netdev - Re: [PATCH] tcp: use RTAX_CWND for outgoing connections properly

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Date:	Wed, 22 Dec 2010 11:00:39 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org, Vojtech Pavlik <vojtech@...e.cz>
Subject: Re: [PATCH] tcp: use RTAX_CWND for outgoing connections properly

On Fri, 17 Dec 2010, Jiri Kosina wrote:

> For outgoing connections, the initial value of cwnd is always set to 2 (in 
> tcp_v4_init_sock()) regardless of setting of RTAX_CWND. For incoming 
> connections, this is handled properly in tcp_init_metrics().
> 
> As a result of this, Linux TCP stack always uses cwnd == 2 at the beginning of
> outgoing TCP session (i.e. waits for ACK after 2 packets once the connection
> has been established) and grows it in accordance with slow-start algorithm
> only after it receives ACK for first two packets.
> 
> When slow-start triggers later during the connection (e.g. after idle), 
> cwnd is properly re-initialized to RTAX_CWND value (if specified) through 
> tcp_cwnd_restart() -> tcp_init_cwnd().
> 
> Initialize tp->snd_cwnd properly so that RTAX_CWND value is being used 
> also in the slow-start phase for the first packets in the connection.

This should of course read RTAX_INITCWND instead of RTAX_CWND in the whole 
changelog, sorry.

Besides that, any comments on this, please?

Thanks.

> 
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> ---
>  net/ipv4/tcp_output.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 61c2463..6dbc55b 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2572,6 +2572,8 @@ static void tcp_connect_init(struct sock *sk)
>  				  &rcv_wscale,
>  				  dst_metric(dst, RTAX_INITRWND));
>  
> +	tp->snd_cwnd = tcp_init_cwnd(tp, dst);
> +
>  	tp->rx_opt.rcv_wscale = rcv_wscale;
>  	tp->rcv_ssthresh = tp->rcv_wnd;
>  
> -- 
> 1.7.3.1
> 
> 

-- 
Jiri Kosina
SUSE Labs, Novell Inc.
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