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 // This does not implement ack train detection because it interacts poorly with
  12 // pacing.
  13 // http://netsrv.csc.ncsu.edu/export/hybridstart_pfldnet08.pdf
  14 // http://research.csc.ncsu.edu/netsrv/sites/default/files/hystart_techreport_2008.pdf
  15
  16 #ifndef NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
  17 #define NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_
  18
  19 #include "base/basictypes.h"
  20 #include "net/base/net_export.h"
  21 #include "net/quic/quic_clock.h"
  22 #include "net/quic/quic_protocol.h"
  23 #include "net/quic/quic_time.h"
  24
  25 namespace net {
  26
  27 class NET_EXPORT_PRIVATE HybridSlowStart {
  28  public:
  29   explicit HybridSlowStart(const QuicClock* clock);
  30
  31   void OnPacketAcked(QuicPacketSequenceNumber acked_sequence_number,
  32                      bool in_slow_start);
  33
  34   void OnPacketSent(QuicPacketSequenceNumber sequence_number);
  35
  36   // ShouldExitSlowStart should be called on every new ack frame, since a new
  37   // RTT measurement can be made then.
  38   // rtt: the RTT for this ack packet.
  39   // min_rtt: is the lowest delay (RTT) we have seen during the session.
  40   // congestion_window: the congestion window in packets.
  41   bool ShouldExitSlowStart(QuicTime::Delta rtt,
  42                            QuicTime::Delta min_rtt,
  43                            int64 congestion_window);
  44
  45   // Start a new slow start phase.
  46   void Restart();
  47
  48   // TODO(ianswett): The following methods should be private, but that requires
  49   // a follow up CL to update the unit test.
  50   // Returns true if this ack the last sequence number of our current slow start
  51   // round.
  52   // Call Reset if this returns true.
  53   bool IsEndOfRound(QuicPacketSequenceNumber ack) const;
  54
  55   // Call for the start of each receive round (burst) in the slow start phase.
  56   void StartReceiveRound(QuicPacketSequenceNumber last_sent);
  57
  58   // Whether slow start has started.
  59   bool started() const {
  60     return started_;
  61   }
  62
  63  private:
  64   // Whether a condition for exiting slow start has been found.
  65   enum HystartState {
  66     NOT_FOUND,
  67     DELAY,  // Too much increase in the round's min_rtt was observed.
  68   };
  69
  70   const QuicClock* clock_;
  71   // Whether the hybrid slow start has been started.
  72   bool started_;
  73   HystartState hystart_found_;
  74   // Last sequence number sent which was CWND limited.
  75   QuicPacketSequenceNumber last_sent_sequence_number_;
  76
  77   // Variables for tracking acks received during a slow start round.
  78   QuicPacketSequenceNumber end_sequence_number_;  // End of the receive round.
  79   uint32 rtt_sample_count_;  // Number of rtt samples in the current round.
  80   QuicTime::Delta current_min_rtt_;  // The minimum rtt of current round.
  81
  82   DISALLOW_COPY_AND_ASSIGN(HybridSlowStart);
  83 };
  84
  85 }  // namespace net
  86
  87 #endif  // NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_