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

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Message-Id: <20100125.234557.00486980.davem@davemloft.net>
Date:	Mon, 25 Jan 2010 23:45:57 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	damian@....rwth-aachen.de
Cc:	denys@...p.net.lb, ilpo.jarvinen@...sinki.fi,
	netdev@...r.kernel.org
Subject: Re: [PATCH] tcp: fix ICMP-RTO war

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'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...
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