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
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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
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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):
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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
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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)
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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)
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