Deprecate quic_bbr_flexible_app_limited which was blocked by a bug that's now closed.  This was added for Quartc, but that project is no longer active.

PiperOrigin-RevId: 331636612
Change-Id: I05aae0bfe0474966646d8dd08ff311e53e0eb054

quic/core/congestion_control/bbr_sender.cc

diff --git a/quic/core/congestion_control/bbr_sender.cc b/quic/core/congestion_control/bbr_sender.cc
index 4de1a07..2948272 100644
--- a/quic/core/congestion_control/bbr_sender.cc
+++ b/quic/core/congestion_control/bbr_sender.cc
@@ -112,7 +112,6 @@
       probe_rtt_round_passed_(false),
       last_sample_is_app_limited_(false),
       has_non_app_limited_sample_(false),
-      flexible_app_limited_(false),
       recovery_state_(NOT_IN_RECOVERY),
       recovery_window_(max_congestion_window_),
       slower_startup_(false),
@@ -219,12 +218,7 @@
 
   // TODO(b/77975811): If the pipe is highly under-utilized, consider not
   // sending a probing transmission, because the extra bandwidth is not needed.
-  // If flexible_app_limited is enabled, check if the pipe is sufficiently full.
-  if (flexible_app_limited_) {
-    return !IsPipeSufficientlyFull();
-  } else {
-    return true;
-  }
+  return true;
 }
 
 bool BbrSender::IsPipeSufficientlyFull() const {
@@ -268,11 +262,6 @@
   if (config.HasClientRequestedIndependentOption(kBBR5, perspective)) {
     sampler_.SetMaxAckHeightTrackerWindowLength(4 * kBandwidthWindowSize);
   }
-  if (GetQuicReloadableFlag(quic_bbr_flexible_app_limited) &&
-      config.HasClientRequestedIndependentOption(kBBR9, perspective)) {
-    QUIC_RELOADABLE_FLAG_COUNT(quic_bbr_flexible_app_limited);
-    flexible_app_limited_ = true;
-  }
   if (config.HasClientRequestedIndependentOption(kBBQ1, perspective)) {
     set_high_gain(kDerivedHighGain);
     set_high_cwnd_gain(kDerivedHighGain);
@@ -885,9 +874,6 @@
   if (bytes_in_flight >= GetCongestionWindow()) {
     return;
   }
-  if (flexible_app_limited_ && IsPipeSufficientlyFull()) {
-    return;
-  }
 
   sampler_.OnAppLimited();
   QUIC_DVLOG(2) << "Becoming application limited. Last sent packet: "

quic/core/congestion_control/bbr_sender.h

diff --git a/quic/core/congestion_control/bbr_sender.h b/quic/core/congestion_control/bbr_sender.h
index d89e3fe..ae6cce9 100644
--- a/quic/core/congestion_control/bbr_sender.h
+++ b/quic/core/congestion_control/bbr_sender.h
@@ -354,9 +354,6 @@
   bool last_sample_is_app_limited_;
   // Indicates whether any non app-limited samples have been recorded.
   bool has_non_app_limited_sample_;
-  // Indicates app-limited calls should be ignored as long as there's
-  // enough data inflight to see more bandwidth when necessary.
-  bool flexible_app_limited_;
 
   // Current state of recovery.
   RecoveryState recovery_state_;

quic/core/crypto/crypto_protocol.h

diff --git a/quic/core/crypto/crypto_protocol.h b/quic/core/crypto/crypto_protocol.h
index a338b47..80273f0 100644
--- a/quic/core/crypto/crypto_protocol.h
+++ b/quic/core/crypto/crypto_protocol.h
@@ -96,8 +96,7 @@
                                                  // per cycle
 const QuicTag kBBR4 = TAG('B', 'B', 'R', '4');   // 20 RTT ack aggregation
 const QuicTag kBBR5 = TAG('B', 'B', 'R', '5');   // 40 RTT ack aggregation
-const QuicTag kBBR9 = TAG('B', 'B', 'R', '9');   // Ignore app-limited calls in
-                                                 // BBR if enough inflight.
+const QuicTag kBBR9 = TAG('B', 'B', 'R', '9');   // DEPRECATED
 const QuicTag kBBRS = TAG('B', 'B', 'R', 'S');   // DEPRECATED
 const QuicTag kBBQ1 = TAG('B', 'B', 'Q', '1');   // BBR with lower 2.77 STARTUP
                                                  // pacing and CWND gain.