QUIC J. Iyengar, Ed.
Internet-Draft I. Swett, Ed.
Intended status: Standards Track Google
Expires: September 14, 2017 March 13, 2017
QUIC Loss Detection and Congestion Control
draft-ietf-quic-recovery-02
Abstract
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. QUIC implements the spirit of known TCP loss detection
mechanisms, described in RFCs, various Internet-drafts, and also
those prevalent in the Linux TCP implementation. This document
describes QUIC loss detection and congestion control, and attributes
the TCP equivalent in RFCs, Internet-drafts, academic papers, and TCP
implementations.
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 September 14, 2017.
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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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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. Constants of interest . . . . . . . . . . . . . . . . . . 5
3.2. Variables of interest . . . . . . . . . . . . . . . . . . 6
3.3. Initialization . . . . . . . . . . . . . . . . . . . . . 7
3.4. On Sending a Packet . . . . . . . . . . . . . . . . . . . 7
3.5. On Ack Receipt . . . . . . . . . . . . . . . . . . . . . 8
3.6. On Packet Acknowledgment . . . . . . . . . . . . . . . . 8
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
3.9. Detecting Lost Packets . . . . . . . . . . . . . . . . . 11
3.9.1. Handshake Packets . . . . . . . . . . . . . . . . . . 11
3.9.2. Pseudocode . . . . . . . . . . . . . . . . . . . . . 11
4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 13
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13
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B.1. Since draft-ietf-quic-recovery-01 . . . . . . . . . . . . 14
B.2. Since draft-ietf-quic-recovery-00: . . . . . . . . . . . 14
B.3. Since draft-iyengar-quic-loss-recovery-01: . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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.
This document first describes pre-requisite parts of the QUIC
transmission machinery, then discusses QUIC's default congestion
control and loss detection mechanisms, and finally lists the various
TCP mechanisms that QUIC loss detection implements (in spirit.)
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.
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o Crypto handshake data is also sent as STREAM data, and uses the
reliability machinery of QUIC underneath.
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
There are some notable differences between QUIC and TCP which are
important for reasoning about the differences between the loss
recovery mechanisms employed by the two protocols. We briefly
describe these 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.
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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
We now describe QUIC's loss detection as functions that should be
called on packet transmission, when a packet is acked, and timer
expiration events.
3.1. Constants of interest
Constants used in loss recovery and 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.
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
sapce 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.
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kDefaultInitialRtt (default 100ms): The default RTT used before an
RTT sample is taken.
3.2. Variables of interest
We first describe the variables required to implement the loss
detection mechanisms described in this section.
loss_detection_alarm: Multi-modal alarm used for loss detection.
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.
smoothed_rtt: The smoothed RTT of the connection, computed as
described in [RFC6298]
rttvar: The RTT variance, computed as described in [RFC6298]
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
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.
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3.3. Initialization
At the beginning of the connection, initialize the loss detection
variables as follows:
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
initial_rtt = kDefaultInitialRtt
3.4. 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_retransmittble: 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):
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()
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3.5. On Ack Receipt
When an ack is received, it may acknowledge 0 or more packets.
Pseudocode for OnAckReceived and UpdateRtt follow:
OnAckReceived(ack):
// If the largest acked is newly acked, update the RTT.
if (sent_packets[ack.largest_acked]):
rtt_sample = now - sent_packets[ack.largest_acked].time
if (rtt_sample > ack.ack_delay):
rtt_sample -= ack.delay
UpdateRtt(rtt_sample)
// Find all newly acked packets.
for acked_packet_number in DetermineNewlyAckedPackets():
OnPacketAcked(acked_packet_number)
DetectLostPackets(ack.largest_acked_packet)
SetLossDetectionAlarm()
UpdateRtt(rtt_sample):
// Based on {{RFC6298}}.
if (smoothed_rtt == 0):
smoothed_rtt = rtt_sample
rttvar = rtt_sample / 2
else:
rttvar = 3/4 * rttvar + 1/4 * (smoothed_rtt - rtt_sample)
smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * rtt_sample
3.6. 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.
Pseudocode for OnPacketAcked follows:
OnPacketAcked(acked_packet_number):
handshake_count = 0
tlp_count = 0
rto_count = 0
sent_packets.remove(acked_packet_number)
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3.7. 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.
3.7.1. 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. Once an RTT measurement is taken, it
MUST replace initial_rtt.
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.
3.7.2. Tail Loss Probe and Retransmission Timeout
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.
3.7.3. 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.
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3.7.4. 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 * 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
loss_detection_alarm.set(now + alarm_duration)
3.8. 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:
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OnLossDetectionAlarm():
if (handshake packets are outstanding):
// 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++
SetLossDetectionAlarm()
3.9. 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.
3.9.1. 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.
3.9.2. Pseudocode
DetectLostPackets takes one parameter, acked, which is the largest
acked packet.
Pseudocode for DetectLostPackets follows:
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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
// Inform the congestion controller of lost packets and
// lets it decide whether to retransmit immediately.
OnPacketsLost(lost_packets)
foreach (packet in lost_packets)
sent_packets.remove(packet.packet_number)
4. Congestion Control
(describe NewReno-style congestion control [RFC6582] for QUIC.)
(describe appropriate byte counting.) (define recovery based on
packet numbers.) (describe min_rtt based hystart.) (describe how
QUIC's F-RTO [RFC5682] delays reducing CWND.) (describe PRR
[RFC6937])
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".
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[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
[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,
"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>.
[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 B. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
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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
B.3. 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
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