Enable packet conservation only when conservativeRecovery is set; dis…

…able cwnd decrease on loss when ignoreLoss is set

Summary:
1. Noticed that bbr2 was not discovering the available BtlBw because it exited startup early due to loss. Disabling packet conservation seems to help, in sims. bbr1 has conservativeRecovery flag set to false by default. In the same spirit, we should enable/disable packet conservation in bbr2 based on that flag.
2. The CWND is decreased by the loss amount. This should be disabled when ignoreLoss is true.

Reviewed By: jbeshay, mjoras

Differential Revision: D55612685

fbshipit-source-id: 0c811de78e4a6f273f340631d29ff9dcd27d898e

quic/congestion_control/Bbr2.cpp

@@ -119,14 +119,18 @@ void Bbr2CongestionController::onPacketAckOrLoss(
             << ")";
   };
 
-  if (lossEvent && lossEvent->lostPackets > 0) {
+  if (lossEvent && lossEvent->lostPackets > 0 &&
+      conn_.transportSettings.ccaConfig.conservativeRecovery) {
     // The pseudo code in BBRHandleLostPacket is included in
     // updateProbeBwCyclePhase. No need to repeat it here.
 
     // We don't expose the loss type here, so always use fast recovery for
     // non-persistent congestion
     saveCwnd();
     inPacketConservation_ = true;
+    // Mark the connection as app-limited so bw samples during recovery are not
+    // taken into account.
+    setAppLimited();
     packetConservationStartTime_ = Clock::now();
     if (lossEvent->persistentCongestion) {
       cwndBytes_ = kMinCwndInMssForBbr * conn_.udpSendPacketLen;
@@ -280,7 +284,7 @@ void Bbr2CongestionController::setCwnd(
   auto inflightMax = addQuantizationBudget(
       getBDPWithGain(cwndGain_) + maxExtraAckedFilter_.GetBest());
   // BBRModulateCwndForRecovery()
-  if (lostBytes > 0) {
+  if (lostBytes > 0 && !conn_.transportSettings.ccaConfig.ignoreLoss) {
     cwndBytes_ = std::max(
         cwndBytes_ - std::min(lostBytes, cwndBytes_),
         kMinCwndInMssForBbr * conn_.udpSendPacketLen);

quic/state/TransportSettings.h

@@ -16,7 +16,7 @@
 namespace quic {
 
 struct CongestionControlConfig {
-  // Used by: BBR1
+  // Used by: BBR1, BBR2
   bool conservativeRecovery{false};
 
   // Used by: BBR1