SQM help | bufferbloat | LTE connection

what is BQL?

BQL ist short for byte queue limits, see https://lwn.net/Articles/469652/ for some information. Effectively BQL removes the need for egress traffic shaping, assuming the modem/nic driver knows when packets are actually sent over the bottleneck. So for LTE it is the LTE modem that needs to implement that. As far as I can tell not even all Ethernet drivers support BQL, so my hopes for more esoteric devices like modems is pretty low.

1 Like

Yes, the most common modem protocols do not provide any feedback from the network. We just push packets to the modem firmware and hope for the best. The firmware will most likely keep a large transmit buffer which is out of our control. Lost battle.

QMI might be slightly better than mbim since there is no additional buffering in the driver. Well, ok, only two packets. You can also tune mbim to buffer less, but defaults are typically 16 kB or more.

That is a real pitty, especially with variable rate interfaces like RF-modems having this feedback would be priceless for proper egress shaping.

A two packet buffer would already help a lot, but I believe there are other larger buffers between the qdisc layer and the actual modem, right?

Yes. This is hidden in the modem firmware. We only control buffering between host and USB device. Buffers between USB device and radio are handled by the modem firmware. These buffers are probably large.