netdev - Re: [PATCH net-next] tcp: add ability to set a timestamp offset

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Message-ID: <CADVnQynxigC7wSW6D1WQNZwxoBd7fpySa8eGN0W4wrh7WtHV4Q@mail.gmail.com>
Date:	Tue, 22 Jan 2013 16:18:04 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	Netdev <netdev@...r.kernel.org>, criu@...nvz.org,
	LKML <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Dave Jones <davej@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH net-next] tcp: add ability to set a timestamp offset

On Tue, Jan 22, 2013 at 3:52 PM, Andrey Vagin <avagin@...nvz.org> wrote:
> If a TCP socket will get live-migrated from one box to another the
> timestamps (which are typically ON) will get screwed up -- the new
> kernel will generate TS values that has nothing to do with what they
> were on dump. The solution is to yet again fix the kernel and put a
> "timestamp offset" on a socket.

One serious issue with this patch is that outgoing timestamp values
will no longer correspond to tcp_time_stamp, so echoed timestamp
values will also no longer have a meaningful relationship to
tcp_time_stamp. That violates assumptions made in several places in
the code, which assumes that we can compare echoed timestamp values to
tcp_time_stamp; for example, there are several places where we do
things like subtracting:
   tcp_time_stamp - tp->rx_opt.rcv_tsecr
to find the estimated RTT for a segment.

neal
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