netdev - Re: [CRIU] TCP_REPAIR MSS issue

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Date:	Tue, 14 Jun 2016 06:03:35 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Eggert, Lars" <lars@...app.com>
Cc:	Pavel Emelyanov <xemul@...tuozzo.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"criu@...nvz.org" <criu@...nvz.org>,
	Andrey Vagin <avagin@...tuozzo.com>,
	"Honda, Michio" <michio@...app.com>
Subject: Re: [CRIU] TCP_REPAIR MSS issue

On Tue, 2016-06-14 at 11:40 +0000, Eggert, Lars wrote:
> On 2016-06-14, at 13:28, Pavel Emelyanov <xemul@...tuozzo.com> wrote:
> > Andrey (in Cc) has played with TCP_REPAIR recently, I guess he can know something.
> 
> Thanks for CC'ing him. We looked a little bit more into this:
> 
> When TCP_REPAIR is on, tcp_connect() directly calls tcp_finish_connect() before returning, passing NULL for skb, which causes sk_rx_dst_set() to be bypassed. Later, when TCP_REPAIR is being turned off, do_tcp_setsockopt() just does tcp_send_window_probe(), but apparently all the "dst" stuff is being bypassed then also, so the mss remains at TCP_MSS_DEFAULT.
> 
> Lars

What gives :

sysctl net/ipv4/ip_no_pmtu_disc net/ipv4/tcp_mtu_probing