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Mini Shell
Mini Shell
from typing import Any, Dict, Iterable
from ..packet_builder import QuicSentPacket
from .base import (
K_INITIAL_WINDOW,
K_MINIMUM_WINDOW,
QuicCongestionControl,
QuicRttMonitor,
register_congestion_control,
)
# cubic specific variables (see https://www.rfc-editor.org/rfc/rfc9438.html#name-definitions)
K_CUBIC_C = 0.4
K_CUBIC_LOSS_REDUCTION_FACTOR = 0.7
K_CUBIC_MAX_IDLE_TIME = 2 # reset the cwnd after 2 seconds of inactivity
def better_cube_root(x: float) -> float:
if x < 0:
# avoid precision errors that make the cube root returns an imaginary number
return -((-x) ** (1.0 / 3.0))
else:
return (x) ** (1.0 / 3.0)
class CubicCongestionControl(QuicCongestionControl):
"""
Cubic congestion control implementation for aioquic
"""
def __init__(self, max_datagram_size: int) -> None:
super().__init__(max_datagram_size=max_datagram_size)
# increase by one segment
self.additive_increase_factor: int = max_datagram_size
self._max_datagram_size: int = max_datagram_size
self._congestion_recovery_start_time = 0.0
self._rtt_monitor = QuicRttMonitor()
self.rtt = 0.02 # starting RTT is considered to be 20ms
self.reset()
self.last_ack = 0.0
def W_cubic(self, t) -> int:
W_max_segments = self._W_max / self._max_datagram_size
target_segments = K_CUBIC_C * (t - self.K) ** 3 + (W_max_segments)
return int(target_segments * self._max_datagram_size)
def is_reno_friendly(self, t) -> bool:
return self.W_cubic(t) < self._W_est
def is_concave(self) -> bool:
return self.congestion_window < self._W_max
def reset(self) -> None:
self.congestion_window = K_INITIAL_WINDOW * self._max_datagram_size
self.ssthresh = None
self._first_slow_start = True
self._starting_congestion_avoidance = False
self.K: float = 0.0
self._W_est = 0
self._cwnd_epoch = 0
self._t_epoch = 0.0
self._W_max = self.congestion_window
def on_packet_acked(self, *, now: float, packet: QuicSentPacket) -> None:
self.bytes_in_flight -= packet.sent_bytes
self.last_ack = packet.sent_time
if self.ssthresh is None or self.congestion_window < self.ssthresh:
# slow start
self.congestion_window += packet.sent_bytes
else:
# congestion avoidance
if self._first_slow_start and not self._starting_congestion_avoidance:
# exiting slow start without having a loss
self._first_slow_start = False
self._W_max = self.congestion_window
self._t_epoch = now
self._cwnd_epoch = self.congestion_window
self._W_est = self._cwnd_epoch
# calculate K
W_max_segments = self._W_max / self._max_datagram_size
cwnd_epoch_segments = self._cwnd_epoch / self._max_datagram_size
self.K = better_cube_root(
(W_max_segments - cwnd_epoch_segments) / K_CUBIC_C
)
# initialize the variables used at start of congestion avoidance
if self._starting_congestion_avoidance:
self._starting_congestion_avoidance = False
self._first_slow_start = False
self._t_epoch = now
self._cwnd_epoch = self.congestion_window
self._W_est = self._cwnd_epoch
# calculate K
W_max_segments = self._W_max / self._max_datagram_size
cwnd_epoch_segments = self._cwnd_epoch / self._max_datagram_size
self.K = better_cube_root(
(W_max_segments - cwnd_epoch_segments) / K_CUBIC_C
)
self._W_est = int(
self._W_est
+ self.additive_increase_factor
* (packet.sent_bytes / self.congestion_window)
)
t = now - self._t_epoch
target: int = 0
W_cubic = self.W_cubic(t + self.rtt)
if W_cubic < self.congestion_window:
target = self.congestion_window
elif W_cubic > 1.5 * self.congestion_window:
target = int(self.congestion_window * 1.5)
else:
target = W_cubic
if self.is_reno_friendly(t):
# reno friendly region of cubic
# (https://www.rfc-editor.org/rfc/rfc9438.html#name-reno-friendly-region)
self.congestion_window = self._W_est
elif self.is_concave():
# concave region of cubic
# (https://www.rfc-editor.org/rfc/rfc9438.html#name-concave-region)
self.congestion_window = int(
self.congestion_window
+ (
(target - self.congestion_window)
* (self._max_datagram_size / self.congestion_window)
)
)
else:
# convex region of cubic
# (https://www.rfc-editor.org/rfc/rfc9438.html#name-convex-region)
self.congestion_window = int(
self.congestion_window
+ (
(target - self.congestion_window)
* (self._max_datagram_size / self.congestion_window)
)
)
def on_packet_sent(self, *, packet: QuicSentPacket) -> None:
self.bytes_in_flight += packet.sent_bytes
if self.last_ack == 0.0:
return
elapsed_idle = packet.sent_time - self.last_ack
if elapsed_idle >= K_CUBIC_MAX_IDLE_TIME:
self.reset()
def on_packets_expired(self, *, packets: Iterable[QuicSentPacket]) -> None:
for packet in packets:
self.bytes_in_flight -= packet.sent_bytes
def on_packets_lost(self, *, now: float, packets: Iterable[QuicSentPacket]) -> None:
lost_largest_time = 0.0
for packet in packets:
self.bytes_in_flight -= packet.sent_bytes
lost_largest_time = packet.sent_time
# start a new congestion event if packet was sent after the
# start of the previous congestion recovery period.
if lost_largest_time > self._congestion_recovery_start_time:
self._congestion_recovery_start_time = now
# Normal congestion handle, can't be used in same time as fast convergence
# self._W_max = self.congestion_window
# fast convergence
if self._W_max is not None and self.congestion_window < self._W_max:
self._W_max = int(
self.congestion_window * (1 + K_CUBIC_LOSS_REDUCTION_FACTOR) / 2
)
else:
self._W_max = self.congestion_window
# normal congestion MD
flight_size = self.bytes_in_flight
new_ssthresh = max(
int(flight_size * K_CUBIC_LOSS_REDUCTION_FACTOR),
K_MINIMUM_WINDOW * self._max_datagram_size,
)
self.ssthresh = new_ssthresh
self.congestion_window = max(
self.ssthresh, K_MINIMUM_WINDOW * self._max_datagram_size
)
# restart a new congestion avoidance phase
self._starting_congestion_avoidance = True
def on_rtt_measurement(self, *, now: float, rtt: float) -> None:
self.rtt = rtt
# check whether we should exit slow start
if self.ssthresh is None and self._rtt_monitor.is_rtt_increasing(
rtt=rtt, now=now
):
self.ssthresh = self.congestion_window
def get_log_data(self) -> Dict[str, Any]:
data = super().get_log_data()
data["cubic-wmax"] = int(self._W_max)
return data
register_congestion_control("cubic", CubicCongestionControl)
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