Record the last packet send time before we start sending the packet

net/quic/quic_connection.cc

 // Copyright (c) 2012 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_connection.h"
 
 #include <string.h>
 #include <sys/types.h>
 
 #include <algorithm>
@@ skipping 1431 matching lines @@
            << ", encryption level: "
            << QuicUtils::EncryptionLevelToString(packet->encryption_level)
            << ", length:"
            << packet->serialized_packet.packet->length()
            << ", encrypted length:"
            << encrypted->length();
   DVLOG(2) << ENDPOINT << "packet(" << sequence_number << "): " << std::endl
            << QuicUtils::StringToHexASCIIDump(
                packet->serialized_packet.packet->AsStringPiece());
 
+  QuicTime packet_send_time = QuicTime::Zero();
+  if (FLAGS_quic_record_send_time_before_write) {
+    // Measure the RTT from before the write begins to avoid underestimating the
+    // min_rtt_, especially in cases where the thread blocks or gets swapped out
+    // during the WritePacket below.
+    packet_send_time = clock_->Now();
+  }
   WriteResult result = writer_->WritePacket(encrypted->data(),
                                             encrypted->length(),
                                             self_address().address(),
                                             peer_address());
   if (result.error_code == ERR_IO_PENDING) {
     DCHECK_EQ(WRITE_STATUS_BLOCKED, result.status);
   }
 
   if (result.status == WRITE_STATUS_BLOCKED) {
     visitor_->OnWriteBlocked();
     // If the socket buffers the the data, then the packet should not
     // be queued and sent again, which would result in an unnecessary
     // duplicate packet being sent.  The helper must call OnCanWrite
     // when the write completes, and OnWriteError if an error occurs.
     if (!writer_->IsWriteBlockedDataBuffered()) {
       return false;
     }
   }
-  QuicTime now = clock_->Now();
+  if (!FLAGS_quic_record_send_time_before_write) {
+    packet_send_time = clock_->Now();
+  }
+  if (!packet_send_time.IsInitialized()) {
+    // TODO(jokulik): This is only needed because of the two code paths for
+    // initializing packet_send_time.  Once "quic_record_send_time_before_write"
+    // is deprecated, this check can be removed.
+    LOG(DFATAL) << "The packet send time should never be zero. "
+                << "This is a programming bug, please report it.";
+  }
   if (result.status != WRITE_STATUS_ERROR && debug_visitor_.get() != nullptr) {
     // Pass the write result to the visitor.
     debug_visitor_->OnPacketSent(packet->serialized_packet,
                                  packet->original_sequence_number,
                                  packet->encryption_level,
                                  packet->transmission_type,
                                  *encrypted,
-                                 now);
+                                 packet_send_time);
   }
   if (packet->transmission_type == NOT_RETRANSMISSION) {
-    time_of_last_sent_new_packet_ = now;
+    time_of_last_sent_new_packet_ = packet_send_time;
   }
   SetPingAlarm();
-  DVLOG(1) << ENDPOINT << "time of last sent packet: "
-           << now.ToDebuggingValue();
+  DVLOG(1) << ENDPOINT << "time "
+           << (FLAGS_quic_record_send_time_before_write ?
+               "we began writing " : "we finished writing ")
+           << "last sent packet: "
+           << packet_send_time.ToDebuggingValue();
 
   // TODO(ianswett): Change the sequence number length and other packet creator
   // options by a more explicit API than setting a struct value directly,
   // perhaps via the NetworkChangeVisitor.
   packet_generator_.UpdateSequenceNumberLength(
       sent_packet_manager_.least_packet_awaited_by_peer(),
       sent_packet_manager_.EstimateMaxPacketsInFlight(max_packet_length()));
 
   bool reset_retransmission_alarm = sent_packet_manager_.OnPacketSent(
       &packet->serialized_packet,
       packet->original_sequence_number,
-      now,
+      packet_send_time,
       encrypted->length(),
       packet->transmission_type,
       IsRetransmittable(*packet));
 
   if (reset_retransmission_alarm || !retransmission_alarm_->IsSet()) {
     retransmission_alarm_->Update(sent_packet_manager_.GetRetransmissionTime(),
                                   QuicTime::Delta::FromMilliseconds(1));
   }
 
   stats_.bytes_sent += result.bytes_written;
@@ skipping 558 matching lines @@
   }
   for (size_t i = 0; i < retransmittable_frames->frames().size(); ++i) {
     if (retransmittable_frames->frames()[i].type == CONNECTION_CLOSE_FRAME) {
       return true;
     }
   }
   return false;
 }
 
 }  // namespace net