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      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 
     17 namespace net {
     18 
     19 // TCP congestion window in QUIC is in packets, not bytes.
     20 typedef uint32 QuicTcpCongestionWindow;
     21 
     22 class 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