netdev - Re: inter-qdisc communication?

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Date:   Mon, 29 Aug 2022 17:03:08 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     Jakub Kicinski <kuba@...nel.org>
cc:     netdev@...r.kernel.org
Subject: Re: inter-qdisc communication?

(note the communication aspect is still open, see near the end)

On Fri, 26 Aug 2022, Jakub Kicinski wrote:

> How do you add latency on ingress? 🤔

I’ve found something that might do it.

https://serverfault.com/a/386791/189656 and
https://wiki.linuxfoundation.org/networking/netem#how_can_i_use_netem_on_incoming_traffic

So, basically:

$ extif=eth0  # or eth1 or wlan0 or…
$ sudo modprobe ifb  # once or via config
$ sudo ip link set dev ifb0 up
$ sudo tc qdisc add dev $extif handle FFFF: ingress
$ sudo tc filter add dev $extif parent FFFF: [… (see below) …]
$ sudo tc qdisc add dev ifb0 root myqdiscname opts…

All references to ifb seem to cargo-cult the following filter…

	protocol ip u32 match u32 0 0 flowid 1:1
	 action mirred egress redirect dev ifb0

… without explaining any of it. (I’ve concerned myself more with
the implementing of the qdisc than with the configuring, which my
coworkers did before, and I found the info quite… not easily
comprehensible.)

I’ve found that mirred means mirror or redirect, so the action
part’s probably fine. I’m very unsure of the protocol/match
part.

I require any and all traffic of all protocols to be redirected.
Not just IPv4, and not just traffic that matches anything. Can I
do that with the filter, and will this “trick” get me the effect
I want to have?

(I could just use netem but there’s still the issue of inter-
qdisc communication which I’d *very* much like to have, not just
for this but also for features that come later… and, perhaps, one
that’s already there — RAN “handover” emulation, i.e. stopping
all traffic for a few dozen ms or so.)

Thanks in advance,
//mirabilos
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