This is wrong. BBR still does retransmission if packets do not arrive on time, and any retransmission in the inner TCP stack automatically becomes badput that w

This is wrong. BBR still does retransmission if packets do not arrive on time, and any retransmission in the inner TCP stack automatically becomes badput that wastes the outer TCP stack which is likely at the same time also doing retransmission for packet losses at lower layer. TCP_NOTSENT_LOWAT is not a solution to this either. It is a solution for HTTP/2 to be more aware of the congestion state and prioritize traffic properly, which itself does not do retransmission. It makes the buffer smaller so congestion is reported to upper layer earlier but still much later than when the actual congestion happens, distorting the upper/inner TCP congestion control. Also, a 128KiB value is hardcoded here for this knob, effectively rendering it only useful for a bandwidth delay product of 5Mbps * 200ms RTT.