From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH net-next] tcp: add ability to set a timestamp offset Date: Tue, 22 Jan 2013 16:24:10 -0500 (EST) Message-ID: <20130122.162410.201366520574331812.davem@davemloft.net> References: <1358887953-28665-1-git-send-email-avagin@openvz.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: avagin@openvz.org, netdev@vger.kernel.org, criu@openvz.org, linux-kernel@vger.kernel.org, kuznet@ms2.inr.ac.ru, jmorris@namei.org, yoshfuji@linux-ipv6.org, kaber@trash.net, edumazet@google.com, ycheng@google.com, xemul@parallels.com, davej@redhat.com, mtk.manpages@gmail.com To: ncardwell@google.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:57211 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751904Ab3AVVYP (ORCPT ); Tue, 22 Jan 2013 16:24:15 -0500 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: From: Neal Cardwell Date: Tue, 22 Jan 2013 16:18:04 -0500 > On Tue, Jan 22, 2013 at 3:52 PM, Andrey Vagin 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.