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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.
// http://netsrv.csc.ncsu.edu/export/hybridstart_pfldnet08.pdf
// http://research.csc.ncsu.edu/netsrv/sites/default/files/hystart_techreport_2008.pdf
#ifndef NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
#define NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
#include "base/basictypes.h"
#include "net/base/net_export.h"
#include "net/quic/quic_clock.h"
#include "net/quic/quic_protocol.h"
#include "net/quic/quic_time.h"
namespace net {
class NET_EXPORT_PRIVATE HybridSlowStart {
public:
explicit HybridSlowStart(const QuicClock* clock);
void OnPacketAcked(QuicPacketSequenceNumber acked_sequence_number,
bool in_slow_start);
void OnPacketSent(QuicPacketSequenceNumber sequence_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,
int64 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 sequence number of our current slow start
// round.
// Call Reset if this returns true.
bool IsEndOfRound(QuicPacketSequenceNumber ack) const;
// Call for the start of each receive round (burst) in the slow start phase.
void StartReceiveRound(QuicPacketSequenceNumber last_sent);
// Whether slow start has started.
bool started() const {
return started_;
}
private:
// Whether a condition for exiting slow start has been found.
enum HystartState {
NOT_FOUND,
ACK_TRAIN, // A closely spaced ack train is too long.
DELAY, // Too much increase in the round's min_rtt was observed.
};
const QuicClock* clock_;
// Whether the hybrid slow start has been started.
bool started_;
HystartState hystart_found_;
// Last sequence number sent which was CWND limited.
QuicPacketSequenceNumber last_sent_sequence_number_;
// Variables for tracking acks received during a slow start round.
QuicTime round_start_; // Beginning of each slow start receive round.
QuicPacketSequenceNumber end_sequence_number_; // End of the receive round.
// Last time when the spacing between ack arrivals was less than 2 ms.
// Defaults to the beginning of the round.
QuicTime last_close_ack_pair_time_;
uint32 rtt_sample_count_; // Number of rtt samples in the current round.
QuicTime::Delta current_min_rtt_; // The minimum rtt of current round.
DISALLOW_COPY_AND_ASSIGN(HybridSlowStart);
};
} // namespace net
#endif // NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
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