netdev - Re: [tcp] The ssthresh reduce during 3WHS

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Message-ID: <1394634390.21721.46.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 12 Mar 2014 07:26:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yong Zhang <fell198904@...il.com>
Cc:	netdev@...r.kernel.org, Jerry Chu <hkchu@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [tcp] The ssthresh reduce during 3WHS

On Wed, 2014-03-12 at 21:46 +0800, Yong Zhang wrote:
> hi，
> 
> I am trying to to apply patch 9ad7c049f0f79c418e293b1b68cf10d68f54fcdb
> to my old version kernel, but I am not understanding the following
> changes.
> 
> The comments int the patch say ssthresh may reduce unnecessarily
> druing 3WHS,  but i can not find the reduce operation of ssthresh
> during 3WHS.
> 
> Can you please explain when dose this may hanppen?
> 

You might CC patch author when asking such questions...


> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index bef9f04..ea0d218 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -880,6 +880,11 @@ static void tcp_init_metrics(struct sock *sk)
> 
>         tp->snd_ssthresh = dst_metric(dst, RTAX_SSTHRESH);
>                  if (tp->snd_ssthresh > tp->snd_cwnd_clamp)
>                          tp->snd_ssthresh = tp->snd_cwnd_clamp;
> +       } else {
> +                /* ssthresh may have been reduced unnecessarily during.
> +                * 3WHS. Restore it back to its initial default.
> +                */
> +                tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
>          }
> 
> 
> Thanks,
> Fel
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