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Diffstat (limited to 'third_party/rust/neqo-transport/src/cc/cubic.rs')
| -rw-r--r-- | third_party/rust/neqo-transport/src/cc/cubic.rs | 211 |
1 files changed, 211 insertions, 0 deletions
diff --git a/third_party/rust/neqo-transport/src/cc/cubic.rs b/third_party/rust/neqo-transport/src/cc/cubic.rs new file mode 100644 index 0000000000..ab3fedb74f --- /dev/null +++ b/third_party/rust/neqo-transport/src/cc/cubic.rs @@ -0,0 +1,211 @@ +// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or +// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license +// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your +// option. This file may not be copied, modified, or distributed +// except according to those terms. + +#![deny(clippy::pedantic)] + +use std::fmt::{self, Display}; +use std::time::{Duration, Instant}; + +use crate::cc::{classic_cc::WindowAdjustment, MAX_DATAGRAM_SIZE_F64}; +use neqo_common::qtrace; +use std::convert::TryFrom; + +// 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_QUOTIENT: 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::try_from(u32::try_from(v >> 21).unwrap_or(u32::MAX)).unwrap(); + f_64 *= 2_097_152.0; // f_64 <<= 21 + f_64 += f64::try_from(u32::try_from(v & 0x1f_ffff).unwrap()).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_QUOTIENT / CUBIC_BETA_USIZE_DIVISOR, + acked_bytes * CUBIC_BETA_USIZE_QUOTIENT / 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; + } +} |