netdev - Re: QDisc Implementation: Setting bit rate and getting RTT

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Date:   Tue, 3 Jul 2018 21:54:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     tflynn@...avis.edu
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: QDisc Implementation: Setting bit rate and getting RTT

On Tue, Jul 3, 2018 at 4:14 PM Taran Lynn <tflynn@...avis.edu> wrote:
>
> Hello,
> I'm new to linux development and am working on creating a qdisc module,
> similar to those under net/sched/sch_*.c. Currently I'm stuck on two
> things.
>
> 1. What's the best way to set the maximum bit rate?
> 2. How do I determine the RTT for packets?
>
> For (1) I'm currently tracking the number of bits sent, and only sending
> packets if they stay within the appropriate rate bound. Is there a better
> way to do this? I've looked at net/sched/sch_netem.c, and as far as I can
> tell it determines the delay between packets for a given rate and sends
> them after that delay has passed. However, when I tried to do this I got
> inconsistent rates, so I think I may be missing something.

It depends on what you want to do for bandwidth control.

If you simply want traffic policing, take a look at gen estimator
in net/core/gen_estimator.c. net/sched/act_police.c is an example
of how it is used.

If you want traffic shaping, it is harder, the simplest one in kernel
is probably net/sched/sch_tbf.c.

>
> As for (2) I'm not sure where to start.

Why do you care about RTT in Qdisc layer? RTT is kinda TCP
thing, while Qdisc is at L2...