Refactor to create a new LossDetectionInterface used by the

net/quic/congestion_control/tcp_loss_algorithm.cc

Index: net/quic/congestion_control/tcp_loss_algorithm.cc
diff --git a/net/quic/congestion_control/tcp_loss_algorithm.cc b/net/quic/congestion_control/tcp_loss_algorithm.cc
new file mode 100644
index 0000000000000000000000000000000000000000..72df0702b26f6bdf286c4caee6663fda950ef4f3
--- /dev/null
+++ b/net/quic/congestion_control/tcp_loss_algorithm.cc
@@ -0,0 +1,51 @@
+// Copyright (c) 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/quic_protocol.h"
+
+namespace net {
+
+namespace {
+// TCP retransmits after 3 nacks.
+static const size_t kNumberOfNacksBeforeRetransmission = 3;
+}
+
+TCPLossAlgorithm::TCPLossAlgorithm() { }
+
+// Uses nack counts to decide when packets are lost.
+SequenceNumberSet TCPLossAlgorithm::DetectLostPackets(
+    const QuicUnackedPacketMap& unacked_packets,
+    const QuicTime& time,
+    QuicPacketSequenceNumber largest_observed,
+    QuicTime::Delta srtt) {
+  SequenceNumberSet lost_packets;
+
+  for (QuicUnackedPacketMap::const_iterator it = unacked_packets.begin();
+       it != unacked_packets.end() && it->first <= largest_observed; ++it) {
+    if (!it->second.pending) {
+      continue;
+    }
+    size_t num_nacks_needed = kNumberOfNacksBeforeRetransmission;
+    // Check for early retransmit(RFC5827) when the last packet gets acked and
+    // the there are fewer than 4 pending packets.
+    // TODO(ianswett): Set a retransmission timer instead of losing the packet
+    // and retransmitting immediately.
+    if (it->second.retransmittable_frames &&
+        unacked_packets.largest_sent_packet() == largest_observed) {
+      num_nacks_needed = largest_observed - it->first;
+    }
+
+    if (it->second.nack_count < num_nacks_needed) {
+      continue;
+    }
+
+    lost_packets.insert(it->first);
+  }
+
+  return lost_packets;
+}
+
+}  // namespace net