netdev - Re: [PATCH 5/5] tcp: skip cwnd moderation in TCP_CA_Open in tcp_try_to

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Date:	Thu, 17 Nov 2011 07:14:15 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Neal Cardwell <ncardwell@...gle.com>
cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH 5/5] tcp: skip cwnd moderation in TCP_CA_Open in
 tcp_try_to_open

On Wed, 16 Nov 2011, Neal Cardwell wrote:

> The problem: Senders were overriding cwnd values picked during an undo
> by calling tcp_moderate_cwnd() in tcp_try_to_open().

I think it's intentional. Because of receiver lying bandwidth cheats all 
unlimited undos are bit dangerous.

> The fix: Don't moderate cwnd in tcp_try_to_open() if we're in
> TCP_CA_Open, since doing so is generally unnecessary and specifically
> would override a DSACK-based undo of a cwnd reduction made in fast
> recovery.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> ---
>  net/ipv4/tcp_input.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index a4efdd7..78dd38c 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2881,7 +2881,8 @@ static void tcp_try_to_open(struct sock *sk, int flag)
>  
>  	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
>  		tcp_try_keep_open(sk);
> -		tcp_moderate_cwnd(tp);
> +		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
> +			tcp_moderate_cwnd(tp);
>  	} else {
>  		tcp_cwnd_down(sk, flag);
>  	}

Wouldn't it be enough if tcp max burst is increased to match IW (iirc we 
had 3 still there as a magic number)?


-- 
 i.
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