---

name: webrtc/modules/guard-empty-transport-feedback-before-delay-update description: When processing TransportPacketsFeedback in congestion controllers, treat empty feedback as a normal no-op early return and omit defensive infinite feedback_time checks rubric: readability severity: should_fix

Rule. When processing transport feedback messages (such as TransportPacketsFeedback) in congestion controllers, treat empty received packet data (e.g. via msg.PacketsWithFeedback().empty() or msg.ReceivedWithSendInfo().empty()) as a normal no-op early return without updating delay metrics or logging warnings. Do not add redundant defensive checks for msg.feedback_time.IsInfinite(), as feedback_time is the local arrival timestamp and is guaranteed to be finite for incoming feedback.

Why. In transport feedback protocols (such as RFC 8888), feedback reports containing only lost packets or keepalives are protocol-valid, so treating them as invalid or updating delay metrics corrupts estimators and spams logs. Conversely, feedback_time records the local reception clock time and is always finite, making defensive infinity checks redundant.

Examples

Bad:

// Bad: Conflating empty feedback with invalid data, missing early returns, or checking feedback_time infinity.
bool ScreamV2::IsFeedbackValid(const TransportPacketsFeedback& msg) const {
  if (msg.ReceivedWithSendInfo().empty()) {
    RTC_LOG(LS_WARNING) << "Received feedback without any received packets.";
    return false;
  }
  if (msg.feedback_time.IsInfinite()) {
    return false;
  }
  return true;
}

void DelayController::OnTransportPacketsFeedback(const TransportPacketsFeedback& msg) {
  last_smoothed_rtt_ = msg.smoothed_rtt;
  for (const PacketResult& packet : msg.SortedByReceiveTime()) {
    UpdateDelayEstimates(packet.receive_time - packet.sent_packet.send_time);
  }
}

Good:

// Good: Treat empty feedback as a normal no-op return; use feedback_time directly.
void DelayController::OnTransportPacketsFeedback(const TransportPacketsFeedback& msg) {
  if (msg.ReceivedWithSendInfo().empty()) {
    return;
  }
  last_smoothed_rtt_ = msg.smoothed_rtt;
  last_feedback_time_ = msg.feedback_time;
  for (const PacketResult& packet : msg.SortedByReceiveTime()) {
    UpdateDelayEstimates(packet.receive_time - packet.sent_packet.send_time);
  }
}