net/quic/congestion_control/tcp_loss_algorithm.cc

   1 // Copyright 2014 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 #include "net/quic/congestion_control/tcp_loss_algorithm.h"
   6
   7 #include "net/quic/congestion_control/rtt_stats.h"
   8 #include "net/quic/quic_protocol.h"
   9
  10 namespace net {
  11
  12 namespace {
  13
  14 // TCP retransmits after 3 nacks.
  15 static const size_t kNumberOfNacksBeforeRetransmission = 3;
  16
  17 // How many RTTs the algorithm waits before determining a packet is lost due
  18 // to early retransmission.
  19 static const double kEarlyRetransmitLossDelayMultiplier = 1.25;
  20
  21 }
  22
  23 TCPLossAlgorithm::TCPLossAlgorithm()
  24     : loss_detection_timeout_(QuicTime::Zero()) { }
  25
  26 LossDetectionType TCPLossAlgorithm::GetLossDetectionType() const {
  27   return kNack;
  28 }
  29
  30 // Uses nack counts to decide when packets are lost.
  31 SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
  32     const QuicUnackedPacketMap& unacked_packets,
  33     const QuicTime& time,
  34     QuicPacketSequenceNumber largest_observed,
  35     const RttStats& rtt_stats) {
  36   SequenceNumberSet lost_packets;
  37   loss_detection_timeout_ = QuicTime::Zero();
  38   QuicTime::Delta loss_delay =
  39       rtt_stats.SmoothedRtt().Multiply(kEarlyRetransmitLossDelayMultiplier);
  40   QuicPacketSequenceNumber sequence_number = unacked_packets.GetLeastUnacked();
  41   for (QuicUnackedPacketMap::const_iterator it = unacked_packets.begin();
  42        it != unacked_packets.end() && sequence_number <= largest_observed;
  43        ++it, ++sequence_number) {
  44     if (!it->in_flight) {
  45       continue;
  46     }
  47
  48     LOG_IF(DFATAL, it->nack_count == 0)
  49         << "All packets less than largest observed should have been nacked."
  50         << "sequence_number:" << sequence_number
  51         << " largest_observed:" << largest_observed;
  52     if (it->nack_count >= kNumberOfNacksBeforeRetransmission) {
  53       lost_packets.insert(sequence_number);
  54       continue;
  55     }
  56
  57     // Only early retransmit(RFC5827) when the last packet gets acked and
  58     // there are retransmittable packets in flight.
  59     // This also implements a timer-protected variant of FACK.
  60     if (it->retransmittable_frames &&
  61         unacked_packets.largest_sent_packet() == largest_observed) {
  62       // Early retransmit marks the packet as lost once 1.25RTTs have passed
  63       // since the packet was sent and otherwise sets an alarm.
  64       if (time >= it->sent_time.Add(loss_delay)) {
  65         lost_packets.insert(sequence_number);
  66       } else {
  67         // Set the timeout for the earliest retransmittable packet where early
  68         // retransmit applies.
  69         loss_detection_timeout_ = it->sent_time.Add(loss_delay);
  70         break;
  71       }
  72     }
  73   }
  74
  75   return lost_packets;
  76 }
  77
  78 QuicTime TCPLossAlgorithm::GetLossTimeout() const {
  79   return loss_detection_timeout_;
  80 }
  81
  82 }  // namespace net