Add kAFF1 enabling the usage of smoothed RTT in the computation of ack delay in AckFrequencyFrame.

Protected by FLAGS_quic_reloadable_flag_quic_can_send_ack_frequency.

PiperOrigin-RevId: 337381712
Change-Id: Icdc57863d57e54a516fe73139903e974097800e5

quic/core/crypto/crypto_protocol.h

diff --git a/quic/core/crypto/crypto_protocol.h b/quic/core/crypto/crypto_protocol.h
index ab0ef01..57c9989 100644
--- a/quic/core/crypto/crypto_protocol.h
+++ b/quic/core/crypto/crypto_protocol.h
@@ -165,6 +165,8 @@
                                                  // 1 RTT of not receiving.
 const QuicTag kAFFE = TAG('A', 'F', 'F', 'E');   // Enable client receiving
                                                  // AckFrequencyFrame.
+const QuicTag kAFF1 = TAG('A', 'F', 'F', '1');   // Use SRTT in building
+                                                 // AckFrequencyFrame.
 const QuicTag kSSLR = TAG('S', 'S', 'L', 'R');   // Slow Start Large Reduction.
 const QuicTag kNPRR = TAG('N', 'P', 'R', 'R');   // Pace at unity instead of PRR
 const QuicTag k2RTO = TAG('2', 'R', 'T', 'O');   // Close connection on 2 RTOs

quic/core/quic_sent_packet_manager.cc

diff --git a/quic/core/quic_sent_packet_manager.cc b/quic/core/quic_sent_packet_manager.cc
index b98f65c..bdad0e4 100644
--- a/quic/core/quic_sent_packet_manager.cc
+++ b/quic/core/quic_sent_packet_manager.cc
@@ -150,6 +150,9 @@
       peer_min_ack_delay_ =
           QuicTime::Delta::FromMilliseconds(config.ReceivedMinAckDelayMs());
     }
+    if (config.HasClientSentConnectionOption(kAFF1, perspective)) {
+      use_smoothed_rtt_in_ack_delay_ = true;
+    }
   }
   if (config.HasClientSentConnectionOption(kMAD0, perspective)) {
     rtt_stats_.set_ignore_max_ack_delay(true);
@@ -725,7 +728,8 @@
 
   QUIC_RELOADABLE_FLAG_COUNT(quic_can_send_ack_frequency);
   frame.packet_tolerance = kMaxRetransmittablePacketsBeforeAck;
-  auto rtt = rtt_stats_.MinOrInitialRtt();
+  auto rtt = use_smoothed_rtt_in_ack_delay_ ? rtt_stats_.SmoothedOrInitialRtt()
+                                            : rtt_stats_.MinOrInitialRtt();
   frame.max_ack_delay = rtt * kAckDecimationDelay;
   frame.max_ack_delay = std::max(frame.max_ack_delay, peer_min_ack_delay_);
 

quic/core/quic_sent_packet_manager.h

diff --git a/quic/core/quic_sent_packet_manager.h b/quic/core/quic_sent_packet_manager.h
index ef5972a..64cd4c0 100644
--- a/quic/core/quic_sent_packet_manager.h
+++ b/quic/core/quic_sent_packet_manager.h
@@ -659,6 +659,9 @@
   // AckFrequencyFrame.
   QuicTime::Delta peer_min_ack_delay_ = QuicTime::Delta::Infinite();
 
+  // Use smoothed RTT for computing max_ack_delay in AckFrequency frame.
+  bool use_smoothed_rtt_in_ack_delay_ = false;
+
   // The history of outstanding max_ack_delays sent to peer. Outstanding means
   // a max_ack_delay is sent as part of the last acked AckFrequencyFrame or
   // an unacked AckFrequencyFrame after that.

quic/core/quic_sent_packet_manager_test.cc

diff --git a/quic/core/quic_sent_packet_manager_test.cc b/quic/core/quic_sent_packet_manager_test.cc
index 4bb16a3..5504e8e 100644
--- a/quic/core/quic_sent_packet_manager_test.cc
+++ b/quic/core/quic_sent_packet_manager_test.cc
@@ -4420,7 +4420,7 @@
   EXPECT_EQ(message_frame->message_length, 0);
 }
 
-TEST_F(QuicSentPacketManagerTest, SendAckFrequencyFrame) {
+TEST_F(QuicSentPacketManagerTest, BuildAckFrequencyFrame) {
   SetQuicReloadableFlag(quic_can_send_ack_frequency, true);
   EXPECT_CALL(*send_algorithm_, SetFromConfig(_, _));
   EXPECT_CALL(*network_change_visitor_, OnCongestionChange());
@@ -4447,6 +4447,35 @@
   EXPECT_EQ(frame.packet_tolerance, 10u);
 }
 
+TEST_F(QuicSentPacketManagerTest, BuildAckFrequencyFrameWithSRTT) {
+  SetQuicReloadableFlag(quic_can_send_ack_frequency, true);
+  EXPECT_CALL(*send_algorithm_, SetFromConfig(_, _));
+  EXPECT_CALL(*network_change_visitor_, OnCongestionChange());
+  QuicConfig config;
+  QuicConfigPeer::SetReceivedMinAckDelayMs(&config, /*min_ack_delay_ms=*/1);
+  QuicTagVector quic_tag_vector;
+  quic_tag_vector.push_back(kAFF1);  // SRTT enabling tag.
+  QuicConfigPeer::SetReceivedConnectionOptions(&config, quic_tag_vector);
+  manager_.SetFromConfig(config);
+  manager_.SetHandshakeConfirmed();
+
+  // Set up RTTs.
+  auto* rtt_stats = const_cast<RttStats*>(manager_.GetRttStats());
+  rtt_stats->UpdateRtt(QuicTime::Delta::FromMilliseconds(8),
+                       /*ack_delay=*/QuicTime::Delta::Zero(),
+                       /*now=*/QuicTime::Zero());
+  // Make sure srtt and min_rtt are different.
+  rtt_stats->UpdateRtt(
+      QuicTime::Delta::FromMilliseconds(16),
+      /*ack_delay=*/QuicTime::Delta::Zero(),
+      /*now=*/QuicTime::Zero() + QuicTime::Delta::FromMilliseconds(24));
+
+  auto frame = manager_.GetUpdatedAckFrequencyFrame();
+  EXPECT_EQ(frame.max_ack_delay,
+            std::max(rtt_stats->SmoothedOrInitialRtt() * 0.25,
+                     QuicTime::Delta::FromMilliseconds(1u)));
+}
+
 }  // namespace
 }  // namespace test
 }  // namespace quic