[Cake] UDP floods and taking advantage of egress signalling at ingress

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Fri May 6 03:48:45 EDT 2016

My asleep brain is better than the awake one :-)

Tcp in essence self servos into equilibrium, udp flows (floods) potentially don't. If the protocol doesn't servo then something else should, ideally before buffer full/memory issues make that happen anyway.

Anyway time to switch brain to dayjob mode :-)


Kevin at Darbyshire-Bryant.me.uk<mailto:Kevin at darbyshire-bryant.me.uk>
Sent from my phone, apologies for brevity, spelling & top posting

On 6 May 2016, at 08:39, Dave Taht <dave.taht at gmail.com<mailto:dave.taht at gmail.com>> wrote:

On Fri, May 6, 2016 at 12:29 AM, Kevin Darbyshire-Bryant
<kevin at darbyshire-bryant.me.uk<mailto:kevin at darbyshire-bryant.me.uk>> wrote:
Hi All,

My brain woke up with this idea rattling around in it this
morning...obviously the subconscious has been busy.  So here it is:

Is there any way to use the egress drop signalling at ingress time to drop
stuff before it gets into the queue so then we don't have to drop it at

Something like: At enqueue if we've a matching flow check to see if that
flow had been in egress 'fast dropping' state *and* know how much data in
terms of time it had to fast drop to get the queue back under the nominal
time threshold.  If say it had to drop 10ms worth of packets to get back to
the nominal 5ms threshold then it dropped 67% of the packets/data.  I'd like
to think of that as an 'unresponsive flow'...hence could it be possible to
use that information at ingress time and in essence drop (some? 66%?) of
them there, we can also signal congestion to the stack at that point to
(cake already does this signalling when getting to its buffer size limit)

Probably a very silly idea.

No, actualy, I'd been thinking about the same thing myself for days.
:) I've always wanted a way to notify userspace that I was dropping
(the heck out of) something on egress to try and stuff it up on
ingress, or do something more intelligent like re-route the traffic

There was a lot of talk about adding the ability to drop stuff really
fast in the rx ring recently using the jit BPF stuff.

I was at a meeting where tom herbert talked about that, and I confess,
extremely dubious. Then... well, I see lots of drops on ingress
already I don't want, the udp flooding episode made me go look harder
at whole system behavior...

and I can't find the lwn article about it.


Kevin at Darbyshire-Bryant.me.uk<mailto:Kevin at darbyshire-bryant.me.uk>
M: +44 7947 355344 H: +44 1256 478597

Cake mailing list
Cake at lists.bufferbloat.net<mailto:Cake at lists.bufferbloat.net>

Dave T?ht
Let's go make home routers and wifi faster! With better software!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/cake/attachments/20160506/aa726f61/attachment.html>

More information about the Cake mailing list