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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use std::{
fmt::{self, Display},
time::{Duration, Instant},
};
use neqo_common::qtrace;
use crate::cc::{classic_cc::WindowAdjustment, MAX_DATAGRAM_SIZE_F64};
// CUBIC congestion control
// C is a constant fixed to determine the aggressiveness of window
// increase in high BDP networks.
pub const CUBIC_C: f64 = 0.4;
pub const CUBIC_ALPHA: f64 = 3.0 * (1.0 - 0.7) / (1.0 + 0.7);
// CUBIC_BETA = 0.7;
pub const CUBIC_BETA_USIZE_DIVIDEND: usize = 7;
pub const CUBIC_BETA_USIZE_DIVISOR: usize = 10;
/// The fast convergence ratio further reduces the congestion window when a congestion event
/// occurs before reaching the previous `W_max`.
pub const CUBIC_FAST_CONVERGENCE: f64 = 0.85; // (1.0 + CUBIC_BETA) / 2.0;
/// The minimum number of multiples of the datagram size that need
/// to be received to cause an increase in the congestion window.
/// When there is no loss, Cubic can return to exponential increase, but
/// this value reduces the magnitude of the resulting growth by a constant factor.
/// A value of 1.0 would mean a return to the rate used in slow start.
const EXPONENTIAL_GROWTH_REDUCTION: f64 = 2.0;
/// Convert an integer congestion window value into a floating point value.
/// This has the effect of reducing larger values to `1<<53`.
/// If you have a congestion window that large, something is probably wrong.
fn convert_to_f64(v: usize) -> f64 {
let mut f_64 = f64::from(u32::try_from(v >> 21).unwrap_or(u32::MAX));
f_64 *= 2_097_152.0; // f_64 <<= 21
f_64 += f64::from(u32::try_from(v & 0x1f_ffff).unwrap());
f_64
}
#[derive(Debug)]
pub struct Cubic {
last_max_cwnd: f64,
estimated_tcp_cwnd: f64,
k: f64,
w_max: f64,
ca_epoch_start: Option<Instant>,
tcp_acked_bytes: f64,
}
impl Default for Cubic {
fn default() -> Self {
Self {
last_max_cwnd: 0.0,
estimated_tcp_cwnd: 0.0,
k: 0.0,
w_max: 0.0,
ca_epoch_start: None,
tcp_acked_bytes: 0.0,
}
}
}
impl Display for Cubic {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(
f,
"Cubic [last_max_cwnd: {}, k: {}, w_max: {}, ca_epoch_start: {:?}]",
self.last_max_cwnd, self.k, self.w_max, self.ca_epoch_start
)?;
Ok(())
}
}
#[allow(clippy::doc_markdown)]
impl Cubic {
/// Original equations is:
/// K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2 RFC8312)
/// W_max is number of segments of the maximum segment size (MSS).
///
/// K is actually the time that W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1) would
/// take to increase to W_max. We use bytes not MSS units, therefore this
/// equation will be: W_cubic(t) = C*MSS*(t-K)^3 + W_max.
///
/// From that equation we can calculate K as:
/// K = cubic_root((W_max - W_cubic) / C / MSS);
fn calc_k(&self, curr_cwnd: f64) -> f64 {
((self.w_max - curr_cwnd) / CUBIC_C / MAX_DATAGRAM_SIZE_F64).cbrt()
}
/// W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
/// t is relative to the start of the congestion avoidance phase and it is in seconds.
fn w_cubic(&self, t: f64) -> f64 {
CUBIC_C * (t - self.k).powi(3) * MAX_DATAGRAM_SIZE_F64 + self.w_max
}
fn start_epoch(&mut self, curr_cwnd_f64: f64, new_acked_f64: f64, now: Instant) {
self.ca_epoch_start = Some(now);
// reset tcp_acked_bytes and estimated_tcp_cwnd;
self.tcp_acked_bytes = new_acked_f64;
self.estimated_tcp_cwnd = curr_cwnd_f64;
if self.last_max_cwnd <= curr_cwnd_f64 {
self.w_max = curr_cwnd_f64;
self.k = 0.0;
} else {
self.w_max = self.last_max_cwnd;
self.k = self.calc_k(curr_cwnd_f64);
}
qtrace!([self], "New epoch");
}
}
impl WindowAdjustment for Cubic {
// This is because of the cast in the last line from f64 to usize.
#[allow(clippy::cast_possible_truncation)]
#[allow(clippy::cast_sign_loss)]
fn bytes_for_cwnd_increase(
&mut self,
curr_cwnd: usize,
new_acked_bytes: usize,
min_rtt: Duration,
now: Instant,
) -> usize {
let curr_cwnd_f64 = convert_to_f64(curr_cwnd);
let new_acked_f64 = convert_to_f64(new_acked_bytes);
if self.ca_epoch_start.is_none() {
// This is a start of a new congestion avoidance phase.
self.start_epoch(curr_cwnd_f64, new_acked_f64, now);
} else {
self.tcp_acked_bytes += new_acked_f64;
}
let time_ca = self
.ca_epoch_start
.map_or(min_rtt, |t| {
if now + min_rtt < t {
// This only happens when processing old packets
// that were saved and replayed with old timestamps.
min_rtt
} else {
now + min_rtt - t
}
})
.as_secs_f64();
let target_cubic = self.w_cubic(time_ca);
let tcp_cnt = self.estimated_tcp_cwnd / CUBIC_ALPHA;
while self.tcp_acked_bytes > tcp_cnt {
self.tcp_acked_bytes -= tcp_cnt;
self.estimated_tcp_cwnd += MAX_DATAGRAM_SIZE_F64;
}
let target_cwnd = target_cubic.max(self.estimated_tcp_cwnd);
// Calculate the number of bytes that would need to be acknowledged for an increase
// of `MAX_DATAGRAM_SIZE` to match the increase of `target - cwnd / cwnd` as defined
// in the specification (Sections 4.4 and 4.5).
// The amount of data required therefore reduces asymptotically as the target increases.
// If the target is not significantly higher than the congestion window, require a very
// large amount of acknowledged data (effectively block increases).
let mut acked_to_increase =
MAX_DATAGRAM_SIZE_F64 * curr_cwnd_f64 / (target_cwnd - curr_cwnd_f64).max(1.0);
// Limit increase to max 1 MSS per EXPONENTIAL_GROWTH_REDUCTION ack packets.
// This effectively limits target_cwnd to (1 + 1 / EXPONENTIAL_GROWTH_REDUCTION) cwnd.
acked_to_increase =
acked_to_increase.max(EXPONENTIAL_GROWTH_REDUCTION * MAX_DATAGRAM_SIZE_F64);
acked_to_increase as usize
}
fn reduce_cwnd(&mut self, curr_cwnd: usize, acked_bytes: usize) -> (usize, usize) {
let curr_cwnd_f64 = convert_to_f64(curr_cwnd);
// Fast Convergence
// If congestion event occurs before the maximum congestion window before the last
// congestion event, we reduce the the maximum congestion window and thereby W_max.
// check cwnd + MAX_DATAGRAM_SIZE instead of cwnd because with cwnd in bytes, cwnd may be
// slightly off.
self.last_max_cwnd = if curr_cwnd_f64 + MAX_DATAGRAM_SIZE_F64 < self.last_max_cwnd {
curr_cwnd_f64 * CUBIC_FAST_CONVERGENCE
} else {
curr_cwnd_f64
};
self.ca_epoch_start = None;
(
curr_cwnd * CUBIC_BETA_USIZE_DIVIDEND / CUBIC_BETA_USIZE_DIVISOR,
acked_bytes * CUBIC_BETA_USIZE_DIVIDEND / CUBIC_BETA_USIZE_DIVISOR,
)
}
fn on_app_limited(&mut self) {
// Reset ca_epoch_start. Let it start again when the congestion controller
// exits the app-limited period.
self.ca_epoch_start = None;
}
#[cfg(test)]
fn last_max_cwnd(&self) -> f64 {
self.last_max_cwnd
}
#[cfg(test)]
fn set_last_max_cwnd(&mut self, last_max_cwnd: f64) {
self.last_max_cwnd = last_max_cwnd;
}
}