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// Copyright (c) 2012 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.
// This class is a helper class to TcpCubicSender.
// Slow start is the initial startup phase of TCP, it lasts until first packet
// loss. This class implements hybrid slow start of the TCP cubic send side
// congestion algorithm. The key feaure of hybrid slow start is that it tries to
// avoid running into the wall too hard during the slow start phase, which
// the traditional TCP implementation does.
// This does not implement ack train detection because it interacts poorly with
// pacing.
#include <cstdint>
#include "net/third_party/quiche/src/quic/core/quic_packets.h"
#include "net/third_party/quiche/src/quic/core/quic_time.h"
#include "net/third_party/quiche/src/quic/platform/api/quic_export.h"
namespace quic {
class QUIC_EXPORT_PRIVATE HybridSlowStart {
HybridSlowStart(const HybridSlowStart&) = delete;
HybridSlowStart& operator=(const HybridSlowStart&) = delete;
void OnPacketAcked(QuicPacketNumber acked_packet_number);
void OnPacketSent(QuicPacketNumber packet_number);
// ShouldExitSlowStart should be called on every new ack frame, since a new
// RTT measurement can be made then.
// rtt: the RTT for this ack packet.
// min_rtt: is the lowest delay (RTT) we have seen during the session.
// congestion_window: the congestion window in packets.
bool ShouldExitSlowStart(QuicTime::Delta rtt,
QuicTime::Delta min_rtt,
QuicPacketCount congestion_window);
// Start a new slow start phase.
void Restart();
// TODO(ianswett): The following methods should be private, but that requires
// a follow up CL to update the unit test.
// Returns true if this ack the last packet number of our current slow start
// round.
// Call Reset if this returns true.
bool IsEndOfRound(QuicPacketNumber ack) const;
// Call for the start of each receive round (burst) in the slow start phase.
void StartReceiveRound(QuicPacketNumber last_sent);
// Whether slow start has started.
bool started() const { return started_; }
// Whether a condition for exiting slow start has been found.
enum HystartState {
DELAY, // Too much increase in the round's min_rtt was observed.
// Whether the hybrid slow start has been started.
bool started_;
HystartState hystart_found_;
// Last packet number sent which was CWND limited.
QuicPacketNumber last_sent_packet_number_;
// Variables for tracking acks received during a slow start round.
QuicPacketNumber end_packet_number_; // End of the receive round.
uint32_t rtt_sample_count_; // Number of rtt samples in the current round.
QuicTime::Delta current_min_rtt_; // The minimum rtt of current round.
} // namespace quic