From mboxrd@z Thu Jan 1 00:00:00 1970 From: Werner Almesberger Subject: Re: snd_cwnd drawn and quartered Date: Thu, 2 Jan 2003 03:08:58 -0300 Sender: netdev-bounce@oss.sgi.com Message-ID: <20030102030858.E1363@almesberger.net> References: <20021224225040.A22201@almesberger.net> <200301020138.EAA32608@sex.inr.ac.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@oss.sgi.com, chengjin@cs.caltech.edu Return-path: To: kuznet@ms2.inr.ac.ru Content-Disposition: inline In-Reply-To: <200301020138.EAA32608@sex.inr.ac.ru>; from kuznet@ms2.inr.ac.ru on Thu, Jan 02, 2003 at 04:38:46AM +0300 Errors-to: netdev-bounce@oss.sgi.com List-Id: netdev.vger.kernel.org kuznet@ms2.inr.ac.ru wrote: > It does the thing which it is supposed to do: prevents reducing cwnd below > 1/4 of original one. Okay, this it does. I guess the only case where this would make a difference is if your network duplicates packets. > This was proposed in one of rete-halving related drafts > with title sort of "...boundary checks...", I forgot exact title, can find > it if you are curious. I searched around but didn't spot anything. A pointer would be welcome, thanks ! > Maybe this is even correct, but I do not see why it can be essential. > cwnd falls too low not due to decrementing due to rate-halving, > but due to draining out in_flight when we are not able to keep pipe full. Yes, but rate-halving is what causes in-flight to drop in the first place (assuming we have enough fresh data to send, of course), no ? > Please, show. I've put graphs of a simulation run (with and without the change) at http://www.almesberger.net/misc/half.eps http://www.almesberger.net/misc/quarter.eps Y-axis is in segments, x-axis is in some arbitrary time unit, RTT is one initial cwnd (100 packets), path is asymmetric with zero-delay and loss-less backward channel. (While unusual, this shouldn't actually affect what TCP does in recovery.) Losses happen right before the packet hits the receiver. I've also asked Cheng if he can send you a copy of his simulator. Thanks, - Werner -- _________________________________________________________________________ / Werner Almesberger, Buenos Aires, Argentina wa@almesberger.net / /_http://www.almesberger.net/____________________________________________/