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

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Message-Id: <20130122.162410.201366520574331812.davem@davemloft.net>
Date:	Tue, 22 Jan 2013 16:24:10 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	ncardwell@...gle.com
Cc:	avagin@...nvz.org, netdev@...r.kernel.org, criu@...nvz.org,
	linux-kernel@...r.kernel.org, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	edumazet@...gle.com, ycheng@...gle.com, xemul@...allels.com,
	davej@...hat.com, mtk.manpages@...il.com
Subject: Re: [PATCH net-next] tcp: add ability to set a timestamp offset

From: Neal Cardwell <ncardwell@...gle.com>
Date: Tue, 22 Jan 2013 16:18:04 -0500

> 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.

Right, this change seems pretty bogus as-is.
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