[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: (draft-iyengar-quic-loss-recovery)
00 01 02 03 04 05 06 07 08 09 10 11
12 13 14 15 16 17 18 19 20 21 22
QUIC J. Iyengar, Ed.
Internet-Draft I. Swett, Ed.
Intended status: Standards Track Google
Expires: December 15, 2017 June 13, 2017
QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-04
Abstract
This document describes loss detection and congestion control
mechanisms for QUIC.
Note to Readers
Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/search/?email_list=quic.
Working Group information can be found at https://github.com/quicwg;
source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/recovery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 15, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
Iyengar & Swett Expires December 15, 2017 [Page 1]
Internet-Draft QUIC Loss Detection June 2017
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Design of the QUIC Transmission Machinery . . . . . . . . . . 3
2.1. Relevant Differences Between QUIC and TCP . . . . . . . . 4
2.1.1. Monotonically Increasing Packet Numbers . . . . . . . 4
2.1.2. No Reneging . . . . . . . . . . . . . . . . . . . . . 4
2.1.3. More ACK Ranges . . . . . . . . . . . . . . . . . . . 5
2.1.4. Explicit Correction For Delayed Acks . . . . . . . . 5
3. Loss Detection . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Algorithm Details . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Constants of interest . . . . . . . . . . . . . . . . 6
3.2.2. Variables of interest . . . . . . . . . . . . . . . . 6
3.2.3. Initialization . . . . . . . . . . . . . . . . . . . 7
3.2.4. On Sending a Packet . . . . . . . . . . . . . . . . . 8
3.2.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . 8
3.2.6. On Packet Acknowledgment . . . . . . . . . . . . . . 9
3.2.7. Setting the Loss Detection Alarm . . . . . . . . . . 10
3.2.8. On Alarm Firing . . . . . . . . . . . . . . . . . . . 12
3.2.9. Detecting Lost Packets . . . . . . . . . . . . . . . 12
3.3. Discussion . . . . . . . . . . . . . . . . . . . . . . . 13
4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 14
4.1. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. Recovery . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Constants of interest . . . . . . . . . . . . . . . . . . 14
4.4. Variables of interest . . . . . . . . . . . . . . . . . . 14
4.5. Initialization . . . . . . . . . . . . . . . . . . . . . 15
4.6. On Packet Acknowledgement . . . . . . . . . . . . . . . . 15
4.7. On Packets Lost . . . . . . . . . . . . . . . . . . . . . 15
4.8. On Retransmission Timeout Verified . . . . . . . . . . . 16
4.9. Pacing Packets . . . . . . . . . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Normative References . . . . . . . . . . . . . . . . . . 16
6.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 17
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 17
B.1. Since draft-ietf-quic-recovery-02 . . . . . . . . . . . . 17
Iyengar & Swett Expires December 15, 2017 [Page 2]
Internet-Draft QUIC Loss Detection June 2017
B.2. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 18
B.3. Since draft-ietf-quic-recovery-00 . . . . . . . . . . . . 18
B.4. Since draft-iyengar-quic-loss-recovery-01 . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
QUIC is a new multiplexed and secure transport atop UDP. QUIC builds
on decades of transport and security experience, and implements
mechanisms that make it attractive as a modern general-purpose
transport. The QUIC protocol is described in [QUIC-TRANSPORT].
QUIC implements the spirit of known TCP loss recovery mechanisms,
described in RFCs, various Internet-drafts, and also those prevalent
in the Linux TCP implementation. This document describes QUIC
congestion control and loss recovery, and where applicable,
attributes the TCP equivalent in RFCs, Internet-drafts, academic
papers, and/or TCP implementations.
1.1. Notational Conventions
The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
document. It's not shouting; when they are capitalized, they have
the special meaning defined in [RFC2119].
2. Design of the QUIC Transmission Machinery
All transmissions in QUIC are sent with a packet-level header, which
includes a packet sequence number (referred to below as a packet
number). These packet numbers never repeat in the lifetime of a
connection, and are monotonically increasing, which makes duplicate
detection trivial. This fundamental design decision obviates the
need for disambiguating between transmissions and retransmissions and
eliminates significant complexity from QUIC's interpretation of TCP
loss detection mechanisms.
Every packet may contain several frames. We outline the frames that
are important to the loss detection and congestion control machinery
below.
o Retransmittable frames are frames requiring reliable delivery.
The most common are STREAM frames, which typically contain
application data.
o Crypto handshake data is sent on stream 0, and uses the
reliability machinery of QUIC underneath.
Iyengar & Swett Expires December 15, 2017 [Page 3]
Internet-Draft QUIC Loss Detection June 2017
o ACK frames contain acknowledgment information. QUIC uses a SACK-
based scheme, where acks express up to 256 ranges. The ACK frame
also includes a receive timestamp for each packet newly acked.
2.1. Relevant Differences Between QUIC and TCP
Readers familiar with TCP's loss detection and congestion control
will find algorithms here that parallel well-known TCP ones.
Protocol differences between QUIC and TCP however contribute to
algorithmic differences. We briefly describe these protocol
differences below.
2.1.1. Monotonically Increasing Packet Numbers
TCP conflates transmission sequence number at the sender with
delivery sequence number at the receiver, which results in
retransmissions of the same data carrying the same sequence number,
and consequently to problems caused by "retransmission ambiguity".
QUIC separates the two: QUIC uses a packet sequence number (referred
to as the "packet number") for transmissions, and any data that is to
be delivered to the receiving application(s) is sent in one or more
streams, with stream offsets encoded within STREAM frames inside of
packets that determine delivery order.
QUIC's packet number is strictly increasing, and directly encodes
transmission order. A higher QUIC packet number signifies that the
packet was sent later, and a lower QUIC packet number signifies that
the packet was sent earlier. When a packet containing frames is
deemed lost, QUIC rebundles necessary frames in a new packet with a
new packet number, removing ambiguity about which packet is
acknowledged when an ACK is received. Consequently, more accurate
RTT measurements can be made, spurious retransmissions are trivially
detected, and mechanisms such as Fast Retransmit can be applied
universally, based only on packet number.
This design point significantly simplifies loss detection mechanisms
for QUIC. Most TCP mechanisms implicitly attempt to infer
transmission ordering based on TCP sequence numbers - a non-trivial
task, especially when TCP timestamps are not available.
2.1.2. No Reneging
QUIC ACKs contain information that is equivalent to TCP SACK, but
QUIC does not allow any acked packet to be reneged, greatly
simplifying implementations on both sides and reducing memory
pressure on the sender.
Iyengar & Swett Expires December 15, 2017 [Page 4]
Internet-Draft QUIC Loss Detection June 2017
2.1.3. More ACK Ranges
QUIC supports up to 256 ACK ranges, opposed to TCP's 3 SACK ranges.
In high loss environments, this speeds recovery.
2.1.4. Explicit Correction For Delayed Acks
QUIC ACKs explicitly encode the delay incurred at the receiver
between when a packet is received and when the corresponding ACK is
sent. This allows the receiver of the ACK to adjust for receiver
delays, specifically the delayed ack timer, when estimating the path
RTT. This mechanism also allows a receiver to measure and report the
delay from when a packet was received by the OS kernel, which is
useful in receivers which may incur delays such as context-switch
latency before a userspace QUIC receiver processes a received packet.
3. Loss Detection
3.1. Overview
QUIC uses a combination of ack information and alarms to detect lost
packets. An unacknowledged QUIC packet is marked as lost in one of
the following ways:
o A packet is marked as lost if at least one packet that was sent a
threshold number of packets (kReorderingThreshold) after it has
been acknowledged. This indicates that the unacknowledged packet
is either lost or reordered beyond the specified threshold. This
mechanism combines both TCP's FastRetransmit and FACK mechanisms.
o If a packet is near the tail, where fewer than
kReorderingThreshold packets are sent after it, the sender cannot
expect to detect loss based on the previous mechanism. In this
case, a sender uses both ack information and an alarm to detect
loss. Specifically, when the last sent packet is acknowledged,
the sender waits a short period of time to allow for reordering
and then marks any unacknowledged packets as lost. This mechanism
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
it, and the sender cannot use ack information to detect its loss.
The sender therefore relies on an alarm to detect such tail
losses. This mechanism is based on TCP's Tail Loss Probe.
o If all else fails, a Retransmission Timeout (RTO) alarm is always
set when any retransmittable packet is outstanding. When this
alarm fires, all unacknowledged packets are marked as lost.
Iyengar & Swett Expires December 15, 2017 [Page 5]
Internet-Draft QUIC Loss Detection June 2017
o Instead of a packet threshold to tolerate reordering, a QUIC
sender may use a time threshold. This allows for senders to be
tolerant of short periods of significant reordering. In this
mechanism, a QUIC sender marks a packet as lost when a packet
larger than it is acknowledged and a threshold amount of time has
passed since the packet was sent.
o Handshake packets, which contain STREAM frames for stream 0, are
critical to QUIC transport and crypto negotiation, so a separate
alarm period is used for them.
3.2. Algorithm Details
3.2.1. Constants of interest
Constants used in loss recovery are based on a combination of RFCs,
papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments.
kMaxTLPs (default 2): Maximum number of tail loss probes before an
RTO fires.
kReorderingThreshold (default 3): Maximum reordering in packet
number space before FACK style loss detection considers a packet
lost.
kTimeReorderingFraction (default 1/8): Maximum reordering in time
space before time based loss detection considers a packet lost.
In fraction of an RTT.
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.
kDelayedAckTimeout (default 25ms): The length of the peer's delayed
ack timer.
kDefaultInitialRtt (default 100ms): The default RTT used before an
RTT sample is taken.
3.2.2. Variables of interest
Variables required to implement the congestion control mechanisms are
described in this section.
loss_detection_alarm: Multi-modal alarm used for loss detection.
Iyengar & Swett Expires December 15, 2017 [Page 6]
Internet-Draft QUIC Loss Detection June 2017
handshake_count: The number of times the handshake packets have been
retransmitted without receiving an ack.
tlp_count: The number of times a tail loss probe has been sent
without receiving an ack.
rto_count: The number of times an rto has been sent without
receiving an ack.
largest_sent_before_rto: The last packet number sent prior to the
first retransmission timeout.
time_of_last_sent_packet: The time the most recent packet was sent.
latest_rtt: The most recent RTT measurement made when receiving an
ack for a previously unacked packet.
smoothed_rtt: The smoothed RTT of the connection, computed as
described in [RFC6298]
rttvar: The RTT variance, computed as described in [RFC6298]
reordering_threshold: The largest delta between the largest acked
retransmittable packet and a packet containing retransmittable
frames before it's declared lost.
time_reordering_fraction: The reordering window as a fraction of
max(smoothed_rtt, latest_rtt).
loss_time: The time at which the next packet will be considered lost
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.
3.2.3. Initialization
At the beginning of the connection, initialize the loss detection
variables as follows:
Iyengar & Swett Expires December 15, 2017 [Page 7]
Internet-Draft QUIC Loss Detection June 2017
loss_detection_alarm.reset()
handshake_count = 0
tlp_count = 0
rto_count = 0
if (UsingTimeLossDetection())
reordering_threshold = infinite
time_reordering_fraction = kTimeReorderingFraction
else:
reordering_threshold = kReorderingThreshold
time_reordering_fraction = infinite
loss_time = 0
smoothed_rtt = 0
rttvar = 0
largest_sent_before_rto = 0
time_of_last_sent_packet = 0
3.2.4. On Sending a Packet
After any packet is sent, be it a new transmission or a rebundled
transmission, the following OnPacketSent function is called. The
parameters to OnPacketSent are as follows:
o packet_number: The packet number of the sent packet.
o is_retransmittable: A boolean that indicates whether the packet
contains at least one frame requiring reliable deliver. The
retransmittability of various QUIC frames is described in
[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.
Pseudocode for OnPacketSent follows:
OnPacketSent(packet_number, is_retransmittable, sent_bytes):
time_of_last_sent_packet = now;
sent_packets[packet_number].packet_number = packet_number
sent_packets[packet_number].time = now
if is_retransmittable:
sent_packets[packet_number].bytes = sent_bytes
SetLossDetectionAlarm()
3.2.5. On Ack Receipt
When an ack is received, it may acknowledge 0 or more packets.
Pseudocode for OnAckReceived and UpdateRtt follow:
Iyengar & Swett Expires December 15, 2017 [Page 8]
Internet-Draft QUIC Loss Detection June 2017
OnAckReceived(ack):
// If the largest acked is newly acked, update the RTT.
if (sent_packets[ack.largest_acked]):
latest_rtt = now - sent_packets[ack.largest_acked].time
if (latest_rtt > ack.ack_delay):
latest_rtt -= ack.delay
UpdateRtt(latest_rtt)
// Find all newly acked packets.
for acked_packet in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet.packet_number)
DetectLostPackets(ack.largest_acked_packet)
SetLossDetectionAlarm()
UpdateRtt(latest_rtt):
// Based on {{RFC6298}}.
if (smoothed_rtt == 0):
smoothed_rtt = latest_rtt
rttvar = latest_rtt / 2
else:
rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - latest_rtt)
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * latest_rtt
3.2.6. On Packet Acknowledgment
When a packet is acked for the first time, the following
OnPacketAcked function is called. Note that a single ACK frame may
newly acknowledge several packets. OnPacketAcked must be called once
for each of these newly acked packets.
OnPacketAcked takes one parameter, acked_packet, which is the packet
number of the newly acked packet, and returns a list of packet
numbers that are detected as lost.
If this is the first acknowledgement following RTO, check if the
smallest newly acknowledged packet is one sent by the RTO, and if so,
inform congestion control of a verified RTO, similar to F-RTO
[RFC5682]
Pseudocode for OnPacketAcked follows:
Iyengar & Swett Expires December 15, 2017 [Page 9]
Internet-Draft QUIC Loss Detection June 2017
OnPacketAcked(acked_packet_number):
// If a packet sent prior to RTO was acked, then the RTO
// was spurious. Otherwise, inform congestion control.
if (rto_count > 0 &&
acked_packet_number > largest_sent_before_rto)
OnRetransmissionTimeoutVerified()
handshake_count = 0
tlp_count = 0
rto_count = 0
sent_packets.remove(acked_packet_number)
3.2.7. 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.
3.2.7.1. Handshake 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.
Endpoints MUST retransmit handshake frames if not acknowledged within
a time limit. This time limit will start as the largest of twice the
rtt value and MinTLPTimeout. Each consecutive handshake
retransmission doubles the time limit, until an acknowledgement is
received.
Handshake frames may be cancelled by handshake state transitions. In
particular, all non-protected frames SHOULD be no longer be
transmitted once packet protection is available.
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.
Version negotiation packets are always stateless, and MUST be sent
once per per handshake packet that uses an unsupported QUIC version,
and MAY be sent in response to 0RTT packets.
Iyengar & Swett Expires December 15, 2017 [Page 10]
Internet-Draft QUIC Loss Detection June 2017
3.2.7.2. Tail Loss Probe and Retransmission Timeout
Tail loss probes [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.
3.2.7.3. Early Retransmit
Early retransmit [RFC5827] is implemented with a 1/4 RTT timer. It
is part of QUIC's time based loss detection, but is always enabled,
even when only packet reordering loss detection is enabled.
3.2.7.4. Pseudocode
Pseudocode for SetLossDetectionAlarm follows:
SetLossDetectionAlarm():
if (retransmittable packets are not outstanding):
loss_detection_alarm.cancel()
return
if (handshake packets are outstanding):
// Handshake retransmission alarm.
if (smoothed_rtt == 0):
alarm_duration = 2 * kDefaultInitialRtt
else:
alarm_duration = 2 * smoothed_rtt
alarm_duration = max(alarm_duration, kMinTLPTimeout)
alarm_duration = alarm_duration * (2 ^ 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
alarm_duration = smoothed_rtt + 4 * rttvar
alarm_duration = max(alarm_duration, kMinRTOTimeout)
alarm_duration = alarm_duration * (2 ^ rto_count)
loss_detection_alarm.set(now + alarm_duration)
Iyengar & Swett Expires December 15, 2017 [Page 11]
Internet-Draft QUIC Loss Detection June 2017
3.2.8. On Alarm Firing
QUIC uses one loss recovery alarm, which when set, can be in one of
several modes. When the alarm fires, the mode determines the action
to be performed.
Pseudocode for OnLossDetectionAlarm follows:
OnLossDetectionAlarm():
if (handshake packets are outstanding):
// Handshake retransmission alarm.
RetransmitAllHandshakePackets()
handshake_count++
else if (loss_time != 0):
// Early retransmit or Time Loss Detection
DetectLostPackets(largest_acked_packet)
else if (tlp_count < kMaxTLPs):
// Tail Loss Probe.
SendOnePacket()
tlp_count++
else:
// RTO.
if (rto_count == 0)
largest_sent_before_rto = largest_sent_packet
SendTwoPackets()
rto_count++
SetLossDetectionAlarm()
3.2.9. Detecting Lost Packets
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.
3.2.9.1. Handshake Packets
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.
3.2.9.2. Pseudocode
DetectLostPackets takes one parameter, acked, which is the largest
acked packet.
Iyengar & Swett Expires December 15, 2017 [Page 12]
Internet-Draft QUIC Loss Detection June 2017
Pseudocode for DetectLostPackets follows:
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 < 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 = now() + delay_until_lost - time_since_sent
// Inform the congestion controller of lost packets and
// lets it decide whether to retransmit immediately.
if (!lost_packets.empty())
OnPacketsLost(lost_packets)
foreach (packet in lost_packets)
sent_packets.remove(packet.packet_number)
3.3. Discussion
The majority of constants were derived from best common practices
among widely deployed TCP implementations on the internet.
Exceptions follow.
A shorter delayed ack time of 25ms was chosen because longer delayed
acks can delay loss recovery and for the small number of connections
where less than packet per 25ms is delivered, acking every packet is
beneficial to congestion control and loss recovery.
The default initial RTT of 100ms was chosen because it is slightly
higher than both the median and mean min_rtt typically observed on
the public internet.
Iyengar & Swett Expires December 15, 2017 [Page 13]
Internet-Draft QUIC Loss Detection June 2017
4. Congestion Control
QUIC's congestion control is based on TCP NewReno[RFC6582] congestion
control to determine the congestion window and pacing rate.
4.1. Slow Start
QUIC begins every connection in slow start and exits slow start upon
loss. While in slow start, QUIC increases the congestion window by
the number of acknowledged bytes when each ack is processed.
4.2. Recovery
Recovery is a period of time beginning with detection of a lost
packet. It ends when all packets outstanding at the time recovery
began have been acknowledged or lost. During recovery, the
congestion window is not increased or decreased.
4.3. Constants of interest
Constants used in congestion control are based on a combination of
RFCs, papers, and common practice. Some may need to be changed or
negotiated in order to better suit a variety of environments.
kDefaultMss (default 1460 bytes): The default max packet size used
for calculating default and minimum congestion windows.
kInitialWindow (default 10 * kDefaultMss): Default limit on the
amount of outstanding data in bytes.
kMinimumWindow (default 2 * kDefaultMss): Default minimum congestion
window.
kLossReductionFactor (default 0.5): Reduction in congestion window
when a new loss event is detected.
4.4. Variables of interest
Variables required to implement the congestion control mechanisms are
described in this section.
bytes_in_flight: The sum of the size in bytes of all sent packets
that contain at least one retransmittable frame, and have not been
acked or declared lost.
congestion_window: Maximum number of bytes in flight that may be
sent.
Iyengar & Swett Expires December 15, 2017 [Page 14]
Internet-Draft QUIC Loss Detection June 2017
end_of_recovery: The packet number after which QUIC will no longer
be in recovery.
ssthresh Slow start threshold in bytes. When the congestion window
is below ssthresh, it grows by the number of bytes acknowledged
for each ack.
4.5. Initialization
At the beginning of the connection, initialize the loss detection
variables as follows:
congestion_window = kInitialWindow
bytes_in_flight = 0
end_of_recovery = 0
ssthresh = infinite
4.6. On Packet Acknowledgement
Invoked at the same time loss detection's OnPacketAcked is called and
supplied with the acked_packet from sent_packets.
Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet):
if (acked_packet.packet_number < end_of_recovery):
return
if (congestion_window < ssthresh):
congestion_window += acket_packets.bytes
else:
congestion_window +=
acked_packets.bytes / congestion_window
4.7. On Packets Lost
Invoked by loss detection from DetectLostPackets when new packets are
detected lost.
OnPacketsLost(lost_packets):
largest_lost_packet = lost_packets.last()
// Start a new recovery epoch if the lost packet is larger
// than the end of the previous recovery epoch.
if (end_of_recovery < largest_lost_packet.packet_number):
end_of_recovery = largest_sent_packet
congestion_window *= kLossReductionFactor
congestion_window = max(congestion_window, kMinimumWindow)
ssthresh = congestion_window
Iyengar & Swett Expires December 15, 2017 [Page 15]
Internet-Draft QUIC Loss Detection June 2017
4.8. On Retransmission Timeout Verified
QUIC decreases the congestion window to the minimum value once the
retransmission timeout has been confirmed to not be spurious when the
first post-RTO acknowledgement is processed.
OnRetransmissionTimeoutVerified()
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
This document has no IANA actions. Yet.
6. References
6.1. Normative References
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport (work in progress), June 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
6.2. Informative References
Iyengar & Swett Expires December 15, 2017 [Page 16]
Internet-Draft QUIC Loss Detection June 2017
[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,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<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>.
Appendix A. Acknowledgments
Appendix B. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
B.1. Since draft-ietf-quic-recovery-02
o Integrate F-RTO (#544, #409)
o Add congestion control (#545, #395)
o Require connection abort if a skipped packet was acknowledged
(#415)
o Simplify RTO calculations (#142, #417)
Iyengar & Swett Expires December 15, 2017 [Page 17]
Internet-Draft QUIC Loss Detection June 2017
B.2. Since draft-ietf-quic-recovery-01
o Overview added to loss detection
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.3. Since draft-ietf-quic-recovery-00
o Improved description of constants and ACK behavior
B.4. Since draft-iyengar-quic-loss-recovery-01
o Adopted as base for draft-ietf-quic-recovery
o Updated authors/editors list
o Added table of contents
Authors' Addresses
Jana Iyengar (editor)
Google
Email: jri@google.com
Ian Swett (editor)
Google
Email: ianswett@google.com
Iyengar & Swett Expires December 15, 2017 [Page 18]
Html markup produced by rfcmarkup 1.129c, available from
https://tools.ietf.org/tools/rfcmarkup/