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// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "net/quic/congestion_control/tcp_loss_algorithm.h"
#include "net/quic/congestion_control/rtt_stats.h"
#include "net/quic/quic_protocol.h"
namespace net {
namespace {
// TCP retransmits after 3 nacks.
static const size_t kNumberOfNacksBeforeRetransmission = 3;
// How many RTTs the algorithm waits before determining a packet is lost due
// to early retransmission.
static const double kEarlyRetransmitLossDelayMultiplier = 1.25;
}
TCPLossAlgorithm::TCPLossAlgorithm()
: loss_detection_timeout_(QuicTime::Zero()) { }
LossDetectionType TCPLossAlgorithm::GetLossDetectionType() const {
return kNack;
}
// Uses nack counts to decide when packets are lost.
SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
const QuicUnackedPacketMap& unacked_packets,
const QuicTime& time,
QuicPacketSequenceNumber largest_observed,
const RttStats& rtt_stats) {
SequenceNumberSet lost_packets;
loss_detection_timeout_ = QuicTime::Zero();
QuicTime::Delta loss_delay =
rtt_stats.SmoothedRtt().Multiply(kEarlyRetransmitLossDelayMultiplier);
for (QuicUnackedPacketMap::const_iterator it = unacked_packets.begin();
it != unacked_packets.end() && it->first <= largest_observed; ++it) {
if (!it->second.pending) {
continue;
}
LOG_IF(DFATAL, it->second.nack_count == 0)
<< "All packets less than largest observed should have been nacked.";
if (it->second.nack_count >= kNumberOfNacksBeforeRetransmission) {
lost_packets.insert(it->first);
continue;
}
// Only early retransmit(RFC5827) when the last packet gets acked and
// there are pending retransmittable packets.
// This also implements a timer-protected variant of FACK.
if (it->second.retransmittable_frames &&
unacked_packets.largest_sent_packet() == largest_observed) {
// Early retransmit marks the packet as lost once 1.25RTTs have passed
// since the packet was sent and otherwise sets an alarm.
if (time >= it->second.sent_time.Add(loss_delay)) {
lost_packets.insert(it->first);
} else {
// Set the timeout for the earliest retransmittable packet where early
// retransmit applies.
loss_detection_timeout_ = it->second.sent_time.Add(loss_delay);
break;
}
}
}
return lost_packets;
}
QuicTime TCPLossAlgorithm::GetLossTimeout() const {
return loss_detection_timeout_;
}
} // namespace net
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