Comment Re:My ISP does not need AQM (Score 2) 119
Of course it is no magic solution, but there is no realistic way to provision in such a way that the receiver capacity in one node is greater than the total sender capacity of all the nodes it is connected to. This is of course depending on your specific network scenario, and some problems are solved with just adding more capacity, but in general it is not possible to solve all your network problems with just "more bandwidth". When downloading files it is almost always so that you have more send capacity than receive capacity, for example.
If you have more total send capacity than your node has receive capacity, the buffers are going to fill up while you tell the send nodes to slow down. Without queue management the senders slow down when they see dropped packets, and then continue to slow down until the total send rate equals the receive capacity, and there are no more dropped packets. At this point the buffers stay at the same level since in and out capacity in the buffer is exactly the same.
But since packets don't get dropped until the buffer is full, this means the buffer stays full until the send rate drops. With big transfers, this can take a long time. Now the buffer does no good, and in fact only introduce latency without having any room for additional bursts. What you want to do here is selectively drop packets so the send rate is a little bit slower than the receive capacity. This allows the buffer to empty while essentially still running at full speed. If you do that until the buffer is almost empty, then you have kept the steady state transfer rate at max, while reducing the buffer latency to zero instead of buffer_max_delay.
Also note that this can happen when the bottle-neck is not the network. If you have enough bandwidth eventually the disk at the receiver becomes the bottle-neck when it tries to write all the received data. This also means the senders need to slow down, and packets need to be buffered.