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Date: Tue, 22 Jan 2013 16:18:04 -0500
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Subject: Re: [PATCH net-next] tcp: add ability to set a timestamp offset
From: Neal Cardwell <ncardwell@google.com>
To: Andrey Vagin <avagin@openvz.org>
Cc: Netdev <netdev@vger.kernel.org>, criu@openvz.org,
        LKML <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        James Morris <jmorris@namei.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Patrick McHardy <kaber@trash.net>, Eric Dumazet <edumazet@google.com>,
        Yuchung Cheng <ycheng@google.com>,
        Pavel Emelyanov <xemul@parallels.com>, Dave Jones <davej@redhat.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
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On Tue, Jan 22, 2013 at 3:52 PM, Andrey Vagin <avagin@openvz.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