netdev - Re: [PATCHv3 net-next 3/4] ipv4: shrink current mss for tcp PMTU blackhole detection

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Date:	Sat, 28 Feb 2015 18:39:58 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	Fan Du <fan.du@...el.com>
Cc:	edumazet@...gle.com, David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Fan Du <fengyuleidian0615@...il.com>
Subject: Re: [PATCHv3 net-next 3/4] ipv4: shrink current mss for tcp PMTU
 blackhole detection

On Fri, Feb 27, 2015 at 10:23 PM, Fan Du <fan.du@...el.com> wrote:
> Reducing current tcp mss instead of search_low will make
> more sense, as current tcp mss still got lost.

> @@ -113,7 +114,8 @@ static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
>                         struct tcp_sock *tp = tcp_sk(sk);
>                         int mss;
>
> -                       mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
> +                       /* try mss smaller than current mss */
> +                       mss = tcp_current_mss(sk) >> 1;
>                         mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
>                         mss = max(mss, 68 - tp->tcp_header_len);
>                         icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
> @@ -176,7 +178,7 @@ static int tcp_write_timeout(struct sock *sk)

I don't believe there's a problem with the code as written.  Modulo
headers, search_low <= mss =< search_high.  This code, on successive
timeouts, widens the search range by adjusting search_low (and mss)
downward.

  -John
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