From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: [PATCH] tcp-illinois: incorrect beta usage Date: Wed, 28 Nov 2007 16:43:31 -0800 Message-ID: <474E0B33.2060108@linux-foundation.org> References: <001001c83063$9adbc9d0$d5897e82@csp.uiuc.edu> <20071128154725.29af2852@freepuppy.rosehill> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , Herbert Xu , shaoliu@princeton.edu, Douglas Leith , Robert Shorten , netdev@vger.kernel.org To: Lachlan Andrew Return-path: Received: from smtp2.linux-foundation.org ([207.189.120.14]:35538 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756843AbXK2Aoi (ORCPT ); Wed, 28 Nov 2007 19:44:38 -0500 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Lachlan Andrew wrote: > Thanks Stephen. > > A related problem (largely due to the published algorithm itself) is > that Illinois is very aggressive when it over-estimates the maximum > RTT. > > At high load (say 200Mbps and 200ms RTT), a backlog of packets builds > up just after a loss, causing the RTT estimate to become large. This > makes Illinois think that *all* losses are due to corruption not > congestion, and so only back off by 1/8 instead of 1/2. > > I can't think how to fix this except by better RTT estimation, or > changes to Illinois itself. Currently, I ignore RTT measurements when > sacked_out != 0 and have a heuristic "RTT aging" mechanism, but > that's pretty ugly. > > Cheers, > Lachlan > > Ageing the RTT estimates needs to be done anyway. Maybe something can be reused from H-TCP. The two are closely related.