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 // Whether slow start has started. 57 bool started() const { 58 return started_; 59 } 60 61 private: 62 // Whether a condition for exiting slow start has been found. 63 enum HystartState { 64 NOT_FOUND, 65 ACK_TRAIN, // A closely spaced ack train is too long. 66 DELAY, // Too much increase in the round's min_rtt was observed. 67 }; 68 69 const QuicClock* clock_; 70 // Whether the hybrid slow start has been started. 71 bool started_; 72 HystartState hystart_found_; 73 // Last sequence number sent which was CWND limited. 74 QuicPacketSequenceNumber last_sent_sequence_number_; 75 76 // Variables for tracking acks received during a slow start round. 77 QuicTime round_start_; // Beginning of each slow start receive round. 78 QuicPacketSequenceNumber end_sequence_number_; // End of the receive round. 79 // Last time when the spacing between ack arrivals was less than 2 ms. 80 // Defaults to the beginning of the round. 81 QuicTime last_close_ack_pair_time_; 82 uint32 rtt_sample_count_; // Number of rtt samples in the current round. 83 QuicTime::Delta current_min_rtt_; // The minimum rtt of current round. 84 85 DISALLOW_COPY_AND_ASSIGN(HybridSlowStart); 86 }; 87 88 } // namespace net 89 90 #endif // NET_QUIC_CONGESTION_CONTROL_HYBRID_SLOW_START_H_ 91
