netdev - Re: [PATCH] tcp: fix ICMP-RTO war

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Date:	Wed, 27 Jan 2010 14:41:56 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	David Miller <davem@...emloft.net>
cc:	damian@....rwth-aachen.de, denys@...p.net.lb,
	Netdev <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH] tcp: fix ICMP-RTO war

On Mon, 25 Jan 2010, David Miller wrote:

> From: Damian Lukowski <damian@....rwth-aachen.de>
> Date: Mon, 25 Jan 2010 16:07:43 +0100
> 
> > @@ -530,7 +530,11 @@ static inline void tcp_bound_rto(const struct sock *sk)
> >  
> >  static inline u32 __tcp_set_rto(const struct tcp_sock *tp)
> >  {
> > -	return (tp->srtt >> 3) + tp->rttvar;
> > +	u32 rto = (tp->srtt >> 3) + tp->rttvar;
> > +	if (unlikely(rto < TCP_RTO_MIN))
> > +		return TCP_RTO_MIN;
> > +	else
> > +		return rto;
> >  }
> 
> The min RTO is now a runtime variable, TCP_RTO_MIN is merely the
> default, so we should use tcp_rto_min() for obtaining that value.
> 
> And if we make this change, we might want to delete the comment in
> tcp_set_rto() which claims:
> 
> 	/* NOTE: clamping at TCP_RTO_MIN is not required, current algo
> 	 * guarantees that rto is higher.
> 	 */
> 	tcp_bound_rto(sk);
> 
> And we have shown here at least one case where that is not true.
> :-)

I went through some history, it seems that this comment about the lower 
bound originates from Alexey [1]:

commit 893e1302654a9bcdc0e7d9ac95159657a8f5e0e8
Author: davem <davem>
Date:   Wed Dec 13 04:10:12 2000 +0000

    Fix numerous RTO bugs.
    Withdraw CWR when DSACKs are seen.
    Do not allow recvmsg on unbound socket, it is
    senseless.

    All from Alexey.


> I've looked at Denys's traces and your analysis, and I still
> can't figure out who the true culprit is that lets us get into
> such a state that RTO is evaluated so low...



-- 
 i.

[1] http://git.kernel.org/?p=linux/kernel/git/davem/netdev-vger-cvs.git;a=commit;h=893e1302654a9bcdc0e7d9ac95159657a8f5e0e8
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