Implement an event-based simulator for QuicConnection

net/quic/core/congestion_control/simulation/link.cc

+#include "net/quic/core/congestion_control/simulation/link.h"
+#include "net/quic/core/congestion_control/simulation/simulator.h"
+
+namespace net {
+namespace simulation {
+
+// Parameters for random noise delay.
+const double kDelayModelMeanUs = 18;
+const double kDelayModelStandardDeviationUs = 3.7;
+const uint64_t kMaxRandomDelayUs = 10;
+
+OneWayLink::OneWayLink(Simulator* simulator,
+                       std::string name,
+                       UnconstrainedPortInterface* sink,
+                       QuicBandwidth bandwidth,
+                       QuicTime::Delta propagation_delay)
+    : Actor(simulator, name),
+      sink_(sink),
+      bandwidth_(bandwidth),
+      propagation_delay_(propagation_delay),
+      next_write_at_(QuicTime::Zero()) {}
+
+OneWayLink::QueuedPacket::QueuedPacket(std::unique_ptr<Packet> packet,
+                                       QuicTime dequeue_time)
+    : packet(std::move(packet)), dequeue_time(dequeue_time) {}
+
+void OneWayLink::AcceptPacket(std::unique_ptr<Packet> packet) {
+  DCHECK(TimeUntilAvailable().IsZero());
+  QuicTime::Delta transfer_time = bandwidth_.TransferTime(packet->size);
+  next_write_at_ = clock_->Now() + transfer_time;
+
+  packets_in_transit_.emplace(
+      std::move(packet),
+      next_write_at_ + propagation_delay_ + GetRandomDelay(transfer_time));
+  ScheduleNextPacketDeparture();
+}
+
+QuicTime::Delta OneWayLink::TimeUntilAvailable() {
+  const QuicTime now = clock_->Now();
+  if (next_write_at_ <= now) {
+    return QuicTime::Delta::Zero();
+  }
+
+  return next_write_at_ - now;
+}
+
+void OneWayLink::Act() {
+  DCHECK(!packets_in_transit_.empty());
+  DCHECK(packets_in_transit_.front().dequeue_time >= clock_->Now());
+
+  sink_->AcceptPacket(std::move(packets_in_transit_.front().packet));
+  packets_in_transit_.pop();
+
+  ScheduleNextPacketDeparture();
+}
+
+void OneWayLink::ScheduleNextPacketDeparture() {
+  if (packets_in_transit_.empty()) {
+    return;
+  }
+
+  Schedule(packets_in_transit_.front().dequeue_time);
+}
+
+QuicTime::Delta OneWayLink::GetRandomDelay(QuicTime::Delta transfer_time) {
+  QuicTime::Delta delta = QuicTime::Delta::FromMicroseconds(
+      simulator_->GetRandomGenerator()->RandUint64() % (kMaxRandomDelayUs + 1));
+  // Have an upper bound on the delay to ensure packets do not go out of order.
+  delta = std::min(delta, transfer_time * 0.5);
+  return delta;
+}
+
+SymmetricLink::SymmetricLink(Simulator* simulator,
+                             std::string name,
+                             UnconstrainedPortInterface* sink_a,
+                             UnconstrainedPortInterface* sink_b,
+                             QuicBandwidth bandwidth,
+                             QuicTime::Delta propagation_delay)
+    : a_to_b_link_(simulator,
+                   StringPrintf("%s (A-to-B)", name.c_str()),
+                   sink_b,
+                   bandwidth,
+                   propagation_delay),
+      b_to_a_link_(simulator,
+                   StringPrintf("%s (B-to-A)", name.c_str()),
+                   sink_a,
+                   bandwidth,
+                   propagation_delay) {}
+
+SymmetricLink::SymmetricLink(Endpoint* endpoint_a,
+                             Endpoint* endpoint_b,
+                             QuicBandwidth bandwidth,
+                             QuicTime::Delta propagation_delay)
+    : SymmetricLink(endpoint_a->simulator(),
+                    StringPrintf("Link [%s]<->[%s]",
+                                 endpoint_a->name().c_str(),
+                                 endpoint_b->name().c_str()),
+                    endpoint_a->GetRxPort(),
+                    endpoint_b->GetRxPort(),
+                    bandwidth,
+                    propagation_delay) {
+  endpoint_a->SetTxPort(&a_to_b_link_);
+  endpoint_b->SetTxPort(&b_to_a_link_);
+}
+
+}  // namespace simulation
+}  // namespace net