Tcp Ecn

Where this is already explained

• A speed test cannot see bufferbloat, and a smaller buffer is not the fix

  That bufferbloat is a hardware defect in somebody else's router, that a speed test will show it, and that the cure is a smaller buffer or a faster link. The queue is built by your own bulk flows, so a speed test — which measures exactly the quantity bufferbloat does not damage — reports the link as healthy while a video call on the same link is unusable. Shrinking the buffer trades latency for throughput and there is no single size that is right at two different link rates. And selecting fq_codel changes nothing at all unless you also shape a few percent below line rate, because until you do, the queue forms in the modem downstream of every qdisc you configured.

• TCP Congestion Control

  That throughput is set by the link's bandwidth, so a slow transfer means buying more of it. A single flow is bounded by window/RTT, and the window is capped first by the receive window, then by the loss rate a loss-based controller has to operate against — upgrading 1 Gbps to 10 Gbps moves none of those, and oversized buffers along the path make the window grow far past the bandwidth-delay product, inflating latency while throughput stays flat.