When switch from QUIC BBR to BBRv2, use BBR's cwnd as the initial cwnd of BBRv2.

Protected by quic_reloadable_flag_quic_copy_bbr_cwnd_to_bbr2.

PiperOrigin-RevId: 330988495
Change-Id: I101432ec8e685a27f310f39ab4aaab98779de420

quic/core/congestion_control/bbr2_sender.cc

diff --git a/quic/core/congestion_control/bbr2_sender.cc b/quic/core/congestion_control/bbr2_sender.cc
index 4b2fac8..94cfc1f 100644
--- a/quic/core/congestion_control/bbr2_sender.cc
+++ b/quic/core/congestion_control/bbr2_sender.cc
@@ -73,8 +73,10 @@
              /*cwnd_gain=*/1.0,
              /*pacing_gain=*/kInitialPacingGain,
              old_sender ? &old_sender->sampler_ : nullptr),
-      initial_cwnd_(
-          cwnd_limits().ApplyLimits(initial_cwnd_in_packets * kDefaultTCPMSS)),
+      initial_cwnd_(cwnd_limits().ApplyLimits(
+          (GetQuicReloadableFlag(quic_copy_bbr_cwnd_to_bbr2) && old_sender)
+              ? old_sender->GetCongestionWindow()
+              : (initial_cwnd_in_packets * kDefaultTCPMSS))),
       cwnd_(initial_cwnd_),
       pacing_rate_(kInitialPacingGain * QuicBandwidth::FromBytesAndTimeDelta(
                                             cwnd_,
@@ -84,6 +86,10 @@
       probe_bw_(this, &model_),
       probe_rtt_(this, &model_),
       last_sample_is_app_limited_(false) {
+  if (GetQuicReloadableFlag(quic_copy_bbr_cwnd_to_bbr2) && old_sender) {
+    QUIC_RELOADABLE_FLAG_COUNT(quic_copy_bbr_cwnd_to_bbr2);
+  }
+
   QUIC_DVLOG(2) << this << " Initializing Bbr2Sender. mode:" << mode_
                 << ", PacingRate:" << pacing_rate_ << ", Cwnd:" << cwnd_
                 << ", CwndLimits:" << cwnd_limits() << "  @ " << now;

quic/core/congestion_control/bbr2_simulator_test.cc

diff --git a/quic/core/congestion_control/bbr2_simulator_test.cc b/quic/core/congestion_control/bbr2_simulator_test.cc
index 0dcf427..2036eb1 100644
--- a/quic/core/congestion_control/bbr2_simulator_test.cc
+++ b/quic/core/congestion_control/bbr2_simulator_test.cc
@@ -912,7 +912,7 @@
   BbrSender old_sender(sender_connection()->clock()->Now(),
                        sender_connection()->sent_packet_manager().GetRttStats(),
                        GetUnackedMap(sender_connection()),
-                       kDefaultInitialCwndPackets,
+                       kDefaultInitialCwndPackets + 1,
                        GetQuicFlag(FLAGS_quic_max_congestion_window), &random_,
                        QuicConnectionPeer::GetStats(sender_connection()));
 
@@ -927,7 +927,11 @@
   }
 
   // Switch from |old_sender| to |sender_|.
+  const QuicByteCount old_sender_cwnd = old_sender.GetCongestionWindow();
   sender_ = SetupBbr2Sender(&sender_endpoint_, &old_sender);
+  if (GetQuicReloadableFlag(quic_copy_bbr_cwnd_to_bbr2)) {
+    EXPECT_EQ(old_sender_cwnd, sender_->GetCongestionWindow());
+  }
 
   // Send packets 5-7.
   now = now + QuicTime::Delta::FromMilliseconds(10);