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Date:	Sun, 1 Mar 2015 11:57:35 -0800
From:	Dave Taht <>
To:	Florian Westphal <>
Cc:	Tom Herbert <>,
	Eric Dumazet <>,
	David Miller <>,
	Linux Netdev List <>
Subject: Re: [PATCH net-next 3/6] flow_dissector: Add hash_extra field to
 flow_keys struct

On Sun, Mar 1, 2015 at 10:24 AM, Florian Westphal <> wrote:
> Tom Herbert <> wrote:
>> On Sat, Feb 28, 2015 at 12:31 PM, Florian Westphal <> wrote:
>> > I've been toying around with reducing skb->cb[] to 44 bytes,
>> > Seems Tom could integrate following patch from my test branch:
>> >
>> >
>> >
>> > It makes sfq use a smaller flow key state.
>> Alternatively, I think we might be able to eliminate the use of
>> flow_keys and flow_dissect from the qdisc code altogether. It looks
>> like this is only being used to determine a hash over the addresses,
>> ports, and protocol so I am thinking that we can just call
>> skb_get_hash for that. Will try to post some patches soon.
> The problem with this is that you'll lose the secret input to jhash
> in sfq_hash().

fq_codel relies on a secret input to the hash also, but it is only
created at instantation time. Retaining that secret input is critical
- otherwise an attacker can fairly easily derive the hash and find
ways of flooding the connection at low rates that are almost as
effective as simple attacks against pfifo_fast.

A note on sfq: it is commonly deployed with perturb 10 - which acts as
a "poor man's AQM" scrambling big flows and causing tcp cwnd
reductions - or at least it used to before linux gained much better
resilience in re-ordering. Either behavior - blowing up tcp flows
(then) - or allowing them to overbuffer (now) is pretty undesirable.

I have gone in detail on everything that is wrong with common SFQ
deployments here, commenting extensively on what was once, one of the
most popular shapers in the world:

> assuming you have packets p1 and p2 (from different flows)
> with skb_get_hash(p1) == skb_get_hash(p2) those flows share same
> queue/bin forever as the hash pertubation will no longer work.
> For sfq, hash collisions may exist as well but they'll be resolved
> after some time when q->perturbation (its part of hash input) is reseeded.
> --
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Dave Täht
Let's make wifi fast, less jittery and reliable again!
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