relnote: Add a new LazyFack loss detection to QUIC triggered by the LFAK connection option.

net/quic/core/congestion_control/general_loss_algorithm.cc

Index: net/quic/core/congestion_control/general_loss_algorithm.cc
diff --git a/net/quic/core/congestion_control/general_loss_algorithm.cc b/net/quic/core/congestion_control/general_loss_algorithm.cc
index 6c82329ab058454db76f88d7ecd0a2fdea6e4498..6d7521c60361c249c92f9a6c5b65725b57341ee3 100644
--- a/net/quic/core/congestion_control/general_loss_algorithm.cc
+++ b/net/quic/core/congestion_control/general_loss_algorithm.cc
@@ -30,7 +30,8 @@ GeneralLossAlgorithm::GeneralLossAlgorithm()
     : loss_detection_timeout_(QuicTime::Zero()),
       largest_sent_on_spurious_retransmit_(0),
       loss_type_(kNack),
-      reordering_shift_(kDefaultLossDelayShift) {}
+      reordering_shift_(kDefaultLossDelayShift),
+      largest_previously_acked_(0) {}
 
 GeneralLossAlgorithm::GeneralLossAlgorithm(LossDetectionType loss_type)
     : loss_detection_timeout_(QuicTime::Zero()),
@@ -38,7 +39,8 @@ GeneralLossAlgorithm::GeneralLossAlgorithm(LossDetectionType loss_type)
       loss_type_(loss_type),
       reordering_shift_(loss_type == kAdaptiveTime
                             ? kDefaultAdaptiveLossDelayShift
-                            : kDefaultLossDelayShift) {}
+                            : kDefaultLossDelayShift),
+      largest_previously_acked_(0) {}
 
 LossDetectionType GeneralLossAlgorithm::GetLossDetectionType() const {
   return loss_type_;
@@ -51,6 +53,7 @@ void GeneralLossAlgorithm::SetLossDetectionType(LossDetectionType loss_type) {
   reordering_shift_ = loss_type == kAdaptiveTime
                           ? kDefaultAdaptiveLossDelayShift
                           : kDefaultLossDelayShift;
+  largest_previously_acked_ = 0;
 }
 
 // Uses nack counts to decide when packets are lost.
@@ -81,6 +84,16 @@ void GeneralLossAlgorithm::DetectLosses(
         packets_lost->push_back(std::make_pair(packet_number, it->bytes_sent));
         continue;
       }
+    } else if (loss_type_ == kLazyFack) {
+      // Require two in order acks to invoke FACK, which avoids spuriously
+      // retransmitting packets when one packet is reordered by a large amount.
+      if (largest_newly_acked > largest_previously_acked_ &&
+          largest_previously_acked_ > packet_number &&
+          largest_previously_acked_ - packet_number >=
+              (kNumberOfNacksBeforeRetransmission - 1)) {
+        packets_lost->push_back(std::make_pair(packet_number, it->bytes_sent));
+        continue;
+      }
     }
 
     // Only early retransmit(RFC5827) when the last packet gets acked and
@@ -108,6 +121,7 @@ void GeneralLossAlgorithm::DetectLosses(
       continue;
     }
   }
+  largest_previously_acked_ = largest_newly_acked;
 }
 
 QuicTime GeneralLossAlgorithm::GetLossTimeout() const {