1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4//
5// Cubic algorithm, helper class to TCP cubic.
6// For details see http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf.
7
8#ifndef NET_QUIC_CONGESTION_CONTROL_CUBIC_H_
9#define NET_QUIC_CONGESTION_CONTROL_CUBIC_H_
10
11#include "base/basictypes.h"
12#include "net/base/net_export.h"
13#include "net/quic/quic_clock.h"
14#include "net/quic/quic_connection_stats.h"
15#include "net/quic/quic_time.h"
16
17namespace net {
18
19// TCP congestion window in QUIC is in packets, not bytes.
20typedef uint32 QuicTcpCongestionWindow;
21
22class NET_EXPORT_PRIVATE Cubic {
23 public:
24  Cubic(const QuicClock* clock, QuicConnectionStats* stats);
25
26  // Call after a timeout to reset the cubic state.
27  void Reset();
28
29  // Compute a new congestion window to use after a loss event.
30  // Returns the new congestion window in packets. The new congestion window is
31  // a multiplicative decrease of our current window.
32  QuicTcpCongestionWindow CongestionWindowAfterPacketLoss(
33      QuicTcpCongestionWindow current);
34
35  // Compute a new congestion window to use after a received ACK.
36  // Returns the new congestion window in packets. The new congestion window
37  // follows a cubic function that depends on the time passed since last
38  // packet loss.
39  QuicTcpCongestionWindow CongestionWindowAfterAck(
40      QuicTcpCongestionWindow current,
41      QuicTime::Delta delay_min);
42
43 private:
44  static const QuicTime::Delta MaxCubicTimeInterval() {
45    return QuicTime::Delta::FromMilliseconds(30);
46  }
47
48  // Update congestion control variables in QuicConnectionStats.
49  void UpdateCongestionControlStats(QuicTcpCongestionWindow new_cubic_mode_cwnd,
50                                    QuicTcpCongestionWindow new_reno_mode_cwnd);
51  const QuicClock* clock_;
52
53  // Time when this cycle started, after last loss event.
54  QuicTime epoch_;
55
56  // Time when we updated last_congestion_window.
57  QuicTime last_update_time_;
58
59  // Last congestion window (in packets) used.
60  QuicTcpCongestionWindow last_congestion_window_;
61
62  // Max congestion window (in packets) used just before last loss event.
63  // Note: to improve fairness to other streams an additional back off is
64  // applied to this value if the new value is below our latest value.
65  QuicTcpCongestionWindow last_max_congestion_window_;
66
67  // Number of acked packets since the cycle started (epoch).
68  uint32 acked_packets_count_;
69
70  // TCP Reno equivalent congestion window in packets.
71  QuicTcpCongestionWindow estimated_tcp_congestion_window_;
72
73  // Origin point of cubic function.
74  QuicTcpCongestionWindow origin_point_congestion_window_;
75
76  // Time to origin point of cubic function in 2^10 fractions of a second.
77  uint32 time_to_origin_point_;
78
79  // Last congestion window in packets computed by cubic function.
80  QuicTcpCongestionWindow last_target_congestion_window_;
81
82  // QuicConnectionStats includes congestion control related stats.
83  QuicConnectionStats* stats_;
84
85  DISALLOW_COPY_AND_ASSIGN(Cubic);
86};
87
88}  // namespace net
89
90#endif  // NET_QUIC_CONGESTION_CONTROL_CUBIC_H_
91