Added support for cubic congestion control (RFC9438)

src/aioquic/quic/configuration.py

@@ -30,7 +30,7 @@ class QuicConfiguration:
     """
     The name of the congestion control algorithm to use.
 
-    Currently supported algorithms: `"reno"`.
+    Currently supported algorithms: `"reno", `"cubic"`.
     """
 
     connection_id_length: int = 8

src/aioquic/quic/congestion/base.py

@@ -1,5 +1,5 @@
 import abc
-from typing import Dict, Iterable, Optional, Protocol
+from typing import Any, Dict, Iterable, Optional, Protocol
 
 from ..packet_builder import QuicSentPacket
 
@@ -21,7 +21,7 @@ def __init__(self, *, max_datagram_size: int) -> None:
         self.congestion_window = K_INITIAL_WINDOW * max_datagram_size
 
     @abc.abstractmethod
-    def on_packet_acked(self, *, packet: QuicSentPacket) -> None:
+    def on_packet_acked(self, *, now: float, packet: QuicSentPacket) -> None:
         ...  # pragma: no cover
 
     @abc.abstractmethod
@@ -40,6 +40,12 @@ def on_packets_lost(self, *, now: float, packets: Iterable[QuicSentPacket]) -> N
     def on_rtt_measurement(self, *, now: float, rtt: float) -> None:
         ...  # pragma: no cover
 
+    def get_log_data(self) -> Dict[str, Any]:
+        data = {"cwnd": self.congestion_window, "bytes_in_flight": self.bytes_in_flight}
+        if self.ssthresh is not None:
+            data["ssthresh"] = self.ssthresh
+        return data
+
 
 class QuicCongestionControlFactory(Protocol):
     def __call__(self, *, max_datagram_size: int) -> QuicCongestionControl:

src/aioquic/quic/congestion/cubic.py

@@ -0,0 +1,212 @@
+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)

src/aioquic/quic/congestion/reno.py

@@ -23,7 +23,7 @@ def __init__(self, *, max_datagram_size: int) -> None:
         self._congestion_stash = 0
         self._rtt_monitor = QuicRttMonitor()
 
-    def on_packet_acked(self, *, packet: QuicSentPacket) -> None:
+    def on_packet_acked(self, *, now: float, packet: QuicSentPacket) -> None:
         self.bytes_in_flight -= packet.sent_bytes
 
         # don't increase window in congestion recovery

src/aioquic/quic/recovery.py

@@ -2,7 +2,7 @@
 import math
 from typing import Any, Callable, Dict, Iterable, List, Optional
 
-from .congestion import reno  # noqa
+from .congestion import cubic, reno  # noqa
 from .congestion.base import K_GRANULARITY, create_congestion_control
 from .logger import QuicLoggerTrace
 from .packet_builder import QuicDeliveryState, QuicSentPacket
@@ -199,7 +199,7 @@ def on_ack_received(
                     is_ack_eliciting = True
                     space.ack_eliciting_in_flight -= 1
                 if packet.in_flight:
-                    self._cc.on_packet_acked(packet=packet)
+                    self._cc.on_packet_acked(packet=packet, now=now)
                 largest_newly_acked = packet_number
                 largest_sent_time = packet.sent_time
 
@@ -334,12 +334,7 @@ def _get_loss_space(self) -> Optional[QuicPacketSpace]:
         return loss_space
 
     def _log_metrics_updated(self, log_rtt=False) -> None:
-        data: Dict[str, Any] = {
-            "bytes_in_flight": self._cc.bytes_in_flight,
-            "cwnd": self._cc.congestion_window,
-        }
-        if self._cc.ssthresh is not None:
-            data["ssthresh"] = self._cc.ssthresh
+        data: Dict[str, Any] = self._cc.get_log_data()
 
         if log_rtt:
             data.update(