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// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
use crate::{
counter::Counter,
event::builder::SlowStartExitCause,
random,
recovery::{
congestion_controller::{self, CongestionController, Publisher},
cubic::{FastRetransmission::*, State::*},
hybrid_slow_start::HybridSlowStart,
pacing::Pacer,
RttEstimator,
},
time::Timestamp,
};
use core::{
cmp::{max, min},
time::Duration,
};
#[cfg(not(feature = "std"))]
use num_traits::Float as _;
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3
//# New Path or +------------+
//# persistent congestion | Slow |
//# (O)---------------------->| Start |
//# +------------+
//# |
//# Loss or |
//# ECN-CE increase |
//# v
//# +------------+ Loss or +------------+
//# | Congestion | ECN-CE increase | Recovery |
//# | Avoidance |------------------>| Period |
//# +------------+ +------------+
//# ^ |
//# | |
//# +----------------------------+
//# Acknowledgment of packet
//# sent during recovery
// This implementation uses Hybrid Slow Start, which allows for
// Slow Start to exit directly to Congestion Avoidance.
#[derive(Clone, Debug, PartialEq, Eq)]
enum State {
SlowStart,
Recovery(Timestamp, FastRetransmission),
CongestionAvoidance(CongestionAvoidanceTiming),
}
impl State {
/// Returns State::CongestionAvoidance initialized with the given `start_time`
fn congestion_avoidance(start_time: Timestamp) -> Self {
Self::CongestionAvoidance(CongestionAvoidanceTiming {
start_time,
window_increase_time: start_time,
app_limited_time: None,
})
}
/// Called when app limited after sending has completed for a round and an ACK has been received.
fn on_app_limited(&mut self, timestamp: Timestamp) {
if let CongestionAvoidance(ref mut timing) = self {
debug_assert!(
timing
.app_limited_time
.map_or(true, |app_limited_time| timestamp >= app_limited_time),
"timestamp must be monotonically increasing"
);
debug_assert!(
timestamp >= timing.window_increase_time,
"timestamp must not be before the window was last increased"
);
timing.app_limited_time = Some(timestamp);
}
}
/// Returns true if the state is `SlowStart`
fn is_slow_start(&self) -> bool {
matches!(self, SlowStart)
}
}
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.2
//# If the congestion window is reduced immediately, a
//# single packet can be sent prior to reduction. This speeds up loss
//# recovery if the data in the lost packet is retransmitted and is
//# similar to TCP as described in Section 5 of [RFC6675].
#[derive(Clone, Debug, PartialEq, Eq)]
enum FastRetransmission {
Idle,
RequiresTransmission,
}
#[derive(Clone, Debug, PartialEq, Eq)]
struct CongestionAvoidanceTiming {
// The time the congestion avoidance state was entered
start_time: Timestamp,
// The time the congestion window was last increased
window_increase_time: Timestamp,
// The last time the current congestion window was underutilized
app_limited_time: Option<Timestamp>,
}
impl CongestionAvoidanceTiming {
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# t is the elapsed time from the beginning of the current congestion avoidance
fn t(&self, timestamp: Timestamp) -> Duration {
timestamp - self.start_time
}
/// Called when the congestion window is being increased.
///
/// Adjusts the start time by the period from the last window increase to the app limited time
/// to avoid counting the app limited time period in W_cubic.
fn on_window_increase(&mut self, timestamp: Timestamp) {
if let Some(app_limited_time) = self.app_limited_time.take() {
//= https://www.rfc-editor.org/rfc/rfc8312#section-5.8
//# CUBIC does not raise its congestion window size if the flow is
//# currently limited by the application instead of the congestion
//# window. In case of long periods when cwnd has not been updated due
//# to the application rate limit, such as idle periods, t in Eq. 1 MUST
//# NOT include these periods; otherwise, W_cubic(t) might be very high
//# after restarting from these periods.
// Adjust the congestion avoidance start time by the app limited duration
self.start_time += app_limited_time - self.window_increase_time;
}
self.window_increase_time = timestamp;
}
}
/// A congestion controller that implements "CUBIC for Fast Long-Distance Networks"
/// as specified in <https://tools.ietf.org/html/rfc8312>. The Hybrid Slow Start algorithm
/// is used for determining the slow start threshold.
#[derive(Clone, Debug)]
pub struct CubicCongestionController {
cubic: Cubic,
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.8
//# CUBIC MUST employ a slow-start algorithm, when the cwnd is no more
//# than ssthresh.
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.8
//# Among the slow-start algorithms, CUBIC MAY choose the
//# standard TCP slow start [RFC5681] in general networks, or the limited
//# slow start [RFC3742] or hybrid slow start [HR08] for fast and long-
//# distance networks.
slow_start: HybridSlowStart,
pacer: Pacer,
max_datagram_size: u16,
congestion_window: f32,
state: State,
//= https://www.rfc-editor.org/rfc/rfc9002#appendix-B.2
//# The sum of the size in bytes of all sent packets
//# that contain at least one ack-eliciting or PADDING frame and have
//# not been acknowledged or declared lost. The size does not include
//# IP or UDP overhead, but does include the QUIC header and
//# Authenticated Encryption with Associated Data (AEAD) overhead.
//# Packets only containing ACK frames do not count toward
//# bytes_in_flight to ensure congestion control does not impede
//# congestion feedback.
// ACK + PADDING packets do not contribute to bytes_in_flight
// See https://github.com/aws/s2n-quic/pull/1514
bytes_in_flight: BytesInFlight,
time_of_last_sent_packet: Option<Timestamp>,
under_utilized: bool,
// The highest number of bytes in flight seen when an ACK was received,
// since the last congestion event.
bytes_in_flight_hi: BytesInFlight,
}
type BytesInFlight = Counter<u32>;
impl CongestionController for CubicCongestionController {
type PacketInfo = ();
#[inline]
fn congestion_window(&self) -> u32 {
self.congestion_window as u32
}
#[inline]
fn bytes_in_flight(&self) -> u32 {
*self.bytes_in_flight
}
#[inline]
fn is_congestion_limited(&self) -> bool {
let available_congestion_window = self
.congestion_window()
.saturating_sub(*self.bytes_in_flight);
available_congestion_window < self.max_datagram_size as u32
}
#[inline]
fn requires_fast_retransmission(&self) -> bool {
matches!(self.state, Recovery(_, RequiresTransmission))
}
#[inline]
fn on_packet_sent<Pub: Publisher>(
&mut self,
time_sent: Timestamp,
bytes_sent: usize,
app_limited: Option<bool>,
rtt_estimator: &RttEstimator,
publisher: &mut Pub,
) {
if bytes_sent == 0 {
// Packet was not congestion controlled
return;
}
self.bytes_in_flight
.try_add(bytes_sent)
.expect("bytes sent should not exceed u32::MAX");
if let Some(app_limited) = app_limited {
// We check both the given `app_limited` value and is_congestion_window_under_utilized()
// as is_congestion_window_under_utilized() is more lenient with respect to the utilization
// of the congestion window than the app_limited check. is_congestion_window_under_utilized()
// returns true if there are more than 3 MTU's of space left in the cwnd, or less than
// half the cwnd is utilized in slow start.
self.under_utilized = app_limited && self.is_congestion_window_under_utilized();
} else {
// We don't externally determine `app_limited` in the Initial and Handshake packet
// spaces, so set under_utilized based on is_congestion_window_under_utilized alone
self.under_utilized = self.is_congestion_window_under_utilized();
}
if let Recovery(recovery_start_time, RequiresTransmission) = self.state {
// A packet has been sent since we entered recovery (fast retransmission)
// so flip the state back to idle.
self.state = Recovery(recovery_start_time, Idle);
}
self.time_of_last_sent_packet = Some(time_sent);
self.pacer.on_packet_sent(
time_sent,
bytes_sent,
rtt_estimator,
self.congestion_window(),
self.max_datagram_size,
self.state.is_slow_start(),
publisher,
);
}
#[inline]
fn on_rtt_update<Pub: Publisher>(
&mut self,
time_sent: Timestamp,
now: Timestamp,
rtt_estimator: &RttEstimator,
publisher: &mut Pub,
) {
// Update the Slow Start algorithm each time the RTT
// estimate is updated to find the slow start threshold.
self.slow_start.on_rtt_update(
self.congestion_window,
time_sent,
self.time_of_last_sent_packet
.expect("At least one packet must be sent to update RTT"),
rtt_estimator.latest_rtt(),
);
if self.state.is_slow_start() && self.congestion_window >= self.slow_start.threshold {
publisher.on_slow_start_exited(SlowStartExitCause::Rtt, self.congestion_window());
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.8
//# In the case when CUBIC runs the hybrid slow start [HR08], it may exit
//# the first slow start without incurring any packet loss and thus W_max
//# is undefined. In this special case, CUBIC switches to congestion
//# avoidance and increases its congestion window size using Eq. 1, where
//# t is the elapsed time since the beginning of the current congestion
//# avoidance, K is set to 0, and W_max is set to the congestion window
//# size at the beginning of the current congestion avoidance.
self.state = State::congestion_avoidance(now);
self.cubic.on_slow_start_exit(self.congestion_window);
}
}
#[inline]
fn on_ack<Pub: Publisher>(
&mut self,
newest_acked_time_sent: Timestamp,
bytes_acknowledged: usize,
_newest_acked_packet_info: Self::PacketInfo,
rtt_estimator: &RttEstimator,
_random_generator: &mut dyn random::Generator,
ack_receive_time: Timestamp,
publisher: &mut Pub,
) {
self.bytes_in_flight_hi = self.bytes_in_flight_hi.max(self.bytes_in_flight);
self.bytes_in_flight
.try_sub(bytes_acknowledged)
.expect("bytes_acknowledged should not exceed u32::MAX");
if self.under_utilized {
self.state.on_app_limited(ack_receive_time);
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.8
//# When bytes in flight is smaller than the congestion window and
//# sending is not pacing limited, the congestion window is
//# underutilized. This can happen due to insufficient application data
//# or flow control limits. When this occurs, the congestion window
//# SHOULD NOT be increased in either slow start or congestion avoidance.
return;
}
// Check if this ack causes the controller to exit recovery
if let State::Recovery(recovery_start_time, _) = self.state {
if newest_acked_time_sent > recovery_start_time {
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.2
//# A recovery period ends and the sender enters congestion avoidance
//# when a packet sent during the recovery period is acknowledged.
self.state = State::congestion_avoidance(ack_receive_time)
}
};
// The congestion window may continue to grow while app-limited due to pacing
// interrupting sending while momentarily not app-limited. To avoid the congestion
// window growing too far beyond bytes in flight, we limit the maximum cwnd to
// the highest bytes_in_flight value seen since the last congestion event multiplied
// by a multiplier depending on the current state.
const SLOW_START_MAX_CWND_MULTIPLIER: f32 = 2.0;
const MAX_CWND_MULTIPLIER: f32 = 1.5;
let max_cwnd = match self.state {
SlowStart => *self.bytes_in_flight_hi as f32 * SLOW_START_MAX_CWND_MULTIPLIER,
Recovery(_, _) => self.congestion_window,
CongestionAvoidance(_) => *self.bytes_in_flight_hi as f32 * MAX_CWND_MULTIPLIER,
}
.max(self.cubic.minimum_window());
if self.congestion_window >= max_cwnd {
// The window is already larger than the max, so we can return early
return;
}
match self.state {
SlowStart => {
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.1
//# While a sender is in slow start, the congestion window increases by
//# the number of bytes acknowledged when each acknowledgment is
//# processed. This results in exponential growth of the congestion
//# window.
self.congestion_window = (self.congestion_window
+ self.slow_start.cwnd_increment(bytes_acknowledged))
.min(max_cwnd);
if self.congestion_window >= self.slow_start.threshold {
// The congestion window has exceeded a previously determined slow start threshold
// so transition to congestion avoidance and notify cubic of the slow start exit
publisher
.on_slow_start_exited(SlowStartExitCause::Other, self.congestion_window());
self.state = State::congestion_avoidance(ack_receive_time);
self.cubic.on_slow_start_exit(self.congestion_window);
}
}
Recovery(_, _) => {
// Don't increase the congestion window while in recovery
}
CongestionAvoidance(ref mut timing) => {
timing.on_window_increase(ack_receive_time);
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# t is the elapsed time from the beginning of the current congestion avoidance
let t = timing.t(ack_receive_time);
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# RTT is the weighted average RTT
// TODO: Linux Kernel Cubic implementation uses min RTT, possibly
// because it is more stable than smoothed_rtt. Other implementations
// have followed Linux's choice, so we will as well. The end result is a more
// conservative rate of increase of the congestion window. This requires
// investigation and testing to evaluate if smoothed_rtt would be a better input.
let rtt = rtt_estimator.min_rtt();
self.congestion_avoidance(t, rtt, bytes_acknowledged, max_cwnd);
}
};
debug_assert!(self.congestion_window >= self.cubic.minimum_window());
}
#[inline]
fn on_packet_lost<Pub: Publisher>(
&mut self,
lost_bytes: u32,
_packet_info: Self::PacketInfo,
persistent_congestion: bool,
_new_loss_burst: bool,
_random_generator: &mut dyn random::Generator,
timestamp: Timestamp,
publisher: &mut Pub,
) {
debug_assert!(lost_bytes > 0);
self.bytes_in_flight -= lost_bytes;
if self.state.is_slow_start() && !persistent_congestion {
publisher
.on_slow_start_exited(SlowStartExitCause::PacketLoss, self.congestion_window());
}
self.on_congestion_event(timestamp);
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.6.2
//# When persistent congestion is declared, the sender's congestion
//# window MUST be reduced to the minimum congestion window
//# (kMinimumWindow), similar to a TCP sender's response on an RTO
//# [RFC5681].
if persistent_congestion {
self.congestion_window = self.cubic.minimum_window();
self.state = State::SlowStart;
self.cubic.reset();
}
}
#[inline]
fn on_explicit_congestion<Pub: Publisher>(
&mut self,
_ce_count: u64,
event_time: Timestamp,
publisher: &mut Pub,
) {
if self.state.is_slow_start() {
publisher.on_slow_start_exited(SlowStartExitCause::Ecn, self.congestion_window());
}
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.1
//# If a path has been validated to support Explicit Congestion
//# Notification (ECN) [RFC3168] [RFC8311], QUIC treats a Congestion
//# Experienced (CE) codepoint in the IP header as a signal of
//# congestion.
self.on_congestion_event(event_time);
}
//= https://www.rfc-editor.org/rfc/rfc8899#section-3
//= type=exception
//= reason=See https://github.com/aws/s2n-quic/issues/959
//# An update to the PLPMTU (or MPS) MUST NOT increase the congestion
//# window measured in bytes [RFC4821].
//= https://www.rfc-editor.org/rfc/rfc8899#section-3
//# A PL that maintains the congestion window in terms of a limit to
//# the number of outstanding fixed-size packets SHOULD adapt this
//# limit to compensate for the size of the actual packets.
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.2
//= type=exception
//= reason=The maximum datagram size remains at the minimum (1200 bytes) during the handshake
//# If the maximum datagram size is decreased in order to complete the
//# handshake, the congestion window SHOULD be set to the new initial
//# congestion window.
#[inline]
fn on_mtu_update<Pub: Publisher>(&mut self, max_datagram_size: u16, _publisher: &mut Pub) {
let old_max_datagram_size = self.max_datagram_size;
self.max_datagram_size = max_datagram_size;
self.cubic.max_datagram_size = max_datagram_size;
self.congestion_window =
(self.congestion_window / old_max_datagram_size as f32) * max_datagram_size as f32;
}
//= https://www.rfc-editor.org/rfc/rfc9002#section-6.4
//# When Initial and Handshake packet protection keys are discarded (see
//# Section 4.9 of [QUIC-TLS]), all packets that were sent with those
//# keys can no longer be acknowledged because their acknowledgments
//# cannot be processed. The sender MUST discard all recovery state
//# associated with those packets and MUST remove them from the count of
//# bytes in flight.
#[inline]
fn on_packet_discarded<Pub: Publisher>(&mut self, bytes_sent: usize, _publisher: &mut Pub) {
self.bytes_in_flight
.try_sub(bytes_sent)
.expect("bytes sent should not exceed u32::MAX");
if let Recovery(recovery_start_time, RequiresTransmission) = self.state {
// If any of the discarded packets were lost, they will no longer be retransmitted
// so flip the Recovery status back to Idle so it is not waiting for a
// retransmission that may never come.
self.state = Recovery(recovery_start_time, Idle);
}
}
#[inline]
fn earliest_departure_time(&self) -> Option<Timestamp> {
self.pacer.earliest_departure_time()
}
}
impl CubicCongestionController {
// max_datagram_size is the current max_datagram_size, and is
// expected to be 1200 when the congestion controller is created.
pub fn new(max_datagram_size: u16) -> Self {
Self {
cubic: Cubic::new(max_datagram_size),
slow_start: HybridSlowStart::new(max_datagram_size),
pacer: Pacer::default(),
max_datagram_size,
congestion_window: CubicCongestionController::initial_window(max_datagram_size) as f32,
state: SlowStart,
bytes_in_flight: Counter::new(0),
time_of_last_sent_packet: None,
under_utilized: true,
bytes_in_flight_hi: Counter::new(0),
}
}
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.2
//# Endpoints SHOULD use an initial congestion
//# window of ten times the maximum datagram size (max_datagram_size),
//# while limiting the window to the larger of 14,720 bytes or twice the
//# maximum datagram size.
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.2
//# If the maximum datagram size changes during the connection, the
//# initial congestion window SHOULD be recalculated with the new size.
#[inline]
fn initial_window(max_datagram_size: u16) -> u32 {
const INITIAL_WINDOW_LIMIT: u32 = 14720;
min(
10 * max_datagram_size as u32,
max(INITIAL_WINDOW_LIMIT, 2 * max_datagram_size as u32),
)
}
#[inline]
fn congestion_avoidance(
&mut self,
t: Duration,
rtt: Duration,
sent_bytes: usize,
max_cwnd: f32,
) {
let w_cubic = self.cubic.w_cubic(t);
let w_est = self.cubic.w_est(t, rtt);
// limit the window increase to half the acked bytes
// as the Linux implementation of Cubic does.
let max_cwnd = (self.congestion_window + sent_bytes as f32 / 2.0).min(max_cwnd);
if w_cubic < w_est {
// TCP-Friendly Region
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.2
//# When receiving an ACK in congestion avoidance (cwnd could be greater than
//# or less than W_max), CUBIC checks whether W_cubic(t) is less than
//# W_est(t). If so, CUBIC is in the TCP-friendly region and cwnd SHOULD
//# be set to W_est(t) at each reception of an ACK.
self.congestion_window = self.packets_to_bytes(w_est).min(max_cwnd);
} else {
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# Upon receiving an ACK during congestion avoidance, CUBIC computes the
//# window increase rate during the next RTT period using Eq. 1. It sets
//# W_cubic(t+RTT) as the candidate target value of the congestion
//# window
// Concave Region
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.3
//# When receiving an ACK in congestion avoidance, if CUBIC is not in the
//# TCP-friendly region and cwnd is less than W_max, then CUBIC is in the
//# concave region. In this region, cwnd MUST be incremented by
//# (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
//# W_cubic(t+RTT) is calculated using Eq. 1.
// Convex Region
//# https://www.rfc-editor.org/rfc/rfc8312#section-4.4
//# When receiving an ACK in congestion avoidance, if CUBIC is not in the
//# TCP-friendly region and cwnd is larger than or equal to W_max, then
//# CUBIC is in the convex region.
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.4
//# In this region, cwnd MUST be incremented by
//# (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
//# W_cubic(t+RTT) is calculated using Eq. 1.
// The congestion window is adjusted in the same way in the convex and concave regions.
// A target congestion window is calculated for where the congestion window should be
// by the end of one RTT. That target is used for calculating the required rate of increase
// based on where the congestion window currently is. Assuming a full congestion window's
// worth of packets will be sent and acknowledged within that RTT, we increase the
// congestion window by this increment for each acknowledgement. As long as all packets
// are sent and acknowledged by the end of the RTT, the congestion window will reach the
// target size. Otherwise it will be smaller, reflecting that the network latency is
// higher than needed to achieve the target window, and thus a smaller congestion window
// is appropriate.
let target_congestion_window = self.packets_to_bytes(self.cubic.w_cubic(t + rtt));
// Decreases in the RTT estimate can cause the congestion window to get ahead of the
// target. In the case where the congestion window has already exceeded the target,
// we return without any further adjustment to the window.
if self.congestion_window >= target_congestion_window {
return;
}
let window_increase_rate =
(target_congestion_window - self.congestion_window) / self.congestion_window;
let window_increment = self.packets_to_bytes(window_increase_rate);
self.congestion_window = (self.congestion_window + window_increment).min(max_cwnd);
}
}
#[inline]
fn on_congestion_event(&mut self, event_time: Timestamp) {
// Reset bytes_in_flight_hi
self.bytes_in_flight_hi = BytesInFlight::new(0);
// No reaction if already in a recovery period.
if matches!(self.state, Recovery(_, _)) {
return;
}
// Enter recovery period.
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.1
//# The sender MUST exit slow start and enter a recovery period when a
//# packet is lost or when the ECN-CE count reported by its peer
//# increases.
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.2
//# If the congestion window is reduced immediately, a
//# single packet can be sent prior to reduction. This speeds up loss
//# recovery if the data in the lost packet is retransmitted and is
//# similar to TCP as described in Section 5 of [RFC6675].
self.state = Recovery(event_time, RequiresTransmission);
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.3.2
//# Implementations MAY reduce the congestion window immediately upon
//# entering a recovery period or use other mechanisms, such as
//# Proportional Rate Reduction [PRR], to reduce the congestion window
//# more gradually.
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.2
//# The minimum congestion window is the smallest value the congestion
//# window can attain in response to loss, an increase in the peer-
//# reported ECN-CE count, or persistent congestion.
self.congestion_window = self.cubic.multiplicative_decrease(self.congestion_window);
// Update Hybrid Slow Start with the decreased congestion window.
self.slow_start.on_congestion_event(self.congestion_window);
}
#[inline]
fn packets_to_bytes(&self, cwnd: f32) -> f32 {
cwnd * self.max_datagram_size as f32
}
/// Returns true if the congestion window is under utilized and should not grow larger
/// without further evidence of the stability of the current window.
#[inline]
fn is_congestion_window_under_utilized(&self) -> bool {
// This value is based on kMaxBurstBytes from Chromium
// https://source.chromium.org/chromium/chromium/src/+/master:net/third_party/quiche/src/quic/core/congestion_control/tcp_cubic_sender_bytes.cc;l=23;drc=f803516d2656ed829e54b2e819731763ca6cf4d9
const MAX_BURST_MULTIPLIER: u32 = 3;
if self.is_congestion_limited() {
return false;
}
// In slow start, allow the congestion window to increase as long as half of it is
// being used. This allows for the window to increase rapidly.
if self.state.is_slow_start() && self.bytes_in_flight >= self.congestion_window() / 2 {
return false;
}
// Otherwise allow the window to increase while MAX_BURST_MULTIPLIER packets are available
// in the window.
let available_congestion_window = self
.congestion_window()
.saturating_sub(*self.bytes_in_flight);
available_congestion_window > self.max_datagram_size as u32 * MAX_BURST_MULTIPLIER
}
}
/// Core functions of "CUBIC for Fast Long-Distance Networks" as specified in
/// https://tools.ietf.org/html/rfc8312. The unit of all window sizes is in
/// packets of size max_datagram_size to maintain alignment with the specification.
/// Thus, window sizes should be converted to bytes before applying to the
/// congestion window in the congestion controller.
#[derive(Clone, Debug)]
struct Cubic {
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# W_max is the window size just before the window is
//# reduced in the last congestion event.
w_max: f32,
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.6
//# a flow remembers the last value of W_max before it
//# updates W_max for the current congestion event.
//# Let us call the last value of W_max to be W_last_max.
w_last_max: f32,
// k is the time until we expect to reach w_max
k: Duration,
max_datagram_size: u16,
}
//= https://www.rfc-editor.org/rfc/rfc8312#section-5.1
//# Based on these observations and our experiments, we find C=0.4
//# gives a good balance between TCP-friendliness and aggressiveness
//# of window increase. Therefore, C SHOULD be set to 0.4.
const C: f32 = 0.4;
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.5
//# Parameter beta_cubic SHOULD be set to 0.7.
const BETA_CUBIC: f32 = 0.7;
impl Cubic {
pub fn new(max_datagram_size: u16) -> Self {
Cubic {
w_max: 0.0,
w_last_max: 0.0,
k: Duration::ZERO,
max_datagram_size,
}
}
/// Reset to the original state
#[inline]
pub fn reset(&mut self) {
self.w_max = 0.0;
self.w_last_max = 0.0;
self.k = Duration::ZERO;
}
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.1
//# CUBIC uses the following window increase function:
//#
//# W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
//#
//# where C is a constant fixed to determine the aggressiveness of window
//# increase in high BDP networks, t is the elapsed time from the
//# beginning of the current congestion avoidance, and K is the time
//# period that the above function takes to increase the current window
//# size to W_max if there are no further congestion events and is
//# calculated using the following equation:
//#
//# K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2)
//#
//# where beta_cubic is the CUBIC multiplication decrease factor
#[inline]
fn w_cubic(&self, t: Duration) -> f32 {
C * (t.as_secs_f32() - self.k.as_secs_f32()).powi(3) + self.w_max
}
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.2
//# W_est(t) = W_max*beta_cubic +
// [3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4)
#[inline]
fn w_est(&self, t: Duration, rtt: Duration) -> f32 {
self.w_max.mul_add(
BETA_CUBIC,
(3.0 * (1.0 - BETA_CUBIC) / (1.0 + BETA_CUBIC)) * (t.as_secs_f32() / rtt.as_secs_f32()),
)
}
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.5
//# When a packet loss is detected by duplicate ACKs or a network
//# congestion is detected by ECN-Echo ACKs, CUBIC updates its W_max,
//# cwnd, and ssthresh as follows. Parameter beta_cubic SHOULD be set to
//# 0.7.
//#
//# W_max = cwnd; // save window size before reduction
//# ssthresh = cwnd * beta_cubic; // new slow-start threshold
//# ssthresh = max(ssthresh, 2); // threshold is at least 2 MSS
//# cwnd = cwnd * beta_cubic; // window reduction
// This does not change the units of the congestion window
#[inline]
fn multiplicative_decrease(&mut self, cwnd: f32) -> f32 {
self.w_max = self.bytes_to_packets(cwnd);
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.6
//# To speed up this bandwidth release by
//# existing flows, the following mechanism called "fast convergence"
//# SHOULD be implemented.
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.6
//# With fast convergence, when a congestion event occurs, before the
//# window reduction of the congestion window, a flow remembers the last
//# value of W_max before it updates W_max for the current congestion
//# event. Let us call the last value of W_max to be W_last_max.
//#
//# if (W_max < W_last_max){ // should we make room for others
//# W_last_max = W_max; // remember the last W_max
//# W_max = W_max*(1.0+beta_cubic)/2.0; // further reduce W_max
//# } else {
//# W_last_max = W_max // remember the last W_max
//# }
//#
//# At a congestion event, if the current value of W_max is less than
//# W_last_max, this indicates that the saturation point experienced by
//# this flow is getting reduced because of the change in available
//# bandwidth. Then we allow this flow to release more bandwidth by
//# reducing W_max further. This action effectively lengthens the time
//# for this flow to increase its congestion window because the reduced
//# W_max forces the flow to have the plateau earlier. This allows more
//# time for the new flow to catch up to its congestion window size.
let w_max = self.w_max;
if w_max < self.w_last_max {
self.w_max = (w_max * (1.0 + BETA_CUBIC) / 2.0)
.max(self.bytes_to_packets(self.minimum_window()));
}
self.w_last_max = w_max;
let cwnd_start = (cwnd * BETA_CUBIC).max(self.minimum_window());
//= https://tools.ietf.org/id/draft-eggert-tcpm-rfc8312bis-01#4.2
//# _K_ is the time period that the above
//# function takes to increase the congestion window size at the
//# beginning of the current congestion avoidance stage to _W_(max)_ if
//# there are no further congestion events and is calculated using the
//# following equation:
//#
//# ________________
//# /W - cwnd
//# 3 / max start
//# K = | / ----------------
//# |/ C
//#
//# Figure 2
//#
//# where _cwnd_(start)_ is the congestion window at the beginning of the
//# current congestion avoidance stage.
self.k =
Duration::from_secs_f32(((self.w_max - self.bytes_to_packets(cwnd_start)) / C).cbrt());
cwnd_start
}
//= https://www.rfc-editor.org/rfc/rfc8312#section-4.8
//# In the case when CUBIC runs the hybrid slow start [HR08], it may exit
//# the first slow start without incurring any packet loss and thus W_max
//# is undefined. In this special case, CUBIC switches to congestion
//# avoidance and increases its congestion window size using Eq. 1, where
//# t is the elapsed time since the beginning of the current congestion
//# avoidance, K is set to 0, and W_max is set to the congestion window
//# size at the beginning of the current congestion avoidance.
#[inline]
fn on_slow_start_exit(&mut self, cwnd: f32) {
self.w_max = self.bytes_to_packets(cwnd);
// We are currently at the w_max, so set k to zero indicating zero
// seconds to reach the max
self.k = Duration::from_secs(0);
}
//= https://www.rfc-editor.org/rfc/rfc9002#section-7.2
//# The minimum congestion window is the smallest value the congestion
//# window can attain in response to loss, an increase in the peer-
//# reported ECN-CE count, or persistent congestion. The RECOMMENDED
//# value is 2 * max_datagram_size.
#[inline]
fn minimum_window(&self) -> f32 {
2.0 * self.max_datagram_size as f32
}
#[inline]
fn bytes_to_packets(&self, bytes: f32) -> f32 {
bytes / self.max_datagram_size as f32
}
}
#[non_exhaustive]
#[derive(Debug, Default)]
pub struct Endpoint {}
impl congestion_controller::Endpoint for Endpoint {
type CongestionController = CubicCongestionController;
fn new_congestion_controller(
&mut self,
path_info: congestion_controller::PathInfo,
) -> Self::CongestionController {
CubicCongestionController::new(path_info.max_datagram_size)
}
}
#[cfg(test)]
mod tests;