net/quic/congestion_control/hybrid_slow_start.h

   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 // This class is a helper class to TcpCubicSender.
   6 // Slow start is the initial startup phase of TCP, it lasts until first packet
   7 // loss. This class implements hybrid slow start of the TCP cubic send side
   8 // congestion algorithm. The key feaure of hybrid slow start is that it tries to
   9 // avoid running into the wall too hard during the slow start phase, which
  10 // the traditional TCP implementation does.
  11 // http://netsrv.csc.ncsu.edu/export/hybridstart_pfldnet08.pdf
  12 // http://research.csc.ncsu.edu/netsrv/sites/default/files/hystart_techreport_2008.pdf
  13
  14 #ifndef NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
  15 #define NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
  16
  17 #include "base/basictypes.h"
  18 #include "net/base/net_export.h"
  19 #include "net/quic/quic_clock.h"
  20 #include "net/quic/quic_protocol.h"
  21 #include "net/quic/quic_time.h"
  22
  23 namespace net {
  24
  25 class NET_EXPORT_PRIVATE HybridSlowStart {
  26  public:
  27   explicit HybridSlowStart(const QuicClock* clock);
  28
  29   void OnPacketAcked(QuicPacketSequenceNumber acked_sequence_number,
  30                      bool in_slow_start);
  31
  32   void OnPacketSent(QuicPacketSequenceNumber sequence_number);
  33
  34   // ShouldExitSlowStart should be called on every new ack frame, since a new
  35   // RTT measurement can be made then.
  36   // rtt: the RTT for this ack packet.
  37   // min_rtt: is the lowest delay (RTT) we have seen during the session.
  38   // congestion_window: the congestion window in packets.
  39   bool ShouldExitSlowStart(QuicTime::Delta rtt,
  40                            QuicTime::Delta min_rtt,
  41                            int64 congestion_window);
  42
  43   // Start a new slow start phase.
  44   void Restart();
  45
  46   // TODO(ianswett): The following methods should be private, but that requires
  47   // a follow up CL to update the unit test.
  48   // Returns true if this ack the last sequence number of our current slow start
  49   // round.
  50   // Call Reset if this returns true.
  51   bool IsEndOfRound(QuicPacketSequenceNumber ack) const;
  52
  53   // Call for the start of each receive round (burst) in the slow start phase.
  54   void StartReceiveRound(QuicPacketSequenceNumber last_sent);
  55
  56   void set_ack_train_detection(bool ack_train_detection) {
  57     ack_train_detection_ = ack_train_detection;
  58   }
  59
  60   bool ack_train_detection() const {
  61     return ack_train_detection_;
  62   }
  63
  64   // Whether slow start has started.
  65   bool started() const {
  66     return started_;
  67   }
  68
  69  private:
  70   // Whether a condition for exiting slow start has been found.
  71   enum HystartState {
  72     NOT_FOUND,
  73     ACK_TRAIN,  // A closely spaced ack train is too long.
  74     DELAY,  // Too much increase in the round's min_rtt was observed.
  75   };
  76
  77   const QuicClock* clock_;
  78   bool ack_train_detection_;
  79   // Whether the hybrid slow start has been started.
  80   bool started_;
  81   HystartState hystart_found_;
  82   // Last sequence number sent which was CWND limited.
  83   QuicPacketSequenceNumber last_sent_sequence_number_;
  84
  85   // Variables for tracking acks received during a slow start round.
  86   QuicTime round_start_;  // Beginning of each slow start receive round.
  87   QuicPacketSequenceNumber end_sequence_number_;  // End of the receive round.
  88   // Last time when the spacing between ack arrivals was less than 2 ms.
  89   // Defaults to the beginning of the round.
  90   QuicTime last_close_ack_pair_time_;
  91   uint32 rtt_sample_count_;  // Number of rtt samples in the current round.
  92   QuicTime::Delta current_min_rtt_;  // The minimum rtt of current round.
  93
  94   DISALLOW_COPY_AND_ASSIGN(HybridSlowStart);
  95 };
  96
  97 }  // namespace net
  98
  99 #endif  // NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_