Add a feature to QuicConnection in which it sends probing retransmissions...

Add a feature to QuicConnection in which it sends probing retransmissions whenever it becomes application limited.

This also corrects the BBR behavior for determining when to probe.

Merge internal change: 174041573

R=rch@chromium.org

Bug: 
Change-Id: I7bdc95af25d83da37ad10f167da33a92fa991f2a
Reviewed-on: https://chromium-review.googlesource.com/777589Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Victor Vasiliev <vasilvv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517708}

net/quic/core/congestion_control/bbr_sender.cc

@@ -197,7 +197,7 @@ bool BbrSender::InRecovery() const {
 }
 
 bool BbrSender::IsProbingForMoreBandwidth() const {
-  return mode_ == PROBE_BW && pacing_gain_ > 1;
+  return (mode_ == PROBE_BW && pacing_gain_ > 1) || mode_ == STARTUP;
 }
 
 void BbrSender::SetFromConfig(const QuicConfig& config,

net/quic/core/quic_connection.cc

@@ -273,7 +273,9 @@ QuicConnection::QuicConnection(
       goaway_received_(false),
       write_error_occurred_(false),
       no_stop_waiting_frames_(false),
-      consecutive_num_packets_with_no_retransmittable_frames_(0) {
+      consecutive_num_packets_with_no_retransmittable_frames_(0),
+      fill_up_link_during_probing_(false),
+      probing_retransmission_pending_(false) {
   QUIC_DLOG(INFO) << ENDPOINT
                   << "Created connection with connection_id: " << connection_id;
   framer_.set_visitor(this);
@@ -2638,12 +2640,46 @@ const QuicTime::Delta QuicConnection::DelayedAckTime() {
       std::min(kMaxDelayedAckTimeMs, kMinRetransmissionTimeMs / 2));
 }
 
+void QuicConnection::MaybeSendProbingRetransmissions() {
+  DCHECK(fill_up_link_during_probing_);
+
+  if (!sent_packet_manager_.handshake_confirmed()) {
+    return;
+  }
+
+  if (!sent_packet_manager_.GetSendAlgorithm()->IsProbingForMoreBandwidth()) {
+    return;
+  }
+
+  if (probing_retransmission_pending_) {
+    QUIC_BUG << "MaybeSendProbingRetransmissions is called while another call "
+                "to it is already in progress";
+    return;
+  }
+
+  probing_retransmission_pending_ = true;
+  SendProbingRetransmissions();
+  probing_retransmission_pending_ = false;
+}
+
 void QuicConnection::CheckIfApplicationLimited() {
-  if (queued_packets_.empty() &&
+  bool application_limited =
+      queued_packets_.empty() &&
       !sent_packet_manager_.HasPendingRetransmissions() &&
-      !visitor_->WillingAndAbleToWrite()) {
-    sent_packet_manager_.OnApplicationLimited();
+      !visitor_->WillingAndAbleToWrite();
+
+  if (!application_limited) {
+    return;
+  }
+
+  if (fill_up_link_during_probing_) {
+    MaybeSendProbingRetransmissions();
+    if (!CanWrite(HAS_RETRANSMITTABLE_DATA)) {
+      return;
+    }
   }
+
+  sent_packet_manager_.OnApplicationLimited();
 }
 
 void QuicConnection::UpdatePacketContent(PacketContent type) {

net/quic/core/quic_connection.h

@@ -718,6 +718,13 @@ class QUIC_EXPORT_PRIVATE QuicConnection
     return last_packet_source_address_;
   }
 
+  bool fill_up_link_during_probing() const {
+    return fill_up_link_during_probing_;
+  }
+  void set_fill_up_link_during_probing(bool new_value) {
+    fill_up_link_during_probing_ = new_value;
+  }
+
  protected:
   // Calls cancel() on all the alarms owned by this connection.
   void CancelAllAlarms();
@@ -772,6 +779,9 @@ class QUIC_EXPORT_PRIVATE QuicConnection
   // If there are no packets to retransmit, does not do anything.
   void SendProbingRetransmissions();
 
+  // Decides whether to send probing retransmissions, and does so if required.
+  void MaybeSendProbingRetransmissions();
+
  private:
   friend class test::QuicConnectionPeer;
   friend class test::PacketSavingConnection;
@@ -1143,6 +1153,18 @@ class QUIC_EXPORT_PRIVATE QuicConnection
   // Consecutive number of sent packets which have no retransmittable frames.
   size_t consecutive_num_packets_with_no_retransmittable_frames_;
 
+  // If true, the connection will fill up the pipe with extra data whenever the
+  // congestion controller needs it in order to make a bandwidth estimate.  This
+  // is useful if the application pesistently underutilizes the link, but still
+  // relies on having a reasonable bandwidth estimate from the connection, e.g.
+  // for real time applications.
+  bool fill_up_link_during_probing_;
+
+  // If true, the probing retransmission will not be started again.  This is
+  // used to safeguard against an accidental tail recursion in probing
+  // retransmission code.
+  bool probing_retransmission_pending_;
+
   DISALLOW_COPY_AND_ASSIGN(QuicConnection);
 };
 

net/quic/core/quic_connection_test.cc

@@ -45,9 +45,11 @@ using testing::_;
 using testing::AnyNumber;
 using testing::AtLeast;
 using testing::DoAll;
+using testing::Ge;
 using testing::InSequence;
 using testing::Invoke;
 using testing::InvokeWithoutArgs;
+using testing::Lt;
 using testing::Ref;
 using testing::Return;
 using testing::SaveArg;
@@ -5489,6 +5491,48 @@ TEST_P(QuicConnectionTest, NotBecomeApplicationLimitedDueToWriteBlock) {
   connection_.SendStreamData3();
 }
 
+// Test the mode in which the link is filled up with probing retransmissions if
+// the connection becomes application-limited.
+TEST_P(QuicConnectionTest, SendDataWhenApplicationLimited) {
+  EXPECT_CALL(visitor_, OnSuccessfulVersionNegotiation(_));
+  EXPECT_CALL(*send_algorithm_, IsProbingForMoreBandwidth())
+      .WillRepeatedly(Return(true));
+  {
+    InSequence seq;
+    EXPECT_CALL(visitor_, WillingAndAbleToWrite()).WillRepeatedly(Return(true));
+    EXPECT_CALL(*send_algorithm_, OnPacketSent(_, _, _, _, _));
+    EXPECT_CALL(visitor_, WillingAndAbleToWrite())
+        .WillRepeatedly(Return(false));
+  }
+  // Fix congestion window to be 2000 bytes.
+  EXPECT_CALL(*send_algorithm_, CanSend(Ge(2000u)))
+      .WillRepeatedly(Return(false));
+  EXPECT_CALL(*send_algorithm_, CanSend(Lt(2000u)))
+      .WillRepeatedly(Return(true));
+
+  EXPECT_CALL(*send_algorithm_, OnApplicationLimited(_)).Times(0);
+  ASSERT_EQ(0u, connection_.GetStats().packets_sent);
+  connection_.set_fill_up_link_during_probing(true);
+  connection_.OnHandshakeComplete();
+  connection_.SendStreamData3();
+
+  // Since the frame data we send is small, we expect a lot of packets from a
+  // 2000 byte window.
+  EXPECT_GT(connection_.GetStats().packets_sent, 10u);
+
+  // Acknowledge all packets sent, except for the last one.
+  QuicAckFrame ack = InitAckFrame(
+      connection_.sent_packet_manager().GetLargestSentPacket() - 1);
+  EXPECT_CALL(*loss_algorithm_, DetectLosses(_, _, _, _, _));
+  EXPECT_CALL(*send_algorithm_, OnCongestionEvent(true, _, _, _, _));
+
+  // Ensure that since we no longer have retransmittable bytes in flight, this
+  // will not cause any responses to be sent.
+  EXPECT_CALL(*send_algorithm_, OnPacketSent(_, _, _, _, _)).Times(0);
+  EXPECT_CALL(*send_algorithm_, OnApplicationLimited(_)).Times(1);
+  ProcessAckPacket(&ack);
+}
+
 TEST_P(QuicConnectionTest, DonotForceSendingAckOnPacketTooLarge) {
   EXPECT_CALL(visitor_, OnSuccessfulVersionNegotiation(_));
   // Send an ack by simulating delayed ack alarm firing.

net/quic/core/quic_sent_packet_manager.cc

@@ -601,7 +601,7 @@ bool QuicSentPacketManager::MaybeRetransmitOldestPacket(TransmissionType type) {
     MarkForRetransmission(packet_number, type);
     return true;
   }
-  QUIC_DLOG(ERROR)
+  QUIC_DVLOG(1)
       << "No retransmittable packets, so RetransmitOldestPacket failed.";
   return false;
 }

net/quic/core/quic_sent_packet_manager.h

@@ -235,6 +235,8 @@ class QUIC_EXPORT_PRIVATE QuicSentPacketManager {
     return largest_packet_peer_knows_is_acked_;
   }
 
+  bool handshake_confirmed() const { return handshake_confirmed_; }
+
  private:
   friend class test::QuicConnectionPeer;
   friend class test::QuicSentPacketManagerPeer;