Quic Session Datagram Queue is ignored when considering whether App limited

[mpelletan]

Doing some prototyping using Quiche with WebTransport+BBR2, I found the datagram queue appeared to be ignored when assessing whether the app is limiting the bitrate:

1. QuicSession::datagram_queue_:

   quiche/quiche/quic/core/quic_session.h

   Line 979 in 9b45510

   QuicDatagramQueue datagram_queue_;

2. QuicSession::WillingAndAbleToWrite() does not consider the datagram queue, and therefore returns false when data is being paced by congestion control:

   quiche/quiche/quic/core/quic_session.cc

   Line 711 in 9b45510

   bool QuicSession::WillingAndAbleToWrite() const {

3. WillingAndAbleToWrite() is used by QuicConnection to determine whether congestion control is app limited:

   quiche/quiche/quic/core/quic_connection.cc

   Line 5277 in 9b45510

   void QuicConnection::CheckIfApplicationLimited() {

In my test, where the pipe is filled with datagrams, BBR constantly marks datapoints as app limited and never gets out of STARTUP phase.

Simply considering a non-empty datagram queue as willingness to write allows BBR to get into PROBE_BW, although I'm sill seeing poor behavior, where the RTT increases quite dramatically from ~40ms (min RTT) to sustained ~200ms when we hit a bitrate close to congestion, but I haven't looked into what's causing it (no draining on latency increase?)

[ianswett]

Wow, good catch. That is indeed a bug.