Deprecate --gfe2_reloadable_flag_quic_bbr2_enable_bbpd_by_default.

PiperOrigin-RevId: 675166014
diff --git a/quiche/common/quiche_feature_flags_list.h b/quiche/common/quiche_feature_flags_list.h
index d526bac..84a1efb 100755
--- a/quiche/common/quiche_feature_flags_list.h
+++ b/quiche/common/quiche_feature_flags_list.h
@@ -14,7 +14,6 @@
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_add_stream_info_to_idle_close_detail, false, true, "If true, include stream information in idle timeout connection close detail.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_allow_client_enabled_bbr_v2, true, true, "If true, allow client to enable BBRv2 on server via connection option 'B2ON'.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_allow_host_in_request2, false, false, "If true, requests with a host header will be allowed.")
-QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr2_enable_bbpd_by_default, true, true, "If true, QUIC BBR2 will use a 0.91 PROBE_DOWN gain by default.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr2_extra_acked_window, false, true, "When true, the BBR4 copt sets the extra_acked window to 20 RTTs and BBR5 sets it to 40 RTTs.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr2_probe_two_rounds, true, true, "When true, the BB2U copt causes BBR2 to wait two rounds with out draining the queue before exiting PROBE_UP and BB2S has the same effect in STARTUP.")
 QUICHE_FLAG(bool, quiche_reloadable_flag_quic_bbr2_simplify_inflight_hi, true, true, "When true, the BBHI copt causes QUIC BBRv2 to use a simpler algorithm for raising inflight_hi in PROBE_UP.")
diff --git a/quiche/quic/core/congestion_control/bbr2_misc.h b/quiche/quic/core/congestion_control/bbr2_misc.h
index 15dcd53..6dbe904 100644
--- a/quiche/quic/core/congestion_control/bbr2_misc.h
+++ b/quiche/quic/core/congestion_control/bbr2_misc.h
@@ -139,7 +139,7 @@
 
   // Pacing gains.
   float probe_bw_probe_up_pacing_gain = 1.25;
-  float probe_bw_probe_down_pacing_gain = 0.75;
+  float probe_bw_probe_down_pacing_gain = 0.91;
   float probe_bw_default_pacing_gain = 1.0;
 
   float probe_bw_cwnd_gain = 2.0;
diff --git a/quiche/quic/core/congestion_control/bbr2_sender.cc b/quiche/quic/core/congestion_control/bbr2_sender.cc
index be7f132..3e4ecb3 100644
--- a/quiche/quic/core/congestion_control/bbr2_sender.cc
+++ b/quiche/quic/core/congestion_control/bbr2_sender.cc
@@ -197,12 +197,6 @@
   if (ContainsQuicTag(connection_options, kB206)) {
     params_.startup_full_loss_count = params_.probe_bw_full_loss_count;
   }
-  if (GetQuicReloadableFlag(quic_bbr2_enable_bbpd_by_default) ||
-      ContainsQuicTag(connection_options, kBBPD)) {
-    QUIC_RELOADABLE_FLAG_COUNT(quic_bbr2_enable_bbpd_by_default);
-    // Derived constant to ensure fairness.
-    params_.probe_bw_probe_down_pacing_gain = 0.91;
-  }
   if (GetQuicReloadableFlag(quic_bbr2_simplify_inflight_hi) &&
       ContainsQuicTag(connection_options, kBBHI)) {
     QUIC_RELOADABLE_FLAG_COUNT(quic_bbr2_simplify_inflight_hi);
diff --git a/quiche/quic/core/congestion_control/bbr2_simulator_test.cc b/quiche/quic/core/congestion_control/bbr2_simulator_test.cc
index 7bae93d..3cc8198 100644
--- a/quiche/quic/core/congestion_control/bbr2_simulator_test.cc
+++ b/quiche/quic/core/congestion_control/bbr2_simulator_test.cc
@@ -2386,39 +2386,6 @@
   ASSERT_TRUE(simulator_result);
 }
 
-TEST_F(Bbr2MultiSenderTest, Bbr2VsBbr2BBPD) {
-  SetConnectionOption(sender_0_, kBBPD);
-  Bbr2Sender* sender_1 = SetupBbr2Sender(sender_endpoints_[1].get());
-  SetConnectionOption(sender_1, kBBPD);
-
-  MultiSenderTopologyParams params;
-  CreateNetwork(params);
-
-  const QuicByteCount transfer_size = 10 * 1024 * 1024;
-  const QuicTime::Delta transfer_time =
-      params.BottleneckBandwidth().TransferTime(transfer_size);
-  QUIC_LOG(INFO) << "Single flow transfer time: " << transfer_time;
-
-  // Transfer 10% of data in first transfer.
-  sender_endpoints_[0]->AddBytesToTransfer(transfer_size);
-  bool simulator_result = simulator_.RunUntilOrTimeout(
-      [this]() {
-        return receiver_endpoints_[0]->bytes_received() >= 0.1 * transfer_size;
-      },
-      transfer_time);
-  ASSERT_TRUE(simulator_result);
-
-  // Start the second transfer and wait until both finish.
-  sender_endpoints_[1]->AddBytesToTransfer(transfer_size);
-  simulator_result = simulator_.RunUntilOrTimeout(
-      [this]() {
-        return receiver_endpoints_[0]->bytes_received() == transfer_size &&
-               receiver_endpoints_[1]->bytes_received() == transfer_size;
-      },
-      3 * transfer_time);
-  ASSERT_TRUE(simulator_result);
-}
-
 TEST_F(Bbr2MultiSenderTest, QUIC_SLOW_TEST(MultipleBbr2s)) {
   const int kTotalNumSenders = 6;
   for (int i = 1; i < kTotalNumSenders; ++i) {
diff --git a/quiche/quic/core/crypto/crypto_protocol.h b/quiche/quic/core/crypto/crypto_protocol.h
index 83e6ab7..e05f869 100644
--- a/quiche/quic/core/crypto/crypto_protocol.h
+++ b/quiche/quic/core/crypto/crypto_protocol.h
@@ -31,7 +31,7 @@
 // "QNZR", "B2HI", "H2PR", "FIFO", "LIFO", "RRWS", "QNSP", "B2CL", "CHSP",
 // "BPTE", "ACKD", "AKD2", "AKD4", "MAD1", "MAD4", "MAD5", "ACD0", "ACKQ",
 // "TLPR", "CCS\0", "PDP4", "NCHP", "NBPE", "2RTO", "3RTO", "4RTO", "6RTO",
-// "PDP1", "PDP2", "PDP3", "PDP5", "QLVE", "RVCM"
+// "PDP1", "PDP2", "PDP3", "PDP5", "QLVE", "RVCM", "BBPD"
 
 // clang-format off
 const QuicTag kCHLO = TAG('C', 'H', 'L', 'O');   // Client hello
@@ -125,7 +125,6 @@
                                                  // bw_lo * bytes_lost/cwnd
 const QuicTag kBBQ0 = TAG('B', 'B', 'Q', '0');   // Increase bytes_acked in
                                                  // PROBE_UP when app limited.
-const QuicTag kBBPD = TAG('B', 'B', 'P', 'D');   // Use 0.91 PROBE_DOWN gain.
 const QuicTag kBBHI = TAG('B', 'B', 'H', 'I');   // Increase inflight_hi in
                                                  // PROBE_UP if ever inflight_hi
                                                  // limited in round