3. Necessary Flow Behaviour

Responsive LL flows can use the L4S ECN codepoint to be classified into the LL queue. Smooth unresponsive flows can use the NQB Diffserv codepoint from RFC 9956 or local classifiers.

The QProt algorithm uses the same variable that drives ECN-marking probability in the LL queue. This makes decisions visible to ECN-capable responsive flows. A flow can objectively avoid QProt by keeping the bit rate of ECN-marked packets, the congestion-rate, below AGING. The default AGING is 2^19 B/s, approximately 4 Mb/s, and a congestion controller should keep its average congestion-rate comfortably below that level, for example around 1 Mb/s, to avoid transient triggering.