netdev - Re: [PATCH 4/5] tcp: allow undo from reordered DSACKs

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Date:	Thu, 17 Nov 2011 07:18:10 +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 4/5] tcp: allow undo from reordered DSACKs

On Wed, 16 Nov 2011, Neal Cardwell wrote:

> Previously, SACK-enabled connections hung around in TCP_CA_Disorder
> state while snd_una==high_seq, just waiting to accumulate DSACKs and
> hopefully undo a cwnd reduction. This could and did lead to the
> following unfortunate scenario: if some incoming ACKs advance snd_una
> beyond high_seq then we were setting undo_marker to 0 and moving to
> TCP_CA_Open, so if (due to reordering in the ACK return path) we
> shortly thereafter received a DSACK then we were no longer able to
> undo the cwnd reduction.
> 
> The change: Simplify the congestion avoidance state machine by
> removing the behavior where SACK-enabled connections hung around in
> the TCP_CA_Disorder state just waiting for DSACKs. Instead, when
> snd_una advances to high_seq or beyond we typically move to
> TCP_CA_Open immediately and allow an undo in either TCP_CA_Open or
> TCP_CA_Disorder if we later receive enough DSACKs.
> 
> Other patches in this series will provide other changes that are
> necessary to fully fix this problem.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> ---
>  net/ipv4/tcp_input.c |   15 ++-------------
>  1 files changed, 2 insertions(+), 13 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 751d390..a4efdd7 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2858,7 +2858,7 @@ static void tcp_try_keep_open(struct sock *sk)
>  	struct tcp_sock *tp = tcp_sk(sk);
>  	int state = TCP_CA_Open;
>  
> -	if (tcp_left_out(tp) || tcp_any_retrans_done(sk) || tp->undo_marker)
> +	if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
>  		state = TCP_CA_Disorder;
>  
>  	if (inet_csk(sk)->icsk_ca_state != state) {
> @@ -3066,17 +3066,6 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
>  			}
>  			break;
>  
> -		case TCP_CA_Disorder:
> -			tcp_try_undo_dsack(sk);
> -			if (!tp->undo_marker ||
> -			    /* For SACK case do not Open to allow to undo
> -			     * catching for all duplicate ACKs. */
> -			    tcp_is_reno(tp) || tp->snd_una != tp->high_seq) {
> -				tp->undo_marker = 0;
> -				tcp_set_ca_state(sk, TCP_CA_Open);
> -			}
> -			break;
> -
>  		case TCP_CA_Recovery:
>  			if (tcp_is_reno(tp))
>  				tcp_reset_reno_sack(tp);
> @@ -3117,7 +3106,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
>  				tcp_add_reno_sack(sk);
>  		}
>  
> -		if (icsk->icsk_ca_state == TCP_CA_Disorder)
> +		if (icsk->icsk_ca_state <= TCP_CA_Disorder)
>  			tcp_try_undo_dsack(sk);
>  
>  		if (!tcp_time_to_recover(sk)) {

How about extending Disorder state until second cumulative ACK that is 
acking >= high_seq?

-- 
 i.
--
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