Add a new QUIC Ack Decimation mode that is reordering tolerant. Protected by FLAG_quic_ack_decimati…

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 1dd656ebaa0a3972ca5842c72bd20d044294903d..d8ab40f485128a3ce639f9d3beaf86d39eb3f127 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -278,9 +278,10 @@ QuicConnection::QuicConnection(QuicConnectionId connection_id,
       received_packet_manager_(&stats_),
       ack_queued_(false),
       num_retransmittable_packets_received_since_last_ack_sent_(0),
+      last_ack_had_missing_packets_(false),
       num_packets_received_since_last_ack_sent_(0),
       stop_waiting_count_(0),
-      ack_decimation_enabled_(false),
+      ack_mode_(TCP_ACKING),
       delay_setting_retransmission_alarm_(false),
       pending_retransmission_alarm_(false),
       arena_(),
@@ -415,7 +416,11 @@ void QuicConnection::SetFromConfig(const QuicConfig& config) {
     debug_visitor_->OnSetFromConfig(config);
   }
   if (config.HasClientSentConnectionOption(kACKD, perspective_)) {
-    ack_decimation_enabled_ = true;
+    ack_mode_ = ACK_DECIMATION;
+  }
+  if (FLAGS_quic_ack_decimation2 &&
+      config.HasClientSentConnectionOption(kAKD2, perspective_)) {
+    ack_mode_ = ACK_DECIMATION_WITH_REORDERING;
   }
 }
 
@@ -1033,13 +1038,16 @@ void QuicConnection::MaybeQueueAck(bool was_missing) {
 
   // Determine whether the newly received packet was missing before recording
   // the received packet.
-  if (was_missing) {
+  // Ack decimation with reordering relies on the timer to send an ack, but if
+  // missing packets we reported in the previous ack, send an ack immediately.
+  if (was_missing && (ack_mode_ != ACK_DECIMATION_WITH_REORDERING ||
+                      last_ack_had_missing_packets_)) {
     ack_queued_ = true;
   }
 
   if (should_last_packet_instigate_acks_ && !ack_queued_) {
     ++num_retransmittable_packets_received_since_last_ack_sent_;
-    if (ack_decimation_enabled_ &&
+    if (ack_mode_ != TCP_ACKING &&
         last_header_.packet_number > kMinReceivedBeforeAckDecimation) {
       // Ack up to 10 packets at once.
       if (num_retransmittable_packets_received_since_last_ack_sent_ >=
@@ -1064,10 +1072,18 @@ void QuicConnection::MaybeQueueAck(bool was_missing) {
     }
 
     // If there are new missing packets to report, send an ack immediately.
-    // TODO(ianswett): Consider allowing 1ms of reordering for the
-    // ack decimation experiment.
     if (received_packet_manager_.HasNewMissingPackets()) {
-      ack_queued_ = true;
+      if (ack_mode_ == ACK_DECIMATION_WITH_REORDERING) {
+        // Wait the minimum of an eighth min_rtt and the existing ack time.
+        QuicTime ack_time = clock_->ApproximateNow().Add(
+            sent_packet_manager_.GetRttStats()->min_rtt().Multiply(0.125));
+        if (!ack_alarm_->IsSet() || ack_alarm_->deadline() > ack_time) {
+          ack_alarm_->Cancel();
+          ack_alarm_->Set(ack_time);
+        }
+      } else {
+        ack_queued_ = true;
+      }
     }
   }
 
@@ -1853,6 +1869,7 @@ void QuicConnection::SendAck() {
   ack_queued_ = false;
   stop_waiting_count_ = 0;
   num_retransmittable_packets_received_since_last_ack_sent_ = 0;
+  last_ack_had_missing_packets_ = received_packet_manager_.HasMissingPackets();
   num_packets_received_since_last_ack_sent_ = 0;
 
   packet_generator_.SetShouldSendAck(true);