quiche / quiche.git / refs/heads/main / . / quiche / quic / core / congestion_control / cubic_bytes.cc

// Copyright (c) 2015 The Chromium Authors. All rights reserved. | |

// Use of this source code is governed by a BSD-style license that can be | |

// found in the LICENSE file. | |

#include "quiche/quic/core/congestion_control/cubic_bytes.h" | |

#include <algorithm> | |

#include <cmath> | |

#include <cstdint> | |

#include "quiche/quic/core/quic_constants.h" | |

#include "quiche/quic/core/quic_packets.h" | |

#include "quiche/quic/platform/api/quic_flag_utils.h" | |

#include "quiche/quic/platform/api/quic_flags.h" | |

#include "quiche/quic/platform/api/quic_logging.h" | |

namespace quic { | |

namespace { | |

// Constants based on TCP defaults. | |

// The following constants are in 2^10 fractions of a second instead of ms to | |

// allow a 10 shift right to divide. | |

const int kCubeScale = 40; // 1024*1024^3 (first 1024 is from 0.100^3) | |

// where 0.100 is 100 ms which is the scaling | |

// round trip time. | |

const int kCubeCongestionWindowScale = 410; | |

// The cube factor for packets in bytes. | |

const uint64_t kCubeFactor = | |

(UINT64_C(1) << kCubeScale) / kCubeCongestionWindowScale / kDefaultTCPMSS; | |

const float kDefaultCubicBackoffFactor = 0.7f; // Default Cubic backoff factor. | |

// Additional backoff factor when loss occurs in the concave part of the Cubic | |

// curve. This additional backoff factor is expected to give up bandwidth to | |

// new concurrent flows and speed up convergence. | |

const float kBetaLastMax = 0.85f; | |

} // namespace | |

CubicBytes::CubicBytes(const QuicClock* clock) | |

: clock_(clock), | |

num_connections_(kDefaultNumConnections), | |

epoch_(QuicTime::Zero()) { | |

ResetCubicState(); | |

} | |

void CubicBytes::SetNumConnections(int num_connections) { | |

num_connections_ = num_connections; | |

} | |

float CubicBytes::Alpha() const { | |

// TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that | |

// beta here is a cwnd multiplier, and is equal to 1-beta from the paper. | |

// We derive the equivalent alpha for an N-connection emulation as: | |

const float beta = Beta(); | |

return 3 * num_connections_ * num_connections_ * (1 - beta) / (1 + beta); | |

} | |

float CubicBytes::Beta() const { | |

// kNConnectionBeta is the backoff factor after loss for our N-connection | |

// emulation, which emulates the effective backoff of an ensemble of N | |

// TCP-Reno connections on a single loss event. The effective multiplier is | |

// computed as: | |

return (num_connections_ - 1 + kDefaultCubicBackoffFactor) / num_connections_; | |

} | |

float CubicBytes::BetaLastMax() const { | |

// BetaLastMax is the additional backoff factor after loss for our | |

// N-connection emulation, which emulates the additional backoff of | |

// an ensemble of N TCP-Reno connections on a single loss event. The | |

// effective multiplier is computed as: | |

return (num_connections_ - 1 + kBetaLastMax) / num_connections_; | |

} | |

void CubicBytes::ResetCubicState() { | |

epoch_ = QuicTime::Zero(); // Reset time. | |

last_max_congestion_window_ = 0; | |

acked_bytes_count_ = 0; | |

estimated_tcp_congestion_window_ = 0; | |

origin_point_congestion_window_ = 0; | |

time_to_origin_point_ = 0; | |

last_target_congestion_window_ = 0; | |

} | |

void CubicBytes::OnApplicationLimited() { | |

// When sender is not using the available congestion window, the window does | |

// not grow. But to be RTT-independent, Cubic assumes that the sender has been | |

// using the entire window during the time since the beginning of the current | |

// "epoch" (the end of the last loss recovery period). Since | |

// application-limited periods break this assumption, we reset the epoch when | |

// in such a period. This reset effectively freezes congestion window growth | |

// through application-limited periods and allows Cubic growth to continue | |

// when the entire window is being used. | |

epoch_ = QuicTime::Zero(); | |

} | |

QuicByteCount CubicBytes::CongestionWindowAfterPacketLoss( | |

QuicByteCount current_congestion_window) { | |

// Since bytes-mode Reno mode slightly under-estimates the cwnd, we | |

// may never reach precisely the last cwnd over the course of an | |

// RTT. Do not interpret a slight under-estimation as competing traffic. | |

if (current_congestion_window + kDefaultTCPMSS < | |

last_max_congestion_window_) { | |

// We never reached the old max, so assume we are competing with | |

// another flow. Use our extra back off factor to allow the other | |

// flow to go up. | |

last_max_congestion_window_ = | |

static_cast<int>(BetaLastMax() * current_congestion_window); | |

} else { | |

last_max_congestion_window_ = current_congestion_window; | |

} | |

epoch_ = QuicTime::Zero(); // Reset time. | |

return static_cast<int>(current_congestion_window * Beta()); | |

} | |

QuicByteCount CubicBytes::CongestionWindowAfterAck( | |

QuicByteCount acked_bytes, QuicByteCount current_congestion_window, | |

QuicTime::Delta delay_min, QuicTime event_time) { | |

acked_bytes_count_ += acked_bytes; | |

if (!epoch_.IsInitialized()) { | |

// First ACK after a loss event. | |

QUIC_DVLOG(1) << "Start of epoch"; | |

epoch_ = event_time; // Start of epoch. | |

acked_bytes_count_ = acked_bytes; // Reset count. | |

// Reset estimated_tcp_congestion_window_ to be in sync with cubic. | |

estimated_tcp_congestion_window_ = current_congestion_window; | |

if (last_max_congestion_window_ <= current_congestion_window) { | |

time_to_origin_point_ = 0; | |

origin_point_congestion_window_ = current_congestion_window; | |

} else { | |

time_to_origin_point_ = static_cast<uint32_t>( | |

cbrt(kCubeFactor * | |

(last_max_congestion_window_ - current_congestion_window))); | |

origin_point_congestion_window_ = last_max_congestion_window_; | |

} | |

} | |

// Change the time unit from microseconds to 2^10 fractions per second. Take | |

// the round trip time in account. This is done to allow us to use shift as a | |

// divide operator. | |

int64_t elapsed_time = | |

((event_time + delay_min - epoch_).ToMicroseconds() << 10) / | |

kNumMicrosPerSecond; | |

// Right-shifts of negative, signed numbers have implementation-dependent | |

// behavior, so force the offset to be positive, as is done in the kernel. | |

uint64_t offset = std::abs(time_to_origin_point_ - elapsed_time); | |

QuicByteCount delta_congestion_window = (kCubeCongestionWindowScale * offset * | |

offset * offset * kDefaultTCPMSS) >> | |

kCubeScale; | |

const bool add_delta = elapsed_time > time_to_origin_point_; | |

QUICHE_DCHECK(add_delta || | |

(origin_point_congestion_window_ > delta_congestion_window)); | |

QuicByteCount target_congestion_window = | |

add_delta ? origin_point_congestion_window_ + delta_congestion_window | |

: origin_point_congestion_window_ - delta_congestion_window; | |

// Limit the CWND increase to half the acked bytes. | |

target_congestion_window = | |

std::min(target_congestion_window, | |

current_congestion_window + acked_bytes_count_ / 2); | |

QUICHE_DCHECK_LT(0u, estimated_tcp_congestion_window_); | |

// Increase the window by approximately Alpha * 1 MSS of bytes every | |

// time we ack an estimated tcp window of bytes. For small | |

// congestion windows (less than 25), the formula below will | |

// increase slightly slower than linearly per estimated tcp window | |

// of bytes. | |

estimated_tcp_congestion_window_ += acked_bytes_count_ * | |

(Alpha() * kDefaultTCPMSS) / | |

estimated_tcp_congestion_window_; | |

acked_bytes_count_ = 0; | |

// We have a new cubic congestion window. | |

last_target_congestion_window_ = target_congestion_window; | |

// Compute target congestion_window based on cubic target and estimated TCP | |

// congestion_window, use highest (fastest). | |

if (target_congestion_window < estimated_tcp_congestion_window_) { | |

target_congestion_window = estimated_tcp_congestion_window_; | |

} | |

QUIC_DVLOG(1) << "Final target congestion_window: " | |

<< target_congestion_window; | |

return target_congestion_window; | |

} | |

} // namespace quic |