Change QUIC's delayed ack timer from 100ms to 25ms.

net/quic/quic_sent_packet_manager.cc

Index: net/quic/quic_sent_packet_manager.cc
diff --git a/net/quic/quic_sent_packet_manager.cc b/net/quic/quic_sent_packet_manager.cc
index a0ce684fe7f9d848c5a4427630254d1f77a5af94..ce6af66e0cd7c91467c661bc062b6f4f3362fde9 100644
--- a/net/quic/quic_sent_packet_manager.cc
+++ b/net/quic/quic_sent_packet_manager.cc
@@ -49,8 +49,6 @@ static const size_t kNumMinRttSamplesAfterQuiescence = 2;
 // Number of unpaced packets to send after quiescence.
 static const size_t kInitialUnpacedBurst = 10;
 
-// Use a 1 minute window for Recent Min RTT with BBR.
-
 bool HasCryptoHandshake(const TransmissionInfo& transmission_info) {
   if (transmission_info.retransmittable_frames == NULL) {
     return false;
@@ -740,6 +738,8 @@ QuicTime::Delta QuicSentPacketManager::TimeUntilSend(
       now, unacked_packets_.bytes_in_flight(), retransmittable);
 }
 
+// Uses a 25ms delayed ack timer. Also helps with better signaling
+// in low-bandwidth (< ~384 kbps), where an ack is sent per packet.
 // Ensures that the Delayed Ack timer is always set to a value lesser
 // than the retransmission timer's minimum value (MinRTO). We want the
 // delayed ack to get back to the QUIC peer before the sender's
@@ -753,7 +753,8 @@ QuicTime::Delta QuicSentPacketManager::TimeUntilSend(
 // any benefits, but if the delayed ack becomes a significant source
 // of (likely, tail) latency, then consider such a mechanism.
 const QuicTime::Delta QuicSentPacketManager::DelayedAckTime() const {
-  return QuicTime::Delta::FromMilliseconds(kMinRetransmissionTimeMs/2);
+  return QuicTime::Delta::FromMilliseconds(min(kMaxDelayedAckTime,
+                                               kMinRetransmissionTimeMs/2));
 }
 
 const QuicTime QuicSentPacketManager::GetRetransmissionTime() const {
@@ -805,7 +806,8 @@ const QuicTime::Delta QuicSentPacketManager::GetTailLossProbeDelay() const {
   QuicTime::Delta srtt = rtt_stats_.SmoothedRtt();
   if (!unacked_packets_.HasMultipleInFlightPackets()) {
     return QuicTime::Delta::Max(
-        srtt.Multiply(1.5).Add(DelayedAckTime()), srtt.Multiply(2));
+        srtt.Multiply(2), srtt.Multiply(1.5)
+          .Add(QuicTime::Delta::FromMilliseconds(kMinRetransmissionTimeMs/2)));
   }
   return QuicTime::Delta::FromMilliseconds(
       max(kMinTailLossProbeTimeoutMs,