Allow FakeNetworkPipe to drop packets in bursts.

webrtc/test/fake_network_pipe.cc

Index: webrtc/test/fake_network_pipe.cc
diff --git a/webrtc/test/fake_network_pipe.cc b/webrtc/test/fake_network_pipe.cc
index ea4e551f7ba7a52046bb3856d0266a639ce3a4dc..1c05a393b0b4de0ce874b2c54cd35ff6b6a1ebc7 100644
--- a/webrtc/test/fake_network_pipe.cc
+++ b/webrtc/test/fake_network_pipe.cc
@@ -34,7 +34,13 @@ FakeNetworkPipe::FakeNetworkPipe(Clock* clock,
       dropped_packets_(0),
       sent_packets_(0),
       total_packet_delay_(0),
-      next_process_time_(clock_->TimeInMilliseconds()) {}
+      bursting_(false),
+      next_process_time_(clock_->TimeInMilliseconds()) {
+  prob_lose_next_ = (1.0 - 1.0 / config_.avg_burst_loss_length);
+  prob_burst_loss_ = (static_cast<double>(config_.loss_percent) /

[terelius, 2016/05/30 14:00:29]

This should probably be
config_.loss_percent / (1-config_.loss_percent) /
(config_.avg_burst_loss_length)
in order to lose on average config_.loss_percent packets.

[philipel, 2016/05/30 14:42:42]

Done.

+                      config_.avg_burst_loss_length) /
+                     100.0;
+}
 
 FakeNetworkPipe::~FakeNetworkPipe() {
   while (!capacity_link_.empty()) {
@@ -118,8 +124,10 @@ void FakeNetworkPipe::Process() {
       NetworkPacket* packet = capacity_link_.front();
       capacity_link_.pop();
 
-      // Packets are randomly dropped after being affected by the bottleneck.
-      if (random_.Rand(100) < static_cast<uint32_t>(config_.loss_percent)) {
+      // Drop packets at an average rate of |config_.loss_percent| with
+      // and average loss burst length of |config_.avg_burst_loss_length|.
+      if (bursting_ || random_.Rand<double>() < prob_burst_loss_) {

[terelius, 2016/05/30 14:00:29]

It would easier to model the process if it was
if (bursting_) {
  // Drop the packet with some probability
}
else {
  // Drop the packet with some other probability
}
bursting = did_we_just_drop_a_packet

[philipel, 2016/05/30 14:42:42]

Done.

[stefan-webrtc, 2016/05/30 15:15:10]

Fyi, burst loss is typically modelled using a gilbert-elliot model. I think we
have an implementation already somewhere in the code

+        bursting_ = random_.Rand<double>() < prob_lose_next_;
         delete packet;
         continue;
       }