Network Working Group J. Iyengar
Internet-Draft I. Swett
Intended status: Experimental Google
Expires: May 4, 2017 October 31, 2016
QUIC Congestion Control And Loss Recovery
draft-iyengar-quic-loss-recovery-01
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.
Status of This Memo
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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 [draft-hamilton-quic-
transport-protocol].
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.)
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 also sent as STREAM data, and uses the
reliability machinery of QUIC underneath.
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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. Variables of interest
We first describe the variables required to implement the loss
detection mechanisms described in this section.
o 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
been retransmitted without receiving an ack.
o tlp_count: The number of times a tail loss probe has been sent
without receiving an ack.
o rto_count: The number of times an rto has been sent without
receiving and ack.
o smoothed_rtt: The smoothed RTT of the connection, computed as
described in [RFC 6298]. TODO: Describe RTT computations.
o reordering_threshold: The largest delta between the largest acked
retransmittable packet and a packet containing retransmittable
frames before it's declared lost.
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o time_loss: When true, loss detection operates solely based on
reordering threshold in time, rather than in packet number gaps.
3.2. 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;
smoothed_rtt = 0;
reordering_threshold = 3;
time_loss = false;
3.3. 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.
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SetLossDetectionAlarm():
if (retransmittable packets are outstanding):
loss_detection_alarm.cancel();
return;
if (handshake packets are outstanding):
alarm_duration = max(1.5 * smoothed_rtt, 10ms) << handshake_count;
handshake_count++;
else if (largest sent packet is acked):
// Set alarm based on short timer for early retransmit.
alarm_duration = 0.25 x smoothed_rtt;
else if (tlp_count < 2):
if (retransmittable_packets_outstanding = 1):
alarm_duration = max(1.5 x smoothed_rtt + delayed_ack_timer,
2 x smoothed_rtt);
else:
alarm_duration = max (10ms, 2 x smoothed_rtt);
tlp_count++;
else:
if (rto_count = 0):
alarm_duration = max(200ms, smoothed_rtt + 4 x rttvar);
else:
alarm_duration = loss_detection_alarm.get_delay() << 1;
rto_count++;
loss_detecton_alarm.set(now + alarm_duration);
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 [draft-
hamilton-quic-protocol]. 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.
Pseudocode for OnPacketSent follows.
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OnPacketSent(packet_number, is_retransmittable):
if is_retransmittable:
SetLossDetectionAlarm()
3.5. 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):
handshake_count = 0;
tlp_count = 0;
rto_count = 0;
UpdateRtt(); // TODO: document RTT estimator.
DetectLostPackets(acked_packet);
SetLossDetectionAlarm();
3.6. 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. OnAlarm returns a list of packet numbers that are
detected as lost.
Pseudocode for OnAlarm follows.
OnAlarm(acked_packet):
lost_packets = DetectLostPackets(acked_packet);
MaybeRetransmitLostPackets();
SetLossDetectionAlarm();
3.7. Detecting Lost Packets
Packets in QUIC are only considered lost once a larger packet number
is acknowledged. DetectLostPackets is called every time there is a
new largest packet or if the loss detection alarm fires the previous
largest acked packet is supplied.
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DetectLostPackets takes one parameter, acked_packet, which is the
packet number of the largest acked packet, and returns a list of
packet numbers detected as lost.
Pseudocode for DetectLostPackets follows.
DetectLostPackets(acked_packet):
lost_packets = {};
foreach (unacked_packet less than acked_packet):
if (unacked_packet.time_sent <
acked_packet.time_sent - 1/8 * 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
(describe NewReno-style congestion control for QUIC.)
5. TCP mechanisms in QUIC
QUIC implements the spirit of a variety of RFCs, Internet drafts, and
other well-known TCP loss recovery mechanisms, though the
implementation details differ from the TCP implementations.
5.1. RFC 6298 (RTO computation)
QUIC calculates SRTT and RTTVAR according to the standard formulas.
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
CWND reduction. However, QUIC retransmits the earliest outstanding
packets rather than the latest, because QUIC doesn't have
retransmission ambiguity. QUIC uses the commonly accepted min RTO of
200ms instead of the 1s the RFC specifies.
5.2. FACK Loss Recovery (paper)
QUIC implements the algorithm for early loss recovery described in
the FACK paper (and implemented in the Linux kernel.) QUIC uses the
packet number to measure the FACK reordering threshold. Currently
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QUIC does not implement an adaptive threshold as many TCP
implementations(ie: the Linux kernel) do.
5.3. RFC 3782, RFC 6582 (NewReno Fast Recovery)
QUIC only reduces its CWND once per congestion window, in keeping
with the NewReno RFC. It tracks the largest outstanding packet at
the time the loss is declared and any losses which occur before that
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)
QUIC always sends two tail loss probes before RTO is triggered. QUIC
invokes tail loss probe even when a loss is outstanding, which is
different than some TCP implementations.
5.5. RFC 5827 (Early Retransmit) with Delay Timer
QUIC implements early retransmit with a timer in order to minimize
spurious retransmits. The timer is set to 1/4 SRTT after the final
outstanding packet is acked.
5.6. RFC 5827 (F-RTO)
QUIC implements F-RTO by not reducing the CWND and SSThresh until a
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)
PRR-SSRB is implemented by QUIC in the epoch when recovering from a
loss.
5.8. TCP Cubic (draft) with optional RFC 5681 (Reno)
TCP Cubic is the default congestion control algorithm in QUIC. Reno
is also an easily available option which may be requested via
connection options and is fully implemented.
5.9. Hybrid Slow Start (paper)
QUIC implements hybrid slow start, but disables ack train detection,
because it has shown to falsely trigger when coupled with packet
pacing, which is also on by default in QUIC. Currently the minimum
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delay increase is 4ms, the maximum is 16ms, and within that range
QUIC exits slow start if the min_rtt within a round increases by more
than ⅛ of the connection mi
5.10. RACK (draft)
QUIC's loss detection is by it's time-ordered nature, very similar to
RACK. Though QUIC defaults to loss detection based on reordering
threshold in packets, it could just as easily be based on fractions
of an rtt, as RACK does.
n
_rtt.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", March 1997.
6.2. Informative References
[draft-hamilton-quic-transport-protocol]
Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
A UDP-Based Multiplexed and Secure Transport", July 2016.
Authors' Addresses
Janardhan Iyengar
Google
Email: jri@google.com
Ian Swett
Google
Email: ianswett@google.com
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