . . . . . . 12
	Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 13		Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 13
	Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13		Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13
	B.1. Since draft-ietf-quic-recovery-00: . . . . . . . . . . . 13		B.1. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 14
	B.2. Since draft-iyengar-quic-loss-recovery-01: . . . . . . . 13		B.2. Since draft-ietf-quic-recovery-00: . . . . . . . . . . . 14
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13		B.3. Since draft-iyengar-quic-loss-recovery-01: . . . . . . . 14
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
	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 41 ¶		skipping to change at page 5, line 34 ¶
	called on packet transmission, when a packet is acked, and timer		called on packet transmission, when a packet is acked, and timer
	expiration events.		expiration events.
	3.1. Constants of interest		3.1. Constants of interest
	Constants used in loss recovery and congestion control are based on a		Constants used in loss recovery and congestion control are based on a
	combination of RFCs, papers, and common practice. Some may need to		combination of RFCs, papers, and common practice. Some may need to
	be changed or negotiated in order to better suit a variety of		be changed or negotiated in order to better suit a variety of
	environments.		environments.
	o kMaxTLPs: 2 Maximum number of tail loss probes before an RTO		kMaxTLPs (default 2): Maximum number of tail loss probes before an
	fires.		RTO fires.
	o kReorderingThreshold: 3 Maximum reordering in packet number space		kReorderingThreshold (default 3): Maximum reordering in packet
	before FACK style loss detection considers a packet lost.		number space before FACK style loss detection considers a packet
			lost.
	o kTimeReorderingThreshold: 1/8 Maximum reordering in time sapce		kTimeReorderingFraction (default 1/8): Maximum reordering in time
	before time based loss detection considers a packet lost. In		sapce before time based loss detection considers a packet lost.
	fraction of an RTT.		In fraction of an RTT.
	o kMinTLPTimeout: 10ms Minimum time in the future a tail loss probe		kMinTLPTimeout (default 10ms): Minimum time in the future a tail
			loss probe alarm may be set for.
			kMinRTOTimeout (default 200ms): Minimum time in the future an RTO
	alarm may be set for.		alarm may be set for.
	o kMinRTOTimeout: 200ms Minimum time in the future an RTO alarm may		kDelayedAckTimeout (default 25ms): The length of the peer's delayed
	be set for.		ack timer.
	o kDelayedAckTimeout: 25ms The length of the peer's delayed ack		kDefaultInitialRtt (default 100ms): The default RTT used before an
	timer.		RTT sample is taken.
	3.2. Variables of interest		3.2. Variables of interest
	We first describe the variables required to implement the loss		We first describe the variables required to implement the loss
	detection mechanisms described in this section.		detection mechanisms described in this section.
	o loss_detection_alarm: Multi-modal alarm used for loss detection.		loss_detection_alarm: Multi-modal alarm used for loss detection.
	o alarm_mode: QUIC maintains a single loss detection alarm, which
	switches between various modes. This mode is used to determine
	the duration of the alarm.
	o handshake_count: The number of times the handshake packets have		handshake_count: The number of times the handshake packets have been
	been retransmitted without receiving an ack.		retransmitted without receiving an ack.
	o tlp_count: The number of times a tail loss probe has been sent		tlp_count: The number of times a tail loss probe has been sent
	without receiving an ack.		without receiving an ack.
	o rto_count: The number of times an rto has been sent without		rto_count: The number of times an rto has been sent without
	receiving an ack.		receiving an ack.
	o 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]
	o rttvar: The RTT variance.		rttvar: The RTT variance, computed as described in [RFC6298]
	o reordering_threshold: The largest delta between the largest acked		initial_rtt: The initial RTT used before any RTT measurements have
			been made.
			reordering_threshold: The largest delta between the largest acked
	retransmittable packet and a packet containing retransmittable		retransmittable packet and a packet containing retransmittable
	frames before it's declared lost.		frames before it's declared lost.
	o use_time_loss: When true, loss detection operates solely based on		time_reordering_fraction: The reordering window as a fraction of
	reordering threshold in time, rather than in packet number gaps.		max(smoothed_rtt, latest_rtt).
	o sent_packets: An association of packet numbers to information		loss_time: The time at which the next packet will be considered lost
	about them.		based on early transmit or exceeding the reordering window in
			time.
			sent_packets: An association of packet numbers to information about
			them, including a number field indicating the packet number, a
			time field indicating the time a packet was sent, and a bytes
			field indicating the packet's size. sent_packets is ordered by
			packet number, and packets remain in sent_packets until
			acknowledged or lost.
	3.3. Initialization		3.3. 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_alarm.reset();		loss_detection_alarm.reset()
	handshake_count = 0;		handshake_count = 0
	tlp_count = 0;		tlp_count = 0
	rto_count = 0;		rto_count = 0
	reordering_threshold = kReorderingThreshold;		if (UsingTimeLossDetection())
	use_time_loss = false;		reordering_threshold = infinite
	smoothed_rtt = 0;		time_reordering_fraction = kTimeReorderingFraction
	rttvar = 0;
	3.4. Setting the Loss Detection Alarm
	QUIC loss detection uses a single alarm for all timer-based loss
	detection. The duration of the alarm is based on the alarm's mode,
	which is set in the packet and timer events further below. The
	function SetLossDetectionAlarm defined below shows how the single
	timer is set based on the alarm mode.
	Pseudocode for SetLossDetectionAlarm follows:
	SetLossDetectionAlarm():
	if (retransmittable packets are not outstanding):
	loss_detection_alarm.cancel();
	return;
	if (handshake packets are outstanding):
	// Handshake retransmission alarm.
	alarm_duration = max(1.5 * smoothed_rtt, kMinTLPTimeout) << handshake_count;
	handshake_count++;
	else if (largest sent packet is acked):
	// Early retransmit alarm.
	alarm_duration = 0.25 x smoothed_rtt;
	else if (tlp_count < kMaxTLPs):
	// Tail Loss Probe alarm.
	if (retransmittable_packets_outstanding = 1):
	alarm_duration = max(1.5 x smoothed_rtt + kDelayedAckTimeout,
	2 x smoothed_rtt);
	else:
	alarm_duration = max (kMinTLPTimeout, 2 x smoothed_rtt);
	tlp_count++;
	else:
	// RTO alarm.
	if (rto_count = 0):
	alarm_duration = max(kMinRTOTimeout, smoothed_rtt + 4 x rttvar);
	else:		else:
	alarm_duration = loss_detection_alarm.get_delay() << 1;		reordering_threshold = kReorderingThreshold
	rto_count++;		time_reordering_fraction = infinite
			loss_time = 0
	loss_detection_alarm.set(now + alarm_duration);		smoothed_rtt = 0
			rttvar = 0
			initial_rtt = kDefaultInitialRtt
	3.5. On Sending a Packet		3.4. 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_retransmittble: A boolean that indicates whether the packet		o is_retransmittble: A boolean that indicates whether the packet
	contains at least one frame requiring reliable deliver. The		contains at least one frame requiring reliable deliver. The
	retransmittability of various QUIC frames is described in		retransmittability of various QUIC frames is described in
	[QUIC-TRANSPORT]. If false, it is still acceptable for an ack to		[QUIC-TRANSPORT]. If false, it is still acceptable for an ack to
	be received for this packet. However, a caller MUST NOT set		be received for this packet. However, a caller MUST NOT set
	is_retransmittable to true if an ack is not expected.		is_retransmittable to true if an ack is not expected.
			o sent_bytes: The number of bytes sent in the packet.
	Pseudocode for OnPacketSent follows:		Pseudocode for OnPacketSent follows:
	OnPacketSent(packet_number, is_retransmittable):		OnPacketSent(packet_number, is_retransmittable, sent_bytes):
	# TODO: Clarify the data in sent_packets.		sent_packets[packet_number].packet_number = packet_number
	sent_packets[packet_number] = {now}		sent_packets[packet_number].time = now
	if is_retransmittable:		if is_retransmittable:
			sent_packets[packet_number].bytes = sent_bytes
	SetLossDetectionAlarm()		SetLossDetectionAlarm()
	3.6. On Ack Receipt		3.5. 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):
	// If the largest acked is newly acked, update the RTT.		// If the largest acked is newly acked, update the RTT.
	if (sent_packets[ack.largest_acked]):		if (sent_packets[ack.largest_acked]):
	rtt_sample = now - sent_packets[ack.largest_acked]		rtt_sample = now - sent_packets[ack.largest_acked].time
	if (rtt_sample > ack.ack_delay):		if (rtt_sample > ack.ack_delay):
	rtt_sample -= ack.delay;		rtt_sample -= ack.delay
	UpdateRtt(rtt_sample)		UpdateRtt(rtt_sample)
	// Find all newly acked packets.		// Find all newly acked packets.
	for acked_packet in DetermineNewlyAckedPackets():		for acked_packet_number in DetermineNewlyAckedPackets():
	OnPacketAcked(acked_packet)		OnPacketAcked(acked_packet_number)
	DetectLostPackets(ack.largest_acked_packet);		DetectLostPackets(ack.largest_acked_packet)
	SetLossDetectionAlarm();		SetLossDetectionAlarm()
	UpdateRtt(rtt_sample):		UpdateRtt(rtt_sample):
			// Based on {{RFC6298}}.
	if (smoothed_rtt == 0):		if (smoothed_rtt == 0):
	smoothed_rtt = rtt_sample		smoothed_rtt = rtt_sample
	rttvar = rtt_sample / 2		rttvar = rtt_sample / 2
	else:		else:
	rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - rtt_sample)		rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - rtt_sample)
	smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * rtt_sample		smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * rtt_sample
	3.7. On Packet Acknowledgment		3.6. 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
	number of the newly acked packet, and returns a list of packet		number of the newly acked packet, and returns a list of packet
	numbers that are detected as lost.		numbers that are detected as lost.
	Pseudocode for OnPacketAcked follows:		Pseudocode for OnPacketAcked follows:
	OnPacketAcked(acked_packet):		OnPacketAcked(acked_packet_number):
	handshake_count = 0;		handshake_count = 0
	tlp_count = 0;		tlp_count = 0
	rto_count = 0;		rto_count = 0
	# TODO: Don't remove packets immediately, since they can be used for		sent_packets.remove(acked_packet_number)
	# detecting spurous retransmits.
	sent_packets.remove(acked_packet);
	3.8. On Alarm Firing
	QUIC uses one loss recovery alarm, which when set, can be in one of		3.7. Setting the Loss Detection Alarm
	several modes. When the alarm fires, the mode determines the action
	to be performed. OnAlarm returns a list of packet numbers that are
	detected as lost.
	Pseudocode for OnAlarm follows:		QUIC loss detection uses a single alarm for all timer-based loss
			detection. The duration of the alarm is based on the alarm's mode,
			which is set in the packet and timer events further below. The
			function SetLossDetectionAlarm defined below shows how the single
			timer is set based on the alarm mode.
	OnAlarm(acked_packet):		3.7.1. Handshake Packets
	lost_packets = DetectLostPackets(acked_packet);
	MaybeRetransmitLostPackets();
	SetLossDetectionAlarm();
	3.9. Detecting Lost Packets		The initial flight has no prior RTT sample. A client SHOULD remember
			the previous RTT it observed when resumption is attempted and use
			that for an initial RTT value. If no previous RTT is available, the
			initial RTT defaults to 200ms. Once an RTT measurement is taken, it
			MUST replace initial_rtt.
	Packets in QUIC are only considered lost once a larger packet number		Endpoints MUST retransmit handshake frames if not acknowledged within
	is acknowledged. DetectLostPackets is called every time there is a		a time limit. This time limit will start as the largest of twice the
	new largest packet or if the loss detection alarm fires the previous		rtt value and MinTLPTimeout. Each consecutive handshake
	largest acked packet is supplied.		retransmission doubles the time limit, until an acknowledgement is
			received.
	DetectLostPackets takes one parameter, acked_packet, which is the		Handshake frames may be cancelled by handshake state transitions. In
	packet number of the largest acked packet, and returns a list of		particular, all non-protected frames SHOULD be no longer be
	packet numbers detected as lost.		transmitted once packet protection is available.
	Pseudocode for DetectLostPackets follows:		When stateless rejects are in use, the connection is considered
			immediately closed once a reject is sent, so no timer is set to
			retransmit the reject.
	DetectLostPackets(acked_packet):		Version negotiation packets are always stateless, and MUST be sent
	lost_packets = {};		once per per handshake packet that uses an unsupported QUIC version,
	foreach (unacked_packet less than acked_packet):		and MAY be sent in response to 0RTT packets.
	if (unacked_packet.time_sent <
	acked_packet.time_sent - kTimeReorderThreshold * smoothed_rtt):
	lost_packets.insert(unacked_packet.packet_number);
	else if (unacked_packet.packet_number <
	acked_packet.packet_number - reordering_threshold)
	lost_packets.insert(unacked_packet.packet_number);
	return lost_packets;
	4. Congestion Control		3.7.2. Tail Loss Probe and Retransmission Timeout
	(describe NewReno-style congestion control for QUIC.)		Tail loss probes [I-D.dukkipati-tcpm-tcp-loss-probe] and
			retransmission timeouts[RFC6298] are an alarm based mechanism to
			recover from cases when there are outstanding retransmittable
			packets, but an acknowledgement has not been received in a timely
			manner.
	5. TCP mechanisms in QUIC		3.7.3. Early Retransmit
	QUIC implements the spirit of a variety of RFCs, Internet drafts, and		Early retransmit [RFC5827] is implemented with a 1/4 RTT timer. It
	other well-known TCP loss recovery mechanisms, though the		is part of QUIC's time based loss detection, but is always enabled,
	implementation details differ from the TCP implementations.		even when only packet reordering loss detection is enabled.
	5.1. RFC 6298 (RTO computation)		3.7.4. Pseudocode
	QUIC calculates SRTT and RTTVAR according to the standard formulas.		Pseudocode for SetLossDetectionAlarm follows:
	An RTT sample is only taken if the delayed ack correction is smaller
	than the measured RTT (otherwise a negative RTT would result), and
	the ack's contains a new, larger largest observed packet number.
	min_rtt is only based on the observed RTT, but SRTT uses the delayed
	ack correction delta.
	As described above, QUIC implements RTO with the standard timeout and		SetLossDetectionAlarm():
	CWND reduction. However, QUIC retransmits the earliest outstanding		if (retransmittable packets are not outstanding):
	packets rather than the latest, because QUIC doesn't have		loss_detection_alarm.cancel();
	retransmission ambiguity. QUIC uses the commonly accepted min RTO of		return
	200ms instead of the 1s the RFC specifies.
	5.2. FACK Loss Recovery (paper)		if (handshake packets are outstanding):
			// Handshake retransmission alarm.
			if (smoothed_rtt == 0):
			alarm_duration = 2 * initial_rtt
			else:
			alarm_duration = 2 * smoothed_rtt
			alarm_duration = max(alarm_duration, kMinTLPTimeout)
			alarm_duration = alarm_duration << handshake_count
			else if (loss_time != 0):
			// Early retransmit timer or time loss detection.
			alarm_duration = loss_time - now
			else if (tlp_count < kMaxTLPs):
			// Tail Loss Probe
			if (retransmittable_packets_outstanding = 1):
			alarm_duration = 1.5 * smoothed_rtt + kDelayedAckTimeout
			else:
			alarm_duration = kMinTLPTimeout
			alarm_duration = max(alarm_duration, 2 * smoothed_rtt)
			else:
			// RTO alarm
			if (rto_count = 0):
			alarm_duration = smoothed_rtt + 4 * rttvar
			alarm_duration = max(alarm_duration, kMinRTOTimeout)
			else:
			alarm_duration = loss_detection_alarm.get_delay() << 1
	QUIC implements the algorithm for early loss recovery described in		loss_detection_alarm.set(now + alarm_duration)
	the FACK paper (and implemented in the Linux kernel.) QUIC uses the
	packet number to measure the FACK reordering threshold. Currently
	QUIC does not implement an adaptive threshold as many TCP
	implementations (i.e., the Linux kernel) do.
	5.3. RFC 3782, RFC 6582 (NewReno Fast Recovery)		3.8. On Alarm Firing
	QUIC only reduces its CWND once per congestion window, in keeping		QUIC uses one loss recovery alarm, which when set, can be in one of
	with the NewReno RFC. It tracks the largest outstanding packet at		several modes. When the alarm fires, the mode determines the action
	the time the loss is declared and any losses which occur before that		to be performed.
	packet number are considered part of the same loss event. It's worth
	noting that some TCP implementations may do this on a sequence number
	basis, and hence consider multiple losses of the same packet a single
	loss event.
	5.4. TLP (draft)		Pseudocode for OnLossDetectionAlarm follows:
	QUIC always sends two tail loss probes before RTO is triggered. QUIC		OnLossDetectionAlarm():
	invokes tail loss probe even when a loss is outstanding, which is		if (handshake packets are outstanding):
	different than some TCP implementations.		// Handshake retransmission alarm.
			RetransmitAllHandshakePackets();
			handshake_count++;
			// TODO: Clarify early retransmit and time loss.
			else if (loss_time != 0):
			// Early retransmit or Time Loss Detection
			DetectLostPackets(largest_acked_packet)
			else if (tlp_count < kMaxTLPs):
			// Tail Loss Probe.
			if (HasNewDataToSend()):
			SendOnePacketOfNewData()
			else:
			RetransmitOldestPacket()
			tlp_count++
			else:
			// RTO.
			RetransmitOldestTwoPackets()
			rto_count++
	5.5. RFC 5827 (Early Retransmit) with Delay Timer		SetLossDetectionAlarm()
	QUIC implements early retransmit with a timer in order to minimize		3.9. Detecting Lost Packets
	spurious retransmits. The timer is set to 1/4 SRTT after the final
	outstanding packet is acked.
	5.6. RFC 5827 (F-RTO)		Packets in QUIC are only considered lost once a larger packet number
			is acknowledged. DetectLostPackets is called every time an ack is
			received. If the loss detection alarm fires and the loss_time is
			set, the previous largest acked packet is supplied.
	QUIC implements F-RTO by not reducing the CWND and SSThresh until a		3.9.1. Handshake Packets
	subsequent ack is received and it's sure the RTO was not spurious.
	Conceptually this is similar, but it makes for a much cleaner
	implementation with fewer edge cases.
	5.7. RFC 6937 (Proportional Rate Reduction)		The receiver MUST ignore unprotected packets that ack protected
			packets. The receiver MUST trust protected acks for unprotected
			packets, however. Aside from this, loss detection for handshake
			packets when an ack is processed is identical to other packets.
	PRR-SSRB is implemented by QUIC in the epoch when recovering from a		3.9.2. Pseudocode
	loss.
	5.8. TCP Cubic (draft) with optional RFC 5681 (Reno)		DetectLostPackets takes one parameter, acked, which is the largest
			acked packet.
	TCP Cubic is the default congestion control algorithm in QUIC. Reno		Pseudocode for DetectLostPackets follows:
	is also an easily available option which may be requested via
	connection options and is fully implemented.
	5.9. Hybrid Slow Start (paper)		DetectLostPackets(largest_acked):
			loss_time = 0
			lost_packets = {}
			delay_until_lost = infinite;
			if (time_reordering_fraction != infinite):
			delay_until_lost =
			(1 + time_reordering_fraction) * max(latest_rtt, smoothed_rtt)
			else if (largest_acked.packet_number == largest_sent_packet):
			// Early retransmit alarm.
			delay_until_lost = 9/8 * max(latest_rtt, smoothed_rtt)
			foreach (unacked less than largest_acked.packet_number):
			time_since_sent = now() - unacked.time_sent
			packet_delta = largest_acked.packet_number - unacked.packet_number
			if (time_since_sent > delay_until_lost):
			lost_packets.insert(unacked)
			else if (packet_delta > reordering_threshold)
			lost_packets.insert(unacked)
			else if (loss_time == 0 && delay_until_lost != infinite):
			loss_time = delay_until_lost - time_since_sent
	QUIC implements hybrid slow start, but disables ack train detection,		// Inform the congestion controller of lost packets and
	because it has shown to falsely trigger when coupled with packet		// lets it decide whether to retransmit immediately.
	pacing, which is also on by default in QUIC. Currently the minimum		OnPacketsLost(lost_packets)
	delay increase is 4ms, the maximum is 16ms, and within that range		foreach (packet in lost_packets)
	QUIC exits slow start if the min_rtt within a round increases by more		sent_packets.remove(packet.packet_number)
	than one eighth of the connection mi
	5.10. RACK (draft)		4. Congestion Control
	QUIC's loss detection is by it's time-ordered nature, very similar to		(describe NewReno-style congestion control [RFC6582] for QUIC.)
	RACK. Though QUIC defaults to loss detection based on reordering		(describe appropriate byte counting.) (define recovery based on
	threshold in packets, it could just as easily be based on fractions		packet numbers.) (describe min_rtt based hystart.) (describe how
	of an rtt, as RACK does.		QUIC's F-RTO [RFC5682] delays reducing CWND.) (describe PRR
			[RFC6937])
	6. IANA Considerations		5. IANA Considerations
	This document has no IANA actions. Yet.		This document has no IANA actions. Yet.
	7. Normative References		6. References
	[QUIC-TLS]		6.1. Normative References
	Thomson, M., Ed. and S. Turner, Ed, Ed., "Using Transport
	Layer Security (TLS) to Secure QUIC".
	[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".		Multiplexed and Secure Transport".
	[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,
	<http://www.rfc-editor.org/info/rfc2119>.		<http://www.rfc-editor.org/info/rfc2119>.
			6.2. Informative References
			[I-D.dukkipati-tcpm-tcp-loss-probe]
			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.
			[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,
			<http://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,
			<http://www.rfc-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,
	<http://www.rfc-editor.org/info/rfc6298>.		<http://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,
			<http://www.rfc-editor.org/info/rfc6582>.
			[RFC6937] Mathis, M., Dukkipati, N., and Y. Cheng, "Proportional
			Rate Reduction for TCP", RFC 6937, DOI 10.17487/RFC6937,
			May 2013, <http://www.rfc-editor.org/info/rfc6937>.
	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-00:		B.1. Since draft-ietf-quic-recovery-01
			o Changes initial default RTT to 100ms
			o Added time-based loss detection and fixes early retransmit
			o Clarified loss recovery for handshake packets
			o Fixed references and made TCP references informative
			B.2. Since draft-ietf-quic-recovery-00:
	o Improved description of constants and ACK behavior		o Improved description of constants and ACK behavior
	B.2. Since draft-iyengar-quic-loss-recovery-01:		B.3. 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. 76 change blocks.
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