Date: Wed, 6 Feb 2013 07:46:43 -0500 From: John Baldwin <jhb@freebsd.org> To: Randall Stewart <rrs@lakerest.net> Cc: Alfred Perlstein <bright@mu.org>, net@freebsd.org Subject: Re: [PATCH] Add a new TCP_IGNOREIDLE socket option Message-ID: <201302060746.43736.jhb@freebsd.org> In-Reply-To: <061B4EA5-6A93-48A0-A269-C2C3A3C7E77C@lakerest.net> References: <201301221511.02496.jhb@freebsd.org> <50FF06AD.402@networx.ch> <061B4EA5-6A93-48A0-A269-C2C3A3C7E77C@lakerest.net>
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On Wednesday, February 06, 2013 6:27:04 am Randall Stewart wrote: > John: > > A burst at line rate will *often* cause drops. This is because > router queues are at a finite size. Also such a burst (especially > on a long delay bandwidth network) cause your RTT to increase even > if there is no drop which is going to hurt you as well. > > A SHOULD in an RFC says you really really really really need to do it > unless there is some thing that makes you willing to override it. It is > slight wiggle room. > > In this I agree with Andre, we should not be *not* doing it. Otherwise > folks will be turning this on and it is plain wrong. It may be fine > for your network but I would not want to see it in FreeBSD. > > In my testing here at home I have put back into our stack max-burst. This > uses Mark Allman's version (not Kacheong Poon's) where you clamp the cwnd at > no more than 4 packets larger than your flight. All of my testing > high-bw-delay or lan has shown this to improve TCP performance. This > is because it helps you avoid bursting out so many packets that you overflow > a queue. > > In your long-delay bw link if you do burst out too many (and you never > know how many that is since you can not predict how full all those > MPLS queues are or how big they are) you will really hurt yourself even worse. > Note that generally in Cisco routers the default queue size is somewhere between > 100-300 packets depending on the router. Due to the way our application works this never happens, but I am fine with just keeping this patch private. If there are other shops that need this they can always dig the patch up from the archives. -- John Baldwin
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