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// 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.
// Cubic algorithm, helper class to TCP cubic.
// For details see
#include <cstdint>
#include "quiche/quic/core/quic_bandwidth.h"
#include "quiche/quic/core/quic_clock.h"
#include "quiche/quic/core/quic_connection_stats.h"
#include "quiche/quic/core/quic_time.h"
#include "quiche/quic/platform/api/quic_export.h"
namespace quic {
namespace test {
class CubicBytesTest;
} // namespace test
class QUIC_EXPORT_PRIVATE CubicBytes {
explicit CubicBytes(const QuicClock* clock);
CubicBytes(const CubicBytes&) = delete;
CubicBytes& operator=(const CubicBytes&) = delete;
void SetNumConnections(int num_connections);
// Call after a timeout to reset the cubic state.
void ResetCubicState();
// Compute a new congestion window to use after a loss event.
// Returns the new congestion window in packets. The new congestion window is
// a multiplicative decrease of our current window.
QuicByteCount CongestionWindowAfterPacketLoss(QuicPacketCount current);
// Compute a new congestion window to use after a received ACK.
// Returns the new congestion window in bytes. The new congestion window
// follows a cubic function that depends on the time passed since last packet
// loss.
QuicByteCount CongestionWindowAfterAck(QuicByteCount acked_bytes,
QuicByteCount current,
QuicTime::Delta delay_min,
QuicTime event_time);
// Call on ack arrival when sender is unable to use the available congestion
// window. Resets Cubic state during quiescence.
void OnApplicationLimited();
friend class test::CubicBytesTest;
static const QuicTime::Delta MaxCubicTimeInterval() {
return QuicTime::Delta::FromMilliseconds(30);
// Compute the TCP Cubic alpha, beta, and beta-last-max based on the
// current number of connections.
float Alpha() const;
float Beta() const;
float BetaLastMax() const;
QuicByteCount last_max_congestion_window() const {
return last_max_congestion_window_;
const QuicClock* clock_;
// Number of connections to simulate.
int num_connections_;
// Time when this cycle started, after last loss event.
QuicTime epoch_;
// Max congestion window used just before last loss event.
// Note: to improve fairness to other streams an additional back off is
// applied to this value if the new value is below our latest value.
QuicByteCount last_max_congestion_window_;
// Number of acked bytes since the cycle started (epoch).
QuicByteCount acked_bytes_count_;
// TCP Reno equivalent congestion window in packets.
QuicByteCount estimated_tcp_congestion_window_;
// Origin point of cubic function.
QuicByteCount origin_point_congestion_window_;
// Time to origin point of cubic function in 2^10 fractions of a second.
uint32_t time_to_origin_point_;
// Last congestion window in packets computed by cubic function.
QuicByteCount last_target_congestion_window_;
} // namespace quic