netdev - Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

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Date:	Mon, 25 Jun 2012 21:55:37 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	subramanian.vijay@...il.com, dave.taht@...il.com,
	hans.schillstrom@...csson.com, netdev@...r.kernel.org,
	ncardwell@...gle.com, therbert@...gle.com, brouer@...hat.com
Subject: Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 26 Jun 2012 06:51:36 +0200

> On Mon, 2012-06-25 at 15:43 -0700, David Miller wrote:
> 
>> I don't agree with this change.
>> 
>> What is the point in having real classification configuration if
>> arbitrary places in the network stack are going to override SKB
>> priority with a fixed priority setting?
>> 
>> I bet the person who set listening socket priority really meant it and
>> does not expect you to override it.
> 
> 
> If I add a test on listener_sk->sk_priority being 0, would you accept
> the patch ? If classification is done after tcp stack, it wont be hurt
> by initial skb priority ?

It's better than your original patch, but it suffers from the same
fundamental problem.

No user is going to expect that TCP on it's own has choosen a
non-default priority and only for some packet types.  It's completely
unexpected behavior.

A SYN flood consumes so much more RX work than the TX for the SYNACK's
ever can.

So whilst I understand your desire to handle all elements of this kind
of attack, this one is reaching too far.

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