QUIC Loss Detection and Congestion ControlGooglejri@google.comGoogleianswett@google.com
Transport
QUICQUIC 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.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 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.)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 .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.Retransmittable frames are frames requiring reliable delivery. The most
common are STREAM frames, which typically contain application data.Crypto handshake data is also sent as STREAM data, and uses the reliability
machinery of QUIC underneath.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.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.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.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.QUIC supports up to 256 ACK ranges, opposed to TCP’s 3 SACK ranges. In high
loss environments, this speeds recovery.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.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.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.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.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
. TODO: Describe RTT computations.reordering_threshold: The largest delta between the largest acked
retransmittable packet and a packet containing retransmittable frames before
it’s declared lost.time_loss: When true, loss detection operates solely based on reordering
threshold in time, rather than in packet number gaps.At the beginning of the connection, initialize the loss detection variables as
follows:QUIC loss detection uses a single alarm for all timer-based loss detection. The
duration of the alarm is based on the alarm’s mode, which is set in the packet
and timer events further below. The function SetLossDetectionAlarm defined
below shows how the single timer is set based on the alarm mode.Pseudocode for SetLossDetectionAlarm follows: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:packet_number: The packet number of the sent packet.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 . 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: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: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: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.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:(describe NewReno-style congestion control for 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.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.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 QUIC does not implement an
adaptive threshold as many TCP implementations (i.e., the Linux kernel) do.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.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.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.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.PRR-SSRB is implemented by QUIC in the epoch when recovering from a loss.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.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 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 one eighth of the connection miQUIC’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.This document has no IANA actions. Yet.Using Transport Layer Security (TLS) to Secure QUICMozillasn3rdQUIC: A UDP-Based Multiplexed and Secure TransportGoogleMozillaKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Computing TCP's Retransmission TimerThis document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. This document obsoletes RFC 2988. [STANDARDS-TRACK]