Cast: reduce the amount of retransmission packets

media/cast/video_sender/video_sender.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 "media/cast/video_sender/video_sender.h"
 
 #include <algorithm>
 #include <cstring>
 
 #include "base/bind.h"
@@ skipping 238 matching lines @@
 
 void VideoSender::ResendCheck() {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
   DCHECK(!last_send_time_.is_null());
   const base::TimeDelta time_since_last_send =
       cast_environment_->Clock()->NowTicks() - last_send_time_;
   if (time_since_last_send > target_playout_delay_) {
     if (latest_acked_frame_id_ == last_sent_frame_id_) {
       // Last frame acked, no point in doing anything
     } else {
-      const uint32 kickstart_frame_id = latest_acked_frame_id_ + 1;
-      VLOG(1) << "ACK timeout, re-sending frame " << kickstart_frame_id;
-      ResendFrame(kickstart_frame_id);
+      VLOG(1) << "ACK timeout; last acked frame: " << latest_acked_frame_id_;
+      ResendForKickstart();
     }
   }
   ScheduleNextResendCheck();
 }
 
 void VideoSender::OnReceivedCastFeedback(const RtcpCastMessage& cast_feedback) {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
 
   base::TimeDelta rtt;
   base::TimeDelta avg_rtt;
@@ skipping 31 matching lines @@
     }
     // We only count duplicate ACKs when we have sent newer frames.
     if (latest_acked_frame_id_ == cast_feedback.ack_frame_id_ &&
         latest_acked_frame_id_ != last_sent_frame_id_) {
       duplicate_ack_counter_++;
     } else {
       duplicate_ack_counter_ = 0;
     }
     // TODO(miu): The values "2" and "3" should be derived from configuration.
     if (duplicate_ack_counter_ >= 2 && duplicate_ack_counter_ % 3 == 2) {
-      // Resend last ACK + 1 frame.
-      const uint32 frame_to_resend = latest_acked_frame_id_ + 1;
-      VLOG(1) << "Received duplicate ACK for frame " << latest_acked_frame_id_
-              << ", will re-send frame " << frame_to_resend;
-      ResendFrame(frame_to_resend);
+      VLOG(1) << "Received duplicate ACK for frame " << latest_acked_frame_id_;
+      ResendForKickstart();
     }
   } else {
+    // Only count duplicated ACKs if there is no NACK request in between.
+    // This is to avoid aggresive resend.
+    duplicate_ack_counter_ = 0;
     transport_sender_->ResendPackets(
         false, cast_feedback.missing_frames_and_packets_);
     uint32 new_bitrate = 0;
     if (congestion_control_.OnNack(rtt, &new_bitrate)) {
       UpdateBitrate(new_bitrate);
     }
   }
 
   RtpTimestamp rtp_timestamp =
       frame_id_to_rtp_timestamp_[cast_feedback.ack_frame_id_ & 0xff];
@@ skipping 17 matching lines @@
     frames_in_flight +=
         static_cast<int32>(last_sent_frame_id_ - latest_acked_frame_id_);
   }
   VLOG(2) << frames_in_flight
           << " frames in flight; last sent: " << last_sent_frame_id_
           << " latest acked: " << latest_acked_frame_id_
           << " frames in encoder: " << frames_in_encoder_;
   return frames_in_flight >= max_unacked_frames_;
 }
 
-void VideoSender::ResendFrame(uint32 resend_frame_id) {
+void VideoSender::ResendForKickstart() {
   DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
   DCHECK(!last_send_time_.is_null());
+  VLOG(1) << "Resending first packet of frame " << last_sent_frame_id_
+          << " to kick-start.";
+  // Send the first packet of the last encoded frame to kick start
+  // retransmission. This gives enough information to the receiver what
+  // packets and frames are missing.
   MissingFramesAndPacketsMap missing_frames_and_packets;
   PacketIdSet missing;
-  missing_frames_and_packets.insert(std::make_pair(resend_frame_id, missing));
+  missing.insert(0);
+  missing_frames_and_packets.insert(
+      std::make_pair(last_sent_frame_id_, missing));
   last_send_time_ = cast_environment_->Clock()->NowTicks();
   transport_sender_->ResendPackets(false, missing_frames_and_packets);
 }
 
 void VideoSender::UpdateBitrate(int new_bitrate) {
   // Make sure we don't set the bitrate too insanely low.
   DCHECK_GT(new_bitrate, 1000);
   video_encoder_->SetBitRate(new_bitrate);
   current_requested_bitrate_ = new_bitrate;
 }
 
 }  // namespace cast
 }  // namespace media