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

net/quic/quic_sent_packet_manager.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/quic_sent_packet_manager.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "base/stl_util.h"
@@ skipping 31 matching lines @@
 // per draft RFC draft-dukkipati-tcpm-tcp-loss-probe.
 static const size_t kDefaultMaxTailLossProbes = 2;
 static const int64 kMinTailLossProbeTimeoutMs = 10;
 
 // Number of samples before we force a new recent min rtt to be captured.
 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;
   }
   return transmission_info.retransmittable_frames->HasCryptoHandshake() ==
       IS_HANDSHAKE;
 }
 
 }  // namespace
 
@@ skipping 669 matching lines @@
     HasRetransmittableData retransmittable) {
   // The TLP logic is entirely contained within QuicSentPacketManager, so the
   // send algorithm does not need to be consulted.
   if (pending_tlp_transmission_) {
     return QuicTime::Delta::Zero();
   }
   return send_algorithm_->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
 // retransmission timer triggers.  Since we do not know the
 // reverse-path one-way delay, we assume equal delays for forward and
 // reverse paths, and ensure that the timer is set to less than half
 // of the MinRTO.
 // There may be a value in making this delay adaptive with the help of
 // the sender and a signaling mechanism -- if the sender uses a
 // different MinRTO, we may get spurious retransmissions. May not have
 // 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 {
   // Don't set the timer if there are no packets in flight or we've already
   // queued a tlp transmission and it hasn't been sent yet.
   if (!unacked_packets_.HasInFlightPackets() || pending_tlp_transmission_) {
     return QuicTime::Zero();
   }
   switch (GetRetransmissionMode()) {
     case HANDSHAKE_MODE:
@@ skipping 31 matching lines @@
   int64 delay_ms = max<int64>(kMinHandshakeTimeoutMs,
                               1.5 * rtt_stats_.SmoothedRtt().ToMilliseconds());
   return QuicTime::Delta::FromMilliseconds(
       delay_ms << consecutive_crypto_retransmission_count_);
 }
 
 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,
           static_cast<int64>(2 * srtt.ToMilliseconds())));
 }
 
 const QuicTime::Delta QuicSentPacketManager::GetRetransmissionDelay() const {
   QuicTime::Delta retransmission_delay = send_algorithm_->RetransmissionDelay();
   // TODO(rch): This code should move to |send_algorithm_|.
   if (retransmission_delay.IsZero()) {
@@ skipping 46 matching lines @@
 
   // Set up a pacing sender with a 5 millisecond alarm granularity.
   using_pacing_ = true;
   send_algorithm_.reset(
       new PacingSender(send_algorithm_.release(),
                        QuicTime::Delta::FromMilliseconds(5),
                        kInitialUnpacedBurst));
 }
 
 }  // namespace net