netdev - Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on retransmit window

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Date:	Mon, 16 Jun 2014 15:39:28 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on
 retransmit window

On Mon, Jun 16, 2014 at 2:19 PM, Michal Kubecek <mkubecek@...e.cz> wrote:
> RFC 5681 says that ssthresh reduction in response to RTO should
> be done only once and should not be repeated until all packets
> from the first loss are retransmitted. RFC 6582 (as well as its
> predecessor RFC 3782) is even more specific and says that when
> loss is detected, one should mark current SND.NXT and ssthresh
> shouldn't be reduced again due to a loss until SND.UNA reaches
> this remembered value.
>
> In Linux implementation, this is done in tcp_enter_loss() but an
> additional condition
>
>   (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)
>
> allows to further reduce ssthresh before snd_una reaches the
> high_seq (the snd_nxt value at the previous loss) as
> icsk_retransmits is reset as soon as snd_una moves forward. As a
> result, if a retransmit timeout ouccurs early in the retransmit
> phase, we can adjust snd_ssthresh based on very low value of
> cwnd. This can be especially harmful for reno congestion control
> with slow linear cwnd growth in congestion avoidance phase.
>
> The patch removes the condition above so that snd_ssthresh is
> not reduced again until snd_una reaches high_seq as described in
> RFC 5681 and 6582.
>
> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

> ---
>  net/ipv4/tcp_input.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 40661fc..768ba88 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1917,8 +1917,7 @@ void tcp_enter_loss(struct sock *sk, int how)
>
>         /* Reduce ssthresh if it has not yet been made inside this window. */
>         if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
> -           !after(tp->high_seq, tp->snd_una) ||
> -           (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
> +           !after(tp->high_seq, tp->snd_una)) {
>                 new_recovery = true;
>                 tp->prior_ssthresh = tcp_current_ssthresh(sk);
>                 tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
> --
> 1.8.4.5
>
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