src/net/quic/congestion_control/cubic.h

// 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.
//
// Cubic algorithm, helper class to TCP cubic.
// For details see http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf.

#ifndef NET_QUIC_CONGESTION_CONTROL_CUBIC_H_
#define NET_QUIC_CONGESTION_CONTROL_CUBIC_H_

#include "base/basictypes.h"
#include "net/base/net_export.h"
#include "net/quic/quic_clock.h"
#include "net/quic/quic_time.h"

namespace net {

// TCP congestion window in QUIC is in packets, not bytes.
typedef uint32 QuicTcpCongestionWindow;

class NET_EXPORT_PRIVATE Cubic {
 public:
  explicit Cubic(const QuicClock* clock);

  // Call after a timeout to reset the cubic state.
  void Reset();

  // 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.
  QuicTcpCongestionWindow CongestionWindowAfterPacketLoss(
      QuicTcpCongestionWindow current);

  // Compute a new congestion window to use after a received ACK.
  // Returns the new congestion window in packets. The new congestion window
  // follows a cubic function that depends on the time passed since last
  // packet loss.
  QuicTcpCongestionWindow CongestionWindowAfterAck(
      QuicTcpCongestionWindow current,
      QuicTime::Delta delay_min);

 protected:
  // Calculates the cubic root using a table lookup followed by one Newton-
  // Raphson iteration.
  uint32 CubeRoot(uint64 a);

 private:
  static const QuicTime::Delta MaxCubicTimeInterval() {
    return QuicTime::Delta::FromMilliseconds(30);
  }

  const QuicClock* clock_;

  // Time when this cycle started, after last loss event.
  QuicTime epoch_;

  // Time when we updated last_congestion_window.
  QuicTime last_update_time_;

  // Last congestion window (in packets) used.
  QuicTcpCongestionWindow last_congestion_window_;

  // Max congestion window (in packets) 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.
  QuicTcpCongestionWindow last_max_congestion_window_;

  // Number of acked packets since the cycle started (epoch).
  uint32 acked_packets_count_;

  // TCP Reno equivalent congestion window in packets.
  QuicTcpCongestionWindow estimated_tcp_congestion_window_;

  // Origin point of cubic function.
  QuicTcpCongestionWindow origin_point_congestion_window_;

  // Time to origin point of cubic function in 2^10 fractions of a second.
  uint32 time_to_origin_point_;

  // Last congestion window in packets computed by cubic function.
  QuicTcpCongestionWindow last_target_congestion_window_;
};

}  // namespace net
#endif  // NET_QUIC_CONGESTION_CONTROL_CUBIC_H_